Abstract
Fossil fuel shortage and severe climate changes due to global warming have prompted extensive research on carbon-neutral and renewable energy resources. Hydrogen gas (H(2)), a clean and high energy density fuel, has emerged as a potential solution for both fulfilling energy demands and diminishing the emission of greenhouse gases. Currently, water oxidation (WO) constitutes the bottleneck in the overall process of producing H(2) from water. As a result, the design of efficient catalysts for WO has become an intensively pursued area of research in recent years. Among all the molecular catalysts reported to date, ruthenium-based catalysts have attracted particular attention due to their robust nature and higher activity compared to catalysts based on other transition metals.Over the past two decades, we and others have studied a wide range of ruthenium complexes displaying impressive catalytic performance for WO in terms of turnover number (TON) and turnover frequency (TOF). However, to produce practically applicable electrochemical, photochemical, or photo-electrochemical WO reactors, further improvement of the catalysts' structure to decrease the overpotential and increase the WO rate is of utmost importance. WO reaction, that is, the production of molecular oxygen and protons from water, requires the formation of an O-O bond through the orchestration of multiple proton and electron transfers. Promotion of these processes using redox noninnocent ligand frameworks that can accept and transfer electrons has therefore attracted substantial attention. The strategic modifications of the ligand structure in ruthenium complexes to enable proton-coupled electron transfer (PCET) and atom proton transfer (APT; in the context of WO, it is the oxygen atom (metal oxo) transfer to the oxygen atom of a water molecule in concert with proton transfer to another water molecule) to facilitate the O-O bond formation have played a central role in these efforts.In particular, promising results have been obtained with ligand frameworks containing carboxylic acid groups that either are directly bonded to the metal center or reside in the close vicinity. The improvement of redox and chemical properties of the catalysts by introduction of carboxylate groups in the ligands has proven to be quite general as demonstrated for a range of mono- and dinuclear ruthenium complexes featuring ligand scaffolds based on pyridine, imidazole, and pyridazine cores. In the first coordination sphere, the carboxylate groups are firmly coordinated to the metal center as negatively charged ligands, improving the stability of the complexes and preventing metal leaching during catalysis. Another important phenomenon is the reduction of the potentials required for the formation of higher valent intermediates, especially metal-oxo species, which take active part in the key O-O bond formation step. Furthermore, the free carboxylic acid/carboxylate units in the proximity to the active center have shown exciting proton donor/acceptor properties (through PCET or APT, chemically noninnocent) that can dramatically improve the rate as well as the overpotential of the WO reaction.