Packing principles for urea and thiourea solvates: structures of urea : morpholine (1 : 1), urea : 1,4-dioxane (1 : 1), thiourea : morpholine (4 : 3) and thiourea : 1,4-dioxane (4 : 1)

Abstract

The solvents 1,4-dioxane and morpholine have been employed to synthesize solvates of urea and thiourea. The structures confirm the tendency of urea to form more rigid systems of hydrogen bonds in the plane of the N₂CO moiety, thus forming layer structures with close complementarity of the donors and acceptors, whereas the more flexible sulfur acceptor of thiourea can also accept hydrogen bonds from donors that lie far from the N₂CS plane, forming three-dimensional packing patterns with much more variable parameters. A database investigation confirms these tendencies. The solvate urea : morpholine (1 : 1) crystallizes in Pbcm with Z = 4. The complete urea molecule lies in the mirror plane, as do the heteroatoms of the morpholine molecule. The molecular packing is a layer structure. The solvate urea : 1,4-dioxane (1 : 1) crystallizes in P2/c with Z = 2. The CO bond of the urea molecule lies along a twofold axis, whereas the dioxane molecule lies across an inversion centre. The molecules form a layer structure analogous to that of the morpholine solvate. The thiourea solvates are more complex, and both involve a more irregular hydrogen bonding geometry at sulfur. The solvate thiourea : morpholine (4 : 3) crystallizes in P2₁/c with Z = 2. The asymmetric unit contains two independent molecules of thiourea, one morpholine on a general position, and one morpholine disordered over an inversion centre. The thiourea molecules combine to form an open framework with a series of channels, in which the morpholine molecules are attached. The solvate thiourea : 1,4-dioxane (4 : 1) crystallizes in P2₁/n with Z = 2. The asymmetric unit contains two independent molecules of thiourea and one molecule of dioxane across an inversion centre. One thiourea molecule and the dioxane combine to form a layer structure. The second thiourea molecule links these layers in the third dimension.