The Reactivity of 1,1-Dialkoxyalkanes with Niobium and Tantalum Pentahalides. Formation of Coordination Compounds, C–H and C–C Bonds Activation and the X-Ray Structure of the Stable Carboxonium Species [Me2C=CHC(=OMe)Me][NbCl5(OMe)]

The reactions of the pentahalides NbX5 (X = Cl, Br) with the 1,1-dialkoxyalkanes CHR′(OEt)2 or 1,3-dioxolane yield the coordination adducts NbX5[κ1-(OEt)CHR′(OEt)] (X = Cl, R′ = H, 2a; X = Br, R′ = H, 2b; X = Cl, R′ = Me, 2c; X = Br, R′ = Me, 2d) or NbCl5(κ1-[upper bond 1 start]OCH2OCH2C[upper bond 1 end]H2), 3, respectively. Compounds 2a–c and 3 are stable at room temperature, while 2d slowly converts into the alkoxide NbBr4[OCH(Me)OEt], 4. Room-temperature fragmentations are observed upon reacting CH2(OMe)2, CHMe(OMe)2 and CMe2(OMe)2 with MX5 (M = Nb, Ta; X = Cl, Br). The complexes MX5[O(Me)(CH2X)], 5, and [MX3(OMe)2]2, 6, form selectively from MX5/CH2(OMe)2 (M = Nb, Ta; X = Cl, Br), while mixtures of [NbX4(OMe)]2 (X = Cl, 8a; X = Br, 8b) and organic halides are obtained from NbX5/CHMe(OMe)2. Instead, the reaction of NbCl5 with CMe2(OMe)2 affords the stable carboxonium species [Me2C[double bond, length as m-dash]CHC([double bond, length as m-dash]OMe)Me][NbCl5(OMe)], 7, as prevalent product. Furthermore, 1,1-dialkoxyalkanes are generally activated by MF5 (M = Nb, Ta) at room temperature: according to the cases, alcohols, ethers, esters and ketones have been detected in the reaction mixtures after hydrolysis. In agreement with NMR data, the carboxonium ion [Me2C[double bond, length as m-dash]CHC([double bond, length as m-dash]OMe)Me]+ is produced from NbF5/CMe2(OMe)2. The complexes NbF5[OEt(Me)], 9a, and NbF5[O[double bond, length as m-dash]CH(OEt)], 9b, obtained by addition of CH2(OEt)2 to NbF5, have been characterised by NMR spectroscopy. Trimethyl formate, CH(OMe)3, reacts with NbX5 (X = Cl, Br) with formation of the alkoxides 6 and 8, in admixture with NbX5[O[double bond, length as m-dash]CH(OMe)] and MeX; otherwise, MF5[O[double bond, length as m-dash]CH(OMe)] and MF5(OMe2) have been synthesised from MF5/CH(OMe)3 (M = Nb, Ta). Complete screening of the organic fragments produced in the distinct reactions has been carried out by both GC-MS and NMR analyses on the reaction mixtures, after treatment with water. The solid state structures of the mixed halo-alkoxy complexes [TaX3(OMe)(μ-OMe)]2 (X = Cl, 6c; X = Br, 6d) and of the stable alkylated ketone containing species 7 have been ascertained by X-ray diffraction studies.