Electrochemical water oxidation using a stable water-soluble mononuclear manganese clathrochelate

Abstract

A stable water-soluble mononuclear manganese clathrochelate complex [Mn^IV(L-6H)]²⁻ is utilized as an electrocatalyst for water oxidation in phosphate buffer solution at pH 8.0. Kinetic tests and electrochemical examinations reveal that this complex homogeneously catalyzes water oxidation to oxygen. Oxygen evolution occurs at a low onset overpotential of only 540 mV, which is lower than the potential of most Mn based homogeneous water oxidation catalysts. The long-term catalytic stability of this complex has been confirmed by collective characterization methods. A water oxidation mechanism involving intermediate species in the form of coordination of hydrogen peroxide on the Mn center is proposed according to the electrochemical test results. An ample amount of the hydrazide structure in the ligand is critical for the stabilization of the Mn(IV) center. It is also found to be beneficial for the generation of active catalytic intermediate species with a higher valent Mn center. This work indicates that the design of a complex with a stable hydrazide structure is an effectual strategy to construct a metal complex with a high valent metal core for efficient water oxidation catalysis.