Ionic Channel Structures in (M+)x([18crown-6)]Ni(dmit)22 Molecular Conductors

The (M+)x[18crown-6)] supramolecular cations (SC+), in which M+ and 18crown-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 18crown-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([18crown-6)2] units in the crystals (type II salts). The K+ and Rb+ are coordinated tightly into the 18crown-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([18crown-6)]Ni(dmit)22, (M+)([18crown-6)(H2O)x(CH3CN)(1.5 - x)]Ni(dmit)23 (x = 1 for Li+ or 0.5 for Na+), and M+([18crown-6)]Ni(dmit)23, 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 (sigma(300 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 2 kF 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 18crown-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.