MATERIALS CHEMISTRY COMMUNICATIONS Charge-transfer complex and radical cation salt of a new donor EDT-TTFCl,: unique conductivities and crystal structures Masahiko Iyoda,"" Hironori Suzuki," Shigeru Sasaki," Harukazu Yoshino," Koichi Kikuchi," Kazuya Saito," Isao Ikemoto," Haruo Matsuyama" and Takehiko Morib "Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan bDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152, Japan X-Ray structural analyses of the charge-transfer complex (EDT-TTFCI,), *TCNQF4, 2, and the radical cation salt EDT-TTFC1, -ClO,, 3, revealed an interesting effect of chlorine substitution on the crystal stacking mode; the electrical conductivity of 2 is unusually high, although it has a mixed-s tacking structure. Recently, weak intermolecular interactions have been recog- nized as one of the most important factors for deciding crystal structures.' In particular, the Cl...Cl interaction has been investigated extensively, because the Cl-..Cl npn-bonded con- tacts, which are sometimes very short (3.2-3.6 A),are estimated as ca.3% of a covalent bond., In addition, the substitution of the chlorine atom on a TC donor may give rise to a small dipole moment from the donor moiety to the chlorine substituent, although the electronegativity of chlorine decreases the donor properties, thus causing difficulties in forming molecular conductor^.^ In a previous paper,, we reported the synthesis and crystal structure of the dichlorinated ethylenedithiotetrathiafulvalene (EDT-TTFCl,), 1, which shows a centrosymmetric head-to- tail stacking. Although there is no strong Cl...Cl interaction in the crystals of 1, some intermolecular S..-S,Cl.-.S, Cl..-C and Cl..-Cl contacts can be detected. Thus, these observations prompted us to prepare single crystals of the charge-transfer complexes or radical cation salts of 1.The first oxidation potential (0.68 V), of 1 is a little higher than that (0.52 V) of bisethylenedithiotetrathiafulvalene (BEDT-TTF). Therefore, 1 and 2,3,5,6-tetrafluoro-p-tetra-cyanoquinodimethane (TCNQF,) can be expected to be a suitable pair for making a charge-transfer complex. A warm solution of 1 in benzene was mixed with a warm solution of TCNQF, in benzonitrile and the resulting mixture was allowed to stand at room temperature.The charge-transfer complex 2 was obtained as black crystals. On the other hand, single crystals of the cation radical salts (3) of 1 were obtained by galvanostatic oxidation of a solution of 1 and Bun4NC10, in chlorobenzene-CS, ( 10:1) (Scheme 1). The molecular structure and packing diagram of 2 are shown TCNQF, 1-w 2 l-TCNQF, 3(EDT-TTFC12) Scheme 1 in Fig. 1.tThe crystal packing of 2 shows that two donor and one acceptor molecules are stacked along the a axis to form a mixed-stacking structure, the donor molecules being stacked in the dimeric mode with 'atom-over-atom' overlap. The donor and acceptor molecules are quasiplanar, and the dithiacy- clohexene ring in the donor shows a slight distortion towards a chair conformation.The fa5e-to-face distance between the donor and acceptor is ca. 3.2 A,whereas the distance between the two donors is ca. 3.8 A. The TCNQF, molecules have crystallographic C, symmetry, and the two-fold axis passes through the centre of the six-membered ring. Furthermore, the packing of the molecules in the crystal shows that a pair of EDT-TTFC1, molecules lie on the position of crystallographic symmetry. t X-Ray diffraction data were collected on a Rigaku AFC7R diffractometer with Mo-Kcr (/I=0.71069 A) radiation up to 28 =55.1". The intensities were corrected for Lorentz and polarization effects, and analytical