A bis(1,2,3-dithiazole) charge transfer salt with 2 : 1 stoichiometry; inhibition of association and JHQHUDWLRQRIVOLSSHG VWDFNV Tosha M. Barclay,a A. Wallace Cordes,a James R. Mingie,b Richard T. Oakley*b and Kathryn E. Preussb aDepartment of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA bDepartment of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. E-mail: oakley@sciborg.uwaterloo.ca Received 30th May 2000, Accepted 6th June 2000, Published 14th June 2000 The first example of a charge transfer salt of a benzo-bridged bis(1,2,3-dithiazole) with 2 : 1 stoichiometry is reported; the steric compounds,5 these KHWHURF\FOHV DUH QRW VWURQJ GRQRUV but can nonetheless be oxidized chemically and electrochemically to stable, highly delocalized radical cations.Disproportionation energies for these radical cations, as estimated by computation and from cell potential data, are comparable to those found for [TTF]+.6 However, the design and engineering of conductive materials using these building blocks is still at an early stage. We have recently reported a variety of charge transfer salts based on 1, 3 and 4.2–4 Simple 1 : 1 salts can be generated, but the crystal structures of these reveal ionic packing with relatively weak cation–cation interactions; the materials are small bandwidth Mott-type semiconductors. More conductive salts with 3 : 2 stoichiometry, consisting bulk of the FKORULQHVDWWKHSRVLWLRQVRIWKHEHQ]HQHULQJLQGXFHVVOLSSHG VWDFNVRIdimer radical cations.In the pursuit of novel heterocyclic systems with potential applications in the design of organic metals we have prepared and characterized the bis(1,2,3-dithiazoles) 1–4.1–4 Unlike conventional tetrathiafulvalene (TTF) based anode with currents of 0.2 to 2 $LQWKHSUHVHQFHRI 0 [t-Bu4N][X] (X– = FeCl4–, GaCl4–)8 as supporting electrolyte. Smaller amounts of a second, less conductive (pressed pellet RT = 10–4 S cm–1) block-like 2 : 1 phase, were also obtained. In the case of X– = GaCl – these crystals were suitable for X-ray analysis. 2 4 2+ + Crystals of [2]2[GaCl4] belong to the monoclinic space group C2/c (see Table 1), and contain crystallographically centrosymmetric [2] + radical cations as the molecular building blocks; the GaCl – anions are located on a twofold rotation axis of the lattice.An ORTEP drawing of a single dimer radical cation is shown in Fig. 1. Within this unit the S–S and S–N bond lengths are fractionally shorter than those seen in the parent molecule 2, but the contraction is not as great as that found in the isolated or dimerized radical cation [3]+ relative to 3.3 At the same time the C–C bonds within the benzene ring of [2]2+ retain the quinoid variations which are characteristic of the parent molecule 2.2 By contrast, in the radical cation [3]+ and also the dimers [3]2 and [4]22+ the C–C bond alternation is lost.3,4 The two halves of the dimer in [2]2 are planar to within 0.03 Å, and the two planes are separated by 3.421 Å.There is a slight (lateral) slippage of the two halves, which we attribute to the need to relieve steric congestion arising from interannular Cl···Cl interactions. Thus the intermolecular Cl···Cl [3.554(2) Å] and S···S contacts [3.616(2) and 3.570(3) Å] are close to the standard Van der RI VWDFNVRIFORVHGVKHOO UDGLFDOLRQdimers and (closed 2 shell) neutral molecules, have also been prepared. In these latter structures the radical ion dimers are cofacially aligned, exhibiting no lateral displacement. In the belief that the presence of the 3,6-chlorines in 2 could act as an inhibitor to directly superimposed association, we have explored the charge transfer chemistry of this material.Here, we report the structural characterization of a charge transfer (CT) salt of 2 exhibiting a 2 : 1 stoichiometry, a ratio common in TTF based materials, but unprecedented in binary or heterocyclic CSN based systems. The crystal structure reveals formally dimeric radical ions, i.e., [2] +, in which superimposed association of the two heterocyclic rings is indeed suppressed. – 4 4–, In our initial report on the properties of 2, we prepared simple 1 : 1 salts by chemical oxidation, but these were not structurally characterized.2 Electrochemical oxidation7 on 2 in the presence of a variety of tetrahedral counterions has provided different stoichiometries and more hospitable crystal morphologies. With small and medium sized anions, e.g., BF4 , ClO –, no crystalline salts were isolated.However, when larger anions were employed, e.g., FeCl GaCl4–, crystalline material was electrodeposited. Highly conductive (pressed pellet RT >10 S cm–1) but fibrous needles of 3 : 2 salts were generated when saturated solutions of 2 in dichloroethane were oxidized at a Pt wire DOI: 10.1039/b004239k 4 CrystEngComm, 2000, 15Table 1 Crystal, data collectiona and refinement