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Cobalt-based Catalysts

A purely structural approach may not be the most adequate one for the elaboration of highly active catalysts. It could be worth taking the problem from another angle, starting from an active or potentially active non-biomimetic system (different ligands, different metals) and introducing new structural or electronic properties inspired from the structure of the active sites of the enzymes.

Figure 1: [Co(dmgBF2)2L2] (L=H2O, CH3CN and has been omitted for clarity, left) [Co(DO)(DOH)pn Br2] (bromide ligands have been omitted for clarity, right)

Cobaloxime is a class of such catalysts. They have been initially developed as mimics of Vitamin B12, which is considered, in the Co(I) super-reduced state, as the most powerful nucleophile designed by Nature. We have reported on the activity of various cobaloximes as electrocatalysts for the reduction of protons in organic solvents. The water-soluble {BF2}-bridged cobaloxime [Co(dmgBF2)2L] (L = CH3CN or DMF) (Figure 1, left) which is quite stable under acidic conditions was shown to be one of the most efficient catalysts for hydrogen evolution in non-aqueous solvent in term of potential and turnover frequency. We recently extended the series with a new generation of Co-based catalysts, ie diimine-dioxime complexes of cobalt (Figure 3 right), that catalyze hydrogen evolution with similar performances. These new catalysts contain a tetradentate ligand that makes them significantly more stable against hydrolysis. In addition, the diimine-dioxime ligand has an open oxime bridge susceptible to proton exchange, which provides the catalyst with a mechanism to adjust its electrocatalytic potential for hydrogen evolution to the acido-basic conditions of the solution and keeps the overvoltage for the reduction of acids within reasonable values over a wide range of pKas. The importance of a basic site in the second coordination sphere of the catalytic metal center has been highlighted above in the case of hydrogenases. This example thus nicely demonstrates that increased understanding of the chemical principles on which the reactivity of a biological active site and fine utilization of the synthetic power of chemistry allow minor modifications of a bio-inspired catalyst resulting in considerable functional improvements.

The catalytic mechanism involves a CoIII-H intermediate formed by protonation of the Co(I) species. A reductive activation, yielding a Co(II)-H is required for a second protonation to occur, generating a coordinated H2 molecule which is finally released. Studies combining theoretical chemistry (in collaboration with Martin Field at IBS) and experiments have been carried out to better characterize catalytic intermediates, gain more insights into the catalytic mechanism and confirm the role of the oxime bridge as a proton relay.

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