Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron-Porphyrin Catalyst.

Iron tetraphenylporphyrin complex modified with four trimethylammonium groups (Fe-p-TMA) is found to be capable of catalyzing the eight-electron eight-proton reduction of CO2 to CH4 photochemically in acetonitrile. In the present work, density functional theory (DFT) calculations have been performed to investigate the reaction mechanism and to rationalize the product selectivity. Our results revealed that the initial catalyst Fe-p-TMA ([Cl-Fe(III)-LR4]4+, where L = tetraphenylporphyrin ligand with a total charge of -2, and R4 = four trimethylammonium groups with a total charge of +4) undergoes three reduction steps, accompanied by the dissociation of the chloride ion to form [Fe(II)-L••2-R4]2+. [Fe(II)-L••2-R4]2+, bearing a Fe(II) center ferromagnetically coupled with a tetraphenylporphyrin diradical, performs a nucleophilic attack on CO2 to produce the 1η-CO2 adduct [CO2•--Fe(II)-L•-R4]2+. Two intermolecular proton transfer steps then take place at the CO2 moiety of [CO2•--Fe(II)-L•-R4]2+, resulting in the cleavage of the C-O bond and the formation of the critical intermediate [Fe(II)-CO]4+ after releasing a water molecule. Subsequently, [Fe(II)-CO]4+ accepts three electrons and one proton to generate [CHO-Fe(II)-L•-R4]2+, which finally undergoes a successive four-electron-five-proton reduction to produce methane without forming formaldehyde, methanol, or formate. Notably, the redox non-innocent tetraphenylporphyrin ligand was found to play an important role in CO2 reduction since it could accept and transfer electron(s) during catalysis, thus keeping the ferrous ion at a relatively high oxidation state. Hydrogen evolution reaction via the formation of Fe-hydride ([Fe(II)-H]3+) turns out to endure a higher total barrier than the CO2 reduction reaction, therefore providing a reasonable explanation for the origin of the product selectivity.