Christopher W. Allen
University of Illinois Urbana, 61801
The Chemistry of Tetrasulfur Tetranitride
The chemistry of sulfur-nitrogen compounds has been a topic of interest since the first synthesis of tetrasulfur tetranitride, S4N4,in 1835 (1). Any comparisons which one might wish to make between these compounds and nitrogen-oxygen compounds are totally absent. This observation may be ascribed to the fact that nitrogen is the less electronegative partner in binary nihgen-oxygen compounds (hence nitrogen oxides) whereas it is the more electronegative partner in binar:f sulfur-nit,rogen compounds (hence sulfur nitrides). The chemistry of sulfur-nitrogen compounds has sevcral general features which are of interest and importance, namely, stability of the sulfur-nitrogen bond, tendency to formsix- and eight-membered rings, ring contract,ion, polymerization, and negative ion formation. There are several older reviews available on various aspects of sulfur-nitrogen chemistry(l4). This paper is limited to the chemistry of one of the most interesting and important of the sulfur-nitrogen compounds, tetrasulfur tetranitride.
Figure l a depicts a slightly distorted tetrahedron of sulfur atoms with nitrogen atoms added out from four of the edges, forming a square. The arrangement shown in Figure l b is spatially the same, with the roles of the atoms reversed. Also in Figure l b the nitrogen atoms on the same side of the plane of the sulfur atoms are bound to each other. The structure indicated in Figure l b , hereafter called the coplanar sulfur structure, was originally proposed on the basis of electron-diffraction studies. Further evidence was cited by Lippincott and Tobin(1l) on the basis of infrared and Raman studies. Since both models have the same symmetry, Dzd, symmetry considerations alone could not distinguish between the two. Lippincott and Tobin's assignment centers around the Raman line at 888 cm-', which they compared to the 883 em-' line ascrihed to the nitrogen-
Preparation and Properties
Tetrasulfur tetranitride can be formed in a wide variety of reactions. Also, it can be obtained from the interconversion of several other sulfur-nitrogen compounds. A few of the general pathways of synthesis, together with specific examples, are illustrated below: 1 . Amrnonolysisof sulfur halides.
2. Disproportionation of elemental sulfur in liquid ammonia (1).
3. Reaction of active nit,rogerl with sulfur or sulfur compounds.
Important physical properties are summarized in Table 1.
Figure 1 . Two geometrical orroys of tetrasulfur tetronitride. nitrogen structure; b, coplonor sulfur structure.
38
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Journal of Chemicol Education
coplanor
nitrogen bond in the hydrazine molecule. Since the spectra of this compound and tetrasulfur tetraimide, S4N4H4, were similar, the authors concluded that the two compounds have analogous structures, with thc nitrogen-nitrogen bond being broken to form nitrogew hydrogen bonds. This structure also has the feature of all the atoms exhibiting their normal valencies. The structure suggested in Figure l a , hereafter called the coplanar nitrogen structure, was proposed by Lu and Donohue(l0) also on the basis of electron-diffractio~~ studies. It was also indicated by a partial X-ray diffraction study(l2) and finally confirmed by Sharma and Donohue(l.9) in 1963 by a three-dimensional reTable 1.
Geometrical and Electronic Structure
The geometrical structure of the SINl molecule has been a subject of controversy for a long period of time and in fact has been settled only in the last few years. The early work in this field was summarized by Lu and Donohne (10). Since 1944, the controversy has revolved around the two geometrical arrays shown in Figure 1.
0.
Physical Properties of S ~ N I
nitrogen mp ('C) 178
(69) bond
Dipole moment
ikcallmole)
strengthb (kcallmole)
(D.) . .
(1)
Xmo~eu~a, (36)
+110f2
73.5+ 1
0.72
-102 X lo-'
AHserm (6BP
-
(c.e.s. un1t.s)
-The indicated thermodynamic instability is demonstrated by the tendency of SINa to explode when struck. "wed on the assumption of only snlfur-nitrogen bonds present.
Table 2.
