Amphoteric Lewis Acid/base Activity Enables an Unprecedented Pathway for Low-Energy C-O Cleavage in the Electrocatalytic Reduction of Carbon Dioxide

Molecular electrocatalysts for CO₂-to-CO conversion often operate at large overpotentials, the cleavage of C-O bond in the intermediate largely contributing to this phenomenon. Additional Lewis acids have been shown to aid in weakening the C-O bond. We herein present computational and experimental evidence, with ruthenium polypyridyl based CO₂ reduction electrocatalysts, for a mechanistic route that involves one metal center acting as both Lewis base and Lewis acid at different stages of the catalytic cycle. The Lewis basic character of Ru is seen in the initial nucleophilic attack at CO₂ to form [Ru-CO₂]⁰, while its Lewis acid character allows the formation of a 5-membered metallacyclic intermediate, [Ru-CO₂CO₂]^0,c, by intramolecular cyclization of a linear [Ru-CO₂CO₂]⁰ species that is formed from [Ru-CO₂]⁰ and a second equivalent of CO₂. [Ru-CO₂CO₂]^0,c is crucial for energy-conserving turnover, as it allows for a third reduction at a more positive potential than that of the starting complex Ru²⁺. The calculated activation barrier for C-O bond cleavage in [Ru-CO₂CO₂]^-1,c is dramatically decreased to 10.5 kcal mol^-1 from 60 kcal mol^-1, the latter required for C-O bond cleavage in the linear intermediate [Ru-CO₂CO₂]⁰. The intermediates are characterized experimentally by FT-IR and ¹³C NMR spectroscopy during electrocatalytic turnover and are corroborated by density functional theory (DFT).