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.

Water‐oxidation catalysts (WOCs) can potentially be improved by installing pendant electron‐donor groups that may also be proton donors or acceptors. We have modified one of the most well‐studied WOCs with alkoxy or hydroxy substituents on the bidentate bipyridine ligand (N,N), thereby forming [(terpy)RuII(N,N)X] (X = Cl, H2O; terpy = 2,2′;6′,2"‐terpyridine). A combination of NMR spectroscopy (particularly 15N chemical‐shift data), UV/Vis spectroscopy, X‐ray diffraction, and oxygen evolution data point to interesting and beneficial effects of an oxygenated group proximal to X. A methoxy group on the 2,2′‐bipyridyl (bipy) ring cis to X = Cl is shown to facilitate ionization of the chloride ligand in aqueous acetone, perhaps by acceptance of a hydrogen bond from the aquo ligand. Hydrogen‐bond donation of a proximal hydroxy group to a bound aquo ligand is shown by X‐ray diffraction. Distinct differences in pKa values for the 4,4′‐ and 6,6′‐dihydroxy bipy complexes are seen. In water oxidation driven by ceric ammonium nitrate, the 6,6′‐dimethoxy species is somewhat faster and longer‐lived than the analogue that lacks the oxygenated groups [a turnover number (TON) of 215 instead of 138 in 10 h, and a turnover frequency (TOF) of 0.36 min–1 instead of 0.23 over the same time period]. Taken together, oxygenated groups near the WOC active site are promising electron or proton donors and/or hydrogen‐bond acceptors, and are the subject of further scrutiny.

Figure 1. Catalystsdiscussed in this work.[a] J.

Water oxidation catalysts (WOCs) are a key part of generating H2 from water and sunlight, consequently, it is a promising process for the production of clean energy. The mechanism of...