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 [Co-II-Cl](+) species results in the formation of [Co-II(kappa(4)-L)(OH2)](2+). Further reduction produces [Co-I(kappa(4)-L)(OH2)](+), which undergoes a rate-limiting structural rearrangement to [Co-I(kappa(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 pK(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.

Automated summary

In this study, the catalytic performance of a series of Co hydrogen evolution catalysts were analyzed by combining electrochemical and transient pulse radiolysis studies. The key factor determining the activity for hydrogen evolution was found to be the hydricity of the [Co-II-H](+) intermediate.