Effect of Included Solvent Molecules on the Physical Properties of the Paramagnetic Charge Transfer Salts β‘ ‘-(bedt-ttf)4[(H3O)Fe(C2O4)3]·Solvent (bedt-ttf = Bis(ethylenedithio)tetrathiafulvalene)

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Abstract
The new charge transfer salt, β‘ ‘-(bedt-ttf)4[(H3O)Fe(C2O4)3]·C5H5N, I, (bedt-ttf = bis(ethylenedithio)tetrathiafulvalene) has a crystal structure closely similar to that of the reported salt β‘ ‘-(bedt-ttf)4[(H3O)Fe(C2O4)3]·C6H5CN, II, which has a superconducting critical temperature of 8.6 K. However, variable temperature magnetic and transport experiments show that I has a metal to insulator transition at 116 K. The crystal structure of I has been determined above (150 K) and below (90 K) the metal to insulator transition and comparisons are made with the structure of II. The pyridine solvate crystallizes in the monoclinic space group C2/c with a = 10.267(2) Å, b = 19.845(4) Å, c = 34.907(7) Å, β = 93.22(3)°, Z = 4 at 150 K and with a = 10.2557(15) Å, b = 19.818(28) Å, c = 34.801(49) Å, β = 93.273(14)°, Z = 4 at 90 K. The structures of I and II both consist of layers of bedt-ttf with +0.5 formal charge per molecule and layers of approximately hexagonal symmetry containing H3O+ and [Fe(C2O4)3]3-. The solvent molecules occupy hexagonal cavities formed by the anionic layer. Changing the solvent molecule from C6H5CN to C5H5N induces disorder in the bedt-ttf layer which accounts for the dramatic difference in observed physical properties. For I, at 150 K, one-half of all the bedt-ttf molecules have identical conformations to all the molecules in II where both terminal ethylene groups of each bedt-ttf molecule are twisted and eclipsed with respect to the opposite end of the molecule. The remaining 50% of bedt-ttf molecules in I have disordered ethylene groups. The disorder persists at 90 K where it can be resolved into two conformations: twisted-twisted eclipsed and twisted-twisted staggered.

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