Chiral Radical Cation Salts of Me-EDT-TTF and DM-EDT-TTF with Octahedral, Linear and Tetrahedral Monoanions ^†

Abstract

Methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF (1) and dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF (2) are valuable precursors for chiral molecular conductors, which are generally obtained by electrocrystallization in the presence of various counter-ions. The number of the stereogenic centers, their relative location on the molecule, the nature of the counter-ion and the electrocrystallization conditions play a paramount role in the crystal structures and conducting properties of the resulting materials. Here, we report the preparation and detailed structural characterization of the following series of radical cation salts: (i) mixed valence (1)₂AsF₆ as racemic, and (S) and (R) enantiomers; (ii) [(S)-1]AsF₆·C₄H₈O and [(R)-1]AsF₆·C₄H₈O where a strong dimerization of the donors is observed; (iii) (1)I₃ and (2)I₃ as racemic and enantiopure forms and (iv) [(meso)-2]PF₆ and [(meso)-2]XO₄ (X = Cl, Re), based on the new donor (meso)-2. In the latter, the two methyl substituents necessarily adopt axial and equatorial conformations, thus leading to a completely different packing of the donors when compared to the chiral form (S,S)/(R,R) of 2 in its radical cation salts. Single crystal resistivity measurements, complemented by thermoelectric power measurements in the case of (1)₂AsF₆, suggest quasi-metallic conductivity for the latter in the high temperature regime, with σ_RT ≈ 1–10 S cm^–1, while semiconducting behavior is observed for the (meso)-2 based salts.

1. Introduction

Substitution of a hydrogen atom at one or both carbon atoms of the ethylene bridge of the ethylenedithio-tetrathiafulvalene (EDT-TTF) precursor generates one or two stereogenic centers, respectively, as in methyl-EDT-TTF (Me-EDT-TTF) 1 and dimethyl-EDT-TTF (DM-EDT-TTF) 2, thus providing chiral tetrathiafulvalenes [1,2] (Scheme 1). These precursors proved to be highly valuable since they afforded by electrocrystallization a series of enantiopure and racemic radical cation salts showing chirality and anion-dependent crystalline packing and conducting properties. For example, donor 1, which has been only recently described [3], provided the complete series of mixed-valence salts (1)₂PF₆, with metal-like conductivity [3], while with the perchlorate anion only the enantiopure salts, formulated as [(S)-1]₂ClO₄ and [(R)-1]₂ClO₄, showed metallic conductivity in the high-temperature regime, the racemic form [(rac)-1]ClO₄ being a very poor conductor because of the formation of strong heterochiral dimers by the radical cations [4]. On the other hand, the dimethylated donor 2, containing two stereogenic centers (S,S) or (R,R) afforded semiconducting 4:2 enantiopure salts and metallic 2:1 racemic salt with PF₆⁻ [5], whereas the use of ClO₄^– proved to be of huge importance across the preparation of the enantiomorphic salts [(S,S)-2]₂ClO₄ and [(R,R)-2]₂ClO₄ which allowed the first observation of the electrical magnetochiral anisotropy (eMChA) effect [6,7] in a bulk crystalline chiral conductor [8]. Thus, the addition of a second stereogenic center on the EDT-TTF platform has a paramount importance for the resulting crystalline materials. Interestingly, the use of 2 in combination with AsF₆⁻ or SbF₆⁻, having larger volumes than PF₆⁻, favored the formation of metallic enantiopure salts (2)₂XF₆ (X = As, Sb) as a consequence of the interplay between the anion size, its propensity to engage in intermolecular hydrogen bonding and chirality of the donor [9], with the much larger double octahedral shaped fluorinated dianion [Ta₂F₁₀O]²⁻ semiconducting salts of 2 with a 3:1 stoichiometry having been obtained [10]. In order to develop the families of chiral conducting materials further, a first objective of the present work is directed to the association of the mono-substituted donor 1 with the larger anion AsF₆⁻. Secondly, when considering the important role played by the linear monoanion I₃^– in the field of molecular conductors through the synthesis of the first ambient pressure organic superconductor (BEDT-TTF)₂I₃ [11,12,13], its use in radical cation salts with the donors 1 and 2 has been investigated and described herein. Note that tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF), the first reported enantiopure TTF derivative [14], provided 1:1 semiconducting radical cation salts with I₃^– [15], besides other conducting materials [16,17,18], thus demonstrating the drastic consequences of the introduction of substituents on the ethylene carbon atoms on the stoichiometry and properties of the resulting materials. Finally, another interesting comparison can be made between the two diastereomers of 2, i.e., the (S,S)/(R,R) pair and the meso form (Scheme 1). Indeed, radical cation salts based on the related donor dimethyl-bis(ethylenedithio)-tetrathiafulvalene (DM-BEDT-TTF) [19,20], which also presents a similar pair of diastereomers (S,S)/(R,R) and meso, show striking differences between them. For example, with the PF₆⁻ anion, semiconducting orthorhombic [(R,R)-DM-BEDT-TTF]₂PF₆ and monoclinic [(rac)-DM-BEDT-TTF]₂PF₆ salts [21] have been described, while the (meso)-DM-BEDT-TTF form provided the triclinic β-[(meso)-DM-BEDT-TTF]₂PF₆ salt which showed a superconducting transition with T_c ~ 4.3 K under 4.0 kbar [22,23]. Thus, the third objective of the present work is to introduce the hitherto unknown (meso)-2 donor and radical cation salts containing, especially, PF₆⁻ and ClO₄⁻ anions, to be compared with the corresponding materials with (S,S)/(R,R)-2 [5,8].

