FULL PAPER DOI: 10.1002/chem.201302507

Charge-Assisted Halogen Bonding: Donor–Acceptor Complexes with Variable Ionicity Julien Lieffrig,[a] Olivier Jeannin,[a] Arkadiusz Fra˛ckowiak,[b] Iwona Olejniczak,[b] Roman S´wietlik,[b] Slimane Dahaoui,[c] Emmanuel Aubert,[c] Enrique Espinosa,[c] Pascale Auban-Senzier,[d] and Marc Fourmigu*[a]

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Abstract: Charge-assisted halogen bonding is unambiguously revealed from structural and electronic investigations of a series of isostructural charge-transfer complexes derived from iodinated tetrathiafulvalene and tetracyanoquinodimethane derivatives, (EDT-TTFI2)2ACHTUNGRE(TCNQFn), n = 0–2, which exhibit variable degrees of ionicity. The iodinated tetrathiafulvalene derivative, EDT-TTFI2, associates with tetracyanoquinodimethane (TCNQ) and its derivatives of increasing reduction potential (TCNQF, TCNQF2) through highly directional CI···N  C halogen-bond interactions. With the

less oxidizing TCNQ acceptor, a neutral and insulating charge-transfer complex is isolated whereas with the more oxidizing TCNQF2 acceptor, an ionic, highly conducting charge-transfer salt is found, both of 2:1 stoichiometry and isostructural with the intermediate TCNQF complex, in which a neutral– ionic conversion takes place upon coolKeywords: charge transfer · conducting materials · crystal engineering · donor–acceptor systems · halogen bonding · noncovalent interactions

Introduction First identified in R3N···X2 complexes (X = Cl, Br, I) by Guthrie[1] and Remsen and Norris,[2] a halogen bond is an attractive interaction between a halogen atom and a lone-pairpossessing atom (frequently nitrogen and oxygen, but also another halogen). This interaction can lead to the formation of supramolecular adducts with short intermolecular distances, as revealed by Hassel from extensive structural investigations.[3] In recent years, this intermolecular interaction has been the subject of renewed interest,[4, 5] owing to its importance in the fields of molecular recognition,[6] crystal engineering,[7] material science,[8, 9] soft matter,[10] nanosciences,[11] catalysis,[12] and biological systems.[13] It has been shown that the behavior of halogen atoms, which acts as an electron acceptor through the so-called s-hole,[14, 15] close to an electron-donor molecule closely resembles the behavior of heteroatom-bound hydrogen atoms (that is, hydrogen-bond donors) in many aspects, the term “halogen bonding” being

[a] Dr. J. Lieffrig, Dr. O. Jeannin, Dr. M. Fourmigu Institut des Sciences Chimiques de Rennes Universit Rennes 1 & CNRS UMR 6226 Campus de Beaulieu, 35042 Rennes (France) Fax: (+ 33) 23-23-67-32 E-mail: [email protected] [b] A. Fra˛ckowiak, Dr. I. Olejniczak, R. S´wietlik Institute of Molecular Physics, Polish Academy of Sciences ul. Mariana Smoluchowskiego 17, 60-179 Poznan´ (Poland) [c] Dr. S. Dahaoui, Dr. E. Aubert, Prof. Dr. E. Espinosa Laboratoire CRM2, UMR CNRS 7036 Institut Jean Barriol, Universit de Lorraine BP 70239, 54506 Vandoeuvre-ls-Nancy (France) [d] Dr. P. Auban-Senzier Laboratoire de Physique des Solides, Universit Paris-Sud UMR CNRS 8502, Bt. 510, 91405 Orsay (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201302507.

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ing. A correlation between the degree of charge transfer and the CI···N  C halogen-bond strength is established from the comparison of the structures of the three isostructural complexes at temperatures from 300 to 20 K, thus demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral–ionic conversion in (EDT-TTFI2)2ACHTUNGRE(TCNQF) is further investigated through the temperature dependence of its magnetic susceptibility and the stretching modes of the C  N groups.

