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About a new class of inorganic polymers: the polythiocyanogens[Sz(CN)2]x
Franco Cataldo a,*, Yeghis Keheyan b
a Societa Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italyb Istituto di Metodologie Chimiche, C.N.R., Area della Ricerca di Roma, P.O. Box 10, Monterotondo Stazione, 00016 Rome, Italy
Received 12 February 2002; accepted 26 April 2002
Abstract
It is shown that all the pseudohalogens belonging to the series of sulfur dicyanide Sy (CN)2 or thiocyanogens polymerise very
easily in the dry state, under moderate heating to produce a series of brick-red or orange solids known as polythiocyanogens with the
general formula [Sy (CN)2]x with y an integer 1, 2, 3, 4. It is shown by FT-IR, electronic spectroscopy, and 13C NMR spectroscopy
that the structure of all the series is essentially the same. The use of model compounds with 1,3,5-triazine structure and of particular
reactions with Na2S and NaCN reveal that the [Sy (CN)2]x polymers have essentially a polyazomethine chain analogous to that of
polycyanogen or paracyanogen (CN)x but crosslinked with sulfur bridges of different length depending on the sulfur chain length in
the original monomer. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Sulfur dicyanide Sy (CN)2; Polythiocyanogens; 13C NMR spectroscopy; Paracyanogen (CN)x
1. Introduction
Polythiocyanogens are a class of inorganic polymeric
compounds derived from the polymerisation of thiocya-
nogens, a class of inorganic molecules belonging to the
pseudohalogens and described by the general formula
[1,2]:
Sy(CN)2 with y an integer number 1; 2; 3; 4; . . .
The thiocyanogens are also called sulfur cyanides.According to the number of sulfur atoms connecting the
two terminal nitrile groups we have respectively the
monosulfur, disulfur, trisulfur and tetrasulfur dicyanide.
The molecules belonging to the thiocyanogen series have
been prepared for a long time by the reaction of sulfur
chlorides with the appropriate inorganic cyanide or
thiocyanate salts; alternatively anodic oxidation of
thiocyanate salts has been used or even other reactions,e.g. [1,2]:
SCl2�2AgCN 0 S(CN)2�2AgCl
Br2�Pb(SCN)2 0 S2(CN)2�PbBr2
2SCN� 0 S2(CN)2�2e (anodic oxidation)
S2Cl2�2AgCN 0 S2(CN)2�2AgCl
SCl2�Pb(SCN)2 0 S3(CN)2�PbCl2
S2Cl2�Pb(SCN)2 0 S4(CN)2�PbCl2
Other alternative synthetic pathways have been de-
scribed in the literature and the superior homologues of
this series (with S�/4) have been reported in the
literature [1].All the series Sy (CN)2 polymerise more or less readily
at room temperature (r.t.) in solution and quickly in the
dry state giving orange or red products which usually
are described by the general formula [Sy(CN)2]x . The
polymerisation may be accelerated by the action of heat,
by refluxing these monomers in solvents like acetic acid.
In the present work we show that all these derivatives
have almost the same FT-IR spectrum and also we willpresent and discuss other characteristic properties and
reactions which will lead us to assign a chemical
structure to this class of compounds.
* Corresponding author. Tel.: �/39-06-205-5084; fax: �/39-06-205-
0800
E-mail address: cdcata@flashnet.it (F. Cataldo).
Polyhedron 21 (2002) 1825�/1835
www.elsevier.com/locate/poly
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 0 6 0 - 4
2. Experimental
All the equipment and procedures used have already
been described in the previous papers [3�/8]. In parti-
cular the synthesis of thiocyanogen and sulfur dicyanide
has already been detailed in Refs. [5,6]. The chemical
synthesis of polythiocyanogen [S2(CN)2]x has already
been reported in Refs. [3,5]. The electrochemical synth-
esis of polythiocyanogen [S2(CN)2]x has been described
both in Refs. [4,7]. The synthesis of all the series of
polythiocyanogens [Sy (CN)2]x with y�/1, 2, 3 and 4 is
detailed in the patent of Ref. [8] and/or will be reported
in another future work. In the reaction of polythiocya-
nogens with NaCN or Na2S we have followed the
procedure already described in Ref. [5]. The triazine-
based monomeric model compounds trithiocyanuric
acid sodium salt has been prepared by reacting trichlor-
otriazine with sodium sulfide (see Refs. [3�/5]). The
polymerisation of trithiocyanuric acid with iodine to
produce compound 1 of Scheme 1 has been described in
Ref. [5]. The procedure to follow the polymerisation of
thiocyanogen on a KBr plate described in Ref. [5] has
been applied in the present work to follow also the
polymerisation of tetrasulfur dicyanide S4(CN)2.
