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Andrea CORNIA

Professore Ordinario
Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche

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Pubblicazioni

2024 - Dual Structure of a Vanadyl-Based Molecular Qubit Containing a Bis(β-diketonato) Ligand [Articolo su rivista]
Imperato, Manuel; Nicolini, Alessio; Boniburini, Matteo; Sartini, Daniele; Benassi, Enrico; Chiesa, Mario; Gigli, Lara; Liao, Yu-Kai; Raza, Arsen; Salvadori, Enrico; Sorace, Lorenzo; Cornia, Andrea
abstract

We designed [VO(bdhb)] (1′) as a new electronic qubit containing an oxovanadium(IV) ion (S = 1/2) embraced by a single bis(β-diketonato) ligand [H2bdhb = 1,3-bis(3,5-dioxo-1-hexyl)benzene]. The synthesis afforded three different crystal phases, all of which unexpectedly contain dimers with formula [(VO)2(bdhb)2] (1). A trigonal form (1h) with a honeycomb structure and 46% of solvent-accessible voids quantitatively transforms over time into a monoclinic solvatomorph 1m and minor amounts of a triclinic solventless phase (1a). In a static magnetic field, 1h and 1m have detectably slow magnetic relaxation at low temperatures through quantum tunneling and Raman mechanisms. Angle-resolved electron paramagnetic resonance (EPR) spectra on single crystals revealed signatures of low-dimensional magnetic behavior, which is solvatomorph-dependent, being the closest interdimer V···V separations (6.7−7.5 Å) much shorter than intramolecular V···V distances (11.9−12.1 Å). According to 1H diffusion ordered spectroscopy (DOSY) and EPR experiments, the complex adopts the desired monomeric structure in organic solution and its geometry was inferred from density functional theory (DFT) calculations. Spin relaxation measurements in a frozen toluene-d8/CD2Cl2 matrix yielded Tm values reaching 13 μs at 10 K, and coherent spin manipulations were demonstrated by Rabi nutation experiments at 70 K. The neutral quasi-macrocyclic structure, featuring nuclear spin-free donors and additional possibilities for chemical functionalization, makes 1′ a new convenient spin-coherent building block in quantum technologies.

2024 - Quantum Spin Coherence and Electron Spin Distribution Channels in Vanadyl-containing Lantern Complexes [Articolo su rivista]
Imperato, Manuel; Nicolini, Alessio; Borsari, Marco; Briganti, Matteo; Chiesa, Mario; Liao, Yu-Kai; Ranieri, Antonio; Raza, Arsen; Salvadori, Enrico; Sorace, Lorenzo; Cornia, Andrea
abstract

We herein investigate the heterobimetallic lantern complexes [PtVO(SOCR)4] as charge neutral electronic qubits based on vanadyl complexes (S = 1/2) with nuclear spin-free donor atoms. The derivatives with R = Me (1) and Ph (2) give highly resolved X-band EPR spectra in frozen CH2Cl2/toluene solution, which evidence the usual hyperfine coupling to the 51V nucleus (I = 7/2) and an additional superhyperfine interaction with the I = 1/2 nucleus of the 195Pt isotope (natural abundance ca. 34%). DFT calculations ascribe the spin-density delocalization on the Pt2+ ion to a combination of π and δ pathways, with the former representing the predominant channel. Spin relaxation measurements in frozen CD2Cl2/toluene-d8 solution between 90 and 10 K yield Tm values (1-6 μs in 1 and 2-11 μs in 2) which match or even outperform those of known vanadyl-based qubits in similar matrices. Coherent spin manipulations indeed prove possible at 70 K, as shown by the observation of Rabi oscillations in nutation experiments. The results indicate that the heavy Group 10 metal is not detrimental to the coherence properties of the vanadyl moiety and that Pt-VO lanterns can be used as robust spin-coherent building blocks in materials science and quantum technologies.

2023 - A remarkably unsymmetric hexairon core embraced by two high-symmetry tripodal oligo-α-pyridylamido ligands [Articolo su rivista]
Nicolini, Alessio; Pankratz, Trey C.; Borsari, Marco; Clérac, Rodolphe; Ranieri, Antonio; Rouzières, Mathieu; Berry, John F.; Cornia, Andrea
abstract

Oligo-α-pyridylamides offer an appealing route to polyiron complexes with short Fe-Fe separations and large room temperature magnetic moments. A derivative of tris(2‐aminoethyl)amine (H6tren) containing three oligo-α-pyridylamine branches and thirteen nitrogen donors (H6L) reacts with [Fe2(Mes)4] to yield an organic nanocage built up by two tripodal ligands with interdigitated branches (HMes = mesitylene). The nanocage has crystallographic D3 symmetry but hosts a remarkably unsymmetric hexairon-oxo core, with a central Fe5(μ5-O) square pyramid, two oxygen donors bridging basal sites, and an additional Fe center residing in one of the two tren-like pockets. Bond Valence Sum (BVS) analysis, Density Functional Theory (DFT) calculations, and electrochemical data were then used to establish the protonation state of oxygen atoms and the formal oxidation states of the metals. To this purpose, a specialized set of BVS parameters was devised for Fe2+-N3- bonds with nitrogen donors of oligo-α-pyridylamides. This allowed us to formulate the compound as [Fe6O2(OH)(H3L)L], with nominally four FeII and two FeIII ions. Mössbauer spectra indicate that the compound contains two unique FeII sites, identified as a pair of closely spaced hydroxo-bridged metal ions in the central Fe5(μ5-O) pyramid, and a substantially valence-delocalized FeII2FeIII2 unit. Broken-symmetry DFT calculations predict strong ferromagnetic coupling between the two iron(II) ions, leading to a local S = 4 state that persists to room temperature and explaining the large magnetic moment measured at 300 K. The compound behaves as a single-molecule magnet, with magnetization dynamics detectable in zero static field and dominated by an Orbach-like mechanism with Ueff/kB = 49(2) K and τ0 = 4(2)·10−10 s.

2023 - An Efficient Transition-metal-free Route to Oligo-α-pyridylamines via Fluoroarenes [Articolo su rivista]
Nicolini, A.; Anderlini, B.; Roncaglia, F.; Cornia, A.
abstract

The new polynucleating tripodal proligand H6tren(dpa)3, containing thirteen nitrogen donors of four different types, was designed, synthesized, and isolated in good yield (~60%) via a transition-metal-free triple N-arylation of H6tren with HXdpa (X = Br or F), using K2CO3 or Cs2CO3 as a base (H6tren = tris(2-aminoethyl)amine, HXdpa = 6-halogeno-N-(pyridin-2-yl)pyridin-2-amine). HFdpa was prepared with excellent yield (90−92%) by reaction of 2,6-difluoropyridine with 2-aminopyridine in LiH/toluene/py and was found more reactive than HBrdpa, affording higher conversion and higher yield. Use of Cs2CO3 turned out to be essential for achieving high regioselectivity and eliminating overarylation almost completely.

2023 - Challenges in the Direct Detection of Chirality-induced Spin Selectivity: Investigation of Foldamer-based Donor-Acceptor Dyads [Articolo su rivista]
Privitera, Alberto; Faccio, Davide; Giuri, Demetra; Latawiec, Elisabeth I.; Genovese, Damiano; Tassinari, Francesco; Mummolo, Liviana; Chiesa, Mario; Fontanesi, Claudio; Salvadori, Enrico; Cornia, Andrea; Wasielewski, Michael R.; Tomasini, Claudia; Sessoli, Roberta
abstract

Over the past two decades, the chirality-induced spin selectivity (CISS) effect was reported in several experiments disclosing a unique connection between chirality and electron spin. Recent theoretical works highlighted time-resolved Electron Paramagnetic Resonance (trEPR) as a powerful tool to directly detect the spin polarization resulting from CISS. Here, we report a first attempt to detect CISS at the molecular level by linking the pyrene electron donor to the fullerene acceptor with chiral peptide bridges of different lengths and electric dipole moment. The dyads are investigated by an array of techniques, including cyclic voltammetry, steady-state and transient optical spectroscopies, and trEPR. Despite the promising energy alignment of the electronic levels, our multi-technique analysis reveals no evidence of electron transfer (ET), highlighting the challenges of spectroscopic detection of CISS. However, the analysis allows the formulation of guidelines for the design of chiral organic model systems suitable to directly probe CISS-polarized ET.

2022 - A tetrairon(III) single-molecule magnet and its solvatomorphs: synthesis, crystal structures and vapor-phase processing [Articolo su rivista]
Cornia, Andrea; Nava, Andrea; Lanzilotto, Valeria; Poneti, Giordano; Mannini, Matteo; Sessoli, Roberta
abstract

With the aim of investigating the impact of lattice solvent on the processability of tetrairon(III) single-molecule magnets by thermal sublimation, two new solvatomorphs of [Fe4(LPh)2(dpm)6] (1) were prepared and structurally characterized along with unsolvated 1 (H3LPh = 2-(hydroxymethyl)-2-phenylpropane-1,3-diol, Hdpm = dipivaloylmethane). All solvatomorphs crystallize in the C2/c space-group whereas solvent-free 1 belongs to a different space group (P21/c). The pitch of the propeller-like tetrairon(III) molecules is distinctly different in solvated vs. unsolvated phases, highlighting the effect of intermolecular interactions and crystal packing. The compounds sublimate at 450 - 490 K (in high vacuum conditions ~ 10^-6-10^-7 mbar) affording thick deposits which display different crystallinity depending on the particular starting material used. However, all sublimated samples retain slow magnetic relaxation with thermal activation parameters comparable to those of microcrystalline 1. The results indicate that factors other than mere molecular structure have a limited influence on the processability of these materials by thermal sublimation.

2022 - Structural Diversity of Lithium Oligo-α-Pyridylamides [Articolo su rivista]
Raza, A.; Mucci, A.; Nicolini, A.; Cornia, A.
abstract

Lithium oligo-α-pyridylamides are useful intermediates in coordination chemistry. Upon trans-metalation they have afforded a variety of extended metal atom chains (EMACs), which are currently investigated as molecular wires and single-molecule magnets. However, structural information on this class of compounds is scarce. Two trilithium salts of a new, sterically encumbered oligo-α-pyridylamido ligand were isolated in crystalline form and structurally characterized in the solid state and in solution. Lithiation of N2-(trimethylsilyl)-N6-{6-[(trimethylsilyl)amino]pyridin-2-yl}pyridine-2,6-diamine (H3 L) with n-BuLi in thf yielded dimeric adduct [Li6 L2 (thf)6 ] (1), which was crystallized from n-hexane/thf as 1·C6 H14. Crystals of a tetra-thf solvate with formula [Li6 L2 (thf)4 ] (2) were also obtained. The compounds feature two twisted L3− ligands exhibiting a cis-cis conformation and whose five nitrogen donors are all engaged in metal coordination. The six Li+ ions per molecule display coordination numbers ranging from 3 to 5. Compound 1·C6 H14 was investigated by multin-uclear 1D and 2D NMR spectroscopy, including1 H DOSY experiments, which indicated retention of the dimeric structure in benzene-d6 solution. To the best of our knowledge, 1 and 2 are the longest-chain lithium oligo-α-pyridylamides structurally authenticated so far, thereby qualifying as appealing intermediates to access high-nuclearity EMACs by trans-metalation.

2022 - The structure of a pentachromium(II) extended metal atom chain at 3 K: Cotton's conjecture proven [Articolo su rivista]
Cornia, Andrea; Nicolini, Alessio; Mcmonagle, Charles J.; Probert, Michael R.
abstract

We provide definitive experimental proof that the archetypal string-like compound [Cr5(tpda)4(NCS)2] has alternating long and short Cr–Cr separations in the solid state, as conjectured by F. A. Cotton, rather than essentially equally spaced Cr atoms, as initially claimed (H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine). Single-crystal X-ray data collected from 292 to 3 K revealed that the misinterpretation is caused by pseudo-merohedral twinning and that bond length alternation is enhanced at low temperature.

2021 - Engineering chemisorption of Fe4 single-molecule magnets on gold [Articolo su rivista]
Poggini, Lorenzo; Tancini, Erik; Danieli, Chiara; Luigi Sorrentino, Andrea; Serrano, Giulia; Lunghi, Alessandro; Malavolti, Luigi; Cucinotta, Giuseppe; Barra, Anne-Laure; Juhin, Amélie; Arrio, Marie-Anne; Li, Weibin; Otero, Edwige; Ohresser, Philippe; Joly, Loïc; Paul Kappler, Jean; Totti, Federico; Sainctavit, Philippe; Caneschi, Andrea; Sessoli, Roberta; Cornia, Andrea; Mannini, Matteo
abstract

Gaining control over the grafting geometry is critically important for any application of surface-supported single-molecule magnets (SMMs) in data storage, spintronics and quantum information science. Tetrairon(III) SMMs with a propeller-like structure are functionalized with thioacetyl-terminated alkyl chains (CH2)nSAc of different lengths (n = 3, 4, and 5) to promote chemisorption on gold surfaces and to evaluate differences in adsorption geometry and magnetic properties as a function of n. The prepared monolayers are investigated using X-ray absorption techniques with linearly and circularly polarized light to extract geometrical and magnetic information, respectively. All derivatives remain intact and form partially oriented monolayers on the gold surface. According to the results of ligand field multiplet analysis, the three-fold molecular axis is invariably biased towards the surface normal (z). Assuming that its deviation from z follows an angular step distribution with maximum angle ξMAX, the three derivatives have ξMAX = 10±5°, 48±2°, and 51±2°, respectively. The five-fold smaller angle spanned by the shortest-chain derivative (n = 3) is in gross agreement with the results of ab initio calculations. Monolayers of the derivative with n = 3 are further studied with an ultra-low temperature X-ray magnetic circular dichroism setup operating down to 350 mK. The isothermal field sweeps with the magnetic field at normal incidence show an open hysteresis loop below 1 K, while measurements at different incidence angles clearly prove the magnetic anisotropy of the monolayers.

2021 - Stereoisomerism in Tetrametallic Propeller-like Complexes: a Solid-state and Solution NMR Study on a Tetragallium(III) Derivative [Articolo su rivista]
Cornia, Andrea; Mucci, Adele; Briganti, Matteo; Bridonneau, Nathalie; Nava, Andrea; Nicolini, Alessio
abstract

Tetragallium(III) complex in [Ga4(L4-Py)2(dpm)6]•EtOH, with H3L4-Py = 2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol and Hdpm = dipivaloylmethane, was investigated as a diamagnetic analogue of tetrametallic, propeller-like single-molecule magnets (SMMs). The chiral molecular structure partitions the six CH2 protons of each (L4-Py)3- tripodal ligand into two diastereotopic sets. The two signals were clearly detected by 1H NMR spectroscopy in C6D6, proving that Λ and Δ enantiomers interconvert slowly over NMR timescale. Density functional theory calculations provided quantitative agreement with the observed values of chemical shifts and scalar coupling constants across both geminal and long-range interaction pathways. The solid-state structure suggests the occurrence of a lower symmetry stereoisomer (27 mol%), which was clearly identified in the NMR spectra. Since Fe3+ forms distinctly more inert complexes than Ga3+, comparable or greater configurational stability is expected for the isostructural FeIII4, FeIII3CrIII, and FeIII3VIII SMMs, which are difficult to investigate by solution NMR because of the strong paramagnetism.

2021 - Tetrairon(II) extended metal atom chains as single-molecule magnets [Articolo su rivista]
Nicolini, A; Affronte, M; SantaLucia, Dj; Borsari, M; Cahier, B; Caleffi, M; Ranieri, A; Berry, Jf; Cornia, A
abstract

Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(II), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and [Fe4(tpda)3Br2] (1Br), obtained by reacting iron(II) dihalides with [Fe2(Mes)4] and N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine (H2tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mossbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with chiMT at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(II) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm-1, |E/D| = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm-1, |E/D| = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.

2020 - Quantum dynamics of a single molecule magnet on superconducting Pb(111) [Articolo su rivista]
Serrano, G.; Poggini, L.; Briganti, M.; Sorrentino, A. L.; Cucinotta, G.; Malavolti, L.; Cortigiani, B.; Otero, E.; Sainctavit, P.; Loth, S.; Parenti, F.; Barra, A. -L.; Vindigni, A.; Cornia, A.; Totti, F.; Mannini, M.; Sessoli, R.
abstract

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(iii) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

2020 - S-functionalized tripods with monomethylene spacers: Routes to tetrairon(III) single-molecule magnets with ultrashort tethering groups [Articolo su rivista]
Cornia, A.; Danieli, C.; Meglioli, F.; Tancini, E.; Nicolini, A.; Rodriguez-Douton, M. J.; Barra, A. -L.; Affronte, M.; Sessoli, R.
abstract

The organization of single-molecule magnets (SMMs) on surfaces is a mainstream research path in molecular magnetism. Of special importance is the control of grafting geometry in chemisorbed monolayers on metal surfaces. We herein present the synthesis, solid-state structure, and magnetic characterization of propeller-like tetrairon(III) SMMs containing the shortest-reported tethering groups for gold surfaces. Functionalization of molecular structure is attained using 2-R-2-(hydroxymethyl)propane-1,3-diol tripodal proligands (H3LR). The R substituents comprise a monomethylene spacer and three different terminations known to act as stable precursors of S-Au bonds (R = CH2SCN, CH2SAc and CH2SSnBu). These chemical groups are shown to be chemically compatible with the tetrairon(III) core and to afford fully-functional SMMs in crystalline form and in fair to excellent yields.

2020 - The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains [Articolo su rivista]
Cornia, A.; Barra, A. -L.; Bulicanu, V.; Clerac, R.; Cortijo, M.; Hillard, E. A.; Galavotti, R.; Lunghi, A.; Nicolini, A.; Rouzieres, M.; Sorace, L.; Totti, F.
abstract

Chromium(II)-based extended metal atom chains have been the focus of considerable discussion regarding their symmetric versus unsymmetric structure and magnetism. We have now investigated four complexes of this class, namely, [Cr3(dpa)4X2] and [Cr5(tpda)4X2] with X = Cl- and SCN- [Hdpa = dipyridin-2-yl-amine; H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine]. By dc/ac magnetic techniques and EPR spectroscopy, we found that all these complexes have easy-axis anisotropies of comparable magnitude in their S = 2 ground state (|D| = 1.5-1.8 cm-1) and behave as single-molecule magnets at low T. Ligand-field and DFT/CASSCF calculations were used to explain the similar magnetic properties of tri- versus pentachromium(II) strings, in spite of their different geometrical preferences and electronic structure. For both X ligands, the ground structure is unsymmetric in the pentachromium(II) species (i.e., with an alternation of long and short Cr-Cr distances) but is symmetric in their shorter congeners. Analysis of the electronic structure using quasi-restricted molecular orbitals (QROs) showed that the four unpaired electrons in Cr5 species are largely localized in four 3d-like QROs centered on the terminal, "isolated" Cr2+ ion. In Cr3 complexes, they occupy four nonbonding combinations of 3d-like orbitals centered only on the two terminal metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level pattern of the terminal ions, whose coordination environment remains quite similar irrespective of chain length. We conclude that the extent of unpaired-electron delocalization has little impact on the magnetic anisotropy of these wire-like molecular species.

2020 - Unbiased evaluation of zero-field splitting D parameter in high-spin molecules from DC magnetic data with incomplete powder averaging [Articolo su rivista]
Cornia, A.; Barra, A. -L.; Poneti, G.; Tancini, E.; Sessoli, R.
abstract

A simple scheme is presented to account for preferential orientation effects in the DC magnetic response of polycrystalline samples of anisotropic high-spin molecules, like single-molecule magnets. A single additional least-squares parameter is introduced in the fitting of isothermal magnetization vs. field data to describe the leading part of a non-spherical distribution of anisotropy axes. The procedure is shown to afford an accurate D parameter and is potentially applicable whenever complete powder averaging cannot be achieved.

2019 - Propeller-Shaped Fe4 and Fe3M Molecular Nanomagnets: A Journey from Crystals to Addressable Single Molecules [Articolo su rivista]
Cornia, Andrea; Mannini, Matteo; Sessoli, Roberta; Gatteschi, Dante
abstract

This Microreview covers the preparation and investigation of Fe4 and Fe3M complexes with a propeller-like structure, most of which function as single-molecule magnets (SMMs). The magnetic characterization of these highly symmetric and chemically versatile complexes has enabled the establishment of important magneto-structural correlations regarding magnetic anisotropy and magnetic relaxation mechanisms in SMM materials. Their chemical stability and easy functionalization have been further exploited to process them into thin films, down to monolayers and submonolayers, using solution or vapor-phase methods. The proof so obtained that surface-supported molecules can largely retain the rich magnetic behavior of bulk phases has been a turning point in SMM research, which has the integration of SMMs into spintronic devices among its most ambitious goals.

2018 - A Pseudo-Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero-Field Single-Molecule Magnet with Easy Axis Anisotropy [Articolo su rivista]
Rigamonti, Luca; Bridonneau, Nathalie; Poneti, Giordano; Tesi, Lorenzo; Sorace, Lorenzo; Pinkowicz, Dawid; Jover, Jesus; Ruiz, Eliseo; Sessoli, Roberta; Cornia, Andrea
abstract

The homoleptic mononuclear compound [Co(bpp-COOMe)2](ClO4)2 (1) (bpp-COOMe=methyl 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate) crystallizes in the monoclinic C2/c space group, and the cobalt(II) ion possesses a pseudo-octahedral environment given by the two mer-coordinated tridentate ligands. Direct-current magnetic data, single-crystal torque magnetometry, and EPR measurements disclosed the easy-axis nature of this cobalt(II) complex, which shows single-molecule magnet behavior when a static field is applied in alternating-current susceptibility measurements. Diamagnetic dilution in the zinc(II) analogue [Zn(bpp-COOMe)2](ClO4)2 (2) afforded the derivative [Zn0.95Co0.05(bpp-COOMe)2](ClO4)2 (3), which exhibits slow relaxation of magnetization even in zero field thanks to the reduction of dipolar interactions. Theoretical calculations confirmed the overall electronic structure and the magnetic scenario of the compound as drawn by experimental data, thus confirming the spin-phonon Raman relaxation mechanism, and a direct quantum tunneling in the ground state as the most plausible relaxation pathway in zero field.

2018 - Filling the Gap in Extended Metal Atom Chains: Ferromagnetic Interactions in a Tetrairon(II) String Supported by Oligo-α-pyridylamido Ligands [Articolo su rivista]
Nicolini, Alessio; Galavotti, Rita; Barra, Anne-Laure; Borsari, Marco; Caleffi, Matteo; Luo, Guangpu; Novitchi, Ghenadie; Park, Kyungwha; Ranieri, Antonio; Rigamonti, Luca; Roncaglia, Fabrizio; Train, Cyrille; Cornia, Andrea
abstract

The stringlike complex [Fe4(tpda)3Cl2] (2; H2tpda = N2,N6-bis(pyridin-2-yl)pyridine-2,6-diamine) was obtained as the first homometallic extended metal atom chain based on iron(II) and oligo-α-pyridylamido ligands. The synthesis was performed under strictly anaerobic and anhydrous conditions using dimesityliron, [Fe2(Mes)4] (1; HMes = mesitylene), as both an iron source and a deprotonating agent for H2tpda. The four lined-up iron(II) ions in the structure of 2 (Fe⋯Fe = 2.94-2.99 Å, Fe⋯Fe⋯Fe = 171.7-168.8°) are wrapped by three doubly deprotonated twisted ligands, and the chain is capped at its termini by two chloride ions. The spectroscopic and electronic properties of 2 were investigated in dichloromethane by UV-vis-NIR absorption spectroscopy, 1H NMR spectroscopy, and cyclic voltammetry. The electrochemical measurements showed four fully resolved, quasi-reversible one-electron-redox processes, implying that 2 can adopt five oxidation states in a potential window of only 0.8 V. Direct current (dc) magnetic measurements indicate dominant ferromagnetic coupling at room temperature, although the ground state is only weakly magnetic. On the basis of density functional theory and angular overlap model calculations, this magnetic behavior was explained as being due to two pairs of ferromagnetically coupled iron(II) ions (J = -21 cm-1 using JŜi·Ŝj convention) weakly antiferromagnetically coupled with each other. Alternating-current susceptibility data in the presence of a 2 kOe dc field and at frequencies up to 1.5 kHz revealed the onset of slow magnetic relaxation below 2.8 K, with the estimated energy barrier Ueff/kB = 10.1(1.3) K.

2018 - Mössbauer spectroscopy of a monolayer of single molecule magnets [Articolo su rivista]
Cini, Alberto; Mannini, Matteo; Totti, Federico; Fittipaldi, Maria; Spina, Gabriele; Chumakov, Aleksandr; Rüffer, Rudolf; Cornia, Andrea; Sessoli, Roberta
abstract

The use of single molecule magnets (SMMs) as cornerstone elements in spintronics and quantum computing applications demands that magnetic bistability is retained when molecules are interfaced with solid conducting surfaces. Here, we employ synchrotron Mössbauer spectroscopy to investigate a monolayer of a tetrairon(III) (Fe4) SMM chemically grafted on a gold substrate. At low temperature and zero magnetic field, we observe the magnetic pattern of the Fe4 molecule, indicating slow spin fluctuations compared to the Mössbauer timescale. Significant structural deformations of the magnetic core, induced by the interaction with the substrate, as predicted by ab initio molecular dynamics, are also observed. However, the effects of the modifications occurring at the individual iron sites partially compensate each other, so that slow magnetic relaxation is retained on the surface. Interestingly, these deformations escaped detection by conventional synchrotron-based techniques, like X-ray magnetic circular dichroism, thus highlighting the power of synchrotron Mössbauer spectroscopy for the investigation of hybrid interfaces.

2018 - Sev and pcu topological nets in one-pot newly synthesized mixed-ligand imidazole-containing Cu(II) coordination frameworks: Crystal structure, intermolecular interactions, theoretical calculations, magnetic behavior and biological activity [Articolo su rivista]
Direm, Amani; Abdelbaky, Mohammed S. M.; Sayın, Koray; Cornia, Andrea; Abosede, Olufunso; García-Granda, Santiago
abstract

Novel mixed-ligand coordination frameworks, occurring concomitantly namely [Cu(Im)3(H2Cit)] (1) and [Cu(Im)2(HCit)]·HIm (2) (with Im = imidazole and H4Cit = citric acid) were obtained as a result of the one-pot reaction between imidazole, citric acid and copper chloride. The complexes were structurally characterized by elemental analysis, FTIR spectroscopy and X-ray diffraction. The two structures were found to be connected through 3D hydrogen-bonding networks examined by means of the Hirshfeld surface analysis which highlighted the presence of O–H⋯O, N–H⋯O and C–H⋯O H-bonds together with the π⋯lp interactions. A topological analysis of the underlying nets corresponding to the two hydrogen-bonded frameworks was carried out. Moreover, quantum chemical calculations were performed using the HF method with 6-31G(d) and LANL2DZ levels in the gas phase, and therefore the optimized structures, the IR, 1H NMR, 13C NMR spectra and the electronic structure descriptors were examined in detail. Furthermore, the magnetic properties of (1) and (2) were also investigated. The complexes showed remarkable antimicrobial and antifungal inhibition activities.

2018 - Solution structure of a pentachromium(II) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy [Articolo su rivista]
Dirvanauskas, Aivaras; Galavotti, Rita; Lunghi, Alessandro; Nicolini, Alessio; Roncaglia, Fabrizio; Totti, Federico; Cornia, Andrea
abstract

The structure of pentachromium(II) extended metal atom chain [Cr5(tpda)4Cl2] (2), which behaves as a single molecule magnet at low temperature, was investigated by Density Functional Theory (DFT) calculations and spectroscopic studies without the constraints of a crystal lattice (H2tpda = N2,N6-bis(pyridin-2-yl)pyridine-2,6-diamine). DFT studies both in the gas phase and including CH2Cl2 solvent effects indicate that an unsymmetric structure (C4 point group), with pairs of formally quadruply-bonded metal ions and one terminal metal center, is slightly more stable (2.9 and 3.9 kcal mol-1) than a symmetric structure (D4 point group). Isotopically-labelled samples (2-d8 and 2-d16) have then been prepared to aid in molecular symmetry determination by combined 1H and 2H NMR studies in dichloromethane solution. The spectra are strongly suggestive of a symmetric (D4) framework, indicating fast shuttling between the two unsymmetric forms over the timescale of NMR experiments. Procedures for a high-yield Pd-free synthesis of H2tpda and for site-selective post-synthetic H/D exchange of aromatic H2tpda hydrogens are also reported.

