crystals, and 77Se NMR spectroscopy [2-4], The measurement of natural abundance 77Se NMR spectra of selenium-rich Se-S rings is particularly difficult since ...
Spectroscopic Characterization of the Selenium-Rich Heterocycles Se5S, Se7S, and l,5 -S e6S 2  R alf Steudel* and M aria Papavassihou Institut für A norg an isch e und A nalytische C hem ie der T echnischen U n iv ersität Berlin, Sekr. C 2 , D-1000 Berlin 12
Frank Baumgart Iw an -N .-S tran sk i-In stitu t für Physikalische und T heoretische Chem ie der T echnischen U n iv ersität Berlin, Sekr. ER 1. D -W -1000 Berlin 12 Z. N atu rfo rsch . 46b, 1 6 7 4 - 1678 (1991); received July 8, 1991 Selenium Sulfides. H P L C . R am an S pectra,
Se N M R Spectra, X -R ay
Se5S in CS-, solution a t 20 C sp o n tan eo u sly decom poses to give a m ixture o f 8 selenium -rich cyclic m olecules w hich have been identified by reversed-phase H PLC and 77Se N M R sp ectro s copy as Se6, Se7, Se8, Se6S, Se7S, l,2-S e6S2, l,5-SegS2, an d l,4-Se4S2. N M R spectra o f Se5S, Se7S, and l,5-Se6S2 have been m easured for the first time; R am an spectra o f solid Se5S and Se7S as well as crystal d a ta o f Se7S are rep o rted in a d d itio n . A m echanism based on nucleophilic a ttack on Se,S is pro p o sed for the fo rm atio n o f the products.
Pure Se-rich SevSv heterocycles (.v> v) o f var ious ring sizes may be prepared by reaction of titanocene polyselenides with chlorosulfanes and have been characterized by HPLC analysis, Ra man spectroscopy, X-ray diffraction on single crystals, and 77Se N M R spectroscopy [2-4], The measurement o f natural abundance 77Se N M R spectra of selenium-rich S e -S rings is particularly difficult since on the one hand many of these com pounds are only sparingly soluble even in carbondisulfide which results in long recording times and, on the other hand, undergo fairly rapid intercon version reactions in solution at 20 C [2,4,5], Therefore, most N M R spectroscopic data on S e -S heterocycles have been derived from the spectra of more or less complex mixtures of such com pounds [3 ,4 ,6 -8 ], We here report for the first time on the N M R spectra o f Se5S, l,5-Se6S2, and Se7S which have been prepared according to eq. (1) and (2), on the R am an spectra of Se5S and Se7S, and on the crystal data of Se7S.
H P L C analysis
(C 5H s)2T iSe5 + SC12
(C 5H 5)2T iC l2 + Se5S
Pure crystalline Se5S was prepared according to the published procedure  and its decomposition in carbondisulfide solution at 20 °C with exclu sion of light investigated by high-pressure liquid chrom atography [2,9,10]. Using retention indices (RS), even unknown species can be identified by HPLC since the retention indices of all possible six-, seven-, and eight-membered S e -S rings have been reliably predicted , In Table I the retention times of four related six-membered rings are listed having all Se atom s in neighboring positions. The retention time of Se5S is interm ediate between T able I. R etention tim es (tR) and lo g arith m s o f the c a p a city factors (k') o f five related six-m em bered an d five eight-m em bered ring m olecules in reversed-phase H P L C (eluent: C H 3O H , statio n ary phase: octadecylsilane, colum n: 8 mm x 10 cm, dead time: 1.40 m in).
