Tetramethyl-Bis(ethylenedithio)-Tetrathiafulvalene (TM-BEDT-TTF) Revisited: Crystal Structures, Chiroptical Properties, Theoretical Calculations, and a Complete Series of Conducting Radical Cation Salts

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Tetramethyl-Bis(ethylenedithio)-Tetrathiafulvalene (TM-BEDT-TTF) Revisited: Crystal Structures, Chiroptical Properties, Theoretical Calculations, and a Complete Series of Conducting Radical Cation Salts

FLAVIA POP,¹STEEVE LAROUSSI,¹THOMAS CAUCHY,¹CARLOS J. GOMEZ-GARCIA,²JOHN D. WALLIS,³ANDNARCIS AVARVARI¹*

1Université d'Angers, CNRS, Laboratoire MOLTECH-Anjou, UMR 6200, UFR Sciences, Angers, France

2Instituto de Ciencia Molecular (ICMol), Catedrático José Beltrán, 2. Universidad de Valencia, Valencia, Spain

3School of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham, United Kingdom

ABSTRACT The (S,S,S,S) and (R,R,R,R) enantiomers of tetramethyl-bis(ethylenedithio)- tetrathiafulvalene (TM-BEDT-TTF) show equatorial conformation for the four methyl groups in the solid state, according to the single-crystal X-ray analyses. Theoretical calculations at the Density Functional Theory (DFT) and time-dependent (TD) DFT levels indicate higher gas phase stability for the axial conformer than the equatorial one by 1.25 kcal · mole^-1 and allow the assignment of the UV–vis and circular dichroism transitions. A complete series of radical cation salts of 1:1 stoichiometry with the triiodide anion I₃^- was obtained by electrocrystallization of both enantiopure and racemic forms of the donor. In the packing the donors are organized in dimers that further interact through S · · · S intermolecular contacts and the triiodide anions lie parallel to pairs of oxidized donors. The conductivity of the racemate, which adopts the same, but disordered, structural type, is considerably lower, with much higher activation energy.

KEY WORDS: chiral tetrathiafulvalenes; circular dichroism; crystalline structures; molecular conductors; theoretical calculations

INTRODUCTION

The synthesis of chiral tetrathiafulvalenes (TTF) and their use as precursors for chiral molecular conductors¹have been continuously developing in the last decade within the general framework of multifunctional materials.²The combination of chirality with electrical conductivity might provide a useful insight into the detection of the electrical magneto-chiral anisotropy effect, evidenced in the case of carbon nanotubes,³ but not observed so far in the TTFfield, or the modulation of the structural disorder, which is known to influence the conducting properties.⁴The access to a large library of chiral precursors is thus of paramount importance, and, accordingly, several families of TTFs, in which the chiral information has been addressed in different ways, have been described.¹For example, derivatives with stereogenic carbon atoms such as bis(ethylenedithio)-tetrathiafulvalenes (BEDT-TTF),^5,6 TTF-oxazolines^7–12 and bis(oxazolines),^13,14 bis(pyrrolo)- TTFs,¹⁵ C₃ symmetric tris(TTF) affording supramolecular helical aggregates,^16,17 or with stereogenic sulfur atoms in TTF-sulfoxides^18,19 have been reported. Complete series of conducting salts, comprising both enantiomeric and racemic forms, have been prepared by electrocrystallization of EDT- TTF-oxazolines (EDT = ethylenedithio). They show either different or similar conductivity between the enantiopure and racemic salts with AsF₆^– and Au(CN)₂^– anions, respectively, depending on the structural disorder in the solid state.^9,12Sur- prisingly, although (S,S,S,S)-TM-BEDT-TTF (TM = tetramethyl), named (S)-1hereafter (Scheme 1), was thefirst reported chiral TTF,²⁰no complete series of radical cation salts of both (S,S,S, S) and (R,R,R,R) enantiomers ((S)-1and (R)-1) and the racemic mixture (rac)-1has been described to date in order to compare their properties.

