**1. Introduction**

In the frame of the increasingly developing field of chiral molecular materials [1], we recently introduced methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) **1** and ethyl-ethylenedithiotetrathiafulvalene (Et-EDT-TTF) **2** (Scheme 1) as valuable precursors for chiral molecular conductors [2,3], through a series of radical cation salts with the PF6 – anion, in which modulation of the donor:anion stoichiometry, crystal packing and conducting properties with the absolute configuration and steric bulkiness of the substituent was observed [4]. Interesting differences were observed, for example, between the enantiopure PF6 – salts of **1** and those of the dimethylated donor DM-EDT-TTF

(Scheme 1), since the former crystallize in the triclinic space group *P*1 and show metallic behaviour in the high-temperature regime, while the latter crystallize in the monoclinic space group *P*21 and are bandgap semiconductors [5]. These results point out the importance of the number of stereogenic centres in the modulation of intermolecular interactions with the anion, through the establishment of C–H···F hydrogen bonding, donor packing, and thus electron transport properties. Moreover, the (*rac*), (*S*) and (*R*) (**1**)2PF6 salts are isostructural with the (DM-EDT-TTF)2XF6 (X = As, Sb) series [6], showing that the size of the anion, in conjunction with the number of stereogenic centres, can also play a paramount role in the modulation of the intermolecular interactions. Enantiopure monoalkylated EDT-TTF **1** and **2** and derivatives [7] have afforded so far only the PF6 – series of radical cation salts. Comparatively, since the first report of an enantiopure TTF precursor, namely (*S*,*S*,*S*,*S*)-TM-BEDT-TTF [8], enantiopure dimethylated EDT-TTF (DM-EDT-TTF) [5,6,9] and BEDT-TTF (DM-BEDT-TTF) [10–12], together with the tetramethylated BEDT-TTF (TM-BEDT-TTF) [13–16] (Scheme 1), provided several series of radical cation salts, culminating in the first observation of the electrical magnetochiral anisotropy effect (eMChA) in a bulk crystalline conductor [17]. Note that diethyl BEDT-TTF (DEt-BEDT-TTF) and tetraethyl BEDT-TTF (TEt-BEDT-TTF) (Scheme 1) precursors have been reported only in the racemic form [18]. Besides the hexafluorophosphate, another anion very often encountered in TTF based radical cation salts, including in the chiral ones, is the perchlorate. Indeed, enantiopure DM-EDT-TTF provided enantiomorphic mixed-valence salts (DM-EDT-TTF)2ClO4 showing the eMChA effect [17], the use of (*S*,*S*,*S*,*S*)-TM-BEDT-TTF afforded a 3:2 salt with metallic conductivity [13], while an orthorhombic semiconducting phase [11] and a metallic monoclinic phase [19], both presenting a 2:1 stoichiometry, have been reported for (*R*,*R*)-DM-BEDT-TTF and (*S*,*S*)-DM-BEDT-TTF, respectively. Moreover, a superconducting transition was suggested for the latter [19], yet this assumption was recently invalidated by more complete physical measurements on both enantiomeric salts [20].

**Scheme 1.** Methyl and ethyl substituted ethylenedithio-tetrathiafulvalene (EDT-TTF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) donors.

In the continuation of our research line dedicated to chiral molecular conductors, we describe herein the synthesis, structural characterization and electron transport properties of racemic and enantiopure radical cation salts of Me-EDT-TTF **1** and Et-EDT-TTF **2** with the perchlorate anion, together with band structure calculations, highlighting some peculiar features of their electronic structures.

#### **2. Materials and Methods**

Donors **1** and **2** were prepared as recently described [4]. For each of the three electrocrystallization experiments with **1**, [NBu4]ClO4 (20 mg) was dissolved in 6 mL THF and the solution was poured into the cathodic compartment of an electrocrystallization cell. The anodic chamber was filled respectively with 5 mg of (*rac*)-**1**, (*S*)-**1** and (*R*)-**1** dissolved in 6 mL THF. Crystals of each salt grown at 3 ◦C over a period of one week on a platinum wire electrode, by applying a constant current of 1 μA. Black plates were obtained on the electrode for [(*rac*)-**1**]ClO4 and brown thin narrow plates for [(*S*)-**1**]2ClO4 and [(*R*)-**1**]2ClO4. Similar experimental conditions were applied for the electrocrystallization of (*rac*)-**2**, (*S*)-**2** and (*R*)-**2** in the presence of [NBu4]ClO4. Black crystalline needles were obtained on the electrode for [(*rac*)-**2**]ClO4, while for (*R*)-**2** a mixture of small green thin plates, formulated as [(*R*)-**2**]2ClO4, and dark violet thick plates, corresponding to [(*R*)-**2**]ClO4, were collected after several trials. The quality of the former was not sufficient to allow a high quality crystal structure, yet it allowed to clearly identify the components and to determine the packing of the donors. Unfortunately, no salt with (*S*)-**2** crystallized under these conditions.

