Lead complexation by novel phenanthroline-containing macrocycles

Two new polyamine macrocycles 2,5,8-triaza[9]-[9](2,9)[1,10]-phenanthrolinophane (L-1) and 2,5,8,11-tetraaza[12]-[12](2,9)[1,10]-phenanthrolinophane (L-2) have been synthesized and characterized. They contain a triamine (L-1) or a tetraamine (L-2) chain linking the 2,9 positions of a phenanthroline moiety. Like L-3, which contains a pentaamine chain connecting the 2,9-phenanthroline positions, they form stable 1 : 1 lead(II) complexes in aqueous solutions. These complexes can readily be extracted in non-aqueous solvents, such as CHCl3 or CH2Cl2. The thermodynamic parameters for lead(II) complexation have been determined by means of potentiometric and microcalorimetric measurements in aqueous solutions. The stability of the complexes decreases from L-1 to L-3 mainly due to a marked decrease of the enthalpy changes related to the formation of the complexes, indicating that the overall metal-ligand interaction decreases from L-1 to L-3. Most likely, in the [PbL1](2+) complex all donors are strongly involved in metal co-ordination, while in [PbL2](2+) and [PbL3](2+) some of the amine groups are weakly bound, or not bound, to the metal. These considerations are supported by the crystal structures of [((PbLBr)-Br-1)(2)(mu-Br)][(PbLBr2)-Br-1]Br . 5H(2)O. In the latter complex the macrocycle wraps around the metal. On the other hand, three nitrogen donors interact at remarkably longer distance than the others. This observation may justify the lower stability of the [PbL3](2+) complex with respect to that of [PbL1](2+).