Enhancing H₂ evolution performance of an immobilised cobalt catalyst by rational ligand design

The catalyst [Co^IIIBr((DO)(DOH)(4-BnPO₃H₂)(2-CH₂py)pn)]Br, CoP , has been synthesised to improve the stability and activity of cobalt catalysts immobilised on metal oxide surfaces. The CoP catalyst contains an equatorial diimine-dioxime ligand, (DOH)₂pn = N²,N²'-propanediyl-bis(2,3-butanedione-2-imine-3-oxime), with a benzylphosphonic acid (4-BnPO₃H₂) group and a methylpyridine (2-CH₂py) ligand covalently linked to the bridgehead of the pseudo-macrocyclic diimine-dioxime ligand. The phosphonic acid functionality provides a robust anchoring group for immobilisation on metal oxides, whereas the pyridine is coordinated to the Co ion to enhance the catalytic activity of the catalyst. Electrochemical investigations in solution confirm that CoP shows electrocatalytic activity for the reduction of aqueous protons between pH 3 and 7. The metal oxide anchor provides the catalyst with a high affinity for mesostructured Sn-doped In₂O₃ electrodes (mesoITO; loading of approximately 22 nmol cm^-2) and the electrostability of the attached CoP was confirmed by cyclic voltammetry. Finally, immobilisation of the catalyst on ruthenium-dye sensitised TiO₂ nanoparticles in aqueous solutions in the presence of a hole scavenger establishes the activity of the catalyst in this photocatalytic scheme. The advantages of the elaborate catalyst design in CoP in terms of stability and catalytic activity are shown by direct comparison with previously reported phosphonated Co catalysts. We therefore demonstrate that rational ligand design is a viable route for improving the performance of immobilised molecular catalysts.