Homogeneous light-driven Systems
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 :
- 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.
Figure 1: Structure of a Ruthenium-Cobalt supramolecular photocatalyst for H2 evolution.
We extend this study shortly after with other Re and Ir-based photosensitizers and could reach 16% quantum yield for light-driven H2 evolution. These pioneering studies were followed by publications from a number of groups worldwide developing similar approaches and using the cobaloxime catalysts we had developed. We also investigated the second generation of cobalt catalysts we recently developed in the context of homogeneous photocatalytic H2 evolution. Combination of cobalt diimine-dioxime complexes with a cyclometallated iridium photosensitizer gave efficient systems for hydrogen generation. Interestingly, addition of PPh3 to the medium results in a significant improvement of the stability of the system, with up to ~700 turnovers achieved within 10 hours. UV-Vis spectroscopic monitoring of the reaction allowed identification of a PPh3-coordinated cobalt(I) intermediate as the active species.