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| - Self-Assembly of Ball-Shaped Molecular Complexes in Water
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| - We present a simple and versatile access to spheroidal molecular assemblies with pronouncedstability in highly polar solvents. These complexes are composed of doubly and triply chargedcomplementary 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 alternatingpositive and negative charges interconnected by a regular network of hydrogen bonds. Associationconstants reach 106 M-1 in methanol and often surpass 103 M-1 in water. The broad range of bindingenergies correlates well with the varying degree of preorganization of both complex partners. As abyproduct of these investigations, new recognition motifs for histidine and arginine esters and theunsubstituted guanidinium ion are proposed. The additional introduction of methyl groups in the2-, 4-, and 6-positions of the central benzene ring in either cations or anions causes a marked dropin the corresponding Ka values of 1 order of magnitude; the related rotational barriers were estimatedat 0.3−2.1 kcal/mol. Spontaneous formation of defined 2:1 complexes from three components hasalso been observed.
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