suggested so as to stress its similarity with hydrogen bonding.[16] This interaction is strongest with the most polarizable halogens (I > Br @ Cl @ F) and, for carbon-bonded halogens, also depends on the carbon hybridization with Csp > Csp2 > Csp3. On the Lewis base side, the strongest halogen bond acceptors appear to be nitrogen and chalcogen atoms.[17] In the most favorable cases, activated carbon-bound halogen atoms can interact with Lewis bases with strengths comparable to, or even larger than, those of competitive hydrogen bonds.[18, 19] For example, the intermolecular hydrogen bond found in pentachlorophenol, C6Cl5OH, has been estimated to be more energetic than the halogen-bonded Cl3 derivative by approximately 3 kJ mol1, whereas the analogous hydrogen bond in the bromo analogue, C6Br5OH was mostly equivalent to the halogen bonded Br3 derivative.[20] Theoretical investigations of the magnitude and origin of the attraction and directionality of halogen bonds have been reported originally for R3N···X2 (X = Cl, Br, I) complexes[21] and recently extended to complexes involving perfluoroalkyl and perfluoroaryl iodides,[22, 23] as well as complexes of tribromomethyl derivatives and bromide anion.[24] Although short-range orbital interactions undoubtedly contribute to the bonding in the strongest halogen bonded systems, as illustrated by the formation of I3 upon reaction of I2 with I, in the less covalent, more weakly bound systems, both electrostatic and orbital (charge-transfer) interactions contribute to the energetic of halogen bonding,[23, 24] which is also influenced by overlap repulsion effects.[25] The comparison with hydrogen bonding also holds for the relationship between interaction energy and intermolecular distances. For example, a systematic examination of series of XH···O hydrogen bonds by X-ray charge density and ab initio methods revealed that the relationship between hydrogen-bonding energies and H···O distances appears to be nonlinear.[26] Very similar effects were found for halogen-bonded systems, thus suggesting that halogen bonds are also much stronger when they are very short.[22]

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In addition, it is well known that when one of the two partners in a hydrogen-bonded complex bears an electric charge, the resulting hydrogen bond is significantly stronger than that between the two neutral species. This charge-assisted phenomenon, to some degree, supports the electrostatic nature of hydrogen bonding.[27, 28] Its transposition to halogen bonding has been very recently addressed in theoretical calculations.[22] Experimentally, charge-assisted halogen bonding can be investigated in several ways, either by the introduction of positive charge on the halogenated molecules (to possibly enhance the s hole), by the use of anions as Lewis bases, or both. The second approach has developed along two lines, namely, toward the elaboration of anion receptors in solution,[29] or the construction of crystalline anionic networks[30] built out of neutral halogenated molecules (for example, tetrabromomethane,[24] diiodoacetylene, tetraiodoethylene, and perfluoroiodobenzenes [31]) interacting through short halogen-bonding interactions with halide,[32] but also pseudohalide[33] or oxo anions.[34] Cationic halogenated molecules have been particularly developed in the context of molecular conductors,[8] where cation radical salts of iodinated tetrathiafulvalenes (TTF) have been shown to interact through halogen bonding with anions acting as halogen-bond acceptors, such as halides,[35] polyhalides,[36] polyhalometallates,[37] polycyanometallates,[38] and organic nitriles.[39] The halogen bond in these salts is often shorter than those in neutral systems, but their fixed stoichiometry—most often two TTF molecules for one anion—imposes an actual charge of + 0.5 for each TTF molecule and the variety of crystal structures does not allow for easy comparisons of halogen-bond lengths. Thus, to investigate charge-assisted halogen bonding, it would be highly desirable to have at hand one single system where the charge could be modified at will, without any other structural changes, a rare property described as a neutral–ionic transition (NIT). Observed in a few organic charge-transfer complexes of p-donor (D) and p-acceptor (A) molecules, NIT is reversible switching between two distinct states under external stimuli such as temperature, pressure, or light.[40] NIT is very rare event and up until now has been essentially observed in 1:1 D–A complexes organized into alternaing stacks, where the ionic phase stabilized at low temperatures (or high pressure) is characterized by a polar structure and having ferroelectric properties. The very first example, which was described by Torrance et al. in 1981,[41, 42] tetrathiafulvalene·p-chloranil (TTF·QCl4) is still today the most investigated compound,[43] which exhibits attractive properties such as a photoinduced phase transition,[44] nonlinear electric transport,[45] current-induced switching of resistance,[45] and a large dielectric constant.[46] Therefore, a very attractive target is a p-donor/p-acceptor system linked through halogen-bond interactions and exhibiting a neutral–ionic transition. Such a system would offer an invaluable opportunity to evaluate the change, if any, of the strength of halogen bonding upon an neutral–ionic transition. Only a handful halogen-bonded, charge-transfer complexes have been described to date, consisting of iodo-TTF

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FULL PAPER derivatives as p-donors and TCNQs as p-acceptors. These complexes are either fully neutral (with TCNQ)[47, 48] or fully ionic (with TCNQF4,[48, 49] TCNQF2,[48] or DDQ)[50] with one single example of a mixed-valence conducting salt.[48] In all cases, the degree of charge transfer is not modifiable and halogen bond lengths cannot be easily compared. We describe here an original series of isostructural donoracceptor salts where the ionicity of electron donor (D) and acceptor (A) molecules, linked together by halogen bonding interactions, can be effectively tuned, either by the proper choice of acceptor molecule, or by temperature, providing an excellent opportunity to evaluate the sensitivity of halogen bonding to charge changes in a model system where charge only is selectively modified.

