Chivers T. - A Guide to Chalcogen-Nitrogen Chemistry (2005)(en)

Chalcogen–Nitrogen Chains and Polymers

281

changing the substituent R attached to carbon is intriguing. However, despite the extensive studies of carbon-containing S–N heterocycles described in Chapters 11 and 12, polymers of this type have not been prepared.

Conducting polymers with p-phenylene groups in the backbone can be generated by the metathetical reaction shown in Eq. 14.1.19 Doping of these polymers with acceptors such as I2, Br2, or AsF5 increases the conductivity to ca. 10-4 –1 cm-1.

Me3SiNSNC6H4NSNSiMe3 + SCl2 :
1/x[-C6H4NSNSNSN-]x + 2Me3SiCl	(14.1)

The use of 1,4-ClSC6H4SCl instead of SCl2 in reaction 14.1 produces the polymer [C6H4NSNSC6H4SNSN]x. Polymers with C6H4SNSNSC6H4 or C6H4SNSNSC6H4SNSNSC6H4 segments separated by flexible spacer groups have also been synthesized.20 These insulators are converted to semiconductors when doped with Br2. Similar polymers have been prepared from 1,3-ClSC6H4SCl.21

14.3 Sulfur–Nitrogen Chains

The simplest chain compounds of the type RSxNyR are the sulfur diimides RNSNR (Section 10.4) The other known thiazyl chains are summarized in Table 14.1. They can be conveniently classified as sulfurrich, nitrogen-rich or even-chain species.

Table 14.1 Sulfur–Nitrogen Chains as a Function of Chain Length

Chain length	S-rich	N-rich	Even-chain
3	[RSNSR]+	RNSNR
4	RS3N2R		RS2N2R
5
6	[RS4N3R]+
7		RN4S3R
8			RS4N4R
9			RS5N4R

282 A Guide to Chalcogen–Nitrogen Chemistry

The electron-counting rules that are used to determine the number of Œ-electrons in S-N rings (Section 4.4) can also be applied to thiazyl chains. Each S atom donates two electrons to the Œ–system and each N is a one-electron donor. The acyclic chain structures favour an even number of Œ-electrons. Consequently, most of the known S–N chains contain an even number of N atoms and are neutral. The exceptions are the cations [RSNSR]+ (R = Cl, Br) and [RS4N3R]+. The cationic systems are stabilized by (a) removal of an antibonding electron from the Œ– system, (b) lowering of the energies of the filled molecular orbitals induced by the positive charge, and (c) the ionic contribution to the lattice energy in the solid state. Conversely, anionic systems are destabilized by addition of an electron to an antibonding level in the already electron-rich Œ–system. Thus, it is not surprising that no stable anionic chains are known. The only selenium analogues of these S-N chain species are the selenium diimides RNSeNR (Section 10.4). The mixed chalcogen species Me3SiNSNSeNSNSiMe3, with a single selenium atom in the middle of a seven-atom chain, has been prepared and structurally characterized.22

Early examples of the synthesis of S–N chains frequently involved the reaction of S4N4 with nucleophilic reagents. For example, trithiadiazenes ArS3N2Ar are obtained by treatment of S4N4 with

Grignard reagents,	e.g.	4-ClC6H4MgBr,23 and the trithiatetrazene
Ph2CNSNSNSNCPh2	is	prepared by the reaction of

diphenyldiazomethane with S4N4.24 In recent years rational syntheses have been developed for chain lengths up to ArS5N4Ar via condensation reactions involving the elimination of Me3SiCl or Me3SiOSiMe3 (Scheme 14.1).25-27 The reagent (NSCl)3, in the presence of AgAsF6, has

also been used to	extend chain length,	e.g.,	in the conversion	of
ArS3N2Ar into [ArS4N3Ar][AsF6] (Ar = Ph,		4-O2NC6H4).28
The S–N chains	ArS2N2Ar• 14.1, Ar =		4-O2NC6H4, Ar•	-

MeOC6H4), PhN4S3Ph (14.2) and ArS4N4SiMe3 (14.3, Ar = 4-O2NC6H4 adopt alternating cis,trans conformations similar to (SN)x.27,29 In contrast

to (SN)x, however, there is a distinct alternation of short and long sulfur– nitrogen bonds in the oligomeric chains, consistent with a more localized [–S–N=S=N–] structure. Other chain conformations are also observed, as illustrated by the structures of ArS3N2Ar (14.4, Ar = 4-ClC6H4),23