Note, however, that conducting salts formulated as [(S)-1]

2X (X = PF₆^–, AsF₆^–, SbF₆^–), [(S)-1]2(I₃)_0.71 and [(S)-1]3X₂

(X = BF₄^–, ClO₄^–, ReO₄^–) were reported shortly after the synthesis of the donor,²¹together with a more recent example of a ferromagnetic metal of the same enantiomer,²²and 1:1 salts of both (S)-1and (R)-1) enantiomers with the racemic trisphat anion.²³ Interestingly, in all these salts the four methyl groups adopt equatorial (eq) positions within“half-chair”or“sofa”conformations of the dihydrodithiin six-membered rings, as in the neutral donor, as shown by the single crystal X-ray structure of the mixed meso/racemic form.²⁴However, the conformation with the methyl groups in axial (ax) positions should also very likely be stable and rather close in energy (Scheme 1), since in the dimethylated donor DM-BEDT-TTF the Me substituents adopt axial positions,²⁴ while in 1:1 cycloadducts between1 and tetrachloro-catecholate both conformations are observed in the solid state.²⁵Clearly, the position of the methyl groups, i.e., eq or ax, has a great impact on the packing in the solid state, and probably on the chiroptical properties. Nevertheless, the relative stabilities of the two conformations has never been estimated by theoretical calculations. Moreover, neither the chiroptical properties of thisﬁrst chiral TTF system have been determined so far, nor the solid state structures of the enantiopure donors. We describe herein a combined experimental and theoretical study on1, addressing the solid state structures of (S)-1 and (R)-1, their chiroptical properties supported by theoretical calculations, and the stability of ax and eq conformations in the neutral and radical cation states.

Additional Supporting Information may be found in the online version of this article.

*Correspondence to: N. Avarvari, Université d'Angers, CNRS, Laboratoire MOLTECH-Anjou, UMR 6200, UFR Sciences, Bât. K, 2 Bd. Lavoisier, 49045 Angers, France. E-mail: narcis.avarvari@univ-angers.fr

Received for publication 01 March 2013; Accepted 31 May 2013 DOI: 10.1002/chir.22210

Published online 9 July 2013 in Wiley Online Library (wileyonlinelibrary.com).

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The ﬁrst complete series of conducting crystalline radical cation salts formulated as (1)(I3) is also reported.

MATERIALS AND METHODS

Reactions were carried out under argon; dry acetonitrile was obtained from distillation machines; chloroform high-performance liquid chroma- tography (HPLC)-grade was purchased from Fisher (Pittsburgh, PA).

Optical rotation was measured on a Jasco P-2000 polarimeter.

Synthesis

Enantiopure (S)-1 and (R)-1 were synthesized using the method previously described.^20,24 The speciﬁc optical rotations for (S)-1 and (R)-1were measured: [α]_D²⁰= 196.6 ± 5% (1.67x10^-3g/mL, CHCl₃) and [α]_D²⁰= +196.6 ± 5% (1.67x10^-3g/mL, CHCl₃), respectively. (rac)-1 was obtained by mixing the corresponding enantiopure derivatives in equal amounts. Single crystals of the neutral (S)-1and (R)-1were obtained by slow evaporation from a mixture of carbon disulﬁde/dichloromethane solution and cyclohexane/dichloromethane solution for (rac)-1.

Dark black single crystals of [(S)-1](I₃), [(R)-1](I₃), and [(rac)-1](I₃) were obtained by electrocrystallization as follows.

[(S)-1](I₃). 69 mg of [NBu₄](I₃) were dissolved in 7 mL of acetonitrile and the solution was poured in the cathodic compartment of the electrocrystallization cell. The anodic chamber wasﬁlled with 5 mg of (S)-1dissolved in 7 mL of acetonitrile. Black prismatic single crystals of the salt were grown at 20°C over a period of 4 weeks on a platinum wire electrode, by applying a constant current of 2μA.

[(R)-1](I₃). This compound was prepared following the same method used for[(S)-1](I₃)but with (R)-1instead of (S)-1.

[(rac)-1](I₃). 69 mg of [NBu₄](I₃) were dissolved in 7 mL of acetonitrile and the solution was poured in the cathodic compartment of the electrocrystallization cell. The anodic chamber wasﬁlled with 5 mg of (rac)-1dissolved in 7 mL of acetonitrile. Single crystals of the salt were grown at 20°C over a period of 7 days on a platinum wire electrode, by applying a constant current of 1μA.

