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Ricercatore t.d. art. 24 c. 3 lett. A
Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche

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

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.

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

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.

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 - 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

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

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 - 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.

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.

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.

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

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 - 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

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.