Synthetic Metals 159 (2009) Contents lists available at ScienceDirect. Synthetic Metals. journal homepage:

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1 Synthetic Metals 159 (2009) Contents lists available at ScienceDirect Synthetic Metals journal homepage: Synthesis and structure of charge transfer salts of tetrathiafulvalene (TTF) and tetramethyl-ttf with 2,4,7-trinitro and 2,4,5,7-tetranitro-9-fluorenone Eric W. Reinheimer a, J.R. Galán-Mascarós b,, Kim R. Dunbar a, a Department of Chemistry, Texas A&M University, College Station, TX , USA b Instituto de Ciencia Molecular, Universidad de Valencia, Polígono de la Coma, s/n, Paterna, Spain article info abstract Article history: Received 23 June 2008 Received in revised form 22 July 2008 Accepted 22 July 2008 Available online 1 October 2008 Keywords: Charge transfer salts Tetrathiafulvalene (TTF) Tetramethyltetrathiafulvalene (TMTTF) Charge transfer salts of tetrathiafulvalene (TTF) and tetramethyltetrathiafulvalene (TMTTF) with the organic acceptors 2,4,7-trinitro-9-fluorenone and 2,4,5,7-tetranitro-9-fluorenone have been prepared and characterized. The compounds (TTF)[TENF] (1), (TTF) 3 [TRNF] 2 (2) and (TMTTF)[TRNF] (3) contain mixed stacks of alternating TTF and nitrofluorenone units. Surprisingly, the degree of charge transfer that occurs in these salts is not controlled solely by the redox potentials of the building blocks, but apparently also by the most effective intermolecular interactions in the solid, as determined from the crystal structures obtained. These three compounds exhibit poor electron delocalization and therefore they behave as diamagnetic insulators. Published by Elsevier B.V. 1. Introduction Charge transfer salts and radical salts containing oxidized species of tetrathiafulvalene (TTF) and its derivatives represent the most numerous and important family of molecular conductors, with many examples of molecular metals and superconductors having been reported in recent years [1]. Most of the successful examples derive from the radical salt approach, in which the overall cationic charge of the oxidized TTFs is counter balanced by inorganic anions [2], innocent or electroactive, which represents a successful approach towards multifunctional conductors [3]. In the case of charge transfer salts, a purely organic approach is possible. The most studied example is that of the 1:1 organic complex TTF-TCNQ (TCNQ = 7,7,8,8- tetracyanoquinodimethane) [4]. In this material, there is partial charge transfer between the donor and acceptor in the solid state, wherein the different molecules stack as segregated cationic and anionic chains of TTF and TCNQ, respectively. This material represents the paradigm for preparing a pure organic conductor. In the same vein, other organic acceptors have been systematically investigated [5 7], but few detailed studies have been reported, especially for organic acceptors other than nitrile- Corresponding authors. Tel.: ; fax: addresses: jose.r.galan@uv.es (J.R. Galán-Mascarós), dunbar@mail.chem.tamu.edu (K.R. Dunbar). containing species. Fluorenone derivatives have recently been explored as potential organic acceptors in the formation of charge transfer complexes. The interest in this area has been heightened by a few examples such as the combination of cyano-derivatives with BEDO-TTF (BEDO-TTF = bis(ethylenedioxy)tetrathiafulvalene) to yield organic metals [8]. Also, nitro-containing derivatives have been combined with trimeric clusters of Au(I) to yield interesting spectroscopic properties [9]. Nitro carboxylated derivatives of fluorene, which are chemically similar to the nitrofluorenone family of acceptors, have also been combined with TTF and their resulting