New (TMTSF)₂X Derivatives: A Change in the Selenium Network Dimensionality Derived From the Molecular and Crystal Structures of (TMTSF)₂(Fso₃) [T=298K, 123K] and (TMTSF)₂(Bro₄) T=298K

We report the first crystallographic analysis, as a function of temperature, of a TMTSF derivative. Both (TMTSF)2(FSO3) and (TMTSF)2(BrO4) are isostructural (triclinic, with space group PI) with superconducting (TMTSF)2(ClO4). (TMTSF)2(FSO3) undergoes a metal-to-insulator transition at 86-90K as observed by microwave conductivity, D.C. conductivity, and magnetic susceptibility. The crystal structure contains 2-dimensional sheets of short Se-Se contacts in the molecular stacking direction and perpendicular to the stacking direction. The temperature dependent variations in these contact distances appear to be of special importance in determining the conduction properties of these materials, and are observed to change in a surprising manner when (TMTSF)2(FSO3) is cooled (298 → 123K). The homoatomic Se separations within each TMTSF molecule appear to increase slightly, but not significantly. At the same time the entire 2-dimensional sheet of intermolecular (intra- and interstack) Se-Se contacts between TMTSF molecules contract quite anisotropically, which results in an increase in “dimensionality” of the Se-Se network. Hence, an increase in electrical conduction, in the absence of insulating phenomena, over the temperature range 298 → 123K is not surprising. The intermolecular Se-Se contact distances in (TMTSF)2(BrO4) are significantly longer than in (TMTSF)2(FSO3) which suggests that the room temperature electrical conductivity of the (BrO4)− salt may be diminished compared to the (FSO3)− analogue.