Results and Discussion Our system is based on the association of the EDT-TTFI2 donor molecule with three different TCNQs (Figure 1), the latter being the parent TCNQ together with the difluoro and monofluoro derivatives, whose reduction potentials are

Figure 1. Association of the EDT-TTFI2 donor molecule with three different TCNQs

0.30 V for TCNQF2,[51] 0.26 V for TCNQF,[52] and 0.14 V for TCNQ (versus SCE). Indeed, the neutral–ionic transition observed for TTF·QCl4 requires a careful tuning of the redox potentials, Eox(D) and Ered(A), of the donor and acceptor molecules, respectively.[41, 53] If Ered(A) is greater than Eox(D), full charge transfer is indeed expected, whereas if DE, that is, Ered(A)Eox(D), is less than approximately 35 V, a neutral charge-transfer complex is isolated. For intermediate DE values, those between approximately0.35 and 0.0 V, a neutral–ionic transition can be eventually observed in alternated stack structures such as TTF·QCl4. The association of EDT-TTFI2 with different TCNQFn acceptors offers therefore an adapted range of DE values to explore simultaneously the ionicity of the compounds an the charge sensitivity of the halogen bonding that can take place be-

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tween the iodine atoms of EDT-TTFI2 and the nitrile substituents of the TCNQ derivatives, as described below. Slow diffusion of CH3CN solutions of TCNQFn (n = 0—2) over a solution of EDT-TTFI2 in CH2Cl2 afforded, over one week, crystals of the title compounds, namely, (EDTTTFI2)2ACHTUNGRE(TCNQ), (EDT-TTFI2)2ACHTUNGRE(TCNQF), and (EDTTTFI2)2ACHTUNGRE(TCNQF2), abbreviated as (I2)2ACHTUNGRE(TCNQF2), (I2)2ACHTUNGRE(TCNQF), and (I2)2ACHTUNGRE(TCNQ), respectively. These compounds can also be obtained from slow concentration of solutions of I2 in CS2 with TCNQFn in CH3CN. The three compounds are isostructural, they crystallize in the triclinic system, space groupP1, with I2 in a general position in the unit cell and the TCNQFn molecules on an inversion center, hence the 2:1 stoichiometry. For TCNQF, the fluorine atom site occupancy is 50 %. As shown in Figure 2 for (I2)2ACHTUNGRE(TCNQF2), dyads of donor molecules alternate with TCNQFn along (ab) into [(D)2A(D)2A]x chains, forming

layers perpendicular to c, linked together by CI···N  C halogen bonds (Figure 2 c). The 2:1 stoichiometry might find its origin in the noncentrosymmetric character of donor molecule I2, linked by halogen bonding to the centrosymmetric TCNQFn acceptor, as already noted in imidazole-based hydrogen-bonded systems.[54] As shown in Table 1, the 293 K I···N halogen-bond length is 3.082(5) and 3.08(3)  in the Table 1. Geometrical characteristics in (I2)2ACHTUNGRE(TCNQFn) complexes (n = 0– 2) at 293 K.[a]

Compound

T [K]

A []

b []

c []

d []

1TCNQFn I···N []

293(2) 1.337 1.439 1.377 1.434 0.2 (I2)2ACHTUNGRE(TCNQ) (I2)2ACHTUNGRE(TCNQF) 293(2) 1.339 1.444 1.383 1.429 0.1 (I2)2ACHTUNGRE(TCNQF2) 293(2) 1.352 1.420 1.409 1.423 1.0

3.082(5) 3.078(33) 2.957(8)

[a] The estimated TCNQFn charge (1TCNQFn) is calculated according to the Kistenmacher formula (ref. [55]) which reads as 1TCNQFn = A[c/ACHTUNGTRENUG(b+d)] + B, with ATCNQ = 41.667 and BTCNQ = 19.833, where a–d are averaged distances. The following A and B values are used for the TCNQF and TCNQF2 complexes: ATCNQF = 57.87, BTCNQF = 27.79, ATCNQF2 = 56.78 and BTCNQF2 = 27.15 (see text).

Figure 2. a) Projection view along the b axis of the unit cell of (I2)2ACHTUNGRE(TCNQF2) at 293 K. b) Projection view along the long molecular axis of donor and acceptor molecule showing the alternated (D)2A(D)2A(D)2 stacks running along (ab). Hydrogen atoms were omitted for clarity. (c) Detail of the halogen-bonding interactions (purple dotted lines).