Single Crystal X-ray Diffraction

X-ray diffraction measurements were performed on a Nonius Kappa CCD diffractometer using graphite-monochromated MoK_αradia- tion (λ= 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least squares techniques based onF². The non-H atoms were refined with anisotropic displacement parameters. Calcula- tions were performed using the SHELX-97 crystallographic software package. A summary of the crystallographic data and the structure refine- ment is given in Table 1. The crystals of (rac)-1were of poor quality;

therefore, its structure will not be detailed. CCDC 926041 (S)-1, CCDC 926042 (R)-1, CCDC 926043 (rac)-1, CCDC 926044 [(S)-1](I₃), CCDC 926045 [(R)-1](I₃), and CCDC 926046 [(rac)-1](I₃) contain the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Electrochemical Studies

Cyclic voltammetry measurements were carried out with a Biologic SP-150 potentiostat under argon, using a three-electrode cell equipped with a platinum millielectrode of 0.126 cm²area, an Ag/Ag + pseudo-ref- erence electrode and a platinum wire counter electrode. The potential values were then readjusted with respect to the saturated calomel electrode (SCE). The electrolytic media involved a 0.1 mol/L solution of (n-Bu₄N)PF₆ in CH₂Cl₂/acetonitrile 1:2. All experiments were performed at room temperature at 0.1 V/s.

CD Measurements

Circular dichroism (CD) spectra were recorded using spectrometric grade solvents in a 0.2 cm cell at sample concentrations of 10^-3M using a Jasco (Tokyo, Japan) J-815 Circular Dichroism Spectrometer (Biosit facility - Université de Rennes 1).

Electrical Conductivity Measurements

The DC conductivity measurements were carried out with the four or two contacts methods (depending on the size of the single crystals) on three single crystals of the enantiomeric pure radical cation salts [(S)-1]

(I₃) and [(R)-1](I₃), and four single crystals of the racemic one [(rac)-1]

Scheme 1.(S) and (R) enantiomers of TM-BEDT-TTF together with the axial (ax) and equatorial (eq) half-chair conformations.

TABLE 1. Crystallographic data, details of data collection and structure reﬁnement parameters

(S)-1 (R)-1 [(S)-1]I₃ [(R)-1]I₃ [(rac)-1]I₃

formula C14H16S8 C14H16S8 C14H16I3S8 C14H16I3S8 C14H16I3S8

M[gmol^-1] 440.75 440.75 821.45 821.45 821.45

T[K] 293(2) 293(2) 293(2) 293(2) 293(2)

crystal system Triclinic Triclinic Monoclinic Monoclinic Monoclinic

space group P1 P1 C2 C2 C2/m

a[Å] 6.9536(4) 6.9558(3) 16.1316(9) 16.1388(16) 16.1035(7)

b[Å] 11.6164(6) 11.6161(4) 10.3443(6) 10.3456(6) 10.3303(6)

c[Å] 12.0632(5) 12.0710(7) 14.4046(12) 14.4216(16) 14.4064(11)

α[°] 74.082(4) 73.992(3) 90.00 90.00 90.00

β[°] 82.633(4) 82.638(4) 93.760(6) 93.761(11) 93.677(6)

γ[°] 89.243(5) 89.209(3) 90.00 90.00 90.00

V[Å³] 929.07(8) 929.54(7) 2398.5(3) 2402.7(4) 2391.6(3)

Z 2 2 4 4 4

ρcalcd[gcm^-3] 1.576 1.575 2.275 2.271 2.281

μ[mm^-1] 0.953 0.953 4.606 4.598 4.619

goodness-of-ﬁt on F² 1.026 1.070 1.001 1.032 1.061

ﬁnal R1/wR2 [I>2(I)] 0.0664/ 0.1318 0.0518/ 0.1078 0.0398/0.0746 0.0360/0.0737 0.0391/0.0758 R1/wR2 (all data) 0.1159/ 0.1535 0.0866/ 0.1183 0.0870/0.0910 0.0730/0.0880 0.0718/0.0825 ChiralityDOI 10.1002/chir

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(I₃) in the temperature range 400–2 K. The results were identical in the cooling and warming scans. The contacts were made with Pt wires (25μm diameter) using graphite paste. The samples were measured in a Quantum Design PPMS-9 equipment connected to an external voltage source (Keithley model 2400 source-meter) and amperometer (Keithley model 6514 electrometer). The samples were measured by applying a constant voltage of 10 or 20 V and measuring the intensity. All the conductivity quoted values were measured in the voltage range where the crystals are Ohmic conductors. The cooling and warming rates were 0.5 or 1.0 K/min.

Theoretical Calculations

Density Functional Theory (DFT) calculations were performed at the DFT/PBE0 level for the geometry optimizations and the time-dependent (TD) DFT/PBE0²⁶level for the electronic transitions with the 6-311++G (3df,2pd) basis set using the Gaussian09²⁷ program package. The rotatory strengths of the transitions, characterizing the CD spectra, were determined for both enantiomers of1. Although the length gauge formu- lation was used for the calculation of the rotatory strengths, since a very large basis set was used, both formulas give practically the same values.

