Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology (BIST), Avinguda Països Catalans, 16, 43007 Tarragona, Spain.
Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDE A Nanociencia), Calle Faraday, 9, 28049 Madrid, Spain.
J Am Chem Soc. 2020 Oct 14;142(41):17434-17446. doi: 10.1021/jacs.0c06515. Epub 2020 Oct 5.
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 () 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 ( and ). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using that is absent in and . 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 compared to and and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a of 140 s at an overpotential of only 200 mV. In contrast, 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 prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.
水氧化催化是设计人工光合作用实用设备最重要的反应之一。使用后过渡金属（TM）配合物为通过配体设计对其电子和结构特性进行精细控制的廉价催化剂的发展提供了极好的平台。然而，难以获得其高氧化态以及其金属-配体键的普遍不稳定性质是重要的挑战。在此，我们探索了一种铜配合物（），其具有扩展的、π离域的四酰胺大环配体（TAML），作为水氧化催化剂，并将其活性与具有较低π离域性的类似体系（和）进行比较。它们的特性表明，在使用的情况下，存在一种特殊的金属-配体协同作用，以容纳所需的氧化当量，而在和中则不存在。这包括通过在狭窄的能量范围内容易进入不同的电子态来促进电荷离域，对应于金属中心或配体中心氧化，我们将其确定为稳定积累的氧化电荷的重要因素。这导致与和相比，的催化性能有了显著提高，并且在中性 pH 下具有 140 s 的为 140 s 的最活跃和最稳健的分子配合物之一，过电势仅为 200 mV。相比之下，在氧化条件下降解，我们将其与通过电荷离域有效地稳定几个氧化当量的能力联系起来，导致氧化配体具有高反应性。最后，由于酸性脱金属作用，的无环结构阻止其在中性 pH 下使用，突出了大环稳定的重要性。
