Single electron reduction of NHC-CO₂-borane compounds

The carbon dioxide radical anion [CO2 center dot-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2 center dot-] 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)(center dot)(-)]. 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 center dot-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2 center dot-]-like species.