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

Li+([12]crown-4)(2)(TCNQ)(2) (1), Li+([15]crown-5)(TCNQ)(2) (2a and 2b), and Li+([18]crown-6)(TCNQ)(2)center dot(H2O)(2) (3) salts (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) were prepared and examined in terms of crystal structures, dielectric constants, magnetic susceptibilities, and electrical conductivities. The dynamic behavior of the small Li+ was easily activated inside the cavities of [15]crown-5 and [18]crown-6. In salt 2a, a structural phase transition at 160 K is thermally activated by a change in the -CN center dot center dot center dot Li+center dot center dot center dot NC- coordination from an asymmetrical to a symmetrical environment. The phase-transition temperature for salt 2a is approximately 25 K lower than that for the isostructural Na+([15]crown-5)(TCNQ)(2), indicating that the behavior of Li+ inside the [15]crown-5 cavity is considerably more dynamic than that of Na+ in the same situation. Crystal polymorphs 2a and 2b are observed for Li+([15]crown-5) supramolecular cations, where the two-dimensional intermolecular interactions of TCNQs in salts 2a and 2b are constructed by the spanning-overlap mode of the pi-dimers and pi-tetramers, respectively. The packing periodicity in the latter salt is twice that in the former, and this structural difference prevents the structural phase transition for salt 2b. In salt 3, the orientational disorder of Li+ inside the cavity of the [18]crown-6 is a combination of the dynamic behavior of Li+ itself and that of Li+-coordinated polar H2O molecules, which are evidenced by two types of dielectric relaxation. Li+ is considerably smaller than Na+, allowing it to form a dynamic Li+center dot center dot center dot O coordination environment.