Structural and Electronic Influences on Rates of Tertpyridine-Amine CoIII-H Formation During Catalytic H2 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 [CoII-Cl]+ species results in the formation of [CoII(κ4-L)(OH2)]2+. Further reduction produces [CoI κ4-L)(OH2)]+, which undergoes a rate-limiting structural rearrangement to [CoI(κ5-L)]+ before being protonated to form [CoIII -H]2+. The rate of [CoIII-H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII-H]2+ estimated from PR experiments, we found that while the protonation of [CoIII-H]2+ is unfavorable, [CoII-H]+ reacts with protons to produce H2. The catalytic activity for H2 evolution tracks the hydricity of the [CoII-H] + intermediate.