Preparation and X-ray crystal structures of the first radical cation salts of 4-iodotetrathiafulvalene: [ITTF .+]2 {Pd[S2C2(CN)2]2}2- and ITTF .+HSO4- Andrei S. Batsanov,a Adrian J. Moore,a Neil Robertson,b Andrew Green,a Martin R. Bryce,*a Judith A. K. Howarda and Allan E. Underhill*b aDepartment of Chemistry, University of Durham, Durham, UK DH1 3L E bDepartment of Chemistry, UCNW Bangor, Bangor, Gwynedd, UK L L 57 2UW The title radical ion salts of 4-iodotetrathiafulvalene 1 have been prepared and their X-ray crystal structures determined at 150 K.The 251 salt [ITTF.+]2 {Pd[S2C2(CN)2 ]2}2- 3 forms a mixed stack structure in which the Pd(mnt)2 anions intermingle with pairs of iodo-TTF radical cations, i.e. a DDADDA stacking arrangement. The structure of the 151 salt ITTF.+ HSO4- 4 comprises stepped stacks of ITTF cations with two alternating modes of overlap.Between the cation stacks there are infinite chains of hydrogensulfate anions, linked by hydrogen bonds. Both structures are characterised by short intermolecular contacts involving the iodine substituent. Within the field of molecular conductors,1 the control of a perchlorate salt by electrocrystallisation in acetonitrile in the presence of tetrabutylammonium perchlorate as supporting intermolecular interactions in the solid state by chemical modification is a challenging topic, and the pivotal role played electrolyte.In these experiments the HSO4- anion must be derived from residual sulfuric acid which had been used for by chalcogen atoms is well established.It is widely recognised within solid-state chemistry that halogen atoms can participate washing the electrodes prior to the electrocrystallisation. in relatively strong and directional intermolecular interactions, thereby providing an effective means of ‘crystal engineering’.2 In this context, halogenated derivatives of tetrathiafulvalene (TTF), ethylenedithio-TTF (EDT-TTF) and ethylenedithiodiselenadithiafulvalene (EDT-DSDTF) are emerging as new p-electron donors in the search for TTF-based radical cation salts and charge-transfer complexes which possess increased dimensionality.3 Although a range of halogenated derivatives of TTF have been synthesised in several laboratories,4 to date, only one radical cation salt has been well characterised, viz.an insulating 151 iodide salt of tetraiodo-TTF, the X-ray crystal structure of which has been reported recently.4e The X-ray crystal structures of a few radical ion salts of halogenated EDT-TTF and EDT-DSDTF derivatives are known,5 and some are molecular semiconductors {e.g.a [Pd(dmit)2]5a and a ClO4 salt}5c and these structures are characterised by relatively strong interactions between the halogen substituent and the anions.It is known that halogenation of the TTF skeleton raises the oxidation potential significantly4 (as would be expected The asymmetric unit of the crystal structure of salt 3 for the attachment of electron-withdrawing substituents). comprises one ITTF.+ radical cation (in a general position) Therefore, we directed our attention to monohalogeno-TTFs,4b and a half of a [Pd(mnt)2]2- anion located at an inversion and we considered iodine to be the most promising substituent centre (Fig. 1, Table 1); both moieties are nearly planar. The for participating in intermolecular interactions, as it is more polarisable than the other halogens. Herein we report the preparation and X-ray crystal structures of the first radical cation salts of 4-iodo-TTF 1.Results and Discussion 4-Iodo-TTF 1 was synthesised by halogenation of 4-tetrathiafulvalenyllithium using perfluorohexyl iodide, as reported by Becker et al.4c This halogenating reagent is more efficient than tosyl iodide which we had used previously.4b The 251 salt [ITTF.+]2 {Pd[S2C2(CN)2]2}2- 3 was obtained as black crystals by mixing compound 1 and K+{Pd[S2C2(CN)2]2}- 2 in acetone.The 151 salt ITTF.