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

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Abstract

Broken-symmetry approximate density functional theory has been used to investigate the electronic and structural properties of the complex Mn2O2(NH3)8z+ in three distinct oxidation states, Mn2IV/IV (z = 4), Mn2III/IV (z = 3) and Mn2III/III (z = 2). In Mn2IV/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 Mn2III/III enter dz2 orbitals aligned along the Mn−Nax axis, resulting in a significant elongation of these bonds. Both dx2-y2 and dz2 orbitals transform as a1 in C2v symmetry, and so electron density can be transferred from the dz2 orbital on one center to the dx2-y2 orbital on the other. In the symmetric dimers, Mn2IV/IV and Mn2III/III, the energetic separation of the dz2 and dx2-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 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 dz2 and dx2-y2 accounts for the structural trends within the series: the loss of electron density from the dz2 orbital at the MnIII site of Mn2III/IV shortens the MnIII−Nax bond relative to that in the symmetric Mn2III/III system. In contrast, the MnIV site in the mixed-valence species is almost identical with that in Mn2IV/IV because the additional electron density enters a Mn−N nonbonding dx2-y2 orbital. The magnetic properties of the dimers are dominated by the symmetric Jxz/xz and Jyz/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 Mn2IV/IV improve the efficiency of the Jyz/Jyz pathway, leading to larger coupling constants in the more oxidized species. The delocalization of the unpaired electron in Mn2III/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.

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