Symmetry constraints, molecular recognition and crystal engineering. Comparative structural studies of urea–butanedioic and urea–E-butanedioic acid (2:1) cocrystals

Abstract
The crystal structures of two urea–dicarboxylic acid (2:1) cocrystals have been determined. Urea–butanedioic acid forms monoclinic crystals, space group P21/c (No. 14), with a = 5.637 (4), b = 8.243 (3), c = 12.258 (3) Å, β = 96.80 (5)°, V = 565.6 (8) Å3, Z = 2. Urea–E-butenedioic acid also forms monoclinic crystals, space group P21/c (No. 14), with a = 5.540 (1), b = 8.227 (1), c = 12.426 (3) Å, β = 97.22 (3)°, V = 561.9 (2) Å3, Z = 2. The geometry and the conformation of both molecular aggregates and the three-dimensional networks formed are very similar. The two strongest hydrogen-bond interactions are constrained in the formation of the heteroaggregates, the third hydrogen-bond interaction is used to self-associate the heteroaggregates in one-dimensional chains, whereas the next three weaker hydrogen bonds interconnect the chains into well organized three-dimensional networks.

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