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

摘要

The synthesis and characterisation of a new bis([9]aneN(3)) ligand (L-4) containing two [9]aneN(3) macrocyclic moieties separated by a 2,6-dimethyleiiepyridine unit is reported. A potentiometric and H-1 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 I : I complexes, the metal is sandwiched between the two [9]aneN(3) 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)-C-.. L-4 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 M-2(OH)(x) (x = 1-3) clusters inside the cavity defined by the two facing [9]aneN(3) units, and pyridine is not involved in metal coordination. A potentiometric and H-1 NMR study on the coordination of halogenide anions by L-4 and its structural analogous L-3 in which the two [9]aneN(3) units are separated by a shorter quinoxaline linkage, shows that bromide is selectively recognised by L-4, while chloride is selectively bound by L-3. Such a behaviour is discussed in terms of dimensional matching between the spherical anions and the cavities generated by the two [9]aneN(3) units of the receptors.