4) S(1Sulfur-nitrogen chains: rational and irrational behaviour* "3 *I Jeremy M. Rawson and John J. Longridge Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 IEW * Dedicated to Arthur Banister, on the occasion of his retirement; an inspiring teacher to all who have been fortunate enough to work for him. Poly(su1fur nitride), [SN],, was the first example of a polymeric metal, and the discovery of its superconducting properties in 1973 fuelled a generation of research into the areas of sulfur-nitrogen chemistry and molecular con-ductors. The synthesis, structure and properties of [SN],now form part of many undergraduate courses and it is an often cited textbook example. Now, in the 1990s, small fragments of [SN], may prove useful as molecular wires in the development of nanoscale technology.Although the preparations of many thiazyl chains can be carried out in a rational high-yielding manner, it is the diverse reaction chemistry, which often involves unexpected changes in the chain size, which provides one of the most rewarding and stimulating aspects of this area. 1 Introduction Poly(su1fur nitride), [SN], is a one-dimensional polymer in which sulfur and nitrogen atoms form an alternating chain (Fig. 1). Its physical properties are it is a conducting material at room temperature and becomes super- conducting below liquid helium temperature. Its one-dimen- sional structure leads to a large degree of anisotropy. Conse- quently, its conductivity is greatest along the chain, where n-orbitals on sulfur and nitrogen overlap to form a conduction band.In addition to its conducting properties, the high electronegativity of [SN],r, even greater than that of gold, produces several further unusual physical properties. The high electronegativity of poly(su1fur nitride) leads to several en- hancements in device efficiency; for example [SN], can act as an efficient barrier electrode in ZnS junctions,2a increasing the quantum efficiency of the blue-emission by a factor of 100over gold; and it can also be used to increase the efficiency of GaAs solar cells (conventionally Au-GaAs) by as much as 35%.*h In addition [SN], is remarkably inert; it does not react with water or acidic solutions, but slowly decomposes in alkaline solutions.Because of the high electronegativity of [SN],, metal ions interact more strongly with a poly(su1fur nitride) surface than with other metal electrodes.2c In some instances this can lead to enhancement of catalytic properties, e.g. [SN], surfaces pre- treated with metal ions have been used as catalysts and can improve the rates of conversion2c of acetylene to ethylene by factors of up to 107. Despite such appealing properties, industrial exploitation3 of [SN], in modern devices has been hampered by synthetic problems; the classical route to [SN], involves the 'cracking' of red S4N4 over silver wool to give colourless crystals of S2N2 which slowly polymerise over a period of weeks to form golden [SN], (Fig.1). The slowness of the polymerisation, coupled with the explosive nature4 of both the S4N4 starting material and intermediate S2N2 molecule, has prompted researchers to investigate other synthetic5 strategies to [SN],. In addition, other processing techniques6 have been sought so that [SN], can be prepared in thin films on a variety of substrates, such as OTEs (optically transparent electrodes), plastics and other metal surfaces. In particular, vacuum sublimation6" of powdered [SN], and electroreduction6h of [S5N5]+ salts have proved valuable routes to the formation of [SN], films. S4N4 S2N2 polymerise// [SNIX Fig. 1 Synthesis and structure of [SN], Jeremy Rawson obtained both his BSc and PhD degrees from the University of Durham.He was a post-doctoral fellow with Dr Arthur Banister (University of Durham, 1990-1 992) and Dr. Richard Winpenny (University of Edinburgh, 1993-1 994) before returning to Durham to take up a temporary lectureship. He moved to Cambridge in 1995 where he is now a University Lecturer and Fellow of Magdalene College. His current research interests include the magnetic properties of main-group x radicals and polynuclear metal complexes bridged by sulfur-nitrogen rings and chains. John Longridge graduated from the University of Durham in 1995, and is presently a graduate student at the University of Cam- bridge. Chemical Society Reviews, 1997 53 The physical properties of [SN], have led to a great resurgence of interest in group 15/16 chemistry in recent years.In particular the inclusion of carbon-based