Ligand-dependent hydrogen evolution rates for (PNNP)Co catalysts.

(PNCH2CH2NP)Co (1) and (PNCHCHNP)Co (2) catalyze the electrocatalytic hydrogen evolution reaction (HER) using phenol as a proton source. We posit that 1 and 2 follow an E1C1E2C2-type mechanism (E = electrochemical step, C = protonation), with an initial ligand-based protonation. Scan-rate dependence studies with phenol and catalyst suggest the protonation step that immediately follows the reduction of 1 is faster than that of 2. The calculated protonation rate constants (kPT1) with 1 mM catalyst and 16 mM phenol, 190 M-1 s-1 for 1 and 38 M-1 s-1 for 2, support the scan-rate dependence studies. Controlled potential electrolysis (CPE) experiments confirmed the production of hydrogen and determined 1 to be more active than 2, attributed to the slow protonation step in the catalytic cycle for 2. Density functional theory (DFT) was used to calculate a series of intermediates and transition states for possible mechanistic pathways and suggested the initial protonation to be ligand-centered.