Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes

The one-electron reduction of (CAAC^Me)BCl₃ (CAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields the dichloroboryl radical [(CAAC^Me)BCl₂]˙. Furthermore, the twofold reduction of (CAAC^Me)BCl₃ in the presence of a range of Lewis bases (L = CAAC^Me, N-heterocyclic carbene, phosphine) yields a series of doubly base-supported (CAAC^Me)LBCl chloroborylenes, all of which were structurally characterised. NMR and UV-vis spectroscopic and electrochemical data for (CAAC^Me)LBCl show that the boron centre becomes more electron-rich and the HOMO-LUMO gap widens as L becomes less π-accepting. A [(CAAC^Me)BCl₂]^- boryl anion coordination polymer was isolated as a potential intermediate in these reductions. In most cases the reduction of the chloroborylenes resulted in the formation of the corresponding hydroborylenes or derivatives thereof, as well as ligand C-H activation products.