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

Abstract

Highly water-soluble lanthanum and cerium citrates or malates with ethylenediaminetetraacetate (NH₄)₈[Ln₂(Hcit)₂(EDTA)₂]·9H₂O [Ln = La, 1; Ce, 2], K₈[La₂(Hcit)₂(EDTA)₂]·16H₂O (3) and K₆[Ln₂(Hmal)₂(EDTA)₂]·14H₂O [Ln = La, 4; Ce, 5] (H₄cit = citric acid, H₃mal = malic acid, and H₄EDTA = 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 ¹³C{¹H} NMR, solid state ¹³C 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 α-hydroxy and α-carboxy groups of citrate and malate chelate in five-membered ring with one lanthanide ion, while one of the β-carboxy group coordinates with the other lanthanide ion, forming a dimeric structure. The other pendent β-carboxy groups in 1–3 form very strong intramolecular hydrogen bond with α-hydroxy groups [O1⋯O7 2.594(4), 2.587(8) and 2.57(1) Å for 1–3 respectively]. ¹³C 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.