Striking transformations of the hydroborylene ligand in a HB:→NiII complex with isocyanides and CO.

For the first time, the reactivity of the metal- and N-heterocyclic carbene-supported monovalent hydroborylene is reported. Isocyanides react with the hydroborylene NiII complex [{cat(TMSL)Si}(Cl)Ni←:BH(NHC)2] 1 (cat = ortho-C6H4O2; TMSL = N(SiMe3)(Dipp); Dipp = 2,6-Pri2C6H3; NHC = :C[(Pri)NC(Me)]2) to form the hydride-bridged hydroborylene-NiII complexes 2. The reaction of 1 with isoelectronic CO, however, is reversible and furnishes the related unprecedented hydride- and CO-bridged hydroborylene NiII complex 2-CO, which undergoes isomerisation through silyl/NHC exchange at ambient temperature to afford the corresponding hydro(silyl)boryl NiII complex 3. Markedly, 2 readily and quantitatively react with one further molar equiv. of isocyanide to give, under borylene liberation and H/Cl ligand exchange, boraketiminium species, which represent cationic BI complexes. These latter compounds are highly reactive in solution, and can undergo quantitative transformation into previously unknown cyanoborenium cations.