Crystal structures of N-aryl-N'-4-nitrophenyl ureas:: Molecular conformation and weak interactions direct the strong hydrogen bond synthon

Hydrogen bond competition was studied in 21 X-ray crystal structures of N-X-phenyl-/N'-p-nitrophenyl urea compounds (X = H, F, Cl, Br, I, CN, C CH, CONH2, COCH3, OH, Me). These structures are classified into two families depending on the hydrogen bond pattern: urea tape structures contain the well-known alpha-network assembled via N-H center dot center dot center dot O hydrogen bonds; however, in nonurea tape structures the N-H donors hydrogen bond with NO2 groups or solvent 0 acceptor atoms. Surprisingly, the urea C=O hardly accepts strong H bonds in nonurea type structures sustained by urea center dot center dot center dot nitro and urea center dot center dot center dot solvent synthons. The carbonyl group accepts intra- and intermolecular C-H center dot center dot center dot O interactions. The molecular conformation and H bonding motifs are different in the two categories of structures: the phenyl rings are twisted out of the urea plane in the tape motif, but they are coplanar in the nonurea category. Even though hydrogen bond synthon energy and urea carbonyl acceptor strength favor the N-H center dot center dot center dot O tape structure, the dominant pattern in electron-withdrawing aryl urea crystal structures is the urea center dot center dot center dot nitro/urea center dot center dot center dot solvent synthon and persistence of intramolecular C-H center dot center dot center dot O interactions. Remarkably, the presence of functional groups that can promote specific C-I center dot center dot center dot O or C-H... 0 interactions with the interfering NO2 group, for example, when X = 1, C CH, NMe2, and Me, steers crystallization toward the N-H center dot center dot center dot O urea tape structure, and now the diaryl urea molecule adopts the metastable, twisted conformation. Molecular conformer energy calculations and difference nuclear Overhauser enhancement NMR experiments show that the planar,trans-trans-N,N'-diphenyl urea conformation is more stable than the N-Ph twisted rotamer. However, the urea C=O is a better hydrogen bond acceptor in the twisted conformer compared to the planar one, based on electrostatic surface potential (ESP) charges. These diaryl ureas together with previously reported crystal structures provide a global structural model to understand how functional groups, molecular conformation, hydrogen bonding, and crystal packing are closely related and influence each other in subtle yet definitive ways. Our strategy simultaneously exploits weak, soft intermolecular interactions and strong, hard hydrogen bonds [supramolecular hard and soft acid-base (HSAB) principle] in the crystal engineering of multifunctional molecules.