LOW-DIMENSIONAL MOLECULAR-METALS (PER)2M(MNT)2 (M = FE AND CO)

Single crystals of the quasi-one-dimensional conductors (Per)2M(mnt)2, M = Fe and Co (Per = perylene, mnt = maleonitriledithiolate), were prepared by electrocrystallization. By X-ray diffraction at room temperature and with synchrotron radiation (lambda = 0.89 angstrom, at Daresbury), the (Per)2Fe(mnt)2 Compound was shown to crystallize in the monoclinic space group C2/c with cell parameters a = 50.571 (6) angstrom, b = 8.212 (2) angstrom, c = 17.726 (9) angstrom, beta = 92.43 (1)-degrees, and Z = 8. An average crystal structure was obtained at 295 K using a conventional X-ray source, on the basis of the monoclinic space group P2(1)/n with a = 17.665 (3) angstrom, b = 4.098 (2) angstrom, c = 25.219 (3) angstrom, beta = 92.43 (2)-degrees, and Z = 2. The average crystal structure of (Per)2Fe(mnt)2 at room temperature consists of a close-packed arrangement of segregated stacks of perylene and Fe(mnt)2 units. Fe-57 Mossbauer spectroscopy shows the dimerization of the Fe(mnt)2 units with the Fe atom in a square pyramidal coordination by sulfur atoms without significant changes within the temperature range 15-295 K. For both compounds, room temperature values of electrical conductivity and absolute thermoelectric power are sigma(RT) congruent-to 200-OMEGA-1 cm-1 and S(RT) = 42-mu-V/K, respectively. They exhibit metallic behavior down to 58 and 73 K, for M = Fe and Co, respectively, where a metal-insulator transition occurs. The paramagnetic susceptibility of the Co compound is due to a Pauli-like contribution of the perylene chains of 3.2 x 10(-4) emu mol-1 at 300 K that vanishes at the metal-insulator transition, while that of the Fe compound is dominated by the contribution of antiferromagnetically coupled pairs of S = 3/2 spins in the [Fe(mnt)2]2(2-) units, in a way similar to the magnetic susceptibility of [(C2H5)4N]2[Fe(mnt)2]2 and [(n-C4H9)4N]2[Fe(mnt)2]2. These properties and the average crystal structure are compared with the correspondent ones for other metallic members of the Per2M(mnt)2 family.