101
Acc. Chem. Res. 1994,27, 101-108
Rings, Radicals, and Synthetic Metals: The Chemistry of SNS+ SIMON PARSONS+ AND JACK PASSMORE* Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, E3B 6E2 Canada Received July 16, 1993
The ONO+cation was first identified in ONO[ClO4] and in solutions of concentrated HN03 and H2S04 (used for the nitration of benzene) by Ingold and co-workers in 1946.' SNS+,the sulfur-containing analog of ONO+, was first prepared and identified by Gillespie and coworkers in 19782 and might have been expected to possess a rather similar chemistry. In fact SNS+ chemistry is substantially different, and in this Account we give a short review of the synthesis, characterization, and known reactions of salts of SNS+. Our understanding of sulfur-nitrogen chemistry has grown substantially over the last 15 yearsS3In the past, developments in this area were hampered by the baffling nature of the chemistry of many binary sulfur-nitrogen compounds, their frequent thermal instability, and the lack of a convenient spectroscopic probe. With today's ready access to X-ray crystallography, multinuclear NMR, ESR, and molecular orbital calculations,progress has become much more rapid. Nevertheless, in spite of several notable and elegant exceptions,4 much of sulfur-nitrogen chemistry remains nonquantitative, mechanistically obscure, and, when viewed in the context of modern organic chemistry, poorly understood. Other than SN+,SNS+is the simplest isolable binary sulfur-nitrogen species. In contrast to neutral binary S-N compounds (e.g., A&[S4N4] = 469 kJ mol-'), it is thermally stable (PHf[SNSAsF61= -1413.8 kJ mol-')? and its reaction chemistry is quantitative and relatively easy to understand. We have shown,6forexample, that 7
its reactions with nitriles and alkynes (forming RCN1
n
SNS+ and RCSNSCR+ salts, respectively, specific examples of the generalized cycloaddition with the unsaturated entity XY shown in eq l),are among the simplest cycloadditions known and constitute a textbook example of the importance of orbital interactions in cycloaddition reactions.
Discovery of SNS+ Interest in SNS+chemistry at the University of New Brunswick was initiated by a discussion in 1977 between Arthur Banister of Durham University and one of us Simon Parsonswas bom in Huntingdonshire, England, In 1965. Afterreceiving his B.Sc. from the University of Durham In 1987, he took his Ph.D. as a CommonwealthScholar at the Universtty of New Brunewlckunder thesupervlaion of Prof. Passmore. Havlng pursued postdoctoral research as a SERC fellow wlth Dr. A. J. Downs at the University of Oxford, he has recently moved to the Universityof Edlnburghto perform postdoctoralresearchIn X-ray crystallography wlth Drs. A. J. Blake and R. 0. Gould. Jack Passmore was born In Barnstaple, Devon, England, and recelved his B.Sc. and D.Sc. from the University of Bristol. England, and hls W.D. &qee (with Dr.Nell Bartlett) from the University of Britlsh Columbia In Vancouver. He did postdoctoral work durlng the year 1968-1969 at MacMaster University, Hamliton, Ontarlo, Canada (with Dr. Ronald J. Qlllesple). He then joined the faculty at the University of New Brunswlck in 1969, where he Is presently Professor of Chemistry.
HI
x-Y
(J.P.), over the nature of the long sulfur-sulfur interacn
tions in Sa2+and between SNSNS*+units in S6N42+ (Figure la,b). We concluded that as preparative chemists we would contribute toward a solution to the problem by preparing S7N+, which is isovalent with Sa2+(cf. the structural and electronic congruence of cyclic S42+ and S2N2). A NATO collaborative grant was subsequently obtained to undertake the project, and we began a fruitful collaboration.
L
J
C.
R
R
Figure 1. SomeA*-+ complexes;see ref 7b. (a) Ss2+,illustrating transannular r*-r* bond formation: S-S bond lengths 2.83, 2.94, and 3.01 A (AsFe-salt). (b) (S3N$+)z,illustratin inter-ring ?r*-r*bond formation: S . 4 bond length ca. 3.00 . (c) The mechanism of the photochemically allowed rearrangement of
fi
n n RCNSNS' to RCNSSN' via the
A*-A*
complex ( R m ) 2 .
