Novel Asymmetric Iron Porphyrins for Photocatalytic CO₂ Reduction to CH₄

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.