Versatility in stabilization of crystalline inclusion complexes of a bulky diol host by various closely related acidic and ester guests

Abstract

In order to stabilize carboxylic acid and ester molecules in a monomeric state for spectroscopic investigation, solid inclusion formation experiments have been carried out with the 2,2′-bis(hydroxydiphenylmethyl)-1,1′-biphenyl host molecule (1) and various carboxylic acid and ester compounds (27 different species). The IR spectra of the included guest molecules have been registered, and single crystals of five selected complexes (1a–e), containing formic acid (1a), acetic acid (1b), propionic acid (1c), butanoic acid (1d), and ethyl acetate (1e) as guest component, respectively, have been studied by X-ray diffraction. The IR spectroscopic investigations yielded some unexpected ν_CO absorption frequencies for two of the guest molecules, namely acetic and propionic acids, but not for the other, closely related guest acid or ester molecules in this study. The X-ray diffraction analyses showed the diol host 1 molecule to assemble into similar hexamer associates in all studied crystals, with various acid/ester guests and some water molecules being included in uniform cyclic cavities inside the hexa-host units. Hence, the five solid inclusion compounds have analogous crystal shapes, unit cell dimensions and crystallographic symmetries, and their host frameworks exhibit a high degree of isostructurality. In spite of the pronounced similarities, the X-ray structural study also indicated remarkable differences in the way the various guest molecules stabilize the respective crystal structures, thus giving a possible explanation for the observed IR absorption anomalies.