DENSITY-FUNCTIONAL STUDIES OF A HEISENBERG SPIN-COUPLED CHROMIUM-SEMIQUINONE COMPLEX AND ITS CHROMIUM-CATECHOL ANALOG

The electronic structure of [Cr(tren)(3,6-DTBSQ)](2+), where tren is t ris(2-aminoethyl)amine and 3,6-DTBSQ is 3, 6-di-tert-butylorthosemiqui none, has been studied by self-consistent-field non-local gradient-cor rected density functional theory. The results are consistent with a He isenberg exchange formulation where a Cr(S = 3/2) ion is antiferromagn etically coupled to the semiquinone(S = 1/2) giving rise to a S = 1 gr ound state. Population analyses were carried out which show net alpha and beta spin densities at the chromium ion and semiquinone, respectiv ely. Some orbital interactions have been identified that allow partial delocalization from the semiquinone toward the chromium ion giving ri se to an antiparallel alignment of their electron spins. The isotropic exchange constant J of the Heisenberg Hamiltonian H-ex = JS(1). S-2 h as been determined from the self-consistent-field energies at the U-BL YP/G-311* and U-B3LYP/6-311** levels and is consistent with previousl y reported magnetic susceptibility data. The tripler state wave functi on shows some spin contamination from the higher-lying quintet state, Accordingly, approximate spin and energy projections were performed to account for the quintet admixture. Some magneto-structural correlatio ns between the Cr-O-SQ and O-SQ-C-SQ bond lengths and the magnitude of J have also been investigated. It was found that J decays in a nearly exponential fashion with increasing bond distances. The electronic st ructure of the free semiquinone ligand has also been studied and corre lated to its bonding with chromium, Finally, a single-crystal X-ray st ructure of the related complex [Cr(tren)(3,6-DTBCat)](+) was obtained and used to carry out similar self-consistent-field calculations, Anal ysis of the quartet ground state wave function of the catecholate comp lex produced spin densities consistent with a Cr(S = 3/2)-catechol(S = 0) formulation.