Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO2 reduction. Herein, photocatalytic conversion of CO2 to CO even at low CO2 concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO2 binding over proton. The following 2 H+-assisted Fe-CO formation is rate-determining for selective CO2-to-CO reduction. A pendant, proton-shuttling alpha-OH group (2) initiates PCET for predominant H-2 evolution, while an alpha-OMe group (3) cancels the selectivity control for either CO or H-2. The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H-2 ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO2RR by photocatalysis.