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

Abstract

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 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.

From non-innocent to guilty: on the role of redox-active ligands in the electro-assisted reduction of CO2 mediated by a cobalt(ii)-polypyridyl complex

“…The structure determined for the dark golden crystals of [Co II (bapbpy)Cl] + [1-Cl] + (Fig. 1B) has already been described elsewhere 55 , reecting a weakening of the metal-chloride bonds in the hexacoordinated complex. In contrast, Co-N bond lengths remain essentially unaffected in both complexes.…”

“…1A) and its cobalt complex were synthesized according to published procedures. 55,58 Interestingly, two sets of single crystalsvarying slightly in their colorssuitable for Xray analysis were obtained by layering of diisopropyl ether over a methanolic solution of the complex. The structure determined for the dark golden crystals of [Co II (bapbpy)Cl] + [1-Cl] + (Fig.…”

“…53,54 In the case of the electro-assisted H 2 evolution, proton relay sites and redox-active units have clearly demonstrated their ability to improve catalytic kinetic features or to signicantly reduce the overpotential required to drive the reaction. 55 Examples are scarcer when it comes to CO 2 reduction and mechanistic deadends have been established in several situations. While Savéant, Costentin and co-workers found that introduction of multiple proton relays dramatically enhanced the CO 2 -to-CO electrocatalytic properties of iron porphyrin, 18 similar modications performed on a ruthenium(II) complex by the group of Fujita resulted in a catalytically-inactive complex under CO 2 atmosphere.…”

“…Similarly, Elgrishi et al investigated the electro-assisted CO 2 -to-CO conversion mediated by [M(tpy) 2 ] 2+ complexes (M ¼ Mn, Fe, Co, Ni, Cu, Zn) and suggested carboxylation of the reduced tpy ligand as a potential deactivation pathway (tpy ¼ 2,2 0 :6 0 ,2 00terpyridine). 32 Herein, we describe the electrocatalytic behavior of a cobalt(II) polypyridyl complex displaying an unusual combination of features, containing both a redox-active moiety and protic sites, 55,57 potentially capable to set up a stabilizing hydrogen bond network around the bound CO 2 molecule. Electrochemical measurements provide insights on the nature of the doubly reduced cobalt catalyst, the rst reduction having a predominantly metal-based character and the second electron transfer being mostly localized on the ligand scaffold.…”

Structure‐Functional Analysis of Hydrogen Production Catalyzed by Molecular Cobalt Complexes with Pentadentate Ligands in Aqueous Solutions

“…The H–H bond formation via the intramolecular proton‐hydride heterolytic coupling in a ligand‐protonated Co II hydride has been suggested as the rate‐determining step in the electrocatalytic H 2 evolution by [Co(bapbpy)Cl] + (bapbpy: 6,6'‐bis(2‐aminopyridyl)‐2,2'‐bipyridine) with HBF 4 used as the proton source, because the effect of the concentration of catalyst on the catalytic midwave potential and the effect of concentration of acid on catalytic current were not observed …”