Anion Coreceptor Molecules. Linear Molecular Recognition in the Selective Binding of Dicarboxylate Substrates by Ditopic Polyammonium Macrocycles

Abstract

AbstractThree macrocyclic hexaamines 1, 2, and 4, and the acyclic tetraamine 5 and hexaamine 6 have been synthesized. The hexaamines 1, 2, and 4 are ditopic coreceptor molecules containing two triamine subunits which may bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrocyclic polyamines and terminal dicarboxylates −O2C−(CH2)m‐ CO2− as well as amino‐acid and dipeptide dicarboxylates have been determined by pH‐metric measurements. Around neutral pH, 1 and 2 give mainly complexes of the fully protonated species 1·6H+ and 2·6H+, whereas 4 yields predominantly complexes of 4·5H+ and 4·4H+. The stability sequences of the complexes formed indicate preferential binding of the dianionic substrates whose length is compatible with the separation of the triammonium binding subunits in the protonated receptor molecules 1, 2, and 4. This selectivity pattern corresponds to a process of linear molecular recognition based on ditopic binding between the two ammonium subunits of the coreceptor and the terminal carboxylates of the substrate of complementary length. The complexes of the acyclic ligands 5 and 6 are much weaker and much less selective, indicating a marked macrocyclic effect on both stability and selectivity of binding, i.e. on recognition.