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Conducting Dimerized Cobalt Complexes with Tetrathiafulvalene Dithiolate Ligands

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posted on 04.02.2008, 00:00 by Emiko Fujiwara, Kazumasa Hosoya, Akiko Kobayashi, Hisashi Tanaka, Madoka Tokumoto, Yoshinori Okano, Hideki Fujiwara, Hayao Kobayashi, Yuichi Fujishiro, Eiji Nishibori, Masaki Takata, Makoto Sakata
To obtain novel single-component molecular metals, we attempted to synthesize several cobalt complexes coordinated by TTF (tetrathiafulvalene)-type dithiolate ligands. We succeeded in the syntheses and structure determinations of (nBu4N)2[Co(chdt)2]2 (1), (nBu4N)2[Co(dmdt)2]2 (2), [Co(dmdt)2]2 (3), and [Co(dt)2]2 (4) (chdt = cyclohexeno-TTF-dithiolate, dmdt = dimethyl-TTF-dithiolate, and dt = TTF-dithiolate). Structure analyses of complexes 14 revealed that two monomeric [Co(ligand)2]- or [Co(ligand)2]0 units are connected by two Co−S bonds resulting in dimeric [Co(ligand)2]22- or [Co(ligand)2]2 molecules. Complex 1 has a cation−anion-intermingled structure and exhibited Curie−Weiss magnetic behavior with a large Curie constant (C = 2.02 K·emu·mol-1) and weak antiferromagnetic interactions (θ = −8.3 K). Complex 2 also has a cation−anion-intermingled structure. However, the dimeric molecules are completely isolated by cations. Complexes 3 and 4 are single-component molecular crystals. The molecules of complex 3 form two-dimensional molecular stacking layers and exhibit a room-temperature conductivity of σrt = 1.2 × 10-2 S·cm-1 and an activation energy of Ea = 85 meV. The magnetic behavior is almost consistent with Curie−Weiss law, where the Curie constant and Weiss temperature are 8.7 × 10-2 K·emu·mol-1 and −0.85 K, respectively. Complex 4 has a rare chair form of the dimeric structure. The electrical conductivity was fairly large (σrt = 19 S·cm-1), and its temperature dependence was very small (σ0.55Krt = ca. 1:10), although the measurements were performed on the compressed pellet sample. Complex 4 showed an almost constant paramagnetic susceptibility (χ300 K = 3.5 × 10-4 emu·mol-1) from 300 to 50 K. The band structure calculation of complex 4 suggested the metallic nature of the system. Complex 4 is a novel single-component molecular conductor with a dimeric molecular structure and essentially metallic properties down to very low temperatures.