level of charge transfer was probed using infrared and Raman spectroscopies [10]. In order to explore and understand the possible role of fluorenone derivatives in the preparation of charge transfer salts we have systematically prepared and crystallized the TTF and tetramethyltetrathiafulvalene (TMTTF) salts of 2,4,7-trinitro-9- fluorenone, and 2,4,5,7-trinitro-9-fluorenone. It should be noted that the (TTF)[2,7-dinitro-9-flourenone] charge transfer complex was first reported in 1989 and later discussed in 1993, and reported to be an insulator [11,12]. This can be understood by the fact that there is a large difference between their redox potentials, E 1/2 (TTF) = V and E 1/2 (2,7-dinitro-9- fluorenone) = 0.68), which is expected to lead to a very small effective charge transfer, with both molecules remaining essentially neutral. By increasing the number of nitro groups one can enhance the electron accepting properties of the fluorenone derivatives, for example E 1/2 (2,4,7-trinitro-9-fluorenone) = 0.42 V (1) and E 1/2 (2,4,5,7-tetranitro-9-fluorenone) = V (2), which /$ see front matter. Published by Elsevier B.V. doi: /j.synthmet

2 46 E.W. Reinheimer et al. / Synthetic Metals 159 (2009) Table 1 X-ray crystallographic data for compounds 1 3 Compound 1: (TTF)[TENF] 2: (TTF) 3[TRNF] 2 3: (TMTTF)[TRNF] Formula C 19H 8N 4O 9S 4 C 44H 22N 6O 14S 12 C 23H 17N 3O 7S 4 Formula weight Space group P-1 P-1 P-1 a (Å) 7.300(2) (2) 7.490(2) b (Å) (2) (2) (2) c (Å) (3) (2) (3) ( ) 92.20(3) 89.45(3) 97.46(3) ˇ ( ) 91.80(3) 83.16(3) 91.85(3) ( ) 97.00(3) 75.31(3) 99.20(3) V (Å 3 ) Z (mm 1 ) Reflns. collected Reflns. I > R1 a wr b c d a R1=[ b wr2 = F o F c ]/ F o. [ [w(fo 2 F c 2)2 ]/ where P = [Fo 2 + 2F c [ 2)]/3. c wr2 = [w(fo 2 F c 2)2 ]/ P], where P = [Fo 2 + 2F c 2)]/3. d wr2 = P = [F 2 o + 2F 2 c )]/3. [ [w(f 2 o F 2 c )2 ]/ [w(f 2 o )2 ]] 1/2, w = 1/[ 2 (F 2 o ) + (0.0596P)2 ], [w(f 2 o )2 ]] 1/2, w = 1/[ 2 (F 2 o ) + (0.0432P)2 + [w(f 2 o )2 ]] 1/2, w = 1/[ 2 (F 2 o ) + (0.1313P)2 ], where should have an effect on the packing and electronic properties of their respective charge transfer salts [13]. 2. Experimental 2.1. Synthesis The adducts 1 and 2 were formed by combining concentrated acetonitrile solutions of TTF with a slight excess of the nitrofluorenone derivatives in warm acetonitrile. Upon addition of the pale yellow nitrofluorenone solution to the intense yellow solutions of TTF, a spontaneous color change to a slight green color ensues. Upon slow evaporation in air, air-stable, X-ray quality black needles of 1 and black blocks of 2 formed over the period of one night and one week for the compounds, respectively. The synthesis of the prod- Table 3 Bond distances for 2 in Å S(1) C(1) 1.743(3) C(1) C(2) 1.326(4) S(1) C(3) 1.756(3) C(3) C(3) 1.362(5) S(2) C(2) 1.742(3) C(4) C(5) 1.327(4) S(2) C(3) 1.749(3) C(6) C(6) 1.344(6) S(3) C(4) 1.741(3) C(7) C(8) 1.324(4) S(3) C(6) 1.766(3) C(9) C(9) 1.335(6) S(4) C(5) 1.737(3) C(10) C(11) 1.375(4) S(4) C(6) 1.764(3) C(10) C(22) 1.379(4) S(5) C(8) 1.741(3) C(11) C(12) 1.378(4) S(5) C(9) 1.772(3) C(12) C(13) 1.481(4) S(6) C(7) 1.744(3) C(12) C(20) 1.508(4) S(6) C(9) 1.759(3) C(13) C(14) 1.483(4) N(1) O(1) 1.227(3) C(14) C(15) 1.409(4) N(1) O(2) 1.224(3) C(14) C(19) 1.412(4) N(1) C(10) 1.469(4) C(15) C(16) 1.377(4) N(2) O(5) 1.230(3) C(16) C(17) 1.383(4) N(2) O(6) 1.223(3) C(17) C(18) 1.383(4) N(2) C(16) 1.474(4) C(18) C(19) 1.387(4) N(3) O(3) 1.230(3) C(19) C(20) 1.508(4) N(3) O(4) 1.210(3) C(20) C(21) 1.400(4) N(3) C(21) 1.473(3) C(21) C(22) 1.387(4) O(7) C(13) 1.216(3) uct formed between TMTTF and 2,4,7-trinitro-9-fluorenone (3)was carried out in a similar manner as described for adducts 1 and 2, the only modification