Iminodiacetic acid complexation with lanthanides under acidic conditions

Abstract

The chemical behavior of the lanthanides and actinides generally converges with increasing number of f-electrons, but growing evidence suggests their chemistries begin to diverge after the f-shell is half-filled. Previous work with dipicolinic acid indicates that the degree of actinide covalent interaction increases at californium and berkelium. The shift in covalency may be unique to dipicolinic acid (DPA) or it may occur in similar ligands. Assessing the potential for covalent interactions with the structurally similar iminodiacetic acid (IDA) may provide an indication to the orbital interactions encouraging covalency in the heaviest actinides. The speciation of f-element complexes with IDA under conditions appropriate for comparison with DPA have not been resolved. To initiate f-block IDA complexation studies, this work considers the formation of lanthanide (Ln):iminodiaceticic acid (IDA) complexes under acidic conditions. Reports of lanthanide and actinide complexes with iminodiacetic acid span two protonated metal-ligand species and four deprotonated species, but these reports do not agree on which species are present. This work focuses only on acidic solution chemistry before metal hydrolysis can occur. Spectrophotometric titration of IDA into lanthanide perchlorate solutions, described in the present work, suggests between 2 < p[H+] < 4, Ln IDA solutions may form LnH2IDA, LnHIDA, and LnIDA. This validates reports of protic lanthanide-IDA complexes. Accurate characterization of lanthanide - IDA complexation will serve as a standard to which actinide - IDA, lanthanide - DPA, and actinide - DPA may be compared. Such a comparison will help to elucidate the contribution of amine aromaticity to weak covalency observed in the heavy actinides.