Design of protonated polyazamacrocycles based on phenanthroline motifs for selective uptake of aromatic carboxylate anions and herbicides

Article abstract of DOI:10.1002/chem.200800993

Three novel large polyazamacrocycles containing two 1,10-phenanthroline (phen) units connected by two polyamine spacers of different length, [32]phen₂N₄, [30]phen₂N₆ and Me ₂[34]phen₂N₆, have been synthesised and their protonated forms used as receptors for binding studies with several aromatic carboxylate anions (benzoate (bzc^-), 1-naphthalate (naphc ^-), 9-anthracenate (anthc^-), pyrene-1-carboxyl-ate (pyrc^-), phthalate, (ph^2-), isophthalate (iph ^2-), terephthalate (tph^2-), 2,5-dihydroxy-1,4- benzenediacetate (dihyac^2-) and, 1,3,5-benzenetricarboxylate (btc^3-)) and three herbicides (4-amino-3,5,6-trichloropyridine-2- carbox-ylate (ATCP^-), dichlorophenoxyacetate (2,4-D^-) and glyphosate (PMG^2-)) in water solution. The [30]phen₂N ₆ receptor was found to be the most suitable for binding the anions considered in a 1:1 stoichiometry. The three receptors exhibit a remarkable binding selectivity towards the extended aromatic anion pyre^- at low pH values. Their binding affinities for the monocarboxylate anions decrease with the extension of the aromatic system in the order pyre^- > anthc^- > naphc^- > bzc^-, which indicates the presence of π-π stacking interactions in the molecular recognition of these anions. Molecular dynamics simulations carried out for the binding of (H₄[30]phen₂N₆}⁴⁺ and {H ₆Me₂[34]phen₂N₆}⁶⁺ with pyre^-, anthc^-, naphc^-, iph^2- and btc^3- in water showed that these receptors adopt a folded conformation with the anion inserted between the two phen heads and that the molecular recognition is governed by π-π stacking interactions and multiple N-H...O=C hydrogen bonds. The binding free energies estimated theoretically are very similar to those found by Potentiometric methods, which supports the proposed binding arrangement.