The [(M+)x(crown-6)] supramolecular cations (SC+), in which M+ and crown-6 are alkali metal ions (M+ = Li+, Na+, and Cs+) and 1,4,7,10,13, 16-hexaoxacyclooctadecane, respectively, form ionic channel structures through the regular stacks of crown-6 in [Ni(dmit)2]-based molecular conductors (dmit2- = 2-thioxo-1,3-dithiole-4,5-dithiolate). In addition to the [Ni(dmit)2] salts that have the ionic channel structures (these salts are abbreviated as type I salts), Li+ and Na+ form dimerized [(M+)2(crown-6)2] units in the crystals (type II salts). The K+ and Rb+ are coordinated tightly into the crown-6 cavity to form typical disk-shape SC+ units in the corresponding [Ni(dmit)2] salts (type III salts). The type I, II, and III salts have typical stoichiometries of [(M+)x(crown-6)]-[Ni(dmit)2]2, [(M+)(crown-6)(H2O)x-(CH 3CN)1.5-x][Ni(dmit)2]3 (x=1 for Li+ or 0.5 for Na+), and [M+(crown-6)][Ni(dmit)2]3, respectively; the salts of the same type are isostructural. In agreement with the trimer structures of [Ni(dmit)2] in the type II and III salts, they exhibit semiconducting behavior with electrical conductivities at 300 K (σ300 K) of 0.01-0.1 S cm-1. Type I salts contain a regular stack of partially oxidized [Ni(dmit)2] units, which form a quasi one-dimensional metallic band within the tight-binding approximation regime. The electrical conductivities at 300 K are 10-30 S cm-1, and an almost temperature-independent conductivity was observed at higher temperatures. However, the one-dimensional electronic structures in these salts are strongly influenced by the static and dynamic structures of the coexisting ionic channel. The Na+ salt is a semiconductor, whose magnetic behavior is described by the disordered one-dimensional antiferromagnetic chain. On the other hand, the Cs+ salt is a exhibits metallic properties with 2kF instability at room temperature. The Li+ salt shows a gradual transition from the high-temperature metallic phase to the low-temperature one-dimensional antiferromagnetic semiconductor phase, which was associated with the freezing of Li+ motion at lower temperatures. The preferential crystallization of type I salts was possible by controlling the equilibrium constant (Kc) of the complex formation between M+ ions and the crown-6 molecule. The ionic channel structures were obtained when the Kc was low in the electrocrystallization solution, while type II or III salts were formed in the high Kc region.
|ジャーナル||Chemistry - A European Journal|
|出版ステータス||出版済み - 2001 11月 19|