We describe herein a series of radical cation salts of Me-EDT-TTF 1 with the AsF₆⁻ and I₃⁻ anions, the complete series of radical cation salts of (S,S)/(R,R)-2 with I₃⁻ and, finally, the synthesis and structural characterization of the new donor (meso)-2 together with its radical cation salts with PF₆⁻ and XO₄⁻ (X = Cl, Re) anions.

2. Results and Discussion

2.1. Radical Cation Salts of Me-EDT-TTF (1) with the AsF₆^– Anion

As outlined in the Introduction, donor 1 provided a complete series of radical cation salts (1)₂PF₆ for which the enantiopure compounds crystallized in the triclinic space group P1, with two independent donor molecules and one anion in the unit cell, while the racemic salt crystallized in the triclinic space group P–1 with one independent donor in the asymmetric unit and the anion located on an inversion center [3]. We describe here the analogous complete series of chiral radical cation salts of Me-EDT-TTF 1 with AsF₆^– and compare their structural features and conducting properties with the previously reported PF₆^– counterparts. Donor 1, prepared according to our published procedure [3], afforded enantiopure and racemic salts by electrocrystallization in THF in the presence of ((n-Bu)₄N)AsF₆. Single crystals were obtained in the form of black plates following experimental conditions identical with those employed for the PF₆ series. Note that, with donor 1, the use of tetrahydrofuran (THF) as an electrocrystallization solvent generally requires working temperatures of 2–3 °C in order to favor crystallization of radical cation salts.

[(rac)-1]₂AsF₆ is isostructural with the previously described [(rac)-1]₂PF₆ metallic salt [3], but also with the racemic [(rac)-2]₂PF₆ [5]. It crystallizes in the triclinic centrosymmetric space group P–1 with one independent donor molecule and half of anion, located on an inversion center, in the asymmetric unit (Figure 1a, Table S1). The ethylene bridge C7–C8 is disordered over two positions A and B with s.o.f. values of 0.62 and 0.38, respectively, whereas the methyl group C9 is not disordered and adopts an equatorial conformation, thus leading to the presence of both enantiomers (S) and (R) on the same crystallographic site. The four equatorial fluorine atoms are disordered on two positions each. The enantiopure salts [(S)-1]₂AsF₆ and [(R)-1]₂AsF₆ crystallize in the non-centrosymmetric triclinic space group P1 with two independent donor molecules and one anion in the asymmetric unit (Figure 1b for the (S) enantiomer). Since they are isostructural, only the structure of [(S)-1]₂AsF₆ will be detailed.

Contrary to the racemic form, the donor molecules of the enantiopure salts have no occupational disorder; the methyl substituent adopts equatorial conformation in both independent molecules, and two of the fluorine atoms were modelled over two positions.

The lengths of the central C = C bond together with the internal C–S bonds, shown in

Table 1. Selected C = C and C–S internal bond distances for [(rac)-1]₂AsF₆ and [(S)-1]₂AsF₆.

Bond lengths (Å)
	[(S)-1]2AsF6		[(rac)-1]AsF6
	C3A–C4A	1.367(9)	C3–C4	1.363(5)
	S1A–C3A	1.714(7)	S1–C3	1.739(4)
A	S2A–C3A	1.759(7)	S2–C3	1.739(3)
	S3A–C4A	1.744(7)	S3–C4	1.740(3)
	S4A–C4A	1.736(7)	S4–C4	1.733(3)
	C3B–C4B	1.374(10)
	S1B–C3B	1.737(7)
B	S2B–C3B	1.753(7)
	S3B–C4B	1.733(7)
	S4B–C4B	1.726(7)

for [(rac)-1]₂AsF₆ and [(S)-1]₂AsF₆, are in agreement with a +0.5 oxidation state of the donor and are comparable with the values measured for the [(rac)-1]₂PF₆ salt [3].

