Role of 2^nd sphere H-bonding residues in tuning the kinetics of CO₂ reduction to CO by iron porphyrin complexes

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO₂. It has been recently established that the reduction of CO₂ by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(ii)-CO₂^2- and a Fe(ii)-COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO₂ reduction. In this report, a series of iron porphyrins that vary only in the distal H-bonding network are further investigated and these exhibit turnover frequencies (TOFs) ranging from 1.0 s^-1 to 10³ s^-1. The experimental TOFs correlate with the H-bonding ability of the distal superstructure of these iron porphyrin complexes and analysis suggests that H-bonding alone can tune the rate of CO₂ reduction by as much as 1000 fold. DFT calculations provide a detailed insight into how the, apparently weak, 2^nd sphere interactions lead to efficient CO₂ activation for reduction. The ability to tune CO₂ reduction rates by changing the H-bonding residue instead of the acid source is a convenient way to tune CO₂ reduction electrocatalysis without compromising selectivity by introducing competitive hydrogen evolution reaction or formate generation.