absorption corrections were carried out.The structures were solved by direct methods and refined by full-matrix least-squares analyses using reflections with I >3.00o(Z). Anisotropic thermal par- ameters were used for non-hydrogen atoms. Crystal data for 2 (EDT-TTFCl,)-0._5(TCNQF4),CI4Cl2F2H4N2S6, M, =501.46, $clinic, space group P1, a =12.449( 2), h =13.153( 2), ~=5.722+(9)A, a=93.85( l), p=93.48(1), ~=70.025( lo)', V= 877.9(2)A3, Z=2, D,= 1.897 g~m-~,F(000)=500.00, R=0.081, R,= 0.083, GOF=6.88 for 2829 observed reflections out of 4076 unique reflections. Crystal data for 3 C8C13H404S,, M, =4t2.84, triclinic, spacegroupPl,a=9.362(1), h=10.129(1), c=8.363(1) A, a=90.41(1), 8=94.28(1), :,=74.56(1)", V=762.2(2) A3, Z=2, D,=2.017 g ~m-~, F(000)=462.00, R =0.038, R, =0.036, GOF = 1.96 for 2442 observed reflections out of 3517 unique reflections. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre.See Information for Authors, Issue No. 1. CI1 fC Fig. 1 Short contacts (in A) between two sulfurs, two chlorines, and sulfur and chlorine in 2 [Cl(l)..-C1(2) 3.705(3); S(l)-..S(2)3.826(3); S(3)*..S(4) 3.852(3); S(5)*..S(6) 3.684(3); C1( 1)4(5) 3.474(3); S(5)*..S(1) 3.410(3); S(l).-.S(3) 3.917(3); S(3)..-S(3) 3.871(4)] J. Muter. Chern., 1996,6(3), 501-503 501 Although there is no intermolecular short distances less than the sum of van der oWaals radii along the a axis (S..-Sdistances are 4.07 and 4.08 A), many intermolecular short contacts are observed between the molecules aligned along the c axis. In addition, there is a Cl;..Cl short contact along the b axis [Cl( l)..-Cl( 1) 3.418(4) A].Thus, the S(l)-..S(5), S(5)-.C1( 1) and Cl(!).-.Cl( 1) distances are 3.410(3), 3.474(3), and 3.418(4) A, respectively, which are much shorter than the>...S, S...C,' and Cl...CI van der Waals distances (S: 1.85 A; C1: 1.80A). Interestingly, the dimeric donor molecules interact along the c axis (Fig. 1); one head-to-tail, and one head-to- head interaction. The S...Snetworks extend along the c axis in the crystal. In contrast, no strong S...Sinteraction exists along the b axis, and the Cl..-Cl interaction may control the packing mode in the crystal.The room-temperature conductivity of 2 is about 1S cm-' which is unusually high in spite of the mixed-stacking struc- ture.' Therefore, we measured the anisotropic conductivities using the method reported by Montgomery.6 The conductivit- ies along the a and c axes at room temperature are 0.28 and 5.6 Scm-' with the activation energies of 83 and 74meV, respectively, These results show that the considerably high conductivity and low activation energy of 2 depend on the S...Snetwork aligned along the c axis. However, the conduc- tivity of the mixed-stacking column along the a axis is also rather high with a low activation energy. In order to estimate the conducting interaction, the overlap integrals of the conduction orbitals have been calculated (Fig.2). The largest interaction is c2 for a head-to-head, side- by-side arrangement, whereas a similar head-to-tail, side-by- side interaction, c4, is about one-third of c2. Along the a axis the donor-donor interaction a2 is large, but the donor-acceptor interaction a1 is only very small, presumably owing to mismatch of the phase between the donor's HOMO and the acceptor's LUMO. Thus, the conductivity along the a axis is prevented by the small interaction of al. As for the interaction along the b axis, there is no overlap integral since there is no S.