GROUND-STATE ELECTRONIC-STRUCTURE OF THE DIMER-OF-DIMERS COMPLEX [(MN2O2)2(TPHPN)2]4+ - POTENTIAL RELEVANCE TO THE PHOTOSYSTEM-II WATER OXIDATION CATALYST

The tetrameric dimer-of-dimers complex [(Mn2O2)2(tphpn)2]4+ possesses structural features and a parallel polarization EPR spectrum which are similar to those observed for the S1 state of the photosystem II water oxidation catalyst. We have probed the ground-state electronic structure of [(Mn2O2)2(tphpn)2]4+ by magnetic susceptibility and isothermal saturation magnetization measurements to understand the origin of the EPR spectrum in terms of the intra- and interdimer magnetic exchange interactions and the ground-state zero-field splitting. Both [MnIIIMnIV] dimers are treated as having an effective spin S'= 1/2 which are coupled (J' = +38.8 cm-1) to yield a zero-field split (D = +1.8 cm-1, E = -0.15 cm-1) triplet ground state. The fictitious J' has been related by vector algebra methods to the real J that describes the exchange through the alkoxide oxygen atoms of the tphpn ligands (J' = -4J(alkoxide)). This treatment shows that the effective ferromagnetic interaction between the two S' = 1/2 cores has its origin in an alkoxide mediated antiferromagnetic exchange interaction. The difference in the magnitude of the singlet-triplet splitting obtained via the effective model and that obtained using a model describing the full exchange symmetry of the tetramer is on the order of 1%. It has been determined that the zero-field splitting is not of single ion or dipolar origin but results from pseudodipolar coupling. We have shown that the electronic structure of [(Mn2O2)2(tphpn)2]4+ parallels the geometric structure of the complex, and these results are presented in light of their relevance to the manganese water oxidation catalyst.