Hal⋯Hal interactions in a series of three isostructural salts of halogenated tetrathiafulvalenes. Contribution of the halogen atoms to the HOMO–HOMO overlap interactions

The halogenated tetrathiafulvalenes, Br₂­EDT­TTF and I₂­EDT­TTF were prepared by the coupling route from the corresponding 4,5­dibromo- or 4,5-diiodo­1,3­dithiole­2­thione. In the isostructural series, (Br₂­EDT­TTF)₂IBr₂, (Br₂­EDT­TTF)₂I₃ and (I₂­EDT­TTF)₂I₃, noted Br₂/IBr₂, Br₂/I₃ and I₂/I₃ respectively, short (3.42–3.60 Å) and directional Hal⋯Hal interactions are identified between donor molecules and with the anions, stabilising rare β' phases. These interactions play not only a structural role but also contribute to the electronic dispersion thanks to sizeable coefficients on the halogen atoms in the HOMOs of Br₂­EDT­TTF and I₂­EDT­TTF. The three salts behave as Mott insulators as reflected by a high room temperature conductivity (0.5 S cm⁻¹) with an activation energy which increases in the order Br₂/IBr₂ (730 K), Br₂/I₃ (1260 K), I₂/I₃ (1330 K) and a weak magnetic susceptibility, which decreases abruptly below 150 K with no sign of an antiferromagnetic ground state.