Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2RR), which promotes multiple synergistic effects to enhance CO2RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2RR activity by 2,000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP), achieving turnover frequencies (TOF) exceeding 109 s-1 with >95% Faradaic efficiency for CO product. Owing to these dual, synergistic second-sphere enhancements, this catalyst also retains high activity and selectivity for homogeneous CO2RR in aqueous media. Notably, the observed TOF value for Fe-ortho-im(H) is 14,000-fold higher than the Fe-para-im(H) positional analog, but only 40-fold higher than the Fe-ortho-im(Me) congener that retains the proximal positive charge but lacks the C2-H hydrogen-bonding moiety, revealing that through-space charge effects have a greater impact on catalytic CO2RR performance compared to hydrogen bonding in this context. This work emphasizes the use of second-sphere pendants that can promote multiple synergistic effects as a design strategy for achieving CO2 reduction catalysis in water.