Chalcogen–Nitrogen Chains and Polymers	283
[ArS4N3Ar]+ (14.5, Ar = 4-MeC6H4),26 ArS5N4Ar	(14.6, Ar = 4-

MeC6H4),30 and tBuN4S3tBu (14.7).31 The small energy difference between various isomers is indicated by the subtle conformational change that occurs upon the replacement of the Ph groups in 14.2 by tBu substituents to give 14.7. The selenium-containing chain (Me3SiNSN)2Se

adopts the same conformation as 14.7.22 Ab initio calculations indicate that weak intramolecular S•••S interactions occur in 14.4.32,33

ArN4S3Ar
	SCl2
		ArSCl
ArNSNSiMe3			ArS2N2Ar
	ArNSO
Li[N(SiMe3)2]
	SOCl2
		ArSCl		ArSCl
Me3SiNSNSiMe3			ArS2N2SiMe3		ArS3N2Ar

SCl2

ArSCl

Me3SiN4S3SiMe3 ArS4N4SiMe3

Scheme 14.1 Syntheses of sulfur–nitrogen chains

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A Guide to Chalcogen–Nitrogen Chemistry

N

S

PhN

N

S

NPh

ArS

NAr'

S

N

S

14.1

14.2

N

S

N

S

ArS

SAr

ArS

N

S

NSiMe3

N

S

N

14.3

14.4

ArS

+

SAr

ArS

S

N

SAr

N

N

S

N

N

N

N

S

S

S

14.5

14.6

tBuN

S

NtBu

S

N

S

N

14.7

The colours of S–N chains are dependent on chain length. The shorter chains are bright yellow or orange ( max 330–475 nm), whereas the longer chains (more than six heteroatoms) produce deep green, blue or purple solutions ( max 520–590 nm) and exhibit a metallic lustre in the solid state. This trend can be rationalized by the decrease in the energy gaps between Œ–molecular orbitals that occurs as chain length increases resulting in lower energy electronic transitions.

The reactions of S–N chains have not been studied in a systematic fashion and those that have been reported are not well understood.2 For example, the reduction of [ArS4N3Ar]Cl (Ar = Ph, 4-MeC6H4) with silver powder promotes chain extension to give ArS5N4Ar.30 By contrast, the addition of sodium metal to a mixture of the ArNSNAr and Ar'NSNAr' results in a scrambling process to give the unsymmetrical sulfur diimide ArNSNAr' (Section 10.4.4).29 Another example of this type of transformation is the generation of unsymmetrical dithiadiazenes

Chalcogen–Nitrogen Chains and Polymers

285

ArNSNSAr' from a mixture of ArNSNAr and Ar'SNSNSAr'.30 The limited data available indicate the occurrence of transformations analogous to the ring expansion or ring contraction processes observed for S–N ring systems.34

The incorporation of S–N chains between metal centres by the use of heteroaryl substituents in complexes of the type 14.7 has been proposed as a way to generate new materials that may function as molecular wires.2 However, the synthesis of thiazyl chains bearing metal-binding sites has yet to be achieved.

14.4Sulfur–Nitrogen Polymers Containing Three-Coordinate Sulfur(IV)

Thiazyl halides form cyclic oligomers, e.g., (NSCl)3 or (NSF)4 (Sections 8.7 and 8.8), rather than polymers. However the thiazyl chloride unit NSCl has been incorporated into the hybrid polymer 14.8, which is

generated by a thermal ring-opening polymerization (ROP) process (Scheme 14.2).3b,35 This process is conveniently monitored by 31P NMR

spectroscopy because, as a general guideline, polymerization is accompanied by an upfield shift of ca. 30 ppm in the 31P NMR resonance, i.e., from / +24.5 for the six-membered ring to / –4.6 for the polymer 14.8.

