Chiral Conducting Me-EDT-TTF and Et-EDT-TTF-Based Radical Cation Salts with the Perchlorate Anion

Nabil Mroweh,Enric Canadell,Narcis Avarvari,Nicolas Vanthuyne,Manuel Almeida,Pascale Auban-Senzier,Elsa B Lopes

doi:10.3390/cryst10111069

Abstract

Introduction of chirality in the field of molecular conductors has received increasing interest in recent years in the frame of modulation of the crystal packing, and hence conducting properties, with the number of stereogenic centers and absolute configuration, e.g., racemic or enantiopure forms. Here, we describe the preparation by electrocrystallization of chiral radical cation salts, based on the donors methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) 1 and ethyl-ethylenedithio-tetrathiafulvalene (Et-EDT-TTF) 2 containing one stereogenic center, with the perchlorate anion. Donor 1 provided the series of crystalline materials [(rac)-1]ClO4, [(S)-1]2ClO4 and [(R)-1]2ClO4, while for donor 2 only the 1:1 salts [(rac)-2]ClO4 and [(R)-2]ClO4 could be prepared as suitable single crystals for X-ray analysis. The enantiopure salts of 1 show β-type packing and metallic conductivity in the high temperature regime, with room temperature conductivity values of 5–10 S cm−1, whereas compound [(rac)-2]ClO4 is a very poor semiconductor. Tight-binding extended Hückel band structure calculations support the metallic conductivity of the enantiopure salts of 1 and suggest that small structural changes, possibly induced by thermal contraction or pressure, could lead to a pseudo-elliptic closed Fermi surface, typical for a 2D metal.

Highlights

In the frame of the increasingly developing field of chiral molecular materials [1], we recently introduced methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) 1 and ethyl-ethylenedithiotetrathiafulvalene (Et-EDT-TTF) 2 (Scheme 1) as valuable precursors for chiral molecular conductors [2,3], through a series of radical cation salts with the PF6 – anion, in which modulation of the donor:anion stoichiometry, crystal packing and conducting properties with the absolute configuration and steric bulkiness of the substituent was observed [4]
(Scheme 1), since the former crystallize in the triclinic space group P1 and show metallic behaviour in the high-temperature regime, while the latter crystallize in the monoclinic space group P21 and are bandgap semiconductors [5]. These results point out the importance of the number of stereogenic centres in the modulation of intermolecular interactions with the anion, through the establishment of C–H···F
The, (S) and (R) (1)2 PF6 salts are isostructural with the (DM-EDT-TTF)2 XF6 (X = As, Sb) series [6], showing that the size of the anion, in conjunction with the number of stereogenic centres, can play a paramount role in the modulation of the intermolecular interactions

Summary

Introduction

In the frame of the increasingly developing field of chiral molecular materials [1], we recently introduced methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) 1 and ethyl-ethylenedithiotetrathiafulvalene (Et-EDT-TTF) 2 (Scheme 1) as valuable precursors for chiral molecular conductors [2,3], through a series of radical cation salts with the PF6 – anion, in which modulation of the donor:anion stoichiometry, crystal packing and conducting properties with the absolute configuration and steric bulkiness of the substituent was observed [4]. Crystals 2020, 10, 1069 for example, between the enantiopure PF6 – salts of 1 and those of the dimethylated donor DM-EDT-TTF (Scheme 1), since the former crystallize in the triclinic space group P1 and show metallic behaviour in the high-temperature regime, while the latter crystallize in the monoclinic space group P21 and are bandgap semiconductors [5]. These results point out the importance of the number of stereogenic centres in the modulation of intermolecular interactions with the anion, through the establishment of C–H···F hydrogen bonding, donor packing, and electron transport properties. Enantiopure DM-EDT-TTF provided enantiomorphic mixed-valence salts (DM-EDT-TTF) ClO4 showing the eMChA effect [17], the use of (S,S,S,S)-TM-BEDT-TTF afforded a 3:2 salt with metallic conductivity [13], while an orthorhombic semiconducting phase [11] and a metallic monoclinic phase [19], both presenting a 2:1 stoichiometry, have been reported for (R,R)-DM-BEDT-TTF and (S,S)-DM-BEDT-TTF, respectively

Methods

Results

Conclusion