TeX4 (X = F, Cl, Br) as Lewis acids – complexes with soft thio- and seleno-ether ligands

Abstract

TeF₄ reacts with OPR₃ (R = Me or Ph) in anhydrous CH₂Cl₂ to give the colourless, square based pyramidal 1 : 1 complexes [TeF₄(OPR₃)] only, in which the OPR₃ is coordinated basally in the solid state, (R = Me: d(Te–O) = 2.122(2) Å; R = Ph: d(Te–O) = 2.1849(14) Å). Variable temperature ¹⁹F{¹H}, ³¹P{¹H} and ¹²⁵Te{¹H} NMR spectroscopic studies strongly suggest this is the low temperature structure in solution, although the systems are dynamic. The much softer donor ligands SMe₂ and SeMe₂ show a lower affinity for TeF₄, although unstable, yellow products with spectroscopic features consistent with [TeF₄(EMe₂)] are obtained by the reaction of TeF₄ in neat SMe₂ or via reaction in CH₂Cl₂ with SeMe₂. TeX₄ (X = F, Cl or Br) causes oxidation and halogenation of TeMe₂ to form X₂TeMe₂. The Br₂TeMe₂ hydrolyses in trace moisture to form [BrMe₂Te–O–TeMe₂Br], the crystal structure of which has been determined. TeX₄ (X = Cl or Br) react with the selenoethers SeMe₂, MeSe(CH₂)₃SeMe or o-C₆H₄(SeMe)₂ (X = Cl) in anhydrous CH₂Cl₂ to give the distorted octahedral monomers trans-[TeX₄(SeMe₂)₂], cis-[TeX₄{MeSe(CH₂)₃SeMe}] and cis-[TeCl₄{o-C₆H₄(SeMe)₂}], which have been characterised by IR, Raman and multinuclear NMR (¹H, ⁷⁷Se{¹H} and ¹²⁵Te{¹H}) spectroscopy, and via X-ray structure determinations of representative examples. Tetrahydrothiophene (tht) can form both 1 : 1 and 1 : 2 Te : L complexes. For X = Br, the former has been shown to be a Br-bridged dimer, [Br₃(tht)Te(μ-Br)₂TeBr₃(tht)], by crystallography with the tht ligands anti, whereas the latter are trans-octahedral monomers. Like its selenoether analogue, MeS(CH₂)₃SMe forms distorted octahedral cis-chelates, [TeX₄{MeS(CH₂)₃SMe}], whereas the more rigid o-C₆H₄(SMe)₂ unexpectedly forms a zig-zag chain polymer in the solid state, [TeCl₄{o-C₆H₄(SMe)₂}]_n, in which the dithioether adopts an extremely unusual bridging mode. This is in contrast to the chelating monomer, cis-[TeCl₄{o-C₆H₄(SeMe)₂}], formed with the analogous selenoether and may be attributed to small differences in the ligand chelate bite angles. The wider bite angle xylyl-linked bidentates, o-C₆H₄(CH₂EMe₂)₂ behave differently; the thioether forms cis-chelated [TeX₄{o-C₆H₄(CH₂SMe)₂}] confirmed crystallographically, whereas the selenoether undergoes C–Se cleavage and rearrangement on treatment with TeX₄, forming the cyclic selenonium salts, [C₉H₁₁Se]₂[TeX₆]. The tetrathiamacrocycle, [14]aneS₄ (1,4,8,11-tetrathiacyclotetradecane), does not react cleanly with TeCl₄, but forms the very poorly soluble [TeCl₄([14]aneS₄)]_n, shown by crystallography to be a zig-zag polymer with exo-coordinated [14]aneS₄ units linked via alternate S atoms to a cis-TeCl₄ unit. Trends in the ¹²⁵Te{¹H} NMR shifts for this series of Te(IV) halides chalcogenoether complexes are discussed.