Iridium and Ruthenium Complexes of <i>N</i>‑Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features and for metal–ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir­(NHC-pyOR)­Cl]­OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir­(6,6′-dxbp)­Cl]­OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)­Ru­(6,6′-dxbp)­Cl]­OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag­(NHC-pyOR)2]­OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1–5. In general, NHC-pyOR complexes 1–3 were modest precatalysts for both reactions. NHC complexes 1–3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5–8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.加氢反应（hydrogenation reactions）可将能量储存于化学键中，若该反应可逆，则可按需释放能量。目前用于二氧化碳加氢反应（CO2 hydrogenation）的最高效过渡金属催化剂中，不乏以2-羟基吡啶基配体（pyridin-2-ol-based ligands）为配体的体系，例如6,6'-二羟基联吡啶（6,6′-dihydroxybipyridine, 6,6′-dhbp），这类配体兼具质子响应特性与金属-配体双功能催化活性。本研究旨在对比双齿2-羟基吡啶基配体与一种新型骨架结构——即结合了氮杂环卡宾（N-heterocyclic carbene, NHC）与2-羟基吡啶的配体。为此，我们合成了一系列[Cp*Ir(NHC-pyOR)Cl]OTf配合物，其中R分别为叔丁基（tBu，1）、氢（H，2）与甲基（Me，3）。为作对照，我们还测试了联吡啶衍生的类似铱配合物作为催化剂，具体为[Cp*Ir(6,6′-dxbp)Cl]OTf，其中x分别为羟基（hydroxy，4Ir）与甲氧基（methoxy，5Ir）；配合物4Ir已有文献报道，而5Ir为全新合成的化合物。此外，我们还采用[(η6-伞花烃（cymene）)Ru(6,6′-dxbp)Cl]OTf测试了类似的钌配合物，其中x分别为羟基（4Ru）与甲氧基（5Ru）；配合物4Ru与5Ru均已有文献报道。所有新合成的配合物均通过光谱学与分析方法进行了完整表征，其中配合物1、2、3、5Ir，以及两种[Ag(NHC-pyOR)2]OTf配合物（R分别为叔丁基（6）与甲基（7））还通过单晶X射线衍射完成了结构解析。我们使用催化剂1-5开展了二氧化碳加氢与甲酸脱氢的水相催化研究。总体而言，NHC-pyOR类配合物1-3在这两类反应中均表现为温和的预催化剂。配合物1-3在碱性二氧化碳加氢反应条件下会发生结构转化，针对配合物3，我们捕获了其转化产物3SP，并通过单晶衍射完成了表征。针对碱性条件下的二氧化碳加氢反应，基于dxbp的催化剂表现出：羟基取代的配合物4Ir的活性是甲氧基取代的5Ir的5~8倍。值得注意的是，钌配合物4Ru的活性达到了4Ir的95%。而在甲酸脱氢反应中，活性趋势则截然不同：催化活性表现为4Ir ≫ 4Ru，且4Ir ≈ 5Ir。在碱性加氢反应条件下，二级配位球效应发挥了重要作用——此时6,6′-dhbp的羟基会发生去质子化，碱金属离子可结合该位点并辅助活化二氧化碳。密度泛函理论（density functional theory, DFT）计算验证了上述活性趋势，并被用于探究二氧化碳加氢与甲酸脱氢的反应机理。