Lead complexation by novel phenanthroline-containing macrocycles

Two new polyamine macrocycles 2,5,8-triaza[9]-[9](2,9)[1,10]-phenanthrolino phane (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-phen anthroline 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) comple xation have been determined by means of potentiometric and microcalorimetri c measurements in aqueous solutions. The stability of the complexes decreas es 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 meta l-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, th ree 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+).