Ionic Channel Structures in [(M+)x([18]crown-6)][Ni(dmit)2]2 Molecular Conductors

Abstract

The [(M+)x[18]crown-6)] supramolecular cations (SC+), in which M+ and [18]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 [18]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([18]crown-6)2] units in the crystals (type II salts). The K+ and Rb+ are coordinated tightly into the [18]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([18]crown-6)][Ni(dmit)2]2, [(M+)([18]crown-6)(H2O)x(CH3CN)(1.5 - x)][Ni(dmit)2]3 (x = 1 for Li+ or 0.5 for Na+), and [M+([18]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 (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 [18]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.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State

“…The peak attributed to a C‐O‐C stretching vibration in [18]crown‐6 changed from a doublet at 1104 and 1114 cm −1 for 1 to a singlet at 1104 cm −1 in the IR spectrum of 1‐K . As the position of the C‐O‐C band depends on the included alkali metal ions, the Li + ions in 1 were believed to have been exchanged by K + in the solution. Meanwhile, the band in the range of 3200–3700 cm −1 derived from O‐H stretching of the crystalline water molecules in the ion channels almost disappeared in this experiment.…”

Effective Na+-Binding Ability and Molecular Assembly of an Alkylamide-Substituted Penta(ethylene)glycol Derivative

“…The ion-recognized crown ether and oligo­(ethylene glycol) derivatives can form interesting ion-encapsulated static and dynamic molecular assemblies. − For instance, a regular array of crown ethers forms an artificial ionic channel that has been applied for the design of functional ion-transport systems in molecular assemblies. − The ionic channels of M + ([18]­crown-6), where M + = Li + , Na + , and Cs + , have been introduced into the electrically conducting [Ni­(dmit) 2 ] salts (dmit 2– = 2-thioxo-1,3-dithiole-4,5-dithiolate); the ion dynamics inside the channels have been investigated based on the electrical conductivity and magnetic susceptibility. − A small-sized Li + cation inside the cavity of [18]­crown-6 has motional freedom, while the dynamics of a Na + cation inside [18]­crown-6 is subtly restricted due to the effective binding of Na + to the six ether oxygen atoms. In addition, the large-sized Cs + cation cannot pass through the cavity of [18]­crown-6 and is fixed between the [18]­crown-6 molecules.…”

Dynamics and Structural Diversity of Li+(Crown Ether) Supramolecular Cations in Electrically Conducting Salts

“…We have previously used M + (crown ether)-based supramolecular cations in [Ni­(dmit) 2 ] salts (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate) to control the crystal structures and, thus, the electronic, magnetic, and dielectric properties. − As a result, a uniform (Li + ) x ([15]­crown-5) array was found to coexist with partially reduced π-stacks of [Ni­(dmit) 2 ] in (Li + ) 0.6 ([15]­crown-5)­[Ni­(dmit) 2 ]·(H 2 O) crystals, where the Li + - and electron-conducting columns exhibited ionic and electrical conductivities of 10 –6 and 100 S cm –1 , respectively . Thus, the electrical conductivity of this crystal is 10 8 -times higher than its Li + ionic conductivity.…”

“…Such hybrid Li + /electron-conducting molecular crystals have been obtained as highly designable supramolecular cation systems. Furthermore, the modulation of the ionic radii from those of Li + , Na + , and NH 4 + to that of Cs + in regular arrays of (M + ) x ([18]­crown-6) in electrically conducting [Ni­(dmit) 2 ] crystals enables the control of the dynamic behavior of the ions in the ionic channels as well as the electron-conducting properties of the material . Specifically, the small and high-mobility Li + in the ionic channels of (Li + ) x ([18]­crown-6) can couple with the conduction electrons in the π-stacks of partially reduced [Ni­(dmit) 2 ], providing a useful approach for the design of Li + dynamics in such supramolecular Li + (crown ether) assemblies.…”