The bandwidth used was 3000 cm^-1for UV-visible and 0.2 eV for CD plots.

RESULTS AND DISCUSSION Neutral Donors

The enantiopure precursors (S)-1 and (R)-1 were synthesized by trimethyl phosphite mediated homocoupling reaction of the corresponding thiones (S,S)-2or (R,R)-2(Scheme 2), according to the described procedure.^20,24

Suitable crystals for single crystal X-ray measurements were obtained by slow evaporation from a mixture of solvents (see Materials and Methods). As mentioned above, the structure of a mixed meso/racemic form of 1 was previously described as a triclinicP–1phase,²⁴which is not isostructural with the enantiopure forms reported here (vide infra). In this case1 was prepared by dimerization of the racemic precur- sor, which also should provide, besides the racemic mixture, the meso compound with two (R) centers on one ethylene bridge and two (S) centers on the other. The reported triclinic structure (room temperature, P–1, R = 0.070) corresponds to this meso compound but disordered 70:30 with respect to a 180° rotation about an axis perpendicular to the molecular plane.

Given the disorder and room temperature measurement it cannot

be excluded that some molecules of the (R,R,R,R) and (S,S,S,S) enantiomers may also be present. Herein the pure racemic form of1was generated by mixing equimolar amounts of (S)-1and (R)-1. A monoclinic polymorphC2/cwas observed in this case, but the structure was only of medium quality and will not be detailed hereafter. The neutral (S)-1and (R)-1are isostructural and crystallize in the triclinic system, chiral space groupP1, with two independent molecules in the unit cell (Fig. 1). The two molecules differ mainly by the torsion angles along the internal S · · · S axes: 22.4° for S3A-S4A, 15.8° for S5A-S6A, 17.4° for S3B-S4B and 27.2° for S5B-S6B in the case of (S)-1.

The internal C = C and S–C bond lengths (Table 2) are typical for neutral donors. The most striking feature is the equatorial position of the methyl substituents, as in the previously described triclinic polymorph of 1.²⁴ Moreover, the donors pack in parallel inclined columns (Fig. 2), reminiscent of aβ”phase of mixed valence salts,²⁸in sharp contrast with the packing diagrams of BEDT-TTF and DM-BEDT-TTF, in which orthogonal pairs of donors are observed.²⁴

Cyclic voltammetry measurements show the expected pair of reversible oxidation processes corresponding to the formation of the radical cation (1)⁺at E¹_1/2= +0.49 V (vs. SCE), and then the dication (1)²⁺at E¹_1/2= +0.81 V (vs. SCE). In the UV–vis absorption spectroscopy three main bands centered at λmax347 nm, 323 nm, and 230 nm are observed. As mentioned, the methylated BEDT-TTF derivatives have never been investi- gated so far for their chiroptical properties. CD spectra for (S)-1 and (R)-1show the mirror-image relationship (Fig. 3), with a ﬁrst weak negative band peaking at 366 nm, a second positive band at 313 nm, and a third intense negative band at 267 nm for the (S) enantiomer (opposite signs for the (R) enantiomer).

The different observed transitions are probably due to both axial and equatorial conformers which coexist in solution.

Theoretical Calculations

In order to estimate the relative stability of the axial and equatorial conformers of1 and to characterize the nature of the electronic transitions and the active CD bands, DFT and TDDFT calculations were performed. The starting geometry for the optimization of the eq conformer was the experimental one provided by the X-ray analysis, which is of“boat”type for the TTF unit, also corresponding to the energy minimum,²⁹ while the distortions about the outer S · · · S hinges are directed outside the“boat”(exo distortion, Fig. 1). Neverthe- less, when input geometries with different distortions, e.g., both endo or one exo and one endo, were considered, they all converged to the same equilibrium geometry globally

Fig. 1.Molecular structure of (S)-1along with the numbering scheme.

Scheme 2.Synthesis of TM-BEDT-TTF1.

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corresponding to the experimental structure. Moreover, both ax and eq conformations are energy minima, thus showing the usual“boat”shape for TTF with exo distortions for the outer S · · · S axes. The difference in stability isΔE = 1.25 kcal · mole^-1 in favor of the ax conformer, which should be thus major in solution (see Supporting Information). However, the energy barrier for the inversion of the dihydrodithiin ring, estimated at 7.6 kcal · mole^-1 by ¹³C NMR measurements,²⁴ is sufﬁ- ciently low to allow a rapid interconversion of the two conformers at room temperature in solution. The calculated geometrical parameters compare very well with the experimental ones, provided by the X-ray data, in the case of the eq conformer, which was the one observed in the solid state.

