Tetraalkyl- and dialkyl-substituted BEDT-TTF derivatives and their cation-radical salts: synthesis, structure, and properties

Tetraalkyl and dialkyl derivatives, where alkyl = ethyl and propyl, of the organic electron donor molecule bis(ethylenedithio) tetrathiafulvalene, BEDT-TTF or ETT have been synthesized via the Diels-Alder approach. Several cation-radical salts of these new donors have been prepared and structurally characterized, and found to contain donor molecules in nominally higher oxidation states (+1, +1.5 and +2) than the typically observed oxidation state of +0.5 in BEDT-TTF sails. The higher solubility of the tetraalkyl and dialkyl derivatives in solvents used for crystal growth is proposed as the principal reason for this finding. Surprisingly, X-ray crystallographic studies reveal that the alkyl groups in the neutral tetraethyl-ET as well as the oxidized tetraethyl-ET and diethyl-ET molecules in their cation-radical salts adopt axial configurations: rather than the expected equatorial configurations. Electrical properties of the cation-radical sails have been found to be either insulating or semiconducting, consistent with the higher oxidation states of the donor molecules in the salts and the crystal structures.