Competition between the C–H···N Hydrogen Bond and C–I···N

Article pubs.acs.org/crystal

Competition between the C−H···N Hydrogen Bond and C−I···N Halogen Bond in TCNQFn (n = 0, 2, 4) Salts with Variable Charge Transfer Published as part of the Crystal Growth & Design virtual special issue In Honor of Prof. G. R. Desiraju Julien Lieffrig,† Olivier Jeannin,† Thierry Guizouarn,† Pascale Auban-Senzier,‡ and Marc Fourmigué*,† †

ISCR, Université Rennes 1, Rennes 35042, France LPS, Université Paris-Sud, Orsay 91405, France

‡

S Supporting Information *

ABSTRACT: The formation of charge-transfer salts with an iodinated tetrathiafulvalene derivative, EDT-TTF-I (1), has been investigated with a series of TCNQ acceptors of different oxidative strengths, namely, TCNQ, TCNQF, TCNQF2, and TCNQF4. These series of compounds have been prepared in order to investigate the effect of the charge in these complexes on the C−I···NC halogen bond interactions which can take place between 1 and the various TCNQs. With the most oxidizing TCNQF4 acceptor, a 1:1 compound formulated as (1)(TCNQF4) was obtained with a charge ρ(1) = +1 on 1, while with TCNQF2, a 2:1 conducting salt formulated as (1)2(TCNQF2) was characterized with ρ(1) ≈ +0.5. With TCNQ itself, both a 1:1 conducting phase, (1)(TCNQ) with ρ(1) ≈ +0.5, and a 2:1 (1)2(TCNQ) compound with ρ(1) = 0 were crystallized. Transport and magnetic properties were rationalized, based on the ρ(1) value and band structure calculations. CTTF−I···NC halogen bond interactions were observed in the mixed valence [ρ(1) = +0.5] salts and even in the neutral [ρ(1) = 0] compound, while they are surprisingly absent from the full charge transfer TCNQF4 salt. It is shown that TTF oxidation also activates the TTF sp2 hydrogen atom located α to the iodine atom, toward the preferential formation of CTTF−Hα···NC hydrogen bonds, also present in the mixed-valence salts. These series provide an opportunity to evaluate the relative strength of competing C−H···N hydrogen and C−I···N halogen bonds and their relative sensitivity to charge.

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hydrogen8 or halogen bond donor character of these substituents and favors the supramolecular interaction with the counterions (X−) acting as a Lewis base through TTF− CONRH···X− hydrogen bonds9 or TTF−I···X− halogen bonds.7a,10,11 By contrast, the introduction of such intermolecular interactions in charge transfer (CT) salts where the X− counterion is also an electroactive molecule (reduced acceptor) has been only scarcely considered up to now. CT salts are formed upon electron transfer between two redox active molecules, a donor (D) and an acceptor (A). Prototypical examples are TTF·TCNQ where segregated stacks and partial charge transfer afford a metallic behavior,12 or TTF·QCl4 (QCl4 = chloranil) where alternated stacks of neutral donor and acceptor molecules turn ionic upon cooling.13 In these DA systems, not only the stoichiometry but also the difference

INTRODUCTION

Following the discovery of superconductivity in cation radical salts of TMTSF (Bechgaard salts)1 and BEDT-TTF salts in 1980s,2 the search for novel molecular conductors3 has been essentially concentrated on similar cations radical salts obtained by electrocrystallization with redox-inactive counterions. In these salts, the degree of charge transfer is controlled by the actual stoichiometry. In order to tentatively control the solid state structures and ultimately the electronic structures of such salts, different supramolecular strategies have been also investigated. For example, the introduction of additional nonbonding interactions such as hydrogen bonding or halogen bonding4 to orient and control the solid state association of radical molecules and the organic/inorganic interface has been considered. These efforts were mainly concentrated on tetrathiafulvalene derivatives acting as hydrogen or halogen bond donor molecules, and accordingly substituted with hydrogen bond donor groups (-CH2OH,5 -CONHR)6 or with halides7 as halogen bond donor moieties. The oxidation of such TTF derivatives into cation radical salts activates the © 2012 American Chemical Society

