Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability.

[Co(bapbpy)Cl]+ (bapbpy: 6,6'-bis(2-aminopyridyl)-2,2'-bipyridine) is a polypyridyl cobalt(II) complex bearing both a redox-active bipyridine ligand and pendant proton relays. This compound catalyzes electro-assisted H2 evolution in DMF with distinct mechanisms depending on the strength of the acid used as the proton source (pKa values ranging from 3.4 to 13.5 in DMF) and the applied potential. Electrochemical studies combining cyclic voltammetry and bulk electrolysis measurements enabled one to bring out four distinct catalytic processes. Where applicable, relevant kinetic information were obtained using either foot-of-the-wave analysis (FOWA) or analytical treatment of bulk electrolysis experiments. Among the different catalytic pathways identified in this study, a clear relationship between the catalyst performances and stability was evidenced. These results draw attention to a number of interesting considerations and may help in the development of future adequately designed catalysts.