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

: 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 ) 2 AsF 6 as racemic, and ( S ) and ( R ) enantiomers; (ii) [( S )- 1 ]AsF 6 · C 4 H 8 O and [( R )- 1 ]AsF 6 · C 4 H 8 O where a strong dimerization of the donors is observed; (iii) ( 1 )I 3 and ( 2 )I 3 as racemic and enantiopure forms and (iv) [( meso )- 2 ]PF 6 and [( meso )- 2 ]XO 4 (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 ) 2 AsF 6 , 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.


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) 2 PF 6 , with metal-like conductivity [3], while with the perchlorate anion only the enantiopure salts, formulated as [(S)-1] 2 ClO 4 and [(R)-1] 2 ClO 4 , showed metallic conductivity in the hightemperature regime, the racemic form [(rac)-1]ClO 4 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 PF 6 − [5], whereas the use of ClO 4 proved to be of huge importance across the preparation of the enantiomorphic salts [(S,S)-2] 2 ClO 4 and [(R,R)-2] 2 ClO 4 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 AsF 6 − or SbF 6 − , having larger volumes than PF 6 − , favored the formation of metallic enantiopure salts (2) 2 XF 6 (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 [Ta 2 F 10 O] 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 AsF 6 − . Secondly, when considering the important role played by the linear monoanion I 3 -in the field of molecular conductors through the synthesis of the first ambient pressure organic superconductor (BEDT-TTF) 2 I 3 [11][12][13], its use in radical cation salts with the donors 1 and 2 has been investigated and described herein. Note that tetramethylbis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF), the first reported enantiopure TTF derivative [14], provided 1:1 semiconducting radical cation salts with I 3 - [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 PF 6 − anion, semiconducting orthorhombic [(R,R)-DM-BEDT-TTF] 2 PF 6 and monoclinic [(rac)-DM-BEDT-TTF] 2 PF 6 salts [21] have been described, while the (meso)-DM-BEDT-TTF form provided the triclinic β-[(meso)-DM-BEDT-TTF] 2 PF 6 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, PF 6 − and ClO 4 − anions, to be compared with the corresponding materials with (S,S)/(R,R)-2 [5,8].
Magnetochemistry 2021, 7, x FOR PEER REVIEW 2 of 17 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 Tc ~ 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 AsF 6 − and I 3 − anions, the complete series of radical cation salts of (S,S)/(R,R)-2 with I 3 − and, finally, the synthesis and structural characterization of the new donor (meso)-2 together with its radical cation salts with PF 6 − and XO 4 − (X = Cl, Re) anions.

Radical Cation Salts of Me-EDT-TTF (1) with the AsF 6 -Anion
As outlined in the Introduction, donor 1 provided a complete series of radical cation salts (1) 2 PF 6 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 AsF 6 and compare their structural features and conducting properties with the previously reported PF 6 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) 4 N)AsF 6 . Single crystals were obtained in the form of black plates following experimental conditions identical with those employed for the PF 6 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] 2 AsF 6 is isostructural with the previously described [(rac)-1] 2 PF 6 metallic salt [3], but also with the racemic [(rac)-2] 2 PF 6 [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] 2 AsF 6 and [(R)-1] 2 AsF 6 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] 2 AsF 6 will be detailed.
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.

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. 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 for [(rac)-1] 2 AsF 6 and [(S)-1] 2 AsF 6 , are in agreement with a +0.5 oxidation state of the donor and are comparable with the values measured for the [(rac)-1] 2 PF 6 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 PF 6 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 AsF 6 than those with PF 6 -, as a consequence of the longer As-F bond lengths (Tables S2 and S3). All fluorine atoms are involved in such hydrogen-bonding interactions. The lengths of the central C = C bond together with the internal C-S bonds, shown in Table 1 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].

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 1.374(10) S1B-C3B 1.737(7) B S2B-C3B 1.753(7) S3B-C4B 1.733(7) S4B-C4B 1.726 (7) 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 (Figures 2a and 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 (Figures 2b and S2). When comparing the structure of the two series, the CH···F distances, such as those for CHvinyl and CHMe, 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 Thus, this (1) 2 AsF 6 series presents the same structural characteristics as the (1) 2 PF 6 series and also as the isostructural one of donor 2 with AsF 6 -, namely (2) 2 AsF 6 [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) 2 AsF 6 were rather small and brittle, single crystal resistivity measurements could be performed for [(rac)-1] 2 AsF 6 and [(R)-1] 2 AsF 6 . The metallic behavior in the high-temperature regime could not be detected, as in the case of two point contact measurements in the (1) 2 PF 6 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 (Figures 4 and S3 for the (R) enantiomer, Table S4).   These values of conductivity are comparable to those in the PF 6 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) 4 N]AsF 6 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]AsF 6 ·C 4 H 8 O and [(R)-1]AsF 6 ·C 4 H 8 O 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).  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 (Figures 4 and 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 mixedvalence 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). The crystallization of these 1:1 phases is very likely favored by the much lower concentration of available AsF 6 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 CH 2 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. 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). 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 (Figures 5b and 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.