2.1. Preparation of S4(CN)2 and study of its
polymerisation over a KBr plate
In a two necked round bottomed flask with condenserand magnetic stirrer, S2Cl2 (2.0 g) was added dropwise
to a stirred suspension of Pb(SCN)2 (5.0 g) in CH2Cl2(80 ml) at r.t. For the addition of S2Cl2 use was made of
a dropping funnel. The reaction is slightly exothermic
since CH2Cl2 is refluxing at the end of the addition.
After having stirred for 1.5 h at r.t. the resulting PbCl2formed was decanted from the solution. The S4(CN)2
solution in CH2Cl2 shows a maximum at 253 nmfollowed by a shoulder at about 300 nm. S2(CN)2 in
the same conditions show a maximum at about 250 nm
followed by another maximum at 294 nm.
The solution of S4(CN)2 is reasonably stable for about
1 week if kept in a cool place in a tightly closed flask in
the dark. However, if a few drops of the solution are put
over a substrate, as soon as almost all the CH2Cl2evaporates a quick polymerisation reaction takes placewith the formation of a brick-red�/orange solid. Thus a
few drops of the solution were placed on a KBr plate
and the polymerisation was followed immediately after
the almost complete evaporation of CH2Cl2. The IR
spectral changes are reported in Figs. 1 and 3. A similar
procedure has been adopted to follow the polymerisa-
tion of S2(CN)2 after its preparation according to the
method reported in Ref. [5]. The spectra of polymerisingS2(CN)2 are reported in Fig. 2.
2.2. Preparation of S(CN)2 and its thermal
polymerisation in refluxing acetic acid
AgCN (9.1 g) was suspended in 70 ml of CH2Cl2 in
the same apparatus described above. Sulfur dichloride
SCl2 (3.3 g) was added dropwise and then the mixture
was stirred at r.t. for 1.5 h before the decantation of theAgCl formed. The evaporation of all the solvent at
reduced pressure in a water bath leaves a residue of
white crystals mixed with orange�/red polythiocyano-
gen. Sublimation of this crude residue gives white
tabular crystals on the top of the flask. The FT-IR
spectrum of these crystals is perfectly coincident with
that reported in Ref. [5] for S(CN)2. The solution of
these crystals in acetonitrile shows a maximum at 224nm and another maximum at 235 nm. A shoulder
appears at 324 nm. The residual matter remained in
the bottom of the flask after the sublimation appears as
a rusty-brown solid. The yield was 1.34 g. The spectrum
of this polymeric mass is shown in Fig. 4(D) and it
corresponds to that of a polythocyanogen.
The white sublimed crystals (abut 500 mg) were
dissolved in 5 ml of acetic acid and refluxed for 15 h.At the end of the thermal treatment the mixture
appeared as orange�/red. The polymer was insoluble in
acetic acid and was recovered by filtration, washed withScheme 1.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/18351826
abundant water and acetone and left to dry. The yield
was almost quantitative. The spectrum of this [S(CN)2]xis shown in Fig. 4(C) and corresponds to that of a
polythiocyanogen.
2.3. Preparation of the triazine-based model compound 1(Scheme 1) by the action of S2Cl2 over trithiocyanuric
acid sodium salt
Trithiocyanuric acid (1.0 g) is dissolved in 20 g of
dimethyl sulfoxide (DMSO) and 0.68 g of NaOH are
added. The mixture is heated and stirred and becomesbluish-green. On cooling the mass solidifies and a small
volume of water (5 ml) was added to re-dissolve the
mass. A rapid precipitation of a yellow mass occurred
on dropwise addition of S2Cl2 (4.0 g) under stirring. The
mixture was left overnight and the following day was
diluted with acetone and filtered under reduced pres-
sure. The filtrate was washed repeatedly with acetone
and dried in air. The yield of recovered bright yellowmaterial was 0.92 g. The FT-IR spectrum of this
material is reported in Fig. 6(C) and agrees reasonably
with the spectrum of polymeric trithiocyanuric acid
prepared by the action of I2 over (SH)3(CN)3 Fig. 6(B),
Ref. [5].
2.4. Thermal treatments of polythiocyanogens
[S2(CN)2]x and [S4(CN)2]x as well as model compound
1 of Scheme 1
The thermal treatment involved the use of 200 mg ofeach compound, respectively [S2(CN)2]x and [S4(CN)2]xas well as model compound 1 of Scheme 1. Each
compound was put into a porcelain crucible with a
cap and heated in a muffle at 300 8C for 1 h. After
cooling the FT-IR spectra shown in Figs. 6(E) and 7(E
and F) have been recorded.
