Modeling of Electron Transfer in a Linear Trinuclear Fe-Co-Fe Complex: Magnetic and Polarizability Properties.

A microscopic approach to the problem of charge transfer-induced spin transitions in a system of interacting trinuclear Fe-Co-Fe clusters, which takes into account the switching of polarization during the transformation FelsII-ColsIII-FelsIII→ FelsIII-CohsII-FelsIII (ls-low spin, hs-high spin), has been developed. The cooperative electron-deformational and dipole-dipole interactions and the intracluster electron transfer have been proved to govern the observed significant changes in the magnetic and polarization characteristics of the examined system. The role of intra- and intercluster interactions in the spin transformation has been elucidated. On the basis of the developed approach, an explanation of the temperature dependence of the magnetic susceptibility and of the polar-nonpolar transformation in the {[FeTp(CN)3]2Co(Meim)4}·6H2O compound in response to thermal stimuli has been given.

Modeling the magnetic properties and Mössbauer spectra of multifunctional magnetic materials obtained by insertion of a spin-crossover Fe(III) complex into bimetallic oxalate-based ferromagnets.

In this article, we present a theoretical microscopic approach to describe the magnetic and spectroscopic behavior of multifunctional hybrid materials which demonstrate spin crossover and ferromagnetic ordering. The low-spin to high-spin transition is considered as a cooperative phenomenon that is driven by the interaction of the electronic shells of the Fe ions with the full symmetric deformation of the local surrounding that is extended over the crystal lattice via the acoustic phonon field. The proposed model is applied to the analysis of the series [Fe(III)(sal2-trien)] [Mn(II)Cr(III)(ox)3]·solv, in short 1·solv, where solv = CH2Cl2, CH2Br2, and CHBr3.

A model of magnetic and relaxation properties of the mononuclear [Pc2Tb](-)TBA+ complex.

The present work is aimed at the elaboration of the model of magnetic properties and magnetic relaxation in the mononuclear [Pc(2)Tb](-)TBA(+) complex that displays single-molecule magnet properties. We calculate the Stark structure of the ground (7)F(6) term of the Tb(3+) ion in the exchange charge model of the crystal field, taking account for covalence effects. The ground Stark level of the complex possesses the maximum value of the total angular momentum projection, while the energies of the excited Stark levels increase with decreasing |M(J)| values, thus giving rise to a barrier for the reversal of magnetization. The one-phonon transitions between the Stark levels of the Tb(3+) ion induced by electron-vibrational interaction are shown to lead to magnetization relaxation in the [Pc(2)Tb](-)TBA(+) complex. The rates of all possible transitions between the low-lying Stark levels are calculated in the temperature range 14 K

Role of orbital degeneracy in the single molecule magnet behavior of a mononuclear high-spin Fe(II) complex.

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

Orbital angular momentum contribution to the magneto-optical behavior of a binuclear cobalt(II) complex.

We report magnetic and magnetic circular dichroism investigations of a binuclear Co(II) compound. The Hamiltonian of the system involves an isotropic exchange interaction dealing with the real spins of cobalt(II) ions, spin-orbit coupling, and a low-symmetry crystal field acting within the (4)T(1g) ground manifold of each cobalt ion. It is shown that spin-orbit coupling between this ground term and the low-lying excited ones can be taken into consideration as an effective g factor in the Zeeman part of the Hamiltonian. The value of this g factor is estimated for the averaged experimental values of Racah and cubic ligand field parameters for high-spin cobalt(II). The treatment of the Hamiltonian is performed with the use of a irreducible tensor operator technique. The results of the calculation are in good agreement with experimental observations. Both a large effective g factor for the ground state and a large temperature-independent part of the magnetic susceptibility arise because of a strong orbital contribution to the magnetic behavior of the Co(II) dimer.

Magnetic, spectroscopic, and structural studies of dicobalt hydroxamates and model hydrolases.

The cobalt(II) urease model complex [Co(2)(mu-OAc)(3)(urea)(tmen)(2)][OTf] (2) prepared from the cobalt model hydrolase [Co(2)(mu-H(2)O)(mu-OAc)(2)(OAc)(2)(tmen)(2)] (1) undergoes facile reaction with acetohydroxamic acid (AHA) to give the monobridged hydroxamate complex [Co(2)(mu-OAc)(2)(mu-AA)(urea)(tmen)(2)][OTf]( )()(3) while 1 gives the dibridged hydroxamate complex [Co(2)(mu-OAc)(mu-AA)(2)(tmen)(2)][OTf] (4). The structures and Co-Co distances of the hydroxamate derivatives of 1 and 2 are very close to those of their nickel analogues and suggest that hydroxamic acids can also inhibit cobalt-based hydrolases as well as inhibiting urease. 1 also reacts with glutarodihydroxamic acid (gluH(2)A(2)) to eliminate hydroxylamine with formation of [Co(2)(mu-OAc)(2)[mu-O(N) (OC)(2)(CH(2))(3)](tmen)(2)][OTf] (5), the structure of which is very close to that of its nickel analogue. Both 1 and 3 show weak antiferromagnetic coupling. Oxidation of 1 with H(2)O(2) gives three dicobalt(III) hydroxy complexes (7-9), the first of which [Co(2)(mu-OAc)(2)(OAc)(2)(mu-OH)(tmen)(2)][OTf] (7) contains a bridging hydroxyl and the second [Co(2)(mu-OAc)(2)(OAc)(mu-OH)(OH)(tmen)(2)][OTf] (8) containing both a bridging and terminal hydroxyl, while the third [Co(2)(mu-OAc) (OAc)(2)(mu-OH)(2)(tmen)(2)][OTf] (9) contains two bridging OH groups with mixed-valence Co(II)/(Co(III) intermediates.