Radical Cation Salts of Me-EDT-TTF (1) and DM-EDT-TTF (2) with the I 3 -Anion
As mentioned above, TM-BEDT-TTF afforded 1:1 radical cation salts with the tris(iodide) anion I 3 − [15]. On the other hand, BEDT-TTF provided the ambient pressure superconducting phase (BEDT-TTF) 2 I 3 [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 I 3 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) 4 N]I 3 .
The racemic and enantiopure salts (1) (Table S7). The asymmetric unit consists of two independent donor molecules and two anions for the latter (Figure 6a (Table S8), are indicative of the +1 oxidation state, in agreement with the 1:1 stoichiometry.

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 ( 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). 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]I 3 ). The packing of the donors is strongly reminiscent of the one observed in the structure of (EDT-TTF)I 3 [24]. Accordingly, one I 3 anion separates the layers of donors, and the other I 3 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 ( 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 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)I 3 , 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)I 3 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) 4 N]I 3 in similar conditions as for 1. A complete series of isostructural crystalline radical cation salts formulated as (2)I 3 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). 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 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 (Figures  9 and S9). Once again, due to the strong dimerization, the conductivity of these materials The intradimer S···S distances of 3.27-3.31 Å for [(rac)-2]I 3 and 3.25-3.34 Å for [(R,R)-2]I 3 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.

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 (ClO 4 [8], PF 6 [5], AsF 6 [9], SbF 6 [9], Ta 2 F 10 O [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)-2,3dibromobutane with dmit 2− , 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 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 (Figures  9 and S9). Once again, due to the strong dimerization, the conductivity of these materials is expected to be poor.

Radical Cation Salts of (meso)-DM-EDT-TTF
Our prime objective was to obtain radical cation salts of (meso)-2 with PF 6 and ClO 4 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) 4 N]PF 6 and [(n-Bu) 4 N]ClO 4 provided 1:1 radical cation salts [(meso)-2]PF 6 and [(meso)-2]ClO 4 , respectively, as black crystalline needles. Additionally, knowing that ClO 4 and ReO 4 − anions usually afford isostructural salts, the use of [(n-Bu) 4 N]ReO 4 leads to the formation of crystalline [(meso)-2]ReO 4 , isostructural with its perchlorate congener (Table S11). A working temperature of 3 • C had to be imposed in the case of ClO 4 and ReO 4 salts, very likely because of the use of THF as co-solvent.
[(meso)-2]PF 6 crystallizes in the monoclinic centrosymmetric space group P2 1 /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
[(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 As mentioned above, [(meso)-2]ClO 4 and [(meso)-2]ReO 4 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 ClO 4 and ReO 4 salts than in the PF 6 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]PF 6 and [(meso)-2]ReO 4 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 ReO 4 salt compared to the PF 6 salt may be in the stronger lateral interdimer interactions observed in the former.
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 (Figures 12b and S14). The lateral S···S contacts are on average shorter in the ClO4 and ReO4 salts than in the PF6 one.
(a) (b) 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 σRT 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 Ea 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.  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 σRT 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 Ea 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. Figure 13. Temperature dependence of the electrical resistivity ρ for one single crystal of [(meso)-2]PF 6 (green curve) and two single crystals of [(meso)-2]ReO 4 (blue curves) measured using two contacts. The red lines are the fit to the activation law ρ = ρ 0 exp(E a /T) in the 160-300 K temperature range giving the activation energy E a .
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.

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) 2 AsF 6 with metal-like behavior in the high-temperature regime, which are isostructural with the previously described PF 6 counterpart, but also with the (2) 2 AsF 6 series. Additionally, 1:1 enantiopure salts formulated as [(S)-1]AsF 6 ·C 4 H 8 O and [(R)-1]AsF 6 ·C 4 H 8 O have been obtained in slightly different conditions. With the tris(iodide) anion I 3 -, both donors afforded 1:1 salts, i.e., (1)I 3 and (2)I 3 , 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 PF 6 − , ClO 4 − and ReO 4 − 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].

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 1 H at room temperature in CDCl 3 solutions. 1 H NMR spectra were referenced to the residual protonated solvent ( 1 H). 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.
(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%); 1  (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 MgSO 4 . 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 %); 1 H NMR (300 MHz, CDCl 3 ) δ ppm: 6.32 (s, 2H, −SCH=), 3 (1) 2 AsF 6 : Single crystals of (1) 2 AsF 6 (rac), (S) and (R) were obtained by electrocrystallization. The electrolyte solution was prepared from 34.9 mg (5 eq.) of [(n-Bu) 4 N]AsF 6 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)AsF 6 ·C 4 H 8 O: Single crystals of (1)AsF 6 ·C 4 H 8 O (S) and (R) were obtained by electrocrystallization. The electrolyte solution was prepared from 34.9 mg (5 eq.) of [(n-Bu) 4 N]AsF 6 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)I 3 : 20 mg of [(n-Bu) 4 N]I 3 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]I 3 , [(S)-1]I 3 and [(R)-1]I 3 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)I 3 : Single crystals of (2)I 3 (rac), (S,S) and (R,R) were obtained by electrocrystallization. The electrolyte solution was prepared from 48.3 mg (5 eq.) of [(n-Bu) 4 N]I 3 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]PF 6 : 20 mg of [(n-Bu) 4 N]PF 6 were dissolved in 6 mL CHCl 3 , 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 CHCl 3 . 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.