Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO₂ reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole-derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized by using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N₂ and CO₂ atmospheres. Photocatalytic reduction of CO₂ to CO and HCO₂^– was analyzed using all of the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO₂ reduction with and without the presence of an external PS with appreciable turnover numbers (TONs) for formate (HCO₂^–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4^Me with a benzimidazole-derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO₂ reduction catalysis via Ni^I or Ni⁰ catalytic cycles. The two active species (Ni^I and Ni⁰) demonstrate distinct reactivity and selectivity which influences the formation of CO vs formate as the product.

中文翻译：

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使用 Ni(II) CNC-Pincer 催化剂在可见光下将敏化和自敏化 CO2 光催化还原为 HCO2– 和 CO

_{光催化CO 2}还原需要坚固的地球丰富的过渡金属基光催化剂。合成了一系列六种 Ni(II) 配合物，其三齿 CNC 钳形配体由两个咪唑或苯并咪唑衍生的 N-杂环卡宾 (NHC) 环和一个具有不同 R 取代基的吡啶环组成（R = OMe、Me、H） )吡啶环 N 的对位。这些复合物已通过光谱、分析和晶体学方法进行了表征。在N ₂和CO ₂气氛下通过循环伏安法研究了所有配合物的电化学性质。在存在和不存在外部光敏剂 (PS) 的情况下^，使用所有配合物对 CO ₂光催化还原为 CO 和 HCO ₂进行了分析。所有这些配合物在存在或不存在外部 PS 的情况下都可作为 CO _{2还原光催化剂，具有可观的甲酸 (HCO 2} ^– ) 生产周转数 (TON)，并且通常产生较低量的 CO。值得注意的是，所有 Ni(II)该系列中的 CNC-pincer 配合物也具有自敏化光催化剂的活性。具有苯并咪唑衍生的 CNC 钳配体的复合物4 ^Me被发现是最活跃的自敏光催化剂。为了了解这些催化剂的机理，进行了超快瞬态吸收光谱（TAS）实验和计算研究。敏化催化涉及卤化物损失以产生更多活性络合物，而自敏化催化需要一些卤化物保持配位，以允许激发的镍络合物和牺牲电子供体之间有利的电子转移。然后这允许镍配合物通过Ni ^I或Ni ⁰催化循环进行CO ₂还原催化。两种活性物质（Ni ^I和 Ni ⁰）表现出不同的反应性和选择性，这会影响 CO 与甲酸盐作为产物的形成。