3. Results and discussion
In the previous works [3�/8] which were limited only
to the study of the structure of parathiocyanogen the
polythiocyanogen with y�/2 derived from the polymer-
isation of S2(CN)2, we have shown that parathiocyano-
gen does not possess a 1,3,5-triazine-based structure 1
(Scheme 1) as reasonably proposed by other researchersfor a number of experimental facts.
i) The FT-IR spectrum of [S2(CN)2]x does not match
the spectrum of a 1,3,5-triazine-based polymer
prepared through a sure route which has the
chemical structure 1 (see Scheme 1) originally
expected for parathiocyanogen [3�/5].
ii) The 13C NMR spectrum of parathiocyanogen has a
relatively broad signal at 186 ppm while trithiocya-
nuric acid (SH)3(CN)3 shows a narrow signal atabout 145 ppm [4,5].
iii) The electronic spectrum of parathiocyanogen shows
a maximum at 267 nm followed by another max-
imum at 440 nm while the 1,3,5-triazine-based
model compound shows a maximum at 275 nm
and another peak at about 350 nm [4,5].
iv) The X-ray powder diffraction pattern of [S2(CN)2]xis very peculiar showing only a relatively broad peakat 2u�/26 (Cu Ka) while the triazine-based model
compound is dominated by a series of reflections
which are similar to those of cyclooctasulfur [5].
Fig. 1. Kinetics of S2(CN)2 and S4(CN)2 polymerisation followed by FT-IR spectroscopy.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/1835 1827
3.1. Aspects of the polymerisation of sulfur dicyanides
(thiocyanogens)
In the present work, we have followed the S2(CN)2
and the S4(CN)2 polymerisation by leaving few drops of
a fresh solution in CH2Cl2 of each pseudohalogen to
evaporate spontaneously on a KBr plate and recording
by FT-IR the spectral changes as function of time.
When almost all the CH2Cl2 has been evaporated the
polymerisation of the non-volatile pseudohalogen
started (t�/0) immediately. The spectral changes are
summarised in Fig. 1 and show two main trends: the
decay in the optical density of the bands associated to
the �/CN stretching at 2162 and 2073 cm�1, and the
concomitant increase of a new main band at about 1220
cm�1. Almost the same trend has been observed for
both S2(CN)2 and S4(CN)2 as shown in Fig. 1 suggesting
that in both cases the �/CN groups were the reaction
sites and the polymer is deriving just from the reaction
of these groups. Visually, the polymerisation can be
observed from the fact that the pseudohalogen thin film
on the KBr plate becomes yellow, then yellow�/orange,
orange and red at the end of the reaction. At this finalstage the film is hard and brittle.
The details of the spectral evolution during the
polymerisation of S2(CN)2 are illustrated in Fig. 2(A
and B). Fig. 2(A) refers to the early polymerisation
stages and is dominated by the -CN stretching bands at
2162 and 2073 cm�1 as well as the C�/S stretching at 670
cm�1 [6]. The incipient polymerisation is suggested in
Fig. 2(A) by the weak bands at 1405, 1220 and 1051cm�1. The mentioned bands develop greatly their
intensity (see Fig. 2(B)) as the polymerisation goes to
completion with the simultaneous decay in intensity of
the �/CN stretching bands. These new bands are not
associated to the 1,3,5-triazine ring formation as will be
discussed in the next section.
Of extreme interest are the spectra shown in Fig. 3(A�/
D) and recorded on S4(CN)2 at increasing degrees ofpolymerisation. In fact these spectra show for the first
time that also the superior homologue of thiocyanogen
S2(CN)2, namely tetrasulfur dicyanide S4(CN)2, poly-
merises exactly as its inferior homologue, suggesting
that all the family of the thiocyanogen or sulfur cyanides
polymerise in the same fashion. In detail, Fig. 3(A)
shows the spectrum of S4(CN)2 at the beginning of the
polymerisation. Again the spectrum is dominated by theasymmetric and symmetric stretching of the �/CN group,
respectively, at 2155 and 2068 cm�1 and by the C�/S
stretching band at 668 cm�1. The polymerisation
involves the gradual development of a band at 1220
cm�1 (see Fig. 3(B�/D)) which is accompanied by weak
bands at 1405 and 1060 cm�1. Simultaneously a decay
on the �/CN stretching band is observed. The new bands
of the polymeric [S4(CN)2]x are located at the samewavenumbers as polymeric [S2(CN)2]x previously dis-
cussed. A comparison between the polymerisation
kinetics of S4(CN)2 in comparison to S2(CN)2 is
illustrated in Fig. 1.
