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6 Ralf Steudel · Bodo Eckert ent solubilities in CS2 (see below) [18], the reaction products Cp2TiCl2, S6 and S12 can easily be separated. Yields: 87% S6, 11% S12 [16]. 2.1.1.2 Preparation of S7 1. Small amounts of S7 are best prepared from titanocene pentasulfide (see the preparation of S6 above) by reaction with dichlorodisulfane S2Cl2 (“sulfur- monochloride”) in CS2 at 0 C [16]: Cp2TiS5þS2Cl2 ! S7þCp2TiCl2 ð6Þ This reaction proceeds quantitatively, but the isolated yield of S7 (23%) is lower owing to its high solubility. Since S7 rapidly decomposes at 20 C, it needs to be handled with cooling and should be stored at temperatures be- low 50 C. 2. Liquid sulfur contains at all temperatures several percent of S7 besides the main constituent S8; in addition, rings of other sizes and, at higher tempera- tures, polymeric sulfur S1 are present [19]. After quenching of the melt at low temperatures it is possible to separate the main components and to iso- late S7 in 0.7% yield; see below under “Preparation of S12, S18, and S20 from S8”. Depending on the crystallization conditions S7 is obtained as either the a, b, g, or d allotrope [20, 21] which are all very well soluble in CS2, CH2Cl2, toluene, and cyclo-alkanes. a-S7 is obtained on rapid cooling of solutions in CS2, CH2Cl2, or toluene and forms intense-yellow needle-shaped crystals of m.p. 38.5 C which are disordered. b-S7 was obtained as a powder from d-S7 by storage at 25 C for 10 min. d-S7 crystallizes from CS2 solutions at 78 C and forms block-shaped, tetragonal-bipyramidal and sarcophagus-like crys- tals. g-S7 was obtained from a solution in CH2Cl2 containing small amounts of tetracyanoethene at 25 C [20]. Regardless of the allotropic modification, solid S7 decomposes at 20 C completely within ten days but can be stored at 78 C for longer periods of time without decomposition. The first signs of the decomposition products S8 and Sm (polymeric sulfur) can be detected already after 30 min at 20 C [20]. In CS2 solution S7 is quite stable. 2.1.1.3 Preparation of Pure S8 cyclo-Octasulfur crystallizes at ambient pressure either as orthorhombic a- S8, monoclinic b-S8 or monoclinic g-S8. Commercially available sulfur sam- ples usually consist of mixtures of a-S8 with some Sm and traces of S7 [22]. It is this S7 content which causes the bright-yellow color of most commercialPDF Image | Topics in Current Chemistry
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