Redox-Induced Changes in the Geometry and Electronic Structure of Di-μ-oxo-Bridged Manganese Dimers

Broken-symmetry approximate density functional theory has been used to investigate the electronic and structural properties of the complex Mn2O2(NH3)8 z + in three distinct oxidation states, Mn2 IV/IV (z = 4), Mn2 III/IV (z = 3) and Mn2 III/III (z = 2). In Mn2 IV/IV the metal-based electrons are almost completely localized on one center or the other, and occupy the single-ion orbitals derived from the t2g subset of the parent octahedron. The additional two electrons in Mn2 III/III enter d z 2 orbitals aligned along the Mn−Nax axis, resulting in a significant elongation of these bonds. Both d x 2 - y 2 and d z 2 orbitals transform as a1 in C 2 v symmetry, and so electron density can be transferred from the d z 2 orbital on one center to the d x 2 - y 2 orbital on the other. In the symmetric dimers, Mn2 IV/IV and Mn2 III/III, the energetic separation of the d z 2 and d x 2 - y 2 orbitals is sufficiently large to prevent significant delocalization of the metal-based electrons along this pathway. In contrast, a combination of low-spin polarization on MnIV and weak axial ligand field in MnIII combine to bring the two orbitals close together in the mixed-valence dimer, and the unpaired electron is significantly delocalized. The delocalization of the unpaired electron between d z 2 and d x 2 - y 2 accounts for the structural trends within the series: the loss of electron density from the d z 2 orbital at the MnIII site of Mn2 III/IV shortens the MnIII−Nax bond relative to that in the symmetric Mn2 III/III system. In contrast, the MnIV site in the mixed-valence species is almost identical with that in Mn2 IV/IV because the additional electron density enters a Mn−N nonbonding d x 2 - y 2 orbital. The magnetic properties of the dimers are dominated by the symmetric J xz/xz and J yz/yz pathways, both of which are ideally oriented for efficient superexchange via the oxo bridges. Redox-induced changes in the Heisenberg exchange coupling constant are caused by changes in geometry of the Mn2O2 core rather than by the generation of new pathways as a consequence of occupation of additional orbitals. The longer Mn−Mn separation and the more acute O−Mn−O angle in Mn2 IV/IV improve the efficiency of the J yz /J yz pathway, leading to larger coupling constants in the more oxidized species. The delocalization of the unpaired electron in Mn2 III/IV along the crossed pathway also provides a possible explanation for the highly anisotropic hyperfine signal observed in the EPR spec