Compound
Bond Lengths and Bond Angles
S-S S-N-S N-S-N N-S-S bond angle angle angle (A) (de- (degrees) (degrees) grees)
S-Nbond (A)
finement of the X-ray data. The bond lengths and bond angles of SINaand some other sulfur-nitrogen ring systems are summarized in Table 2. The coplanar nitrogen structure has a few salient features which are worth considering. The direct sulfur-nitrogen lengths are all equal and shorter than the sulfur-nitrogen single bond length, which is 1.764 A (in sulfamic acidH3N+-SO3-), or the single-bond radius wm of 1.74 A. The intramolecular sulfur-sulfur distance between the atoms on the same side of the nitrogen plane is considerably shorter than the sum of the van der Waals radii, but somewhat l o ~ g e than r the sulfur-sulfur single bond distance of 2.05 A. Attempts to explain these parameters in a consistent theoretical framework have been the object of considerable interest in the last few years. Becke-Goehring (4, 14) made some of the earliest attempts at a qualitative approach to the electronic structure of the SPNl molecule by representing it as a resonant hybrid, of which the most important forms are -
III=S=NI
I
I IS/ I
IS1
I
X=S=N, - a
-
INS--NI
l
""'I1
INS-N b
I/
- - -
- - -
NS=N
NS-N
I1
+il-li+ll
I
1
I
N=S-N - - c
II
N S =
- .-d
These assignmeuts were mostly deduced from chemical evidence which centers around hydrolysis reactions (15, 16). Tetrasulfur tetranitride readily undergoes base hydrolysis, 2S1N4
+ 6 0 H - + 9Hn0
-
2S3Os-
+ S20s- + 8NH3
This result is typical for a substance with sulfur in the + 3 oxidation state, since it can easily undergo disproportionation to sulfur(1V) and sulfur(I1). Other degradative reactions, such as treatment with hydrogen iodide (17) or chloramine-T (18) oxidation, confirm the sulfur(II1) oxidation state. Another conclusion drawn from these studies is that since all the nitrogen present is converted into ammonia rather than hydrazine and the products do contain sulfur-sulfur bonds, then the SINI molecules must have sulfur-sulfur rather than nitrogen-nitrogen bonds (15). Acid hydrolysis is also exhibited by S4Nl, but at a slower rate (16). Sulfur-nitrogen compounds are considered in Craig's (19-21) formulation of aromatic character in inorganic systems. He proposes a cyclic delocalization model for the S4N1molecule, involving p r orbitals on the nitrogen atoms and d r orbitals on the sulfur atoms. That the neighboring nitrogen atoms are more electronegative than the sulfur atoms causes a contraction of sulfur dorbitals to such a degree that they may effectively overlap with the nitrogen orbitals. Thus the sulfur atom changes from a spahybrid to one using a d orbital, thus
leaving one or two d orbitals free for delocalization. Since d orbitals have more multidirectional character than p orbitals, planarity is not a prerequisite for dclocali5ation. If the sulfur atom has strongly electronegativr exocyclic groups, the promotion to the state with available d orbitals is favored. The greater the electronegativity difference, the better the contraction of the d orbitals and hence the better the overlap. A concomitant effect is a tighter binding of the sulfur electrons and thus less tendency for delocalization. This effect is observed in the molerule SIN4F4(28) (Fig. 2). Tetrathiazyl fluoride, S4NiF4, which can be p r e pared from the reaction of silver difluoride on tetrasulfur tetranitride (6), has a puckered eight-membered ring type of molecular st,ructure, with the plane of the F nitrogen atoms above the Figure 2. Tchothioryle fluoride. plane of the sulfur atoms. The sulfur-nitrogen bonds (cf. Table 2) alternate between double and single bonds, which indicates a localized, i.e., cyclooct,atetraene, type of electronic structure. Some experimental justification for the delooalized model can be gained from a study of S4N4derived ions. A solid salt of tetrasulfur tetranitride may be formed by the reaction of tetrasulfnr tetramide with triphenylmethyl sodium (25). The solution undergoes a number of color changes, and finally NalSINl can be isolated. When S4Nl is treated with vacuum-distilled potassium in dry dimethoxyethane, various color changes are observed (24). These changes have been interpreted as indicating the formation of the following sequence of ions:
The electron-spin resonance spectrum of the first paramagnetic species to occur amounted to nine lines of intensitities close to 1:4:10:16:19:16:10:4:1, which is consistent with delocalization involving four equivalent nitrogen atoms. Of course, the possibility of the ion being delocalized although the parent molecule is not must be noted. A somewhat different approach to the bonding in S4N4 has been suggested by Lindqvist (25). The o b s e ~ e dsulfur-sulfur bond lengths in the S,OC and S:04= ions are longer than the generally accepted sulfur-sulfur single-bond length. Lindqvist maintains that the true sulfur-sulfur single-bond length is larger than the generally assumed 2.05 A, and that the sulfursulfur bond length in S4N4 is what one would expect for a bond of zero 8-character, i.e., pure p orbitals between the sulfur atoms. In this case, no resonance forms would exist, and the molecule would have the highly polar structure:
Volume 44, Number I, January 1967
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39
However this model does not explain the short sulfurnitrogen bond lengths. The first more quantitative approach was made by Chapman and Waddington ($6). They considered a delocalized model, with the behavior of the ?i elect,rons being approximated by a free electron on a sphere, and solved the Schrodinger equation for a particle on a sphere. Their energy-level and scheme successfully predicted the main absorption band = 2560 A, Xobs = 2530 A). They were also able to accommodate negative ions in their scheme and m&e predictions as to the magnetic charact,eristics of the species. This modcl has been criticized ($7, $8) on various grounds. It is inconsistent with the observed dipole moment; it cannot predict the lower-energy absorption bands; and it allows for both sulfur-sulfur and nitrogennitrogen bonds. Turner (&8) says that in light of similar calculations (e.g., for CH4),the agreement with the observed spectrum seems fortuitous. Brateman (87) has applied the formalism of Craig to a study of the electronic stmcture of the SaN4molecule. He contends that the resonance forms a and b are unreasonable since they differ by more than two oneelectron wave functions and consequently would not be expected to interact to any great extent. His calculations employ the resonance forms c and d as a basis set. The resulting electronic configuration on each atom is, therefore: N: (lone pair)%(ahs)*(n)'
where aNs is the sigma bond between the nitrogen and sulfur atoms and css is the sigma bond between the two sulfur atoms on the same side of the nitrogen plane. The energy-level scheme resulting from t,he Hiirkel molecular-orbital calculat~ionshas been used to interpret the visible and ultraviolet spectra of tetrasulfur tetranitride. The highest filled orbital is the ass set, and the electniic t,ransitions occur from this level t,o the (ass)* and (r)* levels. This assignment gives added support to t,he importance of the sulfur-sulfur bond, and since the ?r syst,em includes sulfur d orbit,als, it is unnecessary to accept Lindqvist's highly polar st,rurture. A more detailed set of calculations has been performed by Turner and Mortimer (28) using extended Hiickel molecular orbital calculations employing a limited Slater orbital basis set including 2s and 2p orbitals on the nitrogen atom and 3s, 3 p , and 3d orbitals on the sulfur atom. The first result obtained was the comparison of energies obtained for the coplanar nitrogen and coplanar sulfur (recalling Figs. la and lb) models. The coplanar nitrogen structure was shown to be of appreciably lower energy, and the nitrogen-nitrogen bond order was shown to he essentially zero under all vari* tion of parameters. Thus both the theoretical and the experimental approaches confirm the coplanar nitrogen structure. The calculations also indicate a sulfur-sulfur bond order of 0.47 to 0.36. Although d orbital contributions were found to enhance the bond order, they were not required for a nonzero sulfur-sulfur bond order. The maximum d orbital charge density was calculated to be 0.160 for d,, orbitals, which corresponds to the 40
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Journal of Chemical Education
nitrogen-sulfur axis. Therefore the d orhital contribution is primarily involved in the sulfur-nitrogen, not the sulfur-sulfur bond. The calculated charge density is 0.108 on both the sulfur and the nitrogen atoms. This small value suggests delocalization of electronic charge about the ring. The molecular-orbital scheme obtained was found to be lowered, but otherwise unchanged, by the inclusion of sulfur d orbitals. The first 22 levels are filled, these are followed by a low-lying, doubly degenerate E level which allows for the formation of negative ions from SnNn-to S1Nh4-. Chemical Behavior of t h e S4Na Ring System Conversion to Other Sulfur Nitrides ( 1 )
Tetrasulfur tetranitride may be used as a starting material for the preparation of all the other known sulfur nitrides. This is summarized in Table 3. Table 3 Compound
%N.
