The Effect of Exogenous Acid Identity on Iron Tetraphenylporphyrin-Catalyzed CO₂ Reduction

Iron tetraphenylporphyrin (FeTPP) is a privileged electrocatalyst for the 2e^-/2H⁺ reduction of CO₂ to CO. FeTPP-catalyzed CO₂ reduction typically employs phenol as an exogenous acid to promote the rate-limiting proton-coupled electron transfer. Beyond the observation that catalytic rates increase with decreasing pK_a, the effects of acid identity on reaction kinetics are largely unexplored. Herein, we report rates of FeTPP-catalyzed CO₂ reduction with structurally diverse O-H, N-H, and C-H acids. While many of these acids follow the expected Brønsted relationship, there are several notable exceptions: the fluorinated alcohols hexafluoroisopropanol (log(k_cat) = 4.54) and 2,2,2-trifluoroethanol (log(k_cat) = 3.55)─and the N-H acid imidazole (log(k_cat) = 4.41)─display catalytic rates that are several times greater than rates obtained with similarly acidic phenols. Amides with pK_as < 19 (in dimethyl sulfoxide) display similar activity as comparably acidic O-H acids, while rates obtained with less acidic amides are ∼2 orders of magnitude slower than O-H donors of similar pK_a. Each C-H acid affords poor activity. An Eyring analysis suggests that acids enforcing less ordered transition states afford superior kinetics. This study reveals that acid pK_a is only one relevant parameter for altering catalytic rates, and judicious selection of the acid is crucial for enhancing catalytic rates.