fragments into the thiazyl backbone would allow the properties of [SN], to be modified by changing the electronic properties of the chain substituents. This has led not only to the preparation of C/N/ S-based polymers7 but also to the development of C/N/S-based heterocyclic rings,8 and sulfur-nitrogen chains (i.e. small fragments of conducting [SN],). These small fragments of conducting [SN], could find novel applications in the field of nanoscale technology, particularly molecular wires .9 Nanoscale devices function on a molecular rather than macroscopic level and one of the key features required for many nanoscale devices to operate effectively is the molecular wire; a functional group which will conduct electrons between different parts of a molecule, allowing the different components to interact in an effective manner.The molecular wires presently used typically possess delocalised n systems and are frequently small fragments of conducting polymers such as acetylene oligomers or fused aromatics. Fig. 2 illustrates how a molecule of this type can respond to light (so-called photo-induced electron transfer) and is exemplified by a Ru(bipy)3 deriva- tive. Phot o-recept or Photo-emitter "r .R;(blPY h Fig. 2 Schematic representation of a photosensitive molecular device The conductivity of pure [SN], is ca. 1 X 103S cm-1 at room temperature (cf.polyacetylene 1 X 10-7 and 1 X 10-2 S cm-1 for cis and trans forms respectively'o) and we might therefore expect that the efficiency of molecular devices with thiazyl linkages might be superior to those of the corresponding acetylene-bridged molecules.This review article aims to describe the chemistry of some of these sulfur-nitrogen chains; the types of sulfur-nitrogen chain we might expect to form; their relative stabilities; their physical properties; their structures and reactivity. In particular, although some references and comparison will be made to short thiazyl chains, containing two or three heteroatoms, this review will highlight the chemistry of the longer-chain compounds (ix., containing at least two thiazyl, -S=N-, units). 2 Types of thiazyl chain and electron counting Thiazyl chains can be split conveniently into three categories dependent on composition; sulfur-rich, nitrogen-rich and even- chain compounds.These are highlighted in Table 1. We can Table 1 Sulfur-nitrogen chain compounds as a function of chain length, with known derivatives highlighted Chain length S-rich N-rich Even-chain 3 RSNSR+ RNSNR 4 RS2NZR 5 RS3N2R RN3 S2R+/- 6 RS3N3R+/- 7 RS4N3R+ RN4S3R 8 RS4N4R 9 RSSN~R RNSS4R+/- 10 RS5N5R+/- 11 RS6NSR+ RN6S5R utilise the same electron-counting rules11 used for sulfur- nitrogen rings to n-electron count these compounds. In these systems, each S donates two electrons to the n system and each N is a one-electron donor. Unlike the sulfur-nitrogen rings where [4n + 2]n Huckel configurations are preferred,? the chain-like structures only favour even numbers of n electrons (Table 2), i.e.a full, or 'closed-shell', electronic configuration. Because N provides only one electron, virtually all thiazyl chains reported to date (highlighted in bold in Table 1) contain an even number of N atoms and are neutral. There are a small number of chains with odd numbers of N atoms, but these are charged so as to retain a full n-shell, e.g. RSNSR+ (R = C1, Br) and RS4N3R+. Indeed cationic chains should be particularly stabilised via the lowering of the filled molecular orbitals induced by the positive charge on the system, and in addition, the ionic contribution to the lattice energy should also assist their stabilisation in the solid state.Intuitively the observation of both ArS4N3Ar+ and XSNSX+ cations indicates that sulfur-rich compounds will have a tendency to form cationic systems, and this can readily be understood by the number of electrons filling antibonding molecular orbitals; For an acetylene chain, (CH),, each C provides one p-orbital for the formation of n molecular orbitals + A number of sulfur-nitrogen systems are also known which are formally 4nn anti-aromatic molecules, but these do not have planar n-delocalised structures analogous to the thiazyl chains and Huckel [4n + 2]n aromatics. Instead they take up cage structures,g e.g. S4N4, S5N6, S4N5+and S6N5+. Table 2 A comparison of sulfur-nitrogen ring compounds and sulfur-nitrogen chain compounds and their respective n electron counts n-electron count 6 8 10 12 14 Ring N-SI I S-N other examples: chain ~3~2~+ R-S \ IN-R R,SI IS/R kS/NN-S