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TCNQ and TCNQF complexes, respectively, whereas a notably shorter value, 2.957(8) , is found with TCNQF2, a possible indication of different degree of charge transfer in the three compounds. Intramolecular bond distances within the TTF and TCNQ cores are pertinent indicators of the degree of charge transfer in such compounds. Empirical formulae have been reported for TCNQ salts,[55] correlating intramolecular bond lengths and charge. Based on reported X-ray crystal structures for TCNQF and TCNQF2 in neutral[56, 57] and radicalanion states,[58, 59] we have derived analogous formulae for these two acceptor molecules. Applied to the three isostructural compounds described here (Table 1), we find that, at 293 K, the TCNQ and TCNQF compounds are essentially neutral while the TCNQF2 acceptor molecule is actually reduced to the radical-anion state, affording an ionic salt, (I2 + 0.5 )2ACHTUNGRE(TCNQF21). These striking differences are fully in line with the observed shortening of the CI···N  C halogenbond interaction in the TCNQF2 salt (Table 1), thus providing the first evidence of charge-assisted halogen bonding in three isostructural salts at the same temperature. The resistivity of the three isostructural compounds and its temperature dependence (Figure 3) also gives complementary information: the ionic (I2)2ACHTUNGRE(TCNQF2) is highly conducting (s293 = 1 S cm1) with very low activation energy, Eact = 82 meV at 293 K that decreases further upon cooling. On the other hand, (I2)2ACHTUNGRE(TCNQF) is a semiconductor with s293 = 5  104 S cm1 and a low activation energy, Eact, that continuously decreases with temperature. We can estimate Eact to be 108 meV at around 293 K and only 72 meV below 200 K, as shown in Figure 3. Finally, the TCNQ compound, (I2)2ACHTUNGRE(TCNQ), behaves as an insulator, with sRT = 5 

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FULL PAPER Table 2. Geometrical characteristics of the TCNQ core in the crystal structures of the TCNQF compound, (I2)2ACHTUNGRE(TCNQF), at different temperatures.[a] Compound

T [K]

a []

b []

c []

d []

1TCNQFn

I···N []

(I2)2ACHTUNGRE(TCNQ)

293(2) 150(2) 20(2)

1.337 1.349 1.349

1.439 1.447 1.442

1.377 1.374 1.387

1.434 1.443 1.439

0.2 0.0 0.1

3.082 3.050 3.003

(I2)2ACHTUNGRE(TCNQF)

293(2) 150(2) 100(2) 20(2)

1.339 1.350 1.349 1.365

1.444 1.437 1.435 1.429

1.383 1.383 1.387 1.396

1.429 1.432 1.432 1.426

0.1 0.1 0.2 0.5

3.077 3.014 2.993 2.945

(I2)2ACHTUNGRE(TCNQF2)

293(2) 150(2) 20(2)

1.352 1.349 1.357

1.420 1.424 1.428

1.409 1.420 1.412

1.423 1.424 1.421

1.0 1.1 0.9

2.986 2.957 2.925

[a] See Table 1 for details on calculations of the TCNQFn charge (1). Figure 3. Temperature dependence of the resistivity of the three compounds.

105 S cm1 and a much higher and temperature-independent activation energy, Eact = 0.25 eV. The high conductivity of the ionic TCNQF2 compound is directly related to the associated molecular formula, (I2 + 0.5)2ACHTUNGRE(TCNQF21), given above, as full charge transfer to the TCNQF2 acceptor molecule gives indeed a mixed-valence character to EDT-TTFI2 dyads, which interact with each other laterally in the (a,b) plane (Figure 2 b). The much higher resistivity of the TCNQ and TCNQF compounds is in accordance with their essentially neutral character. However, the slightly higher conductivity of the TCNQF compound, in comparison with the TCNQ compound, lets us infer a partial, even if weak, charge transfer in the former at 293 K. This result suggests that (I2)2ACHTUNGRE(TCNQF) could sit under ambient conditions on the verge of a NI transition, which could be activated under external stimuli such as temperature or pressure. The collection of single-crystal X-ray diffraction data and subsequent refinements were accordingly performed at lower temperatures (down to 20 K) for the three complexes. The changes in (i) the intramolecular bond distances within the TCNQFn core, (ii) the calculated TCNQFn charge, and (iii) the intermolecular I···N halogen-bond distances are collected in Table 2 and summarized in Figure 4. All complexes were found to crystallize in the triclinic system, space group P 1, exactly as at 293 K, without indication of any structural transition such as the loss of an inversion center. Upon cooling, the charge of the acceptor molecule was found to remain essentially zero in the TCNQ complex, and essentially 1 in the TCNQF2 salt, just as at 293 K. As shown in Figure 4, the I···N halogen-bond distance slowly decreases in the two systems upon cooling, reflecting the unit cell contraction. On the other hand, in the intermediate TCNQF complex, the calculated charge on TCNQF decreases strongly, from approximately 0 at 293 K, down to approximately 0.5 at 20 K, associated with a parallel unit-cell contraction and I···N halogen-bond strengthening. This striking behavior provides an unambiguous confirmation for the charge-assisted character of the halogen-bond interaction, not only be-

