Mechanism of Electrochemical Proton Reduction Catalyzed by a Cobalt Tetraaza Schiff Base Macrocyclic Complex: Ligand Protonation and/or Influence of the Chloro Ligand

Cobalt complexes with tetra- and pentaaza-macrocyclic ligands, including the pyridyldiimine motif isolated by Busch as early as the 1970s, is a very promising family of catalysts that were only quite recently exploited for both the electro- and photocatalytic HER and CO2RR. In particular, the tetraaza [CoIII(CR14)­Cl2]+ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]­heptadeca-1(17),2,11,13,15-pentaene) appears to be one of the most efficient and stable Co catalysts in pure aqueous solution for the HER. In this work, we reinvestigated the H2-evolving mechanism catalyzed by this complex in an organic solvent (CH3CN) with the acid p-cyanoanilinium tetrafluoroborate as a proton source. By comparison of [CoIII(CR14)­Cl2]+ and [CoIII(CR14)­(CH3CN)2]3+ electrochemical behavior with and without the addition of chloride, we first characterized the thermodynamical coordination and decoordination properties of the chloro ligands at the CoIII, CoII, and CoI formal redox states. Then, we showed (through echem, UV–visible absorption, and EPR) that the addition of p-cyanoanilinium facilitates chloro ligand decoordination at the CoII state rather than protonation of one nitrogen of the ligand. The mechanism of p-cyanoanilinium acid electroreduction catalyzed by [CoIII(CR14)­(CH3CN)2]3+ is then characterized kinetically by a thorough cyclic voltammetry analysis. The resting state in the bulk solution in the course of constant potential electrolysis for p-cyanoanilinium acid reduction was identified as a nonprotonated [CoII(CR14)­(CH3CN) x ]2+ (x = 1 or 2) species, whereas it is proposed that a CoII hydride is the resting state in the diffusion-reaction layer.