Redox-inactive metal single-site molecular complexes: a new generation of electrocatalysts for oxygen evolution?

A molecular pre-catalyst complex, [CuII(indH)(OClO3)(NCCH3)](ClO4)·CH3CN (1·CH3CN) with the 3N pincer ligand 1,3-bis(2′-pyridyl)iminoisoindoline (indH) was immobilized on indium tin oxide (ITO) transparent conducting substrate to generate O2 electrocatalytically for over 20 hours at pH 10 in a carbonated buffer, reaching a turnover of 139 with no signs of CuOx/Cu(OH)2 formation at the surface. Further electrolysis experiments revealed that the catalyst was present in the aqueous phase, despite the poor initial solubility of the pre-catalyst (1). In order to identify the actual form responsible for this important catalytic reaction, the aquo complex [CuII(ind)(OClO3)(OH2)]·CH3OH (2·CH3OH) was structurally characterized. Spectroscopic investigations of a solid isolated from the buffer used in the electrolysis reaction and solution equilibrium studies using 2 indicated that the [CuII(ind)(OH)] form occurs at pH 10. Electron paramagnetic resonance (EPR) spectroscopy and DFT calculations confirmed a distorted {3N,O}eq coordination plane in solution, as found in 2. The buffer (i.e. bicarbonate/carbonate) may affect reactivity in two ways: as an external base facilitating the proton-coupled electron transfer steps; and/or displacing the inner-sphere solvent molecules from the favourable quasi-equatorial position, thus inhibiting the catalysis. Structural features of a tri-nuclear cluster [CuII3(ind)3(μ3-CO3)(CH3OH)(OClO3)] (3) isolated under basic conditions confirmed that beside acting as an external base, the inhibiting effect of carbonate anions may also play a role. In acetonitrile-water solutions, where both 1 and 2 exhibit reasonable solubility, experimental findings supported by DFT calculations suggest that it is the ind− ligand which is being oxidized while the cupric ion remains redox-inactive which is very unusual yet of great significance for the creation of a new generation of low-cost Cu-based water oxidation catalysts as well as potentially other 1st row transition metals.