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{kappa(2)(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{kappa(2)(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 [acacH(2)][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.