SELF-ASSEMBLY THROUGH HYDROGEN-BONDING - PERIPHERAL CROWDING - A NEW STRATEGY FOR THE PREPARATION OF STABLE SUPRAMOLECULAR AGGREGATES BASEDON PARALLEL, CONNECTED CA(3)CENTER-DOT-M(3) ROSETTES

The self-assembly of two new types of stable hydrogen-bonded supramole cular aggregates-bisrosettes-that are based on parallel, connected CA( 3).M(3) rosettes is reported. The minimization of intermolecular steri c hindrance-peripheral crowding within the aggregate-is proposed as th e structural feature responsible for selecting the cyclic CA(3).M(3) r osette 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 isocya nurate (CA). The second series of aggregates-3bis(CA)2:6 (M)-is compos ed of 3 equiv of a bisisocyanurate-bis(CA)(2)-and 6 equiv of a melamin e (M) bearing two bulky substituents. The nine particles in these aggr egates are stabilized by 36 hydrogen bonds in two connected CA(3).M(3) rosettes. Each aggregate has been characterized by H-1 NMR spectrosco py, gel permeation chromatography, and vapor pressure osmometry. Corre lations between the size of the substituents and the preorganization o f the precursors and the stability of the aggregates derived from them are discussed. Observations from H-1 NMR spectroscopy and gel permeat ion chromatography indicate that aggregates comprising two connected C A(3).M(3) rosettes are more stable than those stabilized by a single C A(3).M(3) rosette, for similar extents of peripheral crowding in the r osettes.