S N S S S N S N S S S N N N N S N S S N Cl Cl S N S 1 3 4 2b 2a S N S Cl F5 CH2CN F5 S N S Cl F5 S N S Cl F5 1/2 Ph3Sb Cl 6 5b 5a excess S2Cl2 Sterically protected 1,2,3-dithiazolyl radicals: preparation and structural characterization of 4-chloro-5-pentafluorophenyl-1,2,3-dithiazolyl Tosha M. Barclay,a Leanne Beer,b A. Wallace Cordes,a Richard T. Oakley,*b Kathryn E. Preuss,b Nicholas J.Taylorb and Robert W. Reedb a Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas, 72701, USA b Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. E-mail: oakley@sciborg.uwaterloo.ca Received (in Cambridge, UK) 22nd December 1998, Accepted 9th February 1999 Steric protection afforded by a pentafluorophenyl group at the 5-position facilitates the first isolation and structural characterization of a monocyclic 1,2,3-dithiazolyl as its S–S bonded dimer.The solid state structures of many 1,3,2-dithiazolyl radicals 1 have been established by X-ray crystallography. Dimerization through long S–S bonds is normally observed,1,2 but unassociated radicals are also known.3,4 By contrast, the solid state properties of 1,2,3-dithiazolyls 2 have remained elusive, as have the structural and chemical implications of the two possible resonance formulations 2a and 2b. Recently, however, we demonstrated that the reduction of the 4,5-dichloro-1,2,3-dithiazolylium chloride (Appel’s salt5) leads to C–C coupling at the 5-position, with the eventual formation of the tetrathiadiazafulvalene 3.6 This tendency to associate through carbon (resonance structure 2b), and hence rearrange, illustrates one of the fundamental difficulties associated with the use of 1,2,3-dithiazolyls as building blocks for molecular conductors.7 In our efforts to overcome this problem we have shown that delocalization of spin density away from the 5-position, as in the fused ring derivative 4, suppresses C–C association.8 We are now investigating the effect of sterically bulky substituents on radical association, and have found that the attachment of a pentafluorophenyl group at the 5-position provides an effective block to C–C bond formation. This has allowed us to isolate and characterize structurally, for the first time, a simple monocyclic 1,2,3-dithiazolyl radical as its S···S bonded dimer. The synthesis of the title compound 5 involved the preparation of the corresponding dithiazolylium cation (Scheme 1) by the reaction of pentafluorophenylacetonitrile (2.6 g, 0.125 mmol) with excess (5 ml) S2Cl2 in 10 ml CH2Cl2 containing Bun 4NCl (0.20 g).After 10 days a canary yellow solid (2.7 g, 0.079 mmol, 63%) was filtered off and recrystallized from hot MeCN to afford orange needles of 6, mp 211–213 °C.The reduction of 6 (0.68 g, 2.0 mmol) to 5 was very sensitive to the reaction temperature and the nature of the reducing agent. The best results were obtained using Ph3Sb (0.38 g, 1.06 mmol) as reductant, and the reaction was performed in liquid SO2 (5 ml) at 270 °C. After 5 min the solvent was evaporated from the still cold mixture to leave a black matrix, from which 5 (0.17 g, 0.54 mmol, 27 %) could be sublimed, at 45 °C/1022 Torr, as lustrous green–black blocks, mp > 52 °C (decomp.).† Compound 5 dissolves in CH2Cl2 at room temperature to afford bright yellow solutions which exhibit a strong and persistent EPR signal (g = 2.0089) characteristic of a simple 1,2,3-dithiazolyl radical.9,10 In addition to the expected triplet structure arising from hyperfine coupling to nitrogen, the spectrum (Fig. 1) also displays rich secondary structure arising from coupling to the 4-chlorine and to all five fluorines on the 5-C6F5 group. The aN value (0.61 mT) is consistent with that expected for a simple 1,2,3-dithiazolyl. Cyclic voltammetry on solutions of 5 in MeCN (Pt electrodes, Bun 4NPF6 supporting electrolyte) reveals a reversible oxidation wave with E1 2(ox) = 0.38 V (vs.SCE) and an irreversible reduction wave with a Epc (red) of 21.1 V (vs. SCE). The E1 2(ox) potential is more anodic than that for benzodithiazolyls,8 as expected from the more localized electron distribution in the monocyclic derivative. Indeed the potential is similar to those observed in 1,2,3,5-dithiadiazolyl radicals,11 where spin density is formally restricted to the five-atom CN2S2 ring.The crystal and molecular structures‡ of both 5 and 6 have been established by X-ray crystallography. The structure of the salt 6 (Fig. 2) consists of ion pairs in which the S–S bond of the cation is bridged by a chloride anion, with S···Cl contacts of 2.8947(16) and 2.9617(18) Å.This arrangement is reminiscent of that observed in the halide salts of 1,2,3,5-dithiadiazolylium cations.12 The mean plane of the heterocyclic ring makes a dihedral angle of 68.6° with the plane of the C6F5 group. The internal structural parameters are typical of a 1,2,3-dithiazoly- Scheme 1 Fig. 1 EPR spectrum (293 K, CH2Cl2, SW = 4 mT, g = 2.0089) of radical 5. Hyperfine coupling constants (derived by simulation with Bruker Simfonia) are aN = 0.610, aF = 0.175 (2 F), 0.038 (1 F), 0.031 (2 F), aCl = 0.1050 (37Cl), 0.0875 (35Cl) mT.Chem. Commun., 1999, 531–532 531S N S R Cl S S N R Cl S N S Cl S N S Cl S S N Cl R R S N S Cl R R S N S Cl S N S Cl R R 7c 7a 7b 7d lium salt.13,14 In the structure of 5 the plane of the C6F5 group is again twisted (by 58.3°) away from the plane of the C2S2N ring, thereby providing the desired sterically protected ‘pocket’ for the 5-carbon.The radicals are nonetheless dimerized (Fig. 3). The two halves of the dimer are related by a two-fold axis, and are linked by a single interannular S···S contact of 3.2987(13) Å. This interaction is longer than the corresponding S···S interactions found in any known dimer of 1, and stretches even further the conventional limit of a covalent S–S bond.15 Taken collectively, the changes observed in the internal bond lengths (Fig. 2 and 3) upon reduction of the 6p-cation to the 7pradical dimer are consistent with the bonding properties of the singly occupied molecular orbital of the radical.15 The mode of association of 5 provides an interesting lesson regarding the interplay of steric and electronic requirements.Dimerization cannot occur at the 5-carbons, for steric reasons, and so occurs through the 1-sulfurs, i.e. 7a, as a result of which these atoms become hypervalent (with a SNC double bond). This arrangement is electronically less favored than dimerization at the 2-position, i.e. 7b, which generates a hypervalent sulfur with a SNN double bond, or the more delocalized fourcentre modes 7c and 7d, which are reminiscent of the cofacial dimers commonly observed for 1,2,3,5-dithiadiazolyls.16 Structure 7a is, however, the only cofacial dimerization mode that precludes Cl/R or Cl/Cl and R/R steric interactions.The steric protection afforded by the C6F5 groups also affects the packing of dimers.Thus, instead of forming p-stacks, the dimers are clustered about a �4 site, with the dimers linked by S···SA contacts of 3.7286(9) Å. We thank the NSERC and the State of Arkansas for financial support. We also acknowledge the NSERC for a post-graduate scholarship to K. E. P. Notes and references † Satisfactory chemical analysis were obtained for compounds 5 and 6. ‡ Crystal data for 5 and 6: Data were collected at 293 K on Siemens P4 (5) and Enraf-Nonius (6) automated diffractometers with graphite-monochromated Mo-Ka radiation (l = 0.71073 Å) using omega (5) and q–2q (6) scans.The structures were solved by direct methods and refined by fullmatrix least-squares analysis which minimized Sw(DF)2. Compound 5: ClS2F5NC8, M = 304.66, tetragonal, space group I�42d, with a = 13.8634(8), c = 21.0268(4) Å, V = 4041.2(4) Å3, Z = 16, Dc = 2.00 g cm23, m = 0.82 mm21. 155 Parameters were refined using 1622 unique observed reflections [I > 0.0 s(I)] to give R = 0.029 and Rw = 0.033. Compound 6: Cl2S2F, M = 340.11, monoclinic, space group P21/n, with a = 12.334(2), b = 6.0917(17), c = 15.947(3) Å, b = 110.382(16)°, V = 1123.4(4) Å3, Z = 4, Dc = 2.01 g cm23, m = 0.98 mm21. 163 Parameters were refined using 1502 unique observed reflections [I > 2.0 s(I)] to give R = 0.044 and Rw = 0.050. CCDC 182/1168. 1 G. Wolmershäuser and G. Kraft, Chem. Ber., 1990, 123, 881; G. Heckmann, R. Johann, G. Kraft and G. Wolmershäuser, Synth. Met., 1991, 41–43, 3287. 2 E. G. Awere, N. Burford, R. C. Haddon, S. Parsons, J. Passmore, J. V. Waszczak and P.S. White, Inorg. Chem., 1990, 29, 4821; G. Wolmershäuser, M. Schnauber and T. Wilhelm, J. Chem. Soc., Chem. Commun., 1984, 573. 3 G. Wolmershäuser and R. Johann, Angew. Chem., Int. Ed. Engl., 1989, 28, 920. 4 T. M. Barclay, A. W. Cordes, N. A. George, R. C. Haddon, M. E. Itkis, M. S. Mashuta, R. T. Oakley, T. T. M. Palstra, G. W. Patenaude, R. W. Reed, J. F. Richardson and H.Zhang, Chem. Commun., 1997, 873; T. M. Barclay, A. W. 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Haddon and R. T. Oakley, in The Chemistry of Inorganic Ring Systems, ed. R. Steudel, Elsevier, Amsterdam, 1992, p. 295; A. J. Banister and J. M. Rawson, in The Chemistry of Inorganic Ring Systems, ed. R. Steudel, Elsevier, Amsterdam, 1992, p. 295; A. J. Banister and J. M. Rawson, Adv. Heterocycl. Chem., 1995, 62, 137. Communication 8/09951K Fig. 2 PLUTO drawing of the cation/anion packing in the chloride salt of 6. Bond distances within the heterocyclic ring are: d(S–S) 2.0284(4), d(S–N) 1.605(4), d(S–C) 1.673(4), d(N–C) 1.309(5), d(C–C) 1.393(2) Å. Fig. 3 PLUTO drawings of the radical dimer of 5, showing the orientation of the overlap of the heterocyclic rings (A) and the interannular S···S contact (B). Bond distances within the heterocyclic ring are: d(S–S) 2.0717(8), d(S– N) 1.639(2), d(S–C) 1.724(2), d(N–C) 1.317(3), d(C–C) 1.390(3) Å. 532 Chem. Commun., 1999, 531–532