J. MATER. CHEM., 1994, 4(11), 1719-1722 X-Ray Crystal Structure and Solid-state Properties of a 1 : I Complex of Tetrathiafulvalene (TTF) and 1-Oxo-2,6-dimethyl-4-dicyanomethylenecyclohexa-2,5-diene Andrei S. Batsanov, Martin R. Bryce,* Stephen R. Davies and Judith A. K. Howard Department of Chemistry, University of Durham, Durham, UK DHI 3LE A crystalline 1 :1 complex of tetrathiafulvalene (TTF) and 1-oxo-2,6-dimethyl-4-dicyanomethylenecyclohexa-Z.5-diene has been characterised by IR, UV and EPR spectroscopy, two-probe dc conductivity data and single-crystal X-ray analysis. The complex consists of mixed donor-acceptor stacks of two crystallographically unrelated types and is an electrical insulator. In the search for new electron acceptors suitable for the formation of conducting molecular complexes, analogues of tetracyano-p-quinodimethane (TCNQ) have received con-siderable attention, and a large number of acceptor systems containing one, or more, dicyanomethylene [C(CN),] or cyanoimine (N-CN) groups have been synthesized.l-l' However, with the exception of N,N'-dicyanoquinonediimine radical anions,' there have been few structural studies on salts or complexes of new acceptor^.',^,^^ We have recently synthe- sized a series of 1-oxo-4-dicyanomethylenecyclohexa-2,5-diene derivatives la-c7 and we now describe the properties and X-ray crystal structure of a 1: 1 complex 2 formed between the 2,6-dimethyl derivative la and tetrathiafulvalene (TTF).The solution electrochemis try of acceptors la-c is discussed.Results and Discussion All the solid-state data obtained on the title complex 2 agree and show that there is no intermolecular charge transfer between TTF and acceptor molecule la. The IR spectrum of complex 2 consists of sharp peaks typical of an insulating complex; the frequency of the cyanide absorption peak in the complex (2225 cm-') is very similar to that of neutral compound la (2220cm-I). These IR data are supported by the UV spectrum of a powdered sample of 2 which does not show any absorptions characteristic of the TTF radical cation (2.2-2.8 eV) nor is a charge-transfer absorption band observed.12 Furthermore, the complex did not give an EPR signal. Consistent with these data, the single-crystal conduc- tivity (two-probe dc measurement) is that of an insulator (a,,= < S cm-').The electrochemical redox properties of the acceptors la-c have been studied by cyclic voltammetry. The compounds undergo one-electron reduction at Ered= -0.18, -0.21 and -0.30 V, respectively (data were recorded us. Ag/AgCl, Pt electrode, tetrabutylammonium perchlorate in acetonitrile at 20°C). This redox process is reversible for derivative la and irreversible for derivatives lb and lc; a second reduction wave to form the dianion was not observed. These data establish that, as expected, the acceptor ability decreases (i.e. the stability of the radical anion decreases) with successive methyl substitution in the series of compounds la-c, and these compounds are considerably weaker acceptors than TCNQ (El"'= +0.22 V; E21i2= -0.34 V, under the same electro-chemical conditions).Single crystal X-ray analysis reveals that the asymmetric unit of complex 2 comprises one TTF molecule (A) in a general position, two 'halves' of TTF molecules (B and C), each occupying a special position at an inversion centre and two molecules of 1 (D and E). TTF molecule A adopts a boat conformation, folding along the S(l)--.S(2) and S(3)...S(4) vectors by 4.3"and 10.0", respectively; molecules B and C are essentially planar. Both molecules la are slightly folded along the C(2)..