Self-assembly of ball-shaped molecular complexes in water

We present a simple and versatile access to spheroidal molecular 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 thermodynamic stability is best explained by the formation of a cyclic array of alternating positive and negative charges interconnected by a regular network of hydrogen bonds. Association constants reach 10(6) M(-1) in methanol and often surpass 10(3) 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 investigations, new recognition motifs for histidine and arginine esters and the unsubstituted guanidinium ion are proposed. The additional introduction of methyl 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 K(a) values of 1 order of magnitude; the related rotational barriers were estimated at 0.3-2.1 kcal/mol. Spontaneous formation of defined 2:1 complexes from three components has also been observed.