Inclusive coordination of F-, Cl- and Br- anions into macrobicyclic polyammonium receptors

The interaction of F-, Cl- and Br- with protonated forms of the cage-like macrobicyclic ligands L1 and L2, containing, respectively, N-7 and N6O sets of donor atoms. was Studied by means of potentiometric titrations in aqueous solution, achieving the determination of the stability constants of the complex species formed. While L1 forms halogenide complexes [L1H(n)X]((n-1)+) in various protonation states (n = 2-5 for F- and Cl-, n = 1-5 for Br-), only [L2H(4)X](3+) (X = F-, Cl-, Br-) complexes are formed by L2. For a given anion, the complex stability increases with the ligand charge, in agreement with a fundamental electrostatic character of the anion-receptor interaction. On the contrary, contrasting stability trends are observed when the stability constants of anion complexes with the ligands in a given protonation state are considered. Only in the case of [L1H(5)X](4+) complexes the stability follows the trend F- > Cl- > Br-, while for less protonated L1 species the trend is Cl- > Br- > F-. In the case of [L2H(4)X](3+) complexes the stability trend is F- > Br- > Cl-. Despite the fact that [L2H(4)](4+) forms a largely more stable F- complex than [L1H(4)](4+), L1 shows clear selectivity in halogenide binding with respect to L2 over all the pH range. L2 displays a marked selectivity in the binding of F- over Cl- and Br- while in the case of L1 inversion of binding selectivity is found upon pH variations, the F- complexes being favoured in very acidic media. Molecular modelling calculations performed in the gas phase and/or by adopting minimal solvation shells, furnished useful information for the interpretation of these stability trends. Anion size and anion desolvation upon coordination are the main factors determining the stability of halogenide complexes with L1 and L2.