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Luca RIGAMONTI

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


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Pubblicazioni

2021 - Bringing homogeneous iron catalysts on the heterogeneous side: solutions for immobilization [Articolo su rivista]
Moccia, F.; Rigamonti, L.; Messori, A.; Zanotti, V.; Mazzoni, R.
abstract

Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an FeII-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, FeIII ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C–H bond activation.


2021 - Structure illumination microscopy imaging of lipid vesicles in live bacteria with naphthalimide-appended organometallic complexes [Articolo su rivista]
Maria Ranieri, Anna; Vezzelli, Matteo; Leslie, Kathryn G; Huang, Song; Stagni, Stefano; Jacquemin, Denis; Jiang, Haibo; Hubbard, Alysia; Rigamonti, Luca; Watkin, Elizabeth L J; Ogden, Mark I; New, Elizabeth J; Massi, Massimiliano
abstract

There is a lack of molecular probes for imaging bacteria, in comparison to the array of such tools available for the imaging of mammalian cells. Here, organometallic molecular probes have been developed and assessed for bacterial imaging, designed to have the potential to support multiple imaging modalities. The chemical structure of the probes is designed around a metal-naphthalimide structure. The 4-amino-1,8-naphthalimide moiety, covalently appended through a pyridine ancillary ligand, acts as a luminescent probe for super-resolution microscopy. On the other hand, the metal centre, rhenium(i) or platinum(ii) in the current study, enables techniques such as nanoSIMS. While the rhenium(i) complex was not sufficiently stable to be used as a probe, the platinum(ii) analogue showed good chemical and biological stability. Structured illumination microscopy (SIM) imaging on live Bacillus cereus confirmed the suitability of the probe for super-resolution microscopy. NanoSIMS analysis was used to monitor the uptake of the platinum(ii) complex within the bacteria and demonstrate the potential of this chemical architecture to enable multimodal imaging. The successful combination of these two moieties introduces a platform that could lead to a versatile range of multi-functional probes for bacteria.


2021 - When the metal makes the difference: template syntheses of tridentate and tetradentate salen-type schiff base ligands and related complexes [Articolo su rivista]
Mazzoni, R.; Roncaglia, F.; Rigamonti, L.
abstract

The reaction of organic molecules mediated by a metal center (template synthesis) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N2O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a template effect will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.


2020 - From solid state to in vitro anticancer activity of copper(ii) compounds with electronically-modulated NNO Schiff base ligands [Articolo su rivista]
Rigamonti, L.; Reginato, F.; Ferrari, E.; Pigani, L.; Gigli, L.; Demitri, N.; Kopel, P.; Tesarova, B.; Heger, Z.
abstract

The copper(ii) complexes of general formula [Cu(GL)(Cl)] (1-3, G = OMe, H and NO2, respectively), bearing tridentate Schiff base ligands (GL-) and a chloride as a fourth labile one, are here reported. The Schiff bases derive from the monocondensation of ethylenediamine and substituted salicylaldehyde, where the electronic properties are modulated by the releasing or withdrawing power of the G group. The compounds were structurally characterized through single crystal Synchrotron X-ray diffraction experiments in the solid state, revealing that 1 (OMe) and 2 (H) adopt a dimeric assembly [Cu(μ-Cl)(GL)]2 through apical interaction of the chloride ions of two monomeric units, while 3 embraces a 1D polymeric chain structure [Cu(μ-Cl)(NO2L)]n with a similar bridging fashion, all supported by extended intramolecular or intrachain hydrogen bonds. The redox properties of the complexes were also studied by cyclic voltammetry with no marked effect of the substituent on the potential of the CuII/CuI redox system. UV/Vis spectroscopic studies in mimicked physiological conditions highlighted the intactness and stability of the coordinated NNO tridentate ligand in 1-3 and the lability of the coordinated chloride ion with the formation of the aquo-complexes [Cu(GL)(H2O)]+ in aqueous solution, as confirmed by conductance measurements with a 1 : 1 electrolyte molar conductivity. In vitro tests on cell viability were conducted on malignant cell lines typical for their poor prognosis and curability, revealing time-dependent and differential cytotoxicity given by the substituent G. All compounds were capable of formation of intracellular reactive oxygen species and DNA intercalation, acting as nuclease and producing double-strand DNA breaks. This is especially effective for 3 (NO2), which revealed the highest anticancer activity against malignant triple-negative breast cancer MDA-MB-231 cells, with a two-to-four-fold cytotoxicity enhancement with respect to 1 (OMe) and 2 (H), and, most important, substantial differentiation of cytotoxicity with respect to healthy endothelial HUVEC cell line.


2020 - Selective formation, reactivity, redox and magnetic properties of MnIII and FeIII dinuclear complexes with shortened salen-type schiff base ligands [Articolo su rivista]
Rigamonti, L.; Zardi, P.; Carlino, S.; Demartin, F.; Castellano, C.; Pigani, L.; Ponti, A.; Ferretti, A. M.; Pasini, A.
abstract

The reactivity of the shortened salen-type ligands H3salmp, H2salmen and H2sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F, Cl, CF3, NO2) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M2(m-salmp)2], M = Mn (1a), Fe (2a), [M2(m-salmen)2(m-OR)2)], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M2(m-sal[p-X]ben)2(m-OMe)2), X = tBu, Me, H, F, Cl, CF3, NO2 and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt3, and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a.2AcOEt, 2a.2CH3CN and 3c.2DMF was performed. In the case of iron(III) and H3salmp, when using NaOH as a base instead of NEt3, the dinuclear complexes [Fe2(m-salmp)(m-OR)(salim)2], R = Me, H (2b–c) could be isolated and spectroscopically characterized, including the crystal structure of 2b.1.5H2O, which showed that rupture of one salmp3– to two coordinated salim– ligands and release of one salH molecule occurred. The same hydrolytic tendency could be identified with sal(p-X)ben ligands in the case of iron(III) also by using NEt3 or upon standing in solution, while manganese(III) did not promote such a C–N bond breakage. Cyclic voltammetry studies were performed for 3b, 4b, 5a and 6a, revealing that the iron(III) complexes can be irreversibly reduced to the mixed-valence FeIIFeIII and FeII2 dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence MnIIIMnIV or to the MnIV2 dinuclear species. The super-exchange interaction between the metal centers, mediated by the bridging ligands, resulted in being antiferromagnetic (AFM) for the selected dinuclear compounds 3b, 4b, 5a, 5e, 5f, 6a and 6e. The coupling constants J (–2J Ŝ1·Ŝ2 formalism) had values around –13 cm–1 for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear MnIII2 derivatives, while values between –3 and –10 cm–1 were obtained for iron(III) compounds.


