LANTHANIDE CONTRACTION IN CHELATES OF ETHYLENEDIAMINETETRAACETIC ACID BASED ON CRYSTALLOGRAPHIC DATA: A SHORT REVIEW

Abstract

The lanthanide (Ln) series has been attracted so much attention not only for its physical properties but also for its applications in organic synthesis and biomedical engineering. In the lanthanide chemistry, lanthanide contraction is a crucial concept meaning the gradual decrease in ionic radii as the atomic number of the Ln series increases. As a prominent chelator, ethylenediaminetetraacetic acid (EDTA) has been well-known to give stable complexes with various metals including Ln ions, resulted in forming (EDTA−4H)4− anion by deprotonation; over 100 kinds of Ln-EDTA complexes have hitherto been synthesized and structurally characterized using X-ray crystallography. While the lanthanide contraction phenomenon has been extensively studied in certain complexes of Ln, systematization on the Ln-EDTA remains inadequate though the chelates of EDTA are commonly used. Thus, this study presents a comprehensive analysis based on the data from the Cambridge Crystallographic Data Centre (CCDC). In mononuclear EDTA complexes X[Ln(EDTA−4H)(H2O)m]·nH2O (X+ = NH4+and its analogues) systematic lanthanide contraction depending on their coordination number are presented except for a few cases. However, in the case of non-mononuclear complexes or coordination polymers, the compositions become more complicated, and a greater variation in atomic bond length occurs, which is unusual for the lanthanide contraction studies in previous research. These varieties are considered to be influenced by different types of counter-cations, inorganic anions, solvent molecules and bridging O atoms from another EDTA anions. This suggests the flexibility and versatility of EDTA as a ligand. Such findings in the Ln-EDTA complexes could potentially provide us further development of a novel function in the future.