Thermodynamics of sulfate anion binding by macrocyclic polyammonium receptors

The interaction of SO(4)(2-) with polyammonium cations derived from fourteen polyamines (5 polyazacycloalkanes, 2 polyazacyclophanes, 3 phenanthrolinacyclophanes, 2 dibenzenacyclophanes and 2 acyclic polyamines) in aqueous solution has been studied by means of potentiometric and microcalorimetric techniques. Only 1:1 receptor-anion complexes have been found in solution. Complexed species of considerable stability are formed, although the two acyclic polyamines (dimethylpentaethylenehexaamine and dimethylhexaethyleneheptaamine) and the smallest phenanthrolinacyclophane do not interact with the anion. The complexation reactions are endothermic, or almost athermic, and promoted by invariably favourable entropic contributions, indicating that these pairing processes are mostly determined by the desolvation of the interacting species that occurs upon charge neutralisation. The results are compared with those previously obtained for phosphate binding.