Anion Recognition in Water, Including Sulfate, by a Bicyclam Bimetallic Receptor: A Process Governed by the Enthalpy/Entropy Compensatory Relationship. Issue 21 (14th March 2018)

• Record Type: Journal Article
• Authors: Boiocchi, Massimo; Bonizzoni, Marco; Ciarrocchi, Carlo; Fabbrizzi, Luigi; Invernici, Michele; Licchelli, Maurizio
• Is Part Of: Chemistry. Volume 24:Issue 21(2018)
• Page Start: 5659
• Page End: 5666
• Publication Date: 2018-03-14
• Subjects: anions -- host–guest systems -- macrocycles -- supramolecular chemistry -- thermodynamics; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201800067
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: The dimetallic system [Cu II 2 (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 log K 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 Δ G ° values resulted from the unbalanced contribution of the Δ H ° and T Δ S ° terms: the more favourable the one, the less favourable the other. In particular, a linear dependence of Δ H ° and T Δ S ° 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 Δ S °. 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 Δ H ° and positive T Δ S ° (+/+ signature), and 2) those displaying the opposite behaviour: (−/− signature). The receptor investigated herein connects the two domains, along the Δ H °/ T Δ S ° straight line, thanks to the hidden role of the versatile metal–anion interaction. Abstract : Selectivity outside the molecule : The dicopper(II) bicyclam receptor [Cu II 2 (L)] 4+ includes ambidentate anions (halides, sulfate) in water with poor selectivity (see scheme), which is derived from conflicting enthalpy and entropy contributions: the more favourable the one contribution, the less favourable the other. This indicates the major role of solvational terms (mainly anion dehydration), which mitigate the contribution of the metal–anion interaction.