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 pi-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 (sigma(RT) approximately 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 alpha-spin orbital of pdsigma(-) is distributed at the central part of the complex (CuS(4)), and alpha- and beta-sym-Lpi orbitals have almost the same energies and their spins are distributed mainly in the pdsigma(-) 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-Lpi orbital (the highest occupied molecular orbital) and asym-Lpi(d) orbital (the lowest unoccupied molecular orbital). These results suggest that the alpha-pdsigma(-) state of [Cu(tmdt)(2)] exists just around the Fermi energy of the virtual metal band formed from the asym-Lpi(d) and sym-Lpi states. Thus, as expected, [Cu(tmdt)(2)] is a non-trivial single-component molecular conductor with pi-d multifrontier orbitals. In addition, ((n)Bu(4)N)(2)[Cu(tmdt)(2)] was synthesized, and its crystal structure was determined. Its Curie behavior (chi(rt) = 1.2 x 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.