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.