Selenium Sulfide Ring Molecules.

Abstract

presence of SS, SeS, and SeSe bonds is seen in all samples. The fundamental vibrations of different Se"S8_" molecules have been calculated by using the extended Urey-Bradley force field with the structural parameters and force constants adapted from those of Ss and Se8.8c From the splitting of the Raman lines in the SeS stretching region (400-320 cm-1), it was concluded that the most abundant species in the mixed phases have all selenium atoms adjacent to each other (the structural unitS -Se"-S-) rather than as isolated selenium atoms (the structural unitS -Se-S-). The Raman spectra also indicated the presence of S8 and Se8 in some of the samples.8' Heterocyclic selenium sulfides can also be prepared by various synthetic routes. The commercial "selenium disulfide", which can be prepared by the reaction of aqueous Se02 with H2S, has been shown by Weiss9 to consist of a mixture of eight-membered selenium sulfide rings like those obtained from the melts. Also, the reactions of chlorosulfanes with hydrogen selenide,10 of chloro-or bromoselanes with sulfanes,* 11 and of chlorosulfanes and di-chlorodiselane with potassium iodide12 seem to lead to mixtures of different selenium sulfides. Recently, the preparation of l,2-Se2S5, SeS5, and l,2,3-Se3S5u from bis(cyclopentadienyl)titanium pentasulfide, [Ti(C5H5)2S5], and dichlorodiselane, Se2Cl2, hase been reported:14 [Ti(C5H5)2S5] + Se2Cl2-l,2-Se2S5 + [Ti(C5H5)2Cl2] 2(l,2-Se2S5)-l,2,3-Se3S5 + SeS5 The reaction was followed by HPLC, and it was observed that while the six-and seven-membered products could be isolated as stoichiometrically pure, 1,2,3-Se3S5 contained small amounts of other eight-membered ring molecules as impurities.14 However, all three products could be identified by vibrational analysis.1411,15 Analogous reactions employing bis(cyclopentadienyl)titanium pentaselenide, [Ti(C5H5)2Se5], and S"C12 (n = 1 or 2) have produced l,2-Se5S2 and Se5S.16 The synthetic work combined with Raman spectroscopy and HPLC has also given information about the sulfur-selenium melts.17 It seems that, in addition to the cyclic eight-membered selenium sulfides, there are small amounts of the six-and seven -membered species present. In a recent preliminary report18 we have shown that 77Se NMR spectroscopy is an attractive tool to study the complicated binary system of sulfur and selenium. The 77Se isotope has a natural abundance of 7.58% and a nuclear spin of '/2. As its sensitivity relative to that of 13C is 2.98, the spectroscopic measurements with modern multinuclear NMR instruments are straightforward, the limiting factor being the rapidly decreasing solubility of the selenium sulfides as the selenium content increases. Due to the low natural abundance of the NMR-active 77Se isotope, the 77Se-77Se spin-spin coupling effects are normally seen as small satellites. Isotope enrichment increases the sensitivity and also leads to complete information on the coupling when the (9) Weiss, (13) The numbered atoms in the formulas of SemS" are those for which m or n is the lowest, whether S or Se. (14) (a) Laitinen, R.; Rautenberg, N.; Steidel, J.; Steudel, R. Z. Anorg. Allg. Chem. 1982, 486, 116. (b) Steudel, R.; Strauss, E.-M. Angew. Chem. 1984, 96, 356; Angew. Chem., Int. Ed. Engl. 1984, 23, 362. (15) Laitinen, R.; Steudel, R.; Strauss, E.-M. J. Chem. Soc., Dalton Trans. 1985, 1896. (16) Steudel, R.; Papavassiliou, M.; Strauss, E.-M.; Laitinen, R. Angew. Chem. 1986, 98, 81; Angew. Chem., Int. Ed. Engl. 1986, 25, 99. (17) Steudel, R.; Strauss, E.-M.; Laitinen, R., unpublished work. (18) Laitinen, R. S.; Pakkanen, T. A. Figure 1. Se"Sg.n (n = 1-7) ring molecules indicating the abbreviated notation used for the different species. Selenium atoms are denoted by closed circles, and sulfur atoms, by open circles. enrichment is sufficiently high. The comparison of the spectrum from the natural-abundance sample to that from the 77Se-enriched sample of the same chemical composition is very useful for the identification of different species in complex mixtures. The 77Se NMR spectroscopy has recently been shown to be suitable in the characterization of the mixtures of organic poly-selenides with up to five selenium atoms in the chain.19 Also, work on the selenium-containing cations20 and Zintl anions21 shows the power of the method in the analysis of compounds containing selenium. In this work we complement our preliminary study18 by presenting a detailed interpretation of the NMR spectra of the phases obtained from the sulfur-selenium melts containing up to 50 mol% selenium. The identification of various components is based mainly on the 77Se-77Se coupling data and on the trends in the spectral parameters. The composition of the mixtures as a function of the selenium content in the melts is discussed. Experimental Section Preparation of the Samples. Sulfur (99.999%, Cerac) and selenium (99.9999%, Koch-Light Laboratories Ltd.) were mixed together in molar ratios of 90:10, 70:30, 65:35, 60:40, and 50:50, heated in sealed ampules under reduced pressure (ca. 0.01 kPa) at 770 K for 4 h, and quenched at liquid-nitrogen temperature. The resulting amorphous materials were extracted with CS2, after which the solvent was removed by evaporation. Orange powder was obtained in each case with the color deepening as the selenium content increased. The NMR spectra were recorded after redissolving the powder in CS2. The solutions were nearly saturated, containing 2-5 wt % dissolved material. Two series of samples were thus prepared: (1) by using natural-abundance selenium; (2) by using sele-nium enriched with 77Se (94%, Techsnabexport, Moscow).22 77Se NMR. All NMR spectra were recorded at 300 K23 with a Bruker AM-250 spectrometer (University of Joensuu) operating at 47.705 MHz. (19) Eggert, H.; Nielsen, O.; Henriksen, L. J. Am. Chem. Soc. 1986, 108, 1725. (20) (a) Lassigne, C. R.; Wells, E. J. J. Chem. Soc., Chem. Commun. 1978, 956. (b) Schrobilgen, G. J.; Burns, R. C.; Granger, P. J. Chem. Soc., Chem. Commun. 1978, 957. (c) Carnell, M. M.; Grein, F.; Murchie, M.; Passmore, J.; Wong, C.-M. (22) The composition of the enriched material was checked with an AEI MS702 spark source mass spectrometer. In addition to the 94% 77Se isotope, the material contained other naturally occurring isotopes in approximately the same respective ratio as in the natural-abundance selenium. (23) Eggert et al.19 reported a strong temperature dependence of the 77Se chemical shift in organic polyselenides. Therefore, the probe temperature was held constant within 0.1 K during the measurement of all spectra.