Boron radical cations: facile oxidation of electron rich diborenes

The realization of a phosphine-stabilized diborene, Et3P·(Mes)B=B(Mes)·PEt3 (4), by KC8 reduction of Et3P·B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC-stabilized diborene IMe·(Dur)B=B(Dur)·IMe (1). Importantly, both species feature unusual electron-rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the B-B double bond system. Thus, the stronger s-donor strength and larger electronegativity of the NHC ligand results in an energetically higher-lying HOMO, making 1 a stronger neutral reductant as 4 (1: E1/2=-1.55 V; 4: -1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red-shift of its lowest-energy absorption band with respect to 4. Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron-centered radical cations (5 and 6).