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Figure 4. Changes in the I···N halogen-bond length within the (I2)2ACHTUNGRE(TCNQFn) complexes with temperature. The reported charges (1) on the TCNQFn molecules are taken from Table 2. Solid lines are provided in order to guide the reader.

tween the three TCNQFn compounds at 293 K, but also within the very same TCNQF complex, the ionicity of which varies with temperature. Investigations of the neutral-ionic conversion in (I2)2ACHTUNGRE(TCNQF): The evolution of the charge in (I2)2ACHTUNGRE(TCNQF) has been evaluated from the temperature dependence of the C  N stretching modes of TCNQF. It is well known indeed that the frequency of several vibrational modes of both TTF and TCNQ derivatives is sensitive to charge. The molecule, EDT-TTF-I2, has three vibrational modes related to C=C stretching and, on ionization, these modes are known to exhibit a considerable shift towards lower frequencies.[60] Similarly for the TCNQ derivatives, the C=C stretching modes are very sensitive indicators of the degree of ionization.[61] However, in IR spectra of (I2)2ACHTUNGRE(TCNQF), many vibrational features are a consequence of the coupling of vibrational modes with charge-transfer transitions. On the other hand, the C  N stretching modes of TCNQ derivatives are well separated from other modes and can also be used for determination of the degree of ionization. The TCNQ molecule

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Figure 5. Temperature dependence of (a) the optical conductivity as obtained from the reflectance spectra by Kramers–Kronig transformation (polarization E j j (ab)) and (b) the transmittance spectra (polarization E j j (a + b)) in the region of C  N stretching in (I2)2ACHTUNGRE(TCNQF).

has three modes related to the stretching of the C  N groups: one is Raman active, n2 (ag) = 2229 cm1, and two are IR active, n19 (b1u) = 2228 cm1, n33 (b2u) = 2228 cm1.[61a] For the TCNQ· anion, these modes shift towards lower frequencies by 23, 47, and 61 cm1, respectively.[61b,c] The fluorinated TCNQF molecule has no center of symmetry; nevertheless, its C  N modes are very similar,[62] therefore, in what follows we use an analogous notation. The IR spectrum of neutral TCNQF powder in KBr in the region of C  N stretching show b1u- and b2u-like modes at 2223 and 2217 cm1, respectively (see the Supporting Information, Figure S1); for neutral TCNQF2, these modes are at 2231 and 2219 cm1, respectively. In the Raman spectrum of a single crystal of (I2)2ACHTUNGRE(TCNQF), we see C  N bands at 2199 and 2213 cm1 (see the Supporting Information, Figure S2); both bands are attributed to ag-like modes. Note that in the salt, two out of the four CN groups of TCNQF are involved into halogen bonding, hence the observed band splitting. Polarized IR reflectance and transmittance spectra of single crystals were measured for two polarizations (see the Supporting Information, Figure S3): the electrical vector of polarized IR beam along the (ab) direction (polarization E||(ab), parallel to staking axis) and along the (a + b) direction (polarization E||(a + b), parallel to molecular planes). For polarization E||(ab), a single line at 2175 cm1 at 150 K (see the Supporting Information, Figure S4) is related to the effect of coupling of the symmetric in-phase stretching of C  N groups (ag-like mode) with charge-transfer transition. As shown in Figure 5 a, this band, which is associated with

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a face-to-face transition between donor and acceptor, shows a very strong temperature dependence, a consequence of considerable growing of the electron density involved in the coupling with molecular vibrations when temperature decreases. On the other hand, for the perpendicular polarization, E||(a + b), we see in Figure 5 b four main vibrational features at 2175, 2188, 2208, and 2224 cm1 (at 300 K). The 2175 and 2188 cm1 bands are a consequence of coupling of the C  N stretching with charge transfer (a weak spectral feature at 2188 cm1 can also be found in the conductivity spectrum in Figure 5 a). Most important for our discussion are the bands at 2208 and 2224 cm1, which are associated with b1u-like modes of the anionic TCNQF· and neutral TCNQF0, respectively, thus indicating the coexistence of both species in the solid state. Note that the b2u-like modes are usually of much lower intensity and are not clearly seen in our spectra. The observed bands together with their assignment are collected in the Supporting Information, Table S1. For analysis of the degree of ionization, the b1u-like modes of ionic TCNQF· are the most important because they are not (or rather weakly) coupled with electrons. If we assume that the integral intensity of the suitable band is proportional to the number of TCNQF· ions, we see that the integral intensity of the band at 2208 cm1 grows very strongly below about 150 K (Figure 6). On the other hand, the integral intensity of the 2224 cm1 peak related to neutral TCNQF0 is nearly constant while the peak frequency increases slowly down to about 150 K (lattice contraction) and