Received: June 1, 2012 Revised: July 2, 2012 Published: July 12, 2012 4248

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molecules ranges from 0.17 V in TCNQ to 0.53 V in TCNQF4 (vs. SCE, Scheme 2), to be compared with the first oxidation

between redox potentials are key elements to control the degree of charge transfer.14,15 Examples of purposely introducing hydrogen bond interactions within such CT salts are limited to a very small set of TTF derivatives, the secondary alcohol Me 3 TTF-CHMe(OH) with TCNQ, 8b primary amides16,17 such as EDT-TTF-CONH2 with TCNQF4, a TTF thioamide with TCNQ,18 and more recently a TTF imidazole derivative with an extended series of TCNQs and quinones acting simultaneously as electron acceptor and hydrogen bond acceptor.19 These last series demonstrated that additional intermolecular interactions activate donor and acceptor molecules toward easier CT despite unfavorable redox differences and allows for uncommon stoichiometry such as 2:1 D−A−D hydrogen-bonded triad,19 a stoichiometry potentially associated with high conductivity. Compared with hydrogen bonding, halogen bonding interactions were even less explored in such CT salts with halogenated TTFs (Scheme 1), with only very few examples described so far, two 1:1 iodotetrathiafulvalene/TCNQ CT neutral complexes,20 and two salts, namely, (EDO-TTF-I)2(TCNQ),21 and (EDT-TTFCl2)2(TCNQF4).22

Scheme 2

potential of EDT-TTF-I at 0.46 V (vs. SCE, in CH3CN with Bu4NPF6 0.1 M as electrolyte at 1 V s−1 scan rate).24 They therefore give the opportunity to explore a wide potential window, with opportunities toward mixed valence conducting salts. Indeed, as illustrated in the prototypical TTF·TCNQ compound, mixed valence and metallic behavior are associated in such TCNQ salts to a potential difference ΔE = Ered(acceptor) − Eox(donor) comprised between 0 and ≈ −0.25 V.25 When ΔE > 0, a complete charge transfer is observed. If the same potential difference rule is applied here to the donor molecule 1, more difficult to oxidize than TTF itself, a partial charge transfer should be predicted with stronger oxidants such as TCNQF2 or TCNQF. Note also this mono iodo TTF derivative 1 has been already engaged by electrocrystallization into cation radical salts with various counterions, halides,10,11e polyhalides,26 and cyanometallates10,27,28 as well as two dithiolene complexes, [Pd(dmit)2]− and [Ni(mnt)2]−.29,30 In all examples, formation of halogen bonding interactions between the iodine atom in 1 and the counterions acting as Lewis base is observed. The series of TCNQ acceptors mentioned above can also act as a halogen bond acceptor through the nitrile moieties, and the strength of these interactions could be related to the degree of charge transfer, an interesting point when considering the strong electrostatic contribution to this interaction.31 We report here four different compounds obtained from the reaction of EDTTTF-I 1 with different fluorinated TCNQs, allowing for a detailed analysis of the charge−transfer evolution within these salts, depending on their stoichiometry, presence of halogen bonding, and difference of redox potentials.

Scheme 1. Chemical Structures of Halogenated TTF Involved in CT Salts

Indeed, the functionalization of the TTF core with electronwithdrawing halogen atoms raises the oxidation potential of the TTF molecules (EDT-TTF-I: 0.46 V vs. SCE, EDT-TTFI2: 0.57 V vs. SCE), making often impossible their oxidation with TCNQ itself whose reduction potential amounts to ≈0.17 V (vs. SCE). Thus, only stronger oxidants such as the tetrafluoro TCNQ (TCNQF4; Ered = 0.53 V vs. SCE) were considered. We have also very recently shown that DDQ (dichlorodicyanobenzoquinone, Ered = 0.53 V) behaves just like TCNQF4 in that respect, affording with EDT-TTF-I and EDT-TTFI2, semiconducting CT salts with full CT and halogen bonding interactions with both oxygen and nitrogen atoms of the DDQ radical anion.23 In order to extract characteristic trends from the introduction of halogen bonding interactions in CT salts, it is therefore highly desirable to go beyond the disparate examples described above with different donor molecules and strong acceptors and to investigate an extended series where one parameter only is modified sequentially. Accordingly, we have undertaken a thorough investigation of CT salts of the EDT-TTF−I (1) donor molecule (Scheme 1) with acceptor molecules of different strengths, namely, TCNQ, TCNQF, TCNQF2, and TCNQF4. The first reduction potential of these acceptor

