School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, P. R. China.
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P. R. China.
Phys Chem Chem Phys. 2022 Jul 6;24(26):15802-15810. doi: 10.1039/d2cp01869a.
Water oxidation is the bottleneck of artificial photosynthesis. A novel nickel phenolate complex with a redox-active ligand has been designed to manage multiple electron transfers during water oxidation (D. Wang and C. O. Bruner, ., 2017, , 13638). However, the mechanism of the reaction is not well understood and verified from a theoretical aspect. Density functional theory calculations were conducted to investigate the mechanism of water oxidation catalyzed by the nickel(II)-phenolate complex. Because only two cyclic voltammogram (CV) peaks were observed and the phenolate ligand is redox-active, the active species was proposed to be Ni-OH by the experiment. Based on the calculated results, the first CV peak is phenolate ligand-centered and the second peak is a single two-proton-coupled-two-electron process. In addition, the activation barrier of O-O bond formation of Ni-OH is higher than that of Ni-2OH by 15.3 kcal mol. Thus, the redox-active phenolate ligand does not lower the oxidation state of Ni in the active species to Ni. The oxidation state of the active species is still Ni, the same as other Ni complexes for WOCs. As the phenolate ligand and the hydroxyl ligand can act as an internal base, three pathways are compared for O-O bond formation: normal WNA, phenolate-involving single electron transfer (SET)-WNA, and OH-involving SET-WNA. The OH-involving SET-WNA pathway is the most favorable because the hydroxyl ligand is more nucleophilic than the oxygen radical of the phenolate ligand. Based on the experimental observation and theoretical results, the phenolate ligand is not stable and easily oxidized because of the hydrogen at the benzyl position. Thus, WOC candidates should not have the presence of hydrogen at the benzyl position near the active center.
水氧化是人工光合作用的瓶颈。设计了一种新型的含氧化还原活性配体的镍酚配合物，以管理水氧化过程中的多个电子转移（D. Wang 和 C. O. Bruner，., 2017,, 13638）。然而，从理论角度来看，该反应的机理尚不清楚且未经证实。进行了密度泛函理论计算，以研究镍(II)-酚配合物催化水氧化的机理。由于仅观察到两个循环伏安（CV）峰，并且酚配体是氧化还原活性的，因此实验提出活性物质为 Ni-OH。基于计算结果，第一个 CV 峰是酚配体中心的，第二个峰是单个两质子耦合两电子过程。此外，Ni-OH 的 O-O 键形成的活化势垒比 Ni-2OH 高 15.3 kcal mol。因此，氧化还原活性的酚配体不会将活性物质中 Ni 的氧化态降低到 Ni。活性物质的氧化态仍为 Ni，与其他用于 WOC 的 Ni 配合物相同。由于酚配体和羟基配体可以作为内部碱，因此比较了三种 O-O 键形成途径：正常 WNA、涉及单电子转移（SET）的酚配体-WNA 和涉及 SET 的 OH-WNA。涉及 SET 的 OH-WNA 途径是最有利的，因为羟基配体比酚配体的氧自由基更具亲核性。基于实验观察和理论结果，由于苄基位置的氢，酚配体不稳定且容易氧化。因此，WOC 候选物不应在活性中心附近的苄基位置存在氢。
