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 σRT ≈ 1–10 S cm–1, while semiconducting behavior is observed for the (meso)-2 based salts.
Highlights
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, providing chiral tetrathiafulvalenes [1,2] (Scheme 1)
The dimethylated donor 2, containing two stereogenic centers (S,S) or (R,R) afforded semiafforded semiconducting 4:2 enantiopure salts and metallic 2:1 racemic−salt with PF6− [5], conducting 4:2 enantiopure salts and metallic 2:1 racemic salt with PF [5], whereas the whereas the use of ClO4– proved to be of huge importance across the6 preparation of the use of ClO4 – proved to be of huge importance across the preparation of the enantiomorphic enantiomorphic salts [(S,S)-2]2ClO4 and [(R,R)-2]2ClO4 which allowed the first observation salts [(S,S)-2]2 ClO4 and [(R,R)-2]2 ClO4 which allowed the first observation of the electrical of the electrical magnetochiral anisotropy effect [6,7] in a bulk crystalline chiral magnetochiral anisotropy effect [6,7] in a bulk crystalline chiral conductor [8]
We describe a series of radical cation salts of Me-EDT-TTF 1 with the AsF6 −
Summary
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, 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. Donor 1, which has been only recently described [3], provided the complete series of mixed-valence salts (1) PF6 , with metal-like conductivity [3], while with the perchlorate anion only the enantiopure salts, formulated as [(S)-1]2 ClO4 and [(R)-1]2 ClO4 , showed metallic conductivity in the hightemperature 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]. The addition of a second stereogenic center on the EDT-TTF platform
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