Single crystals of the compounds were mounted on glass fibre loops using a viscous hydrocarbon oil to coat the crystal and then transferred directly to cold nitrogen stream for data collection. Data collection was performed on an Agilent Supernova diffractometer with CuKα (λ = 1.54184 Å). The structures were solved by direct methods with the SIR92 program and refined against all F<sup>2</sup> values with the SHELXL-97 program using the WinGX graphical user interface. All non-H atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations but not refined. Further details about data collection and solution refinement are given in Table 1; Table 2.

Crystallographic data for the five structures were deposited with the Cambridge Crystallographic Data Centre, deposition numbers CCDC 2043591 ([(*rac*)-**1**]ClO4), 2043592 ([(*R*)-**1**]2ClO4), 2043593 ([(*S*)-**1**]2ClO4), 2043594 ([(*rac*)-**2**]ClO4), 2043595 ([(*R*)-**2**]ClO4). These data can be obtained free of charge from CCDC, 12 Union road, Cambridge CB2 1EZ, UK (e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).


**Table 1.** Crystal Data for compounds [(*rac*)-**1**]ClO4, [(*R*)-**1**]2ClO4 and [(*S*)-**1**]2ClO4.


**Table 2.** Crystal Data for compounds [(*rac*)-**2**]ClO4 and [(*R*)-**2**]ClO4.

Electrical conductivity and the thermoelectric power measurements for [(*S*)-**1**]2ClO4 and [(*R*)-**1**]2ClO4 single crystals were made along the long axis of the crystals (*a* axis) in the temperature range of 20/50–310 K, using a measurement cell attached to the cold stage of a closed-cycle helium refrigerator. The thermopower was measured by using a slow AC (ca. 102 Hz) technique [21], by attaching two Ø = 25 μm diameter 99.99% pure Au wires (Goodfellow, UK), thermally anchored to two quartz blocks, with Pt paint (Demetron, Germany, 308A) to the extremities of an elongated sample as in a previously described apparatus [22], controlled by a computer [23]. The oscillating thermal gradient was kept below 1 K and was measured with a differential Au 0.05 at. % Fe vs. chromel thermocouple of the same type. The absolute thermoelectric power of the samples was obtained after correction for the absolute thermopower of the Au leads, by using the data of Huebner [24]. Electrical resistivity measurements were done both in a four-in-line contact configuration, using a low-frequency AC method (77 Hz) with a 5–10 μA current, the voltage is measured with an SRS (Stanford Research Systems, California, USA) Model SR83 lock-in amplifier, and in two points configuration. For resistivity measurements with two contacts, gold wires were glued with silver paste directly on both ends of the crystals and low temperature was provided by a homemade cryostat equipped with a 4 K pulse-tube. For [(*S*)-**1**]2ClO4 and [(*R*)-**1**]2ClO4 single crystals, we applied a DC current (0.1 μA) and measured the voltage with a Keithley 2401 microvoltmeter (Tektronix, Inc Beaverton, OR 97077 United States). Note that, in this configuration, the resistance of the two contacts is added to the resistance of the sample which makes it difficult to evidence a metallic behavior when cooling down. For measuring much higher resistance values in [(*rac*)-**2**]ClO4 and [(*R)*-**2**]ClO4 single crystals, we used a different technique always in two points configuration. We applied a constant voltage (10–15 V) and measured the current using a Keithley 6487 Picoammeter/Voltage Source (Tektronix, Inc Beaverton, OR 97077 United States). However, [(*R)*-**2**]ClO4 single crystals could not be cooled down because the limit in current detection (around 50 pA) was nearly reached at room temperature.

The tight-binding band structure calculations were of the extended Hückel type [25]. A modified Wolfsberg–Helmholtz formula was used to calculate the non-diagonal Hμν values [26]. All valence electrons were taken into account in the calculations and the basis set consisted of Slater-type orbitals of double-ζ quality for C 2s and 2p, S 3s and 3p and of single-ζ quality for H 1s. The ionization potentials, contraction coefficients and exponents were taken from previous work [27].

#### **3. Results and Discussion**

Donors **1** and **2** were prepared as racemic mixtures according to the protocol we recently reported, followed by chiral HPLC separation to afford the pure enantiomers [4]. The racemic and enantiopure forms of both donors were engaged in electrocrystallization experiments in the presence of [NBu4]ClO4 as a supporting electrolyte. Suitable single crystals for X-ray diffraction analysis of radical cation salts with ClO4 − were obtained for (*rac*)-**1**, (*R*)-**1**, (*S*)-**1**, (*rac*)-**2** and (*R*)-**2**.

#### *3.1. Solid-State Structures of the Radical Cation Salts*