+HSO4- 4 was obtained by electrochemical oxidation of donor 1 under constant current in acetonitrile containing sulfuric acid. Remarkably, the same Fig. 1 ITTF and Pd(mnt)2 ions in the crystal structure of 3, projected on their planes hydrogensulfate salt 4 was obtained during attempts to obtain J. Mater. Chem., 1997, 7(3), 387–389 387Table 1 Selected bond distances (A° ) in the structures of 3 and 4 3 3 4 PdMS(5) 2.302(4) C(1)MC(4) 1.38(2) 1.392(7) PdMS(6) 2.288(4) C(1)MS(1) 1.72(1) 1.731(5) S(5)MC(7) 1.72(2) C(1)MS(2) 1.74(2) 1.735(5) S(6)MC(8) 1.73(2) S(1)MC(2) 1.77(2) 1.738(5) C(7)MC(8) 1.39(2) S(2)MC(3) 1.73(2) 1.735(5) 4 C(2)MC(3) 1.35(2) 1.344(7) S(5)MO(1) 1.443(4) C(2)MI 2.06(2) 2.083(5) S(5)MO(2) 1.519(4) C(4)MS(3) 1.72(2) 1.723(5) S(5)MO(3) 1.521(3) C(4)MS(4) 1.737(14) 1.724(5) S(5)MO(4) 1.444(3) S(3)MC(5) 1.74(2) 1.724(5) C(5)MC(6) 1.34(2) 1.337(8) Fig. 4 Crystal structure of 4, showing hydrogen bonds (dashes), the radical cations form a dimer with an interplanar separation of disorder of the H atom in the O(2)HO(2¾) bond, and short 3.4 A° . These dimers intermingle with [Pd(mnt)2]2- anions to contacts (dots) form a mixed DDADDA stack parallel to the [1 1� 0] crystallographic direction (Fig. 2). The cation and anion planes in the stack form a dihedral angle of 9° with an average interplanar separation of 3.6 A° . Short contacts (S,S 3.67–3.73 A° and S,I cations form a stair-like stack with two alternating kinds of 3.81 A° ) between cations and anions of different stacks join the overlap: (i) between two TTF moieties with a lateral shift of stacks into layers parallel to the (0 0 1) plane (Fig. 3). A nearly ca. 0.5 A° , and (ii) between substituted dithiole rings only; the linear CMI,NMC interlayer contact [I,N(2) 3.04 A° ] is interplanar separations are 3.33 and 3.40 A° , respectively. remarkably shorter than the sum of van der Waals radii Parallel to this stack, i.e.in the direction of the crystallographic (3.65 A° ),6 even after the correction for asphericity of the iodine axis y, runs a chain of hydrogensulfate anions, linked by strong atom (3.36 A° ).7 hydrogen bonds O(2),O(2¾) 2.555(7) and O(3),O(3¾) The asymmetric unit of salt 4 comprises an ITTF.+ radical 2.627(7) A° . Both of these bonds are crystallographically centro- cation and a HSO4- anion (Fig. 4, Table 1). Almost planar symmeric. In the former bond the hydrogen atom was found to be disordered over two positions, corresponding to (asymmetric) distances OMH 0.8(1) and H,O 1.9(1) A° . In the latter bond the only peak of electron density was located at the inversion centre, implying a truly symmetrical bond. Although both H atoms were successfully refined, the reliability of this result is limited.The anion–cation contacts I,O(4) 2.92 (CMIMO 168°) and S(3),O(2) 2.82 A° are substantially shorter than the sums of the van der Waals radii (3.5 and 3.2 A° , or if corrected for ellipsoidal shape of the I and S atoms, 3.16 and 3.00 A° , respectively),6,7 and imply significant polarisation of the ‘soft’ I or S atoms. On the other hand, the H(3),O(1) and H(6),O(1) contacts of 2.19(5) and 2.24(6) A° can be regarded as hydrogen bonds.Thus, the anionic chain contributes significantly to the close packing of cations. The geometry of the ITTF moieties in 3 and 4 clearly characterises them as radical cations. The elongation of the central CNC bond distance is usually the most sensitive indicator of the degree of charge transfer.These bonds in 3 [1.38(2) A° ] and 4 [1.392(7) A° ] are significantly longer than in neutral 1 [1.34(1) A° ]4c and in two polymorphs of pure TTF [1.349(3)8a and 1.337(4)8b A° ]. They are also marginally longer than in radical cation salts with ‘soft’ anions, such as I4TTF.