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This essay for EurJIC's special issue on "Redox Catalysis for Artificial Photosynthesis" introduces the reader to the field of water oxidation using molecular catalysts. The most essential challenge our society must address during the 21st century is perhaps the realization of a system for producing sustainable energy on the global scale. Currently, there exists an urgent need to develop effective and economical carbon‐neutral or carbon‐free energy technologies. The production of solar fuels through water splitting constitutes a key enabling element. The construction of robust and efficient catalysts for oxidation of water is therefore essential. In this essay the progress and mechanistic considerations pertaining to molecular water oxidation catalysts are described and discussed from a personal perspective. [ABSTRACT FROM AUTHOR]

The production of clean and sustainable energy is considered as one of the most urgent issues for our society. Mastering the oxidation of water to dioxygen is essential for the production of solar fuels. A study of the influence of the substituents on the catalytic activity of a series of mononuclear Ru complexes ( 2 a- e) based on a tetradentate ligand framework is presented. At neutral pH, using [Ru(bpy)3](PF6)3 (bpy=2,2'-bipyridine) as the terminal oxidant, a good correlation between the turnover frequency (TOF) and the Hammett σmeta parameters was obtained. Additionally, a general pathway for the deactivation of Ru-based catalysts 2 a- e during the catalytic oxidation of water through poisoning by carbon monoxide was demonstrated. These results highlight the importance of ligand design for fine-tuning the catalytic activity of water oxidation catalysts. [ABSTRACT FROM AUTHOR]

Artificial photosynthesis represents an attractive way of converting solar energy into storable chemical energy. The H2O oxidation half-reaction, which is essential for producing the necessary reduction equivalents, is an energy-demanding transformation associated with a high kinetic barrier. Herein we present a couple of efficient Ru-based catalysts capable of mediating this four-proton-four-electron oxidation. We have focused on the incorporation of negatively charged ligands, such as carboxylate, phenol, and imidazole, into the catalysts to decrease the redox potentials. This account describes our work in designing Ru catalysts based on this idea. The presence of the negatively charged ligands is crucial for stabilizing the metal centers, allowing for light-driven H2O oxidation. Mechanistic details associated with the designed catalysts are also presented. [ABSTRACT FROM AUTHOR]

Insight into how H2O is oxidized to O2 is envisioned to facilitate the rational design of artificial water oxidation catalysts, which is a vital component in solar-to-fuel conversion schemes. Herein, we report on the mechanistic features associated with a dinuclear Ru-based water oxidation catalyst. The catalytic action of the designed Ru complex was studied by the combined use of high-resolution mass spectrometry, electrochemistry, and quantum chemical calculations. Based on the obtained results, it is suggested that the designed ligand scaffold in Ru complex 1 has a non-innocent behavior, in which metal-ligand cooperation is an important part during the four-electron oxidation of H2O. This feature is vital for the observed catalytic efficiency and highlights that the preparation of catalysts housing non-innocent molecular frameworks could be a general strategy for accessing efficient catalysts for activation of H2O. [ABSTRACT FROM AUTHOR]

Herein, we describe the use of Pd nanoparticles immobilized on an amino-functionalized siliceous mesocellular foam for the catalytic oxidation of H2O. The Pd nanocatalyst proved to be capable of mediating the four-electron oxidation of H2O to O2, both chemically and photochemically. The Pd nanocatalyst is easy to prepare and shows high chemical stability, low leaching, and recyclability. Together with its promising catalytic activity, these features make the Pd nanocatalyst of potential interest for future sustainable solar-fuel production. [ABSTRACT FROM AUTHOR]

A series of [Ru(bpy)3]2+-type (bpy=2,2′-bipyridine) photosensitisers have been coupled to a ligand for Mn, which is expected to give a dinuclear complex that is active as a water oxidation catalyst. Unexpectedly, photophysical studies showed that the assemblies had very short lived excited states and that the decay patterns were complex and strongly dependent on pH. One dyad was prepared that was capable of catalysing chemical water oxidation by using [Ru(bpy)3]3+ as an oxidant. However, photochemical water oxidation in the presence of an external electron acceptor failed, presumably because the short excited-state lifetime precluded initial electron transfer to the added acceptor. The photophysical behaviour could be explained by the presence of an intricate excited-state manifold, as also suggested by time-dependent DFT calculations. [ABSTRACT FROM AUTHOR]

The development of catalysts for the oxidation of water using ligands with a non ‐ innocent character could potentially be employed for the rational design of efficient water ‐ oxidation catalysts. In their Full Paper on page 10039 ff., B. Åkermark, M. D. Kärkäs, R. ‐ Z. Liao and co ‐ workers explore the mechanistic details associated with a dinuclear Ru ‐ based water ‐ oxidation catalyst. Combining experimental studies and quantum chemical calculations provides insight into the fundamental principles controlling the reactivity of the catalyst, in which metal – ligand cooperation is a vital part. [ABSTRACT FROM AUTHOR]