Obviously, packing forces and intermolecular interactions, which can easily overcome the slight stability of the ax conformer when compared to the eq one, are responsible for the crystallization of the later. The molecular orbitals diagrams of the two conformers show similarities in the TABLE 2. Selected bond distances for neutral 1 and its radical cation salts with I₃^-

Bond length (Å)

(S)-1 (R)-1

C = C C-S C = C C-S

C(7A)-C(8A) 1.342(13) S(3A)-C(7A) 1.769(12) C(7A)-C(8A) 1.377(11) S(3A)-C(7A) 1.759(10)

S(4A)-C(7A) 1.768(11) S(4A)-C(7A) 1.736(10)

S(5A)-C(8A) 1.739(11) S(5A)-C(8A) 1.746(9)

S(6A)-C(8A) 1.758(11) S(6A)-C(8A) 1.773(9)

C(7B)-C(8B) 1.351(13) S(3B)-C(7B) 1.752(11) C(7B)-C(8B) 1.336(11) S(3B)-C(7B) 1.759(9)

S(4B)-C(7B) 1.757(12) S(4B)-C(7B) 1.733(11)

S(5B)-C(8B) 1.758(10) S(5B)-C(8B) 1.771(10)

S(6B)-C(8B) 1.756(11) S(6B)-C(8B) 1.773(9)

[(S)-1](I₃) [(R)-1](I₃) [(rac)-1](I₃)

C = C C-S C = C C-S C = C C-S

C(7)-C(8) 1.392(5) S(3)-C(7) 1.741(12) C(7)-C(8) 1.390(6) S(3)-C(7) 1.711(16) C(4)-C(5) 1.387(8) S(2)-C(4) 1.726(4)

S(4)-C(7) 1.717(12) S(4)-C(7) 1.746(16) S(3)-C(5) 1.724(3)

S(5)-C(8) 1.708(12) S(5)-C(8) 1.733(15)

S(6)-C(8) 1.746(12) S(6)-C(8) 1.728(16)

Fig. 2.Packing diagram of (S)-1in theabplane.

Fig. 3.CD spectra of (S)-1 (red line) and (R)-1(blue line) in CH2Cl2

(c = 1 x 10^–3M) in a rectangular cell with 2 mm pathlength.

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shape and the energy level of the orbitals, excepting the position and the energy of the LUMO + 3 ( 0.39 eV) in1-eq which becomes LUMO + 4 ( 0.24 eV) in1-ax with a quite higher energy (Supporting Information). Therefore, if this orbital is involved in any electronic transitions, differences are expected between the calculated UV–vis and CD spectra of the two conformers. The experimental UV–vis spectrum very likely results from the addition of the transitions of the two species in equilibrium in solution (Fig. 4).

As generally observed for TTFs theπHOMO→σ*LUMOtran- sition is forbidden.²⁹ For both conformers there are clearly two groups of transitions differing slightly in intensity, wavelength, and orbitals involved (Fig. 4). In the lower energy region the most intense transitions are located at 347 and 325 nm for 1-eq, the ﬁrst one being a combination of π^HOMO-1→σ*_LUMO and πHOMO→π*LUMO+2excitations, and at 331 and 320 nm for 1-ax, with theﬁrst one being a mixedπ^HOMO→π*LUMO+1/+2/+3

transition. At higher energy the main excitation appears at

Fig. 4.Theoretical (blue line for eq and black line for ax conformers) and experimental absorption (in CH2Cl2, dashed red line) spectra of (S)-1(left). Calculated electronic transitions for the eq (blue) and the ax conformers (black) (right).