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Figure 6. Temperature dependence of peak integral intensity for the 2208 cm1 b1u mode of TCNQF· (black diamonds) and the 2224 cm1 b1u mode of neutral TCNQF (grey squares). Integral intensities were determined from band decomposition by fitting them with Voigt (or Lorentz) functions.

then is nearly unchanged (see the Supporting Information, Figure S5). These data demonstrate that neutral molecules exist in the salt down to the lowest temperatures and an NI conversion is occurring, with a continuous change of the NI ratio of the crystal with temperature rather than a sharp transition. The temperature dependence of peaks assigned to ag-like modes at 2175 and 2188 cm1 (see the Supporting Information, Figure S6) supports our conclusion that these vibrational features are due to coupling with a charge-transfer transition. Upon cooling, the integral intensities grow in agreement with the increase of electron density participating in electron-molecular vibration coupling. The temperature dependence of the magnetic susceptibility of (I2)2ACHTUNGRE(TCNQF), shown here as cT versus T (Figure 7), completes this preliminary investigation of the NI conversion. Although the compound is essentially neutral and weakly magnetic at 300 K, the cT value strongly increases upon cooling, in accord with the increase of ionicity. A full charge transfer without any intermolecular interaction would afford a cT value of approximately 0.75 (0.375  2). The X-ray structures showed that 1TCNQF does do exceed 0.5 at 20 K, indicating, at best, approximately 50 % conversion and a maximum reachable cT value, in the absence of any interactions, of approximately 0.375 cm3 K mol1. The presence of a maximum in the cT versus T curve around 125 K is therefore due to antiferromagnetic interactions taking place between the growing number of radical species upon further cooling. Notably, the strengthening of the halogen-bonding interaction in (I2)2ACHTUNGRE(TCNQF) is not simply due to the compressibility of the structure upon cooling. Indeed, the temperature dependence of the unit-cell parameters of (I2)2ACHTUNGRE(TCNQF), determined between 300 and 100 K (Figure 8), shows that the c parameter, and to a smaller extent, the b parameter are associated with the halogen-bonding interaction that develops between layers (see Figure 2) and exhibit indeed a clear regime change below a value in the range 150–

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Figure 7. Temperature dependence of the cT product for (I2)2ACHTUNGRE(TCNQF). Molar susceptibility values were corrected for Pascal diamagnetism and estimated to be 6  104 cm3 mol1.

200 K; this behavior is at variance with that of the a parameter, which contracts gradually with decreasing temperature. In other words, within the (I2)2ACHTUNGRE(TCNQF) charge-transfer complex, the change of the ionicity below 200 K, as determined from the IR, Raman, and susceptibility data, is directly correlated with a strengthening of interlayer halogenbond interactions. Another point of discussion concerns the change of the actual charge distribution on the nitrogen atoms of the TCNQFn molecules as a function of (i) the TCNQFn charge and (ii) the number of fluorine atoms. To this end, we have performed calculations on the TCNQFn molecules in their crystalline geometries with net charge transfer of q(e) = 0,

Figure 8. Temperature dependence of a, b, and c unit cell parameters of (I2)2ACHTUNGRE(TCNQF).