-.Sinteraction. It may be concluded that the large side-by- side interaction between the adjacent dimeric EDT-TTFCl, molecules permit the mixed-stacking 2 to have a fairly large conductivity.The molecular structure and packing diagram of 3 are shown in Fig. 33 The donor molecules are stacked in a strongly dimeric mode with 'atom-over-atom' overlap: The face-to-face distance between the two donors is cu. 3.5 A which shows a Fig.2 Crystal packing in 2. Overlap integrals ( x lo3)of the conduction orbitals in 2 are: a1 = -0.10; a2= -2.54; cl=0.50; c2= -5.56; ~3= 0.25; ~4 = -1.84 502 J. Muter. Chem., 1996, 6(3), 501-503 CI1 c12 Fig.3 Crystal structure and packing diagram of 3 [S(l)..-S(5) 3.491 (2); S(3)**.S( 5) 3.575( 2); C1(2)*.-S(3) 3.486( 2); S(3)**.S( 2) 3.574(2); C1(2).*.S(5) 3.538(2); S(1)4(4) 3.537(2); S(2)*-*S(3) ~ 3.377(2)A] very close contact between two TTF framework!. The length of the central double bond C(3)-C(4) [ 1.375(5) A] i! compar- able to that of the TTF radical cation7 [ 1.369(4) A] and is fairly elonsated as compared with that of the neutral l4 [1.323(4) A].In a similar manner, the ring double bonds C( 1)7C(2) and C(5)=C(6) are elongated by about 0.01 and 0.04 A, respectively. Thus, the cationic charge is localized at the fulvalene ring containing the ethylenedithio group. In agreement with these results, the perchlorate ion is located on the fulvalene ring containing the ethylenedithio group. This localized cationic charge may cause the donor molecules to be stacked in a head-to-tail dimeric mode. The S(1)--S(4), S(2)-..S(3), and S(5)...C1(2) distanceso in the dimeric structure are 3.537(2), 3.377(2) and 3.538(2) A, which are much less than the S..-Sand S...Cl van der Waals distances.Furthermore, the EDT-TTFC1, molecules interact along the c axis, and many S.-.Scontacts less than the van der Waals distances are observed, as shown in Fig. 3. Although 3 forms as a 1: 1 radical cation salt, the room temperature conductivity of 3is found to be 2.3 x S cm-l. Thus, 3 shows a rather high conductivity as expected for the 1: 1 radical cation salt. These findings suggest that the head- to-tail dimeric contact and the side-by-side s...S interaction in 3 lead this radical cation salt to be a semiconductor. Recently, attention has focused on iodine-bonded n donors in synthetic metals.' However, our results show that chlorine- bonded TTF derivatives may produce a new type of synthetic metal using the Cl...S and Cl...Cl interactions in crystals.We thank Dr. M. Yoshida, Tokyo Metropolitan University for helpful discussions. Financial support by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture, Japan (06243 105), is gratefully acknowledged. References A. Gavezzotti, Acc. Chem. Res., 1994, 27, 309; J. A. R. P. Sarma and G. R. Desiraju, Acc. Chem. Res., 1986, 19, 222, and references therein. D. E. Williams and L-Y. Hsu, Acta Crystallog., Sect. A, 1985, 41,296. For the synthesis of chlorinated tetrathiafulvalene, see: M. Jarrgensen and K. Bechgaard, Synthesis, 1989,207; M. R. Bryce and G. Cooke, Synthesis, 1990,263. 4 5 U. Kux, H. Suzuki, S. Sasaki and M. Iyoda, Chem. Lett., 1995,183. For a semiconductor with a mixed-stacking structure, see: R. Kato, H. Kobayashi, A. Kobayashi, T. Naito, M. Tamura, H. Tajima and H. Kuroda, Chem. Lett., 1989,1839. 8 C. Wang, A. Ellern, V. Khodorkovsky, J. Bernstein and J. Y. Becker, J. Chem. SOC., Chem. Commun., 1994, 583; R. Gompper, J. Hock, K. Polborn, E. Dormann and H. Winter, Adu. Muter., 1995, 7, 41; T. Imakubo, H. Sawa and R. Kato, 6 7 H. C.Montgomery, J. Appl. Phys., 1971,42,2971. Crystallogr., Sect. B, 1974,30, 763. T. J. Kistenmacher, T. E. Phillips and D. 0. Cowan, Actu J. Chem. SOC., Chem. Commun., 1995,1097. Communication 5/07442H; Received 13th November, 1995 J. Muter. Chem., 1996, 6(3), 501-503 503