Mechanism of Electrochemical Proton Reduction Catalyzed by a Cobalt Tetraaza Schiff Base Macrocyclic Complex: Ligand Protonation and/or Influence of Chloro Ligand?
Résumé
Cobalt complexes with tetra-and penta-aza 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 exploitated for both electroand photo-catalytic HER and CO2RR. In particular, the tetra-aza [Co III (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 catalyst in pure aqueous solution for HER. In this work we reinvestigated the H2-evolving mechanism catalyzed by this complex in organic solvent (CH3CN) with the acid p-cyanoanilinium tetrafluoroborate as a proton source. By comparison of [Co III (CR14)Cl2] + and [Co III (CR14)(CH3CN)2] 3+ electrochemical behavior with and without 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 addition of p-cyanoanilinium facilitates chloro ligand decoordination at the Co II state rather than protonation of a 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 non-protonated [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.
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