Epoxide ring opening and insertion into the M-X bond of niobium and tantalum pentahalides: Synthesis of dihalide-tris(2-haloalcoholato) complexes

The Group 5 pentahalides MX5 (M = Nb, Ta; X = Cl, Br), 1, react with a variety of epoxides (1,2-epoxybutane, styrene oxide, 2,3-dimethyl-2,3-epoxybutane, epoxycyclohexane, ethylene oxide) in a 1:3 molar ratio to afford the dinuclear dihalide-tris(2-haloalcoholato) complexes [MX2(OR)(2)(mu-OR)](2) [M = Nb, X = Cl, R = -CH(Et)CH2Cl, 2a; M = Nb, X = Cl, R = -CH2CH(Ph)Cl, 2b; M = Nb, X = Cl, R = -C(Me)(2)C(Me)(2)Cl, 2c: M = Nb, X = Br, R = -CH2CH(Ph)Br, 2d; M = Nb, X = Br, R = -C(Me)(2)C(Me)(2)Br, 2e; M = Ta, X = Cl, R = -CH(Et)CH2Cl, 2f; M = Ta, X = Cl, -CH(CH2)(4)CHCl, 2g; M = Ta, X = Br, R = -CH2CH2Br, 2h], in moderate to good yields. The products 2a-h, result from multiple epoxide insertion into metal-halide bonds, and the reactions involving 1,2-epoxybutane and styrene oxide proceed with high regioselectivity. The molecular structure of 2h has been elucidated by X-ray diffraction. Differently, NbF5 adds one equivalent of 2,3-dimethyl-2,3-epoxybutane to give the monomer NbF5[O=C(Me)(Bu-t)], 3, in high yield, as a result of epoxide to ketone isomerization. The reactions of MF5 (M = Nb, Ta) with other epoxides proceed nonselectively according to various pathways, including formation of C-O and C-F bonds and C-C and CH cleavages. (C) 2009 Elsevier Ltd. All rights reserved.