Further Insights into the Chemistry of Niobium and Tantalum Pentahalides with 1,2-Dialkoxyalkanes: Synthesis of Bromo- and Iodoalkoxides, Spectroscopic and Computational Studies

The room temperature reactions of a series of 1,2-dialkoxyalkanes ROCH2CH(R′)OR′′ with MX5 (M = Nb, Ta; X = Br, I) in 1:1 ratio result in single C–O bond cleavage and high-yield formation of the halo-alkoxides MBr4[κ2-OCH2CH(R′)OR′′] or [NbI4{κ1-OCH2CH(R′)OR′′}]2, and equimolar amounts of the corresponding alkyl halides RX. The reaction of NbBr5 with 1,2-dimethoxyethane, dme, proceeds with preliminary formation of the ionic species [NbBr4(κ2-dme)(κ1-dme)][NbBr6], 3b, which has been identified by solution NMR at low temperature and conductivity analyses. The gas-phase structure of 3b has been optimized by DFT calculations, confirming that the dme ligands adopt bidentate and monodentate coordination, respectively. Although the formation of NbOBr3(dme), 4b, 1,4-dioxane and MeBr from NbBr5/dme (ratio 1:2) is an exoergonic process (calculated = −115.96 kcal mol−1), it is inhibited at room temperature. High temperature conditions enhance the production of 1,4-dioxane at the expense of selectivity. The dinuclear species NbOBr3(dme)NbBr5 (Nb–O–Nb), 5b, (X-ray) has been isolated in modest yield as byproduct of the room temperature reaction of NbBr5 with dme. In general, the 1:2 molar reactions of NbX5 (X = Br, I) with ROCH2CH(R′)OR′′ occur with the exclusion of nearly one equivalent of organic reactant.