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.