Homogeneous light-driven catalytic systems for hydrogen production and, more generally, efficient photo-activated synthetic multi-electron catalysts remain relatively scarce. Such systems generally consist of :

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• a photosensitizer, often based on the ruthenium tris(diimine) moiety,
• a metal-based catalytic center and in some cases
• an additional redox mediator.

However their efficiency remains to be improved in terms of both turnover numbers (stability) and turnover frequencies and these systems should use inexpensive first-row transition metal catalysts rather than unsustainable noble metals. We and others recently reported that cobaloximes are very efficient and cheap electrocatalysts for hydrogen evolution. We thus decided to couple cobaloximes with ruthenium tris(diimine) moieties in order to make a supramolecular variant of the system previously studied by Lehn et al for hydrogen photo-production. In such a molecular device, the intramolecular electron-transfer from the photoactivated center to the catalytic center can potentially be controlled and the charge recombination processes limited, to an extent larger than in intermolecular systems, by a fine tuning of both the distance between metal centers and the nature of the bridge. Such an organized assembly is found in hydrogen-evolving green-algae where the photosystem I is tightly coupled to hydrogenase enzymes.
 

We have recently reported the synthesis and activity of a series of novel heterodinuclear ruthenium-cobaloxime photocatalysts able to achieve hydrogen photo-production with the highest turnover numbers so far reported for such devices. The influence of the stability, Co(II)/Co(I) redox potential and nucleophilicity of the cobaloxime moiety on the photocatalytic properties has been studied.