Sulfur dicyanide is the most stable molecule of the
series of thiocyanogens: it can be isolated as stable
crystals. Elsewhere [6] we have discussed the reasons of
this unusual stability. However, S(CN)2 is not stableindefinitely. Even at r.t., under the action of moisture,
light and heat it is converted into a brick-red polymeric
material [S(CN)2]x . Fig. 4(A) shows the FT-IR spectrum
of aged crystals of S(CN)2: they are mixed with the
polymeric derivative as suggested by the presence of the
broad band at 1242 cm�1 together with the �/CN band
at 2186 cm�1 and the C�/S band at 678 cm�1 due to the
monomer. The progression of the polymerisation at r.t.is shown in Fig. 4(B) where, as usual, a reduction of the
intensity of the band at 2186 cm�1 (�/CN stretching)
corresponds to the development of the broad band at
1240�/1220 cm�1. S(CN)2 can be completely poly-
Fig. 2. (A) FT-IR of S2(CN)2 polymerisation: spectrum taken 7 min
after the evaporation of the CH2Cl2 solvent from a KBr plate. This is
the spectrum of the monomeric thiocyanogen and the incipient
polymerisation product (band at 1220 cm�1). (B) FT-IR of S2(CN)2
polymerisation product 21 min after solvent evaporation.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/18351828
merised when heated in refluxing acetic acid; the
spectrum of the corresponding [S(CN)2]x is shown in
Fig. 4(C).
The experimental evidences are now sufficient to
affirm that all the sulfur cyanide family Sy (CN)2
polymerise in the same way yielding polymers whose
general structure may be described by the general
formula:
[Sy(CN)2]x with y and integer 1; 2; 3; 4; . . .
and x an high number but unknown
All the polythiocyanogens show the same spectrum
essentially characterised by a very strong and broad
band centred at about 1220 cm�1 and by several other
minor bands. Fig. 5 illustrates the concept by comparingthe spectra of [S2(CN)2]x , [S3(CN)2]x and [S4(CN)2]x : it
is clear the spectral identity and thus the structural
identity of these polymers.
Fig. 3. (A) FT-IR of S4(CN)2 polymerisation: spectrum taken 5 min
after the evaporation of the CH2Cl2 solvent from a KBr plate. This is
the spectrum of the monomeric tetrasulfur dicyanide and the incipient
polymerisation product (band at 1220 cm�1). (B) FT-IR of S4(CN)2
polymerisation product 10 min after solvent evaporation. (C) FT-IR of
S4(CN)2 polymerisation product 15 min after solvent evaporation. (D)
FT-IR of S4(CN)2 polymerisation product 45 min after solvent
evaporation.
Fig. 4. (A) FT-IR of partially polymerised S(CN)2. (B) FT-IR of
polymerised S(CN)2 at r.t. after several weeks. (C) FT-IR of
polymerised S(CN)2 after refluxing in acetic acid for 15 h. (D) FT-
IR of polymerised S(CN)2 formed in the bottom of the flask after the
thermal treatment for the sublimation recovery of pure S(CN)2.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/1835 1829
Thus the provisory conclusions we can draw at this
stage is that despite the different length of the sulfurbridges connecting the �/CN end groups, the latter
polymerise in the same way. Therefore, it appears that
the �/CN groups are transformed into derivatives having
the same chemical structure. Since nitrile groups are
prone to trimerise into 1,3,5-triazine rings, the simplest
explanation for the polymerisation of all sulfur dicya-
nides is the formation of 1,3,5-triazine rings connected
together by sulfur bridges. The length of the sulfurbridges depends on the original chain length of the
starting monomeric sulfur dicyanide. This interpretation
has been proposed by Feher et al. in 1956 [9].
3.2. The FT-IR spectra of the 1,3,5-triazine-based
polymers with sulfur bridges do not match the spectra of
polythiocyanogens
As illustrated in Scheme 1 we have prepared trithio-
cyanuric acid by reacting trichlorotriazine with diso-
dium sulfide and we have polymerised the
trithiocyanuric acid by the action of iodine or by
reacting the sodium salt of trithiocyanuric acid with
sulfur monochloride (Scheme 1, structure 1). In the lasttwo cases we have obtained through two different sure
routes the chemical structures expected from the poly-
merisation of sulfur cyanides or thiocyanogens, suppos-
ing that they trimerise into triazine rings. Fig. 6(A�/C)
illustrate the FT-IR spectra of both trithiocyanuric acid
(Fig. 6(A)) and its polymers with S2 (Fig. 6(B)) and
polysulfide bridges (Fig. 6(C)): if we compare these
spectra with those of [Sy (CN)2]x for instance shown inFig. 5, it appears immediately evident that the poly-
thiocyanogens cannot have the triazine-based structure.
First of all the triazine-based structure is characterised
by the out-of plane bending of the triazine ring which is
located at 783 and 751 cm�1 in the monomeric
trithiocyanuric acid (Fig. 6(A)) and is shifted to higher
wavenumbers after the polymerisation. In fact the
triazine ring bending appears at about 823 cm�1 inFig. 6(B and C) both referring to the polymeric triazine
structure 1 (Scheme 1). This characteristic band is weak
or completely absent in the case of the [Sy(CN)2]x (see
Fig. 5). Other characteristic triazine bands present in the
spectrum of Fig. 6(A) are the very strong bands at 1540
and 1362 cm�1 respectively due to quadrant and
semicircle triazine ring stretch [10]. After polymerisation
these two bands are still detectable at 1480 and 1242cm�1, shifted to shorter wavenumbers (see Fig. 6(B and
C)). In any case these spectra cannot be compared with
those of polythiocyanogens of Fig. 5. Thus, once again
structure 1 is not the structure of polythiocyanogen.
