Mechanism of Water Oxidation Catalyzed by a Mononuclear Manganese Complex

The design and synthesis of biomimetic Mn complexes to catalyze oxygen evolution is a very appealing goal because water oxidation in nature employs a Mn complex. Recently, the mononuclear Mn complex [LMn^II (H₂ O)₂ ]²⁺ [1, L=Py₂ N(tBu)₂ , Py=pyridyl] was reported to catalyze water oxidation electrochemically at an applied potential of 1.23 V at pH 12.2 in aqueous solution. Density functional calculations were performed to elucidate the mechanism of water oxidation promoted by this catalyst. The calculations showed that 1 can lose two protons and one electron readily to produce [LMn^III (OH)₂ ]⁺ (2), which then undergoes two sequential proton-coupled electron-transfer processes to afford [LMn^V OO]⁺ (4). The O-O bond formation can occur through direct coupling of the two oxido ligands or through nucleophilic attack of water. These two mechanisms have similar barriers of approximately 17 kcal mol^-1 . The further oxidation of 4 to generate [LMn^VI OO]²⁺ (5), which enables O-O bond formation, has a much higher barrier. In addition, ligand degradation by C-H activation has a similar barrier to that for the O-O bond formation, and this explains the relatively low turnover number of this catalyst.