Structural and Electronic Influences on Rates of Tertpyridine−Amine Co III −H Formation During Catalytic H 2 Evolution in an Aqueous Environment

In this work, 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 p K a values of [Co III −H] 2+ estimated from PR experiments, we found that while the protonation of [Co III −H] 2+ is unfavorable, [Co II −H] + reacts with protons to produce H 2 . The catalytic activity for H 2 evolution tracks the hydricity of the [Co II −H] + intermediate.