3.3. Some chemical aspects of polythiocyanogens: their
reaction with sodium sulphide and sodium cyanide
Although polythiocyanogens do not display the sameFT-IR spectra of the triazine-based polymer, they are
reduced to polymeric trithiocyanuric acid (Structure 1,
Scheme 1) when they are treated with sodium sulfide in
water solution (see Ref. [5]). This is evident by compar-
ing Fig. 7(A) and especially Fig. 7(B) with the spectra of
Fig. 6(C and B). This is the first indication that
polythiocyanogens undergo a structural isomerisation
to a triazine-based polymer. Thus the Na2S treatmentcauses the passage from structure 4 to 1.
Another interesting reaction, which can be observed
on the polythiocyanogens, involves their treatment with
NaCN. Sodium cyanide is able to abstract sulfur from
the polysulfide bridges of structure 4, with the conse-
quent formation of NaSCN, causing the passage from
structure 4 to 2 of paracyanogen (see Scheme 1). This is
documented by the spectra of Fig. 7(D and E) whichshould be compared with the IR spectra of the pristine
samples shown in Fig. 5. The formation of paracyano-
gen (CN)x by sulfur abstraction from polythiocyano-
Fig. 5. (A) FT-IR of polymerised S2(CN)2: polythiocyanogen
[S2(CN)2]x . (B) FT-IR of polymerised S3(CN)2: polythiocyanogen
[S3(CN)2]x . (C) FT-IR of polymerised S4(CN)2: polythiocyanogen
[S4(CN)2]x .
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/18351830
gens [Sy(CN)2]x is suggested by the development of new
bands at 1630�/1640 and at about 1500 cm�1 which
Fig. 6. (A) FT-IR of trithiocyanuric acid sodium salt (1,3,5-triazine-
based model compound). (B) FT-IR of polymerised trithiocyanuric
acid with iodine catalyst according to Ref. [5] (1,3,5-triazine-based
model compound 1 of Scheme 1). (C) FT-IR of polymerised
trithiocyanuric acid with S2Cl2 according to Ref. [5] (1,3,5-triazine-
based model compound 1 of Scheme 1). (D) FT-IR of 1,3,5-triazine-
based model compound 1 of Scheme 1 after treatment with NaCN.
The spectrum is now that of paracyanogen (structure 3, Scheme 1) see
Refs. [11,13,17]. (E) FT-IR of 1,3,5-triazine-based model compound 1
of Scheme 1 after a thermal treatment at 300 8C. The spectrum is now
that of paracyanogen (structure 3, Scheme 1).
Fig. 7. (A) FT-IR of polythiocyanogen [S2(CN)2]x after the treatmentwith Na2S according to Ref. [5]. The spectrum is now that of 1,3,5-triazine-based model compound 1 of Scheme 1; compare with Fig. 6(Band C). (B) FT-IR of polythiocyanogen [S4(CN)2]x after the treatmentwith Na2S according to Ref. [5]. The spectrum is now that of 1,3,5-triazine-based model compound 1 of Scheme 1; compare with Fig. 6(Band C). (C) FT-IR of polythiocyanogen [S2(CN)2]x after the treatmentwith NaCN according to Ref. [5]. The spectrum is now that ofparacyanogen structure 2, Scheme 1. (D) FT-IR of polythiocyanogen[S4(CN)2]x after the treatment with NaCN according to Ref. [5]. Thespectrum is now that of paracyanogen structure 2, Scheme 1. (E) FT-IR of polythiocyanogen [S2(CN)2]x after thermal treatment at 300 8C.The spectrum is now that of paracyanogen structure 3, Scheme 1. (F)FT-IR of polythiocyanogen [S4(CN)2]x after thermal treatment at300 8C. The spectrum is now that of paracyanogen structure 3,Scheme 1.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/1835 1831
reach the same intensity as the band at 1220 cm�1 as
shown in the spectra of Fig. 7(C and D).
The other aspect, which links again (by chemical
behaviour) the triazine, based polymer and the poly-thiocyanogens is the treatment with sodium cyanide of
the former (according to Ref. [5]). In fact when the
polymeric triazine compound (structure 1, Scheme 1) is
treated with NaCN, sulfur abstraction occurs as well
because of the formation of NaSCN and the IR
spectrum changes into that of paracyanogen (CN)x as
illustrated in Fig. 6(D). This spectrum is characterised
by the intense and broad band at 1640�/1450 cm�1 andby the triazine ring band at 878 cm�1 but by the absence
of the intense band at 1230�/1260 cm�1 observed in the
paracyanogen obtained from polythiocyanogens. There-
fore, the triazine-based polymer of structure 1 is
transformed into the triazine isomer of paracyanogen
(structure 3 of Scheme 1) when treated with NaCN.
