Promoting selective electrochemical CO2 reduction under unconventionally acidic conditions through secondary coordination sphere positioning

Abstract

While several studies have investigated the effects of protic secondary coordination sphere (SCS) groups on the kinetics of iron tetraphenylporphyrin (FeTPP) catalysed CO₂ reduction, few have examined how a protic SCS might alter reaction selectivity. Under mildly acidic conditions, FeTPP-based catalysts are selective towards the 2e⁻/2H⁺ reduction of CO₂ to CO; however, in the presence of more acidic proton donors, indiscriminate proton transfers often result in parasitic H₂ evolution. This report investigates how SCS amide positioning alters CO versus H₂ selectivity during CO₂ reduction with a series of four FeTPP isomers bearing SCS amides at varying positions around the porphyrin core: NH donors are placed at either the meta or ortho position of the meso aryl porphyrin ring, as well as proximal (closer) or distal (farther away) to the porphyrin plane. In the presence of a conventional, weakly acidic proton source (phenol; pK_a = 29.2 in MeCN), all isomers display the expected high faradaic efficiency (FE) towards CO (FE_CO = 67–85%) along with minimal H₂ evolution (FE_H2 = 3–13%). With a significantly stronger acid (3,5-bis(trifluoromethyl)phenol; pK_a = 23.8 in MeCN), H₂ becomes the major product when using the ortho-distal or both meta isomers (FE_H2 = 45–65%) as well as unfunctionalized FeTPP (FE_H2 = 78%). Importantly, the ortho-proximal isomer shows dramatically rescued CO selectivity under these unconventionally acidic conditions (FE_CO = 83 ± 4%). These results show how proper SCS placement impacts reaction selectivity during CO₂ reduction, particularly with respect to minimizing indiscriminate proton transfers that lead to undesirable reactivity.