Self-Assembly through Hydrogen Bonding: Peripheral Crowding-A New Strategy for the Preparation of Stable

The self-assembly of two new types of stable hydrogen-bonded supramolecular aggregates-bisrosettes-that are based on parallel, connected CA3.M3 rosettes is reported. The minimization of intermolecular steric hindrance-peripheral crowding within the aggregate-is proposed as the structural feature responsible for selecting the cyclic CA3'M3 rosette over competing linear hydrogen-bonded networks available to the precursors. One series of aggregates-3bis(M)2:6(CA)-is composed of 3 equiv of a bulky bismelamine-bis(M)2-and 6 equiv of a bulky isocyanurate(CA). Thesecondseriesofaggregates-3bis(CA)2:6(M)-iscomposedof3equivofabisisocyanurate-bis(CA)2-and 6 equiv of a melamine (M) bearing two bulky substituents. The nine particles in these aggregates are stabilized by 36 hydrogen bonds in two connected CA3.M3 rosettes. Each aggregate has been characterized by rH NMR spectroscopy, gel permeation chromatography, and vapor pressure osmometry. Correlations between the size of the substituents and the preorganization of the precursors and the stability of the aggregates derived from them are discussed. Observations from tH NMR spectroscopy and gel permeation chromatography indicate that aggregates comprising two connected CA3'M3 rosettes are more stable than those stabilized by a single CA3.M3 rosette, for similar extents of peripheral crowding in the rosettes.