Tetranuclear Manganese Models of the OEC Displaying Hydrogen Bonding Interactions: Application to Electrocatalytic Water Oxidation to Hydrogen Peroxide.

Toward the development of structural and functional models of the oxygen evolving complex (OEC) of photosystem II, we report the synthesis of site-differentiated tetranuclear manganese complexes featuring three six-coordinate and one five-coordinate Mn centers. To incorporate biologically relevant second coordination sphere interactions, substituents capable of hydrogen bonding are included as pyrazolates with arylamine substituents. Complexes with terminal anionic ligands, OH or Cl, bound to the lower coordinate metal center are supported through the hydrogen-bonding network in a fashion reminiscent of the enzymatic active site. The hydroxide complex was found to be a competent electrocatalyst for O-O bond formation, a key transformation pertinent to the OEC. In an acetonitrile-water mixture, at neutral pH, electrochemical water oxidation to hydrogen peroxide was observed, albeit with low (15%) Faradaic yield, likely due to competing reactions with organics. In agreement, 9,10-dihydroanthracene is electrochemically oxidized in the presence of this cluster both via H-atom abstraction and oxygenation with ∼50% combined Faradaic yield.