Previous results from our laboratory had shown that Sa2+reacted quantitatively with KI to give &I+, KAsF6, and 1/gSg,7a and so Richard Hey, a graduate student from Banister's group, while at the University of New Brunswick working on the joint project, attempted the analogous reaction of Ss(AsF6)2and NaN3, in a 1:l mole ratio. He obtained not the expected S7NAsF6, but yellow SNSAsF6 in about 20 % yield, which we identified + Present address: Department of Chemistry,University of Edinburgh, King's Building, West Mains Road, Edinburgh EH9 355,Scotland. (1)Gillespie, R. J.; Graham, J.; Hughes, E. D.;Ingold, C. K.; Peeling, E. R. A. Nature 1946,158,480. Goddard, D. R.; Hughes, E. D.; Ingold, C. K. Nature 1946,158,480. Ingold, C. K.;Millen, D. J.; Poole, H. G . Nature 1946,158,480. (2)Faggiani, R.; Gillespie, R. J.; Lock, C. J. L.; Tyrer, J. D. Inorg. Chem. 1978,17,2975. (3)Chivers, T.Chem. Rev. 1985,85,341. Oakley, R. T.B o g . Inorg. Chem. 1988,36,299. (4)For example: Bestari, K. T.; BoerB, R. T.; Oakley, R. T. J. Am. Chem. SOC.1989,111,1579. (5)OHare, P. A. G.; Awere, E. G.;Parsons, S.; Passmore, J.J. Chem. Thermodyn. 1989,21,153. (6)Parsons, S.;Passmore, J.; Schriver, M. J.;Sun,X. Inorg. Chem. 1991,30,3342.
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Vol.27,No.4,1994
by a comparison of its IR spectrum to that of SCS and AsFe-. S6(AsF,), + NaN,
-
SNSAsF, + N, 4- NahF,
+ ,/$, (2)
As it happened, the very next day a paper by Gillespie appeared in Inorganic Chemistry2 describing the synthesis of SNSSbCl6, presumably in low yield, by the reaction of STNH with SbCIS, and its characterization by vibrational spectroscopy and X-ray crystallography. This might have been the end of the road in terms of our interest in SNS+were it not for roughly concurrent discoverythat trace amounts of bromine facilitated the oxidation of Sa to S42+by AsF5 (without traces of Brz, the oxidation proceeds only t o Sg2+).7a3d In the light of this, the reaction of sg, S4N4,and AsF5 in the presence of a trace of bromine appeared to be a possible route to large quantities of SNS+ in high yield. A major breakthrough in the chemistry of SNS+ was made at UNB by Greg MacLean, who showed&that this reaction proceeded as anticipated (eq 3), and since then we have used this “one pot” synthesis to obtain over 10 g (ca. 40 mmol) of pure, yellow, crystalline SNSAsF6 in over 90 5% isolated yield.8bThis reaction has been shown to
and toxic AsF5. In addition,” SNSAsF6 has been prepared in a two-step synthesis in 90% overall yield, according to eqs 10a,b, providing a safer alternative to eq 3. Equation 10b has also been applied to the (NSCl), + 3SC1,
-
+ BAgAsF,
3AgC1+ 3N(SC1,)AsF6 (loa)
3C1SNSClAsF6+ 3SnC1,
3SNSAsF, + 3SnC1, (lob)
syntheses of other SNS+salts including AlCb-, FeCb-, CF3S03-, and SbCb-, although these tend to be insoluble, and/or their reactions complicated, possibly as a result of anionic participation.9
Characterization of the SNS+Cation
X-ray crystal structures have been determined for SNSAsFs,loaSNSSbCh2,and SNSA1C4,loband all show discrete, linear SNS+ cations, consistent with the VSEPR model. The SN bond length does not vary significantly between the salts when librational effects are taken into account, and their average (1.51 A) is consistent with a formal SN bond order of 2. Empirical measures of the SN bond strength, such as the force constant (8.256mdyn A-l)I1 and the bond energy term S, + 2S4N4+ 12AsF, 8SNSAsF, + 4AsF, (473 kJ mol-’, obtainedlZfollowing the determination of A&[SNSAsF6] (-1413.8 kJ mol-’ by fluorine comAH = -418 kJ mol-’ (3) bustion calorimetryS),similarly attest to the importance proceedsbvia the rapid formation of S2+, which then of 7~ bonding, and this is confirmed by MO calculations reacts with S4N4 to give (S~NZ)~(ASF~)Z, which is at all levels of theory. While this suggests that SNS+ subsequently oxidized in solution to SNS+ and SN+ can reasonably be represented by the resonance form (although in the solid state this exists as S~NZ(ASF~)Z;S=N+=S, anionic contacts in these salts are formed see below). The SN+ formed reacts rapidly with s8 to exclusivelyto the sulfur atoms. Presumably the positive give more SNS+: charge is delocalized onto the sulfur atoms in SNS+ through polarization effects, although the weakness of so2 the contacts implies that this may not be extreme as S, + 6AsF5 2S4(ASF6)2 + 2AsF, (4) trace Brz indicated by SCF calculations (CNDO and STO-3G
