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 (L(2)) forms stable complexes with this metal ion in the presence of different counterions (perchlorate and thiocyanate). The X-ray crystal structure of [PbL(2)](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 L(3), N,N'-bis[(2-salicylaldimino)benzyl]-4,13-diaza-18-crown-6, derives from L(2) by condensation of salicylaldehyde with the amine group of each side arm. It can be deprotonated to yield cationic complexes of formula [Pb(L(3)-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 (L(7)), a lateral macrobicycle incorporating a phenolyl Schiff-base spacer. Spectrophotometric titrations of L(2) and (L(3)-2H)(2)(-) with lead(II) perchlorate in acetonitrile gave values of log K[PbL(2)] = 7.7(5) and log K[Pb(L(3)-2H)] = 7.2(3), demonstrating that the stability of the lead(II) complexes with these two ligands is very similar.