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RESULTS AND DISCUSSION Reaction of 1 with either TCNQF4, TCNQF2, or TCNQ afforded crystalline materials, with two different phases with the unsubstituted TCNQ molecule. With the most oxidizing TCNQF4 acceptor, a 1:1 compound formulated as (1)(TCNQF4) was obtained while with TCNQF 2, a 2:1 compound formulated as (1)2(TCNQF2) was isolated. With TCNQ itself, we could isolate both a 1:1 (1)(TCNQ) as well as a 2:1 (1)2(TCNQ) compound. Note that with the monofluoro TCNQF, only microcrystalline material could be obtained. In the following, we will first discuss the degree of charge transfer in the different compounds, and then we will investigate their solid state structure, in relationship with their transport and magnetic properties, and finally, we will analyze the characteristics of the weak intermolecular interactions (halogen bonding, hydrogen bonding) taking place in the solid, in connection with the actual degree of CT in the different compounds. 4249

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Table 1. Estimated (ρest) and Calculated (ρcalc, see text) Charges in 1 in the Different Compounds, Together with Averaged Intramolecular Bond Distances within Iodinated TTF Molecule 1, in Various CT Salts or Complexes, and Reference Compounds

compound neutral 1 mol A mol B (1)2[Ag(CN)2] (1)(DDQ) (1)(TCNQF4) mol A mol B (1)2(TCNQF2) (1)(TCNQ) (1)2(TCNQ)

ρest. 0 +0.5 1 1 ≈ 0.5 ≈ 0.5 ≈0

ρcalc

a (Å)

b (Å)

c (Å)

d (Å)

+0.04 +0.00 +0.60 +0.93

1.332(13) 1.323(14) 1.373 1.390(7)

1.763(18) 1.760(17) 1.739 1.721(6)

1.759(19) 1.747(16) 1.746 1.736(6)

1.331(14) 1.331(13) 1.347 1.350(8)

+0.78 +0.75 +0.49 +0.49 +0.10

1.383(8) 1.385(8) 1.364(18) 1.375(16) 1.349(4)

1.732(9) 1.724(9) 1.743(18) 1.739(13) 1.764(3)

1.737(9) 1.740(9) 1.746(19) 1.748(19) 1.758(3)

1.348(10) 1.336(10) 1.344(14) 1.331(9) 1.340(5)

ref 29 10 23 this work this work this work this work

Figure 1. Projection view along the a axis of the unit cell of (1)(TCNQF4).

TCNQ core in TCNQF2 (Table S2), as well as by the νCN stretching frequency, found here at 2164 and 2182 cm−1. For comparison, it has been indeed reported at 2230 cm−1 in neutral TCNQF2, at 2201 cm−1 in K+,TCNQF2−•.33 In the 1:1 TCNQ salt (1)(TCNQ), evaluation of the charge on the TTF derivative, ρest ≈ +0.5, is also confirmed by (i) bond distances within the TCNQ core (Table S3) as shown by application of formulas which relate CT with combination of bond distances in TCNQ salts,34 giving a calculated ρTCNQ value of ≈0.47. Finally, in the 2:1 TCNQ compound formulated as (1)2(TCNQ), bond distances within the TTF core indicate a weak charge transfer, confirmed by the calculated charge (Table S3) on the TCNQ (ρTCNQ = 0.18). The νCN stretching frequencies are less conclusive since they are found at 2180 and 2215 cm−1 in (1)2(TCNQ), at 2179 and 2216 cm−1 in (1)(TCNQ). A mixing of the two phases or a solid state transformation upon KBr pellet elaboration might be at the origin of this result. To summarize this first section, a full CT 1:1 salt has been found with TCNQF4, associated as expected with a positive ΔE value, ΔE = Ered(TCNQF4) − Eox(1) = 0.53 − 0.46 = +0.07 V. With TCNQF2, ΔE = −0.10 V and a 2:1 salt is obtained instead, allowing for an interesting mixed valence on the donor side. With TCNQ itself with a large and negative ΔE value at −0.39 V, one would have expected a simple 1:1 neutral charge transfer complex. However, the 1:1 compound isolated so far exhibits an intermediate and promising ρest = 0.5 value while a 2:1 compound formulated as (1)2(TCNQ) appears to be close to neutral. This rich and somehow unexpected behavior might be also related to the added role of intermolecular interactions

