Multiple conformations of benzil in resoreinarene-based supramolecular host matrixes

Six supramolecular complexes incorporating benzil as a guest, CMCR(.)bipy(.)benzil (alpha) 1 (CMCR = C-methylcalix[4]resorcinarene), CMCR(.)bipy(.)benzil (beta) 2, CMCR(.)2bpe(.)benzil(.)ethanol 3 (bpe = trans-1,4-bis(pyridyl)ethylene), CMCR(.)2bpe(.)benzil(.)2H(2)O 4, CMCR(.)2bpeh(.)benzil(.)ethanol 5 (bpeh = bis-(1-pyridin-4-yl-ethylidene)-hydrazine), and CECR(.)2bpe(.)benzil 6 (CECR = C-ethylcalix[4]resorcinarene), have been synthesized by hydrothermal and conventional methods and characterized by X-ray diffraction. Resorcinarene adopts a boat conformation in 1-4 and a bowl conformation in 5 and 6. Compounds 1-4 show a brick-wall-like framework, in which two benzil molecules are incorporated. For 5, bpeh spacers link CMCR molecules to give a one-dimensional wavelike polymer in which one benzil guest is embedded within the polymer cavity. Complex 6 forms a carcerand-like capsule in which two benzil guests are encapsulated. The O=C-C=O torsion angles vary from 91.8 to 139.3degrees and correlate with the length of the central C-C bond. The benzil concentration, which is similar to6.2 mol/L in the neat crystals, varies between 1.01 and 1.51 mol/L in the structures studied, corresponding to a 6-fold dilution. The benzil molecules are disordered in the larger cavities of 4 and 5. The two benzoyl fragments are almost perpendicular in 3, which has the next largest cavity size when solvent volume is excluded, whereas a nearly trans-coplanar conformation occurs for the cavity with the smallest volume in 6.