Proximity effects and aggregation of Hamilton-receptor barbiturate host–guest complexes probed by cross-metathesis and ESI MS analysis

Abstract

The molecular environment around supramolecular bonding systems significantly affects their stability and the assembly of host–guest complexes, most prominent for hydrogen bonds (H-bonds). Hamilton receptor-barbiturate host–guest complexes are well-known in solution, typically forming a 1 : 1 molar ratio complex. However, within a polymer matrix, these complexes can form higher-order assemblies, deviating from the standard 1 : 1 complex, which are challenging to characterize and often require lab-intensive methods. In this study, a novel Hamilton receptor (H) was equipped with cyclopentene moieties and used as a host to form host-guest complexes (H-B) with allobarbital (B), followed by covalent crosslinking. UV-Vis spectroscopy titration experiments in different solvents and at various temperatures revealed that polar solvents containing additional H-bonding sites significantly reduce the formation of the 1 : 1 H-B complex, as indicated by a reduced association constant. Higher-order aggregates (HH-dimer, HHH-trimer) were subsequently detected via an alkene cross-metathesis (CM) reaction to fix the assemblies covalently, followed by analysis via electrospray ionization mass spectrometry (ESI MS). This two-step method, firstly via CM fixation followed by ESI MS, was extended to study the H-B model complex within a polyisobutylene (PIB) matrix, presenting a direct method to analyze the complex host-guest assembly in solvent-free (polymer) environments.