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···N(pyridyl) hydrogen bonding, because of steric congestion of the urea carbonyl by intramolecular CH···O interations. However, introducing a further competing synthon in the form of alkenic or π-stacking interactions causes a gradual changeover, through a number of interesting intermediate hydrogen bonding motifs, to the urea α-tape arrangement.