+I- [1.369(4) A° ],4e and close to the distances in the salts with strongly electronegative counter ions and complete charge Fig. 2 Stack of ions in the structure of 3 transfer (1.39–1.40 A° ).9 A linear relation b=1.757-0.0385d has been suggested recently10 between the mean length (b) of the four CMS bonds adjacent to the central CNC bond and the degree of charge transfer d. For b=1.730(8) A° in 3 a 1.728(5) A° in 4, this formula gives d=0.7(2) and 0.75(13), respectively.Both salts 3 and 4 exhibit low conductivity values [srt= 2×10-6 S cm-1 (four-probe compressed pellet measurement) and 5×10-7 S cm-1 (two-probe, single-crystal measurement) respectively]. Preliminary static susceptibility data on a small polycrystalline sample of salt 3 over the temperature range 300–4 K suggest that the material behaves as a one-dimensional Heisenberg antiferromagnet with an isotropic nearest neighbour exchange interaction, J#50 K, consistent with the Bonner–Fischer model.11 These magnetic data are qualitatively similar to those of many linear chain charge-transfer com- Fig. 3 Interstack contacts in the structure of 3; projection on the (1 1� 0) plane pounds studied previously,12 including the salt I4TTF.+ I-.4e 388 J. Mater. Chem., 1997, 7(3), 387–389Table 2 Crystal data Conclusions compound 3 4 The first radical ion salts of the electron donor molecule ITTF formula C20H6I2N4PdS12 C6H4IO4S5 1 have been prepared, and the X-ray crystal structures of the M 1047.21 427.29 title salts establish that the iodine substituent participates to a symmetry triclinic triclinic significant extent in intermolecular interactions.These results, a/A° 7.667(1) 8.145(1) combined with those recently obtained by other workers,4e,5a–c b/A° 8.577(1) 8.242(1) c/A° 11.638(2) 9.916(1) auger well for the use of halogenated TTF derivatives in the a/degrees 74.42(1) 99.17(1) synthesis of new charge-transfer materials, in which the solid b/degrees 88.38(1) 100.98(1) state structure can be modified by intermolecular and c/degrees 86.64(1) 104.75(1) interstack interactions involving polarisable halogen atoms.U/A° 3 735.9(2) 616.5(1) space group P 1� P 1� Z 1 2 Experimental m/cm-1 36.0 34.4 Dc/g cm-3 2.36 2.30 Preparation of [ITTF·+]2 {Pd[S2C2(CN)2]2}2- 3 crystal size/mm 0.02×0.2×0.25 0.11×0.2×0.3 2hmax/degrees 50.5 51 Method 1. 4-Iodo-TTF 14c (7 mg, 0.021 mmol) and data total 3126 2718 K+{Pd[S2C2(CN)2]2}- 2 (15 mg, 0.035 mmol) were each data unique 2227 1948 dissolved in separate portions of dry acetone (10 ml) and data observed, I>2s(I) 1745 1909 placed in the outer compartments of a three-compartment Rinta 0.101, 0.060 0.092, 0.027 transmission min, max 0.67, 1.00 0.46, 0.85 diffusion cell.The central section was filled with dry acetone no. of variables 178 162 (10 ml) and separated from the outer compartments by porous wR(F2), all data 0.223 0.081 glass frits.After 13 days, black crystals of complex 3 (2 mg, goodness-of-fit 1.56 1.12 18%) suitable for X-ray analysis were collected from the central R(F), obs. data 0.078 0.029 compartment and washed with acetone. Drmax/e A° -3 2.4 0.79 Drmin/e A° -3 -2.2 -1.03 Method 2. 4-Iodo-TTF 1 (13 mg, 0.038 mmol) and aBefore and after the absorption correction.K[Pd(mnt)2] (28 mg, 0.068 mmol) were each dissolved in dry acetone (10 ml) and the solutions mixed, affording immediately We thank the EPSRC for funding this work and Dr T. Rogers a black precipitate which was collected by filtration, washed for the magnetic data on salt 4. with cold acetone and dried to afford complex 3 (8 mg, 40%) as a fine black powder.Analysis: found C, 22.80; H, 0.90; N, References 5.38; S, 36.51; C20H6I2N4PdS12 requires C, 22.84; H, 0.57; N, 1 Reviews: (a) M. R. Bryce, Chem. Soc. Rev., 1991, 20, 355; (b) A. E. 5.35; S, 36.71%. Underhill, J. Mater. Chem., 1992, 2, 1; (c) J. Mater. Chem., Special Issue on Molecular Conductors, 1995, 5(10), 1469–1760. 2 G. R. Desiraju, Crystal Engineering, T he Design of Organic Solids, Preparation of ITTF·+HSO4- 4 Elsevier, Amsterdam, 1989, ch. 6. 