Studies on iodination reactions of phosphiniminocyclotrithiazenes

Article abstract of DOI:10.1080/15533174.2012.759231

Iodination reactions of various phosphiniminocyclotrithiazenes [R ₃PNS₃N₃; R = (i) phenyl-, (ii) morpholino-, and (iii) p-chlorophenyl-] have been studied under different experimental conditions. Iodination of Ph₃PNS₃N₃ yields interesting products such as [(Ph₃PN)₂S₃N ₃]I₃, ((Ph₃PN)₃S)₂I ₄, (Ph₃PNH₂)I₃, ((Ph ₃PSI₂)₂I₂, and [(Ph ₃PN)₃SO]₂I₆ depending on the experimental conditions. All the products have been isolated and characterized by the spectroscopic and analytical techniques. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2012.759231

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Studies on Iodination Reactions of
Phosphiniminocyclotrithiazenes
Akella Sivaramakrishna a b , U. Swarnalatha a & M. N. Sudheendra Rao b
a Chemistry Division, School of Advanced Sciences , VIT University , Vellore , 632 014 , Tamil
Nadu , India
b Department of Chemistry, Indian Institute of Technology Madras , Chennai , 600 036 , Tamil
Nadu , India
Accepted author version posted online: 03 Jun 2013.
To cite this article: Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2013): Studies on
Iodination Reactions of Phosphiniminocyclotrithiazenes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, DOI: 10.1080/15533174.2012.759231
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Studies on Iodination Reactions of Phosphiniminocyclotrithiazenes
1,2Akella Sivaramakrishna*, 1U. Swarnalatha, 2M. N. Sudheendra Rao
1Chemistry Division, School of Advanced Sciences, VIT University, Vellore – 632 014,
Tamil Nadu, India
2Department of Chemistry, Indian Institute of Technology Madras, Chennai – 600 036,
Tamil Nadu, India
ABSTRACT
Iodination reactions of various phosphiniminocyclotrithiazenes [R3PNS3N3; R = (i)
phenyl-, (ii) morpholino-, (iii) p-chlorophenyl-] have been studied under different
experimental conditions. Iodination of Ph3PNS3N3 yields interesting products such as
[(Ph3PN)2S3N3]I3, ((Ph3PN)3S)2I4, (Ph3PNH2)I3, ((Ph3PSI2)2I2 and [(Ph3PN)3SO]2I6
depending on the experimental conditions. All the products have been isolated and
characterized by the spectroscopic and analytical techniques.
Keywords: phosphiminocyclotrithiazenes, iodination, oxidative addition; ring cleavage
* To whom correspondence should be addressed: [asrkrishna@rediffmail.com]
INTRODUCTION
Heterocyclic compounds are important and an active area of research in the contemporary
inorganic chemistry.[1–6] Sulfur and nitrogen containing cyclothiazenes (e.g. S4N4,
S3N3Cl3, S3N2Cl2, S4N3Cl, etc.) represent an interesting class among inorganic

heterocycles and the chemistry of these cyclothiazenes is well developed and literature
information available on them is fairly extensive.[7-13] Phosphiniminocyclotrithiazenes,
R3PN-S3N3 are a class of electron-rich inorganic heterocycles[10,11] having a
phosphinimino moiety as the exocyclic substituent and have served as stable derivatives
of monosubstituted cyclotrithiazenes (-S3N3).[14,15] In addition to the noticeable structural
differences16, some of the reactivity aspects of R3PN-S3N3 heterocycles have also been
demonstrated by redox reactions,[17] alkene adduct formation,[18] ring transformations,[19]
electrochemical and coordination behavior,[20] substituent-dependent pyrolytic
behavior,[21] and nucleophilic attack by ammonia, amines,[22] and phosphines.[23] These
results suggested the preferential cleavage of the trithiazene ring skeleton over that of
exocyclic substituent.
It has been believed that the presence of a relatively bulky phosphinimino moiety as an
exocyclic part of the S3N3 ring skeleton and the possibility of substituent variation on
phosphorus render them as interesting examples for further studies.[20] These observations
prompted us to undertake this study to probe the studies on iodination reactions of R3PN-
S3N3 ring systems for the first time. In this paper, we have reported the preparation of
R3PN-S3N3 heterocycles with symmetrically substituted phosphines (Eq.1) and studied
the tendency of these heterocyclic systems to oxidative cleavage by iodine.