		Cl
		S		Cl	Cl
	N	N
			90 °C
			S N	P N	P N
	Cl	P	Cl
	P		Cl	Cl	Cl
	Cl	N
			Cl		n

14.8

Scheme 14.2 Synthesis of poly(thiophosphazene)

Polymers containing three-coordinate sulfur(IV) are generally hydrolytically sensitive even when the chloro substituents are replaced by phenoxy groups.36 Consequently, much more attention has been

286 A Guide to Chalcogen–Nitrogen Chemistry

accorded to analogous polymer systems that contain four-coordinate sulfur(VI). Interestingly, nucleophilic substitution in 14.8 occurs preferentially at the sulfur sites.36

14.5Sulfur–Nitrogen Polymers Containing Four-Coordinate Sulfur(VI)

Sulfanuric halides [NS(O)X]x (X = Cl, F) are normally isolated as cyclic trimers (x = 3) (Section 8.9). These ring systems do not undergo ringopening polymerization; they decompose exothermically above 250°C.37 Consequently, an alternative route to sulfanuric polymers of the type 14.9 had to be developed. High molecular weight polymers containing repeating [–N=S(O)R–] (R = Me. Ph) units are generated from an acyclic precursor by a condensation process in which the formation of a very strong Si–O bond provides a driving force for the elimination (Scheme 14.3).

O

O

120 - 127 °C

Me3SiN

S

OR'

N

S

- Me3SiOR'

R

R

n

R' = Ph, CH2CF3

14.9

R = Me, Et, Ph

Scheme 14.3 Synthesis of sulfanuric polymers

The sulfanuric polymers 14.9 are thermoplastics with glass transition temperatures (Tg) in the range 30–85°C, which are considerably higher than those of the better known poly(phosphazenes) (NPX2)n (Tg = – 96°C, X = F; –84°C, X = OEt; –66°C, X = Cl; –6°C, X = OPh). The higher Tgs of the sulfanuric systems are attributed to (a) increased intermolecular interactions as a result of the polar S=O groups and (b) greater side group–main chain interactions as a result of the smaller <NSN bond angle (calculated value 103°) compared to ca. 120° for <NPN in poly(phosphazenes).37b

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287

Hybrid polymers that contain both [NS(O)R] and [NPR2] units (R = Cl, F OAr, NHR) have been the subject of extensive investigations as a result of their promising properties.3 The first example of this type of sulfanuric–phosphazene polymer 14.10a was reported on 1991.38 It was prepared by the thermal ring-opening polymerization of a cyclic precursor at 165°C (Scheme 14.4). The conversion of the six-membered ring into a polymer is accompanied by an upfield shift of 31P NMR resonance of ca. 35 ppm. Small amounts of macrocycles {[NS(O)Cl](NPCl2)2}n (n = 2-6) are also formed during this process and the twelve (n = 2) and twenty-four-membered (n = 4) rings have been isolated and structurally characterized (Section 13.4).39 The polymer is produced as a pale yellow elastomer by adding hexane to a cold, rapidly stirred CH2Cl2 solution of the product. The fluorinated derivative 14.10b may be obtained in a similar manner by using a slightly higher temperature (180°C). Subsequently, it was found that this polymerization process occurs at room temperature in CH2Cl2 in the presence of GaCl3 (10% of the stoichiometric amount) as a catalyst.40 The yield of the polymer 14.10a obtained by the latter method is essentially quantitative.

O

X

S

O

Cl

Cl

165 °C ( X = Cl)

N

N

S

N

P

N

P

N

Cl

Cl

180 °C ( X = F)

P

X

Cl

Cl

P

n

Cl

N

Cl

14.10a, X =Cl

14.10b, X =F

Scheme 14.4 Synthesis of poly(thionylphosphazenes)

The cation [NSO(NPCl2)2]+ (14.11) is the proposed intermediate in this ring-opening polymerization process. This cation is extremely reactive, as illustrated by the isolation of the solvent-derived product 14.12 when it is generated by halide abstraction from the cyclic precursor with AlCl3 in 1,2-dichloroethane.40

288	A Guide to Chalcogen–Nitrogen Chemistry
	O			O	CH2CHCl2
	S+			S
N		N		N	N
Cl		Cl	Cl	P	Cl
P		P			P
Cl	N	Cl	Cl	N	Cl
	14.11			14.12

In order to obtain hydrolytically stable materials, the chlorine substituents in the polymer 14.10a may be replaced by phenoxy or tert- butylamino groups.41,42 The reaction of 14.10a with sodium phenoxide in dioxane at room temperature is regiospecific. It produces the polymer {[NS(O)Cl][NP(OPh)2]2}n (14.13), i.e., the chlorines attached to phosphorus are replaced by OPh groups, but the S–Cl bond remains intact. This observation is different from the behaviour of the analogous sulfur(IV) polymer 14.8 towards nucleophilic reagents. In that case substitution takes place preferentially at the sulfur centre (Section 14.4). The polymer 14.10a is a colourless elastomer, stable towards moisture with an average molecular weight of ca. 3 x 104. The chlorine substituents on phosphorus and sulfur are all replaced upon treatment with an excess of tert-butylamine in dichloromethane at room temperature to give the polymer {[NS(O)NHtBu][NP(NHtBu)2]2}n (14.14), which has an average molecular weight of ca. 2.5 x 104.