S3N3-, S4N42+ER;; :I+ N, ,N, HN R-q p-S, IN--R N-S N-S R-S\ N-S pS SOR I I kS/N other examples RN4SSR 54 Chemical Society Reviews, 1997 and donates one electron to this n manifold. The result is a set of bonding and antibonding n orbitals of which only the bonding orbitals are filled.In comparison, in thiazyl oligomers, each S donates two electrons and each N one electron for n-bonding and some of the formally antibonding orbitals will also be occupied to accommodate the additional electrons, provided by S. Consequently, if the thiazyl chain contains an odd number of electrons, removal of the unpaired electron from its antibonding orbital not only strengthens the n-bonding character, but also lowers the energy of the bonding orbitals through the introduction of the positive charge.In general the removal of an electron to form a cationic system is preferred over the addition of an electron to generate an anionic system since the latter requires the addition of a further electron into an antibonding orbital. This observation also explains the pro- pensity for sulfur-nitrogen rings to form cationic rather than anionic systems, although anionic rings, such as the lox Hiickel S3N3- are known. Compounds with the same chain lengths will also have similar sets of molecular orbitals, and it is perhaps not too surprising to find that the isoelectronic RS4N3R+ and RN&R chains take up similar geometries (see section 5).3 Synthesis of sulfur-nitrogen chains 3.1 Historical background Prior to the 1970s, sulfur-nitrogen chemistry was plagued by structural mis-assignments and the diverse nature of many reactions which typically yielded multiple and sometimes unexpected products. 12 With the development of modern analytical methods (particularly X-ray crystallography and more recently multinuclear NMR) and theoretical studies, the area of sulfur-nitrogen chemistry has been revolutionised and many unusual mechanistic processes have been rationalised. As a consequence, controlled syntheses of many sulfur-nitrogen compounds can now be achieved.In this section we aim to outline early developments in the area of sulfur-nitrogen chains and illustrate more rational synthetic methodologies. 3.2 Initial syntheses of thiazyl chains The first reported syntheses of sulfur-nitrogen chains appeared in the mid-1960s and early 1970s, and de~cribed'3.1~ the preparation of RS3N2R (trithiadiazenes) and the shorter chains, RNSNR (known as sulfur diimides). The chemistry of sulfur diimides is particularly extensive and beyond the scope of this review, except as reagents for tlie synthesis of other sulfur- nitrogen chains. As with many other areas of sulfur-nitrogen chemistry, early syntheses of S/N chains involved the ubiqui- tous S4N4 molecule;13 the reaction of S4N4 with aromatic Grignard reagents or diazomethanes yielded RS3N2R in low yield.The more traditional 'boil-and-bake' approach (involving the condensation of HCl between S-Cl and N-H bonds, and concurrently forming S-N bonds) was also utilised with some success.14 3.3 Rational syntheses For many years the standard synthetic route to inorganic rings and chains has involved condensation reactions, typically with loss of HCl.12J4 Such reactions occur at elevated temperatures, so as to remove the HCl from the reaction mixture. Recently, condensation reactions, particularly involving the loss of Me3SiC1, Me3SiOSiMe3 or metal halides,8 have been used successfully in the synthesis of inorganic rings and chains. These condensation reactions occur smoothly at low tem-peratures and lead to clean products in high yield.This technique has been extensively ernpl~yed'~ in the syntheses of sulfur-nitrogen chains; one of the most common reagents being bis(trimethylsily1)sulfur diimide, Me3SiNSNSiMe3. The syn- thesis of this reagent itself (see Scheme 1) provides a useful example of the use of both LiCl and MesSiOSiMe3 as thermodynamic sinks, the sulfur diimide being formed in excellent yields under mild conditions. l5 ArN4S3Ar t sc12 ArSClArNSNSiMe3-ArS2N2Ar t ArNSO Li[N(SiMe3)2] 1soc12 ArSCl ArSClMe3SiNSNSiMe3--+ ArS2N2SiMe3-ArS3N2Ar J. sc12 ArSCl ArSClMe3SiN4S3SiMe3-ArS4N4SiMe3-ArSSNSAr Scheme 1Rational syntheses of some sulfur-nitrogen chains The stoichiometric condensation of Me3SiNSNSiMe3 with ArSCl in 1 : 1 or 1 :2 mole ratios gives the anticipated ArS2N2SiMe3 and ArS3N2Ar chainsl6 (although an excess of ArSCl yields the ArS4N3Ar+ cationic chain!). The syntheses of other sulfur-nitrogen chains can be designed in an analogous manner17 and