Cd(II) and Pb(II) complexation by dipyridine-containing macrocycles with different molecular architecture. Effect of complex protonation on metal coordination environment

The coordination features of the three dipyridine-containing polyamine macr ocycles 2,5,8,11,14-pentaaza[15][15](2,2')[1,15]-bipyridylophane (L1), 5,8, 11-trimethyl-2,5,8,11,14-pentaaza[15]-[15](2,2)[1,15]-bipyridylophane (L2), and 4,4'-(2,5,8,11,14-pentaaza[15]-[15](2,2')-bipyridylophane) (L3) toward Cd(II) and Pb(II) have been studied by means of potentiometric, microcalor imetric, and spectrophotometric UV-vis titrations in aqueous solutions. All ligands form 1:1 metal complexes. In the L1 and L2 complexes the metals ar e lodged inside the macrocyclic cavity, coordinated to the heteroaromatic n itrogens. On the other hand, the insertion of a rather rigid dipyridine moi ety within a macrocyclic structure does not allow all the aliphatic amine g roups to coordinate to the metals and several protonated complexes are foun d in solution. The particular molecular architecture of L3, which displays two well-separated binding moieties, strongly affects its coordination beha vior. In the [PbL3](2+), complex and in its protonated species, the metal i s lodged inside the macrocyclic cavity, not bound to the heteroaromatic nit rogens. A similar coordination environment is found in [CdL3](2+). In this case, however, protonation of the complex takes place on the aliphatic amin e groups and gives rise to translocation of the metal outside the cavity, c oordinated by the dipyridine moiety.