The Role of Geometry in Cobalt–Polypyridine Complexes in the Electrochemical Reduction of CO2 Using UV-Vis Spectroelectrochemistry

This work explores the effect of geometry and the presence of a site available for carbon dioxide coordination in molecular catalysis of CO2 reduction for cobalt complexes using electrochemical and spectroelectrochemical studies. The octahedral complexes [CoII(bztpen)Br]PF6 and [CoII(bpy)3](BF4)2, along with the trigonal bipyramidal complex [CoII(TPA)Cl]Cl, were selected for this study (where bztepen = N-benzyl-N,N′,N′-tris-(pyridine-2-ylmethyl)-ethylenediamine), TPA = tris (2-pyridimethyl)-amine, and bpy = 2′-2′- pyridine). DFT calculations were performed to predict the geometries of the complexes and to propose the sites at which electron transfer occurs. Among the studied compounds, [CoII(bpy)3(BF4)2] exhibited the highest catalytic rate constant for CO2 reduction (k = 1.22 × 102 M−1·s−1) compared to [CoII(bztpen)Br]PF6 (k = 8.93 × 101 M−1·s−1). The trigonal bipyramidal complex [CoII(TPA)Cl]Cl presented the lowest catalytic rate constant for CO2 reduction (k = 1.47 × 101 M−1·s−1). UV-Vis spectroelectrochemical studies and DFT calculations suggested the formation of [CoI(bztpen)(CO)]+ and [CoI(tpa)(CO)]+ species, which are associated with catalyst deactivation and may account for the lower performance of CO2 reduction.