2020 - Synthesis and reactivity of poly(propyleneimine) dendrimers functionalized with cyclopentadienone n-heterocyclic-carbene ruthenium(0) complexes [Articolo su rivista]
Cesari, C.; Conti, R.; Cingolani, A.; Zanotti, V.; Cassani, M. C.; Rigamonti, L.; Mazzoni, R.
abstract

Ligand design in metal chemistry is a fundamental step when pursuing compounds with specific reactivity. In this paper, the functionalization of the OH group in the lateral chain of the N-heterocyclic-carbene (NHC) ligand bound to a bis-carbonyl cyclopentadienone NHC ruthenium(0) complex allowed the decoration of five generations of poly(propyleneimine) (PPIs) dendrimers with up to 64 organometallic moieties. The coupling was achieved by employing carbonyldiimidazole and the formation of carbamate linkages between dendritic peripheral NH2 and lateral OH groups on ruthenium complexes. The synthetic procedure, chemical purification, and spectroscopic characterization of the five generations of dendrimers (3g1–5) are here described. The ruthenium-modified dendrimers were activated as catalysts in the transfer hydrogenation of the model compound 4-fluoroacetophenone in the presence of cerium ammonium nitrate as their mononuclear congeners. The catalytic activity, being similar for the five generations, shows a decrease if compared to mononuclear complexes. This detrimental effect might be ascribed to the –CH2 NH– functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, but also partially to a dendritic effect caused by steric encumbrance.


2019 - Cytocompatibility of potential bioactive cerium-doped glasses based on 45S5 [Articolo su rivista]
Malavasi, Gianluca; Salvatori, Roberta; Zambon, Alfonso; Lusvardi, Gigliola; Rigamonti, Luca; Chiarini, Luigi; Anesi, Alexandre
abstract

The cytocompatibility of potential bioactive cerium-containing (Ce 3+ /Ce 4+ ) glasses is here investigated by preparing three different glasses with increasing amount of doping CeO 2 (1.2, 3.6 and 5.3 mol% of CeO 2 , called BG_1.2, BG_3.6 and BG_5.3, respectively) based on 45S5 Bioglass®(called BG). These materials were characterized by Environmental Scanning Electron Microscopy (ESEM) and infrared spectroscopy (FTIR) after performing bioactivity tests in Dulbecco's Modified Eagle Medium (DMEM) solution, and the ions released in solution were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Optical Emission Spectrometry (ICP-OES). The data obtained clearly show that the glass surfaces of BG, BG_1.2 and BG_3.6 were covered by hydroxyapatite (HA), while BG_5.3 favored the formation of a cerium phosphate crystal phase. The cytotoxicity tests were performed using both murine long bone osteocyte-like (MLO-Y4) and mouse embryonic fibroblast (NIH/3T3) cell lines. The cerium-containing bioactive glasses show an increment in cell viability with respect to BG, and at long times, no cell aggregation and deformation were observed. The proliferation of NIH/3T3 cells increased with the cerium content in the glasses; in particular, BG_3.6 and BG_5.3 showed a higher proliferation of cells than the negative control. These results highlight and enforce the proposal of cerium-doped bioactive glasses as a new class of biomaterials for hard-tissue applications.


2019 - Push-pull unsymmetrical substitution in nickel(II) complexes with tetradentate N2O2 Schiff base ligands: Synthesis, structures and linear-nonlinear optical studies [Articolo su rivista]
Rigamonti, Luca; Forni, Alessandra; Righetto, Stefania; Pasini, Alessandro
abstract

New push-pull (A-D) nickel(II) compounds of general formula [Ni(5-A-5'-D-saltn)] (3a-3l) with unsymmetrically-substituted N2O2 tetradentate Schiff base ligands are reported here. The ligands 5-A-5'-D-saltn(2-) (H(2)saltn = N,N'-bis(salicylidene) diaminopropane) possess differently-substituted salicylaldehyde ((A/D)sal) moieties condensed to 1,3-diaminopropane (tn), and carry either an electron acceptor (A = H, Br, or NO2) or donor (D = H, Me, or OMe) group in the para position with respect to the coordinated phenoxido oxygen atoms. These compounds could be obtained by template synthesis involving derivatives [Ni(Gsal)(2)(H2O)(2)], 1a-e (G = NO2, Br, H, Me and OMe, respectively) and [Ni(L-G)2], 2a-d (L-G-= (E)2-(( 3-aminopropylimino) methyl)-4-G-phenolate, G = NO2, Br, H and Me, respectively). Scrambling of the ligands and condensation to compounds 3 was suitably achieved by refluxing compounds 1 and 2 that carry the G groups suitable for the desired final A-D combination. Dinuclear intermediates [Ni-2(mu-L-G)2((')G'sal)(2)] (4a, b, e, f, g) were also detected and isolated. The single-crystal X-ray diffraction structures of [Ni(5'-OMe-saltn)]center dot CHCl3 (3c center dot CHCl3), [Ni(5-Br-5'-OMe-saltn)]center dot EtOH (3g center dot EtOH) and [Ni(5-NO2-saltn)] (3j) show different degrees of distortion around the central core, leading to saddle-like (3c), planar (3g) and step-like (3j) molecular conformations. DFT geometrical optimization of the compounds showed that, for isolated molecules, the saddle-like conformation is slightly more stable with respect to the other conformations. UV-visible absorption spectra showed structured absorption profiles at about 320-440 nm, whose intensity was amplified by the presence of the nitro group, and this was assigned to a convolution of one metal-to-ligand charge transfer and two intra-ligand charge transfer transitions by TDDFT computations. Surprisingly, UV-visible spectra of the derivatives with Br were comparable to the derivatives with Me, suggesting that in this case the behaviour of the halogen was as a weak electron donor group. The experimental investigation (through electric-field-induced second-harmonic and solvatochromic measurements) of the second-order NLO responses of compounds 3, in conjunction with the theoretical calculations, revealed that the observed NLO efficiency is determined by the combined effect of two almost orthogonal charge transfer directions within the molecules, one along the axis approximately bisecting the donor and the accepting moieties and the other along the A-D axis.


