Effect of PDI ligand binding pattern on the electrocatalytic activity of two Ru(II) complexes for CO2 reduction

The electrochemical properties of two complexes, [Ru-II(eta(3)-(N,N,N)-(PDI)-P-OMe)Cl-2(PPh3)](0) (1) and [Ru-II(eta(2)-(C,N)-(PDI)-P-OMe-H)Cl (PPh3)(2)](0) (2), were studied. In octahedral complex 1, bis(imino)pyridine (PDI) is a tridentate eta(3)-N,N,N-coordinated ligand, whereas in trigonal-bipyramidal complex 2, the deprotonated PDI ligand adopts the unusual bidentate binding mode eta(2)-C,N to coordinate to the central Ru(II) ion. Bulk electrolysis in two electrolyte solutions of acetonitrile (MeCN) and tetrahydrofuran (THF) suggests that complexes 1 and 2 have very different electrocatalytic CO2 reduction activities. In MeCN solution, complex 1 can selectively electrocatalytic CO2 reduction to CO with a Faradaic efficiency of about 50% and a turnover frequency (TOF) of 4.4 s(-1), whereas complex 2 can perform electrocatalytic of CO2 reduction with a Faraday efficiency of similar to 22% and a TOF of 0.3 S-1. The electrocatalytic CO2 reduction selectivity and activity of the two complexes are poor when the solvent is changed to THF. Combined with the results of the density functional theory calculation, we propose that the binding pattern of the redox-active ligand (PDI)-P-OMe has a significant effect on the electrocatalytic activity for the two Ru(II)PDI complexes.