Halogenation reactions of phosphiniminocyclotrithiazenes: Search for new inorganic heterocycles

Article abstract of DOI:10.1080/15533174.2015.1066804

Though the chemistry of tetrasulfur tetranitride is very well developed and studied, that of phosphiniminocyclotrithiazenes is not. Reactions of symmetrically substituted R₃PN-S₃N₃ [R = Ph, OC₄H₈N, Me

Full text of DOI:10.1080/15533174.2015.1066804

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Halogenation Reactions of
Phosphiniminocyclotrithiazenes: Search for New
Inorganic Heterocycles
Janarthanan Gopalakrishnan, Ulaganathan Swarnalatha & Sudheendra M.
N. Rao
To cite this article: Janarthanan Gopalakrishnan, Ulaganathan Swarnalatha & Sudheendra
M. N. Rao (2016) Halogenation Reactions of Phosphiniminocyclotrithiazenes: Search for New
Inorganic Heterocycles, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 46:9, 1324-1331, DOI: 10.1080/15533174.2015.1066804
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2016) 46, 1324–1331
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2015.1066804
Halogenation Reactions of Phosphiniminocyclotrithiazenes:
Search for New Inorganic Heterocycles
JANARTHANAN GOPALAKRISHNAN1, ULAGANATHAN SWARNALATHA2, and SUDHEENDRA M. N. RAO3
1Department of Applied Sciences, PNG University of Technology, Lae, Morobe Province, Papua New Guinea
2Department of Chemistry, Indian Institute of Technology Madras, Chennai, India
3Department of Chemistry, Indian Institute of Technology Tirupati, Tirupati, Andhra Pradesh, India
Received 10 May 2015; accepted 31 May 2015
Though the chemistry of tetrasulfur tetranitride is very well developed and studied, that of phosphiniminocyclotrithiazenes is not.
Reactions of symmetrically substituted R3PN-S3N3 [R D Ph, OC4H8N, MeNC4H8N, and C5H10N] with halogens and halogen
sources (SO2Cl2, PhICl2, Br2, and I2) gave rise to various products. In case of R D Ph, the ring-retained product, while for others,
ring-degraded products were obtained. A novel cyclotrithiazene heterocycle, [Ph3PN]2S3N3CI3¡ containing a tetracoordinated sulfur
atom with two phosphinimino substituents was obtained, which was further established by IR, NMR (1H-, 13C-, and 31P-) and
elemental analysis. Cyclic voltammetric studies indicated the redox behavior as well as the presence of triiodide. Theoretical
calculations clearly indicate the residence of maximum positive charge (close to unity) on the tetracoordinated sulfur atom.
Keywords: halogenation, inorganic heterocycles, phosphiniminocyclotrithiazenes, cyclotrithiazenes, trisulfur trinitride, sulfur-nitro-
gen heterocycles
Introduction
The chemistry of S4N4 is well developed in terms of
various reactions, namely, coordination, pyrolysis, haloge-
To address the needs of the growing technology and demands nation, oxidation, reduction, and electro- and photochemi-
for sophisticated and more advanced materials, the current cal reactions, to name a few.[6–9] In all these cases, the
research focus spans several sub-branches such as transition products resulted were quite unexpected and novel. On
metal coordination, organometallic, theoretical, polymer, the contrary, despite the availability of many different
organic and inorganic chemistry.[1,2] In these areas of research, kinds of substituents on phosphorus, very little reactivity
inorganic heterocycles are known for a long period of time and aspects and chemistry are known for the –S3N3 deriva-
their chemistry has been investigated for several decades. How- tives: adduct formation, thermolysis, exchange, ring con-
ever, inorganic heterocyclic chemistry especially the search for version, coordination, electrochemical, and hydrolytic
new heterocycle systems is still pursued as one of the significant degradation studies,[10–13] and some are even in the