2019 - Solid-state Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands [Articolo su rivista]
Rigamonti, L.; Forni, A.; Cariati, E.; Malavasi, G.; Pasini, A.
abstract

Salen-type metal complexes have been actively studied for their nonlinear optical (NLO) properties, and push-pull compounds with charge asymmetry generated by electron releasing and withdrawing groups have shown promising results. As a continuation of our research in this field and aiming at solid-state features, herein we report on the synthesis of mononuclear copper(II) derivatives bearing either tridentate N2O Schiff bases L(a-c)- and pyridine as the forth ancillary ligand, [Cu(La-c)(py)](ClO4) (1a-c), or unsymmetrically-substituted push-pull tetradentate N2O2 Schiff base ligands, [Cu(5-A-5'-D-saldpen/chxn)] (2a-c), both derived from 5-substituted salicylaldehydes (sal) and the diamines (1R,2R)-1,2-diphenylethanediamine (dpen) and (1S,2S)-1,2-diaminocyclohexane (chxn). All compounds were characterized through elemental analysis, infrared and UV/visible spectroscopies, and mass spectrometry in order to guarantee their purity and assess their charge transfer properties. The structures of 1a-c were determined via single-crystal X-ray diffraction studies. The geometries of cations of 1a-c and of molecules 2a-c were optimized through DFT calculations. The solid-state NLO behavior was measured by the Kurtz-Perry powder technique @1.907 µm. All chiral derivatives possess non-zero quadratic electric susceptibility (χ(2)) and an efficiency of about 0.15-0.45 times that of standard urea.


2019 - Synthesis, reactivity and preliminary biological activity of iron(0) complexes with cyclopentadienone and amino-appended N-heterocyclic carbene ligands [Articolo su rivista]
Cingolani, Andrea; Zanotti, Valerio; Zacchini, Stefano; Massi, Massimiliano; Simpson, Peter V.; Maheshkumar Desai, Nima; Casari, Ilaria; Falasca, Marco; Rigamonti, Luca; Mazzoni, Rita
abstract

Neutral and cationic cyclopentadienone (CpO) N-heterocyclic carbene (NHC) bis-carbonyl iron(0) complexes bearing, appended to the NHC ligand, either a terminal amino group on the lateral chain, [Fe(η4-CpO)(CO)2(κC-NHC(CH2)nNH2)] with n = 2 (2a) and 3 (2b), or a cationic NMe3+ fragment, [Fe(η4-CpO)(CO)2(κC-NHC(CH2)2NMe3)](I) (3), were prepared and characterized in terms of their structure, stability and reactivity. The photochemical properties of 2a and 2b were examined both in organic solvents and in water, revealing the photoactivated release of one CO ligand followed by the formation of the chelated complex [Fe(η4-CpO)(CO)(κ2C,N-NHC(CH2)2NH2)] (4), whose molecular structure was confirmed by single crystal X-ray diffraction studies. This metallacyclization occurs only in the case of 2a, with the ethylene spacer between NHC ring and NH2 group in the lateral chain, allowing the formation of a stable 6-membered ring. On the other hand, 2b undergoes decomposition upon irradiation. The reactivity in aqueous solutions revealed the chemical speciation of the complexes at different pH and especially under physiological conditions (phosphate buffer solution at pH 7.4 and 37 °C). The lack of data on the biological properties of iron(0) complexes prompted us to preliminarily investigate their cytotoxicity against model cancer cells (AsPC-1 and HPAF-II), along with a determination of their lipophilicity.


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 - Curcumin derivatives and Aβ-fibrillar aggregates: An interactions’ study for diagnostic/therapeutic purposes in neurodegenerative diseases [Articolo su rivista]
Orteca, Giulia; Tavanti, Francesco; Bednarikova, Zuzana; Gazova, Zuzana; Rigillo, Giovanna; Imbriano, Carol; Basile, Valentina; Asti, Mattia; Rigamonti, Luca; Saladini, Monica; Ferrari, Erika; Menziani, Maria Cristina
abstract

Several neurodegenerative diseases, like Alzheimer's (AD), are characterized by amyloid fibrillar deposition of misfolded proteins, and this feature can be exploited for both diagnosis and therapy design. In this paper, structural modifications of curcumin scaffold were examined in order to improve its bioavailability and stability in physiological conditions, as well as its ability to interfere with β-amyloid fibrils and aggregates. The acid-base behaviour of curcumin derivatives, their pharmacokinetic stability in physiological conditions, and in vitro ability to interfere with Aβ fibrils at different incubation time were investigated. The mechanisms governing these phenomena have been studied at atomic level by means of molecular docking and dynamic simulations. Finally, biological activity of selected curcuminoids has been investigated in vitro to evaluate their safety and efficiency in oxidative stress protection on hippocampal HT-22 mouse cells. Two aromatic rings, π-conjugated structure and H-donor/acceptor substituents on the aromatic rings showed to be the sine qua non structural features to provide interaction and disaggregation activity even at very low incubation time (2h). Computational simulations proved that upon binding the ligands modify the conformational dynamics and/or interact with the amyloidogenic region of the protofibril facilitating disaggregation. Significantly, in vitro results on hippocampal cells pointed out protection against glutamate toxicity and safety when administered at low concentrations (1 μM). On the overall, in view of its higher stability in physiological conditions with respect to curcumin, of his rapid binding to fibrillar aggregates and strong depolymerizing activity, phtalimmide derivative K2F21 appeared a good candidate for both AD diagnostic and therapeutic purposes.


2018 - Effect of crystal packing and coordinated solvent molecules on metal-ligand bond distances in linear trinuclear nickel compounds with bridging acetato and Schiff base ligands [Articolo su rivista]
Rigamonti, Luca; Forni, Alessandra
abstract

A combined experimental and theoretical study on the molecular structure of linear trinuclear nickel compounds of general formula [Ni3(μ-Zsaltn)2(μ-AcO)2(S)2] (Z = H, 1S, and Br, 2S, where S identifies coordinated solvent molecules) is here presented. In these complexes the three metal ions are linked together by bridging acetato ions and tetradentate Schiff base ligands, these last derived from the condensation of 1,3-diaminopropane with two equivalents of 5-Z-salicylaldehyde. Two solvent molecules S can also interact with the terminal nickel ions to complete an octahedral coordination environment. We report the synthesis of [Ni3(μ-Brsaltn)2(μ-AcO)2] (2), and its crystal structure with two coordinated dimethylformamide molecules, [Ni3(μ-Brsaltn)2(μ-AcO)2(DMF)2] (2DMF). Its comparison with the crystal structure of 2 without solvent, together with the series of compounds 1, 1DMF, 1MeOH, 1DMSO, 1Py and 1H2O (MeOH = methanol, DMSO = dimethylsulfoxide, Py = pyridine) available in the literature, allows recognising modification in Ni–O and Ni–N coordination bond lengths and peculiar intermolecular contacts between neighbour molecules. In order to discern among the contribution of solvent molecules, crystal packing and substituent Z (H or Br) on bond distances (and angles), we performed a systematic theoretical study on four representative derivatives, namely 1, 1DFM, 2 and 2DMF, with the M06 functional and a careful choice of the best basis set. The results indicate that, while the major modulating factor is the presence or absence of the sixth solvent molecule to the peripheral nickel(II) ion, even if less strongly coordinated with Ni–O/N bond distances > 2.15 Å π-π stacking interactions and C–H⋯π and/or C–Br⋯π non-bonded interactions affect the coordination geometry in a significant way.


