Electrocatalytic CO2 Reduction by Cobalt Bis(pyridylmonoimine) Complexes: Effect of Ligand Flexibility on Catalytic Activity

A series of Co complexes with bis(pyridylmonoimine)-based ligands with different degrees of structural flexibility have been prepared and studied for the electrocatalytic CO2 reduction reaction to CO. First, electrochemical kinetic studies of the structurally rigid [Co(L-L)] complex show that it undergoes a reductive dimerization upon reduction to the Co-I complex. This dimerization is facilitated by the planar geometry of the [Co(L-L)] complex. The dimer structure dissociates after reduction of the ligand, forming a monomer species that is active for CO2 reduction. The reductive dimerization can be sterically prevented either by adding the strong axially coordinating ligand such as triphenylphosphine (PPh3) or by distorting the square planarity of the Co geometry by modulating the flexibility of the ligand scaffold. The more flexible [Co(L-RL)] complexes prevent catalyst dimerization and operate with more positive catalytic onset potentials for CO2 reduction compared to the more rigid [Co(L-L)] complex but operate with lower overall activity in the presence of a proton source. CO-binding and inhibition studies provide evidence that the lower activity for CO2 reduction of the more flexible [Co(L-R-L)] complexes compared to [Co(L-L)] is due to CO poisoning because of the stronger binding affinity of the CO product to the flexible [Co(L-R-L)] complexes. This highlights an important trade-off in catalyst design for this class of molecular electrocatalysts: Co bis(pyridylmonoimine) complexes with higher degrees of structural flexibility prevent dimerization and shift the onset of CO2 reduction catalysis to more positive potentials but decrease the maximum activity due to CO product inhibition.