Exclusion complexes of the HCl salts of benzidine and bis(4-aminophenyl) methane with two methyl-substituted cucurbiturils

Abstract

The interaction between two partially methyl-substituted cucurbiturils, a sym-tetramethyl-substituted cucurbit[6]uril (TMeQ[6]) and a meta-hexamethyl-substituted cucurbituril (m-HMeQ[6]), with the hydrochloride salt of benzidine (g1·HCl) and the analogue bis(4-aminophenyl) methane (g2·HCl) was investigated by single crystal X-ray diffraction determination, ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy. Single crystal X-ray diffraction determination showed the two guest compounds were excluded at the portals of the partial methyl-substituted cucurbiturils in the solid state. The ¹H NMR spectroscopic analysis in aqueous solution supported the crystallographic results in which an excluding or portal interaction occurs between the host and guest. Aqueous absorption spectrophotometric and fluorescence spectroscopic analysis defined the stability of the host–guest exclusion complex at pH 5.6 with a host : guest ratio of 1 : 1, which forms quantitatively as ∼10⁵ L mol⁻¹ for the TMeQ[6]–g1 system. The host : guest ratio of 2 : 1 forms quantitatively as ∼10¹⁰ L² mol⁻² for the m-HMeQ[6]–g2 system. The experimental results are in good agreement with HF and B3LYP computational approaches with a moderate-sized basis set.