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

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 proto ns in aqueous solution above pH 2. The H-1 and C-13 NMR spectra at differen t pH values allowed the determination of the stepwise protonation sites. Th e first three protonation steps take place on amine groups of the macrocycl ic structure, the fourth and fifth on the pyridine nitrogens. Go-ordination of Cu-II, Zn-II, Cd-II and Pb-II in aqueous solution has been studied by m eans of potentiometric, H-1 NMR and/or UV-vis measurements (0.1 M NaClO4, 2 98.1 K). The [ML](2+) complexes show unusually low stabilities, which are c onsidered due to the low sigma-donating properties of tertiary nitrogens as well as to the formation of large chelate rings containing unbound amine g roups. The ligand behaves as a ditopic receptor for Cu-II. The [CuL](2+) co mplex exhibits a marked tendency to bind a second Cu2+ ion, giving binuclea r 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 AT P binding at acidic pH.