Striking transformations of the hydroborylene ligand in a HB:→Ni(II) 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 Ni(II) complex [{cat((TMS)L)Si}(Cl)Ni←:BH(NHC)(2)] 1 (cat = ortho-C(6)H(4)O(2); (TMS)L = N(SiMe(3))(Dipp); Dipp = 2,6-Pr(i)(2)C(6)H(3); NHC = :C[(Pr(i))NC(Me)](2)) to form the hydride-bridged hydroborylene-Ni(II) complexes 2. The reaction of 1 with isoelectronic CO, however, is reversible and furnishes the related unprecedented hydride- and CO-bridged hydroborylene Ni(II) complex 2-CO, which undergoes isomerisation through silyl/NHC exchange at ambient temperature to afford the corresponding hydro(silyl)boryl Ni(II) 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 B(I) complexes. These latter compounds are highly reactive in solution, and can undergo quantitative transformation into previously unknown cyanoborenium cations.