TABLE 3. TD-DFT calculated energies and assignment of the most pertinent singlet transition of the equatorial conformer. HO and LU stands for HOMO and LUMO

Wavenumber (cm^-1) λ(nm) Oscillator strength Assignment Transition

22042 454 0.000 π→σ* HO→LU (98%)

27073 369 0.049 π→π* HO→LU + 1 (80%), HO→LU + 2 (12%)

28844 347 0.137 π(outer CC)→σ* HO-1→LU (43%), HO→LU + 2 (39%)

π→π*

30791 325 0.264 π→π* HO-1→LU (34%), HO→LU + 2 (40%), HO→LU + 7 (12%)

34936 286 0.033 π→σ* HO→LU + 8 (76%)

37365 268 0.034 π→π* outer CC HO-1→LU + 3 (54%)

42926 233 0.299 π(TTF/outer CC)→π* HO-3→LU + 1 (22%), HO-2→LU + 2 (51%)

TABLE 4. TD-DFT calculated energies and assignment of the most pertinent singlet transition of the axial conformer. HO and LU stands for HOMO and LUMO

Wavenumber

(cm^-1) λ

(nm)

Oscillator

strength Assignment Transition

21399 467 0.000 π→σ* HO→LU (98%)

26793 373 0.016 π→π* HO→LU + 1 (71%), HO→LU + 2 (22%)

28339 353 0.049 π(outer CC)→σ* HO-1→LU (61%), HO→LU + 2 (18%)

30198 331 0.180 π→π* HO→LU + 1 (17%), HO→LU + 2 (28%), HO→LU + 3 (37%)

31279 320 0.238 π→π* HO-1→LU (12%), HO→LU + 2 (28%), HO→LU + 3 (43%),

HO→LU + 5 (11%)

39350 254 0.055 π→π* outer CC HO-1→LU + 4 (34%), HO→LU + 11 (17%), HO→LU + 14 (22%), HO→LU + 15 (14%)

42448 236 0.0974 π→π* HO-2→LU + 2 (23%), HO-1→LU + 9 (14%), HO→LU + 20 (20%) 42889 233 0.152 π(TTF/outer CC)→π* HO-3→LU + 1 (11%), HO-2→LU + 2 (19%), HO-1→LU + 9 (43%) ChiralityDOI 10.1002/chir

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233 nm for both of them and has aπ→π* character. The combination of all these transitions (see Tables 3 and 4 for the assignment) gives a very accurate description of the experimental spectrum, showing aﬁrst pair of two low energy bands at 347 and 323 nm, and a second band at 230 nm.

The differences are much larger between the eq and ax conformers when the CD spectra are concerned (Fig. 5).

Indeed, especially in the high energy region, there are transitions with opposite signs and intensities as a consequence of different mixed excitations. For example, the ﬁrst CD active transition for (S)-1-eq has positive rotational strength (R = 60 × 10^-40 esu²cm²) and results from combined πHOMO→π*LUMO+1/+2/+6 excitations (see Table 5 for full assignment), while for (S)-1-ax the ﬁrst CD active transition is negative (R = 14 × 10^-40 esu²cm²) and involves

πHOMO→π*LUMO+1/+2/+3excitations (see Table 6 for full assignment). The experimental CD curve follows roughly the shape of the spectrum of the axial conformer, thus suggesting the predom- inance in solution of the later, in agreement with its theoretically higher stability with respect to the equatorial conformer.

In conclusion, this combined experimental and theoretical CD study gives clear indications of the existence of the ax/eq equilibrium in solution with a dominance of the axial conformer, in contrast to the occurrence of solely the equatorial one in the solid state.

Conducting Radical Cation Salts

Electrocrystallization of (S)-1, (R)-1and (rac)-1in acetonitrile in the presence of [NBu₄](I₃) as supporting electrolyte provides the 1:1 salts formulated as [(S)-1](I3), [(R)-1](I3),

Fig. 5.Theoretical (blue line for eq and black line for ax conformers) and experimental absorption (in CH2Cl2, dashed red line) CD spectra of (S)-1(left).

Calculated electronic transitions for the eq (blue) and the ax conformers (black) (right).

TABLE 5. TD-DFT calculated energies and assignment of the strongest singlet CD transitions of the equatorial conformer Wavenumber

(cm^-1) λ

(nm)

Rotational

27073 369 59.861 π→π* HO→LU + 1 (80%), HO→LU + 2 (12%), HO→LU + 6 (6%)

28844 347 23.855 π→π* HO-1→LU (43%), HO→LU + 2 (39%), HO→LU + 1 (6%), HO→LU + 7 (4%) 30791 325 26.186 π→π* HO-1→LU (34%), HO→LU + 2 (40%), HO→LU + 7 (12%), HO→LU + 1 (8%) 32691 306 17.874 π→π* (outer CC) HO→LU + 5 (74%), HO-1→LU + 1 (4%), HO-1→LU + 4 (3%), HO→LU + 3 (5%),

HO→LU + 8 (6%), HO→LU + 10 (3%)

