Novel Asymmetric Iron Porphyrins for Photocatalytic CO2 Reduction to CH4

Abstract

Developing earth-abundant transition metal catalysts for CO2 reduction is a promising approach for sustainable energy conversion. Here, the synthesis and photocatalytic activity of two novel asymmetric iron porphyrin complexes, namely iron 5-(N-benzyloxycarbonyl-4-aminophenyl)-10,15,20-tris(4-aminophenyl)porphyrin (Fe–p–NH2–Cbz) and iron 5-(N-benzyloxycarbonyl-4-aminophenyl)-10,15,20-tris(4-(trimethylammonio)phenyl)porphyrin (Fe–p–TMA–Cbz) for visible-light-driven CO2 reduction to CO and CH4 are reported. Under blue light (447 nm) irradiation, Fe–p–NH2–Cbz and Fe–p–TMA–Cbz achieve turnover numbers (TONs) of 20 and 23 for CO, and 6 and 10 for CH4, respectively, using a commercially available organic photosensitizer (Phenox), triethylamine (TEA) as sacrificial electron donor and trifluoroethanol (TFE) as proton source. In this reaction conditions, Fe–p–NH2–Cbz and Fe–p–TMA–Cbz demonstrate catalytic activity comparable to its symmetric counterpart iron 5,10,15,20-tetra(4-(trimethylammonio)phenyl)porphyrin (Fe–p–TMA), previously reported by Prof. Marc Robert's group, achieving a TON of 23 for CO and of 11 for CH4. Isotopic labeling studies using 13CO2 confirm that CH4 and CO products come from photocatalytic CO2 reduction. The results highlight the potential of iron porphyrins as tunable molecular catalysts for photocatalytic CO2 reduction beyond two electrons for artificial photosynthesis applications.