| Abstract
| - Recognition of dicarboxylates by bis-functional hydrogen-bonding receptors displays divergentthermodynamics in different solvent systems. NMR titration and isothermal titration calorimetryindicated that neutral bis-urea and bis-thiourea receptors form exothermic complexes withdicarboxylates in DMSO, with a near zero entropic contribution to binding. The increased bindingstrength of bis-guanidinium receptors precluded quantitative measurement of binding constantsin DMSO, but titration calorimetry offered a qualitative picture of the association. Formation ofthese 1:1 complexes was also exothermic, but additional endothermic events occurred at both lowerand higher host−guest ratios. These events indicated multiple binding equilibria but did not alwaysoccur at a discrete 2:1 or 1:2 host−guest molar ratio, suggesting higher aggregates. With increasingamounts of methanol as solvent, bis-guanidinium receptors form more endothermic complexes withdicarboxylates, with a favorable entropy of association. This switch from association driven byenthalpy to one driven by entropy may reflect a change from complexation involving the formationof hydrogen bonds to that promoted by solvent liberation from binding sites.
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