Solid-state NMR as a probe of anion binding: molecular dynamics and associations in a [5]polynorbornane bisurea host complexed with terephthalate

Abstract

A range of solid-state NMR techniques is used to characterise a molecular host:guest complex consisting of a [5]polynorbornane bisurea host binding a terephthalate dianion guest. Detailed information is obtained on the molecular dynamics and associations from the point of view of both the host and guest molecules. The formation of the complex in the solid state is confirmed using ¹H 2D exchange NMR, and the 180° flipping of the ²H-labelled terephthalate guest and its eventual expulsion from the complex at elevated temperatures are quantified using variable-temperature ²H spin-echo experiments. Two-dimensional ¹H–¹³C HETCOR spectra obtained under fast magic angle spinning conditions (60 kHz) show a high resolution despite the poor crystallinity of the solid complex, and clearly reveal changes in the rigidity of the host molecule when complexed. Short-range intra- and intermolecular ¹H–¹H proximities are also detected using 2D SQ–DQ correlation methods, providing insight into the molecular packing in the solid phase.