are illustrated in Scheme 1.SCl2 plays a particularly important role in the syntheses of longer-chain thiazyl oligomers; coupling of two small thiazyl chains containing the trimethylsilyl functional group provides a convenient route to long-chain molecules. It should be noted that for some long chain compounds, the choice of aryl substituent plays a key role in determining the stability and this is discussed further in section 5.2. Using these simple condensation reactions, chain lengths up to ArS5N4Ar have been prepared.4 Structures of sulfur-nitrogen chains 4.1 Structure of [SN], Despite several structure determinations,' the precise structure of [SN], is still open to some debate. The structure determina- tions have been persistently hampered by crystal defects which arise during the polymerisation of S2N2. Although the way in which the atoms are linked together in the alternating cis-trans configuration is not disputed, the bond lengths are not precise. The X-ray diffraction study tends to indicate an alternating set of long and short bonds [ 1.593(5) and 1.628(7) A], intermediate between S-N (ca. 1.69 A) and S=N (ca. 1.54 A) bonds,l6 although the errors on these bond lengths are so large that they are the same (within three esds).The nature of these bonds, intermediate between S-N and S=N, is consistent with the extensive x delocalisation required for conduction. The 'cis- trans' alternating polymeric chain facilitates a set of secondary S--N interactions between atoms in the same chain, composed of electrostatic S*+-N*-interactions, coupled with pn-px interactions. The bond angles at N and S are about 120" and 106O, respectively. Each [SN], chain deviates only slightly (0.17 A) from planarity, and there is a series of weak interactions between chains. 4.2 Thiazyl chains The structural features described in section 4.1 for [SN], are also observed in many of the sulfur-nitrogen chains, which can be considered as small fragments of [SN],.The structures17918 of 02NC6H&N2C6&0Me, PhN4S3Ph and 02NC6H4S4N4SiMe3 chains are shown in Fig. 3. In each of these cases the chains take up similar conformations to [SN], with an alternating 'cis- trans' configuration. In comparison to [SN],, these oligomers definitely exhibit alternating long and short S-N bonds, consistent with a more localised structure of the form [-S-N=S=N-] , although these too are intermediate between S-N and S=N bond lengths. Secondary interactions between non-bonded atoms within the chains are still significant. However, the 'cis-trans' conformation observed in [SN], is not exclusive, and other conformations are observed16319 in which a Chemical Society Reviews, 1997 55 Fig.3 Molecular structures of 02NC6H4S2N2C6H40Me,PhN4S3Ph and 02NC6H4S4N4SiMe3 ‘cis’configuration is replaced by a ‘trans’ arrangement. This ‘defect’ to the [SN],-type structure can arise either at an N atom [such as N(2) in C1C6H4S3N2C6H4Cl] or at an s atom [e.g. s(3) in the cation MeC6H4S4N3C6H4Me+] (Fig. 4). The energy required to introduce such a ‘defect’ primarily arises through the breaking of one of the transannular interactions and theoretical calculations20 have estimated this to be of the order of 25-30 kJ mol-1. This small energy contribution can be overcome by molecular packing forces, or particularly in the case of the cationic ArS4N3Ar+ salts, through significant ionic lattice contributions. This is highlighted by two different conformations to the RN4S3R chain, depending on substituent; for the PhN4S3Ph derivative17 the ‘ideal’ [SN], type configura- tion is observed (Fig. 3), whereas for ButN&But a ‘defect’ is found21 at N(3) (Fig.4). The longest known thiazyl chain to be crystallographically characterised,22 MeC6H4S5N4C6H4Me is also shown in Fig. 4 and shows a ‘defect’ at N(4); this structure can be envisaged as a pair of ArS2N2 and Ar fragments on an S3N2 chain (which has a characteristic ‘open-ring’ structure). The thiazyl chain compounds exhibit similar bond lengths to [SN], although there is considerable variation in the bond angles depending on the length of the sulfur-nitrogen chain and the terminal groups; bond angles are typically in the region 102-124” and 118-129” at S and N respectively.Without exception, diaryl-substituted thiazyl chains are approximately planar, facilitating the x-delocalisation along both the thiazyl chain and over the aryl substituents. However, other substitu- ents can produce a more pronounced deviation from planarity, and for example, u-O~NC~H~S~N~S~M~~ sits17 on the