2018 - Ultralow-temperature device dedicated to soft X-ray magnetic circular dichroism experiments [Articolo su rivista]
Kappler, J. -P.; Otero, E.; Li, Weicheng; Joly, L.; Schmerber, G.; Muller, B.; Scheurer, F.; Leduc, F.; Gobaut, B.; Poggini, L.; Serrano, G.; Choueikani, F.; Lhotel, E.; Cornia, A.; Sessoli, R.; Mannini, M.; Arrio, M. -A.; Sainctavit, Ph.; Ohresser, P.
abstract

A new ultralow-temperature setup dedicated to soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) experiments is described. Two experiments, performed on the DEIMOS beamline (SOLEIL synchrotron), demonstrate the outstanding performance of this new platform in terms of the lowest achievable temperature under X-ray irradiation (T = 220 mK), the precision in controlling the temperature during measurements as well as the speed of the cooling-down and warming-up procedures. Moreover, owing to the new design of the setup, the eddy-current power is strongly reduced, allowing fast scanning of the magnetic field in XMCD experiments; these performances lead to a powerful device for X-ray spectroscopies on synchrotron-radiation beamlines facilities.

2017 - Evidence of crystal packing effects in stabilizing high or low spin states of iron(II) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands [Articolo su rivista]
Bridonneau, Nathalie; Rigamonti, Luca; Poneti, Giordano; Pinkowicz, Dawid; Forni, Alessandra; Cornia, Andrea
abstract

The molecular structures and magnetic properties of homoleptic iron(II) compounds [Fe(bpp-COOMe)2](ClO4)2 (1) and [Fe(bpp-triolH3)2](ClO4)2 (2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CH2OH)3 para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(II) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(II) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with TLIESST = 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH3)2]2+ cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.

2017 - Form Matters: Stable Helical Foldamers Preferentially Target Human Monocytes and Granulocytes [Articolo su rivista]
Del Secco, Benedetta; Malachin, Giulia; Milli, Lorenzo; Zanna, Nicola; Papini, Emanuele; Cornia, Andrea; Tavano, Regina; Tomasini, Claudia
abstract

Some hybrid foldamers of various length, all containing the (4R,5S)-4-carboxy-5-methyloxazolidin-2-one (d-Oxd) moiety alternating with an l-amino acid (l-Val, l-Lys, or l-Ala), were prepared in order to study their preferred conformations and to evaluate their biological activity. Surprisingly, only the longer oligomers containing l-Ala fold into well-established helices, whereas all the other oligomers give partially unfolded turn structures. Nevertheless, they all show good biocompatibility, with no detrimental effects up to 64 μm. After equipping some selected foldamers with the fluorescent tag rhodamine B, a quantitative analysis was performed by dose– and time–response fluorescence-activated cell sorting (FACS) assays with human HeLa cells and primary blood lymphocytes, granulocytes, and monocytes. Among the cell types analyzed, the oligomers associated with monocytes and granulocytes with greatest efficacy, still visible after 24 h incubation. This effect is even more pronounced for foldamers that are able to form stable helices.

2017 - Spintronics: The molecular way [Articolo su rivista]
Cornia, Andrea; Seneor, Pierre
abstract

An invited Meeting Report on the first European Conference on Molecular Spintronics, held in Bologna (Italy).

2017 - Structure, magnetic properties and thermal sublimation of fluorinated Fe4 Single-Molecule Magnets [Articolo su rivista]
Rigamonti, Luca; Piccioli, Marco; Nava, Andrea; Malavolti, Luigi; Cortigiani, Brunetto; Sessoli, Roberta; Cornia, Andrea
abstract

Fluorinated tetrairon(III) Single-Molecule Magnets (SMMs) [Fe4(L1)2(dpmF6)6] (1dpmF6), [Fe4(L2)2(dpmF6)6] (2dpmF6), and [Fe4(L2)2(pta)6] (2pta) were assembled combining the tripodal ligands H3L1 = 2-hydroxymethyl-2-phenylpropane-1,3-diol and H3L2 = S-5-hydroxy-4,4-bis(hydroxymethyl)pentyl ethanethioate with fluorinated β-diketones analogues of dipivaloylmethane (Hdpm), namely 1,1,1-trifluoro-2,6,6-trimethyl-2-(trifluoromethyl)heptane-3,5-dione (HdpmF6) and pivaloyltrifluoroacetone (Hpta). The new compounds, along with [Fe4(L1)2(dpm)6] (1dpm) and [Fe4(L1)2(pta)6] (1pta), were designed in order to investigate the effect of fluorination degree on processability by thermal sublimation. The two different functional groups on the tripodal ligands, i.e. C6H5 in H3L1 and (CH2)3SAc in H3L2, are suitable for promoting physisorption and chemisorption on surfaces, respectively. Direct current magnetic data are typical for the metal-centred triangular topology of Fe4 complexes, with antiferromagnetic nearest-neighbour coupling constants in the range 16–18 cm−1 and an S = 5 ground spin state. Alternating current susceptibility measurements showed that slow magnetic relaxation persists in fluorinated compounds. When heated in high vacuum (10−7 mbar), 2dpmF6 and 2pta undergo thermal decomposition before subliming, while 1dpmF6 was found to sublimate at 497 ± 5 K in the same conditions, being the third sublimable SMM of this family after 1pta (440 ± 5 K) and 1dpm (500 ± 10 K).

2017 - Synthesis, structural characterization and biological evaluation of 4′-C-methyl- and phenyl-dioxolane pyrimidine and purine nucleosides [Articolo su rivista]
Franchini, Silvia; Battisti, Umberto M.; Sorbi, Claudia; Tait, Annalisa; Cornia, Andrea; Jeong, Lak Shin; Lee, Sang Kook; Song, Jayoung; Loddo, Roberta; Madeddu, Silvia; Sanna, Giuseppina; Brasili, Livio
abstract

Nucleoside analogues play an important role in antiviral, antibacterial and antineoplastic chemotherapy. Herein we report the synthesis, structural characterization and biological activity of some 4′-C-methyl- and -phenyl dioxolane-based nucleosides. In particular, α and β anomers of all natural nucleosides were obtained and characterized by NMR, HR-MS and X-ray crystallography. The compounds were tested for antimicrobial activity against some representative human pathogenic fungi, bacteria and viruses. Antitumor activity was evaluated in a large variety of human cancer cell-lines. Although most of the compounds showed non-significant activity, 23α weakly inhibited HIV-1 multiplication. Moreover, 22α and 32α demonstrated a residual antineoplastic activity, interestingly linked to the unnatural α configuration. These results may provide structural insights for the design of active antiviral and antitumor agents.

2017 - Thin Layers of Molecular Magnets [Capitolo/Saggio]
Cornia, A.; Talham, D. R.; Affronte, M.
abstract

Thin films of magnetic molecular materials are increasingly under investigation for such diverse applications as molecular spintronics, quantum computation and on-chip magnetic refrigeration. We herein review recent progress in the preparation and characterization of films of magnetic molecules or network solids with thickness ranging from several hundred nanometers down to monolayers or submonolayers. Particular emphasis is placed on the design, processing and magnetic study of the films, and on changes in magnetic response resulting from reduced dimensionality.

2016 - Diamondoid Structure in a Metal–Organic Framework of Fe4Single-Molecule Magnets [Articolo su rivista]
Rigamonti, Luca; Cotton, Carri; Nava, Andrea; Lang, Heinrich; Rüffer, Tobias; Perfetti, Mauro; Sorace, Lorenzo; Barra, Anne Laure; Lan, Yanhua; Wernsdorfer, Wolfgang; Sessoli, Roberta; Cornia, Andrea
abstract

A 3D metal–organic framework (MOF) having single-molecule magnet (SMM) linkers was prepared in crystalline form by using a tetrairon(III) complex functionalised with two divergent pyridyl groups, namely [Fe4(pPy)2(dpm)6] (1; H3pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane). Reaction of 1 with silver(I) perchlorate afforded [Fe4(pPy)2(dpm)6]2AgClO4(2), which crystallises in a cubic face-centred lattice and exhibits two interlocked diamondoid networks. In 2, the SMMs act as linear ditopic synthons, and silver(I) ions as tetrahedral nodes coordinated by four pyridyl nitrogen atoms. The magnetic properties of 1 (S=5 and D≈−0.4 cm−1in the ground spin state) are largely preserved in 2, which shows slow magnetic relaxation with an anisotropy barrier of Ueff/kB=11.46(10) K in zero field and 14.25(8) K in an applied field of 1 kOe. However, crystal symmetry triggers highly noncollinear magnetic anisotropy contributions oriented at 109.47° from each other along the threefold axes of AgN4tetrahedra, a unique scenario fully confirmed by a single-crystal cantilever torque magnetometry investigation. Magnetisation curves down to 0.03 K demonstrated the occurrence of a wide hysteresis loop when the magnetic field was swept along one of the four Ag−N bonds. By symmetry, the crystalline compound can then be persistently magnetised parallel or antiparallel to the four main diagonals of the unit cell, although the crystals have no overall second-order anisotropy.

2016 - Expansion of a Discrete [3 × 3] Mn9 Metallogrid to a µ-Carboxylato-Bridged Polymeric {Mn11}n Assembly [Articolo su rivista]
Lakma, Avinash; Hossain, Sayed Muktar; Pradhan, Rabindra Nath; Topwal, Dinesh; Cornia, Andrea; Singh, Akhilesh Kumar
abstract

Compounds based on the [3 × 3] nonamanganese(II) square grid motif and featuring additional manganese(II) ions linked to the grid core were isolated through a strict self-assembly approach. Extended tritopic picolinic dihydrazide ligands were used, which contain terminal ester groups and differ in the R substituent on the para position of the central pyridine ring (H2L1, R = OCH3; H2L2, R = H). Upon reaction with Mn(OAc)2·4H2O, the ligand H2L1 undergoes hydrolysis and spontaneous self-assembly to afford [Mn10(L1′)6(H2O)5][Mn(H2O)6]2·11H2O, where a pentaaquomanganese(II) unit is connected to one of the terminal carboxylate functions of the grid through a Mn–O bond. The same reaction carried out with H2L2 affords an unprecedented 2D polymeric structure [Mn11(L2′)6(H2O)8][Mn(H2O)6]·24H2O, in which each nonamanganese grid is connected to its four neighbors through tetraaquomanganese(II) centers, again linked to the carboxylate functions. The two compounds also contain [Mn(H2O)6]2+ complexes in the lattice. These metallogrid-based compounds, for which DC magnetic data were collected and analysed in detail, emphasize the rich binding affinity and bridging versatility of the used ligands, which provide access to both discrete complexes and extended polymeric assemblies.

2016 - The Challenge of Thermal Deposition of Coordination Compounds: Insight into the Case of an Fe4 Single Molecule Magnet [Articolo su rivista]
Lanzilotto, Valeria; Malavolti, Luigi; Ninova, Silviya; Cimatti, Irene; Poggini, Lorenzo; Cortigiani, Brunetto; Mannini, Matteo; Totti, Federico; Cornia, Andrea; Sessoli, Roberta
abstract

Realization of well-controlled hybrid interfaces between solid surfaces and functional complex molecules can be hampered by the presence of contaminants originated by the fragmentation of fragile architectures based on the coordinative bond. Here, we present a morphological and spectroscopic analysis of submonolayer films obtained by sublimation of the [Fe4(L)2(dpm)6] (Fe4) single molecule magnet on different substrates. Though intact tetranuclear molecules can be transferred to surfaces, smaller molecular species are often codeposited. By comparison of substrates characterized by different reactivities, such as Au(111), Cu(100), and Cu2N, and employing a protocol of indirect exposure of the substrate, we infer that the observed fragments do not originate from the reaction of Fe4 molecules with the surface but rather are produced during Fe4 sublimation, which releases Fe(dpm)3 as a very volatile compound. Fe(dpm)3 undergoes substrate-dependent on-surface decomposition to final products that have been identified by combined STM, UPS, XPS, and DFT studies.

2016 - The classical and quantum dynamics of molecular spins on graphene [Articolo su rivista]
Cervetti, Christian; Rettori, Angelo; Pini, Maria Gloria; Cornia, Andrea; Repollés, Ana; Luis, Fernando; Dressel, Martin; Rauschenbach, Stephan; Kern, Klaus; Burghard, Marko; Bogani, Lapo
abstract

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics and electrical spin manipulation. However, the influence of the graphene environment on the spin systems has yet to be unravelled. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets on graphene. Whereas the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly developed model. Coupling to Dirac electrons introduces a dominant quantum relaxation channel that, by driving the spins over Villain's threshold, gives rise to fully coherent, resonant spin tunnelling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin manipulation in graphene nanodevices.

2016 - Torque-Detected Electron Spin Resonance as a Tool to Investigate Magnetic Anisotropy in Molecular Nanomagnets [Articolo su rivista]
Dörfel, María; Kern, Michal; Bamberger, Heiko; Neugebauer, Petr; Bader, Katharina; Marx, Raphael; Cornia, Andrea; Mitra, Tamoghna; Müller, Achim; Dressel, Martin; Bogani, Lapo; van Slageren, Joris
abstract

The method of choice for in-depth investigation of the magnetic anisotropy in molecular nanomagnets is high-frequency electron spin resonance (HFESR) spectroscopy. It has the benefits of high resolution and facile access to large energy splittings. However, the sensitivity is limited to about 10^7 spins for a reasonable data acquisition time. In contrast, methods based on the measurement of the deflection of a cantilever were shown to enable single spin magnetic resonance sensitivity. In the area of molecular nanomagnets, the technique of torque detected electron spin resonance (TDESR) has been used sporadically. Here, we explore the applicability of that technique by investigating molecular nanomagnets with different types of magnetic anisotropy. We also assess different methods for the detection of the magnetic torque. We find that all types of samples are amenable to these studies, but that sensitivities do not yet rival those of HFESR.

2015 - A New and Versatile Synthesis of 1,3-Dioxan-5-yl-pyrimidine and Purine Nucleoside Analogues [Articolo su rivista]
Sorbi, Claudia; Prandi, Adolfo; Battisti, UMBERTO MARIA; Franchini, Silvia; Cornia, Andrea; Balzarini, Jan; Jeong, Lak Shin; Lee, Sang Kook; Song, Jayoung; Brasili, Livio
abstract

1,3-Dioxan-5-yl pyrimidine and purine nucleoside analogues were prepared following a new and versatile synthetic strategy. These analogues were synthesized via nucleophilic addition of the selected nucleobase to a 1,3-dioxane scaffold that presents an appropriate leaving group in position 5. In particular cis and trans isomers of purine/pyrimidine nucleosides and their halogenated homologues were obtained. NMR experiments, carried out on the cis isomers, led to assignment of an equatorial orientation to the 2-hydroxymethyl group and axial orientation to the nucleobase in position 5 of the 1,3-dioxane. The trans isomers showed a diequatorial orientation of these groups. These assignments were confirmed by X-ray crystallographic studies

2015 - Chiral Gold Nanoparticles Decorated with Pseudopeptides [Articolo su rivista]
Fanelli, Rossana; Milli, Lorenzo; Cornia, Andrea; Moretto, Alessandro; Castellucci, Nicola; Zanna, Nicola; Malachin, Giulia; Tavano, Regina; Tomasini, Claudia
abstract

Peptide-functionalized gold nanoparticles are supramolecular systems that can mimic natural proteins or DNA. In this work, we describe the preparation, the analysis, and the biological evaluation of gold nanoparticles linked to pseudopeptide foldamers containing one to eight L-Ala-D-Oxd (Ala = alanine; Oxd = 4-carboxy-5-methyloxazolidin-2-one) residues. The nanoparticles become increasingly organized as the number of such moieties increases. Moreover, from the analysis of chiroptical signals we find that the chirality of the gold surface becomes more evident with decreasing foldamer length. Finally, these systems display no cytotoxicity towards HeLa cells and are good candidates to promote drug delivery once equipped with biologically active moieties.

2015 - Crystal structure of a new homochiral one-dimensional zincophosphate containing L-methionine [Articolo su rivista]
Chouat, Nadjet; Hasnaoui, Mohammed Abdelkrim; Sassi, Mohamed; Bengueddach, Abdelkader; Lusvardi, Gigliola; Cornia, Andrea
abstract

catena-Poly[[(L-me­thio­nine-[kappa]O)zinc]-[mu]3-(hydrogen phosphato)-[kappa]3O:O':O''], [Zn{PO3(OH)}(C5H11NO2S)]n, a new one-dimensional homochiral zincophos­phate, was hydro­thermally synthesized using L-me­thio­nine as a structure-directing agent. The compound consists of a network of ZnO4 and (HO)PO3 tetra­hedra that form ladder-like chains of edge-fused Zn2P2O4 rings propagating parallel to [100]. The chains are decorated on each side by zwitterionic L-me­thio­nine ligands, which inter­act with the inorganic framework via Zn-O coordination bonds. The structure displays inter­chain N-H...O and O-H...S hydrogen bonds.

2015 - Experimental and Theoretical Studies on the Magnetic Anisotropy in Lanthanide(III)-Centered Fe3 Ln Propellers [Articolo su rivista]
Rigamonti, Luca; Nava, Andrea; Boulon, Marie Emmanuelle; Luzon, Javier; Sessoli, Roberta; Cornia, Andrea
abstract

Compounds [Fe3 Ln(tea)2 (dpm)6 ] (Fe3 Ln; Ln= Tb-Yb, H3 tea=triethanolamine, Hdpm=dipivaloylmethane) were synthesized as lanthanide(III)-centered variants of tetrairon(III) single-molecule magnets (Fe4 ) and isolated in crystalline form. Compounds with Ln=Tb-Tm are isomorphous and show crystallographic threefold symmetry. The coordination environment of the rare earth, given by two tea(3-) ligands, can be described as a bicapped distorted trigonal prism with D3 symmetry. Magnetic measurements showed the presence of weak ferromagnetic Fe⋅⋅⋅Ln interactions for derivatives with Tb, Dy, Ho, and Er, and of weak antiferromagnetic or negligible coupling in complexes with Tm and Yb. Alternating current susceptibility measurements showed simple paramagnetic behavior down to 1.8 K and for frequencies reaching 10000 Hz, despite the easy-axis magnetic anisotropy found in Fe3 Dy, Fe3 Er, and Fe3 Tm by single-crystal angle-resolved magnetometry. Relativistic quantum chemistry calculations were performed on Fe3 Ln (Ln=Tb-Tm): the ground J multiplet of Ln(3+) ion is split by the crystal field to give a ground singlet state for Tb and Tm, and a doublet for Dy, Ho, and Er with a large admixture of mJ states. Gyromagnetic factors result in no predominance of gz component along the threefold axis, with comparable gx and gy values in all compounds. It follows that the environment provided by the tea(3-) ligands, though uniaxial, is unsuitable to promote slow magnetic relaxation in Fe3 Ln species.

2015 - Magnetic Bistability in a Submonolayer of Sublimated Fe4 Single-Molecule Magnets [Articolo su rivista]
Malavolti, L.; Lanzilotto, V.; Ninova, S.; Poggini, L.; Cimatti, I.; Cortigiani, B.; Margheriti, L.; Chiappe, D.; Otero, E.; Sainctavit, P.; Totti, F; Cornia, Andrea; Mannini, M.; Sessoli, R.
abstract

We demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations.

2015 - Magnetic fingerprint of individual Fe4 molecular magnets under compression by a scanning tunnelling microscope [Articolo su rivista]
Burgess, Jacob A. J.; Malavolti, Luigi; Lanzilotto, Valeria; Mannini, Matteo; Yan, Shichao; Ninova, Silviya; Totti, Federico; Rolf Pissarczyk, Steffen; Cornia, Andrea; Sessoli, Roberta; Loth, Sebastian
abstract

Single-molecule magnets (SMMs) present a promising avenue to develop spintronic technologies. Addressing individual molecules with electrical leads in SMM-based spintronic devices remains a ubiquitous challenge: interactions with metallic electrodes can drastically modify the SMM’s properties by charge transfer or through changes in the molecular structure. Here, we probe electrical transport through individual Fe4 SMMs using a scanning tunnelling microscope at 0.5 K. Correlation of topographic and spectroscopic information permits identification of the spin excitation fingerprint of intact Fe4 molecules. Building from this, we find that the exchange coupling strength within the molecule’s magnetic core is significantly enhanced. First-principles calculations support the conclusion that this is the result of confinement of the molecule in the two-contact junction formed by the microscope tip and the sample surface.

2015 - Probing transverse magnetic anisotropy by electronic transport through a single-molecule magnet [Articolo su rivista]
Misiorny, M.; Burzurí, E.; Gaudenzi, R.; Park, K.; Leijnse, M.; Wegewijs, M. R.; Paaske, J.; Cornia, Andrea; van der Zant, H. S. J.
abstract

By means of electronic transport, we study the transverse magnetic anisotropy of an individual Fe4 single-molecule magnet (SMM) embedded in a three-terminal junction. In particular, we determine in situ the transverse anisotropy of the molecule from the pronounced intensity modulations of the linear conductance, which are observed as a function of applied magnetic field. The proposed technique works at temperatures exceeding the energy scale of the tunnel splittings of the SMM. We deduce that the transverse anisotropy for a single Fe4 molecule captured in a junction is substantially larger than the bulk value.

2015 - Redox-Controlled Exchange Bias in a Supramolecular Chain of Fe4 Single-Molecule Magnets [Articolo su rivista]
Nava, Andrea; Rigamonti, Luca; Zangrando, Ennio; Sessoli, Roberta; Wernsdorfer, Wolfgang; Cornia, Andrea
abstract

Tetrairon(III) single-molecule magnets [Fe4 (pPy)2 (dpm)6 ] (1) (H3 pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with Ru(II) Ru(II) or Ru(II) Ru(III) paddlewheel complexes. The products [Fe4 (pPy)2 (dpm)6 ][Ru2 (OAc)4 ](BF4 )x with x=0 (2 a) or x=1 (2 b) differ in the electron count on the paramagnetic diruthenium bridges and display hysteresis loops of substantially different shape. Owing to their large easy-plane anisotropy, the s=1 diruthenium(II,II) units in 2 a act as effective seff =0 spins and lead to negligible intrachain communication. By contrast, the mixed-valent bridges (s=3/2, seff =1/2) in 2 b introduce a significant exchange bias, with concomitant enhancement of the remnant magnetization. Our results suggest the possibility to use electron transfer to tune intermolecular communication in redox-responsive arrays of SMMs.

2015 - Single-molecule Magnets on Surfaces [Capitolo/Saggio]
Cornia, Andrea; Mannini, M.
abstract

Encoding and manipulating information through the spin degrees of freedom of individual magnetic molecules or atoms is one of the central challenges in the continuing trend towards molecular/atomic scale electronics. With their large magnetic moment and long spin relaxation time, single-molecule magnets (SMMs) are of special importance in this emerging field. Their electrical addressing at the molecular level appears now well within reach using STM methods, which require to organize SMMs on a conducting surface. In this chapter, we present a critical overview of the latest achievements in the deposition of SMMs as monolayers or submonolayers on native or prefunctionalized surfaces. Special emphasis is placed on the selection and design of molecular structures that withstand solution or vapour-phase processing and that maintain their magnetic functionality on a surface. Chemical strategies to control the strength of molecule–substrate interaction and the molecular orientation on the surface are also illustrated. Rewardingly, these efforts have shown that the distinctive properties of SMMs, i.e. slow spin relaxation and quantum tunnelling of the magnetic moment, persist in metal-wired molecules.

2014 - Adding Remnant Magnetization and Anisotropic Exchange to Propeller-like Single-Molecule Magnets through Chemical Design [Articolo su rivista]
Westrup, Katia Cristina M.; Boulon, Marie Emmanuelle; Totaro, Pasquale; Nunes, Giovana G.; Back, Davi F.; Barison, Andersson; Jackson, Martin; Paulsen, Carley; Gatteschi, Dante; Sorace, Lorenzo; Cornia, Andrea; Soares, Jaisa F.; Sessoli, Roberta
abstract

The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller-shaped single-molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero-field tunneling of the magnetization for the half-integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single-molecule magnets.

2014 - Arylsulfonyl Groups: The Best Cyclization Auxiliaries for the Preparation of ATRC γ-Lactams can be Acidolytically Removed [Articolo su rivista]
Clark, A. J.; Cornia, Andrea; Felluga, F.; Gennaro, A.; Ghelfi, Franco; Isse, A. A.; Menziani, Maria Cristina; Muniz Miranda, F.; Roncaglia, Fabrizio; Spinelli, D.
abstract

The N-arylsulfonyl group, which is the best and most useful cyclization auxiliary for the transition-metal-catalyzed atom transfer radical cyclization (ATRC) of N-allyl-α-polychloroamides, can be effectively removed from the target γ-lactams by using H2SO4–HOAc, without impairing the halogen functions. The reaction involves H+ attack on the aromatic moiety, and is strongly responsive to the electronic properties of the substituent bound to the aromatic ring: electron-donating groups, such as methyl or methoxy are, in fact, required for efficient “deprotection”. The N-p-nitrophenylsulfonyl cyclization auxiliary, in contrast to all the other sulfonyl groups tested, proved to be unsuitable for the ATRC, owing to a side reductive transposition that halts the redox cycle.

2014 - Franck-Condon Blockade in a Single-Molecule Transistor [Articolo su rivista]
E., Burzurì; Y., Yamamoto; M., Warnock; X., Zhong; K., Park; Cornia, Andrea; H. S. J., van der Zant
abstract

We investigate vibron-assisted electron transport in single-molecule transistors containing an individual Fe4 Single-Molecule Magnet. We observe a strong suppression of the tunneling current at low bias in combination with vibron-assisted excitations. The observed features are explained by a strong electron−vibron coupling in the framework of the Franck−Condon model supported by density-functional theory.

2014 - Grafting Single Molecule Magnets on Gold Nanoparticles [Articolo su rivista]
M., Perfetti; F., Pineider; L., Poggini; E., Otero; M., Mannini; L., Sorace; C., Sangregorio; CORNIA, Andrea; R., Sessoli
abstract

The chemical synthesis and characterization of the first hybrid material composed by gold nanoparticles and single molecule magnets (SMMs) are described. Gold nanoparticles are functionalized via ligand exchange using a tetrairon(III) SMM containing two 1,2-dithiolane end groups. The grafting is evidenced by the shift of the plasmon resonance peak recorded with a UV–vis spectrometer, by the suppression of nuclear magnetic resonance signals, by X-ray photoemission spectroscopy peaks, and by transmission electron microscopy images. The latter evidence the formation of aggregates of nanoparticles as a consequence of the cross-linking ability of Fe4 through the two 1,2-dithiolane rings located on opposite sides of the metal core. The presence of intact Fe4 molecules is directly proven by synchrotron-based X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectroscopy, while a detailed magnetic characterization, obtained using electron paramagnetic resonance and alternating-current susceptibility, confirms the persistence of SMM behavior in this new hybrid nanostructure.

2014 - Magnetic Blocking in Extended Metal Atom Chains: a Pentachromium(II) Complex Behaving as a Single-molecule Magnet [Articolo su rivista]
Cornia, Andrea; Rigamonti, Luca; Simone, Boccedi; Rodolphe, Clérac; Mathieu, Rouzières; Lorenzo, Sorace
abstract

Compound [Cr5(tpda)4Cl2] (H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine), an Extended Metal Atom Chain complex featuring two quadruply-bonded {Cr2} units, exhibits field-induced slow relaxation of its magnetization arising from the terminal chromium(II) ion and provides the first example of a chromium(II)-based Single-Molecule Magnet.

