Photo-induced redox catalysis for proton reduction to hydrogen with homogeneous molecular systems using rhodium-based catalysts

• Molecular multi-component photocatalytic systems toward H2 production using rhodium catalysts are reviewed.
• Ruthenium and iridium complexes have been used as photosensitizers in these photocatalytic systems in association with a sacrificial electron donor.
• Assembled molecular structures in which the catalyst and the photosensitizer are covalently linked are also presented.
• Systems functioning in pure aqueous solution are specially highlighted and some of them have proved particularly efficient.
• Mechanistic aspects of photochemical H2 evolution catalyzed by these multi-component systems will be also discussed in detail.

Solar driven water-splitting into hydrogen (H2) and oxygen (O2) has emerged as a very attractive sustainable approach to produce the fuel H2, which can be considered as promising clean and renewable energy carrier for the future. The development of homogeneous multi-component photocatalytic systems for the reduction of protons to H2 based on molecular compounds has experienced considerable growth since the end of the seventies. Such systems are generally composed of a light-harvesting antenna (photosensitizer), a hydrogen-evolving catalyst and a sacrificial electron donor coupled in some cases with an electron mediator. This review will survey the literature on homogeneous molecular multi-component photocatalytic systems for hydrogen production using rhodium complexes as catalysts with a special highlight on those operating in fully aqueous solution. In all of these studies, ruthenium-tris(bipyridine) or heteroleptic cyclometalated iridium complexes have been used as photosensitizers. Rhodium(III) complexes with poly(bipyridyl) ligands have been employed as catalysts in most of the studies. These multi-components photocatalytic systems are based on two different approaches: (i) multi-molecular systems, and (ii) systems involving an assembled structure (photocatalyst) that chemically couples in the same molecule the photosensitizer and the catalyst.

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