curve of a circle with an appproximate radius of 50 A. Intermolecular interactions are very important in stabilising the metallic state of [SN],. In [SN],, the secondary interactions between S atoms in neighbouring chains is 3.48 A.l Strong secondary interactions between heteroatoms are also prevalent in many of the thiazyl chain structures, e.g. in 02NC6H&N2- C6H40Me the interplane distance between molecules is only 3.42 A.l8 However, in many instances, particularly when a chain is terminated with a bulky substituent such as SiMe3, then many of these secondary contacts are often appreciably longer,17 although still less than the sum of the van der Waal’s radii [3.63 (Sa-N) to 4.06 A (S-S)], e.g.S-.S in 02NC6H4S4N4- SiMe3 at 3.73 A. In addition the presence of a positive charge on the thiazyl chain leads to electrostatic repulsion between cations and the closest approaches are close to the sum of the van der Waal’s radii, e. [02NC6H4S&C6H4N02] [AsF6] has close contacts at 3.64 % for S.-N and S.-S contacts in the region 3.9 to 4.0 A.23 5 Correlation of sulfur-nitrogen chain lengths and their physical properties 5.1 Jc-Delocalisation Each sulfur and nitrogen atom in the thiazyl chain contributes one p-orbital towards the formation of a set of x molecular orbitals.The energies of the 3t molecular orbitals for two thiazyl chains, HS3N2H and HSSN4H, are shown in Fig. 5. As the chain- length increases, the number of p-orbitals also increases and the energy gap between x-orbitals becomes smaller, consistent with simple band theory.10 The distribution of the energies of these x molecular orbitals is an important feature of such thiazyl chains, and plays an important role in determining some of their physical properties, particularly their optical properties, both in solution and in the solid state. 56 Chemical Society Reviews, 1997 ArS4N4SiMe3 [AT = o-N02C& or 2,4-(N02)&H3] with SC12 does not lead to the 17-heteroatom chain, ArSgNxAr, but Energy rather to the isolation17 of the decomposition products, ArS5N4Ar. The disproportionation of these thiazyl chains is r-9 -described in section 6.4.For short chain oligomers there appear II-to be no such problems with disproportionation. -10 _-11 st --12 --13 + -1 4 1--15 stI--% st 1-Fig. 5 The energies of the n-molecular orbitals of HSsN4H and HS3N2H 5.2 Optical properties and delocalisation As the chain length increases, the energies between different n-molecular orbitals decreases and the energy, hv, to excite electrons between different n-orbitals becomes smaller. This leads to the ob~ervation~~.’~ of a ‘red-shift’ as the chain-length becomes longer and is indicative of a ‘tight’ n-manifold and more extended n-delocalisation.Electron-withdrawing substi- tuents on the thiazyl chains can also lead to a slightly increased red-shift,’b as can the introduction of a positive charge which produces a lowering of the orbital energies.16 The UV-VIS absorption maxima for a series of thiazyl chains are given in Table 3, and illustrate the effect of chain length, electron- withdrawing substituents and charge on the absorption maxima. In general short sulfur-nitrogen chains (less than six hetero- atoms) tend to be brightly coloured; yellow or orange whereas the longer chains (greater than six heteroatoms) tend to produce very intense deep-coloured solutions; typically deep green, royal blue or purple.In the solid state, a similar set of colours is observed; short thiazyl chains tend to be brightly coloured, whereas the longer chains tend to have a metallic lustre, similar to that observed for ‘golden’ [SN], or ‘silver’ polyacetylene, [CHI,. 5.3 Stability The stability of thiazyl oligomers, particularly long-chain (n > 7) derivatives, is particularly dependent on the terminal functional groups. Zibarev and coworkers have noted17 that electron-withdrawing substituents on aromatic terminal groups are particularly good at stabilising longer chain lengths. For example, reaction of Me3SiN4S3SiMe3 with ArSCl (Ar = Ph, o-N02C6H4) in a 1 :2 mol ratio yielded ArS5N4Ar when Ar = o-NO~C~H~.However, when Ar = Ph, the only recovered products were PhS3NlPh and S4N4.The stability of very long thiazyl chains is questionable and, for example, reaction of 6 Reactivity In comparison to the development of synthetic routes to thiazyl chains, their chemistries are poorly understood. Indeed those reactions which have been carried out appear diverse and, like other areas of sulfur-nitrogen chemistry, somewhat unexpected at first glance. A series of reported reactions are described below which initially seem both unusual and varied. However, a common theme appears in many of them and this is discussed in more detail in section 6.5. 