Synthesis, crystal structure and properties of the semiconducting molecular charge-transfer salt (bedt-ttf)2Ge(C2O4)3·PhCN [bedt-ttf=bis(ethylenedithio)tetrathiafulvalene]

Abstract

The synthesis, crystal structure and physical properties of a new bedt-ttf charge transfer salt containing the tris(oxalato)germanium(IV) anion are described and interpreted. Electrochemical oxidation of neutral bedt-ttf in the presence of (NH₄)₂Ge(C₂O₄)₃·2H₂O in PhCN solution yields crystals of (bedt-ttf)₂Ge(C₂O₄)₃·PhCN. The crystal structure has been solved at 296(2) and at 120(2) K. In contrast to the well known tris(oxalato)-metallate(III) salts of bedt-ttf, the structure does not contain alternating layers of bedt-ttf cations and tris(oxalato)metallate anions, but consists of a ‘checker board’ arrangement of face-to-face bedt-ttf dimers with [Ge(C₂O₄)₃]^2– interspersed by layers of solvent molecules. Each bedt-ttf molecule has a charge close to +1, which is estimated from the empirical relationship between CC and C–S bond lengths and these charges are correlated with Raman spectra. As expected from the presence of strongly dimerised (bedt-ttf⁺)₂ units the salt is a semiconductor between 300 and 120 K with a low activation energy of 0.127 eV.