Method of Preparation
.. .
&N? (54 &N1
Cvelic
30°
L
+ N,
0.01 rnm
cvc1ic
[SNl.
s3N2
300-
115O W
SIN. Probable Configuration
RN2 &NI
He
SIP?
LineaP
L
700 mm H g
lSNlz Linear
SnNz haa been found t o be a eyelie molecule from infrared and Raman spectroscopy 166).
Hiickel-type molecular orbital calculations have been carried out on polysulfur nitride (23), [SKI,, and it was concluded that bond-length alternation was present. This situation would correspond to a basis set along the lines of resonance forms a and b of those drawn for tetrasulfur tetranitride in four atom units throughout the infinite chain. There are many interesting properties of polysulfur nitride, such as its dark blue color and its semiconductor properties, which are shorvn to be consistent with the bond-length alteration model (fi5). This is caused by the splitting of the widely separated hands so that t,he highest filled band is much rloser to the empty conduction band. Reduction ( I )
Reduction of tetrasulfur tetranitride by a variety of reducing agents results in the formation of tetrasulfur tetraimide, SnClz
S S I LS A H *
The redurtion takes place via two separate routes. Each of the nitrogw atoms picks up a hydrogen atom such as in a carbon-carbon or carbon-nitrogen doublebond reduction. The sulfur atom, on the other hand, changes from a + 3 to a +2 oxidation state. Tetrasulfur tetranitride can be regenerated by oxidation, SINIH,
C1,
SINl
+ HC1
As shown by the crystal stmcture determination, the conformation of the ring system in S4NrHn(Fig. 3) is considerably different from that of &N4 (SO). In the SaNaH4 molecule, there exists a regular puckered eight-membered ring of alternating sulfur and nitrogen atoms, with the plane of the nitrogen atoms above the plane of the sulfur atoms. The hydrogen atoms are bound to the nitrogen atoms. The position of the hydrogen atom was deduced from
infrared (11) and chemical data (Sf) and from packing considerations in the crystal (30). Tetrasulfur tetraimide is remarkably similar to the Ss molecule with alternating sulfur atoms replaced by -NH groups. The sulfur bond angles in SaN+H4and Ss are very close, being 108.4" in the former and 107.V in the latter. The parallelism is also reflected in the fact that the ultraviolet spectra of these two molecules are very similar (32). These similarities have led to the designation of S A I 4 , along with SiNH and Se(NH)%,as "pseudo-sulfurs." Although the sulfur-nitrogen bond leugth is longer
Figure 3.
Tetrorulfur tetroimide.
than t,hat of SnNn,it is shorter than the sulfur nitrogen single-bond length. This fact, coupled with the lack of basicity of the nitrogen atoms, suggests the possibility of some donation of nitrogen electron density to energetically favorable sulfur d orbitals (SO). Oxidation
Since sulfur(II1) may be reduced to sulfur(I1) without cleaving the ring, one might expect oxidation of sulfur(II1) to sulfur(1V) to occur. This change has been observed in the formation of the compound SdNdFa. A tetrameric thionylimide is produced from t,he air oxidation of tetrasulfur tetraimide (35). 0 2
SdNJL
ll(t-20" C
(OSNH 14
The formation of a derivative of a tetrameric sulfimide (sulfur in the +6 oxidation state) from an existing ring system has not been reported, but the compound has been formed from SO3and ammonia (5). AaNOn
NHs
SO,
d(NH&SsO,o
HIO
[AgNSOd,
Organic Derivatives
The only organic derivatives of the eight-membered sulfur-nitrogen ring are the nitrogen-substituted derivatives. They may be obtained by the reaction of sulfur dichloride with primary amines:
+ 4 RNH.
