Gradual transition from NH^...pyridyl hydrogen bonding to the NH^{...O tape synthon in pyridyl ureas}

We report the synthesis and X-ray crystal structures of a series of pyridyl ureas of the type currently topical in anion binding and supramolecular gelation applications, along with their anion-binding ability in solution. The structures of the simple pyridyl ureas are dominated by urea center dot center dot center dot N(pyridyl) hydrogen bonding, because of steric congestion of the urea carbonyl by intramolecular CH center dot center dot center dot O interations. However, introducing a further competing synthon in the form of alkenic or pi-stacking interactions causes a gradual changeover, through a number of interesting intermediate hydrogen bonding motifs, to the urea alpha-tape arrangement.