Pyridine-incorporated cyclo[6]aramide for recognition of urea and its derivatives with two different binding modes.

A novel pyridine-incorporated cyclo[6]aramide is designed and synthesised for recognition of urea and its derivatives. Analysis of its single crystal structure reveals the presence of introverted amide NH protons and amide carbonyl groups that are supposed to contribute to the subsequent accommodation of neutral urea-related guest molecules via multiple hydrogen bonding interactions. Thiourea is found to be superior to urea in binding to the receptor. Particularly interesting is the observation of two binding modes in complexing urea/thiourea (contact mode) and ethylurea/diethylurea (threading mode) as supported by both NMR experiments and computational simulations. The finding of the threading mode may open up new opportunities for the development of pesudorotaxanes and related mechanically interlocked structures. [ABSTRACT FROM AUTHOR]