Redox-induced changes in the geometry and electronic structure of di-mu-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(+) in three distinct oxidation states, Mn-2(IV/IV) (z = 4), Mn-2(III/IV) (z = 3) and Mn-2(III/III) (z = 2). In Mn-2/(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 t(2g), subset of the parent octahedron. The additional two electrons in Mn-2(III/III) enter d(z)(2) orbitals aligned along the Mn-N-ax axis, resulting in a significant elongation of these bonds. Both d(x)(2)-(2)(y) and d(z)(2) orbitals transform as a(1) in C-2v symmetry, and so electron density can be transferred from the d(z)(2) orbital on one center to the d(x)2-(2)(y) orbital an the other. In the symmetric dimers, Mn-2(IV/IV) and Mn-2(III/III) the energetic separation of the d(z)(2) and d(x)(2)-(2)(y) 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 Mn-IV and weak axial ligand field in Mn-III 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)-(2)(y) accounts for the structural trends within the series: the loss of electron density from the d(z)(2) orbital at the Mn-III site of Mn-2(III/IV) shortens the Mn-III-N-ax bond relative to that in the symmetric Mn-2(III/III) system. In contrast, the Mn-IV site in the mixed-valence species is almost identical with that in Mn-2(IV/IV) because the additional electron density enters a Mn-N nonbonding d(x)(2)-(2)(y) orbital. The magnetic properties of the dimers are dominated by the symmetric J(xz)/J(xz) and J(yz/yz) pathways, both of which are ideally oriented fur 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 mote acute O-Mn-O angle in Mn-2(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 Mn-2(III/IV) along the crossed pathway also provides a possible explanation for the highly anisotropic hyperfine signal observed in the EPR spectrum of the oxygen-evolving complex.