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

Iron tetraphenylporphyrin complex modified with fourtrimethylammoniumgroups (Fe-p-TMA) is found to be capable of catalyzingthe eight-electron eight-proton reduction of CO2 to CH4 photochemically in acetonitrile. In the present work, densityfunctional theory (DFT) calculations have been performed to investigatethe 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, andR(4) = four trimethylammonium groups with a total chargeof +4) undergoes three reduction steps, accompanied by the dissociationof the chloride ion to form [Fe(II)-L center dot center dot 2-R4](2+). [Fe(II)-L center dot center dot 2-R4](2+), bearing a Fe(II) center ferromagneticallycoupled with a tetraphenylporphyrin diradical, performs a nucleophilicattack on CO2 to produce the (1)eta-CO2 adduct [CO2 (center dot-)-Fe(II)-L center dot-R4](2+). Two intermolecularproton transfer steps then take place at the CO2 moietyof [CO2 (center dot-)-Fe(II)-L center dot-R4](2+), resulting in thecleavage of the C-O bond and the formation of the criticalintermediate [Fe(II)-CO](4+) after releasing a watermolecule. Subsequently, [Fe(II)-CO](4+) accepts threeelectrons and one proton to generate [CHO-Fe(II)-L center dot-R4](2+), which finally undergoes a successivefour-electron-five-proton reduction to produce methane without formingformaldehyde, methanol, or formate. Notably, the redox non-innocenttetraphenylporphyrin ligand was found to play an important role inCO(2) reduction since it could accept and transfer electron(s)during catalysis, thus keeping the ferrous ion at a relatively highoxidation state. Hydrogen evolution reaction via the formation ofFe-hydride ([Fe(II)-H](3+)) turns out to endure ahigher total barrier than the CO2 reduction reaction, thereforeproviding a reasonable explanation for the origin of the product selectivity. DFT calculations were performed to investigatethe mechanismand selectivity of the Iron tetraphenylporphyrin complex-catalyzedphotochemical reduction of CO2 to CH4. Resultsshow that the redox non-innocent tetraphenylporphyrin ligand can acceptand transfer electrons to keep the ferrous ion at a relatively highoxidation state, thus suppressing the H-2 evolution. Besides,the hydrogen-bonding interaction between Et3NH+ and the O atom of critical intermediates facilitates the reductiontoward producing CH4 over other C1 products.

Automated summary

In this study, density functional theory (DFT) calculations were performed to investigate the reaction mechanism and product selectivity of the iron tetraphenylporphyrin complex-catalyzed photochemical reduction of CO2 to CH4. The results showed that the initial catalyst Fe-p-TMA undergoes three reduction steps, accompanied by the dissociation of the chloride ion. The redox non-innocent tetraphenylporphyrin ligand plays an important role in the CO2 reduction, keeping the ferrous ion at a relatively high oxidation state.