3.1.1. [(*rac*)-1]ClO4, [(R)-1]2ClO4 and [(S)-1]2ClO4

Black thick crystalline plates of [(*rac*)-**1**]ClO4 were collected on the electrode. The compound crystallizes in the triclinic centrosymmetric space group *P*–1, with one independent donor molecule and one anion in the asymmetric unit (Figure 1a). The methyl substituent lies in the axial position and the oxygen atom O1 is disordered over two positions O1A (s.o.f. 0.28) and O1B (s.o.f. 0.72).

**Figure 1.** (**a**) Molecular structure of [(*rac*)-**1**]ClO4 (O1 atom is disordered). Only the (*S*) enantiomer of the donor is shown; (**b**) Packing diagram of [(*rac*)-**1**]ClO4 in the *bc* plane, with an emphasis on the intradimer S···S short contacts (3.38 and 3.47 Å, green dotted lines).

The donors form very stable centrosymmetric head-to-tail dimers (*vide infra*) through the establishment of very short S···S intermolecular contacts of 3.385 and 3.466 Å (Figure 1b). These dimers interact especially laterally along the *a*-direction as denoted by the short S···S intermolecular contacts ranging between 3.39 and 3.87 Å (Figure S1). The stoichiometry 1:1 is in favor of a +1 oxidation state of the donors, in agreement with the values of the central C=C and internal C–S bonds (Table 3).


**Table 3.** Selected C=C and C–S bond lengths for [(*rac*)-**1**]ClO4, [(*R*)-**1**]2ClO4 and [(*S*)-**1**]2ClO4.

As already extensively discussed for the (**1**)2PF6 series of salts [4], the anion is also involved here in several hydrogen-bonding interactions between the oxygen atoms and protons of different types (CHvinyl, CHMe, CH2 and CH3) (Figure S2).

The enantiopure salts [(*R*)-**1**]2ClO4 and [(*S*)-**1**]2ClO4 are isostructural and crystallize in the monoclinic non-centrosymmetric space group *C*2, the asymmetric unit containing two independent donor molecules and two anion halves located on *C*<sup>2</sup> symmetry axes. Unlike the racemic salt, now the methyl substituent is located in an equatorial position in both independent molecules (Figure 2a for the (*R*) enantiomer). Since the two compounds are isostructural, only [(*R*)-**1**]2ClO4 will be detailed hereafter. Parallel columns of donors oriented in a head-to-tail manner are present in the organic layer, an arrangement reminiscent of the β-type packing [28]. Intrastack and interstack short S···S contacts of 3.52–3.54 Å and 3.62–3.63 Å, respectively, are observed (Figure 2b).

**Figure 2.** (**a**) Molecular structure of [(*R*)-**1**]2ClO4 along with the atom numbering scheme; (**b**) Donor layer with an emphasis on the S···S short contacts: red dotted lines (3.52–3.54 Å), blue dotted lines (3.81 Å), green dotted lines (3.62–3.63 Å) and violet dotted lines (4.02 Å).

The analysis of the central C=C and internal C–S bond lengths (Table 3) suggests that the charge is equally distributed over the two independent donor molecules which are thus in a mixed-valence state. Again, the anions and the donors engage in a set of hydrogen bond interactions, such as CHvinyl···O (2.43–2.51 Å), HCH2··· O (2.46–2.63 Å) and HMe···O (2.62 Å) (Figure 3).

**Figure 3.** Solid-state structure of [(*R*)-**1**]2ClO4, with an emphasis on the C–H···O short contacts: blue dotted lines for CHvinyl (2.43–2.51 Å), green dotted lines for CH2 (2.46–2.63 Å), brown dotted lines for Me (2.62 Å) and red dotted line for CHMe (2.99 Å).

The crystal structures of [(*R*)-**1**]2ClO4 and [(*S*)-**1**]2ClO4 are thus completely different from those of the analogous salts with the dimethylated donor DM-EDT-TTF, i.e., [(*R,R*)-DM-EDT-TTF]2ClO4 and [(*S,S*)-DM-EDT-TTF]2ClO4, which crystallized in hexagonal enantiomorphic space groups [17], pointing out the importance of the number of stereogenic centers in the modulation of the packing.
