Reactivity of niobium(v) and tantalum(v) halides with carbonyl compounds: Synthesis of simple coordination adducts, C–H bond activation, CO protonation, and halide transfer

Abstract

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.