DENSITY-FUNCTIONAL DESCRIPTION OF THE FERROMAGNETIC EXCHANGE INTERACTIONS BETWEEN SEMIQUINONATO RADICALS MEDIATED BY DIAMAGNETIC METAL-IONS

The electronic structures of Ti(CatNSQ)(2) and Sn(CatNSQ)(2), where Ca tNSQ(2-) is the tridentate radical ligand (3,5-di-tert-butyl-1,2-semiq uinonato 1(2-hydroxy-3,5-di-tert-butyl-phenyl)immine) were investigate d with density functional (DF) calculations, using the local approxima tion for the exchange-correlation functional. The crystal structure of Sn(CatNSQ)(2) was solved. The complex crystallizes in the orthorhombi c space group, C222(1), with Z = 8 in a unit cell of the following dim ensions: a 19.580(5) Angstrom, b = 24.310(5) Angstrom, c = 23.690(5) A ngstrom. The crystals are not isomorphous with similar M(CatNSQ)(2) (M = Ti, V) complexes previously reported. DF calculations showed that t he triplet (S = 1) spin state is stabilized with respect to the first excited singlet (S = 0) state and the computed exchange coupling const ant J is in semiquantitative agreement with the values obtained from m agnetic susceptibility measurements. Using a symmetry-based multiplet structure decomposition in terms of states defined by a single determi nant (single determinant method, SD) the energies of the excited singl et states were also computed in agreement with the experimental data. The calculations have shown that the main exchange mechanism between t he organic radicals, responsible for the ferromagnetism of these compl exes, is a superexchange pathway mediated by the 3d orbitals of Ti and the 4p empty orbitals of Sn. Magnetostructural correlations between t he exchange coupling constant and the M-O and M-N bond distances have been established.