Charge Transfer and Stoichiometry. In the four crystalline compounds, the degree of CT between donor and acceptor can be assessed from several complementary methods: (i) intramolecular bond distances within the TTF core, (ii) intramolecular bond distances within the TCNQ core, (iii) νCN vibration frequency of the TCNQ derivative. In Table 1, intramolecular distances within the TTF core in 1 are compared with those of reference compounds where the oxidation state is nonambiguous, that is, the neutral 1 with ρ(1) = 0, the mixed-valence salt with [Ag(CN)2]− with ρ(1) = 0.5, the DDQ salt with ρ(1) = 1. The recurrent trends observed, that is, the lengthening of the inner and outer CC double bonds, the shortening of the C−S bonds upon oxidation, allows us to give in Table 1 tentative CT degree on the TTF core, ρest., for the four different compounds. Correlations between bond distances and charges have been used for BEDT-TTF salts which are written as ρcal = A + B[(b + c) − (a + d)].32 A linear fit with the reported data for the known EDT-TTF-I compounds reported in Table 1 gives A = 5.825 and B = −6.8253. Applied to the compounds in Table 1, these ρcalc values are in good accordance with the ρest values given above. In (1)(TCNQF4), the full charge transfer (ρest = +1) is confirmed by the intramolecular bond lengths within TCNQF4 (see Table S1 in Supporting Information), as well as by the νCN stretching frequency, observed here at 2172 and 2192 cm−1, while it is observed at 2215 and 2230 cm−1 in neutral TCNQF40 and 2179 and 2195 cm−1 in K+,TCNQF4−•.33 In (1)2(TCNQF2), the mixed valence character on the donor side [ρest ≈ +0.5] implies a full CT on TCNQF2. This is confirmed by the intramolecular bond lengths within the 4250

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alternation along the stacking axis of two overlap patterns (Figure 5a,b). On the other hand, the overlap interaction between the TCNQF2−• radical species shown in Figure 5c is most probably weak. Because of its mixed valence character (for donor molecules), this salt is potentially highly conducting; it exhibits indeed a high room temperature conductivity, σRT = 10 S cm−1. Temperature dependence of the resistivity (Figure 6) shows however a semiconducting behavior in the whole temperature range, but with weak activation energy (0.019 eV). From a magnetic point of view, this salt should combine the susceptibility of the conducting slab with the contribution of the TCNQF2−• stacking along the a axis. The temperature dependence of the χ·T product (Figure 6 inset) confirms this analysis, since the susceptibility is well fitted as a sum χ = χPauli + χBF, with an essentially temperature independent contribution of the conducting donor slab (χPauli = 1.38(8) × 10−4 cm3 mol−1), together with the contribution χBF of a uniform spin chain system (Bonner−Fisher model)36,37 with J/k = −239(3) K (−166 cm−1), attributable to the weakly overlapping (Figure 5c) TCNQF2−• species. Band structure calculations were performed on this salt to understand the origin of the high conductivity (Figure 4b). For such a 3/4 filled system, the lower band is full and the upper band is 1/2 filled. The Fermi level (for a hypothetical metallic behavior) cuts the upper band only in the Γ−Y direction, indicating a one-dimensional character confirmed by the calculated open Fermi surface (Figure 4c) for this β phase. The 1:1 Salt with TCNQ. (1)(TCNQ) crystallizes in the monoclinic system, space group P21/c, with both donor molecule 1 and TCNQ in the general position in the unit cell. From the intramolecular bond distances in both donor and acceptor moieties, we have found a partial charge transfer, with ρ ≈ 0.5 (see above). As shown in Figure 7, the salt adopts a very original structure. The TTF derivatives 1 stack along the c axis while the TCNQ molecules stack along the b axis. Because of the 21 screw axis running along b, the TCNQ stacks are uniform, with a single overlap interaction shown in Figure 8c and a plane-to-plane distance of 3.30 Å. On the other hand, the overlap interaction within the donor slab is more complicated. Molecules are associated two-by-two into inversion-centered dyads with favorable bond-over-ring overlap (Figure 8a) and short plane-to-plane distance (3.52 Å), but these dyads adopt with neighboring molecules along the c stacking axis a so-called spanning overlap interaction (Figure 8b) found in δ phases of BEDT-TTF salts.38 This overlap interaction is most probably much weaker, as confirmed by band structure calculations (see below). The room-temperature conductivity amounts to σRT = 5−8 × 10−2 S cm−1 which is a lower limit value as it is deduced from a two points measurement. The temperature dependence of the resistivity (Figure S2) shows a semiconducting behavior with a weak activation energy (Eact = 0.043 eV, Eact/k = 500 K). Band structure calculations were performed separately for the 2D slabs of molecules 1, and for the 1D stacks of TCNQ molecules. As shown in Figure 9, the strong dimerization of the TTF moieties is clearly seen from the large splitting between the two lowest and two upper bands. A ρ = 0.5 charge transfer is associated with a 3/4-filled system, and hence the Fermi level for a hypothetical metallic behavior in the 2D slabs would cut the two upper bands, characterized however here by a very small dispersion ( 2σ(I)) no. param R1 (I > 2σ(I)) wR2 (all data) goodness-of-fit res. dens (e−·Å−3)