2018 - Experimental and theoretical investigations on magneto-structural correlation in trinuclear copper(II) hydroxido propellers [Articolo su rivista]
Rigamonti, Luca; Forni, Alessandra; Sironi, Maurizio; Ponti, Alessandro; Ferretti, Anna M.; Baschieri, Carlo; Pasini, Alessandro
abstract

The trinuclear copper(II) compounds [Cu3(μ3-OH)(GL1)3](ClO4)2 (1–4) and [Cu3(μ3-OH)(GL2)3](ClO4)2 (5–8) with tridentate NNO Schiff base ligands GL1− and GL2− derived from 5-G-substituted salicylaldehydes (G = NO2, Br, H, Me) and the diamines 1,2-ethanediamine and 1,3-propanediamine, respectively, were investigated aiming at shedding light on possible magneto-structural correlation in this class of complexes. All derivatives contain [Cu3(μ3-OH)(L)3]2+ cations with partial cubane Cu3O4cores, and the metal ions are linked together in a pyramidal fashion by a triple-bridging hydroxido group, giving rise to propellers with three [Cu(L)]+ blades. In these spin-frustrated magnetic systems, the three copper(II) ions within a cluster communicate anti-ferromagnetically (−2J Ŝi·Ŝj convention) through the bridging OH group with coupling constants J ranging from −4.5(1) for 4 (G = Me) to −10.1(1) cm−1 for 1 (G = NO2), and stabilization of the doublet S = 1/2 ground state. The structural features of the complexes reveal very minimal deviations upon variation of G or the diamine flexibility along the whole series of compounds, preserving almost constant magnetic cores. Similar conclusions are also drawn by DFT gas-phase geometry optimizations of the [Cu3(μ3-OH)(L)3]2+ cations. Therefore, confident of excluding structural influences on the magnetic super-exchange path, the modulating factor of J in our derivatives can be sought after the different electronic demand of G. Atomic NBO charges support this point, revealing small but systematic variations in the electron density flow along the blades and the positive charge on copper(II) ions with the electronic nature of G, with the most remarkable effect given by the nitro group. Topological analysis of electron density according to the Quantum Theory of Atoms In Molecules further support the distinguishing role of this group with respect to the other substituents taken into consideration, besides providing indirect information about the super-exchange path.


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 - Highly-Bioreactive Silica-Based Mesoporous Bioactive Glasses Enriched with Gallium (III) [Articolo su rivista]
Sanchez-Salcedo, S.; Malavasi, Gianluca; Salinas, A. J.; Lusvardi, Gigliola; Rigamonti, Luca; Menabue, Ledi; Vallet-Regi, M.
abstract

Beneficial effects in bone cell growth and antibacterial action are currently attributed to Ga3+ ions. Thus, they can be used to upgrade mesoporous bioactive glasses (MBGs), investigated for tissue engineering, whenever they released therapeutic amounts of gallium ions to the surrounding medium. Three gallium-enriched MBGs with composition (in mol %) xSiO₂-yCaO-zP₂O₅-5Ga₂O₃, being x = 70, y = 15, z = 10 for Ga_1; x = 80, y = 12, z = 3 for Ga_2; and x = 80, y = 15, z = 0 for Ga_3, were investigated and compared with the gallium-free 80SiO₂-15CaO-5P₂O₅ MBG (B). 29Si and 31P MAS NMR analyses indicated that Ga3+ acts as network modifier in the glass regions with higher polymerization degree and as network former in the zones with high concentration of classical modifiers (Ca2+ ions). Ga_1 and Ga_2 exhibited a quick in vitro bioactive response because they were coated by an apatite-like layer after 1 and 3 days in simulated body fluid. Although we have not conducted biological tests in this paper (cells or bacteria), Ga_1 released high but non-cytotoxic amounts of Ga3+ ions in Todd Hewitt Broth culture medium that were 140 times higher than the IC90 of Pseudomonas aeruginosa bacteria, demonstrating its potential for tissue engineering applications.


2018 - New Silver(I) Coordination Polymer with Fe4 Single-Molecule Magnets as Long Spacer [Articolo su rivista]
Rigamonti, Luca; Vaccari, Manuela; Roncaglia, Fabrizio; Baschieri, Carlo; Forni, Alessandra
abstract

In continuation of our work on supramolecular architectures of single-molecule magnets (SMMs) as a promising strategy in developing their magnetic performance, in this paper we report the synthesis and single crystal X-ray structure of the centered triangular tetrairon(III) SMM, [Fe4(PhpPy)2(dpm)6], Fe4 (Hdpm = dipivaloylmethane, H3PhpPy = 2-(hydroxymethyl)- 2-(4-(pyridine-4-yl)phenyl)propane-1,3-diol), and its assembly in the coordination polymer {[Fe4(PhpPy)2(dpm)6Ag](ClO4)}n, Fe4Ag, upon reaction with silver(I) perchlorate. Thanks to the presence of the pyridyl rings on the two tripodal ligands, Fe4 behaves as divergent ditopic linker, and due to the Fe4:AgClO4 1:1 ratio, Fe4Ag probably possesses a linear arrangement in which silver(I) ions are linearly coordinated by two nitrogen atoms, forming 1D chains whose positive charge is balanced by the perchlorate anions. The stabilization of such a polymeric structure can be ascribed to the long distance between the two donor nitrogen atoms (23.4 Å) and their donor power. Fe4Ag shows slow relaxation of the magnetization which follows a thermally activated process with Ueff/kB = 11.17(18) K, t0 = 2.24(17) 107 s in zero field, and Ueff/kB = 14.49(5) K, t0 = 3.88(8) 107 s in 1-kOe applied field, in line with what reported for tetrairon(III) SMMs acting as building blocks in polymeric structures.