Conversely, the polythiocyanogens with structure 4 are
converted into ladder paracyanogen of structure 2 uponNaCN treatment.
Elsewhere [11�/13] we have already discussed about
paracyanogen (CN)x and about its formation from the
photopolymerisation of dicyanogen (CN)2 and about
the synthesis and the existence of a triazine-based isomer
of paracyanogen having structure 3, in addition to the
well known ladder paracyanogen of structure 2 (Scheme
1). The spectrum of Fig. 6(D) is analogous to the IRspectrum of the triazine-based isomer of paracyanogen
(CN)x with structure 3 of Scheme 1 which we have
prepared through a sure route [11�/13].
Furthermore, the thermal treatment at 300 8C of
both the polythiocyanogens and the triazine-based
polymer transform both these macromolecules into
paracyanogen as clearly suggested by the spectra of
Figs. 6(E) and 7(E and F), with release of detectable freesulfur vapours. After this thermal treatment there are no
doubts about the fact that both the polythiocyanogens
having structure 4 and the triazine polymer of structure
1 have been transformed into paracyanogen. In fact the
spectra of Figs. 6(E) and 7(E and F) are completely
comparable with those of the paracyanogen prepared by
Bircumshaw et al. [14]. It is of particular interest that all
the spectra just mentioned show an intense band at 807cm�1 which could be assigned also to a triazine ring and
hence the paracyanogen formed from both the model
compound and the polythiocyanogens may be the
triazine isomer of Scheme 1 structure 3.
3.4. Further reflections and discussions about the
structure of polythiocyanogens
Among the data discussed in the preceding para-graph, these are still some puzzling result concerning the
structure assignment of the polythiocyanogens. In fact
the treatment with sodium sulfide of [Sy(CN)2]x favours
the idea that polythiocyanogens may have structure 5
instead of structure 4 (Scheme 1) where we have
represented a polymer with triazine rings having both
possible tautomers (thioketo and sulfidic form). In thiscontext it is useful to recall that thiocyanic acid is an
unstable molecule and trimerises to trithiocyanuric acid
which is in tautomeric equilibrium with its trithioketo
form (isothiocyanuric acid trimer) as illustrated in
Scheme 1 structure 6.
Structure 5 reported in Scheme 1 is in fact able to
justify why the polythiocyanogens show an FT-IR
spectrum different from that of triazine polymers withstructure 1. The strong band at 1220 cm�1 in fact could
reasonably be assigned to the thioketo group C�/S which
absorb just in this spectral region [15]; additionally the
spectrum of 1,3,5-triazine-2,4,6-trithione shows a strong
band at 1140 cm�1 which is shifted to 1210 cm�1 in the
parent compound 1,3,5-triazine-2,4-amino-6-thione [15].
Moreover in these compounds the triazine ring bending
is weak or absent, as in the case of polythiocyanogensspectra of Fig. 5. Structure 5 for polythiocyanogens may
also be able to explain their orange and red colour since
the chromophore will be just the C�/S group and the
quinoid structures associated (see structure 5). Of
course, with structure 1 it is almost impossible to explain
the orange�/red colour of polythiocyanogens.
Thus it appears reasonable to think that when the
sulfur dicyanides polymerise, they form triazine ringscontaining also thioketonic groups as shown in Scheme
1 structure 5. The treatment of these polymers with
sodium sulfide causes the rearrangement of the structure
5 to the structure 1 (Scheme 1). The treatment with
NaCN, being an alkaline agent probably causes the
passage from structure 5 to 1 and simultaneously causes
the partial abstraction of sulfur from the polysulphidic
bridges connecting the triazine rings leading to bothstructure 1 with y�/1 or directly to structure 3 (see
Scheme 1) which is the structure of the triazine-isomer
paracyanogen. However, in the previous paragraph we
have already shown that the treatment of polytiocyano-
gens with NaCN is leading to paracyanogen with the
ladder structure 2 and not with the triazine isomer with
structure 3. Thus this fact strongly supports structure 4
for polythiocyanogens rather than structure 5.Furthermore, the strong heating of both polythiocya-
nogens and also the triazine-based model polymer
(structure 1) causes the extrusion of sulfur and the
passage to structure 3 in both cases. This fact does not
necessarily support structure 5 for polythiocyanogens
since one can expect a rather easy decomposition and
thermal depolymerisation from structure 5 rather than
its rearrangement to structure 3.Additional data, which are against structure 5 for
polythiocyanogens, are provided by 13C NMR spectro-
scopy. We have already excluded that the polythiocya-
nogens have a triazine-based structure 1 because the
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/18351832
trithiocyanuric acid shows a peak at 145 ppm while
[S2(CN)2]x shows a broad peak at 186 ppm [4,5]. Thus
the newly proposed structure 5 is not correct because it
requires at least two 13C NMR signals: one for the C�/Sgroup which is expected at about 200 ppm or at lower
field and the triazine signal at 145 ppm. Since we have
recorded only a unique peak at 186 ppm [5], it is now
allowed to drop the structure 5 as the corrected structure
for polythiocyanogens.