2014 - Mapping of Single-site Magnetic Anisotropy Tensors in Weakly Coupled Spin Clusters by Torque Magnetometry [Articolo su rivista]
RIGAMONTI, Luca; CORNIA, Andrea; NAVA, ANDREA; M. E., Boulon; M., Perfetti; A. L., Barra; X., Zhong; K., Park; R., Sessoli
abstract

Single-crystal torque magnetometry performed on weakly-coupled polynuclear systems provides access to a complete description of single-site anisotropy tensors. Variable-temperature, variable-field torque magnetometry was used to investigate triiron(III) complex [Fe3La(tea)2(dpm)6] (Fe3La), a lanthanum(III)-centred variant of tetrairon(III) single molecule magnets (Fe4) (H3tea = triethanolamine, Hdpm = dipivaloylmethane). Due to the presence of the diamagnetic lanthanoid, magnetic interactions among iron(III) ions (si = 5/2) are very weak (<0.1 cm-1) and the magnetic response of Fe3La is predominantly determined by single-site anisotropies. The local anisotropy tensors were found to have Di > 0 and to be quasi-axial with |Ei/Di| ~ 0.05. Their hard axes form an angle of approximately 70° with the threefold molecular axis, which therefore corresponds to an easy magnetic direction for the molecule. The resulting picture was supported by a High Frequency EPR investigation and by DFT calculations. Our study confirms that the array of peripheral iron(III) centres provides substantially noncollinear anisotropy contributions to the ground state of Fe4 complexes, which are of current interest in molecular magnetism and spintronics.

2014 - Spin-lattice Relaxation via Quantum Tunneling in Diluted Crystals of Fe4 Single-molecule Magnets [Articolo su rivista]
A., Repollés; Cornia, Andrea; F., Luis
abstract

We investigate the dynamic susceptibility of Fe4 single-molecule magnets with integer spin (S = 5) in the form of pure crystals as well as diluted in crystals of isostructural, but nonmagnetic, Ga4 clusters. Below approximately 1 K, the spin-lattice relaxation becomes dominated by a temperature-independent process. The spin-lattice relaxation time τ measured in this “quantum regime” is 12 orders of magnitude shorter than the characteristic time scale of direct phonon-induced processes but agrees with the relaxation times of pure (i.e.,not assisted by phonons) spin tunneling events. The present results show that the latter phenomenon, despite conserving the energy of the ensemble of electronic and nuclear spins, drives the thermalization of electronic spins at very low temperatures. The spin-lattice relaxation time scales with the concentration of Fe4, thus suggesting that the main effect of dipolar interactions is to block tunneling. The data show therefore no evidence for the contribution of collective phonon emission processes, such as phonon superradiance, to the spin-lattice relaxation.

2014 - Synthesis, enantiomeric separation and docking studies of spiropiperidine analogues as ligands of the nociceptin/orphanin FQ receptor [Articolo su rivista]
Battisti, Umberto M.; Corrado, Sandra; Sorbi, Claudia; Cornia, Andrea; Tait, Annalisa; Malfacini, Davide; Cerlesi, Maria Camilla; Calò, Girolamo; Brasili, Livio
abstract

A series of triazospirodecanone derivatives were synthesized as potential NOP ligands. 8-(Chroman-4-yl)-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one (4) and its 5-fluoro analogue (18) proved to be active as agonists with EC50 values in the submicromolar range. Single enantiomers of compound 4 were separated and tested as NOP agonists; the eutomer R-(+)-4 showed a pEC50 of 7.34. Finally docking studies were performed on the NOP receptor to identify the most significant stereospecific interactions.

2014 - Tetrairon(III) Single-Molecule Magnet Monolayers on Gold: Insights from ToF-SIMS and Isotopic Labeling [Articolo su rivista]
P., Totaro; A., Favre; L., Poggini; M., Mannini; Sainctavit, P. h.; CORNIA, Andrea; A., Magnani; R., Sessoli
abstract

To work as magnetic components in molecular electronics and spintronics, single-molecule magnets (SMMs) must be reliably interfaced with metals. The organization on gold of a Fe4 SMM carrying two acetyl-protected thiol groups has been studied by exploiting the surface sensitivity of time-of-flight secondary ion mass spectrometry (ToF-SIMS), additionally powered by the use of an isotopic labeling strategy. Deposition from millimolar dichloromethane solutions results in a higher surface coverage and better packed monolayers as compared with previous protocols based on more diluted solutions. Fe4 complexes are chemically tethered to the surface via a single Au−S bond while they still contain an intact SAc group.

2014 - UHV Deposition and Characterization of a Mononuclear Iron(III) β-diketonate Complex on Au(111) [Articolo su rivista]
Cimatti, Irene; Ninova, Silviya; Lanzilotto, Valeria; Malavolti, Luigi; Rigamonti, Luca; Cortigiani, Brunetto; Mannini, Matteo; Magnano, Elena; Bondino, Federica; Totti, Federico; Cornia, Andrea; Sessoli, Roberta
abstract

The adsorption of the sterically hindered β-diketonate complex Fe(dpm)3, where Hdpm = dipivaloylmethane, on Au(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunnelling microscopy (STM). The high volatility of the molecule limited the growth of the film to a few monolayers. While UPS evidenced the presence of the β-diketonate ligands on the surface, the integrity of the molecule on the surface could not be assessed. The low temperature STM images were more informative and at submonolayer coverage they showed the presence of regular domains characterized by a flat morphology and height of ≈0.3 nm. Along with these domains, tetra-lobed features adsorbed on the kinks of the herringbone were also observed. DFT-simulated images of the pristine molecule and its possible decomposition products allowed to assess the partial fragmentation of Fe(dpm)3 upon adsorption on the Au(111) surface. Structural features with intact molecules were only observed for the saturation coverage. An ex situ prepared thick film of the complex was also investigated by X-ray magnetic circular dichroism (XMCD) and features typical of high-spin iron(III) in octahedral environment were observed.

2014 - α,ε-Hybrid Foldamers with 1,2,3-Triazole Rings: Order versus Disorder [Articolo su rivista]
L., Milli; M., Larocca; M., Tedesco; N., Castellucci; E., Ghibaudi; Cornia, Andrea; M., Calvaresi; F., Zerbetto; C., Tomasini
abstract

Two epimeric series of foldamers characterized by the presence of a repeating α,ε-dipeptide unit have been prepared and characterized by 1H NMR and ECD spectroscopies together with X-ray diffraction. The first series contains L-Ala and D-4-carboxy-5-methyl-oxazolidin-2-one (D-Oxd). The other series contains L-Ala and L-Oxd. The L,D series of oligomers forms ordered β-turn foldamers, characterized by a 311 pattern. The L,L series is not ordered. Simulations show that an ordered L,L trimer lies more than 2 kcal/mol higher than the more stable nonfolded extended conformations. Cu2+ forms complexes with both series but is not able to order the L,L series. Analysis of the EPR spectra shows that the L,D foldamers bear two types of complexation sites that are assigned as a nitrogen donor of the triazole ring and a carboxylate ligand. The L-Ala-D-Oxd-Tri-CO motif may be introduced in any peptide sequence requiring the presence of a stable β-turn conformations, like in the study of protein−protein interactions.

2013 - A New Approach to the Synthesis of Heteronuclear Propeller-like Single Molecule Magnets [Articolo su rivista]
P., Totaro; K. C. M., Westrup; M. E., Boulon; G. G., Nunes; D. F., Back; A., Barison; S., Ciattini; M., Mannini; L., Sorace; J. F., Soares; Cornia, Andrea; R., Sessoli
abstract

Propeller-like [Fe4(L)2(dk)6] complexes, in which Hdk is a β-diketone and H3L is a tripodal alcohol, R–C-(CH2OH)3, exhibit tunable magnetic anisotropy barriers and retain their magnetic memory effect when chemically anchored on metal surfaces. Heteronuclear analogues of these M4 complexes have been sought to afford a library of compounds with different total spin (S) values, but synthetic efforts described so far gave solid solutions containing M4 in addition to the desired M3M’ species. We now present a novel synthetic route to M3M’ complexes featuring a central chromium(III) ion. The three-step preparation goes through coordination of CrIII by two equivalents of tripodal alkoxide (R = Et and Ph), followed by reaction of this complex “core” with the peripheral +III metal ions. Products have been characterised by chemical analyses together with 1H-NMR, FTIR, W-band EPR, DC/AC magnetic susceptibility measurements and single crystal X-ray diffractometry. Due to the chemical inertness of CrIII, this route yields 100% pure Fe3Cr complexes without metal scrambling; what is more, it is suitable for designing novel heteronuclear single molecule magnets (SMMs) with a variety of d- and f-metals and R groups.

2013 - Enhanced Vapor-Phase Processing in Fluorinated Fe4 Single-Molecule Magnets [Articolo su rivista]
Rigamonti, Luca; Marco, Piccioli; Luigi, Malavolti; Lorenzo, Poggini; Matteo, Mannini; Federico, Totti; Brunetto, Cortigiani; Agnese, Magnani; Roberta, Sessoli; Cornia, Andrea
abstract

A new tetrairon(III) single-molecule magnet with enhanced volatility and processability was obtained by partial fluorination of the ancillary β-diketonato ligands. Fluorinated proligand Hpta = pivaloyltrifluoroacetone was used to assemble the bis(alkoxido)-bridged dimer [Fe2(OEt)2(pta)4] (1) in crystalline form, from which the new tetranuclear complex [Fe4(L)2(pta)6] (2) was synthesized in a one-pot reaction with H3L = 2-hydroxymethyl-2-phenylpropane-1,3-diol, NaOEt, and FeCl3 in a Et2O:EtOH solvent mixture. The structure of compound 2 was inferred from (1)H NMR, mass spectrometry, magnetic measurements, and DFT calculations. Direct current magnetic data are consistent with the expected metal-centered triangular topology for the iron(III) ions, with an antiferromagnetic coupling constant J = 16.20(6) cm(-1) between the central iron and the peripheral ones and consequent stabilization of an S = 5 spin ground state. Alternating current (ac) susceptibility measurements in 0 and 1 kOe static applied fields show the presence of a thermally activated process for magnetic relaxation, with τ0 = 2.3(1) 10(-7) s and Ueff/kB = 9.9(1) K at zero static field and τ0 = 2.0(2) 10(-7) s and Ueff/kB = 13.0(2) K at 1 kOe. At a pressure of 10(-7) mbar, compound 2 sublimates at (440 ± 5) K vs (500 ± 10) K for the nonfluorinated variant [Fe4(L)2(dpm)6] (Hdpm = dipivaloylmethane). According to XPS, ToF-SIMS, and ac susceptibility studies, the chemical composition, fragmentation pattern, and slow magnetic relaxation of the pristine material are retained in sublimated samples, suggesting that the molecular structure remains totally unaffected upon vapor-phase processing.

2013 - On-Surface Magnetometry: The Evaluation of Superexchange Coupling Constants in Surface-Wired Single-Molecule Magnets [Articolo su rivista]
Tancini, Erik; Matteo, Mannini; Philippe, Sainctavit; Edwige, Otero; Roberta, Sessoli; Cornia, Andrea
abstract

A comfortable environment: Interaction with a gold surface fully preserves the spin structure of a Fe3Cr single-molecule magnet of current interest in molecular spintronics. Magnetic polarization at metal sites is measured over a broad temperature range (from 10 to 300 K) by X-ray magnetic circular dichroism (XMCD), which provides the required monolayer sensitivity. The data are used to evaluate Fe-Cr superexchange coupling interactions, which compare well with bulk-phase values in both sign and strength.

2013 - Origin and Spectroscopic Determination of Trigonal Anisotropy in a Heteronuclear Single-molecule Magnet [Articolo su rivista]
L., Sorace; M. E., Boulon; P., Totaro; Cornia, Andrea; J., Fernandes Soares; R., Sessoli
abstract

W-band (ν ∼ 94 GHz) electron paramagnetic resonance (EPR) spectroscopy was used for a single-crystal study of a star-shaped Fe3Cr single-molecule magnet (SMM) with crystallographically imposed trigonal symmetry. The high resolution and sensitivity accessible with W-band EPR allowed us to determine accurately the axial zero-field splitting terms for the ground (S =6) and first two excited states (S =5 and S =4). Furthermore, spectra recorded by applying the magnetic field perpendicular to the trigonal axis showed a π/6 angular modulation. This behavior is a signature of the presence of trigonal transverse magnetic anisotropy terms whose values had not been spectroscopically determined in any SMM prior to this work. Such in-plane anisotropy could only be justified by dropping the so-called “giant spin approach” and by considering a complete multispin approach. From a detailed analysis of experimental data with the two models, it emerged that the observed trigonal anisotropy directly reflects the structural features of the cluster, i.e., the relative orientation of single-ion anisotropy tensors and the angular modulation of single-ion anisotropy components in the hard plane of the cluster. Finally, since high-order transverse anisotropy is pivotal in determining the spin dynamics in the quantum tunneling regime, we have compared the angular dependence of the tunnel splitting predicted by the two models upon application of a transverse field (Berry-phase interference).

2012 - CuCl catalyzed radical cyclisation of N-alpha-perchloroacyl-ketene-N,S-acetals: a new way to prepare disubstituted maleic anhydrides [Articolo su rivista]
Cornia, Andrea; F., Felluga; V., Frenna; Ghelfi, Franco; A. F., Parsons; M., Pattarozzi; Roncaglia, Fabrizio; D., Spinelli
abstract

The copper catalyzed radical cyclization (RC) of N-α-perchloroacyl cyclic ketene-N,X(X = O, NR, S)-acetals was studied. While the RC of N-acyl ketene-N,O-acetals was unsuccessful, the 5-endo cyclization provided much better results, with the following order of cyclization efficiency: hexa-atomic cyclic ketene-N,NR-acetals < penta-atomic cyclic ketene-N,S-acetals < hexa-atomic cyclic ketene-N,S-acetals. Invariably the catalytic cycle begins with the formation of a carbamoyl methyl radical. This leads to a cascade of reactions, including a radical polar crossover step, which ends with the formation of the maleimide nucleus, or precursors of this. Products from the RC of the hexa-atomic cyclic ketene-N,S-acetals, were efficiently transformed into disubstituted maleic anhydrides.

2012 - Direct Observation of Magnetic Anisotropy in an Individual Fe4 Single-Molecule Magnet [Articolo su rivista]
E., Burzurì; A. S., Zyazin; Cornia, Andrea; H. S. J., van der Zant
abstract

We study three-terminal charge transport through individual Fe4 single-molecule magnets. Magnetic anisotropy of the single molecule is directly observed by introducing a spectroscopic technique based on measuring the position of the degeneracy point as a function of gate voltage and applied magnetic field. A nonlinear field-dependence is observed which changes by rotating the sample and is, thus, a direct proof of magnetic anisotropy. The sensitivity of this method allows us to observe small changes in the orientation and magnitude of the anisotropy in different charge states. We find that the easy axes in adjacent states are (almost) collinear.

2012 - Magnetic Bistability of Isolated Giant-Spin Centers in a Diamagnetic Crystalline Matrix [Articolo su rivista]
L., Vergnani; A. L., Barra; P., Neugebauer; RODRIGUEZ DOUTON, Maria Jesus; R., Sessoli; L., Sorace; W., Wernsdorfer; Cornia, Andrea
abstract

Polynuclear single-molecule magnets (SMMs) were diluted in a diamagnetic crystal lattice to afford arrays of independent and iso-oriented magnetic units. Crystalline solid solutions of an Fe4 SMM and its Ga4 analogue were prepared with no metal scrambling for Fe4 molar fractions x down to 0.01. According to high-frequency EPR and magnetic measurements, the guest SMM species have the same total spin (S = 5), anisotropy, and high-temperature spin dynamics found in the pure Fe4 phase. However, suppression of intermolecular magnetic interactions affects magnetic relaxation at low temperature (40 mK), where quantum tunneling (QT) of the magnetization dominates. When a magnetic field is applied along the easy magnetic axis, both pure and diluted (x = 0.01) phases display pronounced steps at evenly spaced field values in their hysteresis loops due to resonant QT. The pure Fe4 phase exhibits additional steps which are firmly ascribed to two-molecule QT transitions. Studies on the field-dependent relaxation rate showed that the zero-field resonance sharpens by a factor of five and shifts from about 8 mT to exactly zero field on dilution, in agreement with the calculated variation of dipolar interactions. The tunneling efficiency also changes significantly as a function of Fe4 concentration: the zero-field resonance is significantly enhanced on dilution, while tunneling at +-0.45 T becomes less efficient. These changes were rationalized on the basis of a dipolar shuffling mechanism and transverse dipolar fields, whose effect was analyzed by using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on SMM behavior and disclose the magnetic response of truly isolated giant spins in a diamagnetic crystalline environment.

2012 - Magnetic and Optical Bistability in Tetrairon(III) Single Molecule Magnets Functionalized with Azobenzene Groups [Articolo su rivista]
T. K., Prasad; G., Poneti; L., Sorace; RODRIGUEZ DOUTON, Maria Jesus; A. L., Barra; P., Neugebauer; Costantino, Luca; R., Sessoli; Cornia, Andrea
abstract

Tetrairon(III) complexes known as “ferric stars” have been functionalized with azobenzene groups to investigate the effect of light-induced trans–cis isomerization on single-molecule magnet (SMM) behaviour. According to DC magnetic data and EPR spectroscopy, clusters dispersed in polystyrene (4% w/w) exhibit the same spin (S = 5) and magnetic anisotropy as bulk samples. Ligand photoisomerization, achieved by irradiation at 365 nm, has no detectable influence on static magnetic properties. However, it induces a small but significant acceleration of magnetic relaxation as probed by AC susceptometry. The pristine behaviour can be almost quantitatively recovered by irradiation with white light. Our studies demonstrate that magnetic and optical bistability can be made to coexist in SMM materials, which are of current interest in molecular spintronics.

2012 - Torque-detected ESR of a Tetrairon(III) Single Molecule Magnet [Articolo su rivista]
F., El Hallak; P., Neugebauer; A. L., Barra; J., van Slageren; M., Dressel; CORNIA, Andrea
abstract

Single-crystal studies on anisotropic ESR-active materials can be conveniently carried out using torque-detected (TD) ESR, a novel technique which brings to ESR the sensitivity typical of torque magnetometry (TM). This method, which is easily operated in high magnetic fields and in a wide range of frequencies, was applied to investigate magnetic anisotropy in crystals of a tetrairon(III) single-molecule magnet with an S = 5 ground state. TDESR was supported by TM measurements carried out in situ and provided an accurate estimate of the second-order axial anisotropy parameter D and of the longitudinal fourth-order contribution B40. The results were validated through a parallel angle-resolved investigation by traditional high-frequency ESR on the same material.

2011 - A Novel Tripodal Ligand with Organosulfur Alligator Clips for Deposition of Tetrairon(III) Single Molecule Magnets on Gold [Articolo su rivista]
RODRIGUEZ DOUTON, Maria Jesus; R., Sessoli; Cornia, Andrea
abstract

A propeller-like tetrairon(III) complex functionalized with two 1,2-dithiolan-3-yl groups was synthesized and magnetically characterized. The compound has formula [Fe4(thioctic)2(dpm)6] and was specifically designed to be grafted on gold surfaces. It was prepared by reacting [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane) with a new tripodal ligand, H3thioctic, obtained by esterification of 2,2-bis(hydroxymethyl)-propane-1,3-diol with (±)-alpha-lipoic acid (also known as thioctic acid). Direct current and alternating current magnetic measurements revealed single-molecule magnet behaviour with an effective anisotropy barrier of 14.0(1) K resulting from a high spin (S = 5) ground state and an easy-axis anisotropy.

2011 - Chemical Strategies and Characterization Tools for the Organization of Single Molecule Magnets on Surfaces [Articolo su rivista]
Cornia, Andrea; M., Mannini; Sainctavit, P. h.; R., Sessoli
abstract

Addressing individual bistable magnetic molecules, known as Single Molecule Magnets (SMMs), is a fascinating goal at the borderline between molecular magnetism and spin electronics. This tutorial review focuses on the first step towards single-molecule experiments, namely the organization of SMMs on surfaces. Both preparation and characterization of surface-supported SMMs prove to be quite demanding and a multidisciplinary approach is necessary, which is described here using selected examples. We first illustrate the chemical strategies devised to assemble SMMs and to control their orientation on surfaces. Then, we present characterization tools, which have been selected on the basis of their relevance to address specific points, i.e. the chemical composition of the deposited SMM films, the organization of the molecules on the surface, the intramolecular arrangement of the spins, the magnetic anisotropy of SMMs, and eventually the dynamics of their magnetization on surfaces. Particular attention is devoted to techniques exploiting synchrotron light.

2011 - High-spin and Magnetic Anisotropy Signatures in Three-Terminal Transport through a Single Molecule [Articolo su rivista]
A. S., Zyazin; H. S. J., van der Zant; M. R., Wegewijs; Cornia, Andrea
abstract

We have studied three-terminal electron transport through an individual single-molecule magnet Fe4. Several fingerprints of high spin and magnetic anisotropy have been observed: the Kondo effect in adjacent charge states in an intermediate coupling regime and zero-field splitting in the regime of a weaker coupling. We also observed spin transitions at energies close to the ones reported for bulk Fe4 crystals. Finally, we have demonstrated electric-field control on the magnetic properties of a single molecule.

2011 - One-step Covalent Grafting of Fe4 Single-Molecule Magnet Monolayers on Gold [Articolo su rivista]
RODRIGUEZ DOUTON, Maria Jesus; M., Mannini; L., Armelao; A. L., Barra; TANCINI, ERIK; R., Sessoli; CORNIA, Andrea
abstract

Iron(III)-based single-molecule magnets have been covalently grafted on Au(111) in one step using 1,2-dithiolan-3-yl side groups. Reaction with the substrate quantitatively affords a monolayer of electronically intact clusters doubly linked to the surface via Au–S bonds, as demonstrated by a combination of STM, XAS/XMCD and XPS studies.

2011 - The Spin Structure of Surface-Supported Single-Molecule Magnets from Isomorphous Replacement and X-ray Magnetic Circular Dichroism [Articolo su rivista]
M., Mannini; Tancini, Erik; L., Sorace; P., Sainctavit; M. A., Arrio; Y., Qian; E., Otero; D., Chiappe; L., Margheriti; J. C., Cezar; R., Sessoli; Cornia, Andrea
abstract

Surface-supported arrays of Fe4-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe4(L)2(dpm)6] (1) on Au, an isomorphous compound [Fe3Cr(L)2(dpm)6 ] was synthesized and structurally and magnetically characterized (H3L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr 3+ ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L2,3 edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L2,3 spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe 3+ ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.

2010 - Deposition of Intact Tetrairon(III) Single Molecule Magnet Monolayers on Gold: an STM, XPS, and ToF-SIMS Investigation [Articolo su rivista]
F., Pineider; M., Mannini; Danieli, Chiara; L., Armelao; F. M., Piras; A., Magnani; Cornia, Andrea; R., Sessoli
abstract

The ability of a tetranuclear iron(III) single-molecule magnet functionalized with thioacetyl-terminated ligands to form monolayers on gold has been investigated by a multitechnique approach based on STM, XPS and ToF-SIMS. We discuss in detail several aspects, which are relevant to the reported observation of a memory effect on the monolayer prepared from dichloromethane (Mannini et al., Nat. Mater., 2009, 8, 194). In particular we show that the adsorbate comprises intact surface-bound Fe4 clusters as opposed to microcrystals or multilayers. The influence of the solvent used for the self assembly process on the morphology, composition and structure of the adsorbate is also studied for four different solvents (dichloromethane, n-hexane, toluene, 1,4-dioxane).

2010 - Electric-Field Controlled Magnetic Anisotropy in a Single Molecule [Articolo su rivista]
A. S., Zyazin; J. W. G., van den Berg; E. A., Osorio; H. S. J., van der Zant; N. P., Konstantinidis; M., Leijnse; M. R., Wegewijs; F., May; W., Hofstetter; Danieli, Chiara; Cornia, Andrea
abstract

We have measured quantum transport through an individual Fe4 single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties and, moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition/subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.

2010 - Introduction of Ester and Amido Functions in Tetrairon(III) Single-Molecule Magnets: Synthesis and Physical Characterization [Articolo su rivista]
Rodriguez Douton, Maria Jesus; Cornia, Andrea; R., Sessoli; L., Sorace; A. L., Barra
abstract

Tetrairon(III) complexes with a propeller-like structure derived from [Fe4(OMe)6(dpm)6] (1) (Hdpm = 2,2,6,6-tetramethylheptane-3,5-dione) are providing a growing class of Single Molecule Magnets (SMMs) displaying unprecedented synthetic flexibility and ease of functionalization. Herein we report the synthesis, crystal structures and magnetic properties of two novel tetrairon(III) SMMs, [Fe4(esterC5)2(dpm)6] (2) and [Fe4(amideC5)2(dpm)6]·Et2O·4MeOH (3·Et2O·4MeOH), in whichfunctionalization of the cluster core is achieved using ester and amido linkages, respectively. To this aim, two new tripodal ligands were prepared by acylation of pentaerythritol (2,2-bis(hydroxymethyl)-propane-1,3-diol) and TRIS (2-amino-2-(hydroxymethyl)propane-1,3-diol), namely H3esterC5 =RC(O)OCH2C(CH2OH)3 and H3amideC5 = RC(O)NHC(CH2OH)3 with R = n-butyl. The compounds were structurally investigated by single-crystal XRD, which demonstrated coordination of the tripodal ligands to the cluster core. The products display SMM behavior with anisotropy barriers Ueff/kB~=11 Kdue to a high-spin (S = 5) ground state and an easy axis anisotropy, described by D = -0.421 cm-1 in 2and -0.414 cm-1 in 3·Et2O·4MeOH. The departure of Ueff from the total splitting of the S = 5 ground multiplet, U/kB~=15 K, has to be ascribed to the sizeable rhombic anisotropy that characterizes the two compounds (E = 0.021 cm-1 in 2 and 0.019 cm-1 in 3·Et2O·4MeOH), as confirmed by master matrixcalculations of the temperature-dependent relaxation time.

2010 - Quantum Tunneling of the Magnetization in a Monolayer of Oriented Single Molecule Magnets [Articolo su rivista]
M., Mannini; F., Pineider; Danieli, Chiara; F., Totti; L., Sorace; Sainctavit, P. h.; M. A., Arrio; E., Otero; L., Joly; J. C., Cesar; Cornia, Andrea; R., Sessoli
abstract

A fundamental step towards atomic- or molecular-scale spintronicdevices has recently been made by demonstrating that the spin of an individual atom deposited on a surface, or of a small paramagnetic molecule embedded in a nanojunction, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices. Here we show that a preferential orientation of Fe4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs—their stepped hysteresis loop, which results from resonantquantum tunnelling of the magnetization—can be clearly detectedusing synchrotron-based spectroscopic techniques. With the aid ofmultiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

2010 - Slow Magnetic Relaxation from Hard-axis Metal Ions in Tetranuclear Single-Molecule Magnets [Articolo su rivista]
Tancini, Erik; RODRIGUEZ DOUTON, Maria Jesus; L., Sorace; A. L., Barra; R., Sessoli; Cornia, Andrea
abstract

We report the synthesis of the novel heterometallic complex [Fe3Cr(L)2(dpm)6]⋅Et2O (Fe3CrPh) (Hdpm=dipivaloylmethane, H3L=2-hydroxymethyl-2-phenylpropane-1,3-diol), obtained by replacing the central iron(III) atom by a chromium(III) ion in an Fe4 propeller-like single-molecule magnet (SMM). Structural and analytical data, high-frequency EPR (HF-EPR) and magnetic studies indicate that the compound is a solid solution of chromium-centred Fe3Cr (S=6) and Fe4 (S=5) species in an 84:16 ratio. Although SMM behaviour is retained, the |D| parameter is considerably reduced as compared with the corresponding tetra-iron(III) propeller (D=−0.179 vs. −0.418 cm−1), and results in a lower energy barrier for magnetisation reversal (Ueff/kB=7.0 vs. 15.6 K). The origin of magnetic anisotropy in Fe3CrPh has been fully elucidated by preparing its Cr- and Fe-doped Ga4 analogues, which contain chromium(III) in the central position (c) and iron(III) in two magnetically distinct peripheral sites (p1 and p2). According to HF-EPR spectra, the Cr and Fe dopants have hard-axis anisotropies with Dc=0.470(5) cm−1, Ec=0.029(1) cm−1, Dp1=0.710(5) cm−1, Ep1=0.077(3) cm−1, Dp2=0.602(5) cm−1, and Ep2=0.101(3) cm−1. Inspection of projection coefficients shows that contributions from dipolar interactions and from the central chromium(III) ion cancel out almost exactly. As a consequence, the easy-axis anisotropy of Fe3CrPh is entirely due to the peripheral, hard-axis-type iron(III) ions, the anisotropy tensors of which are necessarily orthogonal to the threefold molecular axis. A similar contribution from peripheral ions is expected to rule the magnetic anisotropy in the tetra-iron(III) complexes currently under investigation in the field of molecular spintronics.

