Crystal Structures and Physical Properties of (M+)(Crown Ethers)x(TCNQ)y Salts with M+ = Li+, Na+, K+, Rb+, and Cs+

Highly designable M+(crown ether)x supramolecular cations were introduced into electrically conducting anion radical salts of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as counter cations. Alkali metal cations of M+ = Li+, Na+, K+, Rb+, and Cs+ were combined with crown ethers of [12]crown-4, [15]crown-5, and [18]crown-6, forming 18 types of single crystals of M+(crown ether)x(TCNQ)y (x = 1 or 2 and y = 2, 1.5, 2.5 or 3). Single-crystal X-ray structural analyses of all of the compounds revealed a variation in the supramolecular cation of M+(crown ether)x and the TCNQ arrangements. A smaller size of Li+ than the [18]crown-6 cavity formed a dynamic cation environment, whereas the size-compatible K+([18]crown-6) supramolecule exhibited a planar disc-shaped structure. When the size of M+ was larger than the crown ether cavity, sandwich-type M+(crown ether)2 and cone-type M+(crown ether) structures were observed in the TCNQ salts. K+([12]crown-4)2 and Cs+([15]crown-5) structures are typical examples of sandwich- and cone-type supramolecules, respectively. Both cation structures were observed in K+([12]crown-4)x and Rb+([12]crown-4)x with x = 1 and 2, and the cone-type structures were also confirmed in M+([15]crown-5) for M+ = Rb+ and Cs+. Depending on the type of the supramolecular cation, the π-stacking arrangements of TCNQs changed from 1D dimer to 1D dimer + monomer, 1D trimer, 1D trimer, 2D dimer, 2D tetramer, 2D spanning-overlap, and 1D trimer + monomer arrangements. The systematic study of all of the combinations of alkali metal ions and crown ethers enables the formation of various types of intermolecular interactions, electrical conductivity, phase transition, and magnetic properties. Among all of the crystals, the sandwich-type M+([12]crown-4)2 with M+ = Na+, K+, and K+([15]crown-5)2 supramolecules demonstrated a thermally activated rotation of crown ethers in the high-temperature phase, whereas the motional freedom of Li+ and Na+ in [15]crown-5 or [18]crown-6 coupled to the phase transition behavior and dielectric response in the TCNQ salts.