1,3,2-Diazaborole-derived carbene complexes of boron.

Reaction of 2-bromo-1,3,2-diazaborole (1) with excess BBr ₃ induces 1,2-hydrogen migration, giving 1,3,2-diazaborole-derived carbene complexes of boron bromide (2). Compound 2 exists in a dynamic solution equilibrium with 1. The ¹ H NMR study shows that the equilibrium lies to the right side of the dissociation reaction of 2. Parallel reaction of 1 with excess BI ₃ gives the corresponding 1,3,2-diazaborole-derived carbene boron iodide complex (3). Notably, in contrast to 2, the dissociation reaction of 3 largely lies to the left side, favouring the formation of 3. The dynamic solution equilibrium behaviours of 2 and 3 are probed by both experimental and theoretical methods.