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 K 4 [Sm(Hcit)(EDTA)]·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 ∼6.5, where the three coordinated water molecules of K[Sm(EDTA)(H 2 O) 3 ]·5H 2 O ( 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 K 4 (NH 4 ) 4 [La 2 (Hcit) 2 (EDTA) 2 ]·17H 2 O ( 2 ) gives a novel tetrameric lanthanum citrate with ethylenediaminetetraacetate K 2 (NH 4 ) 8 [[La(EDTA)(H 2 O) 2 ] 2 [La 2 (Hcit) 2 (EDTA) 2 ]]·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 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.