A new functionalized hexaazamacrocycle. Effect of pyridine pendants on cation and anion binding

Abstract

The macrocyclic ligand 1,4,7,13-tetramethyl-10,16-bis(o-pyridylmethyl)-1,4,7,10,13,16-hexaazacyclooctadecane (L) has been prepared and its protonation studied by means of potentiometric measurements. It binds up to five protons in aqueous solution above pH 2. The ¹H and ¹³C NMR spectra at different pH values allowed the determination of the stepwise protonation sites. The first three protonation steps take place on amine groups of the macrocyclic structure, the fourth and fifth on the pyridine nitrogens. Co-ordination of Cu^II, Zn^II, Cd^II and Pb^II in aqueous solution has been studied by means of potentiometric, ¹H NMR and/or UV-vis measurements (0.1 M NaClO₄, 298.1 K). The [ML]²⁺ complexes show unusually low stabilities, which are considered due to the low σ-donating properties of tertiary nitrogens as well as to the formation of large chelate rings containing unbound amine groups. The ligand behaves as a ditopic receptor for Cu^II. The [CuL]²⁺ complex exhibits a marked tendency to bind a second Cu²⁺ ion, giving binuclear complexes. Protonated forms of L are efficient receptors for ATP and ADP. The binding properties toward these anions are influenced by the presence of the two heteroaromatic moieties, which lead to a higher efficiency in ATP binding at acidic pH.