A classical organic-inorganic segregation occurs in the packing of both enantiopure and racemic salts, with the donors adopting a β-type organization in parallel columns, with short intrastack and interstack S···S distances (Figure 2a and Figure S1). As in the case of the PF₆^– homologous series, the donors engage in a complex set of intermolecular CH···F hydrogen bond interactions with the fluorine atoms of the anion (Figure 2b and Figure S2). When comparing the structure of the two series, the CH···F distances, such as those for CH_vinyl and CH_Me, are either equal or slightly smaller for AsF₆^– than those with PF₆^–, as a consequence of the longer As–F bond lengths (Tables S2 and S3). All fluorine atoms are involved in such hydrogen-bonding interactions.

Thus, this (1)₂AsF₆ series presents the same structural characteristics as the (1)₂PF₆ series and also as the isostructural one of donor 2 with AsF₆^–, namely (2)₂AsF₆ [9]. Since the latter two show metal-like behavior, it is reasonable to envisage similar conducting properties for the former. Although crystals of the series (1)₂AsF₆ were rather small and brittle, single crystal resistivity measurements could be performed for [(rac)-1]₂AsF₆ and [(R)-1]₂AsF₆. The metallic behavior in the high-temperature regime could not be detected, as in the case of two point contact measurements in the (1)₂PF₆ series, in spite of rather high values of the room temperature conductivities (σ_RT ≈ 1–10 S cm^–1) and very small activation energies (9–12 meV) (Figure 3a). Most likely, the presence of structural disorder on the donors and the anions prevents the observation of a metal-like conductivity. However, thermoelectric power measurements suggest metallic behavior in the high-temperature regime according to the small positive values of the Seebeck coefficient decreasing towards zero upon cooling (Figure 3b).

These values of conductivity are comparable to those in the PF₆^– series; moreover, no striking difference has been observed between the racemic and enantiopure materials. This last feature is not really surprising in view of the similar packing of the donors, although one might argue that the enantiopure salts could show in principle higher conductivity since there is no structural disorder on the donors. However, the anion is disordered in all the salts of the series; therefore, this feature can hamper such fine observations.

Surprisingly, in a second set of experiments realized with the enantiopure donor and [(n-Bu)₄N]AsF₆ in separate compartments, totally different 1:1 salts crystallized in the anodic compartment as black prismatic blocks. Once again, a working temperature of 2–3 °C was imposed in order to obtain crystalline materials. Accordingly, [(S)-1]AsF₆·C₄H₈O and [(R)-1]AsF₆·C₄H₈O radical cation salts are isostructural and crystallize in the triclinic system non-centrosymmetric system P1, with two independent donors A and B, two anions and two tetrahydrofuran (THF) molecules in the unit cell (Figure 4 and Figure S3 for the (R) enantiomer, Table S4).

In one of the two donors, the methyl substituent C9B, located in an equatorial position, is disordered over two positions (C9BA and C9BB), which is different to the previous mixed-valence salts. Moreover, all the six fluorine atoms of one anion are disordered on two positions each. The stoichiometry of the two compounds indicates that both independent donors are in a radical cation state, in agreement with the values of the central C = C and internal C–S bond lengths (

Table 2. Selected C = C and C–S internal bond distances for [(S)-1]₂AsF₆ and [(S)-1]₂AsF₆.

Bond Lengths (Å)
	[(S)-1]AsF6·C4H8O		[(R)-1]AsF6·C4H8O
A	C3A–C4A	1.396(13)	C3A–C4A	1.4214(13)
	S1A–C3A	1.737(9)	S1A–C3A	1.734(12)
	S2A–C3A	1.710(10)	S2A–C3A	1.7068(98)
	S3A–C4A	1.705(9)	S3A–C4A	1.6963(91)
	S4A–C4A	1.727(9)	S4A–C4A	1.7142(12)
B	C3B–C4B	1.389(13)	C3B–C4B	1.3577(13)
	S1B–C3B	1.729(10)	S1B–C3B	1.7306(89)
	S2B–C3B	1.710(10)	S2B–C3B	1.7154(12)
	S3B–C4B	1.705(10)	S3B–C4B	1.7128(12)
	S4B–C4B	1.735(9)	S4B–C4B	1.7485(88)

). The crystallization of these 1:1 phases is very likely favored by the much lower concentration of available AsF₆^– counter ion in the anodic compartment, leading to the complete oxidation of the donors before crystallization.

