Mechanism of Water Oxidation Catalyzed by a Mononuclear Manganese Complex

• Published in: ChemSusChem 2017-03, Vol.10 (5), p.903-911
• Main Authors: Li, Ying‐Ying, Ye, Ke, Siegbahn, Per E. M., Liao, Rong‐Zhen
• Format: Article
• Language: English
• Subjects: Aqueous solutions; Barriers; Bonding; Catalysis; Catalysts; Coupling; density functional theory; Ligands; Manganese; Manganese - chemistry; Mathematical analysis; Models, Molecular; Molecular Conformation; Organometallic Compounds - chemistry; Oxidation; Oxidation-Reduction; Quantum Theory; reaction mechanism; Water - chemistry; water splitting
• ISSN: 1864-5631; 1864-564X; 1864-564X
• DOI: 10.1002/cssc.201601538

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 [LMnII(H2O)2]2+ [1, L=Py2N(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 [LMnIII(OH)2]+ (2), which then undergoes two sequential proton‐coupled electron‐transfer processes to afford [LMnVOO]+ (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 [LMnVIOO]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. Water works: The mechanism of water oxidation catalyzed by a mononuclear Mn complex is investigated through DFT calculations. The O−O bond formation can occur through oxo–oxo coupling or nucleophilic water attack because these processes have similar barriers.