2018 - New curcumin-derived ligands and their affinity towards Ga3+, Fe3+and Cu2+: Spectroscopic studies on complex formation and stability in solution [Articolo su rivista]
Rigamonti, Luca; Orteca, Giulia; Asti, Mattia; Basile, Valentina; Imbriano, Carol; Saladini, Monica; Ferrari, Erika
abstract

The metal complexing ability in solution of four substituted curcumin (CUR)-derived ligands K3T, originated by the insertion of the -CH2CH2COOtBu branch on the central atom of the diketonic moiety of CUR and related derivatives with variable meta and para substituents (OH, OMe, H, OCOCH3) on the peripheral aromatic rings, is examined. These molecules can act as new chelators with biological properties comparable to those of CUR but with improved stability. In fact, curcuminoids represent new perspectives for the development of novel therapeutic agents for several diseases including Alzheimer's disease. CUR showed neuroprotective properties, and a probable mechanism of its action is related to the complexation ability towards endogenous metal ions Fe3+and Cu2+. K3T derivatives retain the solvent-dependent diketo-ketoenol tautomerism, with the prevalence of the diketonic form in aqueous solution. They show enhanced stability in simulated physiological conditions (phosphate buffered solution at pH = 7.4) compared to CUR, together with similar or even higher anti-proliferative activity against human colon carcinoma cells HCT116. The addition of the metal ion causes dissociation of the enolic proton creating chelate complexes and shift of the tautomeric equilibrium toward the keto-enol species. The formation of metal complexes was followed and confirmed by both NMR (using Ga3+as a diamagnetic probe for Fe3+) and UV-visible spectroscopy. All the ligands showed high affinity for Fe3+and Ga3+, forming M:L 1:2 species. In view of therapeutic applications, notable is the good affinity of K3T31, i.e. the ligand bearing only OH groups in para positions of the aromatic rings, for Cu2+, and the ability of the Cu:K3T31 1:1 complex to bind to DNA.


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 - 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).


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.


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 - 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.


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 - 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 - Valence electronic structure of sublimated Fe4 single-molecule magnets: An experimental and theoretical characterization [Articolo su rivista]
Silviya, Ninova; Valeria, Lanzilotto; Luigi, Malavolti; Rigamonti, Luca; Brunetto, Cortigiani; Matteo, Manini; Federico, Totti; Roberta, Sessoli
abstract

The valence electronic structures of two single-molecule magnets (SMMs), [Fe4(L)2(dpm)6] and [Fe4(L)2(pta)6], (Hdpm = dipivaloylmethane, Hpta = pivaloyltrifluoroacetone, L3- = Ph–C(CH2O)33-), are investigated by means of ultraviolet photoemission spectroscopy (UPS) and ab initio calculations. The experimental UPS spectra of both compounds are analysed and compared with the total density of states (TDOS) computed with the hybrid functional PBE0. The substitution of half of the methyl groups in [Fe4(L)2(dpm)6] with fluorine atoms in [Fe4(L)2(pta)6] unexpectedly affects the spectrum shape in the Fermi region, thus becoming a useful fingerprint of the two SMMs. Moreover, a computational protocol at DFT + U level of theory is assessed on both compounds, which is in good agreement with the experimental spectroscopic and magnetic data. The basis for the future modelling of the adsorption of Fe4 clusters on surfaces is established.


2013 - Copper 1D coordination polymers and dimers: Role of the carboxylate and the ammonium cation, crystal structures and magnetic studies [Articolo su rivista]
Rigamonti, Luca; Stefano, Carlino; Yassin, Halibi; Francesco, Demartin; Carlo, Castellano; Alessandro, Ponti; Roberta, Pievo; Alessandro, Pasini
abstract

The 1D coordination polymers [Cu2(syn,syn-g1:g1:l-RCOO)4(anti,anti-g1:g1:l-RCOO)(Et3NH)]n, R =Me (1) and H (2), and the dimer [Cu2(syn,syn-g1:g1:l-PhCOO)4(anti-g1-PhCOO)2(Et3NH)2] (3) are formed by reaction of copper carboxylates with the corresponding carboxylic acid and NEt3. Moreover, the compound [Cu2(syn,syn-g1:g1:l-MeCOO)4(anti-g1-MeCOO)2(Et2NH2)2] (4) can be obtained with the use of NHEt2. The crystal structures of 1CH3CN and 2 comprise linear anionic 1D chains of paddle-wheel copper carboxylates [Cu2(RCOO)4], bridged together by anti,anti-g1:g1:l acetato or formato ions. The negative charges are balanced by hydrogen-bonded triethylammonium countercations, and in compound 1 acetonitrile molecules are hosted in the voids between the chains. In compounds 3 and 4, two anti-g1-benzoato or -acetato anions occupy the apical positions of the paddle-wheel dimers [Cu2(RCOO)4], with Et3NH+ or Et2NH2+ cations balancing the negative charges, without the formation of coordination polymer chains. In compound 4, the dimeric units are held together in a 1D network by hydrogen bonds between the apical carboxylates and the diethylammonium cations. In order to evaluate the effect of the simultaneous presence of syn,syn- and anti,anti-g1:g1:l acetato ions on the Cu–Cu magnetic communication, the magnetic susceptibility of 1 has been measured. Data were fitted to a model based on an alternating chain of copper dimers based on the Heisenberg–Zeeman Hamiltonian (2J Sˆ 1Sˆ 2 convention). The results show the presence of a strong antiferromagnetic coupling within the dimer 2J1 = (310.8 ± 0.1) cm1 and a weak antiferromagnetic coupling between dimers 2J2 = (25 ± 1) cm1. The latter coupling is significant since its introduction in the fit model leads to a well-behaved error surface and to a 35% decrease of the sum of squared residuals. The electron paramagnetic resonance (EPR) spectra of powder sample of 1 have been recorded between 5 and 295 K, and they are typical of the triplet state of an antiferromagnetic copper dimer (zero field splitting |D| = 0.34 cm1).


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.


2012 - Copper(II) compounds with NNO tridentate Schiff base ligands: effect of subtle variations in ligands on complex formation, structures and magnetic properties [Articolo su rivista]
Rigamonti, Luca; Alessandra, Forni; Roberta, Pievo; Jan, Reedijk; Alessandro, Pasini
abstract

The formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1 = (E)-2-((3-aminoethylimino)methyl)phenolate or L2 = (E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monocondensation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) and pyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear derivatives, whose nature depends on the length of the diamine used and hence on the size of the corresponding metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) in compound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decomposition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres by substitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c), with exclusion of pyridine, or the trinuclear [Cu3(L2)3(l3-OH)](ClO4)2 (2d), where pyrazine acts as base generating OH, and it does not appear in the product. Reaction of 1a with pyrazine yields only the trinuclear [Cu3(L1)3(l3-OH)](ClO4)2 (1d). Also with 2,20-bipyridine (2,20-bpy) dinuclear complexes are not formed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear compounds [Cu(L1)(2,20-bpy)](ClO4) (1e) and [Cu(L2)(2,20-bpy)](ClO4) (2e). The crystal structures of compounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities vM(T) of 1a and 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling (J = +0.96(3) cm1) through the short dimeric CuO contacts is detected for 2a (tn, six-membered ring). The X-band EPR spectrum of 2a in a frozen methanol solution at 70 K shows the hyperfine coupling of mononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines.


