New organic conductors based on dibromo- and diiodo-TSeFs with magnetic and non-magnetic MX4 counter anions (M = Fe, Ga; X = Cl, Br)

Abstract

Eight cation radical salts based on halogenated tetraselenafulvalene (TSeF) derivatives, dibromo(ethylenedithio)tetraselenafulvalene (DBrETSe) and diiodo(ethylenedithio)tetraselenafulvalene (DIETSe), were prepared using magnetic and non-magnetic MX4 counter anions (M = Fe, Ga; X = Cl, Br). Crystal structures of the DBrETSe salts depend on the halogen species on the MX4 counter anion. The MCl4 salts of DBrETSe are isostructural and crystallize in the orthorhombic space group Ibam and those of the MBr4 salts crystallize in the monoclinic space group C2/c. On the other hand, all four MX4 salts of DIETSe are isostructural and crystallize in the orthorhombic space group Ibam. In all eight crystals, donor molecules form a so-called β-type molecular arrangement and characteristic halogen bonds between the halogen atoms on the edge of the donor molecules and those of the counter anions are observed. (DBrETSe)2MX4 (M = Fe, Ga; X = Cl, Br) show stable metallic behaviour down to 4.2 K. This is in contrast to their iodinated analogues (DIETSe)2MCl4 (M = Fe, Ga), which show a metal–semiconductor transition at 11 K for the FeCl4 salt and at 12 K for the GaCl4 salt. No metal–semiconductor transition is observed for (DIETSe)2GaBr4, which contains the non-magnetic anion, but the transition is observed at 7.2 K for the corresponding FeBr4 salt, which contains a magnetic anion, indicating that the metal–semiconductor transition of (DIETSe)2FeBr4 correlates to the π–d electronic interaction through the I⋯Br halogen bonds. Antiferromagnetic orderings of d spins of the FeX4 anions are observed in (DBrETSe)2FeX4 and (DIETSe)2FeX4 (X = Cl, Br). In contrast to the low Neel temperature (TN ≈ 2.5 K) of the FeCl4 salts, the antiferromagnetic orderings occur at relatively high temperatures, i.e.TN = 7.5 K for (DBrETSe)2FeBr4 and TN = 7.0 K for (DIETSe)2FeBr4. Since the metallic state of (DBrETSe)2FeBr4 remains below TN, this salt is classified as a novel antiferromagnetic organic metal. On the other hand, the antiferromagnetic ordering of the d spins in (DIETSe)2FeBr4 takes place cooperatively with the metal–semiconductor transition around 7 K. These antiferromagnetic orderings of the d spins between the FeX4 anions cannot be explained by direct anion–anion interactions because of their long halogen⋯halogen distances between the FeX4 anions, and the importance of the π–d interaction between the donors and the counter anions through the halogen bonds is strongly suggested.