Isolations and characterization of highly water-soluble dimeric lanthanide citrate and malate with ethylenediaminetetraacetate

Highly water-soluble lanthanum and cerium citrates or malates with ethylenediaminetetraacetate (NH4)(8)[Ln(2)(Hcit)(2)(EDTA)(2)]center dot 9H(2)O [Ln = La, 1; Ce, 2], K-8[La-2(Hcit)(2)(EDTA)(2)]center dot 16H(2)O (3) and K-6[Ln(2)(Hmal)(2)(EDTA)(2)]center dot 14H(2)O [Ln = La, 4; Ce, 5] (H(4)cit = citric acid, H(3)mal = malic acid, and H(4)EDTA = ethylenediaminetetracetic acid) were prepared from the reactions of lanthanide ethylenediaminetetraacetate trihydrates with citric or malic acid at pH 5.0-6.5. These compounds were characterized by elemental analyses, IR, TG-DTG, solution C-13{H-1} NMR, solid state C-13 NMR spectra and X-ray structural analyses. The main structural feature of the compounds consists of a dinuclear unit deca-coordinated by EDTA and citrate or malate. The alpha-hydroxy and alpha-carboxy groups of citrate and malate chelate in five-membered ring with one lanthanide ion, while one of the beta-carboxy group coordinates with the other lanthanide ion, forming a dimeric structure. The other pendent beta-carboxy groups in 1-3 form very strong intramolecular hydrogen bond with alpha-hydroxy groups [O1...O7 2.594(4), 2.587(8) and 2.57(1) angstrom for 1-3 respectively]. C-13 NMR spectra of the lanthanum compounds show obvious downfield shifts based on solid and solution NMR measurements, indicating the coordinations of mixed-ligand in lanthanum complexes, while highfield shifts are observed in cerium complexes.