S
R
P
N
N
S
S
S
R
P
R
P
N
R
2
N
N
R
S
+
R
N
S
N
+
N
R
R
R
S
S
1
2
3
R = Phenyl ( ); Morpholino ( ); p-Chlorophenyl ( )
Eq.1
EXPERIMENTAL
Apparatus
Infrared spectra (4000–400 cm-1) were recorded as Nujol mulls (NaCl windows) on a
Shimadzu 470 spectrophotometer. Electronic spectra were recorded by JASCO –V-550
1
13
31
UV/Vis Spectrophotometer. Spectra for H, C and P–NMR were collected on a Jeol
1
JNM GSX – 400 (160 MHz) NMR spectrometer and all H chemical shifts are reported
relative to residual proton resonance in the deuterated solvents. Heraeus CHNS rapid
micro analyzer was used for elemental analysis.
Materials
All the reactions and subsequent work up were done under an inert atmosphere of dry
nitrogen gas using Schlenk line technique. The organic solvents used were dried and
purified according to standard procedures and stored under nitrogen atmosphere. Melting
points were determined with an apparatus using samples in one side sealed capillaries.
Tetrasulfur tetranitride was synthesized by the reported procedure and crystallized from
toluene[24] (Caution: S4N4 may cause an explosion!!. Recommended safety procedures
should be strictly observed).[25] Morpholine (HNC4H8O), PPh3 and (p-chlorophenyl)3P

(Aldrich) were used as received. The tris(morpholino)phosphine was prepared by the
reaction of PCl3 with the respective amine.[26] Ph3PNS3N3 (1),[27] (OC4H8N)3PNS3N3 (2)
and (p-ClC6H4)3PNS3N3 (3)[20] were synthesized using reported procedures. Freshly
sublimed iodine sample was used.
Preparation of [(Ph3PN)2S3N3]I3 (5)
To a stirred solution of 1 (2.002 g, 4.836 mmol) in 30 mL of toluene at 0 °C, a solution of
iodine (0.365 g, 1.35 mmol) in 20 mL of toluene was added by a dropping funnel for 30
min. The reaction mixture was allowed to room temperature and after 5 h stirring, a dark
brown oily mass was formed. At the end of 6h, the oily mass was separated by
decantation from clear red brown filtrate. The oily mass was then stirred in 10 mL of
CH3CN for 2 h and filtered to obtain a red brown solid which was recrystallized from a
solvent mixture of CH3CN-CH2Cl2 (1:1, 20 mL) to afford 184 mg (8 %) of red brown
crystals of the title compound, [(Ph3PN)2S3N3))I3]; m.p. 174-176 ºC. Anal. Calcd for
C36H30I3N5P2S3: C, 40.35; H, 2.82; N, 6.54; S, 8.98. Found: C, 40.54: H, 3.15: N, 6.72:
1
S, 9.06. The H-NMR spectrum δTMS (CDCl3): 7.26-8.16 (m, 30H, Ph); 13C–NMR: δ
124.62, 125.43, 126.96, 128.46, 129.06, 129.75, 131.09, 134.64 and 31P–NMR: δ 15.6.
Preparation of ((Ph3PN)3S)2I4 (6)
A solution of iodine (0.651 g, 1.44 mmol) in 20 mL of toluene was added to a stirred
solution of 1 (1.06 g, 2.56 mmol) in 15 mL of toluene at room temperature. A similar
procedure was followed as given above to afford 122 mg of the dark red brown crystals
of the compound ((Ph3PN)3S)2I4) (6) (6 %); m.p 174-176 ºC. Anal. Calcd for calculated

for C108H90P6N6S2I4: C, 58.18: H, 4.07: N, 3.77: S, 2.88. Found: C, 58.26: H, 4.26: N,
13
3.84: S, 2.91.The NMR spectrum δTMS (CDCl3): δ 7.12-7.96 (m, 90H, Ph); C–NMR: δ
124.12, 125.36, 129.56, 130.78 and 134.18. 31P NMR: δ 15.6.
Preparation of (Ph3PNH2)I3 (7)
Iodine (0.338 g, 1.33 mmol) in toluene (20 mL) was added to a stirred solution of 1
(0.5028 g, 1.21 mmol) in toluene (10 mL) and 0.5 mL of water. After the addition was
complete (approx. 30min.), the reaction mixture was refluxed for 6h. The formed oily
mass was separated by decantation of the filtrate. The same procedure was adopted for
working up of the reaction mixture further as given in the above to afford 345 mg of
compound 7 (43 %); m.p. 98-100 ºC. Anal. Calcd for calculated for C18H17PNI3: C, 32.8:
H, 2.58: N, 2.12. Found: C, 32.5: H, 2.67: N, 2.18. The NMR spectrum δTMS (CDCl3): δ
7.16-7.88 (m, 15H, Ph), 1.54 (br, 2H, NH); 13C–NMR: δ 124.88, 125.32, 127.40, 128.22,
129.11, 129.84, 131.44, 134.24; 31P–NMR: δ 35.5.
Preparation of ((Ph3PSI2)2I2 (9)
A solution of iodine (1.229 g, 4.84 mmol) in toluene (25 mL) was added dropwise to a
solution of 1 (0.5012g, 1.21mmol) in toluene (10 mL). Then the reaction mixture was
subjected to refluxing for 18 h. The same procedure was adopted for working up of the
reaction mixture further as given in the above to afford 156 mg of compound 9 (19 %);
m.p. 142-144 ºC. Anal. Calcd for calculated for C36H30P2S2I6: C, 32.00: H, 2.24: S, 4.74.
Found: C, 32.02: H, 2.42: S, 4.78. The NMR spectrum δTMS (CDCl3) δ 7.20-8.14 (m,

