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Homoatomic Sulfur Cations 137 sulfur cation solutions contain larger quantities of at least two (radical-)cat- ions Sn·+ (n4). Only the existence of dissolved S5·+ was established in 1976 with great certainty [7] while another cation was tentatively assigned as S7·+ [8]. The driving force for the formation of less charged (radical-)monoca- tions in solutions presumably is due to electrostatics and a “coulomb explo- sion” of the in the solid state lattice stabilized Sn2+ dications. Other than the above-mentioned five species no further sulfur cations were identified with confidence in condensed phases. However, very recently evidence for the ex- istence of D3d symmetric S62+ and square planar S4·+ was presented [3]. 2 Synthesis of Sulfur Cations in Condensed Phases The synthesis of the highly electrophilic sulfur cation salts requires the use of strong one or two electron oxidizing agents such as SO3, S2O6F2, or MF5 (M=As, Sb) in weakly basic and oxidation resistant solvents. From all report- ed synthetic procedures (see [1a] for an overview) the oxidation using MF5 in a suitable solvent such as SO2 emerged as the most straight forward meth- od, i.e., Eqs. (1–3) [(ss)=standard state]: 4=8S8ðssÞ þ 3MF5ðssÞ ! S4ðMF6Þ2ðMF3ÞðssÞDrHðM 1⁄4 AsÞ 1⁄4 348kJmol1 ð1Þ S8ðssÞ þ 3MF5ðssÞ ! S8ðMF6Þ2ðssÞ þ MF3ðssÞDrHðM 1⁄4 AsÞ 1⁄4 278kJmol1 ð2Þ 19=8S8ðssÞ þ 3MF5ðssÞ ! S19 ðMF6 Þ2ðssÞ þ MF3ðssÞ ð3Þ The synthesis of the highest oxidized S42+ salt in Eq. (1) necessitates the presence of a trace amount of halogen (=“facilitator”).[9] Salts containing the S42+ cation are colorless, while those of S82+ appear to be blue [10] and S192+ salts are orange-red. Crystalline samples of all three sulfur cation salts were characterized by a variety of experimental techniques including vibra- tional and UV-Vis spectroscopy as well as X-ray diffraction (see [1a] for a detailed overview on the physical characterization of sulfur cation salts in condensed phases). 3 Solid State Structures of Sulfur Cations The solid state structures of the known Sn2+(n=4, 8, 19) dications including their basic structural parameters are collected in Fig. 1. All known solid state structures shown in Fig. 1 contain sulfur homocy- cles. While the S42+ cation is a regular square, the S82+ cation adopts an ap- proximately Cs symmetric exo-endo conformation with three transannular contacts and S192+ consists of two seven membered rings linked by a penta- sulfur chain and incorporates two tricoordinate sulfur atoms. An analysis of the solid state anion contacts of the S42+ and S82+ cations showed that the positive charge is delocalized almost equally over the entire cations [11, 23].PDF Image | Topics in Current Chemistry
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