Redox Metal–Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

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 (12–) 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 (22– and 32–). Their characterization evidences a special metal–ligand cooperativity in accommodating the required oxidative equivalents using 12– that is absent in 22– and 32–. 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 12– compared to 22– and 32– and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a kobs of 140 s–1 at an overpotential of only 200 mV. In contrast, 22– 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 32– prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.水氧化催化（water oxidation catalysis）是构建实用型人工光合作用（artificial photosynthesis）装置的核心反应之一。采用第一行后过渡金属（transition metal, TM）配合物，可为开发低成本催化剂提供绝佳平台，通过配体设计即可精准调控其电子与结构特性。但这类配合物存在两大关键挑战：其一难以达到高氧化态，其二其金属-配体键普遍具有易解离特性。 本研究针对一种带有扩展π离域四酰胺基大环配体（tetra-amidate macrocyclic ligand, TAML）的铜配合物（12–）作为水氧化催化剂展开探究，并将其催化活性与π离域程度更低的同类配合物（22–与32–）进行对比。表征结果显示，12–在适配反应所需氧化当量的过程中存在特殊的金属-配体协同效应，而22–与32–并无该效应。该协同效应的核心在于：在窄能量区间内可便捷获取不同电子态（对应金属中心氧化或配体中心氧化），从而促进电荷离域，我们认为这是稳定累积氧化电荷的关键因素。 相较于22–与32–，12–的催化性能得到显著提升，成为中性pH条件下活性与稳定性俱佳的分子水氧化催化剂之一，在仅200 mV过电位下的观测速率常数kobs可达140 s–1。与之形成对比的是，22–在氧化条件下会发生降解，这归因于其无法通过电荷离域有效稳定多个氧化当量，最终导致氧化态配体具备高反应活性。最后，32–的非环结构使其在中性pH条件下因酸性脱金属（acidic demetalation）而无法使用，这凸显了大环稳定化策略的重要性。