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.