As is often observed in the case of salts based on fully oxidized donors, the radical cations form strong eclipsed dimers in the packing, with very short S···S intradimer distances of 3.33–3.37 Å and much longer lateral S···S interdimer distances (3.59–3.77 Å) (Figure 5a). The dimers are separated along the stacking direction by THF molecules (Figures S4 and S5 for the (R) enantiomer). The fluorine atoms are engaged in hydrogen bonding with protons of the donors but also with a CH₂ group of THF, thus providing a certain stability of the crystals against the desolvation (Figure 5b and Figure S6 for the (R) enantiomer, Tables S5 and S6). When considering this strong dimerization of the donors, it can be safely concluded that the two salts are insulators.

2.2. Radical Cation Salts of Me-EDT-TTF (1) and DM-EDT-TTF (2) with the I₃^– Anion

As mentioned above, TM-BEDT-TTF afforded 1:1 radical cation salts with the tris(iodide) anion I₃⁻ [15]. On the other hand, BEDT-TTF provided the ambient pressure superconducting phase (BEDT-TTF)₂I₃ [11,12,13], while a 1:1 salt was described with EDT-TTF [24]. Here, we present our results on the complete series of radical cation salts of chiral EDT-TTF derived donors 1 and 2 with the I₃^– anion. The crystalline salts, collected as brown plates for 1 and black prisms for 2, were prepared by electrocrystallization of the respective donors in acetonitrile at 20 °C in the presence of [(n-Bu)₄N]I₃.

The racemic and enantiopure salts (1)I₃ are isostructural, with [(rac)-1]I₃ crystallizing in the centrosymmetric triclinic P–1 space group, while the enantiopure [(S)-1]I₃ and [(R)-1]I₃ are in the non-centrosymmetric triclinic space group P1 (Table S7). The asymmetric unit consists of two independent donor molecules and two anions for the latter (Figure 6a for [(S)-1]I₃) and one donor and two half anions for the former. In both independent molecules of [(S)-1]I₃, the methyl substituent (C9A and C9B) adopts an axial position. The central C = C bond lengths values, together with the internal C–S bond lengths (Table S8), are indicative of the +1 oxidation state, in agreement with the 1:1 stoichiometry.

The donors A and B are strongly dimerized as attested by the eclipsed arrangement and the very short intradimer S···S distances ranging between 3.34 and 3.42 Å (Figure 6b for [(S)-1]I₃). The packing of the donors is strongly reminiscent of the one observed in the structure of (EDT-TTF)I₃ [24]. Accordingly, one I₃ anion separates the layers of donors, and the other I₃ anion is embedded in the layer, with its axis parallel to the long axis of the donors, thus leading to only lateral overlap between the dimers (Figure 7 for [(R)-1]I₃).

Although the interdimer S···S distances for these lateral donor···donor contacts are as short as 3.45–3.48 Å (Figure 7), one can expect very weak conductivity for these radical cation salts of 1:1 composition when considering the strong dimerization of the donors.

Thus, within this series of (1)I₃, the presence of a methyl substituent on the ethylene bridge of EDT-TTF as in donor 1 does not have a strong impact on the packing, which is similar to the one observed in the parent (EDT-TTF)I₃ salt. What would be the consequence of introducing a second methyl substituent as in donor 2? Accordingly, we have proceeded to electrocrystallization experiments of 2 in the presence of [(n-Bu)₄N]I₃ in similar conditions as for 1. A complete series of isostructural crystalline radical cation salts formulated as (2)I₃ has been obtained as well, with the racemic compound having crystallized in the triclinic space group P–1 and the enantiopure counterparts in triclinic P1 (Table S9). The asymmetric unit of the former contains one independent donor and one anion, while those of the latter contain two independent donors and two anions. In contrast to the previous series based on donor 1, now both tris(iodide) ions are aligned with the long axis of the donors, and the methyl groups are located in equatorial positions (eq, eq) (Figure 8).

As it was recently evidenced by DFT calculations, the difference in energy between the axial and equatorial conformers of DM-EDT-TTF amounts only to ≈3 kcal mol⁻¹ [25], and even less for the monomethylated precursors [26], the occurrence of one form or the other in the solid state being determined mainly by the packing and establishment of intermolecular interactions. The analysis of the central C = C and internal C–S bonds lengths values allows the assignment of the oxidation state +1 to the donors, in agreement with the 1:1 stoichiometry. Since in this case both tris(iodide) anions are aligned with the long axis of the donors, the c direction, along which no donor···donor interaction takes place (Figures S7 and S8), mixed donor/anion layers are formed in the ab plane, with strongly dimerized donors interacting laterally (Figure 9a for (rac) and Figure 9b for (R,R)).