34936 286 19.022 π→σ* HO→LU + 8 (76%), HO-1→LU + 1 (5%), HO→LU + 3 (2%), HO→LU + 5 (6%), HO→LU + 13 (7%)

37365 268 59.284 π→π* outer CC HO-1→LU + 3 (54%), HO-2→LU + 1 (4%), HO-1→LU + 8 (3%), HO→LU + 4 (8%), HO→LU + 11 (4%), HO→LU + 14 (5%),

HO→LU + 15 (6%), HO→LU + 17 (3%)

38101 262 26.059 π→π* outer CC HO-1→LU + 3 (15%), HO→LU + 11 (40%), HO→LU + 17 (10%), HO-1→LU + 5 (4%),

HO-1→LU + 8 (4%),

HOMO→LU + 9 (7%), HO→LU + 14 (5%), HO→LU + 15 (4%)

39208 255 31.495 π→π* HO→LU + 9 (22%), HO→LU + 14 (28%), HO→LU + 15 (35%), HO-1→LU + 3 (2%), HO→LU + 17 (2%)

40275 248 40.339 π→π* HO→LU + 14 (11%), HO→LU + 15 (10%), HO→LU + 17 (53%), HO-1→LU + 8 (3%),

HO-1→LU + 13 (4%),

HO→LU + 9 (4%), HO→LU + 11 (5%)

42926 233 62.037 π→π* HO-3→LU + 1 (22%), HO-2→LU + 2 (51%), HO-5→LU (5%), HO-4→LU (4%), HO-4→LU + 3 (3%), HO→LU + 20 (2%)

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and [(rac)-1](I3). The enantiopure salts crystallize in the monoclinic system, chiral space groupC2with one molecule of donor and one molecule of anion in the asymmetric unit (see Fig. 6 for [(S)-1](I3)), while the racemic salt crystallizes in the centrosymmetric monoclinic space group C2/m (Table 1), with the mirror plane passing through the central C4 = C5 bond of the TTF unit and the linear triiodide anion atoms (Fig. 7). In the later the ethylene bridge and methyl groups are disordered at 50:50 on two positions, thus generat- ing the two enantiomers on the same crystallographic site. In the three salts the methyl groups adopt eq positions, as in the case of the neutral donors.

In the three salts the central C = C and internal C–S bonds have typical lengths for oxidized TTF⁺units (Table 2). The similar three packing diagrams show alternating TMBEDT- TTF dimers and I₃^- anions. The TMBEDT-TTF dimers belong- ing to successive layers interact through S · · · S interactions

(Fig. 8) and present the same stair-like organization as in the previously reported 1:1 radical cation salts of (S)-1 and (R)-1with the trisphat anion.²³

The shortest interdimer and intradimer S · · · S distances in [(S)-1](I3) and [(R)-1](I3) amount to 3.44 Å, while in [(rac)-1](I3) they measure 3.43 Å and 3.48 Å, respectively.

These structures are quite different from the [(S)-1]2(I₃)_0.71 structural type, which by analogy of its cell constants with the [(S)-1]2(PF₆) salt is likely to have the I₃ions located in channels bordered by methyl groups,²¹rather than lying parallel to the main donor axis.

Single crystal resistivity measurements show that the three compounds are semiconductors (Fig. 9) with room temperature conductivities of ca. 2x10^-4S.cm^-1 for both enantiomeric pure salts, and ca. 4x10^-7S.cm^-1 for the racemic salt. The activation energies for the two enantiomeric pure salts are also very similar (ca. 250 meV) and are lower TABLE 6. TD-DFT calculated energies and assignment of the strongest singlet CD transitions of the axial conformer Wavenumber

(cm^-1) λ

(nm)

Rotational

26793 373 14.100 π→π* HO→LU + 1 (71%), HO→LU + 2 (22%), HO→LU + 3 (2%)

30198 331 38.797 π→π* HO→LU + 1 (17%), HO→LU + 2 (28%), HO→LU + 3 (37%), HO-1→LU (9%), HO→LU + 8 (3%)

31182 321 24.377 π→π*

(outer CC)

HO→LU + 4 (56%), HO→LU + 6 (37%), HO-1→LU + 1 (2%), HO→LU + 7 (2%) 31279 320 44.058 π→π* HO-1→LU (12%), HO→LU + 2 (28%), HO→LU + 3 (43%), HO→LU + 5 (11%)