30H, Ph); 13C–NMR: δ 124.53, 125.48, 128.30, 128.92, 131.64, 133.94; 31P–NMR: δ
42.2.
Preparation of ((Ph3PN)3SOI6 (10)
A solution of iodine (1.485 g, 5.85 mmol) in toluene (25 mL) was added dropwise to a
solution of 1 (0.606 g, 1.462 mmol) in toluene (10 mL) and 0.5 mL of water. Then the
reaction mixture was subjected to refluxing for 18 h. The same procedure was adopted
for working up of the reaction mixture further as given in the above to afford 118 mg of
compound 10 (15 %); m.p. 168-170 ºC. Anal. Calcd for calculated for C54H45P3N3SOI6:
C, 39.56: H, 2.77: N, 2.56: S, 1.95. Found: C, 39.52: H, 2.81: N, 2.56: S, 1.97. The NMR
spectrum δTMS (CDCl3): δ 7.02-7.68 (m, 45H, Ph); 13C–NMR: δ 125.17, 128.58, 129.96,
131.60, 134.23; 31P–NMR: δ 19.4. In all the above reactions, varying amounts of pale
yellow precipitate, removed by filtration, was identified as a known [Ph3PNH2]I (8).[28]
Preparation of ((p-ClC6H5)3PNH2)I3 (11b)
Iodine (0.245 g, 0.97 mmol) in toluene (20 mL) was added to a stirred solution of 3
(0.5024 g, 0.97 mmol) in toluene (15 mL) at room temperature. The reaction mixture was
refluxed for 8 h. The obtained red brown precipitate was filtered and washed with toluene
(2 x 5 mL) and recrystallized from a solvent mixture of CH2Cl2-CH3CN (1:1) to afford
392 mg of compound 11b (53 %); m.p. 102-104 ºC. Anal. Calcd for calculated for
C18H14PNCl3I3: C, 28.33: H, 1.85: N, 1.84. Found: C, 28.45: H, 1.90: N, 1.86. The NMR
spectrum δTMS (CDCl3): 7.14-7.78 (m, 12H, Ph), 1.56 (br, 2H, NH); 13C–NMR: δ 124.0,
125.0, 130.0 and 134.0; 31P–NMR: δ 35.5.

RESULTS AND DISCUSSION
Halogenation of sulfur-nitrogen compounds is important because of the influence of
halogens on the electronic properties of (SN)x species and their ability to cause
spontaneous polymerization of cyclothiazine compounds.[29-32] The preparation of various
[30]
halogenated cyclothiazenes of the type S3N2Cl2
and S4N3Cl[31] and their chemistry
have been reported.[32] It is also interesting to note that the halogenation does not proceed
analogously with all halogens.
In light of this, the present halogenation study has been prompted by the desire to verify
the possibility of oxidative addition reactions of Ph3PNS3N3 heterocycles (8π to 6π) with
iodine and isolation of hitherto unknown cyclotrithiazine systems with mixed substituents
on sulfur centers through the formation of 4 as shown in Eq.2. The reactions of halogens
like chlorine and bromine with Ph3PNS3N3 did not yield any new products because of
their strong oxidizing capacity to cleave the heterocycle ring. Hence we focused our
further studies with iodine, which is a mild oxidant.
X
N
N
S
S
R1
P
N
N
S
S
Oxidative
Addition
R1
P
R2
N
S
N
+
X2
R2
N
S
N
R3
R3
X
- system
8 
- system
4
6 
Eq.2: Expected halogenation reaction

