Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with...

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (1^2-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (2^2- and 3^2-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 1^2- that is absent in 2^2- and 3^2-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 1^2- compared to 2^2- and 3^2- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a k_obs of 140 s^-1 at an overpotential of only 200 mV. In contrast, 2^2- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 3^2- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.