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1. Accordingly, single-point MP2/aug-cc-pVTZ calculations of TCNQ (293 and 20 K, q(e) = 0), TCNQF (293 K, q(e) = 0), and TCNQF2 (293 K and 20 K, q(e) = 1) were carried out. The net charge of the TCNQFn molecules is clearly reflected by the net integrated charge of their corresponding four nitrogen atoms, ranging from 1.12 to 1.15 e (TCNQ, 293 K and q(e) = 0), 1.14 to 1.16 e (TCNQF, 293 K and q(e) = 0), 1.31 to 1.32 e (TCNQF2, 293 K and q(e) = 1), 1.12 to 1.14 e (TCNQ, 20 K and q(e) = 0), and 1.31 to 1.33 e (TCNQF2, 20 K and q(e) = 1). The accumulation of the transferred charge at the nitrogen atoms of the TCNQ molecule was firstly experimentally reported in the electron-density study of the one-dimensional organic metal bis(thiodimethylene)tetrathiafulvalene-tetracyanoquinodimethane (BTDMTTF-TCNQ) derivative,[62] which crystallizes as a segregated charge-transfer complex. Along the TCNQFn series at 293 K, the modification of the intermolecular distance of the halogen-bonding interaction, I···N, follows the variation of the net charge of the involved nitrogen atom, decreasing in the order dACHTUNGRE(I···N) = 3.082 , 3.077 , and 2.986  as the charge increases in the order q(N) = 1.147, 1.152, and 1.316 e, respectively (see the Supporting Information, Figure S7). The nitrogen atoms belonging to TCNQ exhibit very similar net charges at 293 and 20 K. A similar observation was made for TCNQF2 at the same temperatures. The comparison of the complexes involving the TCNQ (q(e) = 0) and TCNQF2 (q(e) = 1) molecules at 293 and 20 K indicates that the relative decrease of the intermolecular distance, I···N, in going from TCNQ to TCNQF2 (DdI···N/dI···N = 3.1 and 2.6 % at 293 and 20 K, respectively) remains almost equivalent, as well as the corresponding relative increase of the net charge of the nitrogen atom (Dq(N)/q(N) = 15 and 16 % at 293 and 20 K, respectively). Accordingly, at any of both temperatures, a relative variation of Dq(N)/q(N) of approximately 15 % from TCNQ (q(e) = 0) to TCNQF2 (q(e) = 1) is followed by the decrease in DdI···N/dI···N of approximately 3 %, indicating the effect of the net charge of the nitrogen atom on the I···N distance (DdI···N = 0.096 and 0.078  at 293 and 20 K, respectively). Notably, the DdI···N/dI···N decrease of 3 % remains even after shrinkage of the unit cell from 293 to 20 K, a temperature change that leads to the shortening of the I···N distances (DdI···N/dI···N being approximately 2.6 and 2.0 % for complexes involving TCNQ and TCNQF2 molecules, respectively). This feature underlines the significance of the dependence of the halogen-bonding distance with the net charge of the nitrogen atom, which straightforwardly depends on the net charge of the TCNQ derivative at any temperature. Finally, one could also expect that the substitution of electron-withdrawing fluorine atoms in the TCNQ molecule could modify the basicity of the nitrogen lone pair in the halogen-bonding interaction and, therefore, the I···N intermolecular distance. The topological analysis of the negative Laplacian of the electron density, L(r) = 521(r), at the charge concentration site of the nitrogen lone-pair (LCC(N)) is a measure of its basicity character.[20, 63] Accordingly, the

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decrease/increase of LCC(N) is expected to increase/decrease the I···N distance. From TCNQ to TCNQF2, a relative decrease DLCC(N)/LCC(N) is observed (approximately 5 and 4 % at 293 and 20 K). On the other hand, a very small opposite relative increase of DLCC(N)/LCC(N) (approximately 0.4 % at 293 K) is observed in going from TCNQ to TCNQF (in that case, where both molecules are neutral and nitrogen atoms are similarly charged, a relative decrease, DdI···N/dI···N, of approximately 0.2 % stands). Consequently, in TCNQF and TCNQF2, the electron withdrawing effect of fluorine atoms seems to lead only to a small perturbation in the observed I···N distance in front of the main effect associated to the net charge of the nitrogen atom.

Conclusion A series of three isostructural 2:1 donor/acceptor complexes, (I2)2ACHTUNGRE(TCNQFn), provide a rare example of stimulable electroactive compounds where the degree of charge transfer can be controlled either by the proper choice of acceptor molecule (TCNQ versus TCNQF versus TCNQF2), or by temperature in the TCNQF compound, where a neutral– ionic conversion takes place upon cooling without structural transition. Hence, the robustness of the P1 centrosymmetric structure allows for a systematic and nonbiased comparison of the three salts, thus demonstrating a direct and unambiguous link between the ionicity of the compound and the strength of the halogen bond.

Experimental Section Synthesis: EDT-TFFI2 (I2) was prepared as previously described.[65] TCNQ, TCNQF, and TCNQF2 are commercially available and were used as received. Diffusion experiments for crystallization described below were performed in Pasteur pipettes closed at the thinnest end. (I2)2ACHTUNGRE(TCNQ): A solution of I2 (2.2 mg, 4.0  106 mol) in CH2Cl2 (2.5 mL) was layered with a solution of TCNQ (1.2 mg, 5.9  106 mol) in CH3CN (0.5 mL). Slow diffusion at 293 K over a two week period afforded black crystals of the title compound. (I2)2ACHTUNGRE(TCNQF): A solution I2 (24.8 mg, 45.4  106 mol) in CS2 (30 mL) was mixed with a solution of TCNQF (5.0 mg, 22.5  106 mol) in CH3CN (5 mL). Slow evaporation afforded the title compound as black crystals. (I2)2ACHTUNGRE(TCNQF2): A solution of I2 (2.2 mg, 4.0  106 mol) in CH2Cl2 (2.5 mL) is layered with a solution of TCNQF2 (1.4 mg, 5.8  106 mol) in CH3CN (0.5 mL). Slow diffusion at room temperature over a two weeks period afforded black crystals of the title compound. X-ray crystallography: Data at 293 K were collected on a Kappa CCD diffractometer with single crystals mounted on the top of a thin glass fiber. Data at 150 and 100 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. Data at 20 K were collected on an OxfordXcalibur-Atlas CCD-based diffractometer and cooled with an Oxford Diffraction helijet open-flow He gas cryosystem. All diffractometers operate with graphite-monochromated Mo-Ka radiation (l = 0.71073 ). Structures were solved by direct methods (SHELXS-97, SIR97)[64] and refined (SHELXL-97) by full-matrix least-squares methods,[65] as implemented in the WinGX software package.[66] Absorption corrections were applied. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations, and not refined, with

