Trapping an elusive intermediate in manganese-oxo cubane chemistry

A new member of the Mn-oxo cubane core complex family [Mn2III,2IV4O4L6] (1), where L = (p-MeOPh)2PO2-, has been synthesized and characterized. Compound 1 possesses structurally inequivalent MnIII and MnIV with clear valence electron localization in the crystal phase, quite unlike the structurally equivalent sites, tetrahedral core symmetry, and delocalized valence of its analogue where L = Ph2PO2-. Compound 1 exhibits appreciable positive shifts (0.1-0.3 V) of both the oxidation and reduction electrochemical potentials, attributable to the remote electron donating p-MeO groups. Reduction of 1 by methanol yields a novel core complex [MnIII4O2(OMe)2(MeOH)[(p-MeOPh)2PO2]6] (2). Each MnIII of 2 is tetragonally distorted due to the Jahn-Teller effect with elongated Mn-O bonds that are directed at the two micro3-MeO bridges and neither of the two micro3-oxos. These electronically driven distortions provide a structural rationale for the greater basicity of the former sites and explain why 2 of the 4 corner oxos are preferentially reduced to water molecules.