Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal-Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways.

Both [CoII(qpy)(H2O)2]2+ and [FeII(qpy)(H2O)2]2+ (with qpy = 2,2':6',2″:6'',2‴-quaterpyridine) are efficient homogeneous electrocatalysts and photoelectrocatalysts for the reduction of CO2 to CO. The Co catalyst is more efficient in the electrochemical reduction, while the Fe catalyst is an excellent photoelectrocatalyst ( ACS Catal. 2018, 8, 3411-3417). This work uses density functional theory to shed light on the contrasting catalytic pathways. While both catalysts experience primarily ligand-based reductions, the second reduction in the Co catalyst is delocalized onto the metal via a metal-ligand bonding interaction, causing a spin transition and a distorted ligand framework. This orbital interaction explains the experimentally observed mild reduction potential and slow kinetics of the second reduction. The decreased hardness and doubly occupied dz2-orbital facilitate a σ-bond with the CO2-π* in an η1-κC binding mode. CO2 binding is only possible after two reductions resulting in an EEC mechanism (E = electron transfer, C = chemical reaction), and the second protonation is rate-limiting. In contrast, the Fe catalyst maintains a Lewis acidic metal center throughout the reduction process because the metal orbitals do not strongly mix with the qpy-π* orbitals. This allows binding of the activated CO2 in an η2-binding mode. This interaction stabilizes the activated CO2 via a π-type interaction of a Fe-t2g orbital and the CO2-π* and a dative bond of the oxygen lone pair. This facilitates CO2 binding to a singly reduced catalyst resulting in an ECE mechanism. The barrier for CO2 addition and the second protonation are higher than those for the Co catalyst and rate-limiting.