3 For reviews which focus on increased dimensionality in TTF mate- 4-Iodo-TTF 1 (11 mg, 0.033 mmol) was dissolved in dry, rials, see: (a)M. Adam and K. Mu�llen, Adv.Mater., 1994, 6, 439; (b) degassed acetonitrile (25 ml) containing 0.02 ml of concen- M. R. Bryce, J.Mater. Chem., 1995, 5, 1481.trated sulfuric acid and placed in the anode compartment of a 4 For leading references, see: (a) M. Jorgensen and K. Bechgaard, 50 ml H-shaped electrocrystallisation cell. In the cathode com- Synthesis, 1989, 207; (b) M. R. Bryce and G. Cooke, Synthesis, partment, separated from the anode compartment by a porous 1991, 263; (c) C. Wang, A. Ellern, V. Khodorkovsky, J.Bernstein glass frit, was placed dry, degassed acetonitrile (25 ml) contain- and J. Y. Becker, J. Chem. Soc., Chem. Commun., 1994, 983; (d) C. Wang, J. Y. Becker, J. Bernstein, A. Ellern and ing 0.02 ml of concentrated sulfuric acid. A constant current V. Khodorkovsky, J.Mater. Chem., 1995, 5, 1559; (e) R. Gompper, of 1 mA was passed through the cell for 10 days. Black crystals J. Hock, K.Polborn, E. Dormann and H. Winter, Adv. Mater., of complex 4 (10 mg, 71%) were harvested from the anode, 1995, 7, 41. and were also collected from the walls of the anode compart- 5 (a) T. Imakubo, H. Sawa and R. Kato, J. Chem. Soc., Chem. ment. Analysis: found C, 17.01; H, 0.85; S, 35.32; C20H6N4S12I2 Commun., 1995, 1097; (b) T. Imakubo, H. Sawa and R. Kato, J. Chem. Soc., Chem. Commun., 1995, 1667; (c)M.Iyoda, H. Suzuki, requires C, 16.86; H, 0.94; S, 37.47%. S. Sasaki, H. Yoshino, K. Kikuchi, K. Saito, I. Ikemoto, H. Matsuyama and T. Mori, J.Mater. Chem., 1996, 6, 501. X-Ray crystallography 6 A. Gavezzotti, J. Am. Chem. Soc., 1983, 105, 5220. 7 S. C. Nyburg and C. H. Faerman, Acta Crystallogr., Sect. B, 1985, Single-crystal X-ray diffraction experiments for 3 and 4 41, 274. 8 (a)W. F. Cooper, J. W. Edmonds, F.Wudl and P. Coppens, Cryst. were carried out at T=150 K on a Siemens three-circle Struct. Commun., 1974,3, 23; (b) A. Ellern, J. Bernstein, J. Y. Becker, diffractometer, equipped with a CCD area detector (graphite- S. Zamir, L. Shahal and S. Cohen, Chem.Mater., 1994, 6, 1378. monochromated Mo-Ka radiation, l=0.71073 A° , v-scan 9 (a) P.Batail, C. Livage, S. S. P. Parkin, C. Coulon, J. D. Martin mode, semi-empirical absorption correction on Laue equiva- and E. Canadell, Angew. Chem., Int. Ed. Engl., 1991, 30, 1498; (b) lents) and an Oxford Cryosystems open-flow N2 gas cryostat. P. Erk, S. Hu�nig, G. Klebe, M. Krebs and J. U. von Schu�tz, Chem. The structures were solved by Patterson (3) and direct (4) Ber., 1991, 124, 2005; (c) G.Matsubayashi, K. Ueyama and T. Tanaka, J. Chem. Soc., Dalton T rans., 1985, 465; (d) methods and refined by full-matrix least squares against F2 of T. Iamakubo, H. Sawa and R. Kato, J. Chem. Soc., Chem. all data, using SHELXTL software.13 Non-H atoms were Commun., 1995, 1097. refined anisotropically; all H atoms in 4 were refined in 10 D. A. Clemente and A. Marzotto, J. Mater. Chem., 1996, 6, 941. isotropic approximation, in 3 were treated as ‘riding’. Crystal 11 J. C. Bonner and M. E. Fischer, Phys. Rev. A, 1964, 135, 640. data and experimental details are listed in Table 2; atomic 12 (a) J. B. Torrance in L ow-Dimensional Conductors and Superconductors, ed. D. Je�rome and L. G. Caron, NAT O ASI Ser. coordinates, thermal parameters and bond lengths and angles B, 1987, 165, 113; (b) S. D. Obertelli, R. H. Friend, D. R. Talham, have been deposited at the Cambridge Crystallographic Data M. Kurmoo and P. Day, J. Phys., Condens. Matter, 1989, 1, 5671. Centre (CCDC). See Information for Authors, J. Mater. Chem., 13 G. M. Sheldrick, SHELXTL, Version 5, Siemens Analytical X-Ray 1997, Issue 1. Any request to the CCDC for this material Instruments Inc., Madison, WI, USA, 1995. should quote the full literature citation and the reference number 1145/27. Paper 6/06829D; Received 7th October, 1996 J. Mater. Chem., 1997, 7(3), 387