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

• Published in: Inorganic chemistry 2002-08, Vol.41 (17), p.4337-4347
• Main Authors: Esteban, David, Avecilla, Fernando, Platas-Iglesias, Carlos, Mahía, Jose, de Blas, Andrés, Rodríguez-Blas, Teresa
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0255095

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 [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 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(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 (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[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.