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thermal parameters fixed as 1.2 times Ueq of the attached carbon atom. Crystallographic data on X-ray data collections and structure refinements are given in the Supporting Information (Tables S2–S4). CCDC-932712 ((I2)2TCNQ at 150 K), 932713 ((I2)2TCNQ at 20 K), 932714 ((I2)2TCNQ at 293 K), 932715 ((I2)2TCNQF2 at 150 K), 932716 ((I2)2TCNQF2 at 20 K), 932717 ((I2)2TCNQF2 at 293 K), 932718 ((I2)2TCNQF at 100 K), 932719 ((I2)2TCNQF at 150 K), 932720 ((I2)2TCNQF at 20 K), and 932721 ((I2)2TCNQF at 293 K) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif. IR and Raman spectroscopy of (I2)2ACHTUNGRE(TCNQF): The single crystals of (I2)2ACHTUNGRE(TCNQF) were in the form of platelets with the best-developed crystal face parallel to the (001) crystallographic plane. Raman spectra were measured at 300 K on single crystals of the salt, (EDT-TTF-I2)2TCNQF, in a backscattering geometry. The measurements were performed on a Labram Horiba Jobbin–Yvon 800 spectrometer equipped with He–Ne laser (632.8 nm) and a liquid-nitrogen cooled CCD detector. The laser beam was focused on the crystal face (001). Infrared polarized reflectance at near normal incidence were measured from the (001) crystal face of single crystals for the electrical vector of the polarized beam parallel to the maximum of reflected energy (Emax), that is, nearly parallel to the stacking axis. Some selected crystals were so thin that the transmittance spectra for perpendicular polarization (Emin) could be recorded (in this case the electrical vector was parallel to the molecular planes). Unfortunately, for polarization Emax, the transmittance could not be measured. For IR studies we used a FT-IR Bruker Equinox 55 spectrometer equipped with a Bruker Hyperion 1000 microscope in the reflectance or transmittance mode. The reflectance data were normalized to the reflectance of a high-quality aluminum mirror. The optical conductivity spectra were determined by Kramers–Kronig analysis of the reflectance data. The IR spectra were measured as a function of temperature in the region T = 8–300 K. The samples were mounted in a continuous flow vacuum helium cryostat with KBr windows. Additionally, using KBr pellet technique we measured the IR spectra of neutral TCNQF as well as for comparison, TCNQF2.

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Acknowledgements Financial support was obtained from ANR (France) under contracts numbers ANR-08-BLAN-0091–01 and ANR-08-BLAN-0091–02. We also thank CNRS (France) and PAN (Poland) for joint CNRS-PAN projects in the periods 2011–12 and 2013–14. The authors thank the Institut Jean Barriol XRD facility of Universit de Lorraine, and the CDIFX facility in Rennes, for X-ray data collections.

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Charge-Assisted Halogen Bonding

FULL PAPER Halogen Bonding

Charge-assisted halogen bonding is apparent within a series of isostructural charge-transfer complexes with variable degrees of ionicity, (EDT-TTFI2)2-

ACHTUNGRE(TCNQFn), n = 0–2 (see graphic), together with temperature-driven neutral-ionic conversion with TCNQF (n = 1).

J. Lieffrig, O. Jeannin, A. Fra˛ckowiak, I. Olejniczak, R. S´wietlik, S. Dahaoui, E. Aubert, E. Espinosa, P. Auban-Senzier, M. Fourmigu* . . . . . . . . . . . . . . . &&&&—&&&& Charge-Assisted Halogen Bonding: Donor–Acceptor Complexes with Variable Ionicity

Charge-assisted halogen bonding is unambiguously revealed by structural and electronic investigations of a series of isostructural charge-transfer complexes formulated as (EDT-TTFI2)2(TCNQFn), with n = 0–2 which exhibit a variable degree of ionicity. The iodinated tetrathiafulvalene (EDT-TTFI2) units in these systems associates with TCNQ derivatives of increasing reduction potential (TCNQ, TCNQF, TCNQF2) through highly directional CI···NC halogen-bond interactions. A correlation between the degree of charge transfer and the CI···NC halogen-bond strength is established by comparison of the structures of three isostructural complexes at room temperatures to 20 K, demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral–ionic conversion in (EDT-TTFI2)2(TCNQF) is further investigated from the temperature dependence of its magnetic susceptibility and of the stretching modes of the CN groups.

Chem. Eur. J. 2013, 00, 0 – 0

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