2 SzC12
-
+
[SNHRII 4 HC1 R = methyl (54) bensyl (55,36)
or by the reaction of tetrasulfur tetraimide with organic reagents which react with secondary amines (31):
+
-
S ~ N ~ H~ N ~ C O [SN~NH.+I~ Ring Contraction Reactions
A frequent consequence of reactions of tetrasulfur tetranitride is ring contraction. We have already noticed this phenomenon in the formation of tetrasulfur dinitride, SaN2,but it is most commonly observed in chlorination reaction.
Although treatment of SINl with fluorine or dilute fluorine-nitrogen mixtures leads only to degradation to sulfur and nitrogen fluorides (S7), treatment with chlorine or chlorinating agents leads to stable sevenand six-membered sulfur-nitrogen ring systems (6). Thiotrithiazyl chloride, S4N3C1,which is the most stable cyclic derivative of tetrasulfur tetranitride, can be formed in a variety of reactions; in fact, all the known sulfur-nitrogen chlorides can be trnnsformed into it (6). It also is formed from the reaction of SINl mith various rhlorinatiug agents. 3S4N4
+ 2S2Cl3
-
4S1Nscl
The reaction with S2CI, is complicated aud presumably has several intermediates, as %holmby experiments using 35SzC1,(4). As mith most other S,N4 derivatives, thiotrithiazyl chloride may be reconverted to tetrasulfur tetranitride. This may be accomplished by heating it in a vacuum, or by reaction with ammonia (39) or aluminum azide (40). The SIN3catiou is a seven-membered cyclic strurture. This structure was proposed on the basis of chemical evidence, (41) and confirmed by nitrogen-15 ~luclear magnetic resonance (42) and X-ray crystallography (43).
The thiotrithiazyl cation is the only known planar sulfur-nitrogen ring system. The sulfur-nitrogen distances are all equal and equivalent to a full sulfurnitrogen double bond (cf. Table 2). The sulfur-sulfur distance corresponds to a single bond. The short and equivalent sulfur-nitrogen bond lengths suggest a delocalized electronic structure over all t,he ring except across the sulfur-sulfur bond. The stability gained from this u system is most likely a good part of the driving force for the ring contraction. Hiickel molecular-orbital and free-electron calculations have been applied to thethiotrithiazyl cation (44), and a bonding scheme has been obtained which is consistent with the electronic spectrum. This model consists of sulfur atoms and the unique nitrogen atom as being sp2 hybrids while the two remaining nitrogens are sp hybrids. This leaves nitrogen p orbitals which can combine with sulfur p and d orbitals to form a tenelectron rr system above and below the plane of the ring. Chlorination with elemental chlorine (6) produces trithiazyl chloride, a six-membered ring system.
The molecular structure of this compound amounts to a six-membered ring in the chain form. The chlorine atoms are all bound to the sulfur atoms and all in axial positions (Fig. 4). The sulfur-nitrogen distances are all equal and comparable to those in the tetrasulfur tetranitride molecule. This suggests a delocalized electronic Volume 44, Number
I , January 1967
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41
structure, in wh'ich sulfur d orbitals are involved, since the molecule is nonplanar. Trithiazyl fluoride can be prepared by the action of silverdifluoride on trithiazyl chloride (6). Fluorine-19 nuclear-magnetic resonance studies (45) show all of the fluorine atoms to be in the same environment. A full structure d e t e r m i n a t i o n would be of interest to see if an isolated single-double bond structure is present Figure 4. Trithiozyl chloride. here, as in the SaN4F4mole. cule.
