An organometallic half-sandwich supramolecular complex {K(18-Crown-6)(η^:N-C₆H₅B(C₆H₅)₃)} (n = 1-6) exhibiting a reversible breaking-symmetry phase transition and switchable dielectric behaviour

A supramolecular crystal, built from a 1-1 molar ratio of potassium tetraphenylboron (KBPh₄) with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-Crown-6), contains an organometallic half-sandwich superstructure unit {K(18-Crown-6)(ηⁿ-C₆H₅B(C₆H₅)₃)} (1 with n = 1-6), and shows a reversible phase transition at ∼211 K. The analysis of crystal structures at 173 K in the low-temperature phase, and 293 and 423 K in the high-temperature phase revealed that the breaking-symmetry phase transition is associated with the order-disorder transformation of [K(18-Crown-6)]⁺ and the change of the coordination mode of the phenyl ring to K⁺. A dielectric anomaly appears at ca. 212 K and dielectric relaxation occurs above 375 K in 1. The dielectric and thermal anomaly temperatures are close to each other, and the dielectric relaxation is relevant to the relative displacement of [K(18-Crown-6)]⁺ and the tetraphenylboron anion.