When a dark solution of iodine (1:1) was treated with deep red toluene solution of
Ph3PNS3N3 (1) at 0 °C did not go to completion as the unreacted precursor 1 (δ 22.0) was
observed by 31P-NMR spectrum. However, the reaction mixture finally gave a dark red-
brown oily mass and a clear red-brown filtrate after 5 h stirring at room temperature. The
oily mass upon treatment with CH3CN gave a red brown powdery solid which can be
recrystallized from CH3CN-CH2Cl2 solvent mixture. The characterization data of this
solid revealed the formation of a novel iodinated cyclotrithiazene compound,
-
[(Ph3PN)2S3N3]+[I3]-, which has an I3 unit confirmed by elemental analysis. The IR
spectrum showed a strong band for S-N at 1325 cm-1 in nujol. It is found that the product
5 is sensitive to the air- and moisture. It was difficult to separate other products from the
reaction mixture. However, our efforts to identify 4 in the reaction mixtures were
completely failed.
On refluxing, an iodine solution (1:1) of 1 in toluene for 6 h yielded 6 by cleaving the –
S3N3 ring. The IR bands of 6 were almost identical to the known (R3PN)3S+ Cl- (R = Me,
Et, Ph) salts.[33] The same reaction in wet toluene either on refluxing or at room
temperature produced traces of 6 and the corresponding phosphonium salts, 7 and 8 in
high yields (Scheme 1).
Reaction of 4-fold excess of iodine with 1 gave 6 at room temperature. But the formation
of 9 was observed when the same reaction mixture is refluxed for 18 h. The product 9
[(Ph3PSI2)2I2] is an iodine adduct and has been the only isolated product from the crude
reaction mixture. The compound 9 consists of two Ph3PSI2 units interacting with an

iodine molecule. The characterization data of 9 is in good agreement with the earlier
work.[34] It appears that the oxidative degradation of cyclotrithiazene occurs by the initial
separation of phosphazene unit and then the formation of P-S-I species through the
cleavage of a P-N as well as S-N bonds in the presence of iodine.
A new cationic product, 10 was isolated from the iodination of 1 (iodine in 4-fold excess)
in a refluxing wet solution of toluene. The other products in this reaction were found to
be 7 and 8. 31P-NMR of 10 (δ 19.4) shows a sharp singlet which is slightly downfield
shift compared to 6 (δ 15.6).
The IR spectra of the crude reaction mixture show several S-N stretching bands
indicating a mixture of iodinated cyclothiazene species. The iodination of R3PNS3N3
+
always conceded a cyclothiazene cation (S4N3 ) in varying yields depending on the
+
experimental conditions as the evidence for the S4N3 cation is obtained by the IR data of
the crude reaction mixtures. A band at 1320 cm-1 appears in the region expected for S-N
+
stretching frequency due to the formation of S4N3 , which is consistent with the
literature.[35] This was further confirmed by mass spectral data. The ready formation and
stability of S-N cationic compounds is of particular significance in view of the apparent
-
role of I3 ions[36] in enhancing the conductivity and superconductivity of (SN)x.
It is surprising to observe that the iodination reactions of 2 and 3 always yielded 11 and
12 respectively (Eq.3). This could be due to the hydrolysis of products during the work-
up of the reaction mixtures.

I2
-
[(R3PNH2)]+I3
[(R3PNH2)]+I-
+
R3PN
S3N3
11a, R = morpholino;
12a, R = morpholino;
11b, R = p-chlorophenyl
12b, R = p-chlorophenyl
Eq.3
It is found that this iodination reaction is dependent on several factors such as
temperature, iodine concentration, mode of addition of iodine, nature of solvents, time of
reaction and remote substituent R (Scheme 1). Further, it is also possible that the
iodinated products formed during the reaction may undergo a side reaction to cleave –
S3N3 ring completely. Prolonged heating of the reaction mixture resulted by the cleavage
of P-N bond as well as formation of P-S bond.
In all the reactions, [R3PNH2]I is the most common product in varying amounts. All the
+ -
isolated products except the yellow crystalline R3PNH2 I salts are obtained as dark red
brown crystalline solids that are found to be soluble in both dichloromethane as well as
+ -
+
-
acetonitrile. Though R3PNH2 I and R3PNH2 I3 salts show similar spectral properties, the
compounds can be easily identified by their color and melting points from their
respective reaction mixtures. All the products show high melting point (> 170 ºC), which
was quite different to that of Ph3PNS3N3 (126 ºC) and the triiodide salts showed deposits
of iodine crystals in the sealed capillary used for melting point determination.

