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 ...
