NEW VARIETIES OF CRYSTALLINE ARCHITECTURE PRODUCED BY SMALL CHANGES IN MOLECULAR-STRUCTURE IN TAPE-COMPLEXES OF MELAMINES AND BARBITURATES

This paper describes the crystal structures of a series of seven 1:1 complexes between N,N'-bis(m-X-phenyl)-melamine and 5,5-diethylbarbituric acid (X = H, F, Cl, Br, I, CH3, and CF3). This series provides small perturbations on the structures of molecules (N,N'-bis(p-X-phenyl)melamines) used ina previous study (Zerkowski, J. A.; MacDonald, J.C.; Seto, C.T.; Wierda, D.A.; Whitesides, G.M. J. Am. Chem. Soc. In press). Both the meta and para complexes crystallize as hydrogen-bonded ''tapes''. With the phenyl substituent in the meta position, however, the melamines can adopt a greater number of molecular conformations; this feature leads to a greater variety of orientations of packing. The meta series packs in both linear and crinkled tape motifs, and four of the seven complexes are solvates. By contrast, the para series, which used the same set of phenyl substituents, always yielded linear tapes and crystals without solvent in the lattice. Inclusion of solvent increases the packing coefficients of members of the meta series to values approximately equal to those of the para series. The multiplicity of molecular conformations available to the meta series is probably largely responsible for the clathration of solvent, which does not rely upon specific non-covalent interactions except in the case of the m-iodo complex with acetonitrile. The wider range of crystalline architecture in the meta series attests only to the kinetic accessibility of these packing formats; polymorphic phases of greater thermodynamic stability may occur. Polymorphism has not been investigated in the series of meta-substituted compounds.