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 [MathML equation] (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 ([MathML equation] (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.