The urea tape
![](/images/gifchars/alpha.gif)
-network of bifurcated N-H···O hydrogen bonds is a common motif in diaryl ureas and their molecularcomplexes. We analyzed the X-ray crystal structures of
N,
N'-bis(3-pyridyl)urea
3 and some of its derivatives: hydrates of stoichiometry
3·(4/3)H
2O and
3·2H
2O, cocrystals
3·SA and
3·FA·H
2O with succinic acid and fumaric acid, bis pyridine
N-oxide
8, and bis
N-methylpyridinium iodide
9. Crystal packing in pyridyl urea structures is directed by N-H···N
pyridyl, N-H···O
water, N-H···O
acid,and N-H···I
- hydrogen bonds instead of the common one-dimensional N-H···O
urea tape. We postulated that the urea tape isabsent in these structures because the C=O acceptor is weakened by two intramolecular C-H···O
urea interactions (synthon
III) ina planar molecular conformation. Electrostatic surface potential (ESP) charges (DFT-B3LYP/6-31G*) showed that the C-H···Ointeractions sufficiently reduce the electron density at the urea O, and so other electronegative atoms, such as pyridyl N, H
2O,COOH, and I
-, become viable hydrogen-bond acceptors for the strong NH donors.
1H NMR difference nOe confirmed that theplanar conformation of dipyridyl urea
3 in the solid-state persists in solution. Interestingly, even though the strong hydrogen-bondmotifs changed in structures of
3, the C-H···O interactions of synthon
III (energy 4.6-5.0 kcal/mol) occurred throughout thefamily. In addition to dipyridyl urea, other electron-withdrawing diaryl ureas, e.g., those with phenylpyridyl and phenyl-nitrophenylgroups, also deviated from the prototype N-H···O tape because of the interference from weak C-H···O hydrogen bonds. Therefore,when one or both aryl rings have hydrogen-bond acceptor groups (e.g., pyridine, PhNO
2), the NH donor(s) preferentially bond topyridyl N, nitro O, or solvent O atom instead of the urea C=O acceptor. We classify supramolecular organization in diaryl ureasinto those with the
![](/images/gifchars/alpha.gif)
-network (twisted molecular conformation) or non-urea tape structures (stable, planar conformation) dependingon the substituent group. Our results suggest a model to steer urea crystal structures toward the tape synthon (Ph and electron-donating groups) or with non-urea hydrogen-bond motifs and a high probability for urea···solvent hydrogen bonding (electron-withdrawing groups) by appropriate selection of functional aryl and heterocyclic groups.