Structural properties of urea inclusion compounds containing carboxylic acid anhydride guest molecules: anomalous modes of guest-molecule ordering

Abstract

The urea inclusion compounds containing carboxylic acid anhydride guest molecules CH₃(CH₂)_nCO₂CO(CH₂)_nCH₃ with n= 3–8 and 10 have been studied by single-crystal X-ray diffraction at room temperature. All of these urea inclusion compounds possess the conventional (hexagonal) urea host tunnel structure, but the mode of ordering of the guest molecules, at room temperature, differs for the different guest molecules. Specifically, for heptanoic anhydride/urea, the offset along the tunnel axis between the positions of guest molecules in adjacent tunnels is 2.3 Å, whereas for all the other guest molecules studied, this offset is equal to zero. This represents the first family of conventional urea inclusion compounds so far discovered in which there is an anomalous member in terms of the mode of intertunnel ordering of the guest molecules. It is suggested that this anomaly may be due to the heptanoic anhydride molecule assuming a different conformation in order to take up a periodicity, along the tunnel axis, which is commensurate with the periodicity of the host structure. It is clear from this result that the mode of intertunnel ordering of a given guest molecule cannot be predicted simply from a knowledge of the mode of intertunnel ordering for other guest molecules belonging to the same homologous series.