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 MX(5) (M = Nb, Ta; X = F, Cl, Br) react with ketones and acetylacetones affording the octahedral complexes MX(5)(ketone)] () and TaX(4){kappa(2)(O)-OC(Me)C(R)C(Me)O}] (R = H, Me, ), respectively. The adducts MX(5)(acetone)] are still reactive towards acetone, acetophenone or benzophenone, giving the aldolate species MX(4){kappa(2)(O)-OC(Me)CH(2)C(R)(R')O}] (). The syntheses of (M = Ta, X = F, R = R' = Ph) and (M = Ta, X = Cl, R = Me, R' = Ph) take place with concomitant formation of (Ph(2)CO)(2)-H]TaF(6)], and (MePhCO)(2)-H]TaCl(6)], respectively. The compounds acacH(2)]TaF(6)], and TaF{OC(Me)C(Me)C(Me)O}(3)]TaF(6)], have been isolated as by-products in the reactions of TaF(5) with acacH and 3-methyl-2,4-pentanedione, respectively. The molecular structures of, and have been ascertained by single crystal X-ray diffraction studies.