The intradimer S···S distances of 3.27–3.31 Å for [(rac)-2]I₃ and 3.25–3.34 Å for [(R,R)-2]I₃ are even shorter than in the previous series, probably as a consequence of the (eq, eq) conformation of the methyl groups, while the interdimer ones are slightly longer (Figure 9 and Figure S9). Once again, due to the strong dimerization, the conductivity of these materials is expected to be poor.

2.3. Synthesis and Structure of (meso)-DM-EDT-TTF

The fascinating properties shown by the radical cation salts of DM-EDT-TTF (2) in its chiral version, that is (rac), (S,S) and (R,R) forms, with different anions (ClO₄ [8], PF₆ [5], AsF₆ [9], SbF₆ [9], Ta₂F₁₀O [10]) prompted us to investigate the hitherto unknown (meso) form of DM-EDT-TTF ((meso)-2 in Scheme 1). For its preparation, we have envisaged a three-step procedure, starting with a double nucleophilic substitution of (meso)-2,3-dibromobutane with dmit²⁻, generated from the protected precursor 3 in basic conditions, to form (meso)-DM-DDDT 4. Then, a classical phosphite-mediated cross-coupling reaction of 4 with the dicarboxylate dithiolene 5 afforded the TTF (meso)-6, which was subsequently heated in DMF in the presence of lithium bromide to yield (meso)-2 upon a double decarboxylation (Scheme 2).

The neutral donor (meso)-2 crystallizes in the monoclinic centrosymmetric space group P2₁/n, with one independent molecule in the unit cell (Table S11). The dithiin six-membered ring shows a sofa-type conformation (dihedral angles C5−C6−S6-C7 and C6−C5−S5-C8 measure 5.05° and 23.2°, respectively), with axial (C10) and equatorial (C9) orientations of the methyl substituents (Figure 10) and boat-like conformation of the TTF unit. Central C = C and internal C−S bond distances have typical values for neutral donors (Table S12). In the packing, the donors organize in orthogonal dyads (Figure S10), with intermolecular S···S contacts of 3.54–3.87 Å.

2.4. Radical Cation Salts of (meso)-DM-EDT-TTF

Our prime objective was to obtain radical cation salts of (meso)-2 with PF₆^– and ClO₄^– anions in order to compare them with those obtained with donors 1 and 2 (chiral form), previously reported. Accordingly, electrocrystallization of (meso)-2 in the presence of [(n-Bu)₄N]PF₆ and [(n-Bu)₄N]ClO₄ provided 1:1 radical cation salts [(meso)-2]PF₆ and [(meso)-2]ClO₄, respectively, as black crystalline needles. Additionally, knowing that ClO₄^– and ReO₄⁻ anions usually afford isostructural salts, the use of [(n-Bu)₄N]ReO₄ leads to the formation of crystalline [(meso)-2]ReO₄, isostructural with its perchlorate congener (Table S11). A working temperature of 3 °C had to be imposed in the case of ClO₄ and ReO₄ salts, very likely because of the use of THF as co-solvent.

[(meso)-2]PF₆ crystallizes in the monoclinic centrosymmetric space group P2₁/n, with one independent donor molecule and one anion in the unit cell (Figure 11a). While in the chiral form of 2, the methyl substituents can adopt either axial (ax) or equatorial (eq) conformations, the most common being the equatorial one in radical cation salts, thus maximizing the overlap between the donors, in (meso)-2 the conformation is necessarily (ax, eq). In the packing, an organic-inorganic segregation takes place along the c direction, with the donors forming strong centrosymmetric dimers, as attested by the short intermolecular S···S contacts of 3.35–3.37 Å (Figure 11b). The dimers interact only laterally in the ab plane, yet with much longer S···S contacts (3.64–3.81 Å). A network of hydrogen bonding exists between fluorine atoms and the different hydrogen atoms of the donor: H_vinyl···F (2.32–2.36 Å) and H_Me···F (2.53–2.75 Å), H_CH···F (2.45−2.65 Å) (Figure S11).

As mentioned above, [(meso)-2]ClO₄ and [(meso)-2]ReO₄ are isostructural, crystallizing in the triclinic centrosymmetric space group P–1, with one independent donor and one anion in the unit cell (Figures S12 and S13, Table S11). The donors organize in dyads, through short S···S interactions (3.34–3.41 Å), further interacting laterally (Figure 12a), while donors-anions’ segregation establishes along the c direction (Figure 12b and Figure S14). The lateral S···S contacts are on average shorter in the ClO₄ and ReO₄ salts than in the PF₆ one.

