Tetrachloro- and Tetrabromostibonium(V) Cations: Raman and 19F, 121Sb, and 123Sb NMR Spectroscopic Characterization and X-ray Crystal Structures of SbCl4+Sb(OTeF5)6- and SbBr4+Sb(OTeF5)6-

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Abstract
The stable salts, SbCl4+Sb(OTeF5)6- and SbBr4+Sb(OTeF5)6-, have been prepared by oxidation of Sb(OTeF5)3 with Cl2 and Br2, respectively. The SbBr4+ cation is reported for the first time and is only the second example of a tetrahalostibonium(V) cation. The SbCl4+ cation had been previously characterized as the Sb2F11-, Sb2Cl2F9-, and Sb2Cl0.5F10.5- salts. Both Sb(OTeF5)6- salts have been characterized in the solid state by low-temperature Raman spectroscopy and X-ray crystallography. Owing to the weakly coordinating nature of the Sb(OTeF5)6- anion, both salts are readily soluble in SO2ClF and have been characterized in solution by 121Sb, 123Sb, and 19F NMR spectroscopy. The tetrahedral environments around the Sb atoms of the cations result in low electric field gradients at the quadrupolar 121Sb and 123Sb nuclei and correspondingly long relaxation times, allowing the first solution NMR characterization of a tetrahalocation of the heavy pnicogens. The following crystal structures are reported: SbCl4+Sb(OTeF5)6-, trigonal system, space group P3̄, a = 10.022(1) Å, c = 18.995(4) Å, V = 1652.3(6) Å3, Dcalc = 3.652 g cm-3, Z = 2, R1 = 0.0461; SbBr4+Sb(OTeF5)6-, trigonal system, space group P3̄, a = 10.206(1) Å, c = 19.297(3) Å, V = 1740.9(5) Å3, Dcalc = 3.806 g cm-3, Z = 2, R1 = 0.0425. The crystal structures of both Sb(OTeF5)6- salts are similar and reveal considerably weaker interactions between anion and cation than in previously known SbCl4+ salts. Both cations are undistorted tetrahedra with bond lengths of 2.221(3) Å for SbCl4+ and 2.385(2) Å for SbBr4+. The Raman spectra are consistent with undistorted SbX4+ tetrahedra and have been assigned under Td point symmetry. Trends within groups 15 and 17 are noted among the general valence force constants of the PI4+, AsF4+, AsBr4+, AsI4+, SbCl4+ and SbBr4+ cations, which have been calculated for the first time, and the previously determined force constants for NF4+, NCl4+, PF4+, PCl4+, PBr4+, and AsCl4+, which have been recalculated for the P and As cations in the present study. The SbCl4+ salt is stable in SO2ClF solution, whereas the SbBr4+ salt decomposes slowly in SO2ClF at room temperature and rapidly in the presence of Br- ion and in CH3CN solution at low temperatures. The major products of the decompositions are SbBr2+Sb(OTeF5)6-, as an adduct with CH3CN in CH3CN solvent, and Br2.