areas.[3] Several research groups are involved in the inorganic infancy stage (Scheme 1). While acids and bases behave
heterocyclic chemistry mostly because of the complexity of the differently with –S3N3 systems leading to either ring deg-
reactions that often require sophisticated experimental facilities. radation or ring expansion, the behavior of halogens,
A quick, easy, and high yield synthesis are the current targets.[4] which are oxidants, would be of interest. Recent results,
Phosphiniminocyclotrithiazenes, (R)(R’)(R”)PDN-S3N3, from our group, on the iodination reactions of phosphini-
are p-electron rich six-membered inorganic heterocycles, pre- minocyclotrithiazenes suggest that the cyclotrithiazene
pared commonly and easily from a phosphine and tetrasulfur ring reacts with iodine differently with different mole
tetranitride, S4N4 (Eq. 1).[5]
ratios of the reactants.[14]
There are several reports on the use of all four halogens as
well as interhalogen compounds in the reactions with
S4N4.[8,15–18] The results suggest many unknown, unexpected
as well as novel compounds by way of undergoing addition,
ring contraction, fragmentation, and polymerization reac-
tions (Scheme 2). Such vastness clearly indicates the very
reactive nature of the heterocycle and the readiness to
undergo the ring transformation reactions, and thus making
it difficult to predict the reaction mechanisms.
Solvent
R3P C S4N4 ÀÀÀÀÀÀ! R3P H N-S3N3 C R3P D S (1)
Room temp:
Address correspondence to Janarthanan Gopalakrishnan,
Department of Applied Sciences, PNG University of Technol-
ogy, Lae 411, Morobe Province, Papua New Guinea. E-mail:
janarthanang@gmail.com
Fluorination reactions with either fluorine or fluorinat-
ing agents afford to isolate three examples of fluorinated

Halogenation Reactions of Cyclotrithiazenes
1325
Sch. 1. Reported reactions of (R)(R’)(R”)PDN-S3N3.
19]
cyclothiazenes, viz. S4N4F4, S4N4F2 and S3N3F3.[16,
but was subsequently disproved.[21] Reactions with the
Chlorination reactions of S4N4 with Cl2 or SO2Cl2 are interhalogens ICl and IBr at room temperature produce
one of the known routes for preparing cyclotrithiazyl conducting polymers such as [(SN)(IX)0.4]x (XHCl, Br),[18]
chloride, (NSCl)3. It was reported[1] that this reaction pro- while at 100ꢀC an ionic product [S4N3]C[ICl2]¡ is
ceeds through the formation of an unstable intermediate observed.[17] On the contrary, when ClF was reacted with
(NSCl)4 that readily decomposes at room temperature to S4N4 monothiazyl halides NSX (XHCl, F) are
give (NSCl)3. In such reactions, S4N4Cl2, a novel disubsti- obtained.[21] A variety of six membered S-N rings are
tuted cyclotetrathiazene has been isolated and structurally established with or without one or more substituents, and
characterized as well as its usefulness in synthesis has among halogen substituted cyclotrithiazenes S3N3X3 and
been demonstrated.[15] On the contrary, serendipitous S3N3X2 are known while S3N3X2C is not known. A few
C
results have been obtained in the bromination reactions, attempts have been made to prepare S3N3 rings with a
for example, Br2 in the vapor phase yields a highly con- single exocyclic S-X bond[22] but the efforts went in vain.
ducting polymer of [SNBr0.4]x, and in the liquid phase an
Reactions of phosphiniminocyclotrithiazenes with ole-
¡[18]
ionic product of S4N3CBr3
and an unexpected product fins such as norbornadiene gives 1:1 adducts wherein the
¡
of [CS3N2Br]CBr3 in the presence of CS2 solvent.[20] heterocycle transforms from the p-electron rich (8p) to a
Reaction between S4N4 and I2 was found to give S3N3I[17] p-electron precise (6p) system (Scheme 1). A similar
Sch. 2. Reactions of S4N4 with halogens and interhalogens.

