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

Abstract

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, COCH_3, OH, Me). These structures are classified into two families depending on the hydrogen bond pattern: urea tape structures contain the well-known a-network assembled via N-H···O hydrogen bonds; however, in nonurea tape structures the N-H donors hydrogen bond with NO₂ groups or solvent O acceptor atoms. Surprisingly, the urea C=O hardly accepts strong H bonds in nonurea type structures sustained by urea···nitro and urea···solvent synthons. The carbonyl group accepts intra- and intermolecular C-H···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···O tape structure, the dominant pattern in electron-withdrawing aryl urea crystal structures is the urea···nitro/urea···solvent synthon and persistence of intramolecular C-H···O interactions. Remarkably, the presence of functional groups that can promote specific C-I···O or C-H···O interactions with the interfering NO₂ group, for example, when X = I, C=CH, NMe₂, and Me, steers crystallization toward the N-H···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.