Structural And Physical Characterization of Tetranuclear [MnII3MnIV] And [MnII2MnIII2] Valence-Isomer Manganese Complexes

Two tetranuclear Mn complexes with an average Mn oxidation state of +2.5 have been prepared. These valence isomers have been characterized by a combination of X-ray crystallography, X-ray absorption spectroscopy, and magnetic susceptibility. The MnII3MnIV tetramer has the Mn ions arranged in a distorted tetrahedron, with an S = 6 ground spin state, dominated by ferromagnetic exchange among the manganese ions. The MnII2MnIII2 tetramer also has a distorted tetrahedral arrangement of Mn ions but shows magnetic behavior, suggesting that it is a single-molecule magnet. The X-ray absorption near-edge structure (XANES) spectra for the two complexes are similar, suggesting that, while Mn XANES has sufficient sensitivity to distinguish between trinuclear valence isomers (Alexiou et al. Inorg. Chem. 2003, 42, 2185), similar distinctions are difficult for tetranuclear complexes such as that found in the photosynthetic oxygen-evolving complex.