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

Abstract

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.

Ligand-Bound CO2 as a Nonclassical Route toward Efficient Photocatalytic CO2 Reduction with a Ni N-Confused Porphyrin

“…The reaction pathway involves three intermediates of *CO 2 , *COOH, and *CO, among which *CO 2 is found to be a weakly bound adduct without electron transfer from Ni to the CO 2 molecule (Table S3). Throughout the catalysis, *COOH generation represents the most endergonic step, which is in agreement with previous theoretical studies. , Clearly, NiNCP–(N – ) furnishes the most favorable CO 2 RR energetics with the key intermediate *COOH being substantially stabilized (Figure b). On the contrary, NiTPP and NiNCP both exhibit a low stabilization ability for *COOH.…”

Boosting the catalytic performance of metalloporphyrin-based covalent organic frameworks via coordination engineering for CO2 and O2 reduction

“…Although not the focus of this study, a great portion of spin distribution was observed on the porphyrin ligands, as shown in Co–N 2 O 2 –COOH intermediates (Fig S11, ESI†). This indicates the non-innocent role of the porphyrin ligands, which has been suggested by Liao et al 86–89 This observation gives us a hint that modifying the porphyrin ligand could further tune the catalytic ability of the Por-COFs.…”

“…It is worth noting that these newly designed Por-COFs may yield deep-reduction C1 products (CH 3 OH and CH 4 ), especially when combined with photosensitizers, sacrificial electron donors, or supporting materials. 22,82,87,89–91 These aspects merit further exploration, but we have limited this work to CO 2 -to-CO reduction.…”

Local protons enhance photocatalytic CO2 reduction by porphyrinic zirconium-organic frameworks

“…The central Fe atom of the porphyrin ring was well-accepted to be the active site for CO 2 reduction, which was consistent with previous reports. 52,53 The second coordination spheres of polyhydroxy groups offer a high local concentration of protons, which enhanced the photocatalytic efficiencies in terms of both rate constant and catalytic selectivity. To further understand the better catalytic performance of MOF-OH than that of MOF-OCH 3 , the Gibbs free energy diagrams for CO 2 reduction to CO were systematically considered and explored (Fig.…”