1326
Gopalakrishnan et al.
reaction can also be assumed when the heterocycle is Halogenation Reactions
reacted with halogen, which certainly gives scope for the
A typical reaction procedure for the reaction between 1 and
iodine in 1:1 ratio in benzene is described below. Other reac-
tions are summarized in Table 1.
design and development of novel organo-inorganic hybrid
heterocycles that could possibly have potential applica-
tions as molecular-based switches.[2,3,23,24] An example is
provided by taking 1,3-diaminopropane in Scheme 3.
Search for new heterocycles for their further use as pre-
cursors to polymers and materials has also made this
research to look into some of these aspects. Prompted by
all these interesting reactions, especially considering the
application potentials and simultaneously considering the
highly ring-cleaving susceptibility of phosphiniminocyclo-
trithiazenes,[25] herein attempts have been made to study
the halogenation reactions on the selected examples of
symmetrically substituted R3PN-S3N3 under various reac-
tion conditions and results are discussed.
To a stirred solution of the heterocycle 1 (1.20 g,
2.9 mmol) in benzene (20 mL) at room temperature, a solu-
tion of iodine (0.73 g, 2.9 mmol) in benzene (15 mL) was
added in drops in 30 min. The solution turned brown gradu-
ally, and after ca. 5 h, the formation of a dark brown oily
mass was observed. After 24 h of stirring, from the clear red
brown benzene solution, the oily mass was separated by
decantation and was then stirred in CH3CN (25 mL) for 2 h
and filtered to obtain the red brown solid which was recrystal-
lized from CH2Cl2-CH3CN ¡(2:1, 15 mL) to get red brown
crystals of [Ph3PN]2S3N3CI3 (5) (0.80 g, 30%). m.p.: 144–
45ꢀC; IR (cm¡1): 1469(w), 1424(s), 1178(s), 1138(vs), 1107(s),
998(vs), 806(w), 736(m), 717(s), 688(s), 525(s) and 470(w).
UV-Vis [λ, nm (e, dm3mol¡1cm¡1)]: 480 (0.03 £ 103), 363
Experimental
1
(0.07 £ 103) and 293 (1.47 £ 103). H-NMR (d, ppm): 7.1–
All manipulations were done in the inert atmosphere (dry N2 7.3(m). 13C-NMR (d, ppm): 124.0, 125.0, 130.0 and 134.0.
or Ar gas) conditions. Br2, I2, and SO2Cl2, solvents (Caution! 31P-NMR (d, ppm): 22.2(s, 1P). Elemental Anal. Calcd. (%):
Benzene is carcinogenic) and other reagents were purified by C: 40.35, H: 2.83, N: 6.54, and I: 35.00. Found for
standard methods.[26] C6H5ICl2 was prepared using the estab- C36H30N5P2S3I3: C: 39.92, H: 2.62, N: 6.31, and I: 34.89.
lished procedure.[27] R3PN-S3N3, R H Ph (1); Mp, OC4H8N
A further quantity (0.04 g) of 5 and pale yellow powdery
(2); NMP, MeNC4H8N (3); Pip, C5H10N (4) was prepared by solid (0.10 g) identified as Ph3PNH2CI¡ (6) were isolated in
following the literature protocols.[28, 29] Shimadzu UV-240 successive crops by cooling the concentrated acetonitrile
spectrophotometer is used for recording the UV-visible spec- extract. m.p.: 224–26ꢀC (Lit. 228ꢀC[30]). IR (cm¡1): 1430(vs),
tra in the range of 190–800 nm. IR spectra were recorded on 1372(s), 1296(m), 1171(w), 1149(w), 1094(s), 1017(w), 992
a Perkin Elmer 1430 spectrophotometer using KBr windows (w), 910(vw), 749(s), 710(vs), 685(vs), 633(m), 611(vw) and
as nujol mulls. Peaks in the finger print region (1600– 586(w). 31P-NMR (d, ppm): 35.5(s, 1P). The original benzene
400 cm¡1) alone are reported. {1H}31P-NMR spectra (162 solution was concentrated and cooled at ca. 5ꢀC for a day to
MHz) were recorded on JEOL JNM GSX-400 spectrometer obtain 6 (0.25 g) and subsequently, S4N4 (0.10 g) as orange
in CH2Cl2 (provided with a D2O insert) using 85% H3PO4 as crystals.