In our previous works we have proposed a linear
structure for polythiocyanogen [S2(CN)2]x . The possible
linear structures are 7 and 8 (see Scheme 1) and havealso been proposed by Skorobogatova et al. [16].
However structure 8 can be phased out because the13C NMR of [S2(CN)2]x shows only one peak at 186
ppm while from structure 8 two different 13C signals are
expected but have not been observed [4,5]. Furthermore,
the �/SCN stretching band is not always present in the
IR spectra of polythiocyanogens and if present is always
weak while for structure 8 it is expected to be strong.Structure 7 is coherent with the 13C NMR spectrum of
[S2(CN)2]x but it appears to be not extremely stable. The
criticism against this structure is that it could easily
undergo a crosslinking reaction with adjacent chains so
that it cannot survive for long time under the form of 7
but other more complex networked structures should be
derived easily from 7.
The discovery that all polythiocyanogens [Sy (CN)2]xhave essentially the same IR spectrum and hence the
same chemical structure also creates problems with
structure 7 since it becomes difficult to explain why
the introduction of a different number of sulfur atoms in
the polymer backbone does not alter at all either the FT-
IR spectrum or the UV spectra of the series.
In fact, the introduction of more than one sulfur atom
in the chain structure 7 must necessarily reduce theconjugation along the chain and the electronic spectra
should change abruptly by passing from y�/1 to 2 and
then to y �/2 but this is not observed experimentally for
all the polythiocyanogen series as outlined in Fig. 8,
where the electronic spectra in DMSO are very similar
to each other from y�/1 to 4 showing essentially two
absorption bands in all series. One of the absorption
bands is located at about 270 nm and other one in therange of 375�/410 nm. Moreover the colour of all the
polythiocyanogen series is invariably orange to brick-
red but if structure 7 is correct we should expect a
gradual shift from red to orange, to yellow, to pale
yellow as the length of the sulfur bridge increases for
y 0/�.
3.5. Attempt to interpret the spectral data and the
chemical data in a unique coherent model
All the discussion in the previous paragraphs has
shown that structure 5 is not satisfactorily describing the
structure of polythiocyanogens, although it has some
merit. Conversely, structure 8 and especially structure 7
which were originally proposed by us [3�/5] and by
Fig. 8. (A) Electronic spectrum of [S(CN)2]x in DMSO solvent. (B)
Electronic spectrum of [S2(CN)2]x in DMSO solvent. (C) Electronic
spectrum of [S3(CN)2]x in DMSO solvent. (D) Electronic spectrum of
[S4(CN)2]x in DMSO solvent. (E) Electronic spectrum of trithiocya-
nuric acid in DMF solvent. (F) Electronic spectrum of compound 1 of
Scheme 1 in DMF solvent.
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/1835 1833
others [16] present too many flaws in front of the
experimental data available. Thus the structure of
polythiocyanogens can be represented as structure 4 of
Scheme 1. Let us summarise the reason for this choice
and proposal:
i) dicyanogen NC�/CN can be considered the most
inferior homologue of the polythiocyanogen series
of [Sy(CN)2]x ; in fact when y�/0 we get [(CN)2]x . It
polymerise thermally [14] and under the action of
the UV light [11�/13] into a polymer having
essentially a polypyrazino�/pyrazine ladder struc-
ture 2 (see Scheme 1). The ladder structure 2 is a
little bit of an idealised structure because recently
ourselves [11,13] and other authors [17,18] have
proposed a more disordered structure derived from
structure 2. However, in the frame of our discussion
here we can utilise the structure 2. In normal
conditions dicyanogen does not polymerise into
the polytriazine structure 3 [11,13,17,18].
ii) Therefore, it is reasonable to think that also the
superior homologues of dicyanogen, those contain-
ing S (so that when y �/0 the general formula is
[Sy (CN)2]x ) should polymerise in the same fashion
of dicyanogen giving polyazomethine chains con-
nected together by sulfur bridges (structure 4,
Scheme 1) rather than the triazine rings of structure
1. Many experimental facts can be explained with
this model.
iii) First of all, the structure 4 proposed for polythio-
cyanogens explains well both the FT-IR spectra and
the electronic spectra of the series and the fact that
these spectra appear exactly the same for all the
polythiocyanogen series. Concerning the strong
infrared band at about 1220 cm�1 we should not
anymore advocate the thioketonic C�/S stretching
band which has been postulated in the case of
structure 5. Instead the band at 1220 is due to the
polyazomethine chain which, due to the tight sulfur
crosslink of structure 4, causes the shift of this band
from 1600 to 1450 cm�1 (typical paracyanogen
band, see Refs. [11�/14]) to about 1250�/1220 cm�1.