2012 - Hydrogen bonding network of 4-amidiniumpyridine acetate and PtIIbis(triphenylphosphine) complexes with 4-amidinepyridine [Articolo su rivista]
Rigamonti, Luca; Stefano, Carlino; Carlo, Castellano; Francesco, Demartin; Alessandro, Pasini
abstract

The X-ray structures of 4-amidiniumpyridine acetate, (1·H)AcO, and of cis-[Pt(1) 2(PPh 3) 2](NO 3) 2 (2), as well as their IR spectra, reveal intramolecular hydrogen bonding, which held together the cations and the anions. The IR spectroscopic data suggest that this may be so also in cis-[PtCl(1)(PPh 3) 2](BF 4) (3). In (1·H)AcO and in 2 extensive intermolecular hydrogen bonding networks span through the whole crystals.


2011 - Synthesis, crystal structures and magnetic properties of dinuclear copper(II) compounds with NNO tridentate Schiff base ligands and bridging aliphatic diamine and aromatic diimine linkers [Articolo su rivista]
Rigamonti, Luca; Alessandra, Forni; Roberta, Pievo; Jan, Reedijk; Alessandro, Pasini
abstract

The synthesis and the characterization of new dinuclear copper(II) compounds of general formula [(La–d)2Cu2(m-N–N)](ClO4)2 (1–6) with either neutral aliphatic diamine (N–N = piperazine, pip) or aromatic diimine (N–N = 4,4¢-bipyridine, 4,4¢-bipy) linker are reported. The copper ligands L- (La- = (E)-2-((2-aminoethylimino)methyl)phenolate, Lb- = (E)-2-((2-aminopropylimino)methyl)-phenolate, Lc- = (E)-2-((2-aminoethylimino)methyl)4-nitrophenolate, Ld- = (E)-2-((2-aminoethylimino)methyl)4-methoxyphenolate) are NNO tridentate Schiff bases derived from the monocondensation of a substituted salicylaldehyde 5-G-salH (G = NO2, H, OMe) with ethylenediamine, en, or 1,3-propylenediamine, tn. The crystal structures of compounds [(La)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (1·2MeOH), [(Lb)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (2·2MeOH), [(Ld)2Cu2(m-4,4¢-bipy)](ClO4)2 (4), [(La)2Cu2(m-pip)](ClO4)2 (5) and [(Lb)2Cu2(m-pip)](ClO4)2 (6) have been determined, revealing the preferred (e-e)-chair conformation of the bridging piperazine in compounds 5 and 6. The presence of hydrogen-bond-mediated intermolecular interactions, that involve the methanol molecules, yields dimers of dinuclear units for 1·2MeOH, and infinite zig-zag chains for 2·2MeOH. The temperature dependences of the magnetic susceptibilities cM(T) for all compounds were measured, indicating the presence of antiferromagnetic Cu–Cu exchange. For the compounds 2–4 with 4,4¢-bipy, the coupling constants J are around -1 cm-1, while in compound 1 no interaction could be detected. The compounds 5 and 6 with piperazine display higher Cu–Cu magnetic interactions through the s-bonding backbone of the bridging molecule, with J around -8 cm-1, and the coupling is favoured by the (e-e)-chair conformation of the diamine ring. The non-aromatic, but shorter, linker piperazine gives rise to stronger Cu–Cu antiferromagnetic couplings than the aromatic, but longer, 4,4¢-bipyridine. In the latter case, the rotation along the C–C bond between the two pyridyl rings and the consequent non co-planarity of the two copper coordination planes play an important role in determining the magnetic communication. EPR studies reveal that the dinuclear species are not stable in solution, yielding the solvated [(L)Cu(MeOH)]+ and the mononuclear [(L)Cu(N–N)]+ species; it appears that the limited solubility of the dinuclear compounds is responsible for their isolation in the solid state.


2011 - The role of the atomic charges on the ligands and platinum(II) in affecting the cis and trans influences in [PtXL(PPh3)2]+ complexes (X = NO3, Cl, Br, I; L = 4-substituted pyridines, amines, PPh3). A 31P NMR and DFT investigation [Articolo su rivista]
Rigamonti, Luca; Michele, Rusconi; Alessandra, Forni; Alessandro, Pasini
abstract

One bond Pt–P coupling constants 1JPtP of a series of cationic complexes [PtXL(PPh3)2]+ (X = NO3, Cl, Br, I; L = 4-Z-pyridines, Z = electron withdrawing or releasing groups, 4a–k; or X = Cl, L = NH3, PhCH2NH2 and iPrNH2, 5a–c) have been used to establish the trans and cis influence sequences of X and pyridines. The crystal structure of compound 4f(BF4) with Z = tBu has been resolved. In the pyridine complexes 4a–d (Z = H, variable X), both the trans and cis influence series of the anionic ligands X decrease along the same sequence I > Br > Cl > NO3, as previously found for [PtX(PPh3)3]+ (X = NO3, Cl, Br, I, 3a–d), however in 4a–d the cis influence turns out to be more important than the trans. On the contrary, in [PtCl(4-Z-py)(PPh3)2]+ (4b,e–k) the sequence of the trans influence of the 4-Z-pyridines is opposite to that of the cis, the latter being Z = CN > CHO > Br > PhCO > H > Me > tBu > NH2, i.e. the most basic pyridine gives rise to the lowest cis influence. This correlation was found to hold also for complexes 5a–c (L = amines). All the observed trends have been fully reproduced by B3LYP/def2-SVP DFT calculations, by looking at the relevant optimized bond lengths of selected complexes of type 3, 4 and 5. Subsequent evaluation of the atomic charges, by resorting to two independent methods, i.e., the Natural Bond Order analysis of the wavefunction and the Bader’s Quantum Theory of Atoms in Molecules, allowed for rationalization of the origin of the cis and trans influences. The negative charge on the nitrogen atoms of free pyridines becomes more negative upon protonation and even more so when coordinated to the [PtCl(PPh3)2]+ moiety. The least negatively charged nitrogen atom of coordinated pyridines is that of 4-CN-py (the highest cis influencing pyridine derivative), which gives rise to the lowest positive charge on Pt, confirming the relationship between the lowering of the charge on the metal ion and a high cis influence. The trans influence can be described in terms of competition between the charges on the two trans donor atoms. In contrast with the behavior of pyridines, the positive charge on the phosphorous atom of free PPh3 increases upon coordination to Pt(II), moreover the PPh3 ligands acquire a substantial positive charge, thus efficiently delocalising the charge of the cationic complex.