2010 - Thermodynamics of Host-guest Interactions between Methylpyridinium Salts and Phosphonate Cavitands [Articolo su rivista]
D., Menozzi; E., Biavardi; C., Massera; F. P., Schmidtchen; Cornia, Andrea; E., Dalcanale
abstract

In this work, the properties of complexation of tetraphosphonate cavitands towards methylpyridinium guests were investigated via isothermal titration calorimetry (ITC). For this purpose, Tiiii[C3H7, CH3 , Ph], Tiiii[C3H7, H, Ph], TSiiii[C3H7, H, Ph] hosts and three different methylpyridinium guests were synthesised. The role of the following parameters in the host–guest complexation was investigated: (i) solvation, (ii) nature of the guest counterion, (iii) presence of substituents in the apical positions of the receptor, and (iv) P=O versus P=S bridging units. The results showed that (i) switching from dichloroethane to methanol leads to a decrease of the association constant due to the competitive nature of the solvent, (ii) the guest counterion does not affect the thermodynamics of the process, (iii) the apical methyl groups enhance the binding affinity of the receptor and (iv) the comparison between phosphonate and thiophosphonate hosts clearly demonstrates that cation–dipole interactions are necessary for binding.

2010 - XAS and XMCD of Single Molecule Magnets [Relazione in Atti di Convegno]
R., Sessoli; M., Mannini; F., Pineider; Cornia, Andrea; Sainctavit, P. h.
abstract

Molecular magnetism is here presented with emphasis concerning the single molecule magnets (SMMs). The architecture of SMMs is reviewed as well as the various ingredients promoting magnetic anisotropy and the relation between magnetic anisotropy and the dynamics of magnetization. Then it is shown how XAS and XMCD can be unique tools to unravel the magnetic properties of SMM submonolayers grafted on clean surfaces. We bring a special attention to the spectral features associated with the magnetic anisotropy and magnetization dynamics.

2010 - XPS, FTIR-ATR and AFM Structural Study of Silicon Grafted Triol Monolayers for Controlled Anchoring of Single Molecule Magnets [Articolo su rivista]
A., Motta; G. G., Condorelli; G., Pellegrino; Cornia, Andrea; I. L., Fragalà
abstract

The structures of monolayers on Si(100) made up of two acetyl-protected triol ligands, derived from 2-hydroxymethyl-propane-1,3-diol, namely, R-C(CH2OAc)3 with R = CH2═CH−(CH2)9−O−CH2− and R = CH2═CH−(CH2)7−, have been investigated. After deprotection, the two ligands are well-suited receptors for anchoring molecular functionalities, such as Fe4 single molecule magnets, to the Si surface through OH groups. Monolayers were obtained by the hydrosilylation reactions between H-terminated Si(100) and the CH2═CH− end groups and were prepared in pure form or diluted with inert 1-alkenes to achieve an accurate control over the concentration and concerted orientation of the receptors on the surface. Subsequent anchoring of a Fe4 cluster, especially designed for a vertical atom sequence, has been expedient to clarify the influence of ligand mole fraction on the ordering of molecular receptors. We have achieved insight into the related structures and morphologies by combining angle-resolved X-ray photoelectron spectroscopy (AR-XPS), FTIR-attenuated total reflection (ATR) spectroscopy, and atomic force microscopy (AFM), including force lithography experiments. There is evidence that steric hindrances of apical groups are crucial for surface concentrations and, in turn, for geometrical orientation of the tethering units. Aging effects on monolayer structures and possible recovery processes have also been evaluated.

2009 - A novel class of tetrairon(III) single-molecule magnets with graphene-binding groups [Articolo su rivista]
Danieli, Chiara; Cornia, Andrea; C., Cecchelli; R., Sessoli; A. L., Barra; Ponterini, Glauco; Zanfrognini, Barbara
abstract

Tripods of general formula R’–O–CH2C(CH2OH)3 are excellent site-specific ligands for the preparation offunctionalized Fe4 single-molecule magnets. Herein, we describe the synthesis and characterization oftwo novel complexes designed to bind graphene surfaces, in which the R group consists of an alkyl spacer–(CH2)n– (n = 6 and 10) and a terminal pyrenyl moiety. The site-specific ligand substitution on [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane) with the new tripods has been studied with 2H NMR on isotopically-enriched samples, revealing that, once formed, these clusters are stable in solution over longtimescales. It was not possible to isolate the new compounds as crystalline solids, nevertheless they werechemically characterized by elemental analysis and 1H NMR. The presence of the pyrenyl ending groupsprompted us to investigate the effect of metal complexation on fluorescence, and a full pyrene-to-ironcluster excitation energy transfer was observed. The analysis of the magnetic behaviour revealed anS = 5 ground spin state with a negative zero-field splitting parameter D = 0.42 cm1.

2009 - Magnetic Memory of a Single-molecule Quantum Magnet Wired to a Gold Surface [Articolo su rivista]
M., Mannini; F., Pineider; Sainctavit, P. H.; Danieli, Chiara; E., Otero; Sciancalepore, Corrado; Talarico, Anna Maria; M. A., Arrio; Cornia, Andrea; D., Gatteschi; R., Sessoli
abstract

In the field of mol. spintronics, the use of magnetic mols. for information technol. is a main target and the observation of magnetic hysteresis on individual mols. organized on surfaces is a necessary step to develop mol. memory arrays. Although simple paramagnetic mols. can show surface-induced magnetic ordering and hysteresis when deposited on ferromagnetic surfaces, information storage at the mol. level requires mols. exhibiting an intrinsic remnant magnetization, like the so-called single-mol. magnets (SMMs). These have been intensively investigated for their rich quantum behavior but no magnetic hysteresis has been so far reported for monolayers of SMMs on various nonmagnetic substrates, most probably owing to the chem. instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic CD synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temps., we have now found that robust, tailor-made Fe4 complexes retain magnetic hysteresis at gold surfaces. Our results demonstrate that isolated SMMs can be used for storing information. The road is now open to address individual mols. wired to a conducting surface in their blocked magnetization state, thereby enabling investigation of the elementary interactions between electron transport and magnetism degrees of freedom at the mol. scale.

2009 - Magnetostructural Correlations in Tetrairon(III) Single-Molecule Magnets [Articolo su rivista]
Gregoli, Luisa; Danieli, Chiara; A. L., Barra; P., Neugebauer; G., Pellegrino; G., Poneti; R., Sessoli; Cornia, Andrea
abstract

We report three novel tetrairon(III) single-molecule magnetswith formula [Fe4(L)2(dpm)6] (Hdpm=2,2,6,6-tetramethylheptane-3,5-dione), prepared by using pentaerythritol monoether ligands H3L=R’OCH2C(CH2OH)3 with R’=allyl (1), (R,S)-2-methyl-1-butyl (2), and (S)-2-methyl-1-butyl (3), along with a new crystal phase of the complex containing H3L=11-(acetylthio)-2,2-bis(hydroxymethyl)-undecan-1-ol (4). High-frequency EPR (HF-EPR) spectra at low temperature were collected on powder samples in order to determine the zero-field splitting (zfs) parameters in the ground S=5 spin state. In 1–4 and in other eight isostructural compounds previously reported,a remarkable correlation is found between the axial zfs parameterD and the pitch gamma of the propeller-like structure. The relationship is directly demonstrated by 1, which features both structurally and magnetically inequivalent molecules in the crystal. The dynamics of magnetization has been investigated by ac susceptometry, and the results analyzed by master-matrix calculations. The large rhombicities of 2 and 3 were found to be responsible for the fast magnetic relaxation observed in the two compounds. However, complex 3 shows an additional faster relaxation mechanism which is unaccounted for by the set of spin Hamiltonian parameters determined by HF-EPR.

2009 - Muon Spin Relaxation Investigation of Tetranuclear Iron(III) Fe4(OCH3)6(dpm)6 Molecular Cluster [Articolo su rivista]
D., Procissi; P., Arosio; F., Orsini; M., Marinone; Cornia, Andrea; A., Lascialfari
abstract

We present a study of the spin dynamics of Fe4(OCH3)6(dpm)6 single molecule magnet by means of SQUIDmagnetization and muon relaxation (mu+SR) measurements. In longitudinal field mu+SR experiments performedat magnetic fields H = 200, 1000 Oe, the muon asymmetry P(t) could be fitted by means of three components,the first constant, the second fast relaxing through a quasiexponential decay, and the third, the slowest relaxing,showing an exponential decay. The slowest muon relaxation rate Lambda studied as a function of temperature Tdisplayed two structures, a broad peak at T ca. 15/20 K and a shoulder at T < 5 K, both decreasing in amplitudeand displacing toward higher temperatures as the field is increased. To mimic qualitatively the temperaturebehavior Lambda(T) at the investigated fields, we used a function expressed as the sum of two Bloembergen-Purcell-Pound (BPP)-like laws, reproducing the mechanism of relaxation. The exponential data resulted well fitted bymeans of a heuristic model which takes into account two correlation times tau' and tau, related to the ground-statemultiplet barrier (Delta'/kB=7.25 K) and to the intermultiplet separation (Delta/kB=86.4 K) between S=5 andS=4.

2009 - One Pot Grafting of Tetrairon(III) Single Molecule Magnets on Silicon [Articolo su rivista]
G., Pellegrino; A., Motta; Cornia, Andrea; I., Spitaleri; I. L., Fragalà; G. G., Condorelli
abstract

Herein we report on the Si grafting of two Fe4 derivatives, [Fe4(Li)2(tmhd)6], in which tmhd is 2,2,6,6-tetramethylheptane-3,5-dionate and H3Li = R–C(CH2OH)3 is a tripodal ligand with R = CH2=CH–CH2–O–CH2 (H3L1) and CH2=CH–(CH2)9–O–CH2 (H3L2). These complexes were specifically designed to be directlyanchored on the H-terminated silicon surface via the hydrosilylation reaction. The complexes were grafted by a one pot route based on the photoinduced hydrosilylation followed by a ligand exchange step in the same reaction solution. The resulting decorated surfaces were characterized using X-ray photoelectronspectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR) and atomic force microscopy (AFM).

2009 - Organizing and Addressing Magnetic Molecules [Articolo su rivista]
D., GATTESCHI; CORNIA, Andrea; M., MANNINI; R., SESSOLI
abstract

Magnetic molecules ranging from simple organic radicals to single-molecule magnets (SMMs) are intensively investigated for their potential applications in molecule-based information storage and processing. The goal of this Article is to review recent achievements in the organization of magnetic molecules on surfaces and in their individual probing and manipulation. We stress that the inherent fragility and redox sensitivity of most SMM complexes, combined with the noninnocent role played by the substrate, ask for a careful evaluation of the structural andelectronic properties of deposited molecules going beyond routine methods for surface analysis. Detailed magnetic information can be directly obtained using X-ray magnetic circular dichroism or newly emerging scanning probe techniques with magnetic detection capabilities.

2009 - Single-Molecule-Magnet Carbon-Nanotube Hybrids [Articolo su rivista]
L., Bogani; Danieli, Chiara; E., Biavardi; N., Bendiab; A. L., Barra; E., Dalcanale; W., Wernsdorfer; Cornia, Andrea
abstract

The self-assembly of single-molecule magnet (SMM) carbon-nanotube (CNT) hybrids in conditions compatible to the creation of electronic devices is described. The process is controlled at the single-molecule level, and the resulting CNT-field-effect transistors display single-SMM sensitivity at room temp.

2009 - Thermal Deposition of Intact Tetrairon(III) Single Molecule Magnets in High Vacuum Conditions [Articolo su rivista]
L., Margheriti; M., Mannini; L., Sorace; L., Gorini; D., Gatteschi; R., Sessoli; D., Chiappe; R., Moroni; F., Buatier de Mongeot; Cornia, Andrea; F. M., Piras; A., Magnani
abstract

A tetrairon(III) single-molecule magnet is deposited using a thermal evaporation technique in high vacuum. The chemical integrity is demonstrated by time-of-flight secondary ion mass spectrometry on a film deposited on Al foil, while superconducting quantum interference device magnetometry and alternating current susceptometry of a film deposited on a kapton substrate show magnetic properties identical to the pristine powder.High-frequency electron paramagnetic resonance spectra confirm the characteristic behavior for a system with S¼5 and a large Ising-type magnetic anisotropy. All these results indicate that the molecules are not damaged during the deposition procedure keeping intact the single-molecule magnet behavior.

2009 - X-ray Magnetic Circular Dichroism Picks Out Single-Molecule Magnets Suitable for Nano-devices [Articolo su rivista]
M., Mannini; F., Pineider; Sainctavit, P. H.; L., Joly; A., FRAILE RODRIGUEZ; M. A., Arrio; CARTIER DIT MOULIN, C. H.; W., Wernsdorfer; Cornia, Andrea; D., Gatteschi; R., Sessoli
abstract

The surface sensitivity of X-ray magnetic CD in extreme conditions is exploited to investigate the first layers of bulk single-mol. magnets (SMM). Striking differences emerge between two classes of SMM with different structural constraints, thus highlighting the importance of mol. design in the realization of mol. spintronic devices.

2009 - XMCD of a single layer of single molecule magnets [Articolo su rivista]
Mannini, M.; Pineider, F.; Sainctavit, Philippe; Cartier dit Moulin, C.; Arrio, M. -A.; Cornia, A.; Gatteschi, D.; Sessoli, R.
abstract

A detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison with XMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices.

2008 - Magneto-optical Studies on the Molecular Cluster Fe4 in Different Polymeric Environments [Articolo su rivista]
R. L., Novak; F., Pineider; C., DE JULIÁN FERNÁNDEZ; L., Gorini; L., Bogani; Danieli, Chiara; L., Cavigli; Cornia, Andrea; R., Sessoli
abstract

The magnetization of a molecular cluster of the Fe4 family embedded in two different polymeric matrices was investigated at low temperatures employing a magnetic circular dichroism (MCD) setup. The light wavelength used for the experiments was found to have a strong influence on the magnetic field dependence of the MCD intensities. The MCD response of this cluster embedded in a silicone elastomer and in poly(methyl methacrylate) (PMMA) exhibited at 476.5 nm a field dependence similar to the one found in the corresponding magnetization data while at other wavelengths (904 and 632.8 nm) the MCD could not be related to the cluster magnetization. Furthermore, the different polymers used as matrix induced slightly different magnetic and MCD behaviors with the samples of Fe4 in poly(methyl methacrylate) saturating at lower magnetic field values than the ones in the silicone elastomer. This result is believed to arise from the selection rules of the electronic transitions in the MCD measurement process. Furthermore, anisotropies in the spatial distribution of magnetic easy-axes, as well as inhomogeneities in cluster concentration induced by the PMMA casting process not present in the silicone elastomer could also contribute to the different magnetic behaviors observed.

2008 - Molecular Magnetism, Status and Perspectives [Articolo su rivista]
D., Gatteschi; L., Bogani; Cornia, Andrea; M., Mannini; L., Sorace; R., Sessoli
abstract

A short review is made of molecular magnetism, trying to discuss what is alive and well, with perspectives for the future. All the main fields of activity are mentioned, ranging from the so-called spin cross-over systems to the quest for organic (molecular) ferromagnets. Particular attention is devoted to some of the recent advances in these fields, highlighting also the opportunities for the development of applications. Low dimensional magnets are perhaps the best opportunity to use molecules, and the status of single-molecule and single chain magnets is discussed. The last part is devoted to the organization of magnetic molecules and to the development of techniques which allow to measurethe magnetic properties of thin layers and, in perspective, of single molecules.

2008 - Novel Chiral Calix[4]arenes by Direct Asymmetric Epoxidation Reaction [Articolo su rivista]
C., Bonini; L., Chiummiento; M., Funicello; M. T., Lopardo; P., Lupattelli; A., Laurita; Cornia, Andrea
abstract

The first asym. synthesis of trans optically active (+) C2 1,3-bisarylepoxide of calix[4]arene I in excellent chem. yield and >99% ee, and its enantiospecific conversion to the corresponding bis-dioxolane II is reported.

2008 - Site-specific Anchoring of Tetrairon(III) Single Molecule Magnets on Functionalized Si(100) Surfaces [Articolo su rivista]
G. G., Condorelli; A., Motta; G., Pellegrino; Cornia, Andrea; L., Gorini; I. L., Fragalà; C., Sangregorio; L., Sorace
abstract

The site-specific reaction of tetrairon(III) propeller-like clusters with tripodal ligands derived from 2-hydroxymethyl-propane-1,3-diol was investigated as a route to graft these single molecule magnets (SMMs) on Si(100). A silicon surface prefunctionalized with a long-chain trimethylol derivative was reacted with [Fe4(OMe)6(tmhd)6] (1) and [Fe4(L)2(tmhd)6] (2), and the reaction course was followed by X-ray photoelectron spectroscopy (XPS), attenuated total reflection-infrared spectroscopy, and atomic force microscopy (Htmhd = 2,2,6,6-tetramethylheptane-3,5-dione, H3L = 2-(4-chloro-phenyl)-2-hydroxymethyl-propane-1,3-diol). Chlorine-marked complex 2 was specifically designed to monitor the ligand-exchange reaction by XPS. This technique showed that the amount of Fe at the surface closely parallels the surface concentration of tripodal receptors. For low receptor concentrations the Cl/Fe atomicratio is halved as compared with solid 2, as expected for intact molecules linked to the surface via one receptor only. For high receptor concentrations, the Cl/Fe ratio drops to zero, as expected for a complete replacement of L3- ligands by surface-bound receptors.

2008 - Slow Quantum Relaxation in a Tetrairon(III) Single-Molecule Magnet [Articolo su rivista]
Cornia, Andrea; Gregoli, Luisa; Danieli, Chiara; A., Caneschi; R., Sessoli; L., Sorace; A. L., Barra; W., Wernsdorfer
abstract

Monoethers of pentaerythritol, R'O-CH2C(CH2OH)3, are convenient site-specific ligands for the design and prepn. of functionalized Fe4 single-mol. magnets. Herein, we describe the synthesis, crystal and mol. structure and magnetic properties of a novel Fe4 deriv. with R' = Ph, obtained by site-specific ligand substitution on [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane). The compd., which has the lowest mol. symmetry among all Fe4 derivs. so far reported (C1), retains the same S = 5 ground spin state as the parent compd., but shows an enhanced easy-axis anisotropy with D = -0.433(2) cm-1, E = 0.014(2) cm-1 and B40 = +1.5(1)×10-5 cm-1 (from high frequency and X-band EPR). The thermal-activation parameters for magnetic moment reversal are Ueff/kB = 15.7(2) K and τ0 = 3.5(5)×10-8 s (from AC susceptometry). Micro-SQUID measurements on single crystals show that below about 0.2 K the spin dynamics is dominated by quantum tunneling within the MS = ±5 ground doublet. In spite of the low mol. symmetry, which is generally believed to enhance tunneling effects, the relaxation time in the purely quantum regime is as long as ∼2.5×10^4 s (∼7 h).

2008 - XAS and XMCD Investigation of Mn12 Monolayers on Gold [Articolo su rivista]
M., Mannini; Sainctavit, P. H.; R., Sessoli; CARTIER DIT MOULIN, C. H.; F., Pineider; M. A., Arrio; Cornia, Andrea; D., Gatteschi
abstract

The deposition of Mn12 single-molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposedapproach to probe the electronic structure and magnetism of Mn12 complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra,respectively. Partial reduction of metal ions to MnII was clearly observed upon deposition on Au(111) of two different Mn12 derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidationstate, as well as the relative proportions of MnII, MnIII and MnIV species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantlydecreased as compared with bulk Mn12 samples. The results highlight an utmost redox instability of Mn12 complexes at gold surfaces, presumably accompanied by structural rearrangements,which cannot be easily revealed by standard surface analysisbased on X-ray photoelectron spectroscopy and scanning tunnelling microscopy.

2007 - Magneto-optical Investigations of Nanostructured Materials Based on Single Molecule Magnets Monitor Strong Environmental Effects [Articolo su rivista]
L., Bogani; L., Cavigli; M., Gurioli; R. L., Novak; M., Mannini; A., Caneschi; F., Pineider; R., Sessoli; M., CLEMENTE LEÓN; E., Coronado; Cornia, Andrea; D., Gatteschi
abstract

The magnetization of single-mol. magnets (SMMs) in noncryst. materials is probed using high-sensitivity magneto-optical techniques. The environment strongly influences the magnetic properties and dramatically changes the spin dynamics. Comparison between different materials indicates the phonon bath as a possible source of the effect. These results indicate the importance of the environment when designing and modeling SMM-based spintronic devices.

2007 - New Cyclosiloxanolate Cluster Complexes of Transition Metals [Articolo su rivista]
Mortalo', Cecilia; A., Caneschi; Cornia, Andrea; E., Diana; S., Faranda; V., Marvaud; M., Pizzotti; O. I., Shchegolikhina; Zucchi, Claudia; Palyi, Gyula
abstract

New cyclosiloxanolate transition metal cluster complex derivs. were prepd. PhSiO2K reacted with NiX2 (X2 = Cl2 or acac) to give K2{[η6-(PhSiO2)6]2[μ3-(OH)]2Ni4K4}, a mixed Group 1-group 10 metal complex. PhSiO2Na reacted with Ni(NH3)6I2 to give Na{[η6-(PhSiO2)6]2Ni6(μ6-I)} as the 1st example of encapsulated I- ion in siloxanolate complexes. The macrocyclic Na4{[η12-(PhSiO2)12]Cu4} complex reacted with η6-(1,3,5-C7H8)Cr(CO)3 to give the heterobimetallic adduct Na4{[η12-(PhSiO2)12]Cu4}[Cr(CO)3]3 as one of the rare examples of heterobimetallic complexes with different oxidn. nos. of the metals. The Cu deriv. {[η6-(PhSiO2)6]2Cu6(BuOH)5} reacted in MeOH/CHCl3 (1:6) with Et4NCN to give hexanuclear {[η6-(PhSiO2)6]2Cu6(η2-C3H5N2O2)2}, contg. 2-amino-2-oxoethanimidic acid Me ester monoanion ligands, product of an unexpected C-C coupling reaction. This latter complex was characterized also by x-ray diffraction crystal and mol. structure detn.

2007 - New Single-Molecule Magnets by Site-Specific Substitution: Incorporation of “Alligator Clips” into Fe4 Complexes [Articolo su rivista]
A. L., Barra; F., Bianchi; A., Caneschi; Cornia, Andrea; D., Gatteschi; L., Gorini; Gregoli, Luisa; M., Maffini; Parenti, Francesca; R., Sessoli; L., Sorace; Talarico, Anna Maria
abstract

“Alligator clips” suitable for grafting on H-terminated siliconor gold surfaces have been incorporated into single-moleculemagnets (SMMs) of the Fe4 family. Tripodal ligands 2,2-bis-(hydroxymethyl)-10-undecen-1-ol (H3L1) and 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol (H3L2) have been usedto replace the six methoxide bridges in [Fe4(OMe)6(dpm)6],affording new Fe4 derivatives [Fe4(L1)2(dpm)6] (1) and[Fe4(L2)2(dpm)6] (2) in high yield (Hdpm = dipivaloylmethane).The molecular structures have been unambiguously established by XRD combined with ESI-MS and magnetic characterization techniques, including DC and AC magnetometry and high-frequency EPR spectroscopy. The twoSMMs have an S = 5 ground state and an easy-axis magneticanisotropy leading to slow relaxation of the magnetization atlow temperature, with effective anisotropy barriers of15.9(2) K in 1 and 15.1(2) K in 2.

2007 - Self-assembling of Mn12 molecular nanomagnets on FIB-patterned Au dot matrix [Articolo su rivista]
V., Corradini; DEL PENNINO, Umberto; Biagi, Roberto; DE RENZI, Valentina; Gambardella, Alessandro; G. C., Gazzadi; Candini, Andrea; Zobbi, Laura; Cornia, Andrea
abstract

AbstractWe have developed a novel strategy to build arrays of magnetic nanodots on the 100 nm scale, which exploits the potentialities of both bottom-up and top-down approaches, by self-assembling sulfur-functionalized Mn12 single molecule magnets (SMMs) on patterned Au dot matrices nanofabricated by FIB (focus ion beam). In this way, we demonstrate the capability to assemble SMMs in ordered arrays, where the magnetic information can be easily addressed, being the single bit represented by a 2D distribution of few hundred Mn12 clusters, grafted on top of each 100 × 100 nm2 Au dot. Moreover, the chosen Mn12 functionalization is expected to favour a preferential orientation of the grafted molecule with the easy magnetization axis normal to the surface.

2007 - Solvent Effects on the Adsorption and Self-Organization of Mn12 on Au(111) [Articolo su rivista]
F., PINEIDER; M., MANNINI; R., SESSOLI; A., CANESCHI; D., BARRECA; L., ARMELAO; CORNIA, Andrea; E., TONDELLO; D., GATTESCHI
abstract

A sulfur-containing single molecule magnet, [Mn12O12(O2CC6H4SCH3)16(H2O)4], was assembled from solution on a Au(111) surface affording both submonolayer and monolayer coverages. The adsorbate morphology and the degreeof coverage were inspected by scanning tunneling microscopy (STM), while X-ray photoelectron spectroscopy (XPS) allowed the determination of the chemical nature of the adsorbate on a qualitative and quantitative basis. The properties of the adsorbates were found to be strongly dependent on the solvent used to dissolve the magnetic complex. In particular, systems prepared from tetrahydrofuran solutions gave arrays of isolated and partially ordered clusters on the gold substrate, while samples prepared from dichloromethane exhibited a homogeneous monolayer coverage of the whole Au(111) surface. These findings are relevant to the optimization of magnetic addressing of single molecule magnets on surfaces.