being the use of dichloromethane instead of acetonitrile to dissolve the TMTTF. Upon mixing the nitrofluorenone and TMTTF, no spontaneous color change occurred. Over the period of one hour, however, air-stable, X-ray quality black needles formed that were separated from the mother liquor by filtration Structural characterization In order to probe the solid-state structures of the adducts, a black needle of dimensions 0.28 mm 0.16 mm 0.10 mm for complex 1 and a black block of dimensions 0.40 mm 0.17 mm 0.05 mm for complex 2 were secured to a crystallographic loop using silicon oil and transferred to a SMART 1000 CCD diffractometer. A black needle of dimensions 0.39 mm 0.08 mm 0.02 mm for complex 3 was transferred in the same manner to a SMART APEX diffractometer. All data collections were conducted at 110 ± 2 K with graphite monochromated Mo-K ( = Å) radiation. The data were corrected for Lorentz and polarization effects. The Siemens SAINT software package was used to integrate the frames, and the data were corrected for absorption using Table 2 Bond distances for 1 in Å S(1) C(1) 1.771(4) O(9) C(17) 1.225(4) S(1) C(3) 1.763(4) C(1) C(2) 1.350(5) S(2) C(2) 1.753(4) C(3) C(4) 1.374(5) S(2) C(3) 1.793(4) C(5) C(6) 1.340(6) S(3) C(4) 1.784(4) C(7) C(8) 1.388(6) S(3) C(5) 1.748(4) C(7) C(19) 1.400(6) S(4) C(4) 1.763(4) C(8) C(9) 1.406(5) S(4) C(6) 1.756(4) C(9) C(10) 1.415(5) N(1) O(1) 1.229(5) C(10) C(11) 1.516(5) N(1) O(2) 1.246(5) C(10) C(18) 1.424(5) N(1) C(7) 1.501(5) C(11) C(12) 1.411(5) N(2) O(3) 1.247(4) C(11) C(16) 1.421(5) N(2) O(4) 1.244(4) C(12) C(13) 1.400(5) N(2) C(9) 1.486(5) C(13) C(14) 1.397(5) N(3) O(5) 1.248(4) C(14) C(15) 1.385(5) N(3) O(6) 1.236(4) C(15) C(16) 1.406(5) N(3) C(12) 1.485(5) C(16) C(17) 1.499(5) N(4) O(7) 1.243(4) C(17) C(18) 1.511(5) N(4) O(8) 1.239(4) C(18) C(19) 1.395(5) N(4) C(14) 1.497(5) Table 4 Bond distances for 3 in Å S(1) C(6) 1.756(8) C(3) C(4) 1.492(11) S(1) C(7) 1.776(7) C(5) C(6) 1.334(11) S(2) C(6) 1.781(8) C(7) C(8) 1.327(12) S(2) C(8) 1.749(9) C(7) C(9) 1.509(11) S(3) C(2) 1.752(8) C(8) C(10) 1.510(11) S(3) C(5) 1.775(8) C(11) C(12) 1.393(12) S(4) C(3) 1.769(8) C(11) C(23) 1.370(11) S(4) C(5) 1.735(8) C(12) C(13) 1.381(11) N(1) O(1) 1.229(9) C(13) C(14) 1.480(12) N(1) O(2) 1.210(9) C(13) C(21) 1.394(11) N(1) C(11) 1.482(11) C(14) C(15) 1.494(10) N(2) O(5) 1.219(9) C(15) C(16) 1.363(12) N(2) O(6) 1.237(9) C(15) C(20) 1.412(11) N(2) C(17) 1.461(10) C(16) C(17) 1.397(11) N(3) O(3) 1.221(9) C(17) C(18) 1.375(11) N(3) O(4) 1.220(9) C(18) C(19) 1.362(11) N(3) C(22) 1.476(10) C(19) C(20) 1.397(11) O(7) C(14) 1.224(9) C(20) C(21) 1.511(11) C(1) C(2) 1.508(10) C(21) C(22) 1.393(11) C(2) C(3) 1.340(12) C(22) C(23) 1.377(11)

3 E.W. Reinheimer et al. / Synthetic Metals 159 (2009) Fig. 1. X-ray structure of (TTF)[TENF] (1). Fig. 2. X-ray structure of (TTF) 3[TRNF] 2 (2). the SADABS program [14]. The structures were solved by direct methods by the use of the SHELXS-97 program [15], in the Bruker SHELXTL v5.1 interface [16], in the XSEED software package [17]. The SHELXL-97 program [18], was employed to refine all nonhydrogen atoms with anisotropic thermal parameters by full matrix least squares calculations on F 2. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal param- Fig. 4. Projection of the crystal structure of 1 in the bc plane, and representation of the alternating chains that run along the a axis. Fig. 3. X-ray structure of (TMTTF)[TENF] (3). eters. Crystallographic information is shown in Table 1, while bond distances for 1 3 are presented in Tables 2 4. X-ray crystallographic structures and packing diagrams for 1 3 are shown in Figs CCDC contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via request/cif.

