Steric factors influencing the complex formation with cucurbit[6]uril

The complex formation between cucurbit[6]uril and different diamides was studied using proton resonance spectroscopy and calorimetric titrations. The values of the stability constants and reaction enthalpies were determined using calorimetric titrations. In solution diamides obtained from the reaction of α,ω-alkyl diamines with aliphatic carbonic acids form inclusion complexes by threading. The stability of these complexes is independent of the number of methylene groups of the diamine compounds. With increasing chain length of the aliphatic carbonic acids the complex stability raises. The thermodynamic data for the reaction of cucurbit[6]uril with diamides containing branched alky-, cycloaliphatic- or aromatic carbonic acids give no evidence for the formation of inclusion complexes. However, 1H-NMR-experiments give clear evidence of the complex formation between cucurbit[6]uril and branched alkyl diamides. Surprisingly cycloaliphatic and aromatic groups act as blocker groups although the cavity size of cucurbit[6]uril is larger than the diameter of the terminating groups.