-C(6) vector and twisted around the C(4)=C(9) bond, thus falling into three planar fragments, of' which [0(1)C(l)C(2)C(6)]and [C(4)C(9)(CN),] form with the 'central' [C(2)C(3)C(4)C(S)C(6)] moiety dihedral angles of 4.2 and 8.0" in molecule D, 3.8 and 11.2" in molccule E, respectively; The methyl carbon atoms deviate but slightly (0.03-0.06 A) from the central C, plane.For the atom num- bering scheme see Fig. 1. All these molecules and their symmetrical equiva Lents in the crystal are parallel to each other within 12.5" and form nearly planar sheets, approximately coinciding with the planes (xyO), (xy1/4), (xy1/2), (xy3/4) etc. [Fig. 2(u)]. There are two symmetrically independent sheets: those at z=O, l/2, 1 etc. are composed of molecules B, C and E [Fig. 2(u)], those at z= 1/4, 3/4 etc. of molecules A and D [Fig. 2(c)]. Molecular packing within sheets of either type is essentially simitar, with infinite chains of identical (translationally related) molecules running in the x direction. The shortest contacts within the sheets are between donor and acceptor molecules, viz.O(1D)**.C(2A)3.05, O(lE)*.*S(1B) 3.04, O(1E) ..C(2B) 3.02A, which ar,e shorter tgan the sums of van der Waals radii of 0 (1.52 A), C (1.70 A) or S (1.80 A).13 However, these contacts do not form infinite chains [Fig. 2(b),(c)]. On the other hand, each donor or acceptor molecule is sandwiched between two molecules of the opposite kind, belonging to the adjacent sheets. Principal planes of the adjacent molecules are p!rallel to within 2-5", with interplanar separations of 3.30-3.47 A. Thus the structure can be described alternatively as consisting of mixed donor-acceptor (1:1) stacks of two crystallographically unrelated types.Stacks of the first type that lie in the planes (xOz), (x1/2z) etc. are parallel to the direction [loll and comprise molecules in consequence ...B...D...C...D...B... [Fig. 2(d)]. Stacks of the second type (...A...E...A.. .E...) lie in parallel planes (x1/4z), (x3/4z) etc., but run in the [OOl] direction, differing from [loll by 36". The lengths of chemically equivalent bonds coincide within experimental errors. The geometry of the TTF moieties in the complex (Fig. 3) is similar to that of the neutral TTF mol- ecule,14 while differing substantially from that in TTF'+X -salts, or in molecular complexes where charge trmsfer is observed (e.g. TTF-TCNQ, TTF-DEtCNQ)." The 1-0x0-4- dicyanomethylenecyclohexa-2,5-dieneframework has not been studied structurally before, except for benzo-fused derivatives, which are non-planar, presumably due to peri-interactions between the dicyanomethylene group and the fused benzene ring.4,'6 In the present complex, compound la exhibits essen- J.MATER. CHEM., 1994, VOL. 4 S(2b') Fig. 1 Molecular overlap in the structure of the complex of TTF and acceptor la; projection on (001) plane, H atoms omitted, primed atoms are symmetry-related to the reference ones via inversion centres, double-primed by the c glide plane I= Z= y= 1/4 (e) y= 1/2 (d) Fig. 2 Crystal packing in the complex: projection along the z axis (a) and cross-sections along the planes z= 1/2 (b),z= 1;4 (c), y= 1/2 (d), z= 1/4 (e),showing short intermolecular contacts (dashes).H atoms are omitted, for molecular labelling see Fig. 1. J. MATER. CHEM., 1994,VOL. 4 [@=qSJdS Me la TTF e 1.230(6) i 1.368(3) 8 1.336(4) f 1.479(4) j 1.449(10) b 1.763(4) g 1.340(3) k 1.141(6) c 1.740(7)h 1.450(9) d 1.328(2) 1 a R'=R2=H b R' = Me; R2= H c R'=R~=M~ Fig. 3 Average bond distances in complex 2 (A),with G in parentheses (esds of individual values are 0.005-0.010A) tially quinonoid geometry, similar to that of 2,6-dimethyl-4-(a,a-diphenylmethy1ene)-1,4-benzoquinone.l7 Thus the crystal of 2 comprises neutral, rather than ionised, molecules of TTF and acceptor la. The crystallographic evidence correlates, therefore, with