The syntheses of high-valent niobium imido complexes [HB(Im)]Nb(N Bu)Cl (2) and [HB(Im)]Nb(N Bu)Me (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 [(OCHPh)B(Im)]Nb(N Bu)Me (12), which undergoes intramolecular η-arene coordination upon hydrogenation.
Reaction of the free base corrole (Mes(p-OMePh)corrole)H with tantalum trialkyl precursors TaMeCl and TaBnNBu resulted in the formation of the tantalum dichloride (1) and tantalum imido (4) corrole complexes via alkane elimination. The X-ray crystal structures of these two compounds have been determined and the structural parameters are discussed. The Ta centre of 1 was found to sit out of the plane of the corrole ring by 0.903 Å and is cis-ligated, similarly to what has been reported for group 4 porphyrin complexes. From complex 1 we synthesized the dimethyl derivative (2), the reactivity of which is compared to an analogous tantalum dimethyl porphyrin cation. The imido complex 4 reacted with triphenylmethanol and 4-methylbenzyl alcohol, resulting in different extents of protonation of the imido group.
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