Electronic effects on polypyridyl Co complex-based water reduction catalysts.

Three new isomeric cobalt complexes of TPA (tris(2-pyridylmethyl)amine) based on methoxy substitution at the , and positions, respectively, were constructed and their photocatalytic proton reduction efficiencies were compared. It was found that there are good linear correlations with the Hammett constants of the substituents for the computed Co-N bond lengths, redox potentials of Co and Co events, and the photocatalytic activities of the complexes. The -substituted Co complex distinguished itself from the others remarkably in all these comparisons, demonstrating the presence of a steric effect besides the electronic effect. For other examined complexes, a stronger electron-donating substituent may lead to a higher hydrogen evolution efficiency, suggesting that the formation of a Co(iii) hydride intermediate is the rate-limiting step.