Single electron reduction of NHC-CO 2 -borane compounds.

The carbon dioxide radical anion [CO 2 ˙ - ] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO 2 to generate [CO 2 ˙ - ] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO 2 -BR 3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO 2 by N-heterocyclic carbenes (NHCs) and boranes (BR 3 ) - undergo single electron reduction at a less negative potential than free CO 2 . A net gain of more than one volt was notably measured with a CAAC-CO 2 -B(C 6 F 5 ) 3 adduct, which was chemically reduced to afford [CAAC-CO 2 -B(C 6 F 5 ) 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 CO 2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO 2 ˙ - ] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO 2 ˙ - ]-like species.