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[K-2-OCH2CH(R')OR"] or [Nbl(4){K-1-OCH2CH(R')OR")](2), and equimolar amounts of the corresponding alkyl halides RX. The reaction of NbBr5 with 1,2-climethoxyethane, dme, proceeds with preliminary formation of the ionic species (NbBr4(K-2-dme)(K-1-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 Delta G(r)(degrees) = -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.