Anion Recognition in Water, Including Sulfate, by a Bicyclam Bimetallic Receptor: A Process Governed by the Enthalpy/Entropy Compensatory Relationship

The dimetallic system [Cu-2(II)(L)](4+) contains two facing equivalent metallocyclam subunits and incorporates ambidentate anions, mono- (halides) and poly-atomic (sulfate), which bridge the two Cu-II centres. Isothermal titration calorimetry (ITC) experiments in water showed that the logK values of the inclusion equilibria for halides and sulfate varied over a restricted interval (3.6 +/- 0.2), which indicated lack of selectivity and that similarity of Delta G degrees values resulted from the unbalanced contribution of the Delta H degrees and T Delta S degrees terms: the more favourable the one, the less favourable the other. In particular, a linear dependence of Delta H degrees and T Delta S degrees was observed (a typical enthalpy/entropy compensatory diagram), which assigned a major role to hydration terms: 1) a more hydrated anion resulted in a more endothermic dehydration process; and 2) a larger number of water molecules released to the solution resulted in a more positive T Delta S degrees. Limiting cases refer to the complexation 1) of the poorly hydrated iodide (highly exothermic process, entropically disfavoured), and 2) of the highly hydrated sulfate (moderately endothermic process, entropically very favoured). Anion receptors operating in water belong to two main domains: 1)those exhibiting positive Delta H degrees and positive T Delta S degrees (+/+ signature), and 2)those displaying the opposite behaviour: (-/- signature). The receptor investigated herein connects the two domains, along the Delta H degrees/T Delta S degrees straight line, thanks to the hidden role of the versatile metal-anion interaction.