New bis-cresol-bridged bis(1,4,7-triazacyclononane) ligand as receptor for metal cations and phosphate anions

he synthesis and characterization of a new bis([9]aneN(3)) ligand (H(2)L) containing two [9]aneN(3) macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H(2)L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H(5)L)(3+) and (H(6)L)(4+) species at acidic pH values. The coordination properties of H(2)L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements, The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M(2)H(2)L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN(3) and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M(2)H(2)L](4+) species at alkaline pH values affords [M(2)L](2+) complexes, where all amine groups of the [9]aneN(3) moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN(3) moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN(3) subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.