Effect of an Intramolecular Hydrogen Bond Belt and Complexation with the Guest on the Rotation Behavior of Phenolic Units in Pillar[5]arenes

Abstract

The rotation behavior of the phenolic units in pillar[5]arenes has been studied by means of the dynamic 1H NMR measurements. In acetone-d6, the individual conformers of pillar[5]arene were observed under −60 °C, indicating that the rotation movement was slow on the NMR time scale under −60 °C. In permethylated pillar[5]arene in acetone-d6, the peaks were not split even at −90 °C, strongly suggesting that the rotation movement was fast on the NMR time scale at −90 °C. This is due to the lack of the intramolecular hydrogen bond belt in permethylated pillar[5]arene. In the mixture of pillar[5]arene and viologen guest in acetone-d6, the split peaks were found at −30 °C and did not change under −60 °C. By formation of the host−guest complex between pillar[5]arene and viologen guest, the rotation movement was slow on the NMR time scale under −30 °C and almost stopped under −60 °C.