Further insights into the chemistry of niobium and tantalum pentahalides with 1,2-dialkoxyalkanes: Synthesis of bromo- and iodoalkoxides, spectroscopic and computational studies

Abstract

The room temperature reactions of a series of 1,2-dialkoxyalkanes ROCH 2 CH(R′)OR′′ with MX 5 (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 MBr 4 [κ 2-OCH 2 CH(R′)OR′′] or [NbI 4 {κ 1-OCH 2 CH(R′)OR′′}] 2 , and equimolar amounts of the corresponding alkyl halides RX. The reaction of NbBr 5 with 1,2dimethoxyethane, dme, proceeds with preliminary formation of the ionic species [NbBr 4 (κ 2dme)(κ 1-dme)][NbBr 6 ], 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 NbOBr 3 (dme), 4b, 1,4-dioxane and MeBr from NbBr 5 /dme (ratio 1:2) is an exoergonic process (calculated ∆G r° =-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 NbOBr 3 (dme)NbBr 5 (Nb-O-Nb), 5b, (X-ray) has been isolated in modest yield as byproduct of the room temperature reaction of NbBr 5 with dme. In general, the 1:2 molar reactions of NbX 5 (X = Br, I) with ROCH 2 CH(R′)OR′′ occur with the exclusion of nearly one equivalent of organic reactant.

Interaction of niobium and tantalum pentahalides with O-donors: coordination chemistry and activation reactions

“…The difference is probably the consequence of the higher steric hindrance exerted by the heavier halide (Cl À or Br À ) with respect to the fluoride. 48 The two possible outcomes (i. À ion, chelating effect, solvent) concur in determining the nature of the coordination product, and the overall situation is probably more complicated than what was tentatively theorised in the past. 49,50 The species MX 5 L and [MX 4 L 2 ][MX 6 ] can be discriminated on the basis of electrical molar conductivity (L M ) measured in a chlorinated solvent.…”

“…For instance, the generation of NbOBr 3 (dme)/ 1,4-dioxane has not been observed from NbBr 5 /excess dme, although computer calculations suggest that this reaction is highly exoergonic. 48 The ionic derivatives [MF 4 (k 2 -ROCH 2 CH 2 OR) 2 ][MF 6 ], prepared by addition of 1,2-dialkoxyalkanes (R = Me, Et) to MF 5 (see Section 2.2), undergo activation of the organic part at high temperature. The activation is driven by the strength of the M-F bond, preserving the [MF 5 ] frame, and gives selectively OR 2 (R = Me, Et) and 1,4-dioxane, see Scheme 25.…”