Polymeric anions leading to novel packing motifs in donor-radical salts: synthesis and crystal and band electronic structure of (BEDT-TTF)BiI4

Abstract

In an attempt to synthesize a 2:1 salt of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET, C{sub 10}H{sub 8}S{sub 8}) with soft C-H{hor ellipsis} anion contacts, which promote an increase in the superconducting transition temperature, T{sub c}, in {beta}-(ET){sub 2}X salts, the authors carried out slow electrocrystallization of ET with BiI{sub 3} and ((n-C{sub 4}H{sub 9}){sub 4}N)I in 1,1,2-trichloroethane. This led to an unexpected 1:1 salt, (ET)BiI{sub 4}, which was characterized by X-ray single-crystal diffraction. The polymeric anion, a (BiI{sub 4}{sup {minus}}){infinity} chain, contains (BiI{sub 6}) octahedra sharing edges, such that the two terminal iodide ligands are in a cis configuration. ET{sup +} cations form a network of (ET{sup +}){sub 2} dimers, connected by S{hor ellipsis}S contacts shorter than the sum of the van der Waals radii (3.6 {angstrom}). The spin susceptibility as a function of temperature, derived from ESR measurements, follows Curie's law down to 20 K, but a spin count indicates that the ESR signal is due to an impurity (less than 1%). Band electronic structure calculations on the (ET{sup +}){sub 2} layer predict (ET)BiI{sub 4} to be a semiconductor with a band gap of order 0.4 eV. Crystal data for the complex are reported. 25 refs., 4 figs., 3 tabs.