6.1 Scrambling of terminal groups and preparation of ArS2N2Ar’ In the presence of a catalytic quantity (5-25 mol%) of alkali metal (Na, K) mixtures of RNSNR and R’NSNR’ undergo rapid scrambling’x of the terminal groups to yield mixtures of starting materials and the mixed product, RNSNR’.The position of the equilibrium is dependent on the nature of the R groups. If they are similar (e.g.Ph and MeC6H4 or MeOC6H4) then there is an approximately statistical distribution of products whereas for dissimilar groups the equilibrium favours the cross-product. An extension of this reaction is the reaction of PhNSNPh with PhS3NZPh in the presence of alkali metal to yield PhS2N2Ph. In this reaction, the radical anion RNSNR--, formed by reduction of the neutral thiazyl chain with alkali metal, possesses 5n electrons and three n molecular orbitals.Oakley and coworkers proposed that two of these molecules could associate in solution to form a dimer’g (dimerisation processes are well known in other areas of sulfur-nitrogen chemistryx) in which the two molecules are weakly associated via overlap of the two singly occupied molecular orbitals, with the S-.S bridge contributing the greatest extent. This dimeric intermediate can then rearrange to form the mixed thiazyl chain (Scheme 2). 6.2 Reduction of ArS4N3Ar+ Wolmershauser and coworkers recently investigated22 the redox behaviour of the ArS4N3Ar+ cation and observed that reduction led to the unstable ArS4N3Ar- radical which dispro- portionated to ArSSN4Ar and ArS3N2Ar. The disproportiona- tion reaction was postulated to proceed through a four-centred S2N2 ring intermediate; this reaction can be considered to occur in a similar manner to that described in section 6.1 for the scrambling of terminal groups in short-chain thiazyl com-pounds. An analysis of the frontier molecular orbitals23u indicates that one electron reduction of the ArS,N,Ar+ cation yields a neutral radical with the unpaired electron occupying a n-type orbital of the same symmetry to that of the sulfur diimides discussed in section 6.1.6.3 Hydrolysis of [ArS4N3Ar]CI and chain lengthening reactions with (NSC1)3 In 1977 Street and coworkers reported16 the first synthesis of an [ArS4N3Ar]+ chain as its chloride salt, and noted that it slowly Table 3 UV-VIS absorption maxima (nm) for a series of thiazyl chain complexes Thiazyl chain ArNSNSiMe3 ArNSNAr ArS3N2Ar ArN4S3Ar ArS4N3Ar+ ArS4N4SiMe3 ArSsN4Ar ~~~~ Ar = O,~-(NO~)~C~H~4 13 476 538 587 CJ-NO~C~H~ 409 476 521 585 P-NO2ChH4 394 458 580 p-ClC6H4 383 449 565 p-MeC6H4 355 448 530 582 ChH5 332 415 450 522 Chemical Society Reviews, 1997 57 Rt RI+ 2e-RI RNSNR + R'NSNR' "I R R' 2 RNSNR Scheme 2 Skeletal scrambling in sulfur diimides decomposed on the open bench to give the shorter, ArS3N2Ar chain. Although a hydrolysis mechanism evidently takes place, the reaction can be conveniently thought of as loss of thiazyl chloride, NSC1, from the starting material.Recently, work has shown that the reverse reaction, i.e. reaction of NSCl with ArS3N2Ar, occurs under ambient conditions and in high yield.23h The mechanism is proposed to involve a four-centred intermediate which facilitates a n-orbital interaction between S and N (Scheme 3).This use of thiazyl chloride as a chain-building reagent has previously been exploited in the synthesis of shorter chains. For example, reaction of NSCl with SC12 and AgAsF6 leads to insertion24 of an SN unit into the S-C1 bond, and condensation of NSCl with ArSNHSAr provides a convenient route14 to ArS3N2Ar via loss of HC1. 6.4 Disproportionation of long chain sulfur-nitrogen compounds In section 5.3, the instability of long-chain thiazyls was described. Two types of reaction appear to occur: either a competing reaction may take place during the formation of the thiazyl chain or the resultant chain itself can decompose, usually accompanied by the loss of S4N4.6.4.1 Competing reactions Condensation of PhNSNSiMe3 with SC12 did not yield the expected product, PhN&Ph, but instead yielded the benzodi- thiadiazine25 (Scheme 4). This reaction is proposed to occur via an intramolecular loss of HC1 (with the abstracted ortho- hydrogen) from the intermediate PhNSNSCl accompanied by ring closure at the ortho-position. 