parameters for [2]2[GaCl4]b Formula MCrystal system Space group a/Å b/Å c/Å S4Cl4Ga0.5N2C6 404.99 Monoclinic C2/c 8.6809(19) 14.606(3) 20.496(6) 99.53(2) 2563.0(12) /° V/Å3Z 8 N2–C4 1.319(6), C1–C2 1.413(7), C2–C3 1.371(7), C4–C5 T/K Linear abs coeff/mm–1 Measured reflections Unique reflections Waals separation.9 Collectively these features constitute a R for merge much weaker interannular interaction than is found in Reflections in refinement I >1 (I) R(F), Rw(F)c (I) 293 2.56 3563 2251 0.030 1738 0.043, 0.049 0.034 R for I>3 a Data were collected at 293K on an Enraf–Nonius CAD-4 automated diffractometer with graphitemonochromated Mo-.radiation ( = 0.71073 A) using – 2 scans. The structure was solved by direct methods and refined by full-matrix least-squares analysis which minimized w ( ))2. 150 Parameters were refined using 1738 unique observed reflections [I (I)] to give R = 0.043 and R > 1 w = 0.049. Click here for full crystallographic data (CCDC no. 1350/21). b NRC386—PC version of NRCVAX—an interactive program system for structure analysis.11 c R = [ ||Fo| - |Fc||]/[ |Fo|]; Rw = {[ Z||Fo| – |Fc||2]/[ (w|Fo|2)]}1/2.Fig. 2 Packing diagram for [2]2[GaCl4], showing alternating layers of cations and anions. The lateral interdimer S···N� contacts are 3.148(4)Å. Click image or here for a 3D representation of the packing. Fig. 1 ORTEP drawing of a [2]2[GaCl4] unit, showing the atom numbering scheme. Bond lengths (Å): S1–S4 2.0742(19), S2–S3 2.0774(19), S1–N1 1.621(4), S3–N2 1.618(4), N1–C1 1.312(6), 1.446(7), C6–C6 1.358(7). Interannular distances (Å): S1···S3' 3.569(2), S2···S4' 3.613(2). Click image or here for a 3D representation of the [2]2[GaCl4] unit. (closed shell) radical ion dimers3,4,10 and reflects the fact that the entire [2] + assembly is nominally held together by 2 a net of one rather than two bonding electrons.+ 4]– 2 The crystal structure of [2]2[GaCl4] consists of slipped stacks of [2]2 radical cations interspersed by [GaCl anions. The columns along the right and left "sides" of Fig. 2 are related to those in the middle by a c-glide; the mirror is perpendicular to b (parallel to ac). Thus, the molecular planes in one stack are nearly perpendicular to those of neighboring stacks. Within the stacks each dimer interacts with a single neighbor on one side via S···N� contacts similar in both direction and magnitude [3.148(4) Å] to WKRVH VHHQ LQ VWDFNHG dithiadiazolyl radical ion salts.12 Fig. 3VKRZVWKH VWDFNHGDUUD\VRIdimers, and also the inregister nature of the lateral interdimer interactions.The interplanar separation between adjacent layers within the stack is 3.468 Å, i.e., close to that observed within the [2] + units themselves. However, as a result of lateral pageFig. 3 6OLSSHG VWDFNVRI>2]2[GaCl4], illustrating the in-register alignment of adjacent molecules. Click image or here for a 3D representation of the stacking. between layers within the stack, there are no close interlayer S···S or S···N contacts. As a consequence, electronic FRPPXQLFDWLRQDORQJWKH VWDFNLVH[SHFWHGWRbe small. 2 This latter observation can be used to explain the low conductivity of [2]2[GaCl4]. Thus, while the evenly spaced array of [2] + radical cations is nominally associated with 3/4-filled band structure, the narrow bandwidth arising from weak interlayer interactions is presumably insufficient to offset on-site Coulomb repulsion effects, and the material is a Mott insulator.A full analysis of the conductive and magnetic properties of [2]2[GaCl4] and related compounds will be described in a subsequent paper. In the meantime, the present results augur well for the control of both stoichiometry and structure of the charge transfer salts of 2 by means of variations in the nature of the 3,6-substituents. Similar approaches have recently been used to control the structure of the charge transfer salts of tetrathiafulvalene vinylogues.13,14 Acknowledgements We thank the Natural Sciences and Engineering Research Council of Canada and the State of Arkansas for financial support.We also acknowledge the NSERCC for a postgraduate scholarship to K.E.P. and the Department of Education for a doctoral fellowship to T.M.B. References 1 T. M. Barclay, A. W. Cordes, R. T. Oakley, K. E. Preuss and R. W. Reed, Chem. Commun., 1998, 1039; T. M. Barclay, L. Beer, A. W. Cordes, R. C. Haddon, M. I. Itkis, R. T. Oakley, K. E. Preuss and R. W. Reed, J. Am. Chem. Soc., 1999, 121, 6657. 2 T. M. Barclay, A. W. Cordes, J. D. Goddard, R. C. Mawhinney, R. T. Oakley, K. E. Preuss and R. W. Reed, J. Am. Chem. Soc., 1997, 119, 12136. 3 T. M. Barclay, A. W. Cordes, R. T. Oakley, K. E. Preuss and R. W. Reed, Chem. Mater., 1999, 11, 164. 4 T.M. Barclay, I. J. Burgess, A. W. Cordes, R. T. Oakley and R. W. Reed, Chem. 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