2007 - The Origin of Transverse Anisotropy in Axially Symmetric Single Molecule Magnets [Articolo su rivista]
A. L., Barra; A., Caneschi; Cornia, Andrea; D., Gatteschi; L., Gorini; L. P., Heiniger; R., Sessoli; L., Sorace
abstract

Single-crystal high-frequency ESR spectroscopy was employed on a truly axial single mol. magnet [Mn12O12(tBuCH2CO2)16(CH3OH)4]·CH3OH to study the origin of the transverse magnetic anisotropy, a crucial parameter that rules the quantum tunneling of the magnetization. The crystal structure, including the abs. structure of the crystal used for EPR expts., was fully detd. and found to belong to I-4 tetragonal space group. The angular dependence of the resonance fields in the crystallog. ab plane shows high-order tetragonal anisotropy and strong dependence on the MS sublevels with the 2nd-highest-field transition being angular independent. This was rationalized including competing 4th- and 6th-order transverse parameters in a giant spin Hamiltonian which describes the magnetic anisotropy in the ground S = 10 spin state of the cluster. To establish the origin of these anisotropy terms, the exptl. results were further analyzed using a simplified multispin Hamiltonian which takes into account the exchange interactions and the single ion magnetic anisotropy of the MnIII centers. It was possible to establish magnetostructural correlations with spin Hamiltonian parameters up to the 6th order. Transverse anisotropy in axial single mol. magnets was found to originate from the multispin nature of the system and from the breakdown of the strong exchange approxn. The tilting of the single-ion easy axes of magnetization with respect to the 4-fold mol. axis of the cluster plays the major role in detg. the transverse anisotropy. Counterintuitively, the projections of the single ion easy axes on the ab plane correspond to hard axes of magnetization.

2007 - “Functionalized single–molecule magnets for molecular electronics” [Abstract in Atti di Convegno]
Cornia, Andrea; Anne Laure, Barra; Federica, Bianchi; Andrea, Caneschi; Danieli, Chiara; Dante, Gatteschi; Lapo, Gorini; Gregoli, Luisa; Monica, Maffini; Parenti, Francesca; Roberta, Sessoli; Lorenzo, Sorace; Anna Maria, Talarico
abstract

v. allegato

2006 - EPR of Molecular Nanomagnets [Articolo su rivista]
D., Gatteschi; A. L., Barra; A., Caneschi; Cornia, Andrea; R., Sessoli; L., Sorace
abstract

Electron paramagnetic resonance has played a crucial role in the determination of the properties and in explaining the origin of the peculiar behaviour of molecular nanomagnets, i.e., the slow relaxation of the magnetization at low temperature and the observation of quantum tunnelling of the magnetization. This is due to its ability to probe directly the spin levels rather than providing thermodynamic averages as in standard magnetic measurements. In particular, thanks to recent technical improvements in both reliability and sensitivity, high-field high-frequency EPR has gained widespread use for the characterization of these systems. We will briefly review here the different aspects underlying the use of this technique, highlighting some results of general interest obtained in the last decade on different class of magnetic clusters.

2006 - Electron Transport through Single Mn12 Molecular Magnets [Articolo su rivista]
H. B., HEERSCHE; Z., DE GROOT; J. A., FOLK; H. S. J., VAN DER ZANT; C., ROMEIKE; M. R., WEGEWIJS; ZOBBI, Laura; D., BARRECA; E., TONDELLO; CORNIA, Andrea
abstract

The authors report transport measurements through a single-mol. magnet, the Mn12 deriv. [Mn12O12(O2C-C6H4-SAc)16(H2O)4], in a single-mol. transistor geometry. Thiol groups connect the mol. to gold electrodes that are fabricated by electromigration. Striking observations are regions of complete current suppression and excitations of neg. differential conductance on the energy scale of the anisotropy barrier of the mol. Transport calcns., taking into account the high-spin ground state and magnetic excitations of the mol., reveal a blocking mechanism of the current involving nondegenerate spin multiplets.

2006 - Preparation of Novel Materials Using SMMs [Capitolo/Saggio]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Zobbi, Laura; A., Caneschi; D., Gatteschi; M., Mannini; R., Sessoli
abstract

Magnetic hysteresis is considered a distinctive feature of bulk magnetic materials, where it arises from long-range ordering and irreversible motion of domain walls. However, many anisotropic high-spin molecules, referred to as Single-Molecule Magnets (SMMs), display a hysteresis loop at low temperature and have the potential of being used as molecular-scale units for information storage. This work reviews recent advances in the realization of new bulk materials and low-dimensional nanostructures based on SMMs. Particular reference will be made to mixed-valence Mn-12 complexes, which comprise the most versatile and best-performing SMMs available to date. Nonetheless, the described criteria for structure tailoring as well as the guiding principles in material design are of general value and applicability. Examples are given of multifunctional materials of either inorganic or hybrid nature which combine in a unique fashion the magnetic properties of embedded SMMs with the conductive, optical and mechanical characteristics of a polymeric host. In the framework of the molecular approach to electronics, successful attempts to organize SMMs into bidimensional arrays on surfaces and to address them individually have been reported. Recent results in this area will be treated exemplarily, with emphasis on molecular design and surface functionalization. The construction of molecular-scale electronic devices embodying SMMs as active components is finally highlighted among the future developments of this lively research area, which straddles the interface between chemistry and physics.

2006 - Single-ion and Molecular Contributions to the Zero-field Splitting in an Iron-(III)-oxo Dimer Studied by Single Crystal W-band EPR [Articolo su rivista]
P., TER HEERDT; M., STEFAN; E., GOOVAERTS; A., CANESCHI; CORNIA, Andrea
abstract

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a beta-diketonate-alkoxide iron(III) dimer compound, [Fe-2(OCH3)(2)(dbm)(4), Hdbm = dibenzoylmethane] (Fe-2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). The diamagnetic substitution method was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga-Fe dimers (GaFe). The single-ion zero-field splitting (ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe-O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S = 1, 2, 3, which were analyzed using the full Spin Hamiltonian description of all exchange coupled pair of s = 5/2 spins. The anisotropic spin-spin interaction tensor of Fe2 possesses a principal axis close to the Fe-Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves a rhombic component J(E) = (J(X) - J(Y))/2 ca. 0.093 cm(-1) of magnitude comparable to the dipolar interaction, and even to the rhombic part of the single-ion ZFS (E = 0.097 cm(-1)). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe-4 and Fe-8 families of single-rnolecule magnets and the antiferromagnetic iron wheels.

2006 - Tuning Anisotropy Barriers in a Family of Tetrairon(III) Single-molecule Magnets with an S=5 Ground State [Articolo su rivista]
S., ACCORSI; A. L., BARRA; A., CANESCHI; G., CHASTANET; CORNIA, Andrea; FABRETTI COSTANTINO, Antonio; D., GATTESCHI; MORTALO', Cecilia; E., OLIVIERI; PARENTI, Francesca; P., ROSA; R., SESSOLI; L., SORACE; W., WERNSDORFER; ZOBBI, Laura
abstract

Tetrairon(III) Single-Mol. Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6]·Et2O (3·Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) were prepd. by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topol. and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably neg., as found in 5 (D = -0.21 cm-1) and amts. to -0.445 cm-1 in 1, -0.432 cm-1 in 2, -0.42 cm-1 in 3·Et2O, and -0.27 cm-1 in 4 (dominant isomer). The anisotropy barrier Ueff detd. by a.c. magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3·Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and Ueff increase with increasing helical pitch of the Fe(O2Fe)3 core. The 4th-order longitudinal anisotropy parameter B40, which affects the shape of the anisotropy barrier, concomitantly changes from pos. in 1 (compressed parabola) to neg. in 5 (stretched parabola). With the aid of spin Hamiltonian calcns. the obsd. trends were attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.

2006 - Valence band resonant photoemission of Mn-12 single molecules grafted on Au(111) surface [Articolo su rivista]
DEL PENNINO, Umberto; DE RENZI, Valentina; Biagi, Roberto; V., Corradini; Zobbi, Laura; Cornia, Andrea; D., Gatteschi; F., Bondino; E., Magnano; M., Zangrando; M., Zacchigna; A., Lichtenstein; D. W., Boukhvalov
abstract

In the present work, we address the determination of the electronic structure of a monolayer of Mn-12 clusters, grafted on Au(1 1 1) substrate by a suitable functionalization, by means of X-ray absorption and resonant valence band photoemission (RESPES) spectroscopies taken across the Mn 2p-3d absorption edge. The absorption edge of the Mn-12 monolayer is compared with that - reported in literature - of bulk Mn-12, showing that the deposition procedure does not significantly affect the intimate nature of the Mn-12 cluster. Quantitative comparison between RESPES spectra data allow us to extract the Mn 3d density of states from the Mn-12-monolayer valence band spectrum, which is dominated by the Au 5d states. The shape of the experimentally obtained valence band DOS is in good agreement with the theoretical DOS, obtained by first-principle LDA + U calculations, confirming the importance of correlation effects in the determination of the electronic properties of the Mn-12 cluster. (c) 2006 Elsevier B.V. All rights reserved.

2005 - Advances in Single-Molecule Magnet Surface Patterning through Micro-Contact Printing [Articolo su rivista]
M., Mannini; D., Bonacchi; Zobbi, Laura; F. M., Piras; E. A., Speets; A., Caneschi; Cornia, Andrea; A., Magnani; B. J., Ravoo; D. N., Reinhoudt; R., Sessoli; D., Gatteschi
abstract

The authors present an implementation of strategies to deposit single-mol. magnets (SMMs) using microcontact printing (μCP). The authors describe different approaches of μCP to print stripes of a sulfur-functionalized dodecamanganese(III,IV) cluster on gold surfaces. Comparison by at. force microscopy profile anal. of the patterned structures confirms the formation of a chem. stable single layer of SMMs. Images based on chem. contrast, obtained by time-of-flight secondary ion mass spectrometry, confirm the patterned structure.

2005 - Fe-57 NMR and relaxation by strong collision in the tunneling regime in the molecular nanomagnet Fe8 [Articolo su rivista]
S. H., Baek; F., Borsa; Y., Furukawa; Y., Hatanaka; S., Kawakami; K., Kumagai; B. J., Suh; Cornia, Andrea
abstract

Fe-57 NMR measurements have been performed in single crystal and oriented powder of enriched (57)Fe8 molecular cluster in the temperature range 0.05-1.7 K in zero external field and with small perturbing field up to 1 T for both transverse and longitudinal orientation of H with respect to the anisotropy axis. The Fe-57 NMR spectrum is analyzed in terms of a dominant contribution due to the hyperfine interaction arising from core polarization. The measured temperature dependence of the resonance frequency is explained well by calculating the local average magnetic moment of the Fe3+ ion with a simple model which incorporates the effects of thermal average in the low lying energy states. Nuclear spin-lattice relaxation rate (1/T-1) and spin-spin relaxation rate (1/T-2) were investigated via temperature and field dependences. The obtained results are analyzed in terms of both intrawell thermal fluctuations of the hyperfine fields due to spin-phonon interaction, and interwell fluctuations due to phonon assisted quantum tunneling of the magnetization. It is argued that in zero external field and at low T the Fe-57 and the proton 1/T-1 is dominated by a strong collision relaxation mechanism due to the fact that phonon assisted tunneling transitions generate a sudden reversal of the local quantization field at the nuclear site. The data could be explained satisfactorily by assuming that the Fe-57 1/T-1 measures directly the effective tunneling rate. However, in order to fit the data we had to assume a larger in-plane anisotropy than previously reported, resulting in a bigger tunneling splitting in zero field. A comparison with published data of Mn-55 in Mn12 indicates that a strong collision relaxation mechanism may apply also in Mn12. Finally the H and T dependence of Fe-57 1/T-2 is well explained simply in terms of thermal fluctuations of the magnetization without any tunneling contribution. At very low T the 1/T-2 approaches a limiting value which can be explained in terms of the dipolar interaction between proton and Fe-57 nuclei in the quasistatic regime.

2005 - Isolated single-molecule magnets on native gold [Articolo su rivista]
L., Zobbi; M., Mannini; M., Pacchioni; G., Chastanet; D., Bonacchi; Zanardi, Chiara; Biagi, Roberto; DEL PENNINO, Umberto; D., Gatteschi; Cornia, Andrea; R., Sessoli
abstract

The incorporation of thioether groups in the structure of a Mn-12 single-molecule magnet, [Mn12O12(L)(16)(H2O)(4)] with L = 4-(methylthio) benzoate, is a successful route to the deposition of well-separated clusters on native gold surfaces and to the addressing of individual molecules by scanning tunnelling microscopy.

2004 - 57Fe NMR in Oriented Powder of Fe8 in Zero and Applied Field [Articolo su rivista]
S. H., Baek; S., Kawakami; Y., Furukawa; B. J., Suh; F., Borsa; K., Kumagai; Cornia, Andrea
abstract

The NMR of 57Fe was detected for the first time in zero external field at 1.5 K in a isotropically enriched and magnetically oriented powder sample of the octanuclear iron(III) cluster, Fe8. Eight well resolved lines, each about 100 kHz wide, were observed in the frequency range 63.1–72.4 MHz confirming the total lack of symmetry of the Fe8 molecule. The resonance frequency of the different lines are analyzed in terms of the hyperfine interactions.

2004 - Energy Barrier Enhancement by Ligand Substitution in Tetrairon(III) Single Molecule Magnets [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; P., Garrisi; Mortalo', Cecilia; D., Bonacchi; D., Gatteschi; R., Sessoli; L., Sorace; W., Wernsdorfer; Barra, A. –. L.
abstract

A dramatic increase of the energy barrier (Ueff) in tetrairon(III) single-mol. magnets can be achieved by simple chem. modification. Site-specific replacement of the six methoxide bridges in [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane; Ueff/kB = 3.5 K) with two tripodal 1,1,1-tris(hydroxymethyl)ethane (H3thme) ligands affords [Fe4(thme)2(dpm)6] with Ueff/kB = 15.6(2) K and a magnetic relaxation time exceeding 1000 s at T &lt;0.2 K. The prepd. complex is trigonal, space group R-3c, Z = 6, R1 = 0.0370, R2 = 0.1089.

2004 - Inter–multiplet Transitions in the Fe4 Magnetic Cluster [Articolo su rivista]
T., Guidi; G., Amoretti; R., Caciuffo; S., Carretta; Cornia, Andrea; C. D., Frost; E., Liviotti
abstract

The tetrairon(III) clusters Fe4(L)2(dpm)6 where Hdpm = dipivaloylmethane and H3L = MeC(CH2OH)3 or PhC(CH2OH)3 were obtained by site-specific replacement of the six methoxide bridges in Fe4(OMe)6(dpm)6. As compared with the parent compd., the new clusters show a much larger anisotropy in the S = 5 ground spin state (D/kB∼-0.6 K vs. -0.3 K) and a higher energy barrier to the reversal of the magnetization.

2004 - Intra- and Inter-multiplet Magnetic Excitations in a Tetrairon(III) Molecular Cluster [Articolo su rivista]
S., Carretta; P., Santini; G., Amoretti; T., Guidi; R., Caciuffo; A., Candini; Cornia, Andrea; D., Gatteschi; M., Plazanet; J. A., Stride
abstract

We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon(III) molecular cluster with trigonal symmetry, [Fe-4(thme)(2)(dpm)(6)], obtained from a parent compound with C-2 symmetry by site-specific ligand substitution. Intra-multiplet excitations within the anisotropy split S=5 ground spin state, and inter-multiplet transitions toward the first S=4 excited states have been observed. The model spin Hamiltonian used to interpret the experimental data evidences an enhancement of the anisotropy barrier due to the chemical modification of the parent molecular cluster. A rhombic anisotropy term, forbidden in D-3 symmetry, is necessary to reproduce experimental data. The determination of this term is crucial for the understanding of quantum tunneling of the magnetization in this model compound. The calculated temperature dependence of heat capacity for different applied magnetic field values is in good agreement with experimental measurements. Within this model, the estimated temperature dependence of the relaxation time agrees with available experimental data.

2004 - Li-7 Nuclear Magnetic Resonance in the Hexairon(III) Antiferromagnetic Molecular Ring Fe6:Li [Articolo su rivista]
A., Lascialfari; F., Borsa; M., Corti; E., Micotti; Y., Furukawa; Cornia, Andrea; D., Gatteschi
abstract

The mol. nanomagnet [LiFe6(OCH3)12(dbm)6]BPh4·5CH2Cl2 (Fe6:Li in short) is formed by six Fe(III) s = 5/2 spins arranged in an almost coplanar ring with antiferromagnetic (AFM) nearest-neighbor coupling const. J/kB∼20 K. The authors report 7Li NMR spectra and nuclear spin-lattice relaxation rate (NSLR) measurements as a function of temp. from 1.5 to 50 K with applied field H = 1.5 T. No evidence of quadrupole effects was found on the 7Li NMR spectrum within the linewidth resoln. (∼25 kHz) ruling against a mol. distortion. The 7Li NSLR is compared with the 1H NSLR results in Fe6:Li and in other AFM rings.

2004 - Organized single-molecule magnets: direct observation of new Mn12 derivatives on gold [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Pacchioni, Mirko; Zobbi, Laura; D., Bonacchi; A., Caneschi; D., Gatteschi; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; L., Gurevich; AND H. S. J., VAN DER ZANT
abstract

Gold adsorbates of the dodecamanganese(III,IV) single-molecule magnet (SMM) [Mn12O12(L)16(H2O)4] whereL=16-(acetylthio)hexadecanoate have been prepared and investigated by X-ray photoelectron spectroscopy andscanning tunneling microscopy (STM). The successful imaging of Mn12 molecules by STM represents a first step towardthe magnetic addressing of individual SMMs and the development of molecule-based devices for magnetic informationstorage.

2004 - Scaling Behavior of the Proton Spin-lattice Relaxation Rate in Antiferromagnetic Molecular Rings [Articolo su rivista]
S. H., Baek; M., Luban; A., Lascialfari; E., Micotti; Y., Furukawa; F., Borsa; J., VAN SLAGEREN; Cornia, Andrea
abstract

We present new and refined data for the magnetic field (H) and temp. (T) dependence of the proton spin-lattice relaxation rate (1/T1) in antiferromagnetic mol. rings as well as a new explicit scaling formula that accurately reproduces our data. The key ingredients of our formulation are (1) a reduced relaxation rate, R(H,T)=(1/T1)/(Tχ(T)), given by R(H,T)=Aωc(T)/(ω2c(T)+ω2N), where χ=(dM/dH)T is the differential susceptibility, A is a fitting const., and ωN is the proton Larmor frequency, and (2) a temp.-dependent correlation frequency ωc(T) which at low T is given by ωc(T).proportional to.Tα, that we identify as a lifetime broadening of the energy levels of the exchange-coupled paramagnetic spins due to spin-acoustic phonon coupling. The main consequences are (1) R(H,T) has a local max. for fixed H and variable T that is proportional to 1/H; the max. occurs at the temp. T0(H) for which ωc(T)=ωN; (2) for low T a scaling formula applies, R(H,T)/R(H,T0(H))=2tα/(1+t2α), where t = T/T0(H). Both results are confirmed by our exptl. data for the choice α=3.5±0.5.

2004 - Self-assembly of High-nuclearity Metal Clusters: Programmed Expansion of a Metallasiloxane Cage to an Octacopper(II) Cluster [Articolo su rivista]
G. L., Abbati; Cornia, Andrea; A., Caneschi; FABRETTI COSTANTINO, Antonio; Mortalo', Cecilia
abstract

The novel octanuclear Cu(II) cluster [Cu6{(PhSiO2)6}2{NCCu(Me6tren)}2(MeOH)4]2+ (1) was isolated as a perchlorate salt by reacting the hexacopper(II) metallasiloxane cage [Cu6{(PhSiO2)6}2(nBuOH)x] (x = 4, 6) with [Cu(Me6tren)CN]ClO4 in a MeOH/CHCl3 mixt. (Me6tren = tris(2-(dimethylamino)ethyl)amine). Crystal data for 1(ClO4)2·MeOH: monoclinic, space group P21/n, a = 16.8490(3), b = 22.2966(4), c = 17.2508(3) Å, β = 94.7658(5)°, Z = 2. The structure comprises a highly distorted hexagonal Cu6 array linked to two [Cu(Me6tren)] units via cyanide bridges. Magnetic measurements reveal that the addn. of the Cu cyanide complexes dramatically affects the magnetism of the Cu6 unit, whose ground spin state changes from S = 3 to S = 0.

2004 - Site-specific Ligation of Anthracene-1,8-Dicarboxylates to an Mn12 Core: a Route to the Controlled Functionalisation of Single-Molecule Magnets [Articolo su rivista]
M., Pacchioni; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Zobbi, Laura; D., Bonacchi; A., Caneschi; G., Chastanet; D., Gatteschi; R., Sessoli
abstract

A novel single-molecule magnet of the Mn-12 family, [Mn12O12(O2CC6H5)8(L)4(H2O)4].8CH2Cl2, has been synthesised by site-specific ligand exchange using a tailor-made dicarboxylate (L2-), which leads to selective occupation of axial binding sites.

2004 - Spin Dynamics at Level Crossing in Molecular AF Rings Probed by NMR [Articolo su rivista]
A., Lascialfari; F., Borsa; M. H., Julien; E., Micotti; Y., Furukawa; Z. H., Jang; Cornia, Andrea; D., Gatteschi; M., Horvatic; J., VAN SLAGEREN
abstract

The low-temperature spin dynamics in molecular rings with a finite number (N less than or equal to 10) of magnetic ions was studied by means of H-1 NMR. When an external magnetic field (B) induces a crossing between energy levels, peaks are observed in the spin-lattice relaxation rate of protons, 1/T-1(B), at constant temperature. We discuss similarities and differences in the data from three different rings: Fe10, Fe6:Li and Cr8.

2004 - Tuneable Energy Barriers in Tetrairon(III) Single–molecule Magnets [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; P., Garrisi; Mortalo', Cecilia; D., Bonacchi; R., Sessoli; L., Sorace; A. L., Barra; W., Wernsdorfer
abstract

The tetrairon(III) clusters Fe4(L)2(dpm)6 where Hdpm = dipivaloylmethane and H3L = MeC(CH2OH)3 or PhC(CH2OH)3 were obtained by site-specific replacement of the six methoxide bridges in Fe4(OMe)6(dpm)6. As compared with the parent compd., the new clusters show a much larger anisotropy in the S = 5 ground spin state (D/kB∼-0.6 K vs. -0.3 K) and a higher energy barrier to the reversal of the magnetization.

2003 - Direct Observation of Single-Molecule Magnets Organized on Gold Surfaces [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; M., Pacchioni; Zobbi, Laura; D., Bonacchi; A., Caneschi; D., Gatteschi; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; L., Gurevich; H. S. J., VAN DER ZANT
abstract

A dodecamanganese(III,IV) cluster, [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH, is considered to be the prototype of a class of materials referred to as single-mol. magnets (SMMs), whose magnetic behavior is strongly reminiscent of bulk magnets. A method to deposit SMMs on a gold film and to observe them directly using scanning tunneling microscopy (STM) was demonstrated. To obtain suitably derivatized Mn12-type clusters, 16-sulfanylhexadecanoate ligands (L') were introduced around the Mn12 core by exploiting a well-known ligand-exchange reaction. However, treatment of I·4H2O·2AcOH with HL' in toluene or CH2Cl2 afforded intractable solids. The use of the corresponding acetyl-protected acid HL yielded a fully-substituted deriv. [Mn12O12(L)16(H2O)4] (II) which is highly sol. in org. solvents. Deposition of the nanoclusters on a Au(111) surface was carried out by incubating gold substrates in dild. solns. of compd. II in THF together with aq. NH4OH. STM anal. showed complete coverage of the gold substrate by a disordered layer of round particles with an apparent lateral size of ∼5.8 nm, which agreed with the size of compd. II obtained from mol. modeling studies (5.0-5.6 nm).

2003 - High Field Magnetization Process in a Dodecanuclear Fe(III) Ring Cluster [Articolo su rivista]
Y., Inagaki; T., Asano; Y., Ajiro; Y., Narumi; K., Kindo; Cornia, Andrea; D., Gatteschi
abstract

A high field magnetization technique is applied to study the field-dependent behaviors of the molecular magnet, dodecanuclear Iron(III) ring cluster [Fe(OCH3)(2)(dbm)](12), in which twelve numbers of Fe3+ (S = 5/2) ions form a finite chain ring, where dbm = dibenzoylmethane. The low-lying states of the spin manifold are dramatically revealed by the observation of magnetization process using pulsed high magnetic field up to 55 T at temperature down to 0.1 K. We observed four distinct magnetization steps with nearly equal field separation of 10T, arising from energy-level crossings for the discrete quantum states in the finite spin system. We discuss the structure of energy levels and the relaxation phenomena in the magnetization process around the level-crossing fields. It is found that the anomalous magnetization curve observed at 1.3 K, showing an unusual shape with characteristic hysteresis for increasing and decreasing field, is well reproduced by Phonon Bottleneck effect caused by the week heat exchange between sample and liquid-He bath during the fast field passage. Our results are compared and discussed with those obtained in the other iron magnets showing the step-like magnetization process due to quantum discrete energy levels.

2003 - Magnetism from the Molecular to the Nanoscale [Capitolo/Saggio]
D., Gatteschi; R., Sessoli; Cornia, Andrea
abstract

A review on nanoscale molecule-based magnetic materials composed of exchange-coupled metal ions. The review covers simple oligonuclear species, antiferromagnetic rings and Single Molecule Magnets. Finally, it describes the most recent developments in one-dimensional chain-like systems, including the exotic spin dynamics exhibited by Single Chain Magnets.

2003 - Magnetism: General Introduction [Capitolo/Saggio]
D., Gatteschi; R., Sessoli; Cornia, Andrea
abstract

A review on magnetic techniques that are traditionally employed to investigate simple paramagnets, on the origin of the interactions responsible for the various forms of cooperative magnetism in molecular systems and on advanced experimental techniqueswhich are now currently used (or will soon become wide spread) for the investigation of the static and dynamic magnetic properties of molecule-based magnetic materials.

2003 - Microscopic Spin Hamiltonian of a Cr8 Antiferromagnetic Ring from Inelastic Neutron Scattering [Articolo su rivista]
S., Carretta; J., VAN SLAGEREN; T., Guidi; E., Liviotti; C., Mondelli; D., Rovai; Cornia, Andrea; A. L., Dearden; F., Carsughi; Affronte, Marco; C. D., Frost; R. E. P., Winpenny; D., Gatteschi; G., Amoretti; R., Caciuffo
abstract

Exchange integrals and single-ion anisotropy parameters of a ring-shaped molecular cluster, comprising eight chromium(III) ions (s = 3/2) were determined by inelastic neutron scattering. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the relative intensity of magnetic excitations in the neutron spectra. Evidence of decreasing lifetimes with increasing energy of excited spin states was found. The microscopic picture emerging from neutron spectroscopy was confirmed by the temperature dependence of the heat capacity.

2003 - Quantum Level Structure of Molecular Magnets, Fe12 and V15 [Articolo su rivista]
Y., Ajiro; Y., Inagaki; H., Itoh; T., Asano; Y., Narumi; K., Kindo; T., Sakon; H., Nojiri; M., Motokawa; Cornia, Andrea; D., Gatteschi; A., Müller; B., Barbara
abstract

We review our recent work on molecular magnets, Fe12 and V15 with focus on the determination of low-lying quantum energy levels which have permanent importance in understanding their unique quantum magnetism.

2003 - Rational Design of Large Spin Clusters Based on the Hexacopper(II) Siloxanolate Core [Articolo su rivista]
G. L., Abbati; A. L., Barra; A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi; Y. A., Pozdniakova; O. I., Shchegholikhina
abstract

The hexacopper(II) siloxanolate cage [Cu6{(PhSiO2)6}2(BuOH)5]·3BuOH was synthesized by reaction between CuCl2 and potassium phenylsiloxanolate in n-butanol, and characterized by single-crystal x-ray diffraction, magnetic measurements and high-frequency EPR (HF-EPR). The complex exhibits a torus-like structure featuring a layer of six copper(II) ions sandwiched between two cyclic (PhSiO2)66- ligands and surrounded by BuOH mols. Two different mols. are obsd. in the crystal structure of [Cu6{(PhSiO2)6}2(BuOH)5]·3BuOH. One mol. has all six copper atoms bonded to BuOH ligands and the other has only four of the six copper atoms bonded to BuOH ligands. The magnetic properties were characterized by moderate ferromagnetic exchange interactions between the S = 1/2 copper(II) centers to give an S = 3 ground spin state. Variable temp. HF-EPR spectra evidence a hard-axis magnetic anisotropy with gpar = 2.063, gperp = 2.225 and D = 0.31 cm-1. The cage is very sol. in org. solvents and, upon exchange of the labile BuOH ligands, it functions as a high-spin hexatopic receptor for monodentate units. Reaction with the trigonal bipyramidal copper(II) mononuclear [Cu(tmpa)CN]+ in CHCl3/MeOH soln. (tmpa = tris(2-pyridylmethyl)amine) affords the decacopper(II) complex [Cu6{(PhSiO2)6}2{NCCu(tmpa)}4](PF6)4, whose x-ray structure and magnetic behavior are presented.