1
external standard. Upfield shifts are negative. H- and 13C-
NMR spectra (400 and 100 MHz respectively) were recorded
using the above spectrometer in CDCl3 using tetramethylsi- Results and Discussion
lane as the internal standard. All the chemical shift values are
quoted in ppm. Elemental analyses were done on Heraeus The halogenation study has been prompted to verify (a) the
CHN rapid microanalyzer.
possibility of oxidative addition reactions of R3PN-S3N3
Sch. 3. Expected products of a halogenation reaction of Ph3PNS3N3.

Halogenation Reactions of Cyclotrithiazenes
1327
Table 1. Reactions of R3PN-S3N3 (1–4) with halogens/halogen sources at different reaction conditions
Halogen
(source)
Mole
ratio
Addition
period
Reaction
conditions
Products
obtained
Compound
Solvent
C6H6
1
I2
1:1
1:2
1:6
1:1
30 min. at RT
30 min. at RT
30 min. at RT
30 min. at RT
Stirred at RT for 24 h
Stirred at RT for 4 h
Stirred at RT for 4 h
Stirred at RT for 14 h
[Ph3PN]2S3N3CI3¡ (0.84 g, 31.5%)
Ph3PNH2CI¡ (0.35 g) S4N4
(0.10 g)
CH2Cl2
CH2Cl2
CH3CN
[Ph3PN]2S3N3CI3¡ (0.25 g, 19%)
Ph3PNH2CI¡ (0.20 g) S4N4
(0.08 g) I2 (0.05 g)
[Ph3PN]2S3N3CI3¡ (0.20 g, 16%)
Ph3PNH2CI¡ (0.26 g) S4N4
(0.10 g) I2 (0.25 g)
White solida (0.11 g) Ph3PNH2CI¡
(0.60 g) S4N4 (0.20 g)
Br2
C6H5ICl2
1:1
1:1
CH2Cl2
CH2Cl2
30 min. at 0ꢀC
1 h at 0ꢀC
Stirred at RT for 17 h
Stirred at 0ꢀC for 4 h;
RT for 2 h
Ph3PNH2CBr¡ (0.42 g) S4N4 (0.02 g)
Ph3PNH2CCl¡ (0.60 g) S4N4 (0.08 g)
¡d
2
I2
1:1
CH3CN
30 min. at RT
Stirred at RT for 12 h
White solida (0.15 g) Mp3PNH2CI3
(0.92 g) S4N4 (0.25 g)
1:1
1:1
1:1
1:1
1:1
1:1
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
30 min. at RT
30 min. at 0ꢀC
30 min. at RT
30 min. at 0ꢀC
30 min. at RT
30 min. at RT
Stirred at RT for 20 h
Stirred at RT for 12 h
Stirred at RT for 4 h
Stirred at 0ꢀC for 6 h
Stirred at RT for 24 h
Stirred at RT for 24 h
Mp3PNS3N3 (0.48 g) I2 (0.18 g)
Br2
I2
SO2Cl2
I2
Mp3PNH2CBr¡ (0.72 g)
3
4
Brick red solid (0.74 g)b
(NMP)3PNH2CCl¡c
(Pip)3PNS3N3 (0.55 g) I2 (0.20 g)
I2
White solida (0.10 g) (Pip)3PNH2CI¡
(0.36 g) S4N4 (0.18 g)
aShowed broad bands in IR spectrum in the range 1000–1200 cm¡1; qualitatively answered for iodide.
