Structure and Physical Properties of Low-Dimensional Molecular Conductors, [PXX][FeIII(Pc)(CN)2] and [PXX][CoIII(Pc) (CN)2] (PXX = peri-xanthenoxanthene, Pc = phthalocyaninato)

A novel low-dimensional molecular conductor, [PXX][FeIII(Pc)(CN)2], has been synthesized. This salt contains the magnetic FeIII ion (S = 1/2), and is isomorphous with [PXX][CoIII(Pc)(CN)2] which includes the non-magnetic CoIII ion. In both salts, the [MIII(Pc)(CN)2] (M = Fe or Co) units form a two-leg ladder chain. The two salts exhibit a similar temperature dependence of the thermoelectric power and a similar reflectance spectrum. The FeIII salt shows semiconducting behavior in its electrical resistivity over the temperature range measured, while the isomorphous CoIII salt exhibits metallic behavior in its resistivity above 100 K. The difference in the transport properties between the two salts suggests that the conduction electrons in the FeIII salt are seriously scattered by the local magnetic moment. Spontaneous magnetization is observed below 8 K in the FeIII salt. Upon applying a magnetic field, the resistivity of the FeIII salt drastically decreases below 50 K. The decrease in the resistivity is highly anisotropic to the field orientation. The field orientation dependence is highly consistent with the g-tensor anisotropy in the [FeIII (Pc)(CN)2] unit, suggesting that the negative magnetoresistance originates from the large (pi)-d interaction self-contained in the [FeIII(Pc)(CN)2] unit.