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

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 MX counter anions (M = Fe, Ga; X = Cl, Br). Crystal structures of the DBrETSe salts depend on the halogen species on the MX counter anion. The MCl salts of DBrETSe are isostructural and crystallize in the orthorhombic space group Ibam and those of the MBr salts crystallize in the monoclinic space group C2/c. On the other hand, all four MX 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)MX (M = Fe, Ga; X = Cl, Br) show stable metallic behaviour down to 4.2 K. This is in contrast to their iodinated analogues (DIETSe)MCl (M = Fe, Ga), which show a metal-semiconductor transition at 11 K for the FeCl salt and at 12 K for the GaCl salt. No metal-semiconductor transition is observed for (DIETSe)GaBr which contains the non-magnetic anion, but the transition is observed at 7.2 K for the corresponding FeBr salt, which contains a magnetic anion, indicating that the metal-semiconductor transition of (DIETSe)FeBr correlates to the π-d electronic interaction through the I⋯Br halogen bonds. Antiferromagnetic orderings of d spins of the FeX anions are observed in (DBrETSe) FeX and (DIETSe)FeX (X = Cl, Br). In contrast to the low Néel temperature (T ≈ 2.5 K) of the FeCl salts, the antiferromagnetic orderings occur at relatively high temperatures, i.e. T = 7.5 K for (DBrETSe) FeBr and T = 7.0 K for (DIETSe) FeBr. Since the metallic state of (DBrETSe) FeBr remains below T, this salt is classified as a novel antiferromagnetic organic metal. On the other hand, the antiferromagnetic ordering of the d spins in (DIETSe)FeBr takes place cooperatively with the metal-semiconductor transition around 7 K. These antiferromagnetic orderings of the d spins between the FeX anions cannot be explained by direct anion-anion interactions because of their long halogen⋯halogen distances between the FeX anions, and the importance of the π-d interaction between the donors and the counter anions through the halogen bonds is strongly suggested. © The Royal Society of Chemistry 2006.