Mononuclear and homobinuclear lead(II) complexes of macrocyclic Schiff bases

Abstract

Lead(II) salts act as templates for the cyclic condensation of several organic dicarbonyl compounds with a variety of polyfunctional diprimary amines to yield complexes of macrocyclic Schiff-base ligands in moderate to good yield. Three types of macrocyclic lead(II) complex are described: (i) mononuclear complexes of 15- and 17-membered quinquedentate N₅ and N₃S₂ ligands; (ii) mononuclear complexes of 18-membered sexidentate N₃O₃, and N₂O₄ ligands; and (iii) binuclear complexes of a 30-rnembered decadentate N₆O₄ ligand. With the aid of i.r. spectra of solids and of electrical-conductance measurements on solutions, and by reference to X-ray structure determinations of selected complexes, the co-ordination geometries are assigned where possible. The co-ordination number of the lead(II) ion is variable between five and eight and depends on the nature of the anions present as well as on the nature of the macrocycle. Alkaline-earth metal complexes of certain of the macrocycles undergo exchange reactions with Pb^II in solution; possible applications of these reactions in the chelation therapy of lead poisoning are noted.