STRUCTURAL PREFERENCES OF HYDROGEN-BONDED NETWORKS IN ORGANIC SOLUTION - THE CYCLIC CA(3)CENTER-DOT-M(3) ROSETTE

Crystals grown from a solution containing equimolar portions of barbit al (3) and N,N'-bis(4-tert-butylphenyl)-melamine (4) in toluene/isopro pyl alcohol (1:1 v/v) comprise a cyclic CA(3).M(3) ''rosette'' (5). Th e six molecules in this supramolecular motif are held together by 18 h ydrogen bonds. Characterization of solutions of equimolar mixtures of 3 and 4 in chloroform by H-1 NMR spectroscopy, gel permeation chromato graphy, and vapor pressure osmometry demonstrates that the same cyclic CA(3).M(3) rosette (5) is also the most probable structure, when [3] = [4] > 4 mM. H-1 NMR exchange experiments confirm that the CA(3).M(3) rosette (5) is qualitatively much less stable in chloroform solution than the supramolecular aggregate hub(M)(3):3barbital (1) that is preo rganized for self-assembly by a covalent tris(melamine) derivative. Co mplexes formed between 4 and different isocyanurates indicate that int ermolecular interactions, as a consequence of the steric bulk of the s ubstituents on these derivatives, favor the formation of the cyclic CA (3).M(3) rosette over competing linear hydrogen-bonded motifs. There i s inferential evidence for formation of a complex of modest stability with composition CA.M(2) when 2[3] = [4].