Encapsulation of metal cations and anions within the cavity of bis(1,4,7-triazacyclononane) receptors

Abstract

The synthesis and characterisation of a new bis([9]aneN₃) ligand (L⁴) containing two [9]aneN₃ macrocyclic moieties separated by a 2,6-dimethylenepyridine unit is reported. A potentiometric and ¹H NMR study in aqueous solution reveals that ligand protonation occurs on the secondary amine groups and does not involve the pyridine nitrogen. The coordination properties toward Cu(II), Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand can form mono- and binuclear complexes in aqueous solution. In the 1 : 1 complexes, the metal is sandwiched between the two [9]aneN₃ moieties and the pyridine N-donor is coordinated to the metal, as actually shown by the crystal structure of the compound [ZnL⁴](NO₃)₂·CH₃NO₂. L⁴ shows a higher binding ability for Cd(II) with respect to Zn(II), probably due to a better fitting of Cd(II) ion inside the cavity generated by the two facing [9]aneN₃ units. The formation of binuclear complexes is accompanied by the assembly of OH-bridged M₂(OH)_x (x = 1–3) clusters inside the cavity defined by the two facing [9]aneN₃ units, and pyridine is not involved in metal coordination. A potentiometric and ¹H NMR study on the coordination of halogenide anions by L⁴ and its structural analogous L³ in which the two [9]aneN₃ units are separated by a shorter quinoxaline linkage, shows that bromide is selectively recognised by L⁴, while chloride is selectively bound by L³. Such a behaviour is discussed in terms of dimensional matching between the spherical anions and the cavities generated by the two [9]aneN₃ units of the receptors.