Some Experiments in Sulfur-Nitrogen Chemistry Arthur J. Banister and Nigel R. M. Smith University of Durham, South Road, Durham, DH1 3LE The chemistry of sulfur-nitrogen compounds has received intensive study, especially in the last twenty years, but it still retains the capacity to confound the experts. Compounds are regularly found with unexpected structures and properties. Recent examples can be found in: (1) The salts of the unusual cage anion ( I ) S4N50- (Fig. 11 -,.
(2) The stabilities of the cyclic cations (e.g., S4N3+and SsNs+ (Fig. 2)). For instance, warm concentrated nitric acid merely converts SaN3C1t o the nitrate (2).
Figure 1. Structure 01
SINSO- in NH,S4Nr0 (Ref. (I))
I 0I - I I e&
S-N
Perhaps the most remarkable compound of all is poly(sulfur nitride) which has metallic properties (3) (e.g., superconductivity below 4.2 K) and yet it contains only nonmetallic elements. The importance attached to (SN), by chemists and physicists can he gauged from the fact that, during 1978 and 1979, "Chemical Abstracts" reported over 150 papers with significant content devoted to this material. Sulfur-nitrogen chemistry is no longer the passion of a few devotees; it is developing an increasingly important role in inoreanic chemistw and can be exoected to feature to a meater extent, in undergraduate practical and lecture courses. In this DaDer . . we hrieflv survev the main structural t .w.e s of sulfur-nitrogen compounds, and describe syntheses, suitahle as underrraduate exoeriments. which illustrate four of the five types of cyclic species.
'I)
S-N
N-S
N-S
o
@
-
Classlflcatlon of Sulfur-Nitrogen Compounds Sulfur-nitrogen compounds contain sulfur (11, IV, or VI) and two or three coordinate nitrogen and can he conveniently classified as follows. ( 1) lmides Containing S(ll)
The sulfur imides are considered to he derived from octasulfur, by replacing sulfur atoms by >NH groups (4) (Fig. 3). They are puckered rings as in elemental sulfur (5).The imides, including the various isomers, can be separated by chromatography (6).Rr values for thin-layer chromatography (using CCla elutant on silica gel plates), vary from 0.85 (sulfur), through0.54 (S7NH),down to zero (S4(NH)4) as the polarity of the species increases.
Figure 2.
Structures of li) SnN. (ii) S,N.
(iii) S4N2(iv) S5N5+(v)
SaN202.
(2) lmides Containing S(IV) and S(VO In this group are the relatively little studied thionyl and sulfuryl imides containing oxygen, with the structural units 3 isomers of S61NH12
SXH
respecti\dy. The only rep(,rted cyclic thionyl imide (R=H) is the tetramer ( n = 4 ) of which there are nu derivatives 1;). Both the trimer and the tetramer sulfuryl imides are stable as crystalline salts, e.g., (NaNS02)3, whereas the hexamer is only known as the acid. An X-ray study (8)showed trisulfi1058
Journal of Chemical Education
2 isomers of
S51NHI3
Figwe 3. Sulhrr imides.
SsN2CI+(vi)
Figure 6. Shuctures of SINGand S.N@.