1:1 C20H5F4IN4S6 696.54 black plate 0.17 × 0.11 × 0.02 monoclinic P21 293(2) 5.8590(5) 46.950(4) 8.1829(9) 90.0 94.077(7) 90.0 2245.2(4) 4 2.061 2.038 21629 multiscan 0.764, 0.960 9748 0.0412 8239 632 0.0396 0.0936 1.007 −0.47, 1.19

2:1 C14H6FIN2S6 540.47 dark green plate 0.14 × 0.08 × 0.02 triclinic P1̅ 293(2) 6.7120(13) 7.8150(16) 17.686(4) 91.79(3) 99.44(3) 105.00(3) 881.3(3) 2 2.037 2.537 20274 multiscan 0.784, 0.951 4035 0.0865 2852 217 0.0526 0.1214 1.104 −0.80 1.37

1:1 C20H9IN4S6 624.57 black plate 0.23 × 0.15 × 0.04 monoclinic P21/c 293(2) 22.784(5) 6.6158(13) 14.989(3) 90.00 94.86(3) 90.00 2251.2(8) 4 1.843 1.996 25781 multiscan 0.705, 0.923 5163 0.0641 3955 280 0.0486 0.1078 1.177 −0.58, 1.65

2:1 C28H14I2N4S12 1044.95 black plate 0.28 × 0.21 × 0.05 triclinic P1̅ 150(2) 8.2393(5) 8.4412(5) 12.7149(8) 81.295(2) 82.515(2) 80.488(2) 857.07(9) 1 2.025 2.596 12605 multiscan 0.525, 0.878 3873 0.0274 3520 208 0.0254 0.0645 1.054 −0.72, 1.84

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|; wR2 = [Σw(Fo2 − Fc2)2/ΣwFo4]1/2. mg, 34.8 × 10−6 mol) in CH3CN (2 mL). Crystals were harvested after one week and washed with a little CH3CN. IR (ATR) νCN 2180, 2215 cm−1. On the other hand, the slow diffusion of a solution of 1 (1.9 mg, 4.5 × 10−6 mol) in 1,1,2-trichloroethane (2.5 mL), layered with a solution of TCNQ (1.2 mg, 5.9 × 10−6 mol) in CH3CN (0.5 mL) afforded crystals of (1)(TCNQ), harvested after two weeks and washed with a little CH3CN. IR (ATR) νCN 2216, 2179 cm−1. Crystallography. Data at room temperature were collected on a Kappa CCD diffractometer with single crystals mounted on the top of a thin glass fiber. Data at 150 K were collected on a Bruker SMART II diffractometer with single crystals taken in a loop in oil and put directly under the N2 stream at 150 K. Both diffractometers operate with graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å). Structures were solved by direct methods (SHELXS-97, SIR97)45 and refined (SHELXL-97) by full-matrix least-squares methods,46 as implemented in the WinGX software package.47 Absorption corrections were applied. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations, and not refined, with thermal parameters fixed as 1.2 times Ueq of the attached carbon atom. Crystallographic data on X-ray data collection and structure refinements are given in Table 2. Resistivity Measurements. To measure the longitudinal resistivity, gold pads were evaporated on the surface of the crystals in order to improve the quality of the contacts. Then a standard four points technique was used with a low frequency lock-in detection (Iac = 0.1−1 μA) for measured resistances below 50 kΩ and dc measurement for higher resistances (Idc = 0.1−0.01 μA). Low temperatures have been provided by a cryocooler equipment down to 25 K. Crystals of (1)(TCNQ) are small squared platelets with the a axis perpendicular to the surface. The resistivity has been measured in 2 points with the current flowing in the (b, c) plane containing both

sufficient in the TTF neutral state to allow for the formation of a regular halogen bond.

