Polysulfanes

Polysulfanes are compounds of the general type H2S, where x 2 (see structure 15.6). Sulfur dissolves in aqueous solutions of group 1 or 2 metal sulfides (e.g. Na2S) to yield polysulfide salts, (e.g. Na2S, ). Acidification of such solutions gives a mixture of polysulfanes as a yellow oil, which can be fractionally distilled to yield H2S c (x = 2-6). An alternative method of synthesis, particularly useful for polysulfanes with X 6, is by condensation reaction 15.40. 

It is well estabhshed that sulfur compounds as well as elemental sulfur have the tendency to form long chain molecules. All of these substances can be regarded as derivatives of the hydrogen polysulfanes (or polysulfanes) H2S . Polysulfanes form a long series of homologous chain-like molecules since the number n can assume any value. S-S and S-H bonds are frequently found in chemical and biological systems. Thus, polysulfanes have been the subject of numerous experimental and theoretical studies (for a recent review, see [15]). 

The lower members of the polysulfane series with n=2-6 have been prepared as pure compounds, while all members with n values up to 35 have been detected by H-NMR spectroscopy in so-called crude sulfane [31]. The gas-phase structures of the first three members of the polysulfane series are well established from either microwave spectra (H2S2 [32], H2S3 [33]) or high-level ab initio MO calculations (H2S4 [34]). Systematic ab initio studies of the structures, vibrational frequencies and heats of formation of H2S with i up to 6 have also been reported [4, 16]. 

The structures, energies, torsional barriers and vibrational spectra of three rotamers of tetrasulfane, H2S4, have been examined by Drozdova, Miaskiewicz and Steudel at the MP2/6-311G level [34]. Surprisingly, the cis-trans conformation (motif -l-H— symmetry Ci) is found to be most stable, followed by the all-cfs form (h—t symmetry C2), while the helical all- 

Suleimenov and Ha have used high-level G2 and CBS-Q ab initio methods to study the thermochemical properties of gaseous polysulfanes (H2S , =l-6) [4]. The enthalpy of formation were calculated from atomisation energies and from enthalpies of dehydrogenation reactions such as shown in Eq. (1)  

The S-S bond dissociation energies of H2S2, H2S3 and H2S4 have been studied by Steudel et al. at the GCSD(T)//6-311 G(2df,p) level [42]. The calculated enthalpies AH° for the dissociation at the central bonds at 298 K are 247, 201 and 159 kJ mol respectively. The lower stability of the tri- and tetrasulfanes towards homolytic S-S cleavage is attributed to the stability of the generated HSS radical which is stabilized by the formation of a three-electron n bond. 

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Hydrogen sulfide reacts with molten sulfur and depresses the viscosity of the latter, particularly at 130—180°C iafrared spectral studies show that polysulfanes, H2S, form. The average chain length of these polysulfanes is shorter than the equiUbrium chain length of molten sulfur alone at the same temperature consequendy, the viscosity of the molten sulfur is markedly reduced (99,100). 

The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes... 

The polysulfanes were at one time made by fusing cmde Na2S.9H20 with various amounts of sulfur and pouring the resulting polysulfide solution into an excess of dilute hydrochloric acid at — 10°C. The resulting cmde yellow oil is a mixture mainly of H2S (n = 4-7). Polysulfanes can now also be readily prepared by a variety of other reactions, e.g. ... 

Purification is by low-pressure distillation. Some physical properties are in Table 15.12. Polysulfanes are readily oxidized and all are thermodynamically unstable with respect to disproportionation ... 

The sulfur analog of hydrogen peroxide also exists and is an example of a polysulfane, a catenated molecular compound of composition HS—S —SH, where n can take on values from 0 through 6. The polysulfide ions obtained from the polysulfanes include two ions found in lapis lazuli (Fig. 15.15). 

Bei der Reduktion von Aldehyden und Ketonen werden neben den Alkoholen auch Di- und Polysulfane er-halten (s. S. 276), mit aromatischen Nitrilen im UberschuB5, mit Oxiranen (S. 422) und mit Thiiranen (S. 449) werden Thioamide, Dihydroxy-disulfane oder Polysulfane gebildet. Mit Thiolen entstehen Borsaure-trithioester, die mit Aldehyden und Ketonen Thioacetale ergeben6. Prim. Nitro-alkane Iiefern Nitrile, sek. Ketone und Oxime (S. 472), Nitro-aromaten die entspreehenden Amine (S. 478). Oxime konnen selektiv zu Hydroxylaminen Oder weiter zu Aminen reduziert werden (S. 375). 

Mit Natrium-trithio-dihydrido-borat werden aus Thiiranen die entsprechenden Polysulfane gebildet, die durch Lithiumalanat zu 1,2-Dithiolen reduziert werden. Polyaryl-thiirane werden dagegen auch mit dieser Methodein Polyaryl-athylene iiber-gefiihrt2 ... 

Ober die Reduktion der Schwefel-Analoga (Di- und Polysulfane) s.S. 466ff. 

H2S2 (hydrogenpersulfide), the smallest member of the polysulfane series [15], has been studied extensively by molecular spectroscopy and theoretical calculations [16] (and references therein). By now, accurate knowledge of its structure, torsional potential and vibrational modes has been established. Ab initio calculations readily reproduce these properties [17]. The value of the SSH angle in hydrogen disulfide was a subject of controversies for some time. However, recent experiments led to a different value which is in favour of the ab initio calculated value [17]. 

Mixtures of polysulfanes H2S are easy to make and probably occur even naturally but the pure compounds are relatively unstable and can therefore be prepared only with some experimental skills and observing certain technical precautions. Detailed prescriptions exist how to make di-, tri-, tetra-, and higher sulfanes in gram quantities. However, as longer the sulfur chain grows as more difficult it becomes to obtain pure compounds. In general, all... 

The H2S usually escapes and the sulfur crystallizes as a consequence the reaction will always be quantitative after long enough reaction times or in the presence of a catalyst. On the other hand, hquid sulfur and H2S react to give a mixture of long-chain polysulfanes ... 

Since hydrogen sulfide and elemental sulfur occur together in hot underground deposits of natural gas (so-called sour gas ) the formation of sulfur-rich polysulfanes under these high-pressure conditions is very likely. If the gas is produced and in this context the pressure and temperature lowered, the decomposition reaction (Eq. 1) takes place and the precipitated sol-... 

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