Redox-induced changes in the geometry and electronic structure of di-μ-oxo-bridged manganese dimers

Abstract

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 t(2g) subset of the parent octahedron. The additional two electrons in Mn2(III/III) enter d(z2) orbitals aligned along the Mn--N(ax) axis, resulting in a significant elongation of these bonds. Both d(x2-y2) and d(z2) orbitals transform as a1 in C(2v) symmetry, and so electron density can be transferred from the d(z2) orbital on one center to the d(x2-y2) orbital on the other. In the symmetric dimers, Mn2(IV/IV) and Mn2(III/III) the energetic separation of the d(z2) and d(x2-y2) 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(z2) and d(x2-y2) accounts for the structural trends within the series: the loss of electron density from the d(z2) orbital at the Mn(III) site of Mn2(III/IV) shortens the Mn(III)-N(ax) bond relative to that in the symmetric Mn2(III/III) system. In contrast, the Mn(IV) 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(x2-y2) 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 spectrum of the oxygen-evolving complex. Show more