6.4.2 Decomposition reactions To date, there have been no systematic studies of the decomposition of thiazyl chains. However there are several 58 Chemical Society Reviews, 1997 possible mechanisms (Scheme 5); the first mechanism, postu- latedl7 by Zibarev and coworkers, involves either trimethylsilyl or aryl group migration (A), with concomitant extrusion of thiazyl units, as S2N2, from the thiazyl chain; alternatively, the reaction can be considered to involve the breaking and reformation of S-N bonds and a similar loss of S2N2 (B).The ease with which thiazyl chains can undergo rearrangement processes, in comparison to the strength of the C-S bond, indicates that the latter mechanism would appear more favourable. 6.5 On the prevalence of four-centred S2N2 intermediates Although these few reactions constitute the greater part of the known reactivity of long-chain thiazyl compounds, a key mechanistic feature proposed by three different groups (and described in sections 6.1 to 6.3) is the four-centred cyclic S2N2 intermediate formed by the association of two S/N-containing molecules.In organic chemistry the four-centred intermediate, typically formed by a [2 + 21 cycloaddition of two alkene functional groups, is formally symmetry forbidden and tends to occur, for example, upon photolysis or at elevated temperatures where access to excited states alters the orbital symmetry.26 In Ar Ar \ &\ / /& S/s \N c1-0-y "-4 N, ,N, / \ ,N-sS S S 1ci Scheme 3 Mechanism for the reaction of ArS3N2Ar with NSCl SCI2 N, SiMe, -Me3SiC1 Ic1 closure -HC1 Scheme 4 Formation of benzodithiadiazine from PhSNSNSiMes ‘S I I .S N’”\ I7 Q \ I/N=S N‘ A N=S ‘S’ \ B \ asI I I N‘ ’N‘S’ Scheme 5 Proposed chain-shortening mechanisms, for the conversion of ArS5N4Ar to ArS3N2Ar: (A) via aryl group migration; (B) via SzN2extrusion comparison, such four-centred intermediates are favoured in sulfur-nitrogen chemistry since each thiazyl group possesses 3n electrons (2 from S and 1 from N) and the resulting [3 + 3]n interaction yields a favourable 6n electronic interaction.This occurs because the frontier orbitals occupied by thiazyl compounds are different to those occupied by C-based com- pounds. For example, cyclobutadiene possesses 4n electrons, leading to two partially occupied frontier orbitals whereas cyclic-&N2 possesses 6n electrons and the two partially occupied orbitals of butadiene become completely occupied in S2N2.The reactions described in sections 6.1 and 6.2 both involve a radical mechanism involving association via a SOMO-SOMO interaction. The mechanism in 6.3, although involving a four- centred intermediate does not proceed in the same manner but occurs through a HOMO-LUMO interaction in which a pair of electrons from an antibonding ArS3N2Ar n orbital are donated into the LUMO of NSC1; the orbital interaction in this case would therefore appear to be identical but in sections 6.1 and 6.2 the interaction is SOMO-SOMO whereas in 6.3 it is a HOMO-LUMO interaction. 6.6 Other reactions In comparison to sulfur diimides whose chemistries have been examined extensively, the chemistries of the longer S/N chains, as outlined above, have been poorly studied.For example, the sulfur diimides possess a diverse coordination chemistry, but little coordination chemistry of the longer thiazyl chains has been reported. Woollins and coworkers reported27 that reaction of ArS NSNSiMe3 with PtC12 (dppe) gives (ArSNSN)2Pt (dppe) (dppe = 1,2-bisdiphenylphosphinoethane)whereas reaction of ArS3N2Ar with Pt(PPh3), led to reduction of the thiazyl chain and formation28 of (ArS),Pt(PPh,),. The decomposition of thiazyl chains on metal centres is not unexpected and Vrieze and coworkers have shown that simple sulfur diimides undergo unusual rearrangements when coordinated to metals.29 7. Sulfur-nitrogen chains as molecular wires: making the contact 7.1 Thiazyl chains as molecular wires Theoretical calculations show that the majority of frontier orbitals of aryl-substituted S/N chains (irrespective of chain- length) are of n-character, and the majority of these also exhibit some degree of n-delocalisation onto the aryl substituents.Such delocalisation onto the terminal groups is important if these materials are to function as molecular wires; if the aryl groups were nodal in this respect (i.e. did not contribute to the n-framework) then they would insulate the sulfur-nitrogen chain from its surroundings. Instead, this extended n-delocal- isation means that such aryl-substituted thiazyl chains can form an effective means of communication between the two terminal Chemical Society Reviews, 1997 59 groups. The extent to which the x-orbitals are delocalised over the whole molecule and the energy gap between bonding and antibonding orbitals enables us to assess the effectiveness of the intramolecular interactions.