Furthermore, structure 4 explains well the electronic
spectra of polythiocyanogens of Fig. 8 and the
colour of these polymeric compounds which does
not change as function of the sulfur chain length.
The polyazomethyne chain of structure 4 is a
sequence of single and double bonds and this
explains well the long wavelength transition at
380�/410 nm observed in the electronic spectra of
Fig. 8(A�/D) for all the polythiocyanogen series.
Thus, the polyazomethyne chain is the chromo-
phore for all the polythiocyanogen series. As
expected, the triazine-based structure 1 cannot dis-
play any transition at so long wavelengths due to
the lack of conjugated chains. In fact, both the
compound having structure 1 and the parent
compound trithiocyanuric acid (structure 6) show
the longest transition at only 344 nm (see Fig. 8(E
and F)). In the case of the alternative structure 5for polythiocyanogen series we were forced to
introduce the ad hoc hypothesis of the presence
of the thioketo (C�/S) groups which should cause
the transition at 400 nm or even at longer wave-
length.
iv) The 13C NMR spectrum of polythiocyanogen
[S2(CN)2]x shows a unique broad band at 186
ppm [5]. The paracyanogen 13C MAS NMR spec-trum appears at about 158�/150 ppm [17] while the
spectrum of triazine ring shows just one unique
sharp carbon signal at 145 ppm [4]. Initially we have
thought that the signal at 186 ppm was the proof for
the linearity of [S2(CN)2]x but in the preceding
section we have explained the reasons which have
led us to abandon the linear structure 7: the intrinsic
instability of that structure together with the factthat it cannot explain many properties of all
[Sy(CN)2]x . At this point, the peculiar peak at 186
ppm is assigned to the carbon atoms present in the
azomethine chains connected with sulfur bridges of
structure 4 (Scheme 1). The connection of the
azomethine chains with sulfur bridges causes a de-
shielding of the carbon atoms which appear at 20
ppm downfield in comparison to the usual chemicalshift of an azomethine chain not connected with
sulfur. In fact the azomethine chain is expected to
give a 13C NMR signal at about 167 ppm in imines,
oximes and hydrazones [19] and it is reported to
appear at 158�/150 ppm in the solid state spectra of
paracyanogen. Thus a shift of 20 ppm downfield is
expected for sulfur substitution [19] and leads to a
value just at about 186 ppm as experimentallyfound.
v) The treatment of the polythiocyanogen series with
NaCN causes the formation of ladder parathiocya-
nogen as suggested by the FT-IR spectra of Fig. 7(C
and D) confirming that the correct structure of
polythiocyanogen is 4 of Scheme 1. Conversely, the
triazine-based polymer with structure 1 is trans-
formed into the triazine-isomer of paracyanogenwith structure 3 when treated with NaCN. The
thermal treatment of both polythiocyanogens with
structure 4 and the triazine polymer with structure 1
involves the elimination of sulfur and causes their
transformation into paracyanogen with presumable
structure 3 as documented by FT-IR spectroscopy
(Figs. 6(E) and 7(E�/F)). There is no clear mechan-
ism for the passage from structure 2 to 3, but theFT-IR of the thermally processed polythiocyano-
gens suggest that the final structure is structure 3,
the same obtained by thermal processing of the
triazine-based structure 1 (see discussion in the
F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/18351834
preceding sections). The sodium sulfide treatment of
the polythiocyanogen series converts structure 4
into 1. This is again the passage from a ladder
structure to a triazine structure and is documentedby the FT-IR spectra of Fig. 7(A and B). Sulfur
abstraction may cause the direct isomerisation and
cyclisation from structure 4 to 3 although the
mechanism is not yet clear.
4. Conclusions
On the basis of a considerable body of experimentalfacts which have been reviewed step-by-step the conclu-
sion has finally been reached that polythiocyanogens of
[Sy(CN)2]x , the red�/orange polymeric materials derived
from the polymerisation of the thiocyanogens or sulfur
cyanides of Sy(CN)2 (with y an integer 1, 2, 3, 4. . .) have
the structure 4 of Scheme 1. This structure is charac-
terised by two long polyazomethine chains connected
together by sulfur bridges of different length. The linearstructure originally proposed has now been rejected due
to various reasons including the experimental evidences.
Acknowledgements
We are deeply indebted with Agenzia Spaziale Itali-
ana (ASI), Viale Liegi 26, Rome, Italy for the financialsupport of the present work.
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Recommended