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

Electrocrystallization experiments with the chiral ethylenedithiotetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors (R)-, (S)-, and (rac)-1 have provided two series of mixed-valence salts with the PF6- 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 (1) over bar for the racemic forms and noncentrosymmetric P1 for the enantiopure salts. In the racemic salt [(rac)-1](2)PF6, 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 Scm(-1) for [(rac)-1](2)PF6, in which disorder was observed. whereas for [(R)-1](2)PF6 and [(S)-1](2)PF6 the average value is around 100S cm(-1). In the second series of salts the conductivity at room temperature is 125130 Scm(-1) for [(rac)-1](2)[Au(CN)(2)] and [(R)-1](2)[Au(CN)(2)]. Extended Huckel 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.