Ortho-Substituent Effects on Diphenylurea Packing Motifs

Abstract

Hydrogen bonding between urea groups is a widely used motif in crystal engineering and supramolecular chemistry studies. In an effort to discern how the steric and electronic properties of substituents affect the molecular conformation and crystal packing of ortho-substituted N,N′-diphenylureas (oPUs), herein we report the synthesis, characterization, and polymorph screening of eight members of this family. Of the 16 total oPU structures known (including nine structures from this study and seven previously reported), only two are isostructural. These 16 structures are sorted into three general architecture types based on their hydrogen bond topologies. In Type I, urea molecules related by translation form linear one-dimensional (1D) hydrogen bonded chains. In Type II, urea molecules rotate about a 1D hydrogen bond axis forming twisted chains. Urea groups do not hydrogen bond to one another in Type III. Energy calculations performed at the B3LYP/6-31G(d,p) level show a higher rotational barrier about the amide bond in oPUs compared to meta-substituted diphenylureas (mPUs), which may explain the smaller range of torsion angles observed in oPUs compared to mPUs. Although ortho-substitution does not seem to limit the hydrogen bonding between urea groups in most cases, a notably higher percentage of oPU phases are polar compared to PUs with other substitution patterns. This suggests restricted conformations might offer some advantage in achieving acentric materials.