The chemistry of niobium and tantalum halides, MX 5, with haloacetic acids and their related anhydrides: Anhydride C–H bond activation promoted by MF 5

Abstract

Niobium and tantalum pentahalides, MX 5 ( 1), react with acetic acid and halo-substituted acetic acids, in 1:1 ratio, to give the dinuclear complexes [MX 4(μ-OOCMe)] 2 [M = Nb, X = Cl, 2a; M = Ta, X = Cl, 2b; Br, 2c] and [MCl 4(μ-OOCR)] 2 [M = Nb, R = CH 2Cl, 4a; CHCl 2, 4c; CCl 3, 4e; CF 3, 4g; CHBr 2, 4i; CH 2I, 4j; M = Ta, R = CH 2Cl, 4b; CHCl 2, 4d; CCl 3, 4f; CF 3, 4h]. The solid state structures of 2b and 4e have been ascertained by X-ray diffraction studies. The reactions of 1 with acetic anhydride and halo-substituted acetic anhydrides result in C–O bond activation and afford 2 and 4, respectively, with concomitant formation of acetyl halides. Moreover, the complexes MCl 5[OC(Cl)Me] [M = Nb, 3a; M = Ta, 3b] have been detected in significant amounts within the mixtures of the reactions of MCl 5 with acetic anhydride. TaI 5 is unreactive, at room temperature, towards both MeCOOH and (MeCO) 2O. MF 5 react with RCOOH (R = Me, CH 2Cl) in 1:1 molar ratio, to afford the ionic compounds [MF 4(RCOOH) 2][MF 6], 5a– d, in high yields. The additions of (RCO) 2O (R = Me, CH 2Cl) to MF 5 give 5, suggesting that anhydride C–H and C–O bonds activation is operative during the course of these reactions.