Evaluation of the binding ability of a novel dioxatetraazamacrocyclic receptor that contains two phenanthroline units: selective uptake of carboxylate anions.

The novel dioxatetraaza macrocycle [26]phen2N4O2, which incorporates two phenanthroline units, has been synthesized, and its acid-base behavior has been evaluated by potentiometric and 1H NMR methods. Six protonation constants were determined, and the protonation sequence was established by NMR. The location of the fifth proton on the phen nitrogen was confirmed by X-ray determinations of the crystal structures of the receptor as bromide and chloride salts. The two compounds have the general molecular formula {(H5[26]phen2N4O2)Xn(H2O)(5-n)}X(n-1) x mH2O, where X = Cl, n = 3, and m = 6 or X = Br, n = 4, and m = 5.5. In the solid state, the (H5[26]phen2N4O2)(5+) cation adopts a "horseshoe" topology with sufficient room to encapsulate three or four halogen anions through the several N-H...X hydrogen-bonding interactions. Two supermolecules {(H5[26]phen2N4O2)Xn(H2O)5-n}(5-n)(+) form an interpenetrating dimeric species, which was also found by ESI mass spectrum. Binding studies of the protonated macrocycle with aliphatic (ox(2-), mal(2-), suc(2-), cit(3-), cta(3-)) and aromatic (bzc(-), naphc(-), anthc(-), pyrc(-), ph(2-), iph(2-), tph(2-), btc(3-)) anions were determined in water by potentiometric methods. These studies were complemented by 1H NMR titrations in D2O of the receptor with selected anions. The Hi[26]phen2N4O2(i+) receptor can selectively uptake highly charged or extended aromatic carboxylate anions, such as btc(3-) and pyrc(-), in the pH ranges of 4.0-8.5 and <4.0, respectively, from aqueous solution that contain the remaining anions as pollutants or contaminants. To obtain further insight into these structural and experimental findings, molecular dynamics (MD) simulations were carried out in water solution.