2002 - 1H Nuclear Magnetic Resonance and Spin Dynamics in the Tetranuclear Iron(III) cluster {Fe4} [Articolo su rivista]
D., Procissi; B. J., Suh; A., Lascialfari; F., Borsa; A., Caneschi; Cornia, Andrea
abstract

The spin dynamics of a cluster of four iron(III) ions characterized by a total spin ground state S-T = 5 and Ising anisotropy have been investigated by the H-1 nuclear magnetic resonance (NMR) linewidth and H-1 NMR spin-lattice relaxation rate 1/T-1 as a function of the temperature (0.5-295 K) and external magnetic field (0.3-7.2 T). At very low T (0.5 K) the spectrum becomes very broad indicating freezing of the Fe3+ moments in a superparamagnetic state. The temperature dependence of T-1(-1) and T-2(-1) shows strong field dependent enhancement below 50 K which must be related to the spin dynamics of the Fe3+ moment freezing process, although no quantitative theory of this is currently available.

2002 - Disorder effects in Mn12–acetate at 83 K [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; R., Sessoli; L., Sorace; D., Gatteschi; A. L., Barra; C., Daiguebonne; T., Roisnel
abstract

The structure of hexadeca-mu-acetato-tetraaquadodeca-mu3 oxo-dodecamanganese bis(acetic acid) tetrahydrate, [Mn12O12 (CH3COO)16(H2O)4].2CH3COOH.4H2O, known as Mn-12- acetate, has been determined at 83(2) K by X-ray diffraction methods. The fourfold (S-4) molecular symmetry is disrupted by a strong hydrogen-bonding interaction with the disordered acetic acid molecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn-12 isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen-bonded acetic acid molecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B36, 2042-2046].

2002 - Intra and Inter-Multiplet Neutron Transitions in an Fe4 Magnetic Cluster [Articolo su rivista]
G., Amoretti; R., Caciuffo; S., Carretta; Cornia, Andrea; D., Gatteschi; J., Kulda; E., Liviotti
abstract

We report the results of neutron spectroscopy for a tetranuclear iron cluster, Fe-4(OCH3)(6)(dpm)(6))6 (Hdpm = dipivaloylmethane), having an S = 5 spin ground state. The experiments were carried out on a deuterated powder sample at ILL, Grenoble. The transitions within the ground multiplet, below 0.3 meV, were measured with a high-resolution time-of-flight spectrometer. Taking into account the presence of three different isomers, the parameters of the zero-field-splitting Hamiltonian have been determined. The results are discussed with reference to high-field electron paramagnetic resonance data. A non-dispersive peak at 7.8 meV, detected at T = 1.4 K using a three-axis spectrometer with polarization analysis, is interpreted as the transition between the S = 5 ground state and the two degenerate S = 4 excited multiplets. The Q dependence of the cross section is discussed in comparison with the theoretical model.

2002 - Magnetic Anisotropy of the Antiferromagnetic Ring [Cr(8)F(8)Piv(16)] [Articolo su rivista]
J., VAN SLAGEREN; R., Sessoli; D., Gatteschi; A. A., Smith; M., Helliwell; R. E. P., Winpenny; Cornia, Andrea; A. L., Barra; A. G. M., Jansen; E., Rentschler; G. A., Timco
abstract

A new tetragonal (P42(1)2) crystalline form of [Cr8F8(Piv)16] (HPiv = pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the Cr-III ions is antiferromagnetic (J = 12 cm(-1)) which results in a S = 0 spin ground state, The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S = 1 and S = 2, respectively) are D-1 = 1.59(3) cm(-1) or 1.63 cm(-1) (from CTM and HFEPR, respectively) and D-2 0.37 cm(-1) (from HFEPR). The dipolar contributions to the ZFS of the S = 1 and S = 2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1)degrees with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic Cr-III rings.

2002 - Magnetic Properties of Fe12 Ring: ESR and Magnetization Measurements [Relazione in Atti di Convegno]
Y., Inagaki; T., Asano; Y., Ajiro; Y., Narumi; K., Kindo; H., Nojiri; M., Motokawa; Cornia, Andrea; D., Gatteschi
abstract

An account on recent EPR/ESR and high field magnetization measurements revealing the electronic structure of a ring-like Fe12 antiferromagnetic cluster

2002 - Observation of magnetic level repulsion in Fe6 : Li molecular antiferromagnetic rings [Articolo su rivista]
Affronte, Marco; Cornia, Andrea; Lascialfari, A; Borsa, F; Gatteschi, D; Hinderer, J; Horvatic, M; Jansen, Agm; Julien, Mh
abstract

Heat capacity (C), magnetic torque, and proton NMR relaxation rate (1/T-1) measurements were performed on Fe6:Li single crystals in order to study the crossings between S = 0 and S = 1 and between S = 1 and S = 2 magnetic states of the molecular rings, at magnetic fields B-c1 = 11.7 T and B-c2 = 22.4 T, respectively. C vs B data at 0.78 K show that the energy gap between two states remains finite at B-c 's (Delta(1)/k(B) = 0.86 K and Delta(2)/k(B) = 2.36 K) thus proving that levels repel each other. The large Delta(1) value may also explain the anomalously large width of the peak in 1/T-1 vs B, around B-c1. This anticrossing, unexpected in a centrosymmetric system, requires a revision of the Hamiltonian.

2002 - Origin of second-order transverse magnetic anisotropy in Mn12-acetate [Articolo su rivista]
Cornia, Andrea; R., Sessoli; L., Sorace; D., Gatteschi; A. L., Barra; C., Daiguebonne
abstract

The symmetry breaking effects for quantum tunneling of the magnetization in Mn12-acetate, a molecular nanomagnet, represent an open problem. We present structural evidence that the disorder of the acetic acid of crystallization induces sizable distortion of the Mn(III) sites, giving rise to six different isomers. Four isomers have symmetry lower than tetragonal and a nonzero second-order transverse magnetic anisotropy, which has been evaluated using a ligand field approach. The result of the calculation leads to an improved simulation of electron paramagnetic resonance spectra and justifies the tunnel splitting distribution derived from the field sweep rate dependence of the hysteresis loops.

2002 - Post-synthetic Isotopic Labeling of an Azamacrocyclic Ligand [Articolo su rivista]
Pacchioni, Mirko; Bega, Andrea; FABRETTI COSTANTINO, Antonio; D., Rovai; Cornia, Andrea
abstract

A simple post-synthetic method for the preparation of 1,4,7-triazacyclononane-d(12) (90% atom D) is reported. The macrocycle is first converted into 1,4,7-trinitroso-1,4,7-triazacyclononane, whose solid-state and solution properties are herein described. In CD3OD/D2O solution, the trinitroso derivative undergoes fast base-catalyzed H,D exchange on the whole set of methylene hydrogens and can be subsequently denitrosated by reduction with Ni/Al alloy.

2002 - Towards Stepwise Cluster Assembly: a Decacopper(II) Complex Obtained by Controlled Expansion of a Metallasiloxane Cage [Articolo su rivista]
G. L., Abbati; A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Y. A., Pozdniakova; O. I., Shchegolikhina
abstract

Clusters can be expanded and their magnetic properties tuned by the addn. of suitable metal-complex ligands. By using this method, [Cu6{(PhSiO2)6}2{(NC)Cu(tmpa)}4](PF6)4 was rationally designed by self-assembly of four {(NC)Cu(tmpa)}+ units around a robust hexacopper(II)-siloxanolate cage (tmpa = tris(2-pyridylmethyl)amine).

2001 - High-field Torque Magnetometry for Investigating Magnetic Anisotropy in Mn-12-acetate Nanomagnets [Articolo su rivista]
Cornia, Andrea; Affronte, Marco; D., Gatteschi; A. G. M., Jansen; A., Caneschi; R., Sessoli
abstract

The single-molecule superparamagnet [Mn12O12(OAc)16(H2O)4].2AcOH.4H2O (Mn-12-acetate) has attracted considerable attention because it shows exceedingly slow paramagnetic relaxation at low temperature. The cluster has S-4 symmetry in the solid state and comprises four Mn(IV) ions (S = 3/2) and eight Mn(III) ions (S = 2) which are magnetically coupled to give an S=10 ground state. The ground manifold is largely split in zero magnetic field and many efforts have been spent to determine the zero-field splitting (zfs) parameters alpha, beta and gamma appearing in the fourth-order spin-Hamiltonian H = alpha Sz^2 + beta Sz^4 + gamma (S+^4 + S-^4) + muB.B.g.S. These are of paramount importance for defining the magnetic anisotropy of the cluster, which in turn determines the slow relaxation of the magnetization and quantum tunneling effects at low temperatures. We want to show that cantilever torque magnetometry in high fields is a suitable technique for determining second- and fourth-order anisotropic contributions in high-spin molecules, such as Mn12-acetate. The main advantage of the method lies in its high sensitivity which allows to use very small single crystals. Torque curves have been recorded at 4.2 K by applying the magnetic field (0-28 T) very close to the ab-plane of the tetragonal unit cell. The zfs parameters obtained by this procedure [alpha = -0.389(5) cm-1 and beta = -8.4(5) x 10^-4 cm-1] are in excellent agreement with those determined by spectroscopic techniques, such as high frequency EPR and inelastic neutron scattering.

2001 - Isotopic Effect on the Quantum Tunneling of the Magnetization of Molecular Nanomagnets [Articolo su rivista]
R., Sessoli; A., Caneschi; D., Gatteschi; L., Sorace; Cornia, Andrea; W., Wernsdorfer
abstract

The molecular cluster [Fe8(tacn)6O2(OH)12]Br8.9H2O, Fe8, characterized by S = 10, with biaxial magnetic anisotropy is an ideal system to investigate quantum effects in the dynamics of the magnetization. Resonant quantum tunneling gives rise to stepped hysteresis cycles and below 0.35 K pure quantum tunneling of the magnetization has been observed. The role of hyperfine, fields in promoting the tunneling as a source of dynamic broadening of the states involved in the tunnel transition is investigated by preparing and characterizing two isotopically enriched samples using Fe-57 and H-2. The relaxation rate in the tunneling regime is proportional to the hyperfine field generated by the nuclei. Also, the intrinsic linewidth of the tunneling resonance scales with the hyperfine field as confirmed by calculations of the super (or transfer) and direct hyperfine interactions. Preliminary results on a novel cluster of formula [Fe4(OCH3)6(dpm)6], Fe4, suited for a more dramatic isotope effect on the tunneling rate are also reported.

2001 - Neutron spectroscopy within the S=5 groud multiplet and low-temperature heat capacity in Fe4 magnetic cluster [Articolo su rivista]
Amoretti, G.; Carretta, S.; Caciuffo, R.; Casalta, H.; Cornia, A.; Affronte, Marco; Gatteschi, D.
abstract

The transitions within the S=5 ground state of the tetrairon(III) molecular cluster Fe4(OCH3)6(dpm)6 (Hdpm = dipivaloylmethane) have been measured by inelastic neutron scattering. The spectra have been interpreted by means of an effective spin Hamiltonian for an isolated S=5 multiplet, taking into account the presence of three different isomers in the compound. It has been shown that for the two dominant isomers the symmetry of the zero-field splitting tensor is not purely axial, but a nonzero rhombic coefficient E is needed to fit the neutron spectra. The results have been discussed in comparison with recent high-field electron paramagnetic resonance data in single crystals. Heat-capacity measurements in the range 2 to 20 K have been performed and shown to be compatible with the neutron results for the zero-field splitting.

2001 - New Experimental Techniques for Magnetic Anisotropy in Molecular Materials [Articolo su rivista]
Cornia, Andrea; D., Gatteschi; R., Sessoli
abstract

Magnetic anisotropy is a very important property in magnetochemistry. Its knowledge allows us to obtain a fundamental understanding of the electronic structure of both simple paramagnetic species and of magnetically coupled systems. The difficulties associated with its experimental determination have so far limited its investigation. Recently new experimental techniques using superconducting quantum interference devices (SQUID) and micro-SQUID arrays as well as cantilever torque magnetometry have drastically reduced the size of the crystals needed for the measurements, thus opening new perspectives. We will briefly review here these techniques, with the aim to advertise their use in the chemical community.

2001 - Single-Ion vs Dipolar Origin of Magnetic Anisotropy in Iron(III)-oxo Clusters: a Case Study [Articolo su rivista]
G. L., Abbati; L. C., Brunel; H., Casalta; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi; A. K., Hassan; A. G. M., Jansen; A. L., Maniero; L., Pardi; C., Paulsen; Segre, Ulderico
abstract

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(m)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6]ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR), bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with D-Fe = 0.43(1) cm-1 and E-Fe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = D-Fe[Sz^2 - S(S + 1)/3] + E-Fe[Sx^2 - Sy^2]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster. Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, D-Fe(ZZ) = - 0.164(1) cm-1. Due to the dominant antiferromagnetic coupling, a negative D-Fe(ZZ) value determines a hard-axis molecular anisotropy in 1 as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D-1 = 4.70(9) cm-1, is in excellent agreement with that determined by inelastic neutron scattering experiments at 2K, D-1 = 4.57(2) cm-1. Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz, The single-ion anisotropy is of the hard-axis type as well. but the D-Fe parameter is significantly larger [D-Fe = 0.770(3) cm-1, E-Fe = 0.090(3) cm-1]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

2001 - Theory of Nuclear Magnetic Resonance in the Molecular Ring Fe10 [Articolo su rivista]
Cornia, Andrea; A., Fort; M. G., Pini; A., Rettori
abstract

The proton nuclear spin-lattice relaxation rate in the molecular ring Fe10 is calculated by linear response theory, starting from the unperturbed eigenvalues and eigenvectors, and introducing dipolar superhyperfine interactions as a perturbation. A strong enhancement of the rate at critical values of the applied magnetic field, recently observed by solid-state NMR at low temperatures, is explained in terms of level crossing between states with \DeltaS\ = 1 and \DeltaM\ = 1.

2000 - Effects of Nuclear Spins on the Quantum Relaxation of the Magnetization for the Molecular Nanomagnet Fe-8 [Articolo su rivista]
W., Wernsdorfer; A., Caneschi; R., Sessoli; D., Gatteschi; Cornia, Andrea; V., Villar; C., Paulsen
abstract

The strong influence of nuclear spins on resonant quantum tunneling in the mol. cluster Fe8 is demonstrated for the 1st time by comparing the relaxation rate of the std. Fe8 sample with two isotopic modified samples: (i) 56Fe is replaced by 57Fe, and (ii) a fraction of 1H is replaced by 2H. By a recently developed hole digging method, we measured an intrinsic broadening which is driven by the hyperfine fields. Our measurements are in good agreement with numerical hyperfine calcns. For T > 1.5 K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.

2000 - Kinetics and Equilibria in the Hydrolysis of [Pt(terpy)Cl]Cl; Crystal and Molecular Structure of [Pt(terpy)OH]ClO4.H2O (terpy=2,2 '-6,2 ''-terpyridine) [Articolo su rivista]
G., Annibale; L., Cattalini; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; F., Guidi
abstract

The cationic platinum(II) complex [Pt(terpy)Cl]+ (terpy = 2,2'-6',2"-terpyridine), monomer in water at low concns. (<1.5 × 10-5 mol dm-3), undergoes basic and acid hydrolysis. The equil. const. for the acid hydrolysis is 1.4 × 10-5 mol dm-3 {25°C, [CH3SO3H] = 0.1 mol dm-3}. Basic hydrolysis (pH > 10) leads to the complete formation of [Pt(terpy)(OH)]+ with a two terms rate law, kobs = k1 + k2[OH-] (k1 = 2.24 × 10-2s-1, k2 = 0.7 mol-1 dm3 s-1). The mechanism of substitution is discussed and the system is compared to that of the congener [Pt(dien)Cl]+ species (dien = 1,5-diamino-3-aza-pentane). The crystal structure of [Pt(terpy)OH]ClO4·H2O has been detd.: monoclinic, space group P21/n, a 7.937(1), b 17.251(1), c 12.245(1) Å; β 95.50(1)°. The Pt-O distance is 1.999(5) Å, and the hydroxo group forms strong hydrogen bonds with the mol. of water of crystn. At. coordinates are given.

2000 - Landau-Zener Method to Study Quantum Phase Interference of Fe-8 Molecular Nanomagnets (invited) [Articolo su rivista]
W., Wernsdorfer; R., Sessoli; A., Caneschi; D., Gatteschi; Cornia, Andrea; D., Mailly
abstract

We present details about an exptl. method based on the Landau-Zener model which allows one to measure very small tunnel splittings Δ in mol. clusters Fe8. The measurements are performed with an array of microsuperconducting quantum interference devices. The obsd. oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topol. quantum interference of two tunnel paths of opposite windings. Transitions between M=-S and (S-n), with n even or odd, revealed a parity (symmetry) effect which is analogous to the suppression of tunneling predicted for half integer spins. This observation is the first direct evidence of the topol. part of the quantum spin phase (Berry phase) in a magnetic system. The influence of intermol. dipole interactions on the measured tunnel splittings Δ is shown.

2000 - Low temperature specific heat of molecular rings: a study on the effects of the internal guest substitution and on the lattice contribution [Articolo su rivista]
Affronte, Marco; J. C., Lasjaunias; Cornia, Andrea
abstract

The effects of the internal guest substitution within hexa-iron molecular rings (Na:Fe6 and Li:Fe6) have been investigated by means of low temperature specific heat (LTSH) measurements. By changing Na to Li as central metal ion, the Schottky anomaly shifts towards lower temperatures. The data analysis is supported by the study of the LTSH of the non-magnetic Na:Ga6 compound which has the same core structure as the iron rings. For the non-magnetic Na:Ga6, significant deviations from the simple C similar to T-3 Debye law were found and the use of conventional C vs. T fitting curves is critically discussed. The singlet-triplet energy gap (E-1/k(B) = 12.1 +/- 0.5 K) and its zero-field splitting (D-1/k(B) = 1.5 +/- 1 K) evaluated from the magnetic contribution of the Li:Fe6 LTSH are compared to values estimated by high field torque magnetometry.

2000 - Low-temperature Theory of Proton NMR in the Molecular Antiferromagnetic Ring Fe10 [Articolo su rivista]
Cornia, Andrea; A., Fort; M. G., Pini; A., Rettori
abstract

The proton nuclear spin-lattice relaxation rate 1/T1 in the mol. "ferric wheel" Fe10(OCH3)20(C2H2O2Cl)10 (Fe10) is calcd. in the framework of the linear response theory, starting from the unperturbed eigenvalues and eigenstates of the magnetic ring. The dipole-dipole superhyperfine interaction between the proton nuclear spins and the electronic spins of the magnetic ions is shown to induce transitions between states |S, M> which differ by 1 both in the quantum no. S (the total spin of the ring) and in M (the projection of the total spin along the quantization axis). The theory predicts that level crossing between the states |S + 1, -(S + 1)> and |S, -S> induced by an applied magnetic field should be accompanied by a strong enhancement of T1-1, in good agreement with recent low-temp. NMR exptl. data on Fe10.

2000 - Low-temperature specific heat of Li : Fe6 molecular magnets [Articolo su rivista]
Affronte, Marco; Jc, Lasjaunias; Cornia, Andrea
abstract

We have studied the low-temperature (T) specific heat (C) of Li : Fe6 molecular magnets. The Schottky anomaly related to the singlet-triplet energy gap is shifted towards lower temperature as compared with that observed on Na:Fe6. This evidences the effects of the internal-guest substitution in these antiferromagnetically coupled iron(III) rings. The singlet-triplet energy gap (E(1)/k(B) = 14.2 +/- 0.4 K) and its zero-field splitting (D(1)/k(B) = 3.9 +/- 1.0 K) evaluated by the C-versus-T data fitting are compared to the values estimated by the high-field torque experiments. (C) 2000 Elsevier Science B.V, All rights reserved.

2000 - Magnetic Anisotropy of Mn12-acetate Nanomagnets from High-field Torque Magnetometry [Articolo su rivista]
Cornia, Andrea; Affronte, Marco; A. G. M., Jansen; D., Gatteschi; A., Caneschi; R., Sessoli
abstract

The zero-field splitting (zfs) parameters of the ground S = 10 state in the easy-axis mol. nanomagnet [Mn12O12(OAc)16(H2O)4]·2AcOH·4H2O were detd. by cantilever torque magnetometry (CTM) in high magnetic fields. Torque curves were recorded in the superparamagnetic regime (4.2 K) by applying the magnetic field (0-28 T) very close to the hard magnetic plane of a single-crystal sample. The 2nd- and 4th-order anisotropy parameters obtained by this procedure [α=-0.389(5) cm-1 and β=-8.4(5)×10-4 cm-1 in the spin-Hamiltonian Hzfs = αSz2+βSz4+γ(S+4+S-4)] are in excellent agreement with those detd. by spectroscopic techniques, such as high-frequency EPR and inelastic neutron scattering. High-field CTM provides a simple macroscopic method for studying magnetic anisotropy in high-spin mols.

2000 - Magnetic and Structural Properties of an Octanuclear Cu(II) S=1/2 Mesoscopic Ring: Susceptibility and NMR Measurements [Articolo su rivista]
A., Lascialfari; Z. H., Jang; F., Borsa; D., Gatteschi; Cornia, Andrea; D., Rovai; A., Caneschi; P., Carretta
abstract

Magnetic susceptibility, 1H NMR and 63Cu NMR-NQR expts. on two slightly different species of the mol. S = 1/2 antiferromagnetic (AF) ring Cu8, [Cu8(dmpz)8(OH)8]·2C5H5N (Cu8P) and [Cu8(dmpz)8(OH)8]·2C6H5NO2 (Cu8N), are presented. The magnetic energy levels are calcd. exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic field. From the magnetic susceptibility measurements the authors est. the AF exchange coupling const. J ∼ 1000 K and the resulting gap Δ∼500 K between the ST = 0 ground state and the ST = 1 1st excited state. The 63,65Cu NQR spectra indicate four crystallog. inequivalent copper nuclei in each ring. From the combination of the 63Cu NQR spectra and of the 63Cu NMR spectra at high magnetic field, the authors est. the quadrupole coupling const. vQ of each site and the av. asymmetry parameter η of the elec.-field gradient tensor. The nuclear spin-lattice relaxation rate (NSLR) decreases exponentially on decreasing temp. for all nuclei studied. The gap parameter extd. from 63Cu NQR-NSLR is the same as for the susceptibility while a smaller value was obtained from the 63Cu NMR-NSLR in an external magnetic field of 8.2 T.

2000 - Nonadiabatic Landau Zener Tunneling and Quantum Phase Interference in Fe-8 Molecular Nanomagnets [Articolo su rivista]
W., Wernsdorfer; R., Sessoli; A., Caneschi; D., Gatteschi; Cornia, Andrea
abstract

The Landau-Zener method allows to measure very small tunnel splittings Δ in mol. clusters Fe8. The obsd. oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topol. quantum interference of two tunnel paths of opposite windings. Studies of the temp. dependence of the Landau-Zener transition rate P gives access to the topol. quantum interference between excited spin levels. The influence of nuclear spins is demonstrated by comparing P of the std. Fe8 sample with two isotopically substituted samples. The need of a generalized Landau-Zener transition rate theory is shown.

2000 - Nonadiabatic Landau–Zener Tunneling in Fe8 Molecular Nanomagnets [Articolo su rivista]
W., Wernsdorfer; R., Sessoli; A., Caneschi; D., Gatteschi; Cornia, Andrea
abstract

The Landau-Zener method allows to measure very small tunnel splittings ∆ in molecular clusters Fe8. The observed oscillations of ∆ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Studies of the temperature dependence of the Landau-Zener transition rate P gives access to the topological quantum interference between excited spin levels. The influence of nuclear spins is demonstrated by comparing P of the standard Fe8 sample with two isotopically substituted samples. The need of a generalised Landau-Zener transition rate theory is shown.

2000 - Reaction of N,N'-dimethylimidazolidine-2-selone (4) with TCNQ. Characterisation and X-ray crystal structure of the mixed-valence compound 4.(TCNQ)(1.167) [Articolo su rivista]
F. A., Devillanova; A., Garau; F., Isaia; V., Lippolis; G., Verani; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; A., Girlando
abstract

The reaction of 1,3-dimethyl-2-imidazolidineselone (I) with TCNQ affords lustrous dark crystals with elemental anal. corresponding to the formula I·(TCNQ)1.167. X-Ray crystal structure detn. shows that this compd. is formed by discrete [I2]2+ dications, in which two oxidized imidazolidine mols. are bonded through an -Se-Se- bridge [2.5187(7) Å]. These dications interact on one side with a third disordered mol. of I (occupancy factor 0.5) via Se···Se soft interactions of 2.715(4) and 2.899(4) Å to give almost linear -Se-Se···Se- arrangements [Se(2)-Se(1)···Se(3), 177.64(6)° and Se(2)-Se(1)···Se(3a), 167.89(5)°]. The unit cell contains six imidazolidine mols. bearing a total of four pos. charges, balanced by four neg. charges distributed on seven TCNQ mols., stacked in two dimers and a trimer. Structural data suggest that the neg. charges are unevenly distributed on the TCNQ mols. Single crystal IR spectra also suggest the presence of at least three differently charged TCNQ's, and in addn. indicate a room temp. optical gap of ∼0.3 eV, placing this compd. among wide bandgap semiconductors (σ = 3.45 × 10-3 S cm-1). Cyclic voltammetry of I is discussed in comparison with those of previously studied 2-imidazolineselone derivs. [i.e., 1,3-dihydro-1,3-dimethyl-2H-imidazole-2-selone, 1,1'-methylenebis[1,3-dihydro-3-methyl-2H-imidazole-2-selone], 1,1'-(1,2-ethanediyl)bis[1,3-dihydro-3-methyl-2H-imidazole-2-selone]], and shows that for this donor the achievement of a mixed valence compd. with TCNQ is possible in MeCN.

2000 - Single-molecule Magnets Based on Iron(III) Oxo Clusters [Articolo su rivista]
D., Gatteschi; R., Sessoli; Cornia, Andrea
abstract

Polynuclear compounds of magnetic transition metal ions are attracting large interest after the discovery that their magnetisation may relax very slowly at low temperature. Since their behaviour is similar to that of bulk magnets they may be called single molecule magnets. Here we review the magnetic properties of iron(III) clusters showing such features which may be interesting for future applications, as well as strategies for designing new molecules with increased performances.

2000 - [Fe(OCH3)2(dbm)]12: Synthesis, Solid-state Characterization and Reactivity of a New Molecular Ferric Wheel [Articolo su rivista]
G. L., Abbati; A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

A novel Fe(III) mol. alkoxide, [Fe(OCH3)2(dbm)]12 (1), was synthesized by the reaction of ferric chloride with dibenzoylmethane (Hdbm) and an excess of either K or Cs methylate in anhyd. MeOH. The 12 octahedrally coordinated Fe(III) ions are linked by double OCH3 bridges in a nonplanar twisted-ring structure, to give the largest cyclic ferric cluster so far reported, contg. chem. equiv. bridging units. The quant. conversion of 1 into hexairon(III) rings [MFe6(OCH3)12(dbm)6]+ (M = Li, Na) by reaction with excess alkali-metal tetraphenylborates in CH2Cl2/MeOH 3:1 was studied by 1H NMR and UV-visible spectroscopies. The reactions follow pseudo-first-order kinetics with respect to 1, with kpo(Li) = 1.07(2) × 10-2 and kpo(Na) = 1.63(3) × 10-2 s-1 at 25°. The magnetic susceptibility of 1 in the solid state is consistent with antiferromagnetic exchange interactions between the high-spin Fe(III) ions and closely follows the behavior predicted for a Heisenberg s = 5/2 quantum-spin chain down to 50 K (J = 22.2 cm-1 for H=J Σi Si·Si+1 and g = 2.00). At low temps. the magnetic behavior is dominated by the energy gap between the ground S = 0 and the 1st excited S = 1 state, which is a consequence of finite-size effects.

