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Solid Sulfur Allotropes 9 mechanism for the formation of S10 will be explained below (see “Prepara- tion of S15”). 2. If only small amounts of S10 are needed and S6 or S7 are available, the oxida- tion of either one with trifluoroperoxoacetic acid provides S10 in a reaction of unknown mechanism. The intermediates S6O2 or S7O decompose at 5 C in CS2 or CH2Cl2 solution within several days to give S10, some insoluble sulfur as well as SO2: 2S6þ4CF3CO3H ! 21⁄2S6O2þ4CF3CO2H ð13Þ 21⁄2S6O2 ! S10þ2SO2 ð14Þ 2S7þ2CF3CO3H ! 2S7O þ 2CF3CO2H ð15Þ 2S7O ! S10þ3SmþSO2 ð16Þ Since the homocyclic oxides do not have to be isolated, the solution of S6 or S7 after addition of the peroxoacid (prepared from H2O2 and trifluo- roacetic acid anhydride in CH2Cl2) is simply kept in the refrigerator until S10 has formed which is then crystallized by cooling and purified by recrystalli- zation [34, 35]. 2.1.1.6 Preparation of S6·S10 When S6 and S10 are dissolved together in CS2 and the solution is cooled, then, under special concentration conditions, a stoichiometric well ordered solid solution of the two components crystallizes as orange-yellow opaque crystals of m.p. 92 C [34]. The structure of S6·S10 consists of alternating lay- ers of S6 and S10 molecules in their usual conformations of D3d respectively D2 symmetry [35]. In liquid solutions the molecular mass of S6·S10 was de- termined as 258 corresponding to 8 atoms per molecule indicating complete dissociation [34]. This is the only example of an allotrope of a chemical ele- ment consisting of molecules of different sizes. 2.1.1.7 Preparation of S11 The sulfur transfer reaction using titanocene pentasulfide and dichloropoly- sulfanes SnCl2 is very versatile and has made it possible to prepare S11 after the necessary S6Cl2 became accessible in sufficient purity: Cp2TiS5þS6Cl2 ! S11þCp2TiCl2 ð17Þ The reaction is carried out in CS2 at 0 C and provides pure S11 (m.p. 74 C) in 7% yield as yellow crystals [36]. The precursor S6Cl2 is best prepared by carefully controlled chlorination of cyclo-S6 with elemental chlo-PDF Image | Topics in Current Chemistry
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