Self-Assembly of Ball-Shaped Molecular Complexes in Water.
The authors present a simple and versatile access to spheroidal mol. assemblies with pronounced stability in highly polar solvents. These complexes are composed of doubly and triply charged complementary building blocks based on ammonium or amidinium cations and phosphonate anions. Their high thermodn. stability is best explained by the formation of a cyclic array of alternating pos. and neg. charges interconnected by a regular network of H bonds. Assocn. consts. reach 106 M-1 in methanol and often surpass 103 M-1 in water. The broad range of binding energies correlates well with the varying degree of preorganization of both complex partners. As a byproduct of these studies, new recognition motifs for histidine and arginine esters and the unsubstituted guanidinium ion are proposed. The addnl. introduction of Me groups in the 2-, 4-, and 6-positions of the central benzene ring in either cations or anions causes a marked drop in the corresponding Ka values of 1 order of magnitude; the related rotational barriers were estd. at 0.3-2.1 kcal/mol. Spontaneous formation of defined 2:1 complexes from three components also was obsd.
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