Single electron reduction of NHC–CO2–borane compounds

Abstract

The carbon dioxide radical anion [CO₂˙⁻] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO₂ to generate [CO₂˙⁻] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC–CO₂–BR₃ species – generated from the Frustrated Lewis Pair (FLP)-type activation of CO₂ by N-heterocyclic carbenes (NHCs) and boranes (BR₃) – undergo single electron reduction at a less negative potential than free CO₂. A net gain of more than one volt was notably measured with a CAAC–CO₂–B(C₆F₅)₃ adduct, which was chemically reduced to afford [CAAC–CO₂–B(C₆F₅)₃˙⁻]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO₂ moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO₂˙⁻] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO₂˙⁻]-like species.