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

Journal of Materials Chemistry Pub Date: 2006-07-19 DOI: 10.1039/B605420J

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)₂GaBr_4, 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 Néel temperature (T_N ≈ 2.5 K) of the FeCl₄ salts, the antiferromagnetic orderings occur at relatively high temperatures, i.e.T_N = 7.5 K for (DBrETSe)₂FeBr₄ and T_N = 7.0 K for (DIETSe)₂FeBr₄. Since the metallic state of (DBrETSe)₂FeBr₄ remains below T_N, 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.