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

Abstract

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.