Single-Component Molecular Conductor [Cu(tmdt)₂] Containing an Antiferromagnetic Heisenberg Chain

Traditional molecular conductors are composed of more than two chemical species and are characterized by low-dimensional electronic band structures. By contrast, the single-component molecular metals [M(tmdt)2] (M = Ni, Pt, Au; tmdt = trimethylenetetrathiafulvalenedithiolate) possess three-dimensional electronic structures that can be widely tuned by exchanging the central transition metal atom (M). In this study, the Cu atom was used to realize a new magnetic single-component molecular conductor exhibiting strong π−d interactions. The crystal structure of [Cu(tmdt)2] was found to be essentially the same as those of the Ni, Pt, or Au-based systems with metallic states down to low temperature, but different from the structure of [Cu(dmdt)2] (dmdt = dimethyltetrathiafulvalenedithiolate) with its tetrahedrally coordinated dmdt ligands. A compressed pellet of microcrystals exhibited fairly high room-temperature conductivity (σRT ∼ 7 S·cm−1), which increased almost linearly with pressure, reaching 110 S·cm−1 at 15 kbar. This strongly suggests that the single crystal of [Cu(tmdt)2] is metallic at high pressure. Magnetic susceptibility measurements indicated one-dimensional Heisenberg behavior with |J| = 117 cm−1 and an antiferromagnetic transition at 13 K. Density functional theory molecular orbital calculations revealed that the α-spin orbital of pdσ(-) is distributed at the central part of the complex (CuS4), and α- and β-sym-Lπ orbitals have almost the same energies and their spins are distributed mainly in the pdσ(-) orbital. This is in contrast to the first single-component molecular metal [Ni(tmdt)2], which has stable metal bands formed from an almost degenerated sym-Lπ orbital (the highest occupied molecular orbital) and asym-Lπ(d) orbital (the lowest unoccupied molecular orbital). These results suggest that the α-pdσ(-) state of [Cu(tmdt)2] exists just around the Fermi energy of the virtual metal band formed from the asym-Lπ(d) and sym-Lπ states. Thus, as expected, [Cu(tmdt)2] is a non-trivial single-component molecular conductor with π−d multifrontier orbitals. In addition, (nBu4N)2[Cu(tmdt)2] was synthesized, and its crystal structure was determined. Its Curie behavior (χrt = 1.2 × 10−3 emu mol−1; C = 0.36 emu·K mol−1) indicates the existence of an isolated S = 1/2 spin on each dianionic molecule.