4 48 E.W. Reinheimer et al. / Synthetic Metals 159 (2009) Fig. 5. Projection of the crystal structure of 2 in the bc plane, and representation of the alternating chains that run parallel to this plane.

5 E.W. Reinheimer et al. / Synthetic Metals 159 (2009) Fig. 6. Projection of the crystal structure of 3 in the bc plane, and representation of the alternating chains that run parallel to this plane Magnetic characterization Bulk magnetization measurements were carried out on polycrystalline samples with a Quantum Design MPMS7 magnetometer. The thermal dependence of the magnetic susceptibility was studied in an applied field of 1000 G in the temperature range K. Susceptibility measurements revealed that all three compounds are essentially diamagnetic, as expected, with no temperature independent paramagnetic (TIP) contribution. 3. Results and discussion In all cases, the interaction between donor and acceptor favors the formation of mixed solids with strong intermolecular interactions between the TTF and nitrofluorenone units. These strong intermolecular donor acceptor interactions also dictate the packing in the solid state, wherein, rather than segregated stacks, one observes an alternating integrated stacking of the molecular species. The strength of these and other intermolecular interactions determine the crystal structure of these materials. The organic molecules appear with non-integer charge, due to the presence of partial charge transfer, as estimated from the TTF bonding distances which are very sensitive to its oxidation level, whereas the nitrofluorenone molecules did not reveal any obvious structural alterations (TTF)[TENF] (1) The solid state structure of 1 contains 1:1 mixed, alternating stacks of donor and acceptor molecules along the a axis forming 1D chains (Fig. 4) in a pseudo-tetragonal packing arrangement.

6 50 E.W. Reinheimer et al. / Synthetic Metals 159 (2009) The TTF molecule is not planar, and displays a slight distortion from planarity by an angle of 7 between the planes of the dithiole rings. From the C C and C S bonding distances, a charge of per TTF molecule can be estimated according to Coppens method [19]. The nitrofluorenone molecule displays a torsion angle collinear with the C 2 axis that bisects the central five-membered ring resulting in a twisted conformation for the central fluorenone moiety. In most cases, the fluorenone moiety of the nitrofluorenone molecules remain planar, while their constituent nitro groups deviate from planarity. A single nitro group is coplanar with each half of the molecule (torsion angles below 3 ), while the second nitro group in each half, in the 4- and 5-positions, differs from planarity by torsion angles of 25 and 30. This conformation involves weak intermolecular interactions between these two nitro groups, with N O distances as short as 2.694(5) Å. Another interesting feature of the tetranitrofluorenone molecule is the bond length of the carbonyl group defined by C(12) and O(3) is 1.226(4) Å, whereas the neutral molecule shows a carbonyl bond distance of 1.205(3) Å. This increase in carbonyl bond distance is indicative of electron donation from TTF to the carbonyl group of tetranitrofluorenone. There are no close interactions between chains, with all contacts being essentially the sum of van der Waals radii. There is a weak interaction between adjacent chains between a nitro group and a sulfur atom (O3 S1 = 2.981(4) Å) (TTF) 3 [TRNF] 2 (2) In this case, the stoichiometry is 3:2 instead of 1:1, as in the other compounds, and is composed of mixed stacks of alternating donor and acceptor units as well as isolated TTF molecules (Fig. 5). The asymmetric unit contains a single trinitrofluorenone molecule and three half-molecules of TTF (A C). Two of the three crystallographically independent TTF molecules (A and B) form alternating stacks with the trinitrofluorenone acceptor parallel to the [0 1 1] direction on the bc plane, with a sequence A TRNF B TRNF [9]. Molecule C appears in the interchain separation with the plane perpendicular to the chains. From the