1999 - A Tetracopper(II) Complex Containing Two μ-Oxamidato-dicopper(II) Units Linked by Croconate Anions [Articolo su rivista]
G. L., Abbati; Cornia, Andrea; FABRETTI COSTANTINO, Antonio
abstract

Crystals of the title compd., di-μ-croconato-bis{μ-[N,N'-bis(3-amino-2,2-dimethylpropyl)oxamidato(2-)-N,O':N',O]dicopper(II)} tetrahydrate, are triclinic, space group P-1, with a 9.496(1), b 10.6620(8), c 12.966(2) Å, α 71.481(8), β 75.780(10), γ 66.160(10)°; Z = 1, dc = 1.647, dm = 1.65(1); R = 0.032, Rw(F2) = 0.087 for 6246 reflections. The structure comprises two μ-oxamidato-dicopper(II) units bridged by croconate ligands to give a tetracopper(II) ring system with crystallog. -1 symmetry; all the Cu centers are five-coordinate.

1999 - Alkylcobalt Carbonyls, Part 13. Preparation and Molecular Structures of Benzyl- and Phenylacetylcobalt Carbonyls [Articolo su rivista]
Zucchi, Claudia; Cornia, Andrea; R., Boese; E., Kleinpeter; H., Alper; Palyi, Gyula
abstract

The benzyl-type Co carbonyl complexes (para-t-BuC6H4CH2)Co(CO)3PPh3 (I) and [para-ClC6H4CH2C(O)]Co(CO)3PPh3 (II) were prepd. and characterized by analyses, spectra and x-ray single-crystal diffraction. The overall structures both of the alkylcobalt-type I and the acylcobalt-type II display trigonal bipyramidal geometry, with the two noncarbonyl ligands in the two axial positions. The relevance of the stereochem. of complexes I and II to the supposed mechanism of the CO insertion/deinsertion on Co is discussed.

1999 - High-field Torque Magnetometry on Fe6 and Fe10 Molecular Magnets [Articolo su rivista]
Cornia, Andrea; A. G. M., Jansen; Affronte, Marco
abstract

The authors have applied torque magnetometry to the study of magnetic anisotropy in six- and ten-membered rings of iron(III) ions. By using magnetic fields up to 23 T, the authors have partially overridden the intramol. antiferromagnetic ordering of the spins (J = 20.0 cm-1 and 9.6 cm-1 in Fe6 and Fe10, resp.) to obtain magnetic ground states with S = 1, 2, 3, etc.,. Torque measurements on microgram single crystals at temps. down to 0.45 K showed that spin-flip transitions are accompanied by sharp variations of magnetic anisotropy. The angular dependence of transition fields provided spectroscopic-quality information about the spin-Hamiltonian parameters in the two compds. By analyzing the field dependence of the torque signal the authors detd. the exchange energies and zero-field splitting parameters of the total-spin multiplets with S ≤ 5 in the Fe10 sample. Torque magnetometry represents a high-sensitivity, powerful technique for studying field-induced level crossing in mol. magnets.

1999 - Host–guest Interactions for Tuning the Magnetic Coupling in Metal Ion Clusters [Capitolo/Saggio]
G. L., Abbati; A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

A review on recent success in the use of supramolecular host-guest interactions for promoting the formation of large metal ion clusters and for controlling their magnetic behavior

1999 - LSb(μ-I)2(μ-S)SbL. A neutral triply bridged complex obtained from Sb powder and diiodine activated by tetraphenyldithioimidodiphosphine (HL) [Articolo su rivista]
M., Arca; A., Garau; F. A., Devillanova; F., Isaia; V., Lippolis; G., Verani; G. L., Abbati; Cornia, Andrea
abstract

The synthesis of the complex LSb(μ-I)2(μ-S)SbL (I) was accomplished by reacting Sb powder with I2 activated by tetraphenyldithioimidodiphosphine (SPPh2NHPPh2S) (HL). X-ray diffraction (tetragonal, M = 1426.30, space group I41/a (No. 88), Z = 8, a = 18.020(2), c = 33.037(4) Å, V = 10,728(2) Å3, ρc = 1.766 g/cm3, μ(MoKα) = 25.07 cm-1, R1 = 0.0593, wR2 = 0.2065) shows that I is a dinuclear Sb(III) complex, in which the 2 metal ions are bridged by 1 sulfide and 2 iodide anions. An anionic bidentate L ligand completes the coordination sphere of each metal with its 2 S atoms, leading to a slightly distorted pyramidal coordination geometry, since each metal ion shows the presence of a sterically active lone-pair in trans position to the bridging sulfide. 31P CP-MAS NMR and IR spectroscopies are in accordance with the structural features of the complex.

1999 - Low-temperature specific heat of Fe-6 and Fe-10 molecular magnets [Articolo su rivista]
Affronte, Marco; Lasjaunias, Jc; Cornia, Andrea; Caneschi, A.
abstract

The energy splitting of the low-lying levels has been investigated on two magnetic molecular clusters Fe-6 and Fe-10 by means of low-temperature zero-field specific-heat measurements. Significant deviations from the usual C similar to T-2 law were observed above the maximum of the main Schottky anomalies as a result of nonnegligible contributions from the excited spin states with S>1 and the estimated lattice contributions follow a phenomenological power law C/R similar to T-alpha with alpha-2.7 for both these compounds. The singlet-triplet energy gaps evaluated by the Schottky anomaly, T-0=19.2 K for Fe6 and 4.56 K for Fe-10, are smaller than what we can estimate by a simplified spin-Hamiltonian approach in the strong exchange approximation and using the energy levels obtained by the high-field magnetization and susceptibility measurements. This discrepancy asks for a more complex description of the low-lying states of these molecular clusters, beyond the strong exchange approximation. At very low temperatures T<<1 K, two low-energy Schottky anomalies were also observed in Fe-10, probably due to a small fraction of defected rings or to hyperfine contributions.

1999 - Magnetic Anisotropy of Fe6 and Fe10 Molecular Rings by Cantilever Torque Magnetometry in High Magnetic Fields [Articolo su rivista]
Cornia, Andrea; A. G. M., Jansen; Affronte, Marco
abstract

We studied the magnetic anisotropy of 2 mol. magnets, Fe6 and Fe10, which comprise 6- and 10-membered rings of antiferromagnetically coupled Fe (III) ions (Si = 5/2), resp. Spin-flip transitions induced by the applied magnetic field (≤23 T) were investigated by cantilever torque magnetometry on microgram single crystals at very low temp. (down to 0.45 K). From the sharp, steplike variations of magnetic anisotropy at the transition fields, we detd. the singlet-triplet energy gap (Δ1) and the axial zero-field splitting parameter (D1) for the triplet state of Fe6 [Δ1 = 15.28(1) cm-1, D1 = 4.32(3) cm-1] and Fe10 [Δ1 = 4.479(4) cm-1, D1 = 2.24(2) cm-1]. By analyzing the addnl. steps obsd. in the Fe10 sample, we evaluated the ΔS and DS parameters for the total-spin multiplets with S ≤5. On the basis of our findings, we discuss the origin of magnetic anisotropy in Fe(III) rings and the application of torque magnetometry to the study of field-induced level crossing in mol. magnets.

1999 - Manganese(III) Formate: A Three-dimensional Framework that Traps Carbon Dioxide Molecules [Articolo su rivista]
CORNIA, Andrea; A., CANESCHI; P., DAPPORTO; FABRETTI COSTANTINO, Antonio; D., GATTESCHI; MALAVASI, Wanda; C., SANGREGORIO; R., SESSOLI
abstract

The reaction of KMnO4 with 95% formic acid led to the formation of "manganese(III) formate" of compn. Mn(HCOO)3·1/2CO2·1/4HCOOH·2/3H2O, which was characterized spectroscopically, by TGA, and by x-ray crystallog. (cubic, space group Im-3, R1 = 0.0520). The structure of "manganese(III) formate" is a three-dimensional framework of manganese ions bridged by six nearest formate ligands, and the resulting open cages trap carbon dioxide and formic acid mols. Jahn-Teller distortion of the MnIII ions was shown by the magnetic properties, which also indicated ferromagnetic intralayer and weaker antiferromagnetic interlayer interactions.

1999 - Molecular Magnetic Clusters: a Bridge between Molecules and Classical Magnets [Relazione in Atti di Convegno]
A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

The synthetic aspects associated to the formation of large metal ion clusters are reviewed with the aim to show the relevance of these materials to develop new magnetic properties at the interface between the quantum and the classical world. Particular emphasis is given to the structural aspects of the clusters and to the relation between structure and magnetic properties.

1999 - Quantum Effects in Single–Molecule Nanomagnets [Relazione in Atti di Convegno]
R., Sessoli; A., Caneschi; D., Gatteschi; C., Sangregorio; Cornia, Andrea; W., Wernsdorfer
abstract

We review recent results on the spin dynamics in molecular clusters formed by interacting paramagnetic centers. When the spin of the ground state is large and characterized by Ising type magnetic anisotropy the molecule behaves like a nanomagnet exhibiting slow relaxation of the magnetization at low temperature. One of the most interesting examples of single molecule magnet is an octanuclear iron cluster of formula [Fe8O2(OH)12(tacn)6]Br8.9H2O. This cluster has a S = 10 ground state and an easy axis of magnetization with strong rhombic anisotropy. Below 0.4 K the relaxation of the magnetization is temperature independent suggesting a pure tunneling process between the M = +- 10 states. Evidences of the tunneling mechanism also come from the observation of the oscillation of the relaxation rate when the field is applied along the easy axis. Other oscillations due to the topological interference effect (Berry phase) are observed when the field is along the hard axis. In the tunneling process also the magnetic moment of the nuclei plays an important role as suggested by the marked isotope effect observed in 2H and 57Fe enriched samples.

1999 - Single-molecule Magnet Behavior of a Tetranuclear Iron(III) Complex. The Origin of Slow Magnetic Relaxation in Iron(III) Clusters [Articolo su rivista]
A. L., Barra; A., Caneschi; Cornia, Andrea; F., FABRIZI DE BIANI; D., Gatteschi; C., Sangregorio; R., Sessoli; L., Sorace
abstract

The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear Fe(III) methoxo-bridged cluster Fe4(OMe)6(dpm)6 (Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = -0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To study the origin of the magnetic anisotropy the projection of the single-ion and dipolar contributions has been evaluated to the zfs of the ground state. The zfs tensors of the 3 structurally independent Fe(III) centers have been calcd. from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded to compare the calcd. and exptl. zfs parameters.

1999 - Structure and Magnetic Properties of a Dodecanuclear Twisted-ring Iron(III) Cluster [Articolo su rivista]
A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

[Fe(OMe)2(dbm)]12 (I; Hdbm = dibenzoylmethane) was prepd. by the reaction of FeCl3 with Hdbm in presence of KOMe or CsOMe in anhyd. MeOH. I.6CHCl3 is monoclinic, space group P21/c, Z = 4, R = 0.0967, Rw = 0.3174. Each Fe(III) in I has a distorted octahedral environment with 6 O atoms from methoxide and dbm with Fe-O band lengths of 1.94-2.07 Å, forming a twisted ring which is not planar and has C1 point group symmetry. NMR data of I in presence of NaBPh4 or LiBPh4 indicate that I is quant. converted to hexairon(III) coronates, [MFe6(OMe)12(dbm)6]+. Magnetic susceptibility data indicate antiferromagnetic coupling in I between high-spin Fe(III) atoms.

1999 - The Molecular Approach to Nanoscale Magnetism [Articolo su rivista]
A., Caneschi; D., Gatteschi; C., Sangregorio; R., Sessoli; L., Sorace; Cornia, Andrea; M. A., Novak; C., Paulsen; W., Wernsdorfer
abstract

Molecular clusters of paramagnetic metal ions have been widely investigated as model for magnetism at the nanoscale, especially for quantum effects like the tunneling of the magnetic moment. We present here some recent results obtained on derivatives of the well-known Mn12 cluster, especially on the half-integer spin compounds. The role of the transverse anisotropy in the dynamics of the magnetization is here elucidated through the comparison of the tunneling rate of the magnetization in two Fe8 cluster compounds, which differ only in the transverse anisotropy. Local dipolar fields and nuclear hyperfine fields have also revealed to strongly affect the relaxation in the pure tunneling regime and recent experiment has allowed to determine the intrinsic linewidth of the tunneling resonance. The transverse field dependence of the relaxation rate of Fe8 has revealed oscillations that are analog to the topological constructive-destructive interference of the spin phase (Berry phase) and we review here some very recent results. The magnetic behavior of antiferromagnetic ring-shaped clusters is also discussed fur their potential interest as models for antiferromagnetic particles. Some recent results obtained by other chemists in the synthesis of large spin dusters are also reviewed.

1999 - Tuning of Magnetic Anisotropy in Hexairon(III) Rings by Host–Guest Interactions. An Investigation by High–Field Torque Magnetometry [Articolo su rivista]
Cornia, Andrea; Affronte, Marco; A. G. M., Jansen; G. L., Abbati; D., Gatteschi
abstract

Magnetic anisotropy of the antiferromagnetic iron(III) rings [LiFe6(OMe)12(dbm)6]+ (I) and [NaFe6(OMe)12(pmdbm)6]+ (II), where Hdbm=1,3-diphenyl-1,3-propanedione and Hpmdbm=1,3-di(4-methoxyphenyl)-1,3-propanedione, was investigated. Staircase structures were obsd. on the torque curves of I-PF6 and II-ClO4 and were assigned to spin-level crossings from the spin ground state. The host-guest interactions responsible for the large difference in magnetic coupling also yield a dramatic difference in the magnetic anisotropies.

1998 - A Ferromagnetic Ring of Six Manganese(III) Ions with a S=12 Ground State [Articolo su rivista]
G. L., Abbati; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; A., Caneschi; D., Gatteschi
abstract

A novel cyclic cluster comprising six Mn(III) ions, [NaMn6(OMe)12(dbm)6]+ (1, Hdbm = dibenzoylmethane ), was synthesized by a procedure similar to that already reported for the Fe(III) analog. MnCl2, NaOMe and dibenzoylmethane were reacted in anhyd. MeOH and the solid product obtained was recrystd. from MeOH/CHCl3 in the presence of NaBPh4. The resulting complex [1]BPh4.2CHCl3 crystallizes in triclinic space-group P-1 with (at 188 K) a 14.392(3), b 16.790(2), c 17.344(3) Å, α 61.42(1), β 72.36(1), γ 88.96(2)°, Z = 1. The mol. structure of 1 comprises a [Mn6(OMe)12] crown with idealized S6 point-group symmetry acting as a host for an alkali-metal ion. The metal ions show remarkably distorted coordination spheres, as expected for high-spin MnIII. Variable temp., solid state magnetic susceptibility measurements performed on powders of [1]BPh4·xCHCl3 at 1 kG and 10 kG evidenced a S = 12 spin ground state. A fitting of high temp. data (T > 15 K) based on a Heisenberg model for exchange-coupling interactions gave j12 = j23 = j45 = j56 = -23.8 cm-1 and j34 = j61 = -2.6 cm-1 with g = 2.00. The structure and magnetic behavior of 1 are compared with those of antiferromagnetic Fe(III) rings previously reported. The results are relevant to the development of new strategies for designing high-spin mol. clusters.

1998 - Bimetallic Cyclooligosiloxanolate Complexes of Copper and Nickel [Articolo su rivista]
Zucchi, Claudia; M., Mattioli; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Gavioli, Giovanna; M., Pizzotti; R., Ugo; Y. A., Pozdniakova; O. I., Shchegolikhina; A. A., Zhdanov; Palyi, Gyula
abstract

The bimetallic cyclosiloxanolate cluster complexes Na[(PhSiO2)6Cu4Ni2(μ6-Cl)(PhSiO2)6] (1) and Na[(PhSiO2)6Cu3Ni3(μ6-Cl)(PhSiO2)6] (2) were prepd. by Na+ and Ni2+ ion exchange from in situ generated Na2{[(PhSiO2)6]2Na4Ni4(OH)2}. Complexes 1 and 2 were characterized by anal., spectroscopic and electrochem. methods as well as complex 2 by single-crystal x-ray diffraction (2·7DMSO·H2O: orthorhombic, space group P212121, R = 0.076). The x-ray structure shows a sandwich-type array comprising two superimposed cyclosiloxanolate rings and an M6Cl unit in between. For the 1st time the regioselectivity of the metal ion exchange could be deduced from the x-ray structural parameters.

1998 - Comparison of the Spin Dynamics in Different Types of Molecular Magnetic Rings from H-1 NMR [Articolo su rivista]
A., Lascialfari; Z. H., Jang; F., Borsa; D., Gatteschi; Cornia, Andrea
abstract

Proton spin-lattice relaxation (NSLR) measurements were performed in two mol. Cu magnetic rings contg. 6 and 8 spins (S = 1/2), resp., in an almost coplanar arrangement to probe the spin dynamics of the spins in the ring. The NSLR results obtained in the Cu6 and Cu8 rings as a function of temp. and of applied magnetic field are compared with previous results of NSLR of 1H in the Fe(III) rings Fe6 and Fe10 (S = 5/2). At room temp., common features are found in the spin dynamics while at low temp., when correlation effects become important, important differences are obsd. in the rings depending on the kind of coupling between magnetic spins (ferromagnetic or antiferromagnetic) and on the spin value S.

1998 - Heterobimetallic cyclosiloxanolate sandwich clusters: Na{[η6-cyclo(PhSiO2)6]2[Fe(OR)]2Ni4(μ6-Cl)} (R = H, Me) [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Gavioli, Giovanna; Zucchi, Claudia; R., Pizzotti; A., Vizi Orosz; O. I., Shchegolikhina; Y. A., Pozdniakova; Palyi, Gyula
abstract

The heterobimetallic cyclosiloxanolate sandwich clusters Na{[η6-cyclo(PhSiO2)6]2[Fe(OR)]2Ni4(μ6-Cl)} (R = H, Me) (1) were prepd. from Na2{[(PhSiO2)6]2Na4Ni4(OH)2} as solvates. The new clusters 1 were characterized by spectra (UV-visible, IR, 1H-NMR), cyclic voltammetry, cond., magnetic susceptibility, and single-crystal x-ray diffraction for 1·10MeOH (R = Me).

1998 - Magnetic and Electronic Properties of Hexairon(III) Nanocluster with Cyclic Structure: a Mossbauer Study [Articolo su rivista]
A., Caneschi; M., Capaccioli; L., Cianchi; Cornia, Andrea; F., DEL GIALLO; F., Pieralli; G., Spina
abstract

A molecular cluster containing a coplanar ring of iron(III) ions with spin 5/2 was investigated by Mossbauer spectroscopy. The iron spins are antiferromagnetically coupled so that the ground state has total spin S = 0. Spectra in the absence of an applied magnetic field consisted of a quadrupolar doublet, the linewidth of which monotonically increased with the temperature. A quadrupolar splitting of about 0.35 mm/s was found. Calculations of the iron-orbital electronic populations were carried out and the quadrupolar splitting was estimated. Its value was in agreement with the experimental one. In addition, the trend of the linewidth was explained in terms of isotropic spin fluctuations. Spectra in the presence of a 4.5 T longitudinal magnetic field were also collected. The hyperfine field was obtained from their fitting. Differences with respect to the hyperfine field obtained from susceptibility data were also interpreted in terms of spin fluctuations.

1998 - Nuclear-spin Relaxation in Magnetic Rings [Articolo su rivista]
A., Lascialfari; D., Gatteschi; Cornia, Andrea; U., Balucani; M. G., Pini; A., Rettori
abstract

In magnetic rings of transition-metal ions, electron-spin correlations are found to exhibit rather unusual dynamical features, as a result of a subtle interplay between finite-size effects and magnetic interactions distinctly smaller than the dominant Heisenberg exchange Hamiltonian. These findings, mostly obtained by a mode-coupling approach, are shown to have important consequences on the nuclear-spin (proton) relaxation rates T-1(-1) as measured in two specific rings, comprising six Cu or six Fe ions (respectively, ferromagnetic and antiferromagnetic). In both systems the magnetic-field and temperature dependence of the theoretical results is found to compare rather well with the experimental findings.

1998 - Reaction between CuCl2 and 2-S-methyl-5,5-dimethylimidazoline-4-thione. X-ray crystal structure of catena-chloro(μ-N(1),S(4)(2-S-methyl-5,5-dimethylimidazoline-4-thione))copper(I) [Articolo su rivista]
F. A., Devillanova; A., Diaz; F., Isaia; V., Lippolis; G., Verani; Cornia, Andrea; FABRETTI COSTANTINO, Antonio
abstract

Catena-Chloro(μ-N(1),S(4)(2-S-methyl-5,5-dimethylimidazoline-4-thione))copper(I) was prepd. from reaction of the ligand with CuCl2, and structurally characterized. The structure exhibits linear polymeric chains disposed parallel to the b axis. The structure of each chain consists of Cu(I) ions coordinated in a planar trigonal geometry by the N(1) and S(4) atoms of two different ligand mols. and one chloride. The ligand acts as an N(1),S(4) bidentate mol. and the orientation of the imidazoline rings is detd. by a H bond between the NH of each ring and the coordinated chloride. The quite strong interaction between the thioether S atom of one chain, S(2), and the Cu(I) of the next chain [2.898(2) Å] is significant in the structure.

1998 - Structure and Magnetic Properties of a Mixed-valence Heptanuclear Manganese Cluster [Articolo su rivista]
G. L., Abbati; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; A., Caneschi; D., Gatteschi
abstract

Two novel polynuclear Mn(II,III) complexes were synthesized by exploiting controlled methanolysis. A 1-pot reaction of MnCl2, NaOMe, dibenzoylmethane (Hdbm), and O2 in anhyd. MeOH, followed by recrystn. from MeOH/CHCl3 mixts., afforded the alkoxomanganese complexes [Mn7(OMe)12(dbm)6]·CHCl3·14MeOH (2) and [Mn2(OMe)2(dbm)4] (3). 2 Crystallizes in trigonal space group R-3 with a = 14.439(2) Å, α = 86.34(1), and Z = 1. 3 Crystallizes in triclinic space group P-1 with a = 9.612(1), b = 10.740(1), c = 13.168(1) Å, α = 80.39(1), β = 87.66(1), γ = 83.57(1)°, and Z = 1. The solid-state structure of 2 comprises a [Mn6(OMe)12(dbm)6] crown with crystallog. imposed 6-fold symmetry plus a central Mn ion. The layered Mn/O core mimics a fragment of the Mn oxide mineral lithiophorite. Cond. measurements confirmed the nonionic character of 2 and suggested a mixed-valence MnII3MnIII4 formulation and the metrical parameters of the core were analyzed with the aid of bond-valence sum calcns. The central ion is essentially a valence-trapped MnII ion, whereas the av. Mn-O distances for the Mn ions of the crown are consistent with the presence of two MnII and four MnIII ions. However, 1H NMR spectra in soln. strongly support valence localization and suggest that the obsd. solid-state structure may be a result of static disorder effects. Magnetic susceptibility vs. T and magnetization vs. field data at low temp. are consistent with an S = 17/2 ground state. 3 Is a sym. alkoxo-bridged dimer and the two high-spin MnIII ions are antiferromagnetically coupled with J = 0.28(4) cm-1, g = 1.983(2), and D = -2.5(4) cm-1.

1998 - Valence Tautomerism in a Cobalt Complex of a Schiff Base Diquinone Ligand [Articolo su rivista]
A., Caneschi; Cornia, Andrea; A., Dei
abstract

The cobalt(III) complex of a Schiff base diquinone ligand Co-III(Cat-N-BQ)(Cat-N-SQ) undergoes an entropy-driven valence tautomeric equilibrium in solution yielding the Co-II(Cat-N-BQ)(2) species. This suggestion is supported by variable temperature electronic and H-1 NMR spectra, as well as by solution magnetic susceptibility data. The calculated thermodynamic quantities associated to the redox equilibrium are Delta H = 42.0(0.8) kJ mol(-1) and Delta S = 140 (3) J mol(-1) K-1.

1997 - A Cyclic Octadecairon(III) Complex, the Molecular 18–Wheeler [Articolo su rivista]
S. P., Watton; P., Fuhrmann; L. E., Pence; A., Caneschi; Cornia, Andrea; G. L., Abbati; S. J., Lippard
abstract

[Fe(OH)(XDK)Fe2(OMe)(O2CCH3)2]6 (1; XDK = m-xylenediamine bis(Kemp's triacid imide)), a cyclic octadecairon(III) complex, the mol. 18-wheeler, was prepd., characterized by single crystal x-ray diffraction anal., and its magnetic behavior was studied. This complex, which the authors designate the "mol. 18-wheeler", is the largest cyclic ferric cluster reported so far, contg. 18 iron(III) atoms in a ring. The compn. of the cryst. compd., a double salt having the formula 1·6Et4N(NO3)·15MeOH·6Et2O·24H2O, was established by elemental anal. and single crystal x-ray diffraction anal. Each iron in 1 has a distorted octahedral symmetry. A plot of the magnetic susceptibility per Fe atom vs. temp. measured on a microcryst. sample at an applied field of 1 T in the range 2.2<T<260 K displays a broad max. at about 60 K. This behavior is indicative of antiferromagnetic interactions between the high spin iron(III) ions.

1997 - Electronic Structure of Manganese(III) Compounds from High-frequency EPR Spectra [Articolo su rivista]
A. L., Barra; D., Gatteschi; R., Sessoli; G. L., Abbati; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; M. G., Uytterhoeven
abstract

The high field EPR spectra of a Mn(III) compd. was obtained. The HF-EPR spectra of Mn(dbm)3, where Hdbm = 1,3-diphenyl-1,3-propanedione, can be used to provide information on g-factors and zero field splitting of Mn(III) complexes. The ligand field anal. can be used to obtain structure and electronic structure information of the complex.

1997 - Modulated Magnetic Coupling in Alkoxoiron(III) Rings by Host-guest Interactions with Alkali Metal Cations [Articolo su rivista]
G. L., Abbati; FABRETTI COSTANTINO, Antonio; Cornia, Andrea; Malavasi, Wanda; Schenetti, Luisa; A., Caneschi; D., Gatteschi
abstract

Two novel host-guest complexes of alkali-metal ions with cyclic iron(III) clusters, [LiFe6(OCH3)12(dbm)6]X (X = PF6-, ClO4-; Hdbm = dibenzoylmethane), were synthesized. The x-ray structure of [LiFe6(OCH3)12(dbm)6]PF6 ([1]PF6) is described and compared with that of sodium complexes, previously reported. [1]PF6.1/2CHCl3.9MeOH crystallizes in trigonal space-group R-3 with (at 298 K) a = 14.523(2) Å, α = 84.35(1), V = 3021.3(7) Å3 and Z = 1. The radius of the encapsulated alkali-metal ion has a marked influence on the geometry of the [Fe6(OCH3)12] host. Substitution of sodium with lithium decreases nearest-neighbor Fe···Fe sepns. and Fe-O-Fe angles, while it does not affect Fe-O distances. A quant. anal. of the magnetic susceptibility of solid samples revealed that weaker Fe-Fe antiferromagnetic interactions occur in the lithium complexes. The authors suggest that host-guest interactions modulate the magnetic coupling by detg. the exact geometry at the bridging methoxide ligands. 7Li and 23Na NMR expts. have beautifully illustrated the soln. behavior of the reported complexes and the selectivity of six-membered iron(II) rings for alkali-metal ions. The structural and functional analogy between iron(III) rings and crown ethers is stressed.