2010 - Cooperation between cis and trans influences in cisPt(II)PPh3)2 complexes: structural, spectroscopic and computational studies [Articolo su rivista]
Rigamonti, Luca; Alessandra, Forni; Mario, Manassero; Carlo, Manassero; Alessandro, Pasini
abstract

The relevance of cis and trans influences of some anionic ligands X and Y in cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] complexes have been studied by the X-ray crystal structures of several derivatives (X2 = (AcO)2 (3), (NO3)2 (5), Br2 (7), I2 (11); and XY = Cl(AcO) (2), Cl(NO3) (4), and Cl(NO2) (13)), density functional theory (DFT) calculations, and one bond Pt-P coupling constants, 1JPtP. The latter have allowed an evaluation of the relative magnitude of both influences. It is concluded that such influences act in a cooperative way and that the cis influence is not irrelevant when rationalizing the 1JPtP values, as well as the experimental Pt-P bond distances. On the contrary, in the optimized geometries, evaluated through B3LYP/def2-SVP calculations, the cis influence was not observed, except for compounds ClPh (21), Ph2 (22), and, to a lesser extent, Cl(NO2) (13) and (NO2)2 (14). A natural bond order analysis on the optimized structures, however, has shown how the cis influence can be related to the s-character of the Pt hybrid orbital involved in the Pt-P bonds and the net atomic charge on Pt. We have also found that in the X-ray structures of cis-[PtX2(PPh3)2] complexes the two Pt-X and the two Pt-P bond lengths are different each other and are related to the conformation of the phosphine groups, rather than to the crystal packing, since this feature is observed also in the optimized geometries.


2010 - Quantification of cis and trans influences in [PtX(PPh3)3]+ complexes. A 31P NMR study [Articolo su rivista]
Rigamonti, Luca; Michele, Rusconi; Carlo, Manassero; Mario, Manassero; Alessandro, Pasini
abstract

In [PtX(PPh3)3]+ complexes (X = F, Cl, Br, I, AcO, NO3, NO2, H, Me) the mutual cis and trans influences of the PPh3 groups can be considered constants in the first place, therefore the one bond Pt–P coupling constants of P(cis) and P(trans) reflect the cis and trans influences of X. The compounds [PtBr(PPh3)3](BF4) (2), [PtI(PPh3)3](BF4) (3), [Pt(AcO)(PPh3)3](BF4) (4), [Pt(NO3)(PPh3)3](BF4) (5), and the two isomers [Pt(NO2-O)(PPh3)3](BF4) (6a) and [Pt(NO2-N)(PPh3)3](BF4) (6b) have been newly synthesised and the crystal structures of 2 and 4CH2Cl20.25C3H6O have been determined. From the 1JPtP values of all compounds we have deduced the series: I > Br > Cl > NO3 > ONO > F > AcO > NO2 > H >Me (cis influence) and Me > H > NO2 > AcO > I > ONO > Br > Cl > F > NO3 (trans influence). These sequences are like those obtained for the (neutral) cis- and trans-[PtClX(PPh3)2] derivatives, showing that there is no dependence on the charge of the complexes. On the contrary, the weights of both influences, relative to those of X = Cl, were found to depend on the charge and nature of the complex.


2010 - Schiff Base Metal Complexes for Second Order Nonlinear Optics [Articolo su rivista]
Rigamonti, Luca
abstract

The research in the development of new materials suitable for NLO purposes has increased in the last few decades. Coordination metal complexes with tetradentate Schiff base ligands are a good target to aim to, since they are robust, chemically and physically stable, and the degrees of variability that can lead to an enhancement of the NLO responses are great. All the chemical variations on the molecular structures (changes of the ligand skeleton and metals) studied are here reported, without going into details, in order to underline the fundamental role of the metal centre in the NLO properties and all the possible developments in the research in this specific field.


2009 - Length-Dependent Convergence and Saturation Behavior of Electrochemical, Linear Optical, Quadratic Nonlinear Optical, and Cubic Nonlinear Optical Properties of Dipolar Alkynylruthenium Complexes with Oligo(phenyleneethynylene) Bridges [Articolo su rivista]
Bandar, Babgi; Rigamonti, Luca; Marie P., Cifuentes; T., Christopher Corkery; Michael D., Randles; Torsten, Schwich; Simon, Petrie; Robert, Stranger; Ayele, Teshome; Inge, Asselberghs; Koen, Clays; Marek, Samoc; Mark G., Humphrey
abstract

The syntheses of trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (19), trans-[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4-CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppe)2] (21), trans-[Ru{4,4′,4′′,4′′′- CtCC6H4-CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (22), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23), and trans-[Ru{4,4′,4′′,4′′′,4′′′′-CtCC6H4-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24) are reported, together with those of precursor alkynes, complexes with the donor-π-bridge-acceptor formulation that affords efficient quadratic and cubic NLO compounds; the identity of 19 was confirmed by a structural study. The electrochemical properties of 19-24 and related complexes with shorter π-bridge ligands were assessed by cyclic voltammetry, and the linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties were assayed by UV-vis-NIR spectroscopy, hyper-Rayleigh scattering studies at 1064 and 1300 nm, and broad spectral range femtosecond Z-scan studies, respectively. The RuII/III oxidation potentials and wavelengths of the optical absorption maxima decrease on π-bridge lengthening, until the tri(phenyleneethynylene) complex is reached, further chain lengthening leaving these parameters invariant; theoretical studies employing time-dependent density functional theory have shed light on this behavior. The quadratic nonlinearity 1064 and two-photon absorption cross-section reach maximal values at this same π-bridge length, a similar saturation behavior that may reflect a common importance of ruthenium-to-alkynyl ligand charge transfer in electronic and optical behavior in these molecules.


2009 - Metal-containing Oligo(phenylenethynylene)s: Syntheses and Nonlinear Optical Properties [Articolo su rivista]
Marie P., Cifuentes; Mark G., Humphrey; Marek, Samoc; Bandar, Babgi; Gulliver T., Dalton; Rigamonti, Luca
abstract

Oligo(p-phenyleneethynylene)s (OPEs) have been of considerable interest in recent years. Introduction of ligated metal units into an OPE structure can add a new dimension: the resultant transition metal OPE complexes can potentially possess a number of useful properties such as luminescence, reversible redox chemistry and accessible mixed valence states. Such complexes have been examined for molecular electronics and liquid crystal applications, but physical properties of series of systematically-varied metal-terminated OPEs are yet to be explored. We report here syntheses of a series of metal-terminated OPEs (up to a hepta(phenyleneethynylene)), and examples with phenylenevinylene segments, the evolution of linear optical properties in metal-containing OPEs and hybrid OPE/OPVs (oligo(phenylenevinylene)s) upon chain lengthening, and a wavelength-dependence study of the cubic nonlinear optical (NLO) properties of metal-containing OPEs by fs Z-scan measurements.


