Mechanism of Electrochemical Proton Reduction Catalyzed by a Cobalt Tetraaza Schiff Base Macrocyclic Complex: Ligand Protonation and/or Influence of the Chloro Ligand
Margaux Willery, Paul‐Gabriel Julliard, Florian Molton, Fabrice Thomas, Jérôme Fortage, Cyrille Costentin, Marie‐Noëlle Collomb
Abstract
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 CO 2 RR. 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 (CH 3 CN) with the acid p -cyanoanilinium tetrafluoroborate as a proton source. By comparison of [Co III (CR14)Cl 2 ] + and [Co III (CR14)(CH 3 CN) 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)(CH 3 CN) 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)(CH 3 CN) 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.