Hydroboration Reactivity of Niobium Bis(N-heterocyclic carbene)borate Complexes.

The syntheses of high-valent niobium imido complexes [H2B(MesIm)2]Nb(N tBu)Cl2 (2) and [H2B(MesIm)2]Nb(N tBu)Me2 (3) bearing a bis(NHC)borate (NHC = N-heterocyclic carbene) supporting ligand are described. The reaction of the dimethyl complex (3) with excess CO generates an equivalent of acetone, which inserts into a B-H bond of the bis(NHC)borate ligand to form a boryl isopropoxide/niobium(III) dicarbonyl complex (4). This mode of hydroboration reactivity also occurs readily upon the treatment of either 2 or 3 with ketones, aldehydes, and isocyanates. Modification of the bis(carbene) ligand of 3 via the hydroboration of benzophenone produces the dimethylniobium complex [(OCHPh2)2B(MesIm)2]Nb(N tBu)Me2 (12), which undergoes intramolecular η6-arene coordination upon hydrogenation.