Structural and Electronic Influences on Rates of Tertpyridine-Amine Co III -H Formation During Catalytic H 2 Evolution in an Aqueous Environment
In this paper, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [Co II -Cl] + species results in the formation of [Co II (κ 4 -L)(OH 2 )] 2+ . Further reduction produces [Co I κ 4 -L)(OH 2 )] + , which undergoes a rate-limiting structural rearrangement to [Co I (κ 5 -L)] + before being protonated to form [Co III -H] 2+ . The rate of [Co III -H] 2+ formation is similar for all complexes in the series. Using E 1/2 values of various Co species and pKa values of [Co III -H] 2+ estimated from PR experiments, we found that while the protonation of [Co III -H] 2+ is unfavorable, [CoII-H]+ reacts with protons to produce H 2 . The catalytic activity for H 2 evolution tracks the hydricity of the [Co II -H] + intermediate.