Novel type of hydrogen-bonded assemblies based on the melamine-cyanuric acid motif

This paper reports the formation of novel hydrogen-bonded assemblies 1(3).CA obtained upon mixing cyanuric acid (CA) with melamine derivatives 1, in which two of the three possible H-bonding arrays have been blocked. The four components are held together by 9 hydrogen bonds and form a rigid planar structure in which a central CA (three ADA motifs: A = acceptor, D = donor) is hydrogen bonded to three peripheral melamine derivatives (DAD motif). Furthermore, the synthesis and assembly studies are described of hydrogen-bonded assemblies 2-4.CA, comprised of three melamine derivatives that are covalently connected, and CA. The overall thermodynamic stability of assemblies 2-4.CA is superior to 1(3).CA (I-Tm = 9 vs 3.6). The presence of the 2.CA complex in chloroform was confirmed by H-1 NMR spectroscopy and MALDI-TOF mass spectrometry. Substitution of the trimelamines with chiral or fluorescent groups (R-3) enabled the study of the assemblies by CD and fluorescence spectroscopy. Titration experiments revealed strongly enhanced stabilities even in the presence of polar solvents, such as THF and CH3OH. Depending on the polarity of the solvent, stacking between the planar assembly units was observed.