A strong dimerization of radical cations, with little interdimer interactions, is generally detrimental for good electron transport properties. Indeed, single crystal resistivity measurements for [(meso)-2]PF₆ and [(meso)-2]ReO₄ show semiconducting behavior with room temperature conductivities σ_RT of 1.4 10^–5 S cm⁻¹ for the former and 1.6 10⁻⁴ S cm⁻¹ for the latter, the activation energies E_a being around 300–340 meV (Figure 13). The origin of the higher conductivity of the ReO₄ salt compared to the PF₆ salt may be in the stronger lateral interdimer interactions observed in the former.

Across the preparation and conducting properties of these radical cation salts based on (meso)-2, it is clear that the mutual orientation of the methyl substituents on the ethylene bridge plays a paramount role on the donor:anion stoichiometry and the packing of the donors. Indeed, in (meso)-2, one of the methyl substituents is necessarily axial, thus precluding strong axial overlap between donors, at the difference with (S,S)- and (R,R)-2 where both methyl groups are equatorial in the most conducting salts.

3. Conclusions

In the continuation of our research lines on chiral materials and, more specifically, on chiral molecular conductors, we reported here a new series of radical cation salts based on the chiral donors Me-EDT-TTF (1) and DM-EDT-TTF (2) as racemic and enantiopure forms, obtained by electrocrystallization. The former provided mixed valence salts (1)₂AsF₆ with metal-like behavior in the high-temperature regime, which are isostructural with the previously described PF₆ counterpart, but also with the (2)₂AsF₆ series. Additionally, 1:1 enantiopure salts formulated as [(S)-1]AsF₆·C₄H₈O and [(R)-1]AsF₆·C₄H₈O have been obtained in slightly different conditions. With the tris(iodide) anion I₃^–, both donors afforded 1:1 salts, i.e., (1)I₃ and (2)I₃, as racemic and enantiopure forms, yet the disposition of the anions with respect to the donors is drastically different in the two series. Finally, the synthesis, characterization and single crystal X-ray structure of the new donor (meso)-2 are described, together with its poorly semiconducting 1:1 radical cation salts with the PF₆⁻, ClO₄⁻ and ReO₄⁻ anions. The striking difference between the salts formed by (meso)-2 and those resulting from the chiral form (S,S)/(R,R)-2 with the same anions is very likely due to the location of the methyl substituents which is necessarily axial, equatorial in the former, while in the latter they can adopt an equatorial, equatorial conformation, thus maximizing the packing. Throughout these different series of radical cation salts, the importance of the number of stereogenic centers, of their mutual arrangement in the case of (meso)-2 and (S,S)/(R,R)-2 and of the nature of the counter-ion on the donor:anion stoichiometry, packing of the donors and, ultimately, electron transport properties, are highlighted. Future work will be devoted to conductivity measurements under high pressures, and the use of these chiral precursors in electrocrystallization in the presence of magnetic anions. This last direction is particularly interesting when considering the possibility of combining chirality with conducting and magnetic properties [17,27], since in magnetic conductors, the existence of delocalized π-electrons and localized d-electrons may lead to exotic phenomena such as magnetic-field-induced superconductors, magnetoresistance effects and magnetic-field-switchable conductors, with applications in molecular electronics and spintronics [28,29,30,31,32].

4. Materials and Methods

All commercially available reagents and solvents were used as received unless otherwise noted. Dry tetrahydrofuran was directly used from the purification machines. Chloroform as a solvent for synthesis was passed through a short column of basic alumina prior to use. Chromatography purifications were performed on silica gel, and thin layer chromatograhy (TLC) was carried out using aluminum sheets precoated with silica gel. NMR spectra were acquired on a Bruker Avance DRX 300 spectrometer operating at 300 MHz for ¹H at room temperature in CDCl₃ solutions. ¹H NMR spectra were referenced to the residual protonated solvent (¹H). All chemical shifts are expressed in parts per million (ppm) downfield from external tetramethylsilane (TMS) using the solvent residual signal as an internal standard, and the coupling constant values (J) are reported in Hertz (Hz). The following abbreviations have been used: s, singlet; d, doublet; m, multiplet. Mass spectrometry MALDI–TOF MS spectra were recorded on a Bruker Biflex-IIITM apparatus equipped with a 337-nm N2 laser.

Precursors Me-EDT-TTF 1 [3], (S,S)- and (R,R)-DM-EDT-TTF 2 [5] were synthesized according to the literature procedures, while the preparation of (meso)-2 is described in this report.

