Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO2 Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

A series of five ruthenium (II) complexes containing a tridentate CNC pincer ligand, a bidentate 2,2′-bipyridine (bpy) ligand, and a monodentate ligand (chloride, bromide, or acetonitrile) were synthesized. The CNC pincer ligands used imidazole or benzimidazole-derived N-heterocyclic carbenes (NHCs) as the C donors and a 4-methoxy-substituted central pyridyl ring as the N donor. All of the complexes were characterized by analytical, spectroscopic, and single-crystal X-ray diffraction methods. These complexes were used as catalysts for visible-light-driven CO2 reduction in the presence and absence of an external photosensitizer (PS). Notably, complex 4C with a benzimidazole-derived CNC pincer ligand and bromide as the monodentate ligand was the most active catalyst tested for both sensitized and self-sensitized CO2 reduction. Thus, this catalyst was the subject of further mechanistic studies using transient absorption spectroscopy (TAS), absorption spectroelectrochemistry (SEC), and computational studies. A mechanism has been proposed for self-sensitized CO2 reduction involving (1) light excitation of the catalyst, (2) reduction by sacrificial donors, (3) halide loss, and (4) CO2 binding to form [RuCO2]+ as the catalyst resting state. The timeline for these steps and the structures of key intermediates are all supported by experimental observations (including TAS and SEC) and supporting computational studies. Subsequent steps in the cycle past [RuCO2]+ were not experimentally observable, but they are supported by computations. Experiments were also used to explain the differences observed for sensitized catalysis. Catalyst 4C is an unusually active catalyst for both sensitized and self-sensitized CO2 reduction, and thus being able to understand how it functions and which steps are turnover-limiting is an important development facilitating the design of commercially viable catalysts for solar fuel formation.一系列包含三齿CNC钳型配体、双齿2,2′-联吡啶（2,2′-bipyridine，bpy）配体以及单齿配体（氯离子、溴离子或乙腈）的钌(II)配合物被成功合成。所用的CNC钳型配体以咪唑或苯并咪唑衍生的氮杂环卡宾（N-heterocyclic carbenes，NHCs）作为碳供体，以4-甲氧基取代的中心吡啶环作为氮供体。所有配合物均通过分析化学、光谱学与单晶X射线衍射方法完成表征。该系列配合物被用作可见光驱动CO₂还原的催化剂，分别在存在与不存在外部光敏剂（photosensitizer，PS）的条件下开展测试。值得注意的是，带有苯并咪唑衍生CNC钳型配体且单齿配体为溴离子的4C配合物，是所有受试催化剂中活性最高的，适用于敏化与自敏化两种CO₂还原模式。因此，该催化剂成为后续机理研究的核心对象，研究手段涵盖瞬态吸收光谱（transient absorption spectroscopy，TAS）、吸收光谱电化学（absorption spectroelectrochemistry，SEC）以及计算化学研究。针对自敏化CO₂还原过程，研究人员已提出如下反应机理：(1) 催化剂的光激发；(2) 牺牲性给体介导的还原过程；(3) 卤离子解离；(4) CO₂与活性中心结合形成作为催化剂静止态的[RuCO₂]+。上述反应步骤的时间线与关键中间体的结构，均得到了实验观测（包括TAS与SEC数据）以及辅助计算研究的佐证。[RuCO₂]+之后的循环后续步骤未通过实验直接观测到，但得到了计算结果的支持。本研究同时对敏化催化体系中观察到的活性差异进行了解释。4C催化剂在敏化与自敏化CO₂还原中均展现出异乎寻常的高活性，因此阐明其催化机制与限速步骤，对于设计具备商业应用潜力的太阳能燃料合成催化剂具有重要的推动意义。