A high temperature reversible phase transition in a supramolecular complex of 15-crown-5 with tetraphenylboron sodium

A supramolecular crystal, [Na(15-crown-5)][BPh₄] (1), (15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane, NaBPh₄ = sodium tetraphenylboron), has been obtained by mixing the ethanol solution of 15-crown-5 and NaBPh₄ in the molar ratio of 1:1. The crystal structure was determined at 293 K, revealing that two [Na(15-crown-5)]⁺ cations form a supramolecular dimer via sharing one side-edge of coordination pentagonal pyramids; also, there are significant H-bonding interactions between anions and supramolecularly dimeric cations. Differential scanning calorimetry (DSC) showed that 1 undergoes a reversible first-order phase transition at ca. 391 K (T_c) upon heating, with a thermal hysteresis of 19 K. ΔH and ΔS were estimated to be 6.9 kJ mol^-1 and 17.7 J mol^-1 K^-1, respectively, in the heating run. The variable-temperature powder X-ray diffraction and dielectric spectra were collected, and both disclosed no significant difference between the low- and high-temperature phases. These results suggest that the phase transition is an ordered-disordered type, which probably involves the change of anion configuration.