1997 - New Perspectives in Phosphonodithioate Coordination Chemistry. Synthesis and X-ray Crystal Structure of Trans-bis-[O-ethyl(4-methoxyphenyl)phosphonodithioato] nickel(II) [Articolo su rivista]
M., Arca; Cornia, Andrea; F. A., Devillanova; FABRETTI COSTANTINO, Antonio; F., Isaia; V., Lippolis; G., Verani
abstract

Attempts to optimise the synthesis of a new class of nickel(ll) dithiolene complexes starting from 1,3-dialkylimidazolidine-2-thione-4,5- dione (1) and Lawesson's reagent (2) have afforded a new and easy one step synthesis of phosphonodithioate nickel(II) complexes. This consists of the direct reaction between NiCl2 and Lawesson's reagent in the appropriate alcohol R'OH (R=Me, Et, i-Pr, Bu, Bz) as solvent, to give the bis-[O-alkyl/aryl-(4-methoxyphenyl)phosphonodithioato] nickel(II) complexes (5a-e) with high yields (64-91%). Among them trans-bis-[O-ethyl-(4-methoxyphenyl) phosphonodithioato] nickel(II) (5b) was characterised by means of X-ray diffraction measurements on a single crystal. The compound crystallises in the triclinic, Pi space group with a= 6.4840(7), b = 7.6032(9), c= 13.055(2) Angstrom, alpha = 99.481(9), beta = 99.43(1), gamma = 104.487(9)degrees and Z = 1. The compound is centrosymmetric and exhibits discrete monomeric units with the ligand acting as S,S-bidentate. To explain the reactivity of Lawesson's reagent, the formation of 4-methoxyphenyldithiophospborane (6) as intermediate has been suggested. The reaction was also tested for Pd(II) and Pt(II), but in these cases the yields were not as satisfactory as for Ni(II).

1997 - Spin Dynamics in Mesoscopic Size Magnetic Systems: A H-1 NMR Study in Rings of Iron(III) Ions [Articolo su rivista]
A., Lascialfari; D., Gatteschi; F., Borsa; Cornia, Andrea
abstract

Two magnetic mol. clusters contg. almost coplanar rings of iron (III) ions with spin S = 5/2 were studied by 1H NMR and relaxation measurements. The 1st system, which will be referred to as Fe6, is a mol. [NaFe6(OCH3)12(C17O4H15)6]+ClO4- or [NaFe6(OCH3)12(C15H11O2)6]+ClO4- or [LiFe6(OCH3)12(C15H11O2)6]+ClO4- while the 2nd type of ring, denoted Fe10, corresponds to the mol. [Fe10(OCH3)20(C2H2O2Cl)10]. The 1H NMR linewidth is broadened by the nuclear dipolar interaction and by the dipolar coupling of the protons with the iron (III) paramagnetic moment. The nuclear spin-lattice relaxation rate, T1-1, of the proton is a sensitive probe of the Fe spin dynamics. In both clusters, T1-1 decreases with decreasing temps. from room temp., goes through a peak just .ltorsim.30 K in Fe6 and 10 K in Fe10, and it drops exponentially to very small values at helium temp. The temp. dependence of the relaxation rate is discussed in terms of the fluctuations of the local spins within the allowed total spin configurations in the framework of the weak collision theory to describe the nuclear relaxation. The authors use the calcd. energy levels for the Fe6 ring based on a Heisenberg Hamiltonian and the value of J obtained from the fit of the magnetic susceptibility to describe semiquant. the behavior of T1-1 vs. T. The exponential drop of T1-1 at low temp. is consistent with a nonmagnetic singlet ground state sepd. by an energy gap from the 1st excited triplet state. The values obtained for the gap energies are ET/k = 12 K for Fe10 and ET/k = 38 K for Fe6 which are almost twice as big as the values deduced from susceptibility measurements. At all temps. the relaxation rate decreases with increasing magnetic field, i.e., NMR resonance frequency. This effect could be related to the long time persistence of the spin correlation functions typical of diffusive modes in low dimensional magnetic systems. It is argued that the data presented are a direct exptl. study of spin dynamics in mesoscopic spin rings and should afford a test for exact anal. and/or numerical solns.

1997 - Synthesis, Crystal Structures and Magnetic Characterization of Four β-diketonate-alkoxide Iron(III) Dimers. Dependence of the Magnetic Properties on Geometrical and Electronic Parameters [Articolo su rivista]
F., LE GALL; F., FABRIZI DE BIANI; A., Caneschi; P., Cinelli; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

Synthesis, crystallog. characterization and magnetic properties of the dinuclear Fe(III) complexes [Fe(OMe)(dbm)2]2 (1), [Fe(OMe)(dpm)2]2 (2), [Fe(OEt)(bpm)2]2 (3) and [Fe(OPr)(npm)2]2 (4) are reported. 1 And 2 have sym. β-diketonate ligands dbm (dibenzoylmethanate) and dpm (dipivaloylmethanate), resp., whereas complexes 3 and 4 contain asym. β-diketonate ligands bpm (benzoylpivaloylmethanate) and npm (naphthoylpivaloylmethanate), resp. 1 Crystallizes in the triclinic system, space group P-1 a = 9.634(1), b = 10.946(2), c 13.079(1) Å, α = 79.95(1), β = 88.01(1), γ = 82.57(1)°, Z = 1. 2 Crystallizes in the triclinic system, space group P-1, a = 10.980(2), b = 14.255(2), c = 17.979(1)Å, α = 85.70(1), β = 89.63(1), γ = 71.65(1)°, Z = 2. 3 Crystallizes in the monoclinic system, space group P21/c, a = 11.546(2), b = 18.539(1), c = 13.595(2)Å, β = 113.18(1)°, Z = 2. 4 Crystallizes in the monoclinic system, space group P21/c, a = 13.746(2), b = 18.933(2), c = 14.158(2)Å, β = 117.37(1)°, Z = 2. Each complex consists of two Fe(III) ions that are sym. bridged by two alkoxide groups. The antiferromagnetic couplings between the S = 5/2 Fe centers were fitted by using the Hamiltonian H = JS1·S2 to give values of J = 15.4(1) cm-1 with g = 1.96(1) for 1, J = 19.0(6) cm-1 with g = 1.98(1) for 2, J = 14.8(5) cm-1 with g = 1.98(1) for 3 and J = 18.0(5) with g = 2 for 4. The structural and magnetic parameters of 1-4 and other Fe(III)-alkoxo clusters synthesized in the authors' lab. are compared. No simple correlation exists between the isotropic exchange-coupling const. J and the av. Fe-O(bridge) bond distance, whereas an approx. linear correlation is found between the J value and the Fe-O-Fe angle. The obsd. trend and the predictions of Extended Hueckel calcns. on the model [Fe(OH)H4]24- are compared and discussed.

1997 - The bonding of thiazoles to platinum(II) complexes. X-ray crystal structure of cis- and trans-[Pt(dimethyl sulfoxide)(thiazole)Cl-2] [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; M., Bonivento; L., Cattalini
abstract

The crystal structures of neutral cis- and trans-[Pt(dimethylsulfoxide)(thiazole)Cl2] have been determined by X-ray diffraction. The longer C4-C5 bond distance of the N-coordinated thiazole in both the square planar complexes, as compared to the free ligand, is interpreted in terms of a negative charge density transfer toward the antibonding orbital system of the thiazolic ring.

1996 - Cyclooligosiloxanolate cluster complexes of transition metals and lanthanides [Articolo su rivista]
Zucchi, Claudia; Oi, Shchegolikhina; Borsari, Marco; Cornia, Andrea; Gavioli, Giovanna; Ac, Fabretti; E., Rentschler; D., Gatteschi; R., Ugo; R., Psaro; Pozdniakova, Y. U. A.; Sv, Lindeman; A. A., Zhdanov; Palyi, Gyula
abstract

Cyclooligosiloxanolate complexes of higher valent transition metals and lanthanides were synthesized and characterized by single-crystal X-ray diffraction experiments, spectra, cyclic voltammetry, conductivity and magnetic behaviour. These complexes are sandwich-type clusters containing assemblies of 4 to 8 metals between siloxanolate ligand 'layers'. A high degree of electron delocalization in the metal containing fragment of these complexes is detected by electrochemical and magnetic measurements, The synthesis, crystal and molecular structure determined by X-ray diffraction of Na-6{[C6H5SiO2)(8)]Gd-2(4)(mu(4)-O)} is reported.

1996 - Magnetic Properties of Dodecanuclear Mixed Valence Iron Clusters [Articolo su rivista]
A., Caneschi; Cornia, Andrea; S. J., Lippard; G. C., Papaefthymiou; R., Sessoli
abstract

Slow addn. of dioxygen to a basic MeOH soln. of Fe(II) chloride afforded the mixed valent polyiron oxo cluster [Fe12(O)2(O2CCH2Cl)5.3Cl0.7(CH3O)18(MeOH)4] (1). The structure of 1 was detd. in a single crystal x-ray diffraction study; crystallog. data at 202 K: space group P-1, a = 10.606(2), b = 12.173(2), c = 13.199(1) Å, α = 100.48(1), β = 96.63(1), γ = 91.43(1)°, Z = 1, Mr = 1907.3, ρcalc = 1.91 g cm-3. For 3499 unique obsd. reflections with F2 > 3σ(F2), R = 0.060, Rw = 0.063. The core of the cluster contains a fcc. array of O atoms with eight Fe(II) ions and four Fe(III) ions occupying octahedral sites. Two μ6-oxo ligands link 10 of the 12 Fe atoms. The mixed valent nature of this air-sensitive compd. was established by analyzing the Fe-O distances, by charge considerations, and by electronic and Moessbauer spectroscopy. Three Fe subsites are resolvable in the Moessbauer spectrum, corresponding to localized Fe(III) and Fe(II) ions and consistent with the crystallog. data. The magnetic properties of 1 were satisfactorily reproduced by a simple model which assumes that exchange coupling interactions mediated by μ2-methoxo ligands are responsible for the obsd. magnetic behavior in the temp. range 20-300 K, whereas at lower temp. the effects of the μ3-methoxo, μ2-carboxylato and μ6-oxo ligands become more important. This behavior is reflected by the appearance of magnetic features in the Moessbauer spectra with decreasing temp. The magnetic properties of 1 may be understood as arising from two subclusters, a and b, for which the following two sets of parameters are proposed and discussed: (i) g(a) = 2.319(3), g(b) = 1.831(4), J = 15.9(1) cm-1 and (ii) g(a) = 2.23(3), g(b) = 2.1(1), J = 19(2) cm-1, where J is the exchange coupling const. through μ2-methoxo bridges within the a subcluster, and the spins of subcluster b remain largely uncoupled.

1996 - Magnetism of Large Iron-Oxo Clusters [Articolo su rivista]
D., Gatteschi; A., Caneschi; R., Sessoli; Cornia, Andrea
abstract

The role of large iron-ore clusters in chemistry, material science, and fundamental physics is briefly reviewed. Examples of clusters with nuclearity ranging from 6 to 19 are reported as well as their magnetic properties. Relevant topics, such as the role of spin topology and spin frustration in antiferromagnetically coupled systems and the observation of superparamagnetic like behaviour and molecular hysteresis, are discussed. The use of strong magnetic fields in the characterization of magnetic clusters is emphasized and a theoretical approach for the rationalization of the magnetic properties of clusters is briefly reported.

1996 - Metal Binding of Polyalcohols. 4. Structure and Magnetism of the Hexanuclear, μ6-Oxo-Centered [OFe6(H-3thme)3(OCH3)3Cl6]2- (thme = 1,1,1-Tris(hydroxymethyl)ethane) [Articolo su rivista]
Cornia, Andrea; D., Gatteschi; K., Hegetschweiler; L., HAUSHERR PRIMO; V., Gramlich
abstract

The addn. of [NMe4]OH to a methanolic soln. of FeCl3 and thme (thme = 1,1,1-tris(hydroxymethyl)ethane) yielded [NMe4]2[OFe6(H-3thme)3(OCH3)3Cl6]·2H2O. Crystal data: C26H64Cl6Fe6N2O15, trigonal space group P31c, a = 12.459(2), c = 18.077(4) Å, Z = 2. The complex anion exhibits the known μ6-O-Fe6-(μ2-OR)12 structure with three μ2-methoxo bridges, three triply deprotonated H-3thme ligands, where each alkoxo group bridges two FeIII centers, and six terminally coordinating Cl- ligands. In contrast to two previously described ferric complexes with an analogous structure of the complex core, this compd. is stable in air. Variable-temp. magnetic susceptibility measurements established antiferromagnetic exchange coupling interactions with Jtrans(Fe-μ6-O-Fe) = 24.5 cm-1, Jcis(Fe-μ2-Othme-Fe) = 11.5 cm-1, and Jcis'(Fe-μ2-OCH3-Fe) = 19.5 cm-1. The unexpectedly high value for Jtrans is explained by a superexchange pathway and is discussed for a simplified model by using MO calcns. at the extended Hueckel level.

1996 - Molecule-based Magnets: Ferro- and Antiferromagnetic Interactions in Copper(II)-polyorganosiloxanolate Clusters [Articolo su rivista]
E., Rentschler; D., Gatteschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; A. L., Barra; O. I., Shchegolikhina; A. A., Zhdanov
abstract

The magnetic behavior of the clusters [(PhSiO2)6Cu6(O2SiPh)6]·6EtOH (1), Na4[(PhSiO2)12Cu4]·8BuOH (2), and K4[(C2H2SiO2)12Cu4]·6BuOH (3) was studied by combined magnetic susceptibility measurements and variable-temp. EPR techniques (9.25 and 245 GHz). The six Cu(II) ions in the core of 1, which approaches 6/mmm symmetry, are ferromagnetically coupled as a result of the geometry at the bridging siloxanolate O atoms (Cu-O-Cu = 91.5-94.6°; J = -42 cm-1). The ground S = 3 spin state is split in zero field mainly due to anisotropic exchange contributions (D = 0.30 cm-1). Notably, both the magnitude and the sign of the zero-field splitting parameter were detd. from HF-EPR spectra. Large antiferromagnetic Cu-Cu interactions (J ∼ 200 cm-1) and an S = 0 ground state were detected in the tetranuclear clusters 2 and 3 as a consequence of the larger Cu-O-Cu angles. The results are relevant to the search for new mol.-based magnetic materials.

1996 - Synthesis, Crystal Structure, Magnetism, and Magnetic Anisotropy of Cyclic Clusters Comprising Six Iron(III) Ions and Entrapping Alkaline Ions [Articolo su rivista]
A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; S., Foner; D., Gatteschi; Grandi, Romano; Schenetti, Luisa
abstract

The reaction of ferric chloride and β-diketones (HL) in alk. MeOH soln. represents a good synthetic route to hexairon(III) clusters [MFe6(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual 6-fold mol. symmetry. Single-crystal x-ray diffraction of [NaFe6(OMe)12(pmdbm)6]ClO4·xCHCl3·yMeOH (pmdbmH = 1,3-bis(p-methoxyphenyl)-1,3-propanedione) showed that the six octahedrally coordinated Fe(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in org. soln., as demonstrated by 23Na and 7Li NMR expts. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field d.c. and pulsed-field differential magnetization expts. at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temp. revealed a sizeable magnetic anisotropy, which was successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters.

1996 - Synthesis, crystal structure, magnetism, and magnetic anisotropy of cyclic clusters comprising six iron(III) ions and entrapping alkaline ions [Articolo su rivista]
Caneschi, A.; Cornia, A.; Fabretti, A. C.; Foner, S.; Gatteschi, D.; Grandi, R.; Schenetti, L.
abstract

The reaction of ferric chloride and β-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6-(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field DC and pulsed-field differential magnetization experiments at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters. © VCH Verlagsgesellschaft mbH, 1996.

1995 - A Cyclic Hexairon(III) Complex with an Octahedrally Coordinated Sodium Ion at the Center, an Example of the [12]Metallacrown–6 Structure Type [Articolo su rivista]
A., Caneschi; Cornia, Andrea; S. J., Lippard
abstract

[NaFe6(OMe)12(dbm)6]Cl·12MeOH·CHCl3 (I; dbmH = dibenzoylmethane) was prepd. and characterized by elemental anal., spectral (UV, IR), x-ray crystallog., and magnetic methods. The 6 Fe atoms are located at the vertexes of an almost regular hexagon, with Fe···Fe sepns. in the range 3.18-3.20 Å and vertex angles of 119.5-120.5°. The ring serves as a host for the Na ion. Antiferromagnetic exchange coupling interactions exist between the paramagnetic Fe(III) centers (J = 20 cm-1). I is a good structural model for cyclic intermediates proposed for the polyalc.-promoted hydrolysis of Fe(III) salts.

1995 - Molecule-Based Magnets: Ferro- and Antiferromagnetic Interactions in Nickel(II) Cyclohexasiloxanolate Sandwich Complexes [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi; Palyi, Gyula; E., Rentschler; O. I., Shchegolikhina; A. A., Zhdanov
abstract

Ferromagnetic exchange-coupling interactions mediated by siloxanolate ligands have been detected in the two nickel(II) nanoscale clusters Na2[(PhSiO2)6Na4Ni4(OH)2(O2SiPh)6]. 16Bu(n)OH (1) and Na2[(PhSiO2)6Ni6(O2SiPh)6Cl].12CH3OH. H2O (2), which qualify as interesting precursors for new molecule-based magnetic materials. Complex 1 has an S = 4 spin ground state, whereas complex 2 has an S = 0 spin ground state. due to the presence of additional antiferromagnetic interactions through the encapsulated mu(6)-chloride ion.

1995 - Polyiron(III)–Alkoxo clusters: a Novel Trinuclear Complex and Its Relevance to the Extended Lattices of Iron Oxides and Hydroxides [Articolo su rivista]
A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi; Malavasi, Wanda
abstract

The synthesis, solid-state structure and magnetic characterization of KFe3(OMe)7(dbm)3·4MeOH (I; Hdbm = dibenzoylmethane) are reported. I readily assembles in methanolic solns. of Fe(III) chloride and Hdbm in the presence of an excess of K methoxide; it crystallizes in the triclinic space group P-1 with a = 13.044(1), b = 14.174(1), c = 18.657(2) Å, α = 97.94(1), β = 104.13(1), γ = 114.12(1)°, and Z = 2 at 190 K, R = 0.046 and Rw = 0.051. The solid-state mol. structure of I consists of a triangular array of Fe(III) ions connected by a triply bridging methoxide and three μ2-methoxide ligands. The O donors of a monodentate methoxide and of a chelating dbm ligand complete the coordination sphere of each metal ion. The resulting mononeg. Fe3(μ3-OMe)(μ2-OMe)3(OMe)3(dbm)3 moiety coordinates the K counterion through the oxygens of the μ2-OMe ligands. The O atoms in the core of I are arranged in two essentially parallel layers and display a closest-packing which is found in Fe oxides and hydroxides. The high-spin Fe(III) ions are antiferromagnetically coupled with an S = 1/2 spin ground state. Assumption of a C2v point-group symmetry for the cluster leads to a satisfactory reprodn. of the obsd. magnetic behavior with g = 2.0 and either J = 10.6(1), J' = 15.3(2) cm-1 or J = 12.9(2), J' = 9.7(1) cm-1, where the spin-only Heisenberg Hamiltonian is defined as H = .sum.i>jJijSi·Sj and J' is the unique coupling const. The relative contributions of geometrical distortions and of antisym. exchange to the splitting of the 4-fold 2E electronic ground state of an idealized system with trigonal symmetry were estd. from X-band EPR spectra recorded on powd. samples.

1995 - Structure and Magnetic Properties of a Decanuclear Oxoiron(III) Cluster: A Further Step to Understanding Iron Aggregation Processes [Articolo su rivista]
A., Caneschi; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; D., Gatteschi
abstract

[Fe10O4(OMe)16(dbm)6]·xCHCl3 (1) was prepd. and characterized by x-ray crystallog. The crystal lattice contains decairon clusters and highly disordered chloroform mols. The temp. dependence of the magnetic susceptibility of 1 was investigated and the magnetic behavior was indicative of strong antiferromagnetic interactions.

1994 - Magnetic Exchange Coupling in the FeIII6(μ6-O) Core: A Hint to the Magnetic Properties of Higher-Nuclearity Spin Clusters [Articolo su rivista]
Cornia, Andrea; D., Gatteschi; K., Hegetschweiler
abstract

Variable-temperature magnetic susceptibility measurements performed on two hexairon(III) clusters containing a μ6-oxo bridge show that this unusual bridging mode leads to weak antiferromagnetic exchange-coupling interactions between the metal centers, with J(trans) and J(cis) values of 19(2) and 9(1) cm-1, respectively. The result is relevant to the search for magneto-structural correlations in oxo-bridged polyiron aggregates.

1994 - Structure of catena–(2–Amino–1,3,4–Thiadiazolium, 2–Amino–1,3,4–Thiadiazole bis(mu–Iodo)–di–Iodo–Bismuth(III)) [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Grandi, Romano; Malavasi, Wanda
abstract

[(C2H4N3S) (C2H3N3S) BiI4]infinity; [(HL) (L) BiI4]infinity (L = 2-amino-1,3,4-thiadiazole), M(r) = 919.86, monoclinic, P 1 2(1)/a 1, a = 7.622(1), b = 18.977(2), c = 12.821(1)angstrom, beta = 102.77(1)-degrees, final R = 0.033. The crystals consist of HL+ cations, neutral L molecules and polymeric [BiI4]infinity- anions built up of edge-sharing [BiI6]3- octahedra, with Bi-I(bridging) distances in the range 3.320(1)-3.057(1) angstrom and Bi-I(terminal) distances in the range 2.917(1)-2.929(1)angstrom. The H-1 and C-13 spectra in DMSO-d6 solution are used in conjunction with the X-Ray structural results to establish the preferred protonation site of the thiadiazole ring.

1993 - Electron Transfer in the Reactions of Organic Trichloromethyl Derivatives with Iron(II) Chloride [Articolo su rivista]
Cornia, Andrea; Folli, Ugo; S., Sbardellati; Taddei, Ferdinando
abstract

Reduction of trichloromethyl derivatives RCCl3 [1, R = Ph;2, R = PhC(O); 3, R = EtOC(O)] with iron(II) chloride in acetonitrile, has been studied in order to examine the mechanism of the electron transfer (ET) process and the reactions of the radicals formed. Substrates 1-3 afforded different product compositions and the cause was identified as differences in the reactivity of radicals which is substantially of two types: reductive coupling and proton abstraction after further reduction to a carbanion coordinated to the metal ion. Compound 1 gave only coupling products, compound 2 only hydrogenated products and compound 3 a mixture of coupling and hydrogenated products depending on experimental conditions. Proton abstraction by the carbanion was found to occur from water molecules, which should be present in the coordination shell of the metal ion, and not from the solvent. The different behaviour of compounds 1-3 is attributed to the presence of substituents which are able to stabilize the radical and carbanionic intermediates. Rate constants at different temperatures were measured and the activation parameters calculated. The three substrates differ only slightly in reaction rates, in the order 1 > 2 > 3. Activation enthalpies are very close to each other and this agrees with the almost equal values of C-Cl bond dissociation energies of compounds 1-3, empirically determined. Large, negative entropies of activation were found, suggesting that an ordered activation complex should be formed in order that electron transfer from the metal ion to the organic halide can take place.

1993 - Magnetic Properties and Crystal Structure of a Linear-chain Copper(II) Compound with Bridging Chloride and Oxamidate Ligands [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; F., Ferraro; D., Gatteschi; Giusti, Aleardo
abstract

The compound [Cu2(L)Cl2] [H2L = N,N'-bis(3-amino-2,2-dimethylpropyl)oxamide] has been prepared and characterized by means of structural and magnetic measurements. The crystals are triclinic, space group P-1, with a = 6.630(1), b = 7.443(1), c = 9.702(1) angstrom, alpha = 95.38(1), beta = 102.72(1), gamma = 101.12(1)° and Z = 2. The structural unit comprises one independent copper ion bonded to one half of a trans-oxamidate ligand and one chloride ion. Two centres of symmetry lead to a polymeric molecule. The copper ions are in distorted square-pyramidal environments and the basal co-ordination sites are occupied by O,N,N' donors from the trans-oxamidate ligand and by a chloride ion. The fifth contact occurs with the chloride ion of an adjacent asymmetric unit. The nearest-neighbour Cu...Cu distance is 3.2989(9) angstrom and a remarkably acute angle is found at the bridging chloride [79.02(3)°]. The magnetic properties of the compound are well reproduced by the dimer model with pairwise interactions, owing to the overwhelming importance of the oxamidate bridge in determining the exchange coupling interactions [J = 409(3) cm-1]. The EPR spectra, however, indicate that the compound intimately behaves as an alternating chain.

1993 - Molecular Structure and Magnetic Properties of Copper(II), Manganese(II) and Iron(II) Croconate Tri-hydrate [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Giusti, Aleardo; F., Ferraro; D., Gatteschi
abstract

MC5O5.3H2O (I - III; M = Cu, Mn, Fe, resp., H2C5O5 = croconic acid) were prepd. and characterized by structural and magnetic measurements. The crystals are orthorhombic, space group Pbca, Z = 8. The previously reported structures of I and II, refined here, and the novel structure of III were solved by direct methods (R = 0.029, 0.025 and 0.025, resp.). The overall structure is similar for the 3 compds. and consists of infinite 1-dimensional chains of metal ions bridged by croconate ligands acting in a monodentate and bidentate fashion. The magnetic properties of I, II and III are reported and discussed on the basis of intrachain and interchain exchange couplings and, for III, of single-ion axial zero-field splitting contributions. Weak antiferromagnetic intrachain interactions via the croconate dianion (J = 0.99(3), 0.22(1) and ≤0.54 cm-1 for I, II and III, resp.) appear to be responsible of most of the obsd. magnetic behavior.

1993 - Synthesis, Crystal and Molecular Structure, and Infrared Characterization of Two Amino Derivatives of 1,3,4–Thiadiazole [Articolo su rivista]
Antolini, Luciano; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Malavasi, Wanda
abstract

2-Amino-5-methyl-1,3,4-thiadiazole, 2-amino-5-methyl-3H-1,3,4-thiadiazolium bromide [(amtzH)(amtz)Br] and bis(2,5-diamino-3H-1,3,4-thiadiazolium) sulfate [(a2tzH)2SO4] were prepd. and characterized by x-ray structural anal. and IR spectroscopy. The crystals of (amtzH)(amtz)Br are monoclinic. Its asym. unit contains one protonated amtzH cation, one neutral amtz mol., and one bromide ion. The crystals of (a2tzH)2SO4 are monoclinic. The structure is built up of one crystallog. independent a2tzH cation and one half sulfate ion with crystallog. dictated twofold symmetry.

1993 - Synthesis, Crystal and Molecular Structure, and Spectroscopic Characterization of 5–(1–Hydroxycyclohexylthio)–1,3,4–thiadiazole–2–thione [Articolo su rivista]
Antolini, Luciano; Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Schenetti, Luisa
abstract

2,5-Dimercapto-1,3,4-thiadiazole (DMTDA), when dissolved in cyclohexanone, interacts strongly with the solvent and the title monothiohemiacetal (MTH) is formed. MTH has been characterized in the solid state by means of X-ray diffraction, IR spectroscopy and thermogravimetric analysis, and in cyclohexanone solution by means of NMR spectroscopy. The title compound crystallizes in the triclinic space group P-1 (No. 2), with cell dimensions a = 6.481 (1), b = 9.322(1), c = 9.553(1) angstrom, alpha = 72.31 (1), beta = 89.74(1), gamma = 80.83(1)° and Z = 2. The structure was solved by direct methods and least-squares refinement of structural parameters led to a conventional R factor of 0.041 for 1074 independent reflections. In the solid state, MTH easily undergoes decomposition with loss of cyclohexanone.