H₂-Evolving Cobalt–Protic-NHC Catalysts: Kinetic Zone Diagram Analysis and Mechanistic Insights

A series of systematically designed cobalt–protic-NHC complexes containing pendant proton-shuttle groups was synthesized. The proton-shuttle motifs enabled these complexes to act as efficient electrocatalysts for the hydrogen evolution reaction (HER) from various acids as proton sources. The effect of acid strength on the mechanism of HER was investigated by varying the proton source ( CH3COOH, pKaCH3CN = 23.51), triethylammonium tetrafluoroborate (Et3NHBF4, pKaCH3CN = 18.57), and trifluoroacetic acid (CF3COOH, pKaCH3CN = 12.70). Additionally, by changing experimental parameters such as substrate/catalyst concentration and scan rate, the single-electron EC′ zone diagram could be extended to the present multielectron reaction system where all of the zones were accessed with little deviation in some of the waveforms from the original. From the kinetic zone diagram analysis, some of the performance parameters such as the observed rate constant (kobs), turnover frequency (TOF), and the rate constant of the first chemical step (k1) were determined. Also, the zone diagram provided insight into the mechanistic cycle and the nature of the rate-limiting step. The investigation suggested that the protic proton of the proton-shuttle functionality triggered a hydrogen evolution reaction via intramolecular proton-hydride coupling from the Co(II)–H intermediate. This intramolecular dihydrogen elimination step, which was independent of the acid concentration, acted as the rate-limiting step and the turnover frequency of HER was fully controlled by this step.