Oxidation State Dependence of the Geometry, Electronic Structure, and Magnetic Coupling in Mixed Oxo- and Carboxylato-Bridged Manganese Dimers

摘要

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn_2(#mu#-O)_2(O_2CH)(NH_3)_6]~(n+) and [Mn_2(#mu#-O)(O_2CH)_2(NH_3)_6]~(n+) in the Mn~(IV)Mn~(IV), and Mn~(III)Mn~(IV), and Mn~(III)Mn~(III) oxidation states. The magnetic coupling in the dimer is profoundly affected by changes in both the bridging ligands and Mn oxidation state. In particular, change in the bridging structure has a dramatic effect on the nature of the Jahn-Teller distortion observed for the Mn~(III) centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn_2(#mu#-O)_2(O_2CH)(NH_3)_6]~(n+) involve the J_(xz/xz) and J_(yz/yz) pathways but due to the tilt of the Mn_2O_2 core, they are less efficicnt than in the planar di-#mu#--oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the Mn~(III) centers are high-spin, and the Jahn-Teller gives rise to axially elongated Mn~(III) geometries with the distortion axis along the Mn-O_c bonds. In the III/IV dimer, the tilt of the Mn_2O_2 core enhances the crossed exchange J_(x~2-y~2/z~2) pathway relative to the planar di-#mu#-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the Mn~(IV) ion into olw-spin Mn~(III), the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between Mn~(IV) (S = 3/2) and high-spin Mn~(III) (S = 2), the other is the result of coupling between Mn~(IV) (S = 3/2) and low-spin Mn~(III) (S = 1). IN [Mn_2(#mu#-O)(O_2CH)_2(NH_3)_6]~(n+), both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin Mn~(III), but the Jahn-Teller axis is now orientated along the Mn-oxo bond, giving rise to axially compressed Mn~(III) geometries with long Mn-O_c equatorial bonds. In the IV/IV dimer, the ferromagnetic crossed exchange J_(yz/z~2) pathway partially cancels J_(yz/yz) and, as a consequence, the antiferromagnetic J_(xz/xz) pathway dominates the magnetic coupling. In the III/III dimer, the J_(yz/yz) pathway is minimized due to the smaller Mn-O-Mn angle, and since the ferromagnetic J_(yz/z~2) pathway largely negates J_(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high-spin and low-spin Mn~(III) are almost degenerate. In the high-spin case, the odd electron is localized on the Mn~(III) center, and the resulting antiferromagnetic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly delocalized due to the crossed J_(yz/z~2) pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalization partially converts the Mn~(IV) ion into high-spin Mn~(III), and consequently, the spin ladders arising from coupling of Mn~(IV) (S = 3/2) with high-spin (S = 2) and low-spin (S = 1) Mn~(III) are configurationally mixed. Thus, in principle, the ground-state magnetic coupling in the mixed-valence dimer will involve contributions from three spin-ladders, two associated with the delocalized olw-spin structure and the third arising from the localized high-spin structure.