Highly Acidic Second Coordination Spheres Promote In Situ Formation of Iron Phlorins Exhibiting Fast and Selective CO ₂ Reduction

Protic functional groups in the secondary coordination sphere (SCS) can lower reaction barriers for reductive electrocatalytic transformations by directing proton transfer from an exogenous acid to a bound substrate. In a recent report with iron tetraphenylporphyrin (Fe-TPP) catalysts bearing SCS amides, we found that pairing a more acidic SCS with a more acidic exogenous phenol acid provides the fastest kinetics for CO₂ reduction to CO. Expanding on this precedent, we report a new series of Fe-TPP catalysts bearing highly acidic SCS thioamides (p<i>K</i>_as of 17.5 ± 0.1 to 18.7 ± 0.1 in MeCN) and describe their catalytic activity in the presence of exogenous benzoic acid. Despite the uncommonly acidic conditions, we observe the selective 2e^-/2H⁺ reduction of CO₂ to CO with minimal competitive H₂ evolution or porphyrin decomposition. Decreases in SCS p<i>K</i>_a continue to provide significant rate enhancements, and the catalyst bearing the most acidic thioamide displays kinetics (log(<i>k</i>_cat) = 8.65 ± 0.09) that are comparable to those of the leading molecular systems. Cyclic voltammetry, UV-visible spectroelectrochemistry, kinetic analysis, and density functional theory show that the highly acidic SCS thioamide groups change catalyst speciation by promoting in situ protonation of the reduced iron porphyrin to form an iron phlorin. This iron phlorin is reduced to form a highly active and selective catalyst for CO₂ reduction that operates at more positive potentials compared with traditional Fe-TPP catalysts. This work therefore reveals a new role for the SCS in promoting beneficial changes to catalyst speciation and motivates further investigation of reduced metalloporphyrinoids.