N–H…N(pyridyl) and N–H…O(urea) hydrogen bonding and molecular conformation of N-aryl-N′-pyridylureas

Analysis of N-Ar-N′-4-pyridylureas (Ar = 4-X-phenyl, 4-py) crystal structures shows a gradual transition from urea…pyridyl N–H…N hydrogen bond in bifurcated synthon of R21 (6) graph set (X = H, Me, Cl) to single N–H…O and N–H…N hydrogen bonds in X = Br, I to urea N–H…O tape synthon (R21 (6) graph set) in N-phenyl-N′-tetrafluoropyridylurea. Thus the two extremes of urea pyridyl N–H…N synthon in N,N′-di(4-pyridyl) urea, N–H…O tape characteristic of diphenyl urea together with an in-between state are now visualized as snap shots in the solid-state. Most remarkably, molecular conformations too show a transition: the diaryl urea molecule is in the twisted metastable conformation in N–H…O tape structures, it is coplanar in the flat stable conformation for N–H…N structures, and one NH is in-plane while the other is oriented slightly outwards in the intermediate N–H…O and N–H…N hydrogen-bonded structure of N-4-Br-phenyl-N′-4-pyridylurea. Subtle and yet observable one-to-one molecular conformation to crystal structure systematic effects are analyzed. A one-dimensional zigzag chain in the crystal structure of N,N′-di(4-pyridyl) urea is consistent with the hydrogen bonding model for organic gelators.