CARBON-MONOXIDE INSERTION AND COUPLING REACTIONS OF NIOBIUM ALKYNE COMPLEXES

Treatment of the dichloroniobium alkyne complexes Tp'NbCl2(PhC=CR') (R' = CH3 (1), CH2CH3 (2)) (Tp' = hydridotris(3,5-dimethylpyrazolyl)borate) with sodium methoxide gives good yields of the corresponding chiral chloro methoxy derivatives Tp'Nb(Cl)(OCH3)(PhC=CR') (R' = CH3 (3), CH2CH3 (4)). Methyl and phenyl methoxy complexes Tp'Nb(CH3)(OCH3)(PhC=CR') (R' = CH3 (5), CH2CH3 (6)) and Tp'Nb(C6H5)(OCH3)(PhC=CH2CH3) (8) are obtained in high yields upon reaction of either 3 or 4 with methyllithium or phenyllithium. These complexes have been characterized spectroscopically. Due to chirality at niobium, the ethyl groups in 4 and 6 give AMX3 type H-1 NMR spectra; in complex 8, a deceptively simple A2X3 pattern is observed, regardless of the solvent or temperature, even though rotation about the niobium-phenyl bond can be frozen out. Complexes 5,6, and 8 react with carbon monoxide to form five-membered oxaniobiacycles Tp'(CH3O)Nb[C(Ph)C(R)C(CH3)O] (R = CH3 (9), CH2CH3 (10)) and Tp'(CH3O)Nb[C(Ph)C(CH2CH3)C(Ph)O] (12), respectively, resulting from the regioselective coupling of the coordinated alkyne with an acetyl or benzoyl group. An X-ray structure has been obtained for 9 (monoclinic, P2(1)/n, a = 12.804(3) angstrom, b = 16.822(4) angstrom, c = 13.496(3) angstrom, beta = 110.92(2)degrees, Z = 4, refinement of 334 parameters over 2890 unique reflections with I > 2.5sigma(I) led to R = 0.043 and R(w) = 0.057). The oxaniobiacycle is folded like an envelope about the C(alpha)-O axis with the C(alpha)-Nb-O triangle constituting the flap. Examination of metric parameters within the ring and comparison with related complexes indicate that an analogy between the oxaniobiacycle and the bonding description for a metallacyclopentatriene is appropriate.