Order Versus Disorder in Chiral Tetrathiafulvalene‐Oxazoline Radical‐Cation Salts: Structural and Theoretical Investigations and Physical Properties

Abstract

Electrocrystallization experiments with the chiral ethylenedithio-tetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors (R)-, (S)-, and (rac)-1 have provided two series of mixed-valence salts with the PF(6) (-) and [Au(CN)(2)](-) anions. Within each series the cell parameters are the same for the three R, S, and rac compounds, except for the space group, which is centrosymmetric triclinic P$\bar 1$ for the racemic forms and noncentrosymmetric P1 for the enantiopure salts. In the racemic salt [(rac)-1](2)PF(6) the two enantiomers crystallize disordered on the same crystallographic site with a site occupational factor of 0.6:0.4, whereas this type of disorder is not possible in the enantiopure salts. Both s-cis and s-trans conformations, when taking into account the mutual orientation of the TTF and oxazoline moieties, are present in this first series. In sharp contrast, in the series of salts [1](2)[Au(CN)(2)], only the s-trans conformation is observed with no structural disorder. Theoretical calculations at the DFT level of theory revealed a very small energy difference between the two stable planar s-cis and s-trans conformations, which are both energy minima in either neutral or oxidized states. Single-crystal conductivity measurements showed metallic-like behavior for all the salts down to 220-250 K with a smooth increase in resistivity at lower temperatures. The conductivity at room temperature is 5 S cm(-1) for [(rac)-1](2)PF(6), in which disorder was observed, whereas for [(R)-1](2)PF(6) and [(S)-1](2)PF(6) the average value is around 100 S cm(-1). In the second series of salts the conductivity at room temperature is 125-130 S cm(-1) for [(rac)-1](2)[Au(CN)(2)] and [(R)-1](2)[Au(CN)(2)]. Extended Hückel band structure calculations revealed identical features for the three salts of the [1](2)[Au(CN)(2)] series and are consistent with the electronic structures of quasi-one-dimensional conductors.

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…The introduction of chirality in these materials represents one of the most recent advances [144] in material science and one of the milestones is represented by the first observation of the electrical magneto-chiral anisotropy (eMChA) effect in a bulk crystalline chiral conductor [145], as a synergy between chirality and conductivity [146][147][148]. However, the combination of chirality with electroactivity in chiral TTF-based materials afforded several other recent important results, particularly the modulation of the structural disorder in the solid state , [130][131][132][133][134][135][136][137][138] and hence a difference in conductivity between the enantiopure and racemic forms [149][150][151] and the induction of different packing patterns and crystalline space groups in mixed valence salts of dimethylethylenedithio-TTF (DM-EDT-TTF), showing semiconducting (enantiopure forms) or metallic (racemic form) behaviour [152]. Although the first example of an enantiopure TTF derivative, namely the tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF), was described almost 30 years ago as the (S,S,S,S) enantiomer [153,154], the number of TM-BEDT-TTF based conducting radical cation salts is still rather limited.…”

Electrical magnetochiral anisotropy in a bulk chiral molecular conductor

“…15), only much more recently there has been an increasing interest in the introduction of chiral information in TTF derivatives 16 , mainly motivated by the possible detection of the eMChA effect. Nevertheless, as mentioned earlier, no such evidence has been provided so far in bulk conductors, including the TTF-based materials, although the influence of chirality on the conducting properties of TTF conductors has been indirectly expressed through the modulation of the structural disorder in a few series of conducting radical cation salts based on chiral TTFoxazoline 17,18 or TM-BEDT-TTF donors 19 , in which the enantiopure materials were more conducting than the racemic ones because of the structural disorder present in the latter. Compared with the relatively large number of chiral TTF precursors 16,20 , only a few examples of derived conducting materials have been described, such as those based on TM-BEDT-TTF 13,19,21 dimethyl-bis(ethylenedithio)-TTF (DM-BEDT-TTF) 22,23 , or TTF-oxazoline 17,18 .…”

“…Nevertheless, as mentioned earlier, no such evidence has been provided so far in bulk conductors, including the TTF-based materials, although the influence of chirality on the conducting properties of TTF conductors has been indirectly expressed through the modulation of the structural disorder in a few series of conducting radical cation salts based on chiral TTFoxazoline 17,18 or TM-BEDT-TTF donors 19 , in which the enantiopure materials were more conducting than the racemic ones because of the structural disorder present in the latter. Compared with the relatively large number of chiral TTF precursors 16,20 , only a few examples of derived conducting materials have been described, such as those based on TM-BEDT-TTF 13,19,21 dimethyl-bis(ethylenedithio)-TTF (DM-BEDT-TTF) 22,23 , or TTF-oxazoline 17,18 . Moreover, complete series of conducting salts containing both enantiomers and the racemic forms, in order to compare their conducting properties, are even more scarce [17][18][19] , and in all of them the enantiomeric pairs crystallize within the same space group, although there is in principle the possibility that enantiomers crystallize in one of the 11 enantiomorphic space groups, which remains, however, a relatively rare event.…”

Enantiopure Radical Cation Salt Based on Tetramethyl-Bis(ethylenedithio)-Tetrathiafulvalene and Hexanuclear Rhenium Cluster

“…As expected, the cluster contains six Re atoms in an octahedral arrangement capped by six µ 3 -S and two µ 3 -Cl atoms forming a cube, with Re-S µ and Re-Cl µ distances in the normal range [28]. The coordination sphere of each Re ion is completed by an apical Cl ligand, with Re-Cl ap distances ranging between 2.362 and 2.383 Å. counterparts were also observed as a consequence of the structural disorder in TTF-oxazoline [11,12] based conductors [13,14], or the different packings in DM-EDT-TTF salts [15]. Other interests of chiral TTFs are related to the modulation of the chiroptical properties [16,17] or the preparation of electroactive helical fibers [18][19][20][21].…”

“…The interest in chiral TTF precursors and derived materials [8] is mainly related to the combination of chirality with conducting properties through the electrical magneto-chiral anisotropy effect [9], recently described for enantiopure crystalline metallic salts of the dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) donor [10]. Nevertheless, differences in conducting properties between the enantiopure and racemic counterparts were also observed as a consequence of the structural disorder in TTF-oxazoline [11,12] based conductors [13,14], or the different packings in DM-EDT-TTF salts [15]. Other interests of chiral TTFs are related to the modulation of the chiroptical properties [16,17] or the preparation of electroactive helical fibers [18][19][20][21].…”