bCould not be further purified and characterized (M.pt. >250ꢀC). Analogous reaction performed in C6H6 and CH3CN as well as 1:1 bromination reaction
gave only intractable pale coloured high melting solids (>250ꢀC) which eluded from further characterization; the IR spectrum showed the presence of PDN
bond.
cFrom the oily mass, no other product could be isolated; IR spectrum indicated the presence of PDN bond and 31P-NMR spectrum showed the presence of
(NMP)3PNH2CCl¡.
dCharacterization data: Dark red brown crystals. M.pt.: 210–12ꢀC. IR (cm¡1): 3190(w), 3060(w), 2928(vs), 1560(m), 1440(vs), 1360(s), 1280(m), 1240(s), 1120
(s), 1080(s), 920(s), 830(m), 700(m), 590(m), and 450(m). UV-Vis [λ, nm (e, dm3mol¡1cm¡1)]: 363 (1.9 £ 103) and 296 (3.7 £ 103). 31P-NMR (d, ppm): 33.1
(s, 1P).
Iodination of 1
heterocycles (8p, electron-rich) with halogens (SO2Cl2 and
PhICl2 as chlorine source, liquid bromine and solid iodine) to When the dark brown solution of iodine was treated with
produce electron-precise (6p) heterocycles with sulfur atoms deep red dichloromethane of 1, no visual inference on the
containing mixed substituents (Scheme 3); and (b) the influ- occurrence of the reaction could be assessed as the result-
ence of nature of substituents attached on phosphorus center, ing solution was also dark red brown. However, the 31P-
where 1 has carbon groups (phenyl) attached while 2–4 have NMR spectrum revealed three peaks at 35.7, 31.2, and
nitrogen groups (amino).
21.4 ppm. The reaction mixture finally gave a dark red
Three solvents C6H6, CH2Cl2 and CH3CN with dif- brown oily mass and a clear pale red brown filtrate. The
fering polarity indices (2.7, 3.1, and 5.8, respectively), oily mass with acetonitrile and dichloromethane yielde¡d
mole ratio of reactants (1:1, 1:2, and 1:6), reaction tem- shining dark red brown crystals of [Ph3PN]2S3N3CI3
perature (0ꢀC and room temperature), and reaction (5). The other two products are phosphiniminium iodide,
period (4–24 h) have been used. It is known that in Ph3PNH2CI¡ (6), and cyclotetrathiazene, S4N4. They all
cyclothiazene chemistry chlorination and bromination have varying yields from all the reactions (1:1, 1:2, and
reactions proceed more violently and pose problems due 1:6) carried out either in benzene or dichloromethane,
to their high reactivity and afford complex reaction mix- though with increasing iodine content, the yield of the
tures, and that temperature plays a vital role in deciding triiodide product dropped from 31.5% to as low as 16%.
the nature of the products thus formed.[22] Hence, milder Surprisingly, an equimolar reaction carried out in more
of the halogens in terms of reactivity, namely, iodine polar acetonitrile solvent gave high yields of 6 and a
and a more stable one of the cyclotrithiazenes, namely white solid (which could not be characterized further),
Ph3PN-S3N3 (1), were considered for a detailed study.
rather than 5. Such white solids are frequently

1328
Gopalakrishnan et al.