Figure 4. Resonance canonicals of (NSO*-h
Figure 7. Poly(su1furnitride).(SNh. Figure 5. Struchrres of (NSCIIJand (NSOCI).. mide salt, A ~ ~ ( N S O Z ) ~ . ~toH be Z Ochair-shaped with C3" symmetry. Two main canonical forms can be drawn (Fig. 4--where structure (11) is a sulfanurate anion (Cf. section 3 below)). (3) Thiazenes with Exocyclic Groups at Sulfur These contain the structural units
where sulfur is four or six valent, respectively. An example of the former (X = chlorine, n = 3) is trithiazyl trichloride or 1,3,5-trichlorocyclotrithiazene,(NSC1)s. (See experimental section.) Of the sulfanuric halides, (NSOC1)3and (NSOF)3.4 are known and there are numerous substitution products (9). There are also some mixed valent sulfur species, e.g., [(NSOCl)(NSCI)z] (10). Where structures are known, the six-membered rings adopt a chair conformation with the halogen atoms in the axial positions to minimize lone pair repulsions (Fig. 5). ( 4 ) Thiazenes without Exocyclic Groups
These thiazenes, S,N,.(x > y) are known as neutral molecules, cations and anions, for example (Fig. 2): (1) S4N2 (experimental section), S2N2 (2) S4N3+, S5N5+ (experimental section) (3) S3N3Both SaNz and S2N2 are unstable a t room temperature. The stahle species S4N3+, S5N5+ and S3N3- (11) are virtually planar (12) and can be reearded as containine a a-bonded framework (with two-coorhate sulfur and niGogen), and a a-system above and below the molecular plane, in which each sulfur provides two, and each nitrogen one a-electron. These species thus conform to the Huckel rule (4n 2 T-electrons where n is an integer). High bond angles a t nitrogen (e.g., up to ~ 1 5 5 "in ) some cyclic species cannot be attributed entirely t o cyclic strain (13) and are probably due to the presence of a further a-system in the plane of the ring.involving - nitrogen lone pairs. X-ray studies show that the S3NzC1+ cation in S3NzC12 (see experimental section) is puckered with the -S2N2unit coplanar (14). It can be considered as a 6 a SzNz unit expanded
+
by an >S+-C1 group. The oxygen analogue, S3N202 has an acyclic structure (Fig. 2). (5) Cage Species
Tetrasulfur tetranitride, (experimental section), first prepared in 1835, is one of the best known sulfur-nitrogen compounds (15). It is an important starting material for preparing many other sulfur-nitrogen species. The structure of S4N4(16) and of the recently discovered species SaNs-, S4N5O- (Fig. 1) and SsNs (Fig. 6) are all based on a tetrahedral cage of four sulfur atoms. The molecule of tetrasulfur tetranitride is comnosed of an eight-membered ring, buckled so that sulfur atoms lie a t the corners of a sliehtlv distorted tetrahedron. with nitroeen atoms bridgingfou; of the six edges. All s k edge S- - y-s distances (2 X 2.58 A and 4 X 2.69 A) suggest considerable sulfur-sulfur interaction, though in each case less than a single bond. In S5N6 an -N==S=Nunit bridges two sulfw atoms of an S4N4 cradle (17). The moat usual large-scale preparation of S4N4 is that reported by Jolly (18); however, this is not advisable a s a student preparation for reasons ofsafety. Safer small-scale preparations have been published recently (191(28) (.6.) Chains
Theone important member of thisgroup ispoly(sulfur nitride). (See ex~erimentalsection.) It can be nrenared either in a two-stage reaction (via S2Nz) from ~ ~ ~ ~ or( from 2 0 ) SaN3C1 (21), or in a single-stage reaction from SsN5+FeC14as described in section (6) below. I t has also been prepared recently from S4Nz (22). Its structure is a chain of alternating sulfur and nitrogen atoms (23) (Fig. 7). This interesting compound is the first known example of a metallic polymer. Recently, halogenated derivatives have been discovered, among them, (SNBr), (24) and an iodinated polymer (25). Experimental CAUTION. In the pure, recrystallized state, SdN4is potentially explosive; thus, proper safety precautions (26) must be adopted for handling the material once isolated as a solid. S I N I CAN BE AVOIDED by the student in sections (4) and (5) as indicated; oth-
erwise, it must be provided for the procedure described in section 15) ~-,-
Figure 8 shows schematicallysome of the main preparative routes to sulfur-nitrogencompounds and includes (marked with filled arrows) those reactions which form the basis for the exoeriments described in sections (2) to (6). Volume 59
Number 12 December 1982
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anhydrous calcium sulphate drying tube
air condenser
t
Figure 8. Preparative routes to sulfur-nilragen mpounds (reference numbers in parentheses).
