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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