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Solid Sulfur Allotropes 5 Na2 S2 O3 þ2HClðaqÞ ! 1=nSn þSO2 þ2NaCl þ H2 O ð1Þ The sulfur rings are extracted from the aqueous reaction mixture by tolu- ene from which S6 as the major product (69 mol%) crystallizes as orange crystals on cooling to 20 C. However, the evolution of large quantities of poisonous SO2 gas makes this preparation somewhat unpleasant. 2. A more convenient but also slightly more expensive method to prepare S6 uses the thermal instability of diiododisulfane which is generated in situ from simple chemicals [11]. Commercial dichlorodisulfane (“sulfur- monochloride”), dissolved in CS2, is stirred with aqueous potassium or so- dium iodide at 20 C for 15 min whereupon iodine and elemental sulfur are formed. The latter is composed of mainly S6 and S8 with small concentra- tions of larger even-membered rings [12] of which S12, S18 and S20 have been isolated from this mixture: S2Cl2þ2KI ! S2I2þ2KCl ð2Þ nS2I2 ! S2nþI2 ð3Þ The iodine is reduced by reaction with stoichiometric amounts of aqueous sodium thiosulfate before the sulfur rings are separated by fractional pre- cipitation with pentane and recrystallization from CS2 (yield of S6: 36%) [11]. The formation of S6 is likely to proceed via the intermediates S4I2 and S6I2 with subsequent ring closure by intramolecular elimination of I2. The larger rings probably result from the intermolecular reaction of the diio- dosulfanes to give sulfur-rich homologs such as S8I2 and S12I2 which then undergo ring closure. The thiosulfate solution contains sodium iodide and, after all thiosulfate has been oxidized, may be used again for another reac- tion with S2Cl2 [11]. 3. Titanocene pentasulfide Cp2TiS5 (Cp=h5-C5H5) is commercially available but can easily be prepared from Cp2TiCl2 and aqueous sodium or ammoni- um polysulfide solution [13]. Using chloroform as a solvent a yield of 88% was obtained [14]. The organometallic pentasulfide forms dark-red air-sta- ble crystals soluble in several organic solvents. The molecules contain a six- membered metallacycle in a chair conformation [15]. The pentasulfide re- acts with many S-Cl compounds at 0–20 C as a sulfur transfer reagent with formation of Cp2TiCl2. For example, with SCl2 the two rings S6 and S12 are formed [16]: Cp2TiS5þSCl2 ! S6þCp2TiCl2 ð4Þ 2Cp2TiS5þ2SCl2 ! S12þ2Cp2TiCl2 ð5Þ Commercial “sulfurdichloride” is a mixture of SCl2, S2Cl2, and Cl2 which are in equilibrium with each other [17]. Therefore, this mixture needs to be dis- tilled to obtain pure SCl2 immediately prior to use. Due to their very differ-PDF Image | Topics in Current Chemistry
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