α-CO provides access to the cavity in acyl calix[4]arenes: A comparative study of crystal structures

Abstract

The structures of partially self-included forms of para-alkanoyl calix[4]arenes, with different lengths of acyl chain (C4–C16), were compared using X-ray diffraction data. The carbonyl group in the calix[4]arenes fixes the position of the two first carbon atoms in the plane of the phenyl ring and effectively expands the cavity. The accessibility of the cavity to small molecules strongly depends on the length of alkanoyl arm. For short arms (C4–C8), the host–host van der Waals interactions are not strong enough and enable easy access by potential guests to the cavity. Long alkanoyl arms compensate the directing effect of the carbonyl group and close the cavity to potential guests. A comparison of para-alkanoyl calix[4]arene with para-alkyl examples allows one to estimate the chain lengths that will enable easy inclusion of potential guests. It was found that the former should be ca. 1.5 times longer than the length of the rigid calixarene fragment.