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 [Co-III(CR14)Cl-2](+) (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 H-2-evolving mechanism catalyzed by this complex in an organic solvent (CH3CN) with the acid p-cyanoanilinium tetrafluoroborate as a proton source. By comparison of [Co-III(CR14)Cl-2](+) and [Co-III(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 Co-III, Co-II, and Co-I 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 Co-II state rather than protonation of one nitrogen of the ligand. The mechanism of p-cyanoanilinium acid electroreduction catalyzed by [Co-III(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 [Co-II(CR14)(CH3CN)(x)](2+) (x = 1 or 2) species, whereas it is proposed that a Co-II hydride is the resting state in the diffusion-reaction layer.