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EXPERIMENTAL SECTION

Syntheses. The TTF derivative 1 and TCNQF4 were prepared as previously described.11b,44 TCNQ, TCNQF, and TCNQF2 were commercially available and used without further purification. With TCNQF4. Donor molecule 1 (4.2 mg, 10.0 × 10−6 mol) dissolved in CH2Cl2 (2.5 mL) was layered with a solution of TCNQF4 (2.9 mg, 10.5 × 10−6 mol) in CH3CN (0.5 mL) in small glass tubes (Pasteur pipettes) and the solutions left to diffuse in the dark. Crystals of (1)(TCNQF4) were harvested after two weeks and washed with a little CH3CN. IR (ATR) νCN 2172, 2192 cm−1. With TCNQF2. Donor molecule 1 (1.9 mg, 4.5 × 10−6 mol) dissolved in 1,1,2-trichloroethane (2.5 mL) was layered with a solution of TCNQF2 (1.4 mg, 5.8 × 10−6 mol) in CH3CN (0.5 mL). Crystals of (1)2(TCNQF2) were harvested after two weeks and washed with a little CH3CN. IR (ATR) νCN 2164, 2182 cm−1. With TCNQF. Donor molecule 1 (4.1 mg, 9.7 × 10−6 mol) dissolved in 1,1,2-trichloroethane (2.5 mL) was layered with a solution of TCNQF (3.5 mg, 15.7 × 10−6 mol) in CH3CN (0.5 mL). Very small polycrystals were harvested after two weeks and washed with a little CH3CN. IR (ATR) νCN 2160, 2196 cm−1, to be compared with reported data for neutral (2223, 2217 cm−1) and ionic (2188, 2200 cm−1) TCNQF, indicating a large charge transfer in this salt. The very limited quantities and poor crystallinity did not allow us to investigate this salt further. With TCNQ. Diffusion techniques or slow evaporation of mixed solutions afforded different compounds. (1)2(TCNQ) was obtained by slow evaporation of a solution of 1 (14.7 mg, 35.0 × 10−6 mol) in 1,1,2-trichloroethane (20 mL) mixed with a solution of TCNQ (7.1 4255

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EDT-TTF-I chains (along the c axis) and TCNQ chains (along the b axis). Considering the small dispersion of room temperature conductivity measured on different samples, the anisotropy of conductivity in the plane should be small (about a factor 2) unless the resistance of the contacts is much higher than that of the sample itself. Magnetic Measurements. The magnetic susceptibility measurements were obtained with the use of a Quantum Design SQUID magnetometer MPMS-XL. This magnetometer works between 1.8 and 400 K for dc applied fields ranging from −5 to 5 T. Measurements were performed on polycrystalline samples of (1)(TCNQF4) (15 mg) and (1)2(TCNQF2) (1.6 mg). The magnetic data were corrected for the sample holder and the diamagnetic contributions. Band Structure Calculations. The tight-binding band structures were calculated with the effective one-electron Hamiltonian of the extended Hückel method,48 as implemented in the Caesar 1.0 chain of programs.49 The off-diagonal matrix elements of the Hamiltonian were calculated according to the modified Wolfsberg−Helmholz formula.50 All valence electrons were explicitly taken into account in the calculations and the basis set consisted of double-ζ Slater-type orbitals for all atoms except H (ζ Slater-type orbital) using the RoothaanHartree−Fock wave functions of Clementi and Roetti.51

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic files in CIF format, tables of bond distances in TCNQs and temperature dependence of the resistivity of different salts (as pdf file). This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (33) 2 23 23 52 43. Fax: (33) 2 23 23 67 32. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work comes from CNRS, Université Rennes 1, and ANR through Contract No. ANR-08-BLAN0091-02. We also thank Th. Roisnel (CDIFX Rennes) for access to the diffraction facilities.

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REFERENCES

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