‘7 We have already seen (section 5.2) that the longer thiazyl chains produce a more delocalised n-framework with relatively small energy gaps between n-orbitals. If the aryl substituent is provided with metal-binding sites then the sulfur-nitrogen chains can be utilised to facilitate communication between the metal ions. 7.2 Making contacts In order for such thiazyl chains to act as molecular wires betwen metal ions, not only must the x-orbitals extend over both the S/N chain and its substituents, but the substituents must be capable of binding metal centres and also be of the correct symmetry to interact strongly with the metal orbitals.In order for the metals to be able to communicate effectively through the thiazyl linkage, the metal orbitals must interact with the n-cloud. This can be achieved by binding the metals in the plane, facilitating dn-pn orbital interaction, e.g. in pyridine or bipyridine complexes, or via an out-of-plane interaction commonly observed for sandwich and half-sandwich com-plexes such as ferrocenes, in which the d and p- orbitals on the metal interact directly with the x-framework of the sulfur- nitrogen chain (Fig. 6). Fig. 6 Two possible modes of communication between aromatic sub- stituents and metal centres; (a)in-plane interactions and (b)out-of-plane interactions The key to success in this field remains the preparation of thiazyl chains with suitable substituents for binding active metal sites. To date, only a very few short acyclic sulfur-nitrogen chains have been prepared containing metal-binding sites, e.g.the sulfur diimide containing just one coordinating group (perfluoropyridyl) PhNSNC5F4N has been reported,17 but its complexation chemistry has not been explored. Nevertheless, the synthetic methodologies required to prepare such deriva- tives are established and promise to provide some novel properties. 8 Future prospects There are many aspects of the chemistry of thiazyl oligomers which remain unexplored. In section 2 we indicated that the compounds prepared to date constitute just a few examples of a potentially large class of heterocyclic chains. The syntheses of ionic chains are yet to be fully exploited, and we anticipate that the lowering of the energies of the n-molecular orbitals (associated with the presence of a positive charge) will produce 60 Chemical Society Reviews, 1997 a series of new stable chains.In addition we have seen that the reactions of thiazyl chains are sometimes unexpected (section 6) and diverse products can appear from seemingly simple reactions. Further work is required to fully establish these reaction mechanisms and subsequently these can be utilised to assist in the design of new molecules. The coordination chemistry of these chains has scarcely been touched and should provide a wealth of valuable information which can be utilised in the design of molecular devices incorporating sulfur-nitrogen chains as the molecular wire.Oakley and coworkers 18 have already proposed that thiazyl chains with combinations of electron-withdrawing and -donat- ing substituents may prove to be active for non-linear optical applications. Futhermore, it seems apparent that these chains can also fulfil a number of other criteria which provide them with properties desirable for a range of other applications; difunctional materials such as that illustrated in Fig. 7(a)have a rod-like nature and may be suitable for development as liquid crystalline materials; the compound illustrated in Fig. 7(b),in its coordinated form, would facilitate communication between multiple metal ions; and the molecule shown in Fig.7(c) illustrates one of a family of novel macrocyclic systems in which thiazyl chains link the coordinating bipyridyl functional groups. The incorporation of other heteroatoms, such as R-P, into the backbone of the chain will allow the electronic properties of the S *NbN I I I I N, 0N ‘S’ ((.1 Fig. 7 Structures of some molecules ontaining thiazyl linkages chain to be modified by fine-tuning the R-group on the chain The potential of these thiazyl-substituted derivatives has yet to be explored but opens-up fascinating new areas of chemistry coupled with the opportunity to develop new molecular materials with unusual properties. 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