By the introduction of methoxycarbonyl-methoxy groups and hydroxyl groups into a pillar structure, a pillararene entirely with two types of functional groups was successfully prepared, which can form a stable 1:1 complex with a bis(imidazolium) salt in CHCl₃/acetone solution (V:V=1:1).

It is well known that pillar[5]arenes have two most stable conformations (pS and pR) in their crystal structures. Because of the intramolecular H-bonding interactions, substituents, temperature, solvent and so on, the rotational behaviors of the phenolic units on pillararenes are also common. This paper showed some other kinds of conformations in the functionalized pillar[5]arenes and gave evidence for a bulky unit (1,4-methoxycarbonylmethoxybenzene unit) flipping in the solid state. The presence of hydrogen bonding facilitated the intermolecular self-assembly in terms of energy-minimized packing in the crystals. Thus, the main driving force for the flipping of this bulky unit might be both the intramolecular hydrogen bonding between the phenolic units on pillararenes and quadrupolar hydrogen bonding between the host and water. This paper helps us to have a better understanding on the conformations of pillar[5]arenes.

The isomeric tetrahydroxylated pillar[5]arenes were synthesized through partial oxidation of 1,4-dimethoxypillar[5]arene or 1,4-dimethoxypillar[4]arene[1]quinone by (NH₄)₂[Ce(NO₃)₆] and then reduction by Na₂S₂O₄. The intermediate 1,4-dimethoxypillar[3]arene[2]quinones have two constitutional isomers. The yield of the 1,3-isomer 1 can reach 42 %, and was far higher than that of the 1,2-isomer 2 (formed in trace amounts). The approach provides an efficient way to prepare constitutional-isomeric selectively functionalized pillar[5]arenes 3 and 4. Furthermore, the host–guest complexation of 3 and 4 with a bis(imidazolium) salt was investigated.