Binding and Removal of Sulfate, Phosphate, Arsenate, Tetrachloromercurate, and Chromate in Aqueous Solution by Means of an Activated Carbon Functionalized with a Pyrimidine-Based Anion Receptor (HL). Crystal Structures of [H3L(HgCl4)]•H2O and [H3L(HgBr4)]•H2O Showing Anion-π Interactions

Binding of anions of great environmental concern such as SO42-, PO43-, AsO43-, HgCl42-, and CrO42- by the protonated forms of a tren-like (tren = tris(2-aminoethyl)amine) ligand (HL) functionalized with a pyrimidine residue was studied by means of potentiometric measurements and isothermal titration calorimetry (ITC) affording log K, Delta H-o, and T Delta S-o values for the formation of the relevant complexes. The complexes show high to very high stability due to the particular topology and electronic properties of the ligand which is able to use two separated coordination environments to host the anions, the protonated tren site where electrostatic and hydrogen bond interactions are operating, and the pyrimidine ring which may act via anion-pi interaction. A contribution of -8.9 +/- 0.4 kJ/mol for pyrimidine-anion interaction in water was derived for SO42- binding. The crystal structures of [H3L(HgCl4)]center dot H2O (1), [H3L(HgBr4)]center dot H2O (2), and that previously reported for [H3L(CdI4)], clearly show these binding features in the solid state. A hybrid AC-HL material obtained by adsorption of HL on commercial activated carbon (AC) was used to study the removal of these anions from water. AC-HL shows enhanced adsorption capacity toward all the anions studied with respect to AC. This behavior is ascribed to the stronger interaction of anions with the HL function of AC-HL than with the C pi-H3O+ sites of the unfunctionalized AC.