(meso)-DM-DDDT 4: Compound 3 (4.5 g, 14.8 mmol) was added into a 500 mL Schlenk round bottomed flask under argon, and then dry THF (400 mL) was poured into the flask. After 10 min, a solution of caesium hydroxide (7 g, 41.6 mmol) in dry methanol (50 mL) was added dropwise, and the color started to turn violet. Then, the resulting solution was left under stirring for one hour at room temperature, followed by the addition of (meso)-2,3-dibromobutane (5.5 g, 25 mmol) and reflux for one night. After cooling to room temperature, the solvent was removed under vacuum, and the solid residue was extracted twice with DCM and water (250/500 mL). The combined organic phases were concentrated, and the residue was purified by column chromatography (petroleum ether/DCM 8:2) to give compound 4 (0.36 g, 10%); ¹H NMR (300 MHz, CDCl₃):δ 3.57 (m, 2H, -SCH), 1.36 (d, 6H, -CH₃) ppm.

(meso)-DM-EDT-TTF-(COOMe)₂ 6: Compound 4 (0.3 g, 1.19 mmol) and dimethyl 2-oxo-1,3-dithiole-4,5-dicarboxylate 5 (0.6 g, 2.5 mmol) were mixed under argon in freshly distilled trimethyl phosphite (10 mL), and the mixture was heated at 110 °C for 5 h. After this period, the solvent was evaporated in a rotary evaporator, and then toluene (20 mL) was added and evaporated. The last procedure was repeated twice. The product was solubilized in dichloromethane and passed through a silica column to remove the remaining phosphite and then purified by chromatography using petroleum spirit/dichloromethane 1/1 to afford a red-brown solid (0.255 g, 49%); ¹H NMR (300 MHz, CDCl₃) δ ppm: 3.83 (s, 6H, −OCH₃), 3.54 (m, 2H, -SCH), 1.36 (d, 6H, −CH₃); MS (MALDI-TOF) m/z: 437.4 (M_th = 437.92).

(meso)-DM-EDT-TTF (meso)-1: Compound 6 (0.25 g, 0.57 mmol) and LiBr (0.75 g, 8.6 mmol) were mixed in dimethylformamide (25 mL). The solution was stirred at 150 °C for 30 min, the formation of the product being monitored by TLC. The product was extracted with dichloromethane, and the organic phase was washed with brine and water and then dried over MgSO₄. The solvent was removed under vacuum, and the product was purified by chromatography on a silica gel column with petroleum spirit/dichloromethane 6/4 to afford a red solid (62 mg, 34 %); ¹H NMR (300 MHz, CDCl₃) δ ppm: 6.32 (s, 2H, −SCH=), 3.54 (m, 2H, -SCH), 1.36 (d, 6H, −CH₃); MS (MALDI-TOF) m/z: 321.89 (M_th = 321.91).

(1)₂AsF₆: Single crystals of (1)₂AsF₆ (rac), (S) and (R) were obtained by electrocrystallization. The electrolyte solution was prepared from 34.9 mg (5 eq.) of [(n-Bu)₄N]AsF₆ dissolved in 12 mL of tetrahydrofuran. The anodic chamber was filled with 5 mg of the corresponding donor dissolved in 6 mL of the previously prepared electrolyte solution, whereas the rest of the electrolyte solution (6 mL) was added in the cathodic compartment of the electrocrystallization cell. Single crystals, as black crystalline plates, of the salts were grown at 2–3 °C over a period of 5 days on a platinum wire electrode by applying a constant current of 1 μA.

(1)AsF₆·C₄H₈O: Single crystals of (1)AsF₆·C₄H₈O (S) and (R) were obtained by electrocrystallization. The electrolyte solution was prepared from 34.9 mg (5 eq.) of [(n-Bu)₄N]AsF₆ dissolved in 6 mL of tetrahydrofuran. The anodic chamber was filled with 5 mg of the corresponding donor dissolved in 6 mL of tetrahydrofuran, and the previously prepared electrolyte solution was added in the cathodic compartment of the electrocrystallization cell. Single crystals, as black crystalline plates, of the salt were grown at 2–3 °C over a period of 4 to 6 days on a platinum wire electrode by applying a constant current of 1 μA.

(1)I₃: 20 mg of [(n-Bu)₄N]I₃ were dissolved in 6 mL of acetonitrile, and the solution was poured in the cathodic compartment of an electrocrystallization cell. The anodic chamber was filled with 5 mg of the donor dissolved in 6 mL of acetonitrile. Single crystals of the salts [(rac)-1]I₃, [(S)-1]I₃ and [(R)-1]I₃ were grown at 20 °C over a period of two weeks on a platinum wire electrode by applying a constant current of 0.5 μA. Black crystalline plates were grown on the electrode.

