Serguei A. Borshch

A mixed-valence polyoxovanadate(III,IV) cluster with a calixarene cap exhibiting ferromagnetic V(III)-V(IV) interactions.

A series of compounds (cat)[V6O6(OCH3)8(calix)(CH3OH)] was obtained under anaerobic conditions and solvothermal reaction of VOSO4 with p-tert-butylcalix[4]arene (calix) in methanol using different types of bases (Et4NOH, NH4OH, pyridine, Et3N). All compounds contain the same polyoxo(alkoxo)hexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) exhibiting a mixed valence {VIIIVIV5O19} core with the so-called Lindqvist structure coordinated to a calix[4]arene macrocycle and cocrystallizing with the conjugated acid of the base (cat = Et4N+, NH4(+), pyridinium, Et3NH+) involved in the synthesis process. The structures have been fully established from X-ray diffraction on single crystals and the mixed valence state has been confirmed by bond valence sum calculations. The magnetic behavior of all compounds are the same because of the polyalkoxohexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) and have been interpreted by DFT calculations. Thus the V(III)...V(IV) interactions are found to be weakly ferromagnetic (<5.5 cm(-1)) while the V(IV)...V(IV) are antiferromagnetic (-17.6; -67.6 cm(-1)). The set of the coupling exchange parameters allows a good agreement with the magnetic experimental data.

Theoretical Analysis of Spin Crossover in Iron(II) [2 × 2] Molecular Grids

We use quantum-chemical density functional theory calculations to elucidate the origin of spin-crossover pathways in two iron(II) [2×2] molecular grids with carbohydrazide-based bridging ligands. The complexes are characterized energetically and structurally in five available spin states. Special attention is paid to analysis of the structural distortion induced on each iron center by spin transition on any of its neighbors. The evolution of coordination polyhedra is monitored using the Continuous Shape Measures. It is demonstrated that a succession of spin transitions on different centers depends on the character of the induced distortion, either approaching or getting them away from a more regular low-spin geometry. These effects, resulting from the elasticity of bridging ligands, can be modulated by weak perturbations such as a change of the positions of the hydrogen atoms.

Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

This paper reports the synthesis of a family of mononuclear complexes [Fe(L)]X(2) (X=BF(4), PF(6), ClO(4)) with hexadentate ligands L=Hpy-DAPP ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine}), Hpy-EPPA ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and Hpy-DEPA ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}). The systematic change of the length of amino-aliphatic chains in these ligands results in chelate rings of different size: two six-membered rings for Hpy-DAPP, one five- and one six-membered rings for Hpy-EPPA, and two five-membered rings for Hpy-DEPA. The X-ray analysis of three low-spin complexes [Fe(L)](BF(4))(2) revealed similarities in their molecular and crystal structures. The magnetic measurements have shown that all synthesized complexes display spin-crossover behavior. The spin-transition temperature increases upon the change from six-membered to five-membered chelate rings, clearly demonstrating the role of the ligand strain. This effect does not depend on the nature of the counter ion. We discuss the structural features accountable for the strain effect on the spin-transition temperature.

Spin Crossover in Tetranuclear Cyanide-Bridged Iron(II) Square Complexes: A Theoretical Study

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.

EPR study of the magnetic states of a mixed-valence VIV4VV2 alkoxypolyoxovanadium cluster

The EPR studies of the hexanuclear vanadium cluster in a [VIV4VV2O7(OCH3)12] single crystal are reported. The spectrum of the mixed valence clusters consists of a single lorentzian line and additional signals in the magnetic field range ∼240–440 mT and is observed in the 4.2–300 K temperature range. At about 190 K, on cooling, an abrupt change of the spectrum due to the structural phase transition, followed by a strong hysteresis, is discovered. Temperature dependence of the EPR spectra indicates an electron transfer between VIV and VV. Obtained results are discussed in the framework of the Heisenberg–Anderson–Zener Hamiltonian approach. The possibilities of other approaches is also considered.

Exchange interactions in oxovanadium phosphates: towards the understanding of the magnetic patterns

Abstract We report the calculations of exchange magnetic constants in a series of vanadium phosphorus oxides for which structural and magneto-chemical data are available. Four different types of dimers have been extracted from the crystal lattices in all solids studied. On the basis of a combined density functional theory broken symmetry approach, our calculations on molecular models allow us to define schemes of magnetic interactions. The largest absolute magnetic interaction is provided by the double O–P–O and di-μ-oxo bridges, suggestive of an alternating dimer chain model and an isolated dimer one for the VO(HPO4)·0.5H2O and α-VO(HPO4)·2H2O phases, respectively. Conversely, VO(HPO4)·4H2O is consistent with a bi-dimensional magnetic pattern whereas VO(H2PO4)2 and α-VO(PO3)2 with three-dimensional magnetic schemes. The use of dimer models has been justified by the analysis of higher-nuclearity clusters.

Spin filtering in molecular junction: Magnetoresistance evaluation from wave-function calculations

The conductance of magnetic molecules opens new ways to probe the electronic structure of correlated systems. Based on a 2-electron/2-molecular orbital prototype system, the current-potential characteristics is inspected as a function of the differential magnetization of the electrodes sandwiching the molecule within a multideterminantal framework. The bias-dependent magnetoresistance effect along the junction reflects the nature and energetics of the different multiplets, obtained within the multiconfigurational wave-function approach. From the wave-function description, a modulation of the magnetoresistance ratio is anticipated and both direct and inverse regimes are observed depending on the electronic structure of the junction.

Theoretical analysis of the vanadyl pyrophosphate (VO)2P2O7 31P NMR spectra

Abstract We present a theoretical analysis of the temperature dependence of the vanadyl pyrophosphate VO2P2O731P NMR spectra. Four distinct phosphorus sites responsible for four signals are identified in the crystal structure. The magnetic states of the crystal are described by two alternative models: the spin ladder and the dimer chain. Within both models, finite clusters with and without periodic conditions are considered. The fit of the experimental NMR data allows us to define combinations of hyperfine coupling parameters which are found to be similar in both spin models.

Comparison of polyoxo and polyoxothiometallate rings: a theoretical approach

The electronic structures of a synthesized oxothiomolybdate ring and its hypothetic oxo analog were studied by DFT calculations. Different intermediate structures, which can appear during ring formation, were analyzed and their stabilities systematically compared. No significant difference between the two systems has been found in the ring assembly processes from elementary bricks [Mo2O2X2(H2O)6]2+ (X=S, O). It is suggested that the assembly of the oxo ring can be perturbed by the coexistence of two close-in-energy isomers for the oxo dication [Mo2O4(H2O)6]2+, having short and long Mo–Mo distances.

Electron Transport Through a Molecular Junction Using a Multi-configurational Description

The importance of the electronic description of the junction electronic structure is investigated in quantum transport through molecular devices. Using an accurate wave-function-based description of the low-energy spectroscopy, transport through a 2-electron/2-molecular orbital prototype is evaluated. The contributions arising from the presence of competing singlet and triplet states in magnetic systems are analyzed. It is shown that the electronic conductivity provides a signature of the full multiplet energy spectrum, as well as of the multideterminant structure of wave-functions. We then inspected the current-potential characteristics as a function of the differential magnetization of the electrodes. From the wave-function description, a modulation of the magnetoresistance ratio is anticipated and both direct and inverse regimes are observed depending on the electronic structure of the junction.