A sharp singlet with 31P chemical shift value revealed that all phosphorus atoms are in the
1
same environment. H-NMR spectra showed the presence of aromatic protons in the
region of δ 7.02-8.16 as well as 13C- NMR spectra also indicated the phenyl carbons at δ
124-134 as expected for compounds 5-7, 9 and 10. The other products, 11a and 12a
1
13
derived from 2 clearly show the H-NMR and C-NMR signals for morpholine ring in
the region of δ 3.15 (N-CH2), 3.78 (O-CH2) and δ 43.5 (N-CH2), 66.4 (O-CH2)
1
respectively[37]. The compounds 11b and 12b show the signals in H- and 13C-NMR
spectra for the aromatic phenyl rings.
-
The isolated iodinated products containing I3 anionic units display the UV-visible
absorption (λmax) at 478, 351, 288 nm (5), 364, 294 nm (6), 351, 288 nm (7) and 362, 294
(11) respectively, where the precursor 1 shows a characteristic λmax at around 480 and
330 nm (Fig.1).[38] Apart from the presence of cyclotrithiazene ring (λmax at 478 nm), an
-
additional absorption for 5 around 351, 288 nm indicates the presence of I3 . On
comparison of the electronic spectra of the compounds 5-7, 10 with Ph3PNS3N3 (1) and
-
products having I3 anion, it is clearly found that the compound has triiodide ion and thus
-
confirming the ionic nature of the compound (Fig.1). All the products with I3 anion
showed similar absorption patterns having λmax around 360 and 288 nm which are
completely different from the precursor.
The mass spectra of these crude reaction mixtures derived from different experiments
showed a mixture of products containing [(Ph3PN)2(S3N3)]+, [Ph3PNH2]+, [(Ph3PN)3S]+,
+
+
S4N3 , S3N3 and the less intense mass fragments of smaller S-N species i.e. S2N, S2N2,
S3N2, S3N3 and S8. The spectra clearly showed that there is no evidence of the higher

mass species in the mass spectra. The formation of elemental sulfur could be due to
thermal decomposition of S-N compounds. These results were consistent with the
literature reports.[33] It was interesting to note that the relative stability of iodinated
products strongly dependent on the nature of ligand: PPh3 > (p-chlorophenyl)3P >
-
(OC4H8N)3P. The presence of excess of iodine is believed to cause the formation of I3 . It
is observed that the wet solvents change the product distribution and yield several
+ -
+
-
-
additional products of the type PR3NH2 I , PR3NH2 I3 and [(PR3N)3SO]2+(I3 )2.
It has been reported that solid state pyrolysis of 1 leads to the formation of acyclic
derivatives (13) from a thermally prompted and controlled ring fragmentation reaction.[39]
But the same thermolysis of 1 in the presence of solid iodine at 175 °C yielded 9 in 20%
yield by the cleavage of S-N and P-N bonds (Eq.4).
Ph3P
N
S N
S
(a)
I2
Ph3P
S
I
I
.I2
1
2
+
9
Ph3P
N
S N
(b)
13
S
S
+ unknown products
Eq.4
Conclusions
The electron-rich symmetrical heterocycles, R3PNS3N3 undergo a smooth oxidative
addition with iodine to give various new and interesting halogenated products. The

products are characterized by various spectroscopic and analytical techniques. The
product distribution strongly depends on the experimental conditions. The iodination
reactions can be influenced by various factors such as iodine concentration, mode of
addition of iodine, temperature, nature of solvent, and nature of substituent R. Iodination
of Ph3PN-S3N3 serves as a source of the “S(NP)3” species while (OC4H8N)3PN-S3N3 and
(p-ClC6H4)3PN-S3N3 offer good yields of the corresponding phosphonium triiodide salts.
Acknowledgements. The authors (ASRK and MNSR) thank DST, India for the financial
assistance.
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Ph3PN
S
S
N
2+
+
[I3]
N
S
NPPh3
N
(I3 )2
7
8
+
+
Ph3PN
S
O
Ph3PN
NPPh3
5
10
(i)
+
NPPh3
S
(v)
(ii)
(I3) I
1
Ph3PN
NPPh3
(iv)
(iii)
2
6
[(Ph3PSI2)2I2]
9
[(Ph3PNH2)]+I-
-
[(Ph3PNH2)]+I3
+
7
8
Scheme-1: (i) RT, toluene, 5 h, iodine (1:1); (ii) Reflux, toluene, 6 h, iodine (1:1); (iii)
Reflux, toluene (wet), 6 h, iodine (1:1); (iv) Reflux, toluene, 18 h, iodine (1:4); (v) RT,
toluene (wet), 18 h, iodine (1:4)

4.5
4
3.5
3
10
7
2.5
2
1
5
1
11
6
1.5
1
5
7
6
0.5
0
260
310
360
410
460
510
560
Wavelength, nm
Fig.1: UV-Vis absorption spectra in CH2Cl2