Mixed Polyatomic Cations of Sulfur, Selenium and Tellurium

Abstract

<p>The study of the behavior of the chalcogen elements sulfur, selenium and tellurium with the strong Lewis acids SbF₅ and AsF₅ in SO₂ solution has been continued. A large number of new salts of polyatomic cations of the chalcogens have been prepared and characterized by their X-ray crystal structures, namely (Te₂Se₈) (AsF₆)₂, (Te₄.₅Se₅.₅) (AsF₆)₂, (Te₂Se₆) (Te₂Se₈) (ȦsF₆)₄ (SO₂)₂, (Te₆)(Se₈)(AsF₆)₆(SO₂), (Te₂Se₄)(SbF₆)₂, (Te₂.₇Se₃.₃) (SbF₆)₂, (Te₃.₄Se₂.₆) (SbF₆)₂, (Te₂Se₄) (Sb₃F₁₄) (SbF₆), (Te₂.₁Se₃.₉) (SbF₆)₂ and (S₃.₀Se₁.₀)₂ (Sb₄F₁₇) (SbF₆)₃. Many of the cations in these compounds, those with non-integral subscripts, possess occupational disorder with some or all of the cation sites partially occupied by two different chalcogen elements. The previously known, but incompletely characterized compounds (Te₂Se₂) (Sb₃F₁₄) (SbF₆) and (Te₃.₀Se₁.₀) (Sb₃F₁₄) (SbF₆) and reprepared from stoichiometric reactions and their compositions confirmed by ⁷⁷Se and ¹²⁵Te NMR spectroscopy and X-ray crystallography. The new compounds (Te₃S₃) (SbF₆)₂, (Te₂Se₈) (SbF₆)₂ and (Te₂Se₈) (SbF₆)₂ (SO₂) were prepared and were found to be isomorphous with their hexafluoroarsenate analogues. The new Te₂Se₆²⁺ cation, which co-crystallizes with Te₂Se₈²⁺ in the compound (Te₂Se₆) (Te₂Se₈) (AsF₆)₄ (SO₂)₂, has a novel cube-like structure, which is very different from that of S₈²⁺ and Se₈²⁺, the known homopolyatomic cations of the same average oxidation state. The Te₂Se₆²⁺ structure is compared with the structures of the other polyatomic cations and with the P₁₁³⁻ and As₁₁³⁻ anions. Selenium-77 and ¹²⁵Te NMR studies indicate that the Te₂Se₆²⁺ and Se₈²⁺ cations retain their structures in solution. The spectra for a Te₂Se₆²⁺ sample enriched to 77.3% in ¹²⁵Te are dominate by AA'X and AA'XX' satellite subspectra. The compound (S₃.₀Se₁.₀)₂ (Sb₄F₁₇) (SbF₆)₃ provides the first example of a mixed Se-S cation as well as the only example of the mixed-valence, Sb(III)-Sb(V), Sb₄F₁₇⁻ anion. The orientations of secondary bonds to Sb(III) atoms in the Sb₄F₁₇⁻ and Sb₃F₁₄⁻ anions and to divalent and trivalent chalcogen atoms of the cations are described. These secondary bonds form in directions that cap faces or bridge edges of polyhedra defined by the primary bonds and lone pairs of electrons. Unit cell volumes for the known structures of the polyatomic cations are compared to determine effective volumes for the AsF₆⁻, SbF₆⁻ and Sb₃F₁₄⁻ anions and the S, Se and Te atoms. These volumes have some predictive value in determining the composition of unknown compounds of known cell volume. Selenium-77 and ¹²⁵Te NMR data are presented for many of the cations, including spin-lattice relaxation time (T₁) measurements for the SᵪSe₄_ᵪ²⁺ cations.</p> <p>Several new salts of MX₃⁺ cations (M = S, Se, Te; X = F, Cl, Br, I) were prepared and crystal structures were determined for the compounds (TeCl₃) (AlCl₄) - triclinic, (TeCl₃) (AsF₆), (TeCl₃) (SbF₆) and (TeF₃)₂(SO₄). The geometries of these and other MX₃⁺ cations are compared with each other and with the isoelectronic neutral molecules. Trends are explained in term of VSEPR arguments and the strengths of anion-cation interactions (secondary bonds). The ¹⁹F, ⁷⁷Se and ¹²⁵Te NMR spectra of the SeF₃⁺ and TeF₃⁺, cations were recorded and the reduced density M-F coupling constants compared and related to the probable geometries of the cations in solution.</p> <p>Solutions of the mixed F/OTeF₅ compounds TeFᵪ(OTeF₅)₄_ᵪ and [TeFᵪ (OTeF₅)₃_ᵪ]⁺[AsFy (OTeF₅)₆_y]⁻ were prepared and characterized by ¹²⁵Te NMR spectroscopy. Trends in the ¹⁹F-¹²⁵Te(IV), ¹⁹F-¹²⁵Te(VI) and ¹²⁵Te(IV)-¹²⁵Te(VI) scalar spin-spin coupling constants are explained in terms of the greater ionic character of the Te--F bond compared to the Te--OTeF₅ bond. The As(OTeF₅)₆- anion was of sufficiently high symmetry for ⁷⁵As NMR spectra to be recorded in CH₃CN and SO₂ solvents. The activation energy for Berry pseudorotation in Te(OTeF₅)₄ was determined from variable temperature ¹⁹F and ¹²⁵Te NMR measurements.</p>