L
N
Lewis-Acid and Lewis-Base Behavior
Because of the differing electronegativities of sulfur and nitrogen atoms, one would expect partial negative charges on the nitrogen atoms and partial positive charges on the sulfur atoms in tetrasulfur tetranitride. Consequently, the compound may act as either a Lewis acid through the sulfur atoms or as a Lewis base through the nitrogen atoms. The general reaction may be considered to be: &N4 acting as a Lewis base (or acid) adding some Lewis acid (or base) in a stoichiometry from 1:1 up to 1:4. The Lewis-base behavior of tetrasulfur tetranitride usually leads to stable adducts, whereas Lewis-acid behavior is usually followed by ring contraction or degradation. Since the reactions of tetrasulfur tetranitride with Lewis acids have been more widely studied, they are considered first. The most stable adducts contain metal halides such as SbCla, CNClr, TiCL, MoCls, WClr, and VCll(46). One of the easiest adducts to prepare and handle has the formula S4N4.SbCI5. The structure of this compound has been determined by X-ray crystallography (47) (Fig. 5). Here again, as in the case of tetrathiazyl fluoride, S~NIFI,and tetrasulfur tetraimide, the sulfur-nitrogen ring has undergone a conformational change. The ring in this case adopts the same conformation as in the SrNPr molecule. This conformational change is a result of the weakening of the sulfur-sulfur bond as a consequence of the drain of electron density from the ring by the SbC15 group. The sulfur-sulfur bond, being the weakest bond present, is the first to break, and its rupture allows the ring to change to a less sterically hindered (in the case of SaN4,SbCls) conformation. The sulfur-nitrogen bond lengths from the nitrogen atom coordinated to the antimony atom are considerably lengthened to 1.74 A. However the sulfurnitrogen bond lengths from the nitrogen atom which is opposite the coordinated one are virtually unaffected. This suggests the possibility of further coordination through this site. A structure of this type has been proposed for the adducts WCL.SIN4 and VClP.SpN4 (47). Here the S4Nagroup would act as a bidentate ligand through the two opposite nitrogen atoms. Another route to metal halide adducts is through the reaction of metal halides with S3Nc0, (48).
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Journal of Chemical Educafion
An interesting structure has been proposed for this last compound (48). It consists of a dimer of titanium atoms each in an octahedral environment with two bridging chlorine atoms and a bridging SINl molecule. Obviously, further structural studies, especially of the "bidentate" species, would be desirable. Two adducts of sulfur trioxide with tetrasulfur tetranitride can be isolated, namely S4N4.2SOaand S I N I . ~ S O( I~) . Thus all four basic sites may be used. An unstable, poorly defined adduct with boron trifluoride can be prepared. It has the composition 4S4N4.BF3(6). As was previously mentioned, adductsformedbetween tetrasulfur tetranitride and Lewis bases are usually unstable. The only one which may have the ring presewed is the thiazyl fluoride adduct, which can be made by the reaction of various fluorinating agents on S4N4 (@), e.g., However, this compound is not well characterized and consequently nothing can be said concerning its structure. One mole of ammonia adds to SnNI to give a compound of the composition SlN4.NH3, but this compound is identical to the adduct of SpNz with ammonia. It is assumed, therefore, that the ring was cleaved during the reaction (1).
Figure 5. S,N~.Sbclbone of the adducts of tetrasulfur tetranitride.
Ring contraction reactions appear to result from the reaction of triphenyl phosphine and triphenyl phosphine methylene (60) with tetrasulfur tetranitride. The following structures have been proposed for 4sPNrSa (60). --
S-N // -\S-N=P& I N \ S-P; // -
--
- -
IS-NS I or
/
I S=N-PC
The reaction with cyanide ion (50) is very complex. By analogy with its reaction with elemental sulfur ($8) the cyanide ion is assumed to attack the sulfur atom. The structure of the product, Kz(CzNloSs).2KCN,has been formulated in terms of two six-membered sulfur-
nitrogen rings. Reaction with Grigrlard reagents causes ring degradation (61).
CH.
S-N
1
I
S-N
)Ni