identical stereochemical environment around the phos-
phorus centers in 5. This value is quite close, but signifi-
cantly different to that of the parent heterocycle 1
(23.8 ppm[10]) indicating a similar structure. The signals
Table 2. Cyclic voltammetric data for 5
Peak potential (mV)
Peak current (IP)
Species Cathodic
Anodic
Cathodic Anodic
1
observed in its H- and 13C-NMR spectra are consistent
5 ¡
174
155
288
290
1.0589 £ 10¡4 1.0540 £ 10¡4
1.4643 £ 10¡4 1.5818 £ 10¡4
with the aromatic protons and phenyl carbons, respec-
tively. Thus, it appears that the 8p-heterocycle 1 experi-
ences one- or two- electron oxidation under the influence
of the relatively mild oxidant iodine towards yielding the
7p- or 6p-heterocycle (Scheme 4). This hypothesis is con-
I3
encountered in similar reactions[12] and exhibited broad sistent with the observation of stable 7p-heterocycle
¡
IR bands (in the nNH and nSO regions) suggesting their S3N2CAsF6 and the ionization of the substituted cyclo-
ionic and water-soluble nature. In the reaction of 1 with trithiazene X3S3N3 in producing N3S3X2CY¡.[34, 35]
halogens / halogen sources, one of the definitive prod-
However, comparison of the analytical results indi-
ucts obtained is the eight-membered heterocycle S4N4 cates that the compound 5 is neither of the two composi-
that also formed during the halogenatio¡n [3r1e]actions of tions considered in Table 3, but it should be
six-membered cyclotrithiazene anion S3N3
.
Reported [Ph3PN]2S3N3CI3¡. Eight different trials have been con-
data indicates that as many as five different types of ducted for estimating the iodine content[36] and the
products could be obtained in the case of R H Ph[14] results obtained (34.17–35.77%) undoubtedly suggest the
indicating the highly reactive nature of the ring in the formation of 5.
formation of different products - ring retained and/or
Scheme 5 highlights a possible mode of attack and the
cleaved. In the present study, yet another novel product product formation, based on the nature of the com-
5, hitherto unreported, has been obtained suggesting the pounds obtained thus far either through charged inter-
ring sensitivity towards mole ratios of the reactants, sol- mediates[37] or radical intermediates as in solid state
vents and reaction periods.
pyrolysis reactions.[5] In the present study, as solvent has
been used, the most likely mechanism could be through
the charged intermediates. The observation of phosphini-
minium salt formation is a clear indication of the attack
of residual moisture available in the solvents on the
more sensitive heterocycles and the intermediate products
as well during the reaction. Thus, it appears that very
often such salts are encountered[31] in these reactions
irrespective of the rigorous inert atmospheric manipula-
tive conditions and the dry solvents. This presumption is
supported by the ascertainment that larger quantities of
salt were obtained in the presence of polar acetonitrile
solvent (cf. Table 1). The phosphinimino substituent is
highly susceptible for cleavage from the S3N3 ring as
observed in the mass spectral fragmentation of such
cyclotrithiazene heterocycles.[25] The S-N ring cleaves the
“NS” species[11] and either polymerizes to the more sta-
ble S4N4,[38] or combines with “S3N3” to produce S4N4.
Despite the observation of phosphiniminium salt, no sign
of SO2 or SO3 gas could be identified during the reac-
tion, though their occurrence was reported for such simi-
lar reactions.[31] From the structure of 5, as shown in
Scheme 5, it is clear that the low-energy empty 3d orbi-
tals of the tetracoordinated S-atom are involved in the
Characterization of 5
Compound 5 is a dark red brown crystalline solid, air stable
and more soluble in polar solvents like CH2Cl2 and CH3CN.
Its color clearly indicates the presence of the triiodide ion. Its
melting point 144–45ꢀC is higher, around 15ꢀC, than the par-
ent heterocycle 1 that melts at 126–28ꢀC and shows deposition
of iodine crystals in the sealed capillary during the determina-
tion of melting point. The IR spectral features of 5 are found
to be very similar to those of 1.[10] Upon comparing the UV-
Visible spectra of 5, 1 and triiodide anion, it could be unam-
biguously concluded that 5 possesses: (a) triiodide anion, the
strong absorption at 293 nm,[32] thus confirming the ionic
nature; and (b) the phosphiniminocyclotrithiazene moiety, the
absorption at 480 nm.[10] Subsequent, cyclic voltammetric
studies on 5, at different sweep rates in the potential range
¡1500 to C1500 mV (Table 2), suggest that the values are in
close comparison with those of triiodide ion,[33] indicating the
redox behavior of 5 as well as the presence of triiodide ion.
