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 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.