l,2-S e6S2 + >- l,5-S e6S2 + z Se7S + Seg (2)
* R eprint requests to Prof. D r. R. Steudel. Verlag der Zeitschrift für N a turforschun g. D-7400 Tübingen 0 9 3 2 - 0 7 7 6 91 1 20 0-16 74 SOI .00/0
t R (min) s6
SeS,  1,2-Se4S2  SesS S e jll] sx l,2,3-S e3S5  1,2-Se6S;  Se7S S e jll]
2.79 2.67 2.94 3.07 3.40 4.18 4.26 4.94 5.24 5.86
Ink' -0 .0 0 7 -0 .0 9 7 0.095 0.176 0.357 0.685 0.714 0.928 1.009 1.157
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R. Ste udel et al. ■ S e l e n i u m - R ic h H eterocycles
those of Se6  and l,2-Se4S2 , as expected (octadecylsilane as a stationary phase, m ethanol as eluent; UV detector at 254 nm). The chrom atogram o f freshly dissolved Se5S (in CS2) showed one large peak for this molecule, but since the dissolution rate is low the mixture al ready contained a num ber o f decomposition prod ucts as indicated by three small peaks. When the solution was kept at 20 C in the dark and ana lyzed by HPLC from time to time, the Se5S peak decreased, while eight other peaks increased. The retention times and indices are given in Table II. The assignment is based on com parison with the retention data o f reference compounds and with the retention indices predicted for six-, seven-, and eight-membered S e -S heterocycles , In addi tion, the results o f the 77Se N M R analysis (see be low) have been taken into account. The intensity behavior of the H PLC peaks during the decom position shows that Se5S primarily yields Se6, Se6S, and Se7S. However, the Se belance requires that at least one species richer in sulfur than Se5S is formed in addition. Such species normally possess a lower extinction coefficient at 254 nm and there fore their HPLC peaks are smaller at indentical concentration. There are three peaks (no. 1, 6, and 7 of Table II) which we assign to such species, namely l,4-Se4S2, l,2-Se6S2, and l,5-Se6S2 (the numbers preceding the formulae give the positions of the m inority atom s in the ring).
l,2-Se6S2 and l,5-Se6S2 have also been identified by N M R spectroscopy (see below), while the con centration of l,4-Se4S2 remained always too low. On further standing of the Se5S solution the peaks of Se6 and Se6S decreased, Se7 increased and then decreased, and Se7S and Se8 finally increased indicating reactions o f type Se6 —> Se7 —» Se8 and Se5S —> Se6S —* Se7S. After three days the peak of Se7S was by far the largest, followed by Se6S, 1,5Se6S2, l,2-Se6S2, and Se8, while l,4-Se4S2, Se5S, Se6, and Se7 were present at very low concentration lev els. On cooling of a solution of this type Se7S crys tallized and was separated under a microscope from other com ponents and in this way isolated as practically pure material. It has been shown by HPLC analysis  that Se5S and Se7S are also com ponents of mixed S e -S melts obtained by heating elemental sulfur and selenium.
T able II. R esults o f the H P L C analysis o f Se5S solutions in C S, decom posing to give elem ental selenium and five S e - S heterocycles. F o r the calcu latio n o f reten tio n in d i ces (R S)  the reten tio n tim es t R o f fo u r sulfur hom ocycles have also been m easured (dead time: t0 = 1.36 min; capacity factor: k' = (tR—t0)/t0).
T able III. R am an spectrum o f solid Se5S at - 1 0 0 C in the region 2 0 -5 0 0 cm -1 (exp.) as co m p ared to the wavenum bers calculated w ith force co n sta n ts ad ap te d from S6 and Se6 (relative intensities in brackets); for a figure o f the spectrum see ,
R am an spectra
The Ram an spectra of Se5S and Se7S measured at - 100 C using the red line of a krypton laser to minimize photodecom position are given in Tables III and IV. The assignments are based on experi ences made with Urey-Bradley force field calcula tions of related molecules [12, 13]. Both com pounds do not show any signals in the SS stretch ing region (400-500 cm “1), but the expected two SeS stretching lines o f each molecule occur near
W avenum bers (cm ') Peak
A ssignm ent 1.4-Se4S, Se5S Se6 Se6S Se7 1.5-Se6S, l,2-Se6S, SevS Sea
1 2 3 4 5 6 7 8 9
3.29 3.63 4.03 4.39 4.97 5.71 6.10 6.48 7.30
0.3506 0.5114 0.6761 0.8001 0.976 1.162 1.249 1.326 1.474
610 657 705 741 792 846 872 894 938
s6 Ss S9 Sio
3.24 4.97 6.71 8.68
0.323 0.977 1.369 1.683
602 793 907 999
A ssignm ent
C alcd. 
