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

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn2(μ-O)2(O2CH)(NH3)6] n+ and [Mn2(μ-O)(O2CH)2(NH3)6] n+ in the MnIVMnIV, MnIIIMnIV, and MnIIIMnIII 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 MnIII centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn2(μ-O)2(O2CH)(NH3)6] n+ involve the J xz / xz and J yz / yz pathways but due to the tilt of the Mn2O2 core, they are less efficient than in the planar di-μ-oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the MnIII centers are high-spin, and the Jahn−Teller effect gives rise to axially elongated MnIII geometries with the distortion axis along the Mn−Oc bonds. In the III/IV dimer, the tilt of the Mn2O2 core enhances the crossed exchange J x 2 - y 2 / z 2 pathway relative to the planar di-μ-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the MnIV ion into low-spin MnIII, the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between MnIV (S = 3/2) and high-spin MnIII (S = 2), the other is the result of coupling between MnIV (S = 3/2) and low-spin MnIII (S = 1). In [Mn2(μ-O)(O2CH)2(NH3)6] n+ , both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin MnIII, but the Jahn−Teller axis is now orientated along the Mn−oxo bond, giving rise to axially compressed MnIII geometries with long Mn−Oc 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 MnIII are almost degenerate. In the high-spin case, the odd electron is local