Enhancing Electrocatalytic CO₂ Reduction of Nickel Porphyrin via Sulfur-Coordination and Through-Space Effects

Metalloporphyrin-based complexes featuring stable tetrapyrrole structures and tunable coordination environments are promising electrocatalysts for CO₂ reduction. Modifying the first-coordination atoms that bonded with the central metal or changing the outer-sphere functional groups was found to affect the catalytic performance. However, the combined effect of these two strategies remains unexplored. Herein, using nickel tetraphenyl porphyrin (NiN₄-TPP, 1) as the original catalyst, we first investigated the effect of first-coordination modification by replacing one N atom with a S atom, yielding NiN₃S-TPP (2). Density functional theory calculations revealed that, as an electrocatalyst, NiN₃S-TPP outperforms NiN₄-TPP in converting CO₂ to CO. Notably, the S-substituted catalyst 2 exhibits a positively shifted reduction potential─a trend consistent with experimental observations. This is attributed to the lowered LUMO energy level after introducing the S atom. Moreover, the S-substitution stabilizes the [*–COOH] intermediate and shifts the rate-determining step from the protonation of [1–CO₂]⁻ to form [1–COOH]⁰ to the absorption of CO₂ on [2]⁻, reducing the barrier. Charge analysis reveals that the S atom donates electrons to the metal center and porphyrin ligands of the catalyst. Then, upon binding to COOH, these reserved electrons are transferred to COOH, thus facilitating the interaction and stabilizing the intermediate. Moreover, peripheral substituent modifications with the cationic –N(Me)₃⁺ functional group yield great enhancement in the catalytic performance. It shifts the reduction potential to be more positive and further stabilizes the [*–COOH] intermediate via through-space electrostatic interaction. This work demonstrates how first- and peripheral-coordination modifications synergistically enhance CO₂ reduction catalysis, offering a strategy for designing efficient porphyrin-based electrocatalysts through core coordination environment regulation and through-space interactions.