per-Hydroxylated Pillar[6]arene: Synthesis, X-ray Crystal Structure, and Host–Guest Complexation

A per-hydroxylated pillar[6]arene was prepared. Single-crystal X-ray analysis demonstrated that its molecules are arranged in an up-to-down manner to form infinite channels in the solid state. Its host–guest complexation with a series of bispyridinium salts in solution was further investigated. It was found that the per-hydroxylated pillar[6]arene could form a 1:1 complex with paraquat in acetone with an association constant of 2.2 × 102 M–1. This complex is a [2]pseudorotaxane as shown by its crystal structure, which is the first pillar[6]arene-based host–guest complex crystal structure.