1.3 crn
heating mantle
( 1) Purification and Dryhg of Solvents and Reagents
Carlrm tetrachloride is dried over phuvphorus pentoride and distilled. 'l'hionyl chloride must be purifiwl by stirrlnp with triphenvl phusphite (120 ml per literl, fdlowcd bv fractional d ~ s t ~ l l a t i(the m middle 40% is colle&d) using a vacuumjacket fractionating column. Monoglyme(l,2-dimethoxyethane) is refluxed over potassium and distilled. Disulfur dichloride can usually he used without further purification, though it can be distilled if found necessary. Iron(II1) chloride is purified by refluxing in dry thionyl chloride prior to use. Powdered sulfur can usually be used without further purification. Ammonium chloride must be baked dry in an oven a t 120°C. Iron turnings are taken from a bar of mild steel. Chlorine gas from a cylinder is dried by passing through a phosphorus pentoxide column, as shown in Fieure 10. Silver wire is wound into a hall and flattened tc, makea diw (thickness -2rnmJ. This is drgreascd in hexanc, and heated i n caruo (2UIPC) for 12 hr prior to usr. Nitrogen gas w a s h e d by passage thnmyh a phosphorus pentoxidr tower.
I
Figure 9. Preparation of SsN&l2.
(a Preparation of Thiodithiazyl Dichloride, &N2C12 4SzCIz t 2NH4CI = S3NzCIz + 8 H C l t 5 s (1) The followineannaratus should be set uu at the hack of the fume cupboard (to m h & i i temperature variatLon) and glassware should at 120°C he hn'wd drv loven ~ - or ahnve). Place oven-baked ammonium chloride (200 g, 3.75 mole), disulfur dichloride, SzC1z (100 ml) and powdered sulfur (20 g, 0.06 mole) in a 700-ml flanged-top, round-bottomed flask with a B34 ground glass socket and attach an air condenser (100 cm X 3 cm diameter) terminating in an anhydrous calcium sulfate drying tube (see Fig. 9). The eround .. elass ioiute should he lubricated with silicone erease to prr\,ent them from "w~lding"togrther. A flange top tlask i's used to t~cilitateeasy rlenninc. .Aluminum foil, wrapped around the expsed pans of the re~ction nark, helps to check random fluctuations in retlux lrvel induced by draughts. Heat the flask with an electric mantle, until the disulfur diehloride refluxes (the h.p. of SzCI is 136O),and regulate the heat to maintain a reflux level half way up the air condenser. Once stabilized, it is important to keep the fumes hood front closed to avoid raising the reflux level (which results in product being washed back into the flask). Orange-brown crystals of S3N2C1zwill form on the condenser walls after 1-2 hr. The yield can he improved if the reflux position in the air condenser is controlled so that (in approximately hourly i n t e ~ a l s ) it gradually falls to just ahove the joint. This is probably the most critical maneuver in the experiment. The refluxing S2C12contains dissolved NSCI, and as the mixture cools in the condenser, S3NzClz crystallizes out: ~
.
c1
S,CL
+ 2 NSCI.--
N
\ .s-s. / \
@
c1-
N
Figure 10. (1)Apparahrs far purging wim nlnogen (11) Apparatus fcf chlorination of SSN~CIZ. heatingis stabilized, the student should be encouraged to set up the apparatus for stage three (Fig. lO(1)).
(3) Preparation of Trithiazyl Trichloride, (NSCh The apparatus for this stwe is shown in Fieure 10. Set uo the aoparatusiil in a fume cupbo&d and leave it purging with n ~ t r o ~ e n . Allow the flask and air condenser \from stage 2) ro cool down, then transfer the air rundenser rapidly to the 500-ml flask of the rhlorination apparatusasrhown in 1111 I'urgc withdry nitrugen fur 10min. Then runner1 up the chlorine rylindrr and adlust fur an exit &,w rate of -3-6 h u h h l ~per ~ ser. The chlorine attacks the SxNzCI2 forming dark red SCI2, and the liquid washe. thecrwtals into the flask. As thisoccur sand the reacti& slows down Ci.