It is interesting to note that only a singlet is observed
in the 31P-NMR spectrum at 22.2 ppm, emphasizing the
Sch. 4. Expected modes of attack of I2 on 1.

Halogenation Reactions of Cyclotrithiazenes
1329
exocyclic atoms (2 nitrogen C 2 phosphorus atoms) of the
two substituents. Based on these findings, it can be
assumed that different resonating structures (I–VII) con-
tribute towards bonding, where delocalization starts from
the phosphorus atom on the left and extends up to the
other phosphorus atom on the right passing through the
cyclotrithiazene ring (structure VIII), with varying degrees
of bond elongation and shortening.[39]
Table 3. Comparison of analytical data
Compound
C % H % N %
I %
Ph3PNS3N3CI3¡ (calcd.)
[Ph3PNS3N3]2C 2I3¡ (calcd.)
[Ph3PN]2S3N3CI3¡ (calcd.)
27.17 1.91 7.05 47.92
18.37 1.29 4.77 64.80
40.35 2.83 6.54 35.00
Dark red brown crystals (obsvd.) 39.92 2.62 6.31 34.89
The main idea of undertaking the theoretical study was to
determine: (a) the total charge, (b) the atomic charge densi-
ties, and (c) the site of preference for the positive charge on
the heterocyclic cation. Using the available structural infor-
mation of the molecule,[39] semiempirical calculations were
performed with different modes of parameterization, namely,
MNDO and PM3 and the results are given in the Table 4.
The numbering of endocyclic atoms is as given in structure
VIII of the Scheme 6.
The data analysis and conclusions drawn from MNDO
calculations clearly reveal that the maximum positive charge
resides on the tetracoordinate sulfur atom and the overall
charge of the heterocyclic cation is close to unity (as shown in
structure I, Scheme 6). The results further substantiate that
other resonance structures (II–VII) do not contribute to any
appreciable extent to the resonance hybrid.
Iodination of 2–4
Sch. 5. Proposed mode of attack for the formation of 5.
The behavior of the other R3PNS3N3 ring systems, 2–4 for
iodination reaction was found to be quite different to that
of 1. For example, in dichloromethane solvent, an equimo-
lar reaction of 3 and iodine gave an insoluble brick red pre-
cipitate that did not melt even at 250ꢀC, while 2 and 4 did
not react at all. Although these observations underscore the
importance of the substituents, the complex nature of the
obtained results is difficult to explain easily. Analogous
reactions carried out in CH3CN showed differences com-
pared to CH2Cl2 reactions, suggesting that the choice of sol-
vent is vital for halogenation reactions. For example, a 1:1
bond formation. In fact, compound 5 appears to be the
first example of a cyclotrithiazene to contain a tetracoor-
dinated sulfur atom while the other two sulfur centers
are dicoordinated.
Theoretical Calculations on 1,1-Bis(triphenylphosphinim¡ino)-
1,3,5-trithia-2,4,6-triazenium triiodide, [Ph3PN]2S3N3CI3 (5)
Scheme 6 highlights the possible seven resonance struc- reaction of 3 and iodine in three different solvents C6H6,
tures for 5. These delocalized structures indicate the resi- CH2Cl2 and CH3CN gave substantial amounts of high melt-
dence of mono-positive charge on all the six endocyclic ing solids (nearly 60% of the weight of reactants) with colors
atoms (3 sulfur C 3 nitrogen atoms) and the four of brick red, yellow and white, respectively. The
Sch. 6. Resonance structures of 5.

1330
Gopalakrishnan et al.
are found to be mixtures of corresponding phosphiniminium
bromide or chloride salts. In some cases, relatively pure sam-
ples of these salts could as well be isolated along with small
amounts of S4N4. Overall, our results suggest that though tris
(amino)phosphiniminium halide salts are difficult to prepare
due to the formation of highly air and moisture sensitive pen-
tavalent phosphorus intermediates, by judicially involving
R3PX2 compounds in the reaction sequence pure crystalline
solids of these halides could be prepared easily, as demon-
strated in the present study (Scheme 7).