35213 284 30.608 π→π*

TTF-outer CC

HO-1→LU + 2 (16%), HO→LU + 9 (52%), HO-1→LU + 1 (9%), HO→LU + 6 (3%), HO→LU + 7 (4%), HO→LU + 10 (2%), HO→LU + 13 (3%)

35656 280 22.962 π→π* HO-2→LU (15%), HO-1→LU + 1 (30%), HO-1→LU + 2 (12%), HO→LU + 9 (27%), HO→LU + 7 (9%)

36856 271 26.528 π→σ* HO-2→LU (59%), HO-1→LU + 2 (17%), HO-1→LU + 3 (4%), HO-1→LU + 8 (5%), HO→LU + 7 (5%), HO→LU + 9 (2%)

38026 263 32.982 π(outer CC,

EDT)→π* (EDT)

HO-1→LU + 3 (39%), HO→LU + 13 (25%), HO-3→LU (3%), HO-1→LU + 1 (4%), HO-1→LU + 5 (9%), HO→LU + 7 (7%)

40016 250 52.157 π→π* outer

CC

HO-1→LU + 4 (13%), HO-1→LU + 6 (51%), HO→LU + 16 (12%), HO-2→LU + 2 (4%), HO-1→LU + 9 (2%), HO→LU + 5 (2%), HO→LU + 14 (3%), HO→LU + 15 (2%) 42023 238 20.169 π→π* TTF HO-2→LU + 1 (70%), HO-4→LU (6%), HO-1→LU + 4 (6%), HO-1→LU + 7 (3%),

HO→LU + 20 (5%)

42448 236 43.628 π(TTF) n_s

(EDT)→π*

HO-2→LU + 2 (23%), HO-1→LU + 9 (14%), HO→LU + 20 (20%), HO-5→LU (2%), HO-4→LU (5%), HO-3→LU + 1 (6%), HO-2→LU + 1 (4%), HO-1→LU + 7 (5%), HO→LU + 15 (3%)

42632 235 33.111 π→π* HO→LU + 23 (42%), HO→LU + 24 (24%), HO-3→LU + 2 (3%), HO-1→LU + 9 (7%), HO-1→LU + 18 (4%), HO→LU + 20 (7%)

Fig. 6.Molecular structure of [(S)-1](I3) along with the numbering scheme.

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than the value observed in the racemic salt (ca. 470 meV).

This behavior is not surprising since the racemic salt presents the donor positions disordered between either enantiomeric orientations, partially precluding the electronic

delocalization and, therefore, the electrical conductivity. A similar behavior was observed in conducting salts based on chiral EDT-TTF-oxazolines, where the enantiopure forms show conductivities one order of magnitude higher than the racemic form.^9,12

CONCLUSION

In conclusion, the two enantiomers of the TM-BEDT-TTF molecule prefer to crystallize with equatorial methyl groups to optimize crystal packing, while in solution the conformation with four axial methyl groups is slightly favored according to UV–vis and CD spectra, supported by TD DFT calculations.

The 1:1 triiodide salts of both enantiomers show semiconduct- ing behavior, in contrast to the common observation of conductivity in partially oxidized salts. The racemate, while structurally similar, but with the donor positions disordered between either enantiomer, shows a lower conductivity and a higher activation energy. These results suggest extension to further chiral tetrasubstituted BEDT-TTF donors will be a proﬁtable area to investigate for candidates showing magnetochiral anisotropy, although the initial challenges will be in their synthesis.

Fig. 7.Molecular structure of [(rac)-1](I3) along with the numbering scheme.

Fig. 8.Distribution of donor and anion in the packing of [(S)-1](I3) (left) and [(R)-1](I3) (right) with details of the S · · · S contacts (middle). Hydrogen atoms have been omitted.

Fig. 9.Resistivityvs. T curves for [(S)-1](I3), [(R)-1](I3) and [(rac)-1](I3).

Inset shows the Arrhenius plot of the three compounds (same legend).

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ACKNOWLEDGMENTS

Financial support from the National Agency for Research (ANR, Project 09-BLAN-0045-01) and the COST Action D35 is gratefully acknowledged. This work was also supported by the CNRS (France). Financial support of the Spanish MEC (grant CTQ2011-26507) and Generalitat Valenciana (Prometeo/2009/95 and ISIC) is acknowledged. J.W. thanks EPSRC for support (EP/C510488/1). Biosit facility (Université de Rennes 1) is acknowledged for the access to the CD instrument. M. Allain (MOLTECH-Anjou) is thanked for help with the X-ray structures.

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