the spectroscopic and conductivity data in confirming the absence of any significant charge transfer from donor to acceptor within the crystal.It is known that in molecular complexes the charge-transfer stabilisation energy tends to be maximised in the observed donor-acceptor orientation (in the absence of hydrogen bonds, electrostatic interactions or other large intermolecular forces)." However, in complex 2, four crystallographically independent donor-acceptor contacts exhibit no prevailing mode of overlap (Fig. 1). Using the guidelines presented by Mayerle andTorrance et the formation of a neutral complex between TTF and acceptor la is in accord with the electrochemical data, which establish that there is a large difference (AE =0.53 V) between the first oxidation potential of TTF (Elli2= +0.34 V us.Ag/AgCl) and the reduction potential of acceptor la, reported above. Acceptor 1b also forms an insulating complex with TTF of 1: 1 stoichiometry, although single crystals could not be obtained. It is noteworthy that tetramethyl derivative lc does not form a complex with TTF, probably due to a deviation from planarity in the acceptor, by analogy with tetramethyl-TCNQ2' and tetra- methyl-N77,7-tricyanoquinomethaneimine.'f7 Experimental Preparation of Complex 2 A hot saturated solution of TTF (40 mg, 0.2 mmol) dissolved in ethanol was added to a hot saturated solution of compound la7 (36 mg, 0.2 mmol) in ethanol. The solution was slowly cooled to 20 "C and then stored at -20 "C for 168 h.Dark green plates of complex 2 (35 mg, 46%) were harvested by filtration; mp 128-132°C. Analysis, found: C, 52.1; H, 3.0; N, 6.0%: calc. for C1,H12N,0S,: C, 52.5; H, 3.1; N, 7.2%. IR vmax (KBr)/cm-': 3065, 2225 (CN), 1630, 1590, 1365, 1200, 890, 800, 780, 775, 735, 680, 660 and 440. t IUPAC-recommended name: 3-cyanoimino-6-dicyanomethylene-1,2,4,5-tetramethylcyclohexa-1,4-diene. X-Ray Structure Determination A dark-green plate-like single crystal of 2 (0.08 mm x 0.30mm x 0.40 mm) was obtained from ethanol. The X-ray diffraction experiment was carried out on a Rigaku AFC6S four-circle diffractometer at 150 K, using a Cryostream (Oxford Cryosystems) liquid-nitrogen device with an open-flow gas cryostat.21 Crystal data: CllH,N20C,H,S,2 M = 388.5.Mopoclinic, space Froup P2,/c, a=8.061(2) A, ho 33.065(7)A, c=13.746(3) A, p=105.21(2)", U=3535(1)A3 (from 24 reflections with 10"-=6'< 12"), Z =8, dcalc= 1.46g cmP3, F(000)=, 1600, graphite-monochromated 440-Ka radiation, 2 =0.7107 A, ,u =5.4 cm -'. 5998 independent reflections were measured in o-sca n mode (28< 50", no absorption correction). The structure was solved by direct methods.22 All non-hydrogen atoms were refined with anisotropic displacement parameters by full-matrix least squares (total 433 variables; all H atoms treated in riding model) against Fs of 3241 reflections having IF/>,40(1'), with w = [02(F)+ 0.0002F2]-' weights, using SHELXTLPLUS programs.23 The refinement converged at =0.045.wR = 0.046, goodness-of-fit 1.21, Apmax=0.34 e AP3. Additional material available from the Cambridge Crystallographic Data Centre comprises atomic coordinates and thermal parameters, bond lengths and angles. We thank SERC for the award of a studentship (to S.R.D.) and the Royal Society and the Nuffield Foundation for financial support (to A.S.B. and M.R.B., respectively). References 1 S. Hiinig and P. Erk, Adv. Mater., 1991,3,225. 2 K.Yui, Y. Aso, T. Otsubo and F. Ogura, J. Chem. Sol ., Chem. Commun., 1987,1816. 3 T.Czekanski, M.Hanack, J. Y. Becker, J. Bernstein, S Bittner, L. Kaufman-Orenstein and D. Peleg, J. Org. 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Commun., 1985,286. Paper 4/021161; Received 1lth April, 1994