2009 - Organometallic Complexes for Nonlinear Optics. 43. Quadratic Optical Nonlinearities of Dipolar Alkynylruthenium Complexes with Phenyleneethynylene/Phenylenevinylene Bridges [Articolo su rivista]
Rigamonti, Luca; Bndar, Babgi; Marie P., Cifuentes; Rachel L., Roberts; Simon, Petrie; Robert, Stranger; Stefania, Righetto; Ayele, Teshome; Inge, Asselberghs; Koen, Clays; Mark G., Humphrey
abstract

The syntheses of trans-[Ru(4,4′-C≡CC6H4C≡CC6H4NO2)Cl(dppe)2] (19) and the systematically varied complexes trans-[Ru(4,4′,4′′-C≡CC6H4X2C6H4Y2C6H4NO2)Cl(L2)2] [L2 ) dppe, X2 ) C≡C, Y2 ) (E)-CH)CH (12), C≡C (18); L2 ) dppe, X2 ) (E)-CH)CH, Y2 ) C≡C (14), (E)-CH)CH (16); L2 ) dppm, X2 ) C≡C, Y2 ) (E)-CH)CH (13); L2 ) dppm, X2 ) (E)-CH)CH, Y2 ) C≡C (15), (E)-CH)CH (17)] are reported, the latter being donorbridge-acceptor complexes varying in bridge composition by replacement of yne with E-ene linkages, together with their cyclic voltammetric data, linear optical, and quadratic nonlinear optical response data. RuII/III oxidation potentials increase on replacing yne linkage by E-ene linkage at the phenylene adjacent to the metal center, and on replacing dppe by dppm co-ligands. The low-energy optical absorption maxima occur in the region 20400-23300 cm-1 and are metal-to-ligand charge-transfer (MLCT) in origin; these bands undergo a blue-shift upon π-bridge lengthening by addition of phenyleneethynylene units, and on replacing E-ene linkages by yne linkages. Time-dependent density functional theory calculations on model complexes have suggested assignments for the low-energy bands. The optical spectra of selected oxidized species contain low-energy ligand-to-metal charge transfer (LMCT) bands centered in the region 9760-11800 cm-1. Quadratic molecular nonlinearities from hyper-Rayleigh scattering (HRS) studies at 1064 nm reveal an increase in the two-level-corrected 0 value on π-bridge lengthening, a trend that is not seen with values because of the blue-shift in λmax for this structural modification. Replacing yne linkages by E-ene linkage at the phenylene adjacent to the metal center or dppm co-ligand by dppe results in an increase in beta and beta(0) values. In contrast, quadratic molecular nonlinearities by HRS at 1300 nm or electric field-induced second harmonic generation (EFISH) studies at 1907 nm do not afford clear trends.


2009 - cis Influence in trans-Pt(PPh3)2 Complexes [Articolo su rivista]
Rigamonti, Luca; Carlo, Manassero; Michele, Rusconi; Mario, Manassero; Alessandro, Pasini
abstract

The cis influence of a series of anionic ligands X and Y has been evaluated through the magnitude of the Pt–P coupling constants for compounds of formula trans-[PtXY(PPh3)2]. The order of decreasing cis influence was found to be I > Cl > SePh ~ SPh ~ SEt > NO3 > AcO ~ NO2 > H > Me > Ph > mtc (mtc = N,N-dimethylmonothiocarbamato-S); moreover, the cis influences of the various ligands was found to be additive. The X-ray structures of three representative compounds (t-4: X = Cl, Y = NO3; t-5: X = Cl, Y = AcO and t-12: X = Y = NO2) have also been determined.


2008 - Copper(II) Complexes of Tridentate Schiff Bases of 5-Substituted Salicylaldehydes and Diamines – The Role of the Substituent and the Diamine in the Formation of Mono-, Di- and Trinuclear Species - Crystal Structures and Magnetic Properties [Articolo su rivista]
Rigamonti, Luca; Antonio, Cinti; Alessandra, Forni; Alessandro, Pasini; Olivo, Piovesana
abstract

We describe some copper(II) complexes with tridentate anionic Schiff bases, L–, derived from the monocondensation of some 5-substituted salicylaldehydes (5-G-salH, G = H, NO2 and OMe) with ethylenediamine (en) or 1,3-diaminopropane (tn). Mononuclear [CuL(py)]+, dinuclear [Cu2L2]2+ and trinuclear [Cu3L3(μ3-OH)]2+ compounds have been obtained, and factors affecting the nuclearity of complexes were investigated and found to depend on reaction conditions, the nature of G and the length of the diamine. The crystal structures of seven representative compounds have been determined. Magnetic studies on a dinuclear and a trinuclear species were also carried out, and the results were interpreted on the basis of the structural data and current superexchange models.


2006 - Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push-Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties [Articolo su rivista]
Rigamonti, Luca; Francesco, Demartin; Alessandra, Forni; Stefania, Righetto; Alessandro, Pasini
abstract

This paper presents some copper(II) complexes of salen analogues in which the two salicylaldehyde moieties carry different (electron donor, D, and acceptor, A) substituents in position 5, producing a push-pull charge asymmetry. The X-ray structures of some compounds show the presence of pairs of stacked molecules with head-to-tail intermolecular associations. The geometries of all complexes have been optimized through density functional theory (DFT) studies, which have shown that a major influence on the coordination bond lengths is given by the presence of the electron acceptor NO2 group. Such an influence operates mainly on the Cu-phenolato bonds: elongation of the Cu-O distance of the 5-nitrosalicylaldehyde moiety, with a concomitant decrease of the other Cu-O distance; the Cu-N bonds are less affected. The D groups have only a minor influence. The nonlinear optical responses, mu(g)beta(vec), of some molecules have been determined by EFISH measurements, and the beta(vec) values have been obtained using the DFT-calculated mu(g) values because solubility problems hampered the experimental measurements of mu(g) of some derivatives; the former, however, have been found to be in agreement with the experimental values that could be obtained. Deconvolution of the absorption bands in the near-UV region has allowed recognition of the charge-transfer (CT) transition, assigned to a ligand-to-metal CT (LMCT) by time-dependent DFT computations; we have then used the solvatochromism of this transition to obtain beta(0) and beta(CT) values using the two-state model. These values were compared with those obtained by computational studies, which have also allowed evaluation of the influence of the substituents on the directions of mu(g) and beta(tot).