Reactivity of Niobium(V) and Tantalum(V) Halides with Carbonyl Compounds: Synthesis of Simple Coordination Adducts, C–H bond Activation, C=O Protonation, and Halide Transfer

The metal halides of Group 5 MX5 (M = Nb, Ta; X = F, Cl, Br) react with ketones and acetylacetones affording the octahedral complexes [MX5(ketone)] (2a–j) and [TaX4{κ2(O)-OC(Me)C(R)C(Me)O}] (R = H, Me, 5a–e), respectively. The adducts [MX5(acetone)] are still reactive towards acetone, acetophenone or benzophenone, giving the aldolate species [MX4{κ2(O)-OC(Me)CH2C(R)(R′)O}] (3a–e). The syntheses of 3b (M = Ta, X = F, R = R′ = Ph) and 3d (M = Ta, X = Cl, R = Me, R′ = Ph) take place with concomitant formation of [(Ph2CO)2-H][TaF6], 4a, and [(MePhCO)2-H][TaCl6], 4b, respectively. The compounds [acacH2][TaF6], 6, and [TaF{OC(Me)C(Me)C(Me)O}3][TaF6], 7, have been isolated as by-products in the reactions of TaF5 with acacH and 3-methyl-2,4-pentanedione, respectively. The molecular structures of 2i, 3c, 4a, 4b and 7 have been ascertained by single crystal X-ray diffraction studies.