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

Our continued research on lanthanide citrate with ethylenediaminetetraacetate results in an isolation of a monomeric samarium complex K-4[Sm(Hcit)(EDTA)]center dot 10H(2)O (1) (H(4)cit = citric acid and H(4)EDTA = ethylenediaminetetraacetic acid) from the reaction of lanthanide ethylenediaminetetraacetate trihydrates with citric acid at pH similar to 6.5, where the three coordinated water molecules of K[Sm(EDTA) (H2O)center dot 5H(2)O (4) were substituted by the alpha-hydroxy, alpha-carboxy and beta-carboxy groups of citrate. When involving lanthanum element, the reaction of lanthanum ethylenediaminetetraacetate trihydrates with dimeric lanthanum complex K-4(NH4)(4)[La-2(Hcit)(2)(EDTA)(2)]center dot 17H(2)O (2) gives a novel tetrameric lanthanum citrate with ethylenediaminetetraacetate K-2(NH4)(8)[[La(EDTA)(H2O)(2)](2)[La-2(Hcit)(2)(EDTA)(2)]]center dot 22H(2)O (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 beta-carboxy groups. It is interesting to note that 1-3 contain strong intramolecular hydrogen bonds between alpha-hydroxy and beta-carboxy groups with very good solubilities in water. The relationships between the isolations of mixed EDTA-citrate lanthanides and modified Pechini method are discussed. (C) 2018 Elsevier Ltd. All rights reserved.