Electrostatic versus Hydrogen Bonding Control of Selectivity in CO₂ Reduction by Iron Porphyrins

Multielectron, multiproton CO2 reduction selectively to C1 products is an important area of research. In nature, metalloenzymes use second-sphere interactions like hydrogen bonding and electrostatic interactions to control the rate and selectivity to these multielectron and multiproton reactions, e.g., NO2–, SO2, H+, etc. Recent developments have suggested that hydrogen bonding as well as electrostatic interactions in molecular catalysts, like iron porphyrins, results in selective 2e–/2H+ reduction of CO2, as well, to CO or HCOOH and suppresses competitive reduction of protons to H2, which occurs at similar reduction potentials. However, until date, there is no direct and systematic investigation of these two different second-sphere effects on multielectron, multiproton transformation like CO2 reduction. A series of iron porphyrins is synthesized where second-sphere hydrogen bonding and electrostatic interactions are installed in the ortho position of a mesophenyl group in an iron tetraphenyl framework. The results show that both hydrogen bonding and electrostatic interactions can facilitate the selective reduction of CO2 to CO by iron porphyrins using H2O as a proton source. In iron porphyrins with hydrogen bonding interactions, the selectivity for CO increases with an increase in H2O concentrations. However, in the iron porphyrins with electrostatic interactions, the selectivity for CO decreases, and the iron porphyrin becomes more selective for H2 evolution instead at higher H2O concentrations. Excited-state lifetime measurements and molecular dynamics simulations of porphyrins suggest that the solvation of the cationic groups in the periphery of the porphyrin by H2O leads to an increased concentration of water near the metal center, which promotes H2 evolution over CO2 reduction.