Lead(II) Complexes with Macrocyclic Receptors Derived from 4,13-Diaza-18-crown-6

The complexation properties of three related macrocycles derived from 4,13-diaza-18-crown-6 toward lead(II) are
reported. The flexible macrocycle N,N‘-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) forms stable complexes with
this metal ion in the presence of different counterions (perchlorate and thiocyanate). The X-ray crystal structure of
[PbL2](SCN)2 indicates that, in the solid state, the lead(II) ion is eight-coordinated and fits quite well into the crown
hole favoring an anti arrangement of the organic receptor, which generates a very infrequent cubic coordination
polyhedron around the Pb(II) ion. In solution both complexes are fluxional and the nature of the counterion seems
to affect the dynamic behavior. Ligand L3, N,N‘-bis[(2-salicylaldimino)benzyl]-4,13-diaza-18-crown-6, derives from
L2 by condensation of salicylaldehyde with the amine group of each side arm. It can be deprotonated to yield
cationic complexes of formula [Pb(L3-H)]+ where the metal ion lies asymmetrically on the cavity of the ligand, being
seven-coordinated and pushed out from the crown hole, the bibracchial lariat ether presents a syn arrangement,
and one of the arms remains uncoordinated. The lead(II) ion also lies asymmetrically on the cavity of the third
macrocycle (L7), a lateral macrobicycle incorporating a phenolyl Schiff-base spacer. Spectrophotometric titrations of L2 and (L3-2H)2- with lead(II) perchlorate in acetonitrile gave values of log K[PbL2] = 7.7(5) and
log K[Pb(L3-2H)] = 7.2(3), demonstrating that the stability of the lead(II) complexes with these two ligands is very
similar.