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 macrocycles 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, microcalorimetric, and spectrophotometric UV-vis titrations in aqueous solutions. All ligands form 1:1 metal complexes. In the L1 and L2 complexes the metals are lodged inside the macrocyclic cavity, coordinated to the heteroaromatic nitrogens. On the other hand, the insertion of a rather rigid dipyridine moiety within a macrocyclic structure does not allow all the aliphatic amine groups to coordinate to the metals and several protonated complexes are found in solution. The particular molecular architecture of L3, which displays two well-separated binding moieties, strongly affects its coordination behavior. In the [PbL3](2+), complex and in its protonated species, the metal is lodged inside the macrocyclic cavity, not bound to the heteroaromatic nitrogens. A similar coordination environment is found in [CdL3](2+). In this case, however, protonation of the complex takes place on the aliphatic amine groups and gives rise to translocation of the metal outside the cavity, coordinated by the dipyridine moiety.