Flash Communication: Bismuthinidenes as Lewis Bases in Reversible C(sp³)–O Bond Cleavage

Herein, we describe a unique example of a low-valent monocoordinated Bi-(I) complex acting as a Lewis base partner in a FLP activation reaction. The interplay between a bulky and rigid Bi-(I) and the Lewis acidic tris-(pentafluorophenyl)-borane (BCF) enables the opening of tetrahydrofuran (THF), delivering a zwitterionic Bi-(+)/B(-) product, which could be isolated and characterized spectroscopically and through SC-XRD. The zwitterionic bismuth complex exhibits remarkable spectroscopic features in the visible range, namely an LMCT band corresponding to the transition from the aromatic fluorene to the highly electron-poor cationic bismuth center. Interestingly, excitation of this band results in ring-closing C-O bond formation and subsequent recovery of the Bi-(I), thus constituting a photoreversible FLP activation with bismuth.