The Role of Redox Potential and Molecular Structure of Co(II)-Polypyridine Complexes on the Molecular Catalysis of CO2 Reduction

Abstract

In this work, we report the electrochemical response of a family of Co(II) complexes, [CoII(L)3]2+ and [CoII(L’)2]2+ (L = 2,2’-bipyridine, 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline; L’ = terpyridine and 4-chloro-terpyridine), in the presence and absence of CO2 in order to understand the role of the redox potential and molecular structure on the molecular catalysis of CO2 reduction. The tris chelate complexes exhibited three electron transfer processes [CoII(L)3]2+ ⇄ [CoIII(L)3]3+ + 1e−, [CoΙΙ(L)3]2++1e− ⇄ [CoΙ(L)3]+, and [CoΙ(L)3]+ + 2e- ⇄ [CoΙ(L)(L−)2]−. In the case of complexes with 1,10-phen and 2,2-bipy, the third redox process showed a coupled chemical reaction [CoΙ(L)(L−)2]− → [CoΙ(L−)2]− + L. For bis chelate complexes, three electron transfer processes associated with the redox couples [CoΙΙ(L)2]/[CoIII(L)2]3+, [CoΙΙ(L)2]2+/[CoΙ(L)2]+, and [CoΙ(L)2]+/[CoΙ(L)(L−)] were registered, including a coupled chemical reaction only for the complex containing the ligand 4-chloro-terpyridine. Foot to the wave analysis (FOWA) obtained from cyclic voltammetry experiments allowed us to calculate the catalytic rate constant (k) for the molecular catalysis of CO2 reduction. The complex [Co(3,4,7,8-tm-1,10-phen)3]2+ presented a high k value; moreover, the complex [Co(4-Cl-terpy)3]2+ did not show catalytic activity, indicating that the more negative redox potential and the absence of the coupled chemical reaction increased the molecular catalysis. Density functional theory (DFT) calculations for compounds and CO2 were obtained to rationalize the effect of electronic structure on the catalytic rate constant (k) of CO2 reduction.