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(μ-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)2C(Me)2Cl, 2c; M = Nb, X = Br, R = –CH2CH(Ph)Br, 2d; M = Nb, X = Br, R = –C(Me)2C(Me)2Br, 2e; M = Ta, X = Cl, R = –CH(Et)CH2Cl, 2f; M = Ta, X = Cl, , 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)(But)], 3, in high yield, as a result of epoxide to ketone isomerization. The reactions of MF5 (M = Nb, Ta) with other epoxides proceed non-selectively according to various pathways, including formation of C–O and C–F bonds and C–C and C–H cleavages.