Iridium and Ruthenium Complexes of N-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-py^OR)Cl]OTf complexes where R = ^t Bu (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 (4_Ir ) or methoxy (5_Ir ); 4_Ir was reported previously, but 5_Ir is new. The analogous ruthenium complexes were also tested using [(η⁶-cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy (4_Ru ) or methoxy (5_Ru ); 4_Ru and 5_Ru 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, 5_Ir , and for two [Ag(NHC-py^OR)₂]OTf complexes 6 (R = ^t Bu) 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-py^OR 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, 3_SP , which we characterized crystallographically. For CO₂ hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4_Ir ) is 5-8 times more active than x = methoxy (5_Ir ). Notably, ruthenium complex 4_Ru showed 95% of the activity of 4_Ir . For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4_Ir ≫ 4_Ru and 4_Ir ≈ 5_Ir . 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.