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 = ^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 (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 = ^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-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.

Fossil fuels (coal, oil, natural gas) are becoming increasingly disfavored as long-term energy options due to concerns of scarcity and environmental consequences (e.g., release of anthropogenic CO₂). Hydrogen gas, on the other hand, has gained popularity as a clean-burning fuel because the only byproduct from its reaction with O₂ is H₂O. In recent decades, hydrogen derived from water splitting has been a topic of extensive research. The bottleneck of the water splitting reaction is the difficult water oxidation step (2H₂O → O₂ + 4H⁺ + 4e⁻), which requires an effective and robust catalyst to overcome its high kinetic barrier. Research in water oxidation by molecular ruthenium catalysts enjoys a rich history spanning nearly 40 years. As the diversity of novel ligands continues to widen, the relationship between ligand geometry or electronics, and catalyst activity is undoubtedly becoming clearer. The present review highlights, in the authors’ opinion, some of the most impactful discoveries in the field and explores the evolution of ligand design that has led to the current state of the art.