intramolecular bonding distance, there are two TTF molecules that appear neutral (B and C) while molecule A has an estimated charge of This was expected for molecule C because it is completely isolated which usually leads to neutral TTF. The difference charges observed for the TTFs in the chains are in good agreement with the intermolecular distances. Indeed, the chains do not show a regular stack, with very close contacts between molecule A and the TRNF molecules (3.01(5) Å between S1 and the mean plane of the two adjacent TRNF molecules), and much longer for molecule B (3.35(5) and 3.43 Å between S3 and the mean plane of the two adjacent TRNF molecule). Thus the structure can be considered as being composed of sandwiched trimers TRNF TTF(A) TRNF with effective charge transfer, separated by neutral TTF molecules. Along the chain, the molecules are related by a center of symmetry, and thus consecutive TRNF molecules show opposite orientation, regarding the C O group and appear with completely planar conformation including the nitro groups, which exhibit a maximum deviation of 7 from planarity. In the case of the 4-nitro group, this coplanarity is the result of weak intramolecular H-bonding interactions, as short as O3 C22 = 2.633(5) and O4 C18 = 2.832(6) Å. There are no close contacts between stacks or with the neutral and isolated TTF molecule. The most relevant intermolecular interactions come between the C O group with sulfur atoms from TTF A molecules, the ones with highest positive charge (S1 O7 = 2.904(8) Å) (TMTTF)[TRNF] (3) This compound also exhibits mixed 1:1 stacks of alternating TMTTF and nitrofluorenone units (Fig. 6) which form along the a axis in a pseudo-hexagonal packing motif. There is only one crystallographically independent neutral TTF molecule, as estimated from the intermolecular bonding distances [19]. The interplanar distance is Å. The trinitrofluorenone molecule also shows a planar configuration, with torsion angles up to 27 for the central 4-nitro group. The loss of coplanarity of this group with respect to compound 2 is the result of stronger intermolecular interactions. In this case in addition to weak intramolecular H- bonding (O3 C22 = 2.633(7) Å) there are also O N intermolecular interactions as short as O4 C18 = 2.832(6) Å. Interchain interactions are much weaker, and no significant H-bonding is observed, with the shortest contact appearing between a nitro group and aromatic carbons from fluorenone molecules in adjacent chains (O4 C19 = 2.844(6) Å). It is obvious that the packing arrangement is dominated by interactions. 4. Conclusion We have analyzed the crystal structures of products formed by the direct reaction of TTF and TMTTF with tri- and tetranitrofluorenone. We have found that there is partial charge transfer in the salts containing TTF, while none occurs in the TMTTF. The crystal packing in all cases consists of mixed donor acceptor stacks. The absence of segregated stacks of donors and acceptors must be related to the low ionicity of the TTF molecules, that as almost neutral entities do not show such strong S S interactions. Our results indicate that the degree of charge transfer in these compounds depends on the solid state structure, controlled mostly by and other very weak intermolecular interactions. It is surprising to observe that in the TTF case, the stronger electron donor, tetranitrofluorenone, did not yield a product with the most efficient charge transfer, such as was found for trinitrofluorenone with an estimated +0.3 charge for TTF. The reason for this is obviously structural since, in the trinitrofluorenone derivative, two fluorenone molecules are accepting electron density from only one TTF molecule, thereby forming trimers with close interactions, while in the tetranitrofluorenone there is a regular stack in the chain. Apart from the interesting structural features controlling the degree of charge transfer from the TTF molecules, the weak charge transfer found in all cases renders these compounds diamagnetic insulators. 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