Hydrogen bonding in crystal structures of N,N '-bis(3-pyridyl)urea. Why is the N-H center dot center dot center dot O tape synthon absent in diaryl ureas with electron-withdrawing groups?

The urea tape a-network of bifurcated N-H center dot center dot center dot O hydrogen bonds is a common motif in diaryl ureas and their molecular complexes. 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)H2O and 3 center dot 2H(2)O, cocrystals 3 center dot SA and 3 center dot FA center dot H2O 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 center dot center dot center dot N-pyridyl, N-H center dot center dot center dot O-water, N-H-O-acid, and N-H center dot center dot center dot I- hydrogen bonds instead of the common one-dimensional N-H center dot center dot center dot O-urea tape. We postulated that the urea tape is absent in these structures because the C=O acceptor is weakened by two intramolecular C-H center dot center dot center dot O-urea interactions (synthon III) in a planar molecular conformation. Electrostatic surface potential (ESP) charges (DFT-B3LYP/6-31G*) showed that the C-H center dot center dot center dot O interactions sufficiently reduce the electron density at the urea O, and so other electronegative atoms, such as pyridyl N, H2O, COOH, and I-, become viable hydrogen-bond acceptors for the strong NH donors. H-1 NMR difference nOe confirmed that the planar conformation of dipyridyl urea 3 in the solid-state persists in solution. Interestingly, even though the strong hydrogen-bond motifs changed in structures of 3, the C-H center dot center dot center dot O interactions of synthon III (energy 4.6-5.0 kcal/mol) occurred throughout the family. In addition to dipyridyl urea, other electron-withdrawing diaryl ureas, e.g., those with phenylpyridyl and phenyl-nitrophenyl groups, also deviated from the prototype N-H center dot center dot center dot O tape because of the interference from weak C-H center dot center dot center dot O hydrogen bonds. Therefore, when one or both aryl rings have hydrogen-bond acceptor groups (e.g., pyridine, PhNO2) the NH donor(s) preferentially bond to pyridyl N. nitro O, or solvent O atom instead of the urea C=O acceptor. We classify supramolecular organization in diaryl ureas into those with the alpha-network (twisted molecular conformation) or non-urea tape structures (stable, planar conformation) depending on the substiment 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 center dot center dot center dot solvent hydrogen bonding (electron-withdrawing groups) by appropriate selection of functional aryl and heterocyclic groups.