355 sh 351(26) 253(100) 237(23)
358 345 253 237 227 209 184 156
188(5) 154(20) 130(4) 111( 11) 108(11) 76(11) 71 (11)
116 106 105 95
v(SeS), a' v(SeS), a" \’(SeSe), a' v(SeSe), a" \’(SeSe), a' v(SeSe), a" S, a' c>, a' c o m b in atio n vibr. ö , a' ö , a" t, a' Ö+ r, a"
R. Ste udel et al. • S e l e n i u m - R ic h H e tero e y ele s
T able IV. R am an spectrum o f solid Se7S at - 1 0 0 C (relative intensities in brackets); for a figure o f the spec trum see ,
lines observed for Se5S and Se7S have to be as signed as follows (relative intensities in brackets):
A ssignm ent
Se7S 364 sh 351(18) 257(100) 250(37) 240 sh 174 sh 161 (8 ) 125(36) 110(37) 96(9) 89(23) 76(25) 54(10) 44(25) 36(25) 28 (35)
« 762.2(2) 686.3(2)
7 645.3(1) 615.4(2)
The atoms neighboring the sulfur are termed as a followed by ß and so forth.
’-S8 (2.08 g cm -3 ) and a-Se8 the density o f our Se7S phase corresponds to a com position of Se:S = 6.9:1. The HPLC analysis (see above) showed that Se7S was contam inated by traces of 1,5-Se6S2 which may explain the S e: S atomic ratio. The crystal structure o f Se5S based on X-ray dif fraction on a single crystal has already been re ported ; this com pound crystallizes in the same rhom bohedral space group as S6 and Se6.
The study of the decomposition of Se5S shows that the main products are those with a maximum of hom onuclear bonds (Se6, Se7, Se8, Se6S, Se7S, l,2-Se6S2) which are more stable  than those se lenium sulfides which have 4 or more heteronuclear bonds in the ring and which therefore are m i nority products (l,4-Se4S2, l,5-Se6S2). No dimerization of Se5S to Se1(JS2 has been observed. The reaction of Se5S to the many products of Table II probably proceeds via a nucleophilic ring opening, followed by chain-growth finally resulting in o th er, more stable eight-, seven-, and to a certain de gree six-membered rings. The first step will be the attack on Se5S by a nucleophilic impurity (Nu) pre sent in the solution to give a new nucleophile (zwitterion) which attacks another Se5S and so forth. The weakest bonds (S e-S e) in the molecules will
R. Steudel et aL ■Selenium -R ich H eterocycles
thus be opened, but depending on the site of the at tack of Nu different chains will result, e.g.\ N u + Se5S " N u - S e - S - S e 4" | + Se5S
+N u ~ S e - S - S e 4- S e , - S - S e , Se6S + +N uSeSSe,
+N u - S e ,- S - S e - , 1 + Se5S fN u - S e ,- S - S e ,- S e - S - S e 4 " / \ Se-,S + +N uSe,S Se"
In this way the form ation of all observed com pounds can be explained, but additional decom position products are also expected; obviously, these are degradated more rapidly and therefore cannot be detected in the spectra or chrom ato grams. Similar mechanisms have been discussed for the decomposition o f SeS5  and Se2S5 .
MHz). All results have been reproduced at least once. Se5S: To 1.15 g (2.0 mmol) (C5H 5)2TiSe5 in 215 ml CS2 are added 0.21 g (2.0 mmol) SC12 in 13 ml CS2 resulting in a color change from violet to red and a precipitation o f Se5S and (C5H 5)2TiCl2. Cooling of this m ixture to -7 8 C yields more pre cipitate which is isolated and washed at 20 C with 300 ml CHC13 or C H 2C12 and then with 100 ml pentane (0 C). Yield: 600 mg Se5S, dark-red hex agonal crystals. Se content: 92.1% (by neutron ac tivation ); calcd: 92.5% , Se. Se7S: A solution of 50 mg Se5S in 150 ml CS2 is stirred at 20 °C for 10 h, followed by cooling to -7 8 C for 20 h. The precipitate is isolated and the dark-red rod-like crystals of Se7S separated from the red powder under a microscope. Yield 23 mg Se7S.
A cknowledgemen t
The equipm ent used for HPLC analysis , R a man spectroscopy  and X-ray diffraction  has been described before. Two Bruker NM R spectrometers have been used: MSL 400 (frequen cy 76.313 M Hz) and WM 360 (frequency 57.315
We are grateful to Dr. H. Förster at Bruker (Karlsruhe) for some 7Se N M R measurements and to the Deutsche Forschungsgemeinschaft and the Verband der Chemischen Industrie for sup port.
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