Synthesis of Unsymmetrical Diboron(5) Compounds and Their Conversion to Diboron(5) Cations

Reaction of the bis-catecholatodiboron-NHC adducts B2Cat2(NHC) (NHC = IMe (tetramethylimidazol-2-ylidene), IMes (1,3-dimesitylimidazol-2-ylidene), IDIPP (1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene)) with BCl3 results in the replacement of the catecholato group bound to the four-coordinate boron atom with two chlorides to yield diboron(5) Lewis acid–base adducts of the formula CatBBCl2(NHC). These compounds are precursors to diboron(5) monocations, accessed by adding AlCl3 or K­[B­(C6F5)4] as halide abstraction agents in the presence of a Lewis base. The substitution of the chlorides of CatBBCl2(NHC) for hydrides was achieved using Bu3SnH and a halide abstracting agent to form 1,1-dihydrodiboron(5) compounds, CatBBH2(NHC). Attempts to generate diboron(4) monocations of formula [CatBB­(Y)­(NHC)]+ (Y = Cl, H) led to the rapid formation of CatBY.