Molecular Dynamics Studies of Aromatic Guests in Three Isostructural Inclusion Compounds with Molecular Boron–Nitrogen Hosts

Abstract

The dynamics of three guests: benzene (ben), naphthalene (nap), and xylene (xyl) included in the isostructural cavities of tetrameric [(2fbe)2(bpy)⊃guest]4 cages were examined using single-crystal X-ray diffraction, thermogravimetric analysis, variable-temperature solid-state 2H NMR spectroscopy, and molecular dynamics simulations. The inclusion compounds were selected from a series of boron–nitrogen host adducts assembled in a 2:1 stoichiometric ratio from the catechol ester of 2,4-difluorophenylboronic acid (2fbe) and 4,4′-bipyridine (bpy) in the presence of aromatic guests. The dynamic characterization studies revealed that ben is highly mobile with fast in-plane jumps around the C6-axis above T = 150 K, with a calculated activation energy, Ea, of 4.9 kJ mol–1. Surprisingly, the larger guest nap experiences 180° jumps above 296 K. On the contrary, xyl guests can only experience 120° in-plane jumps at temperatures close to the decomposition point because the methyl groups act as effective stoppers.