Conversions of monomeric, dimeric and tetrameric lanthanum and samarium citrates with ethylenediaminetetraacetates in aqueous solutions

Abstract

Our continued research on lanthanide citrate with ethylenediaminetetraacetate results in an isolation of a monomeric samarium complex K4[Sm(Hcit)(EDTA)]·10H2O (1) (H4cit = citric acid and H4EDTA = ethylenediaminetetraacetic acid) from the reaction of lanthanide ethylenediaminetetraacetate trihydrates with citric acid at pH ∼6.5, where the three coordinated water molecules of K[Sm(EDTA)(H2O)3]·5H2O (4) were substituted by the α-hydroxy, α-carboxy and β-carboxy groups of citrate. When involving lanthanum element, the reaction of lanthanum ethylenediaminetetraacetate trihydrates with dimeric lanthanum complex K4(NH4)4[La2(Hcit)2(EDTA)2]·17H2O (2) gives a novel tetrameric lanthanum citrate with ethylenediaminetetraacetate K2(NH4)8[[La(EDTA)(H2O)2]2[La2(Hcit)2(EDTA)2]]·22H2O (3). The main structural feature of 2 consists of a dinuclear unit deca-coordinated by citrate and EDTA, while tetramer 3 is a dinuclear unit bridged with two lanthanum ethylenediaminetetraacetates through pendent β-carboxy groups. It is interesting to note that 1–3 contain strong intramolecular hydrogen bonds between α-hydroxy and β-carboxy groups with very good solubilities in water. The relationships between the isolations of mixed EDTA–citrate lanthanides and modified Pechini method are discussed.