(2)I₃: Single crystals of (2)I₃ (rac), (S,S) and (R,R) were obtained by electrocrystallization. The electrolyte solution was prepared from 48.3 mg (5 eq.) of [(n-Bu)₄N]I₃ dissolved in 12 mL of acetonitrile/chloroform 1/1. The anodic chamber was filled with 5 mg of the corresponding donor dissolved in 6 mL of the previously prepared electrolyte solution, whereas the rest of the electrolyte solution (6 mL) was added in the cathodic compartment of the electrocrystallization cell. Single crystals of the salts, as black crystalline blocks, were grown at 20 °C over a period of 5 days on a platinum wire electrode by applying a constant current of 1 μA.

[(meso)-2]PF₆: 20 mg of [(n-Bu)₄N]PF₆ were dissolved in 6 mL CHCl₃, and then the solution was poured into the cathodic compartment of an electrocrystallization cell. The anodic chamber was filled with 5 mg of [(meso)-2] dissolved in 6 mL CHCl₃. Single crystals of the salt were grown at 20 °C over a period of one week on a platinum wire electrode, by applying a constant current of 0.5 μA. Black crystalline needles were collected on the electrode.

[(meso)-2]ClO₄: 20 mg of [(n-Bu)₄N]ClO₄ were dissolved in 6 mL of a mixture of 1,1,2-trichloroethane: tetrahydrofuran 1:1, and then the solution was poured in the cathodic compartment of an electrocrystallization cell. The anodic chamber was filled with 5 mg of [(meso)-2] dissolved in 6 mL of a mixture of 1,1,2-trichloroethane/tetrahydrofuran 1/1. Single crystals of the salt were grown at 3 °C over a period of one week on a platinum wire electrode by applying a constant current of 1 μA. Black crystalline needles were grown on the electrode.

[(meso)-2]ReO₄: The same procedure as previously was applied by using 20 mg of [(n-Bu)₄N]ReO₄ instead of [(n-Bu)₄N]ClO₄. Black crystalline needles were grown on the electrode.

Details about data collection and solution refinement are given in Tables S1, S4, S7, S9 and S11. 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. X-ray data collection was performed at 150 K on an Agilent Supernova with CuKα (λ = 1.54184 Å). The structures were solved by direct methods with the SHELXS-97 and SIR92 programs and refined against all F2 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. Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre, deposition numbers CCDC 2085699 ([(rac)-1]₂AsF₆), 2085700 ([(S)-1]₂AsF₆), 2085701 ([(R)-1]₂AsF₆), 2085702 ([(S)-1]AsF₆·C₄H₈O), 2085703 ([(R)-1]AsF₆·C₄H₈O), 2085704 ([(rac)-1]I₃), 2085705 ([(S)-1]I₃), 2085706 ([(R)-1]I₃), 2085707 ([(rac)-2]I₃), 2085708 ([(S,S)-2]I₃), 2085709 ([(R,R)-2]I₃), 2085710 ((meso)-2), 2085711 ([(meso)-2]ClO₄), 2085712 ([(meso)-2]PF₆) and 2085713 ([(meso)-2]ReO₄). These data can be obtained free of charge from CCDC, 12 Union road, Cambridge CB2 1EZ, UK (e-mail: [email protected] or http://www.ccdc.cam.ac.uk).

Thermoelectric power and electrical conductivity measurements for [(rac)-1]₂AsF₆ and [(R)-1]₂AsF₆ were made along the longer axis of the crystals in the temperature range of 100–310 K. The measurement cell used was attached to the cold stage of a closed cycle helium refrigerator. The thermopower was measured using a slow AC (ca. 10⁻² Hz) technique [33], by attaching two ∅ = 25 μm diameter 99.99% pure Au wires (Goodfellow), thermally anchored to two quartz blocks, with Pt paint (Demetron 308A) to the extremities of an elongated sample using a previously described apparatus [34], controlled by a computer [35]. The oscillating thermal gradient was kept below 1 K and was measured with a differential Au-0.05 at. % Fe vs. chromel thermocouple. The absolute thermoelectric power of the samples was obtained after correction for the absolute thermopower of the Au leads by using the data of Huebener [36]. Electrical resistivity measurements were conducted in a four-in-line contact configuration where a low-frequency AC method (77 Hz) was used; the measurements were conducted with a SRS Model SR83 lock-in amplifier, and a 5 µA current was applied. Electrical resistivity of [(meso)-2]PF₆ and [(meso)-2]ReO₄ was measured in two points on needle-shaped single crystals 0.5 mm long. Gold wires were glued with silver paste on gold-evaporated contacts. Different techniques were used to measure resistivity, either applying a constant voltage (1–5 V) and measuring the current with a Keithley 486 or applying a DC current (0.1–0.01 µA) and measuring the voltage with a Keithley 2400. We have checked for each crystal that both techniques give the same resistance value at room temperature. A low temperature was provided by a homemade cryostat equipped with a 4 K pulse-tube.