+ SCI,
c1-
(2)
\s/ Continue heatine far about 4-5 hr or until onlv a little disulfur dichloride remains. ~!~~POH.I.ANT: Ensure t h a t i h e flask does not boil "dry."sioce subliming ammonium chloride and sulfur will contaminate the product. The yield of S8N2Cl2is approximately ~~~~~
~~
~~
~~~~~
~
~~~~~~
~~~~~~
~~
~~
The atudent should not leave the apparatus unattended, especially toward the end of the experiment, since anysublimingsolid will seal the neck and the pressure build-up could be dangerous. After the
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Journal of Chemical Education
(i.e.. after -15 min.). adiust the cvlinder valve so that the exit flow ;&falls to 1-2 huhi;lesisec. The ~hloriustionwill be comolete after ahout an hour. Replace the air condenser with astupper and atrnrh the othcr neck uf the flask to a vacuum line and pump off the SC12 from the viscous liquid mixture. Empty and clean the muck trap. This 15aCOnVenirnt point for thestudent tustopat threndolthe tint day hecauw tbe tlask rn he cmvemenrlg left under dry nitrogen overnight ~~
~
~~
~
~
~~~
Second Day Connect the 2-necked flask (with crude (NSC1)dto the chlorination line and repeat the slow chlorination (-30 min.) and pump down to remove SC12. The uude (NSCI)3 can be recrystallized from dry carbon tetrachloride, but it is normally pure enough (pale yellow) to continue with the next stage. The pale yellow crystals of (NSCI)S are very moisture sensitive and should be manipulated in a dry box or dry bag. Purity should he assessed hy determining the melting point and i.r. spectrum. Run the infrared spectrum of a mull made up in the dry box using liquid paraffin (Nujol)@andKBr plates. Place a sample of (NSCl)3in a melting point capilliary tube, cover the top with Paraf~lm tape and remove from the dry box. Rapidly seal the tube with a glassblowing torch and determine the melting point in the usual way. The yield of (NSC1)3 is approximately 5-10 g. Meltingpoint: 91°C Infrared Absorptions of NSCIS 1017(w), 693(ms), 621(w), 514(m), 493(m), 385(m) and 320(m) cm-I Impurities of SflzClz can be detected by absorptions at: 1015(s), 936(w), 893(m,sh),717(s,sh), 712(vs), 578(s), 458(s) and 400(s,hr) cm-'. [vs = very strong, s = strong, ms = medium strong, m = medium, w = weak, sh = shoulder, br = hroad.] (4)Preparation of Tetrasulfur Tetranitride, S4N4 Take alittleof the trithiazyl trichloride (-0.5 g) from stage3 and place it, under dry-nitrogen, in a 50 ml, 2-necked round-bottomed flask. Stir it with iron turnings ("0.25 g) for 1 hr in refluxing, dry monoglyme (25 ml). After 30 min, remove a small sample in a syringe and run on a silica gel thin-layer chromatographic plate (with a standard of tetrasulfur tetranitride in carbon tetrachloride). Elute the plate in carbon tetrachloride and measure Rrvalues for the major components. Typical values are as follows: S4Naf (Rf = 0.00 yellow); S4N4 (Rr = 0.26 +/- 0.06 yellow); SdNz (Rr = 0.70 +/- 0.02 red, fades); Ss (Rr = 0.85 +/- 0.02). The TLC technique is very useful for the identification of complex mixtures of sulfur-nitrogen and sulfur-nitrogen-chlorine compounds, and of sulfur and the sulfur imides. After refluxing for an hour, coal the reaction mixture and do the following test for SdNd Add a few drops of tin(1V)chloride to the mixture and observe what happens. (Care should be exercised in the handling of SnClr. Since rapid hydrolysisoccurs in the air, the compound should be syringed from a sealed flask under an atmosphere of dry nitrogen.) What stoichiometry do you expect for the precipitated product? Filter the solution with a grade 3 s i n k toisolate the solid, wash it with 2 M hydrochloric acid (10 ml), methanol (10 ml), and diethyl ether (10 ml), and pump dry on a vacuumline. Record a yield and run the infrared spectrum to identify the product. Test the effect of water on the solid and comment. The adduct formed is bis(tetrasu1fur tetranitride)tin(IV)chloride, (&N4)2SnC14 which is a purple solid and has i.r. absorptions at 1043(vs), 965(s), 930(ms), 812(s), 791(m), 700(ms), 621(mw), 568(s), 522(s), 420(m,hr), 370(s), 350(mw),312(s,hr) cm-l. The yield is approximately 0.1-0.2 g, but the ahove instructions could be fallowed using reagent quantities scaled up by a factor of 1.X20, for a largescale preparation of the adduct, (S4Na)zSnCk. Alternative Isolation of S4N4 After refluxine" for 1hr.,cool thereaction ~.~~ mixture and w m n w tha monoglyme by evapwatim iundcr reduced pressure. l'etrasulfur rerran~trideis Soxhlet-extracted frum rhe crude product with c a r l ~ m tetra