O

OPh

OPh

O

NHtBu

NHtBu

S

N

P

N

P

N

S

N

P

N

P

N

NHtBu

NHtBu

NHtBu

Cl

OPh

OPh

n

n

14.13

14.14

As expected, the Tgs of the hybrid sulfanuric–phosphazene polymers are much closer to the values reported for poly(phosphazenes) than those of sulfanuric polymers (vide supra). The values for the polymers 14.10a

Chalcogen–Nitrogen Chains and Polymers

289

and 14.10b are –46°C and –56°C, respectively. Ab initio molecular orbital calculations, including geometry optimizations, on short-chain model compounds of 14.10a and 14.10b show that they adopt a nonplanar trans, cis structure.43 The calculations also reveal a decrease in the torsional barriers for rotation of the SNP and PNP bond angles when changing from Cl on sulfur in 14.10a to F attached to sulfur in 14.10b. This calculated increase in flexibility is thought to account for the lower Tgs of fluorine-containing poly(phosphazenes) as well as the hybrid polymer 14.10b. The Tg of –16°C for the butylamino-substituted hybrid polymer 14.14 is significantly lower than the value of +8°C reported for the corresponding poly(phosphazene) [NP(NHtBu)2]n as a result of the presence of the small S=O group and only five tBuNH substituents in 14.14 compared to six tBuNH groups in the phosphazene polymer.

Polythionylphosphazenes have considerable potential as components of the matrices for phosphorescent oxygen sensors.44 These sensors contain transition-metal-based dyes with oxygen-quenchable excited states dispersed in a polymer matrix. They can be used, for example, to determine the air pressure distribution on the wings of an aircraft in wind tunnel experiments. The high solubility and high diffusion coefficient for oxygen of the alkylamino derivatives of these hybrid polymers are crucial properties in this application.45 Although 14.14 forms sticky films, the block co-polymer polythionylphosphazene-b- polytetrahydrofuran, prepared from the reaction of the cation of 14.10a with THF, forms free-standing films.46

Hybrid polymers containing an equal number of alternating [NS(O)R] and [NPR2] units in the backbone can formally be derived by ROP of the appropriate eight-membered ring. In practice, however, this approach is not successful because the ring strain in eight-membered rings is considerably smaller than that in six-membered inorganic ring systems. This type of copolymer has, however, been synthesized by the ingenious exploitation of condensation processes as exemplified in Scheme 14.5.47 The formation of the acyclic precursor 14.15 is highly regiospecific with respect to the elimination of the Me3Si group attached to the PN unit and the CF3CH2O group bonded to sulfur in the form of Me3SiOCH2CF3. Molecular weight determinations indicate that the hybrid polymer 14.16 produced in this way consists of ca. 30 repeat units.

290	A Guide to Chalcogen–Nitrogen Chemistry

The 31P NMR spectrum of the polymer 14.16 shows two singlets of approximately equal intensities that differ in chemical shift by only 0.17 ppm. This observation is attributed to the formation of equal amounts of isotactic (14.16a, both S=O groups on the same side of the polymer chain) and syndiotactic (14.16b, S=O groups on opposite sides of the polymer chain) forms of the polymer. The atacticity is confirmed by the 1H and 13C NMR spectra. Two resonances are observed for the Me2P groups of 14.16a, whereas 14.16b gives rise to only one resonance.

Me

O

CF3CH2O

CF3CH2O

S

NSiMe3

P

NSiMe3

+

Me

Me

- Me3SiOCH2CF3

Me

O

CF3CH2O P

N

S

NSiMe3

14.15

Me

Me

140 °C

- Me3SiOCH2CF3

Me

O

14.16

P

N

S

N

Me

Me

n

Scheme 14.5 Synthesis of a polymer with alternating phosphazene and oxathiazene units