Photocatalytic hydrogen evolution by a molecular cobalt complex with polypyridyl-phenolate ligand in alkaline aqueous solutions.

Solar-driven water-splitting represents a promising method of producing H2 gas as a clean, renewable, and alternative fuel source. While proton reduction is thermodynamically more favorable in acidic pHs, hydrogen evolution reaction (HER) in neutral and basic media may be preferred for its coupling to the oxidation of water for overall water splitting. To further understand the electronic and steric effects that govern the structure-function relationships in catalytic HER, molecular Co complexes based on a pentadentate tetrapyridylamine ligand containing a phenol group were synthesized, characterized, and studied for photocatalytic HER. The octahedral CoII-chloride complex with a dangling phenol group can be converted to a trigonal bipyramidal CoII complex featuring a bound phenolate and a dangling pyridine moiety in aqueous solutions. The resulting Co complex with trigonal bipyramidal geometry shows considerable activity and improved stability for photocatalytic HER in purely basic aqueous solutions with turnover numbers ranging from 11 900-13 700 in pH 7-9 solutions. Results from DFT computations suggest either a modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) or mod-CEEC pathway for HER depending on the protonation state of the ligand.