Trapping an elusive intermediate in manganese-oxo cubane chemistry

A new member of the Mn-oxo cubane core complex family [Mn ^2III,2IV₄O₄L₆] (1), where L = (p-MeOPh)₂PO₂^-, has been synthesized and characterized. Compound 1 possesses structurally inequivalent Mn^III and Mn^IV 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 = Ph ₂PO₂^-. 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 [Mn^III ₄O₂(OMe)₂(MeOH){(p-MeOPh)₂PO ₂}₆] (2). Each Mn^III of 2 is tetragonally distorted due to the Jahn-Teller effect with elongated Mn-O bonds that are directed at the two μ₃-MeO bridges and neither of the two μ₃-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.