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.

References

[1] Gmellins Handbuch der Anorganischen Chemie, vol.14/11, Sys-

tem C14, TL D5, Springer-Verlag, Berlin, 1978, pp. 161�/237.

[2] P. Brun, A. Fontaine, P. Pascal, Nouveau Traite de Chimie

Minerale, vol. 8, Masson, Paris, 1961, p. 973.

[3] F. Cataldo, Polyhedron 11 (1992) 79.

[4] F. Cataldo, P. Fiordiponti, Polyhedron 12 (1993) 279.

[5] F. Cataldo, J. Inorg. Organomet. Polymer 7 (1997) 35.

[6] F. Cataldo, Polyhedron 19 (2000) 681.

[7] F. Cataldo, Italian Patent No. 1273004, originally filed 30 June,

1994.

[8] F. Cataldo, Italian Patent No. 1231927, originally filed 19 June,

1989.

[9] F. Feher, D. Hirschfeld, K.H. Linke, Z. Naturfosch. B17 (1962)

624.

[10] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The

Handbook of Infrared and Raman Characteristic Frequencies of

Organic Molecules, Academic Press, San Diego, CA, 1991, pp.

295�/300.

[11] F. Cataldo, Eur. Polymer J. 35 (1999) 571.

[12] F. Cataldo, Meteoritics Planetary Sci. (2001) A38.

[13] F.Cataldo Intern., J. Astrobiol. 1 (2002) 25.

[14] L.L. Bircumshaw, F.M. Tayler, D.H. Whiffen, J. Chem. Soc.

(1954) 931.

[15] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared

and Raman Spectroscopy, 3rd ed. (see also the spectra No. 412

and 415 at page 452), Academic Press, San Diego, CA, 1990, pp.

377�/379.

[16] E.V. Skorobogatova, V.R. Karthashov, Russian Chem. Rev. 67

(1998) 423.

[17] L. Maya, J. Polymer Sci. Polymer Chem. 31 (1993) 2595.

[18] R.P. Subrayan, P.G. Rasmussen, Trends Polymer Sci. 3 (1995)

165.

[19] E. Pretsch, W. Simon, J. Seibl, T. Clerc, Tables of Spectral Data

for Structure Determination of Organic Compounds, 2nd ed.,

Springer-Verlag, Berlin, 1989, pp. C201�/C203.

F. Cataldo, Y. Keheyan / Polyhedron 21 (2002) 1825�/1835 1835