Table 4. Atomic charges of ring atoms for 5
Atom
MNDO
PM3
S1
0.5731
1.7992
0.7601
1.4678
0.9048
¡0.9184
¡0.8567
¡0.6641
S2 (tetracoordinated)
S3
0.6120
N1
N2
N3
¡0.8765
¡0.4323
¡0.8601
corresponding IR spectra suggested the presence of N-meth-
ylpiperazino group and PHN bond as well as the absence of
trithiazene ring. Subsequent qualitative analyses of the sol-
ids revealed the presence of iodide, sulfur, and phosphorus
that in turn suggests that these compounds are mixtures of
products. In general, the corresponding phosphiniminium
salts, R3PNH2CI¡ and S4N4 are the only isolable products
from these reactions. Interestingly, in the case of morpho-
Conclusions
The quest for novel p-electron systems has paved the way
to explore many new heterocyclic systems, and cyclic SN
systems (e.g., S4N4 and S3N3) stand out as special exam-
ples. Importantly, their synthesis, chemistry, and structure
elucidation offer several favorable opportunities. In addi-
tion, the continuing interest in these systems has moti-
vated us to undertake a systematic investigation on the
cyclic SN systems, especially to gain better insights on the
bonding and conformation. Herein, we demonstrated that
upon reacting iodine with Ph3PNS3N3, a novel inorganic
heterocycle could be obtained wherein one of the sulfur
atoms is tetracoordinated while the other two are dicoor-
dinated. Subsequent spectroscopic characterization along
with the theoretical calculations suggest the structure as
[Ph3PN]2S3N3CI3¡. In general, the halogenation reactions
of phosphiniminocyclotrithiazenes proceed at different
rates and at variable complexities depending on: (a) the
nature of halogen used for the reaction, (b) the nature of
substituent (whether P-C or P-N bonded) on the P-atom,
(c) the nature of solvents used, and (d) the mole ratio of
the reactants. However, more cyclic SN systems need to
be synthesized to tap their complete potential and
functionality.
¡
lino derivative, a triiodide salt (OC4H8N)3PNH2CI3 was
isolated.
Bromination and Chlorination of 1–4
Though bromination and chlorination reactions were carried
out under relatively mild conditions (1:1 ratio, 0ꢀC and short
reaction time) in CH2Cl2 and CH3CN solvents, they pro-
gressed far more rapidly and revealed more complex path-
ways. This is further supported by the 31P-NMR data of the
filtrates of the reactions that clearly revealed the presence of
many phosphorus-based products. For example, the filtrate
of the reaction between 3 and SO2Cl2 exhibited signals at
74.3, 35.7, 24.6, 22.6, 19.9, and 13.3 ppm. However, the com-
plexity gradually decreases from chlorination to bromination
to iodination, which very well correlates with their oxidizing
ability. Expectedly, in almost all the cases, the starting hetero-
cycle was found to be absent and the presence of correspond-
ing iminium salt, R3PNH2CX¡, as evidenced by the presence
of a strong signal at around 35 ppm.[40] Strikingly, in all these
spectra, a signal around 74 ppm that could be attributable to
the aminophosphine sulfide R3PS[40] has been observed stress-
Acknowledgments
ing the thiophilic nature of the phosphorus atom.[41] This Prof. Pranav Deshmukh of Physics Department (IITM) is
observation further exemplifies the more complex nature of greatly acknowledged for his valuable inputs on the theoreti-
these reactions. Considerable amounts of colored precipitate cal calculations. Dr. Gopalakrishnan thanks PNGUT for the
(yellow or red brown) or oily mass (yellow) are obtained and facilities provided for writing the manuscript.
Sch. 7. Ways of producing phosphiniminium halide salts.

Halogenation Reactions of Cyclotrithiazenes
1331
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The authors thank IITM for the financial support and SAIF
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