Lead-citrate chemistry. Synthesis, spectroscopic and structural studies of a novel lead(II)-citrate aqueous complex

The interaction of lead with biologically relevant ligands, such as citrate, constitutes a major research thrust relating to toxic effects exerted on humans. Reaction of Pb(NO3)(2) with citric acid in water at pH 2 and various ratios (1:1-1:3) yielded the first Pb-citrate species, which was characterized analytically and spectroscopically (FT-IR). X-ray crystallographic studies were carried out and showed the novel compound to be [Pb(C6H6O7)](n).nH(2)O. The geometry around Pb2+ appears to be distorted trigonal bipyramidal with Pb-O distances ranging from 2.397(7) to 2.527(6) Angstrom and the lone pair on Pb2+ occupying one of the three basal plane positions. Citrate binds Pb2+ forming rhomboidal Pb2O2 units, while it subtends its terminal carboxylate onto a proximal Pb2+, thus linking adjacent Pb2O2 units into a chain along the a axis. The involvement of citrate in hydrogen bonds with water molecules and abutting Pb-citrate moieties leads to a stable structure. The stability of the species formed and eventually isolated from aqueous solutions as well as its reactivity toward bases present useful information on the chemistry in biological media and its relevance to the known biotoxicity of lead. (C) 2000 Elsevier Science S.A. All rights reserved.