The reactivity of 1,1-dialkoxyalkanes with niobium and tantalum pentahalides. Formation of coordination compounds, C–H and C–C bond activation and the X-ray structure of the stable carboxonium species [Me2CCHC(OMe)Me][NbCl5(OMe)]

Abstract

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-OCH₂OCH₂CH₂), 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 [Me₂CCHC(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 [Me₂CCHC(OMe)Me]⁺ is produced from NbF₅/CMe₂(OMe)₂. The complexes NbF₅[OEt(Me)], 9a, and NbF₅[OCH(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[OCH(OMe)] and MeX; otherwise, MF₅[OCH(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.