For the past 30 years or so, we were involved into the separation of this series of
transition elements . The separation of these elements poses considerable challenges due to the marked similarity in their chemical and physical properties and also due to their flexibility in the formation of
coordination compounds. It began by our first attempt to separate
lanthanide (Ln) complexes by HPLC and solvent extraction. The work was later extended to the separation of lanthanide mixture by ion chromatography, whereby the emphasized was on the development of separation method by the addition of crown ethers in the mobile electrolyte as the ion association to facilitate the separation. The successful separation of lanthanides by solvent extraction and ion chromatographic methods led to our deep involvement in the finding of the similarities as well as the differences of lanthanides in their coordination behaviour with
cyclic and
acyclic polyethers. In combination with multiple donor atoms from ligands, very interesting similarities and dissimilarities in the formation of complexes in their solid state and liquid forms have been observed. In the present paper, the discussion will be focused on the formation of complexes with oxygen donor from acyclic (pentaethylene glycol,EO5) and cyclic (18-crown-6 ,18C6) polyethers. The observation on the coordination behavior of the donoracceptor atoms and the contribution of the counter anions to the stability of the complexes will also be elaborated. The presence of bulky counter anion picrate (Pic) having oxygen and nitrogen donor atoms have produced several solid state complexes with unique coordination behaviour and hydrogen-donor atoms interaction leading towards strong and weak hydrogen bondings. From the observation on the ternary complex systems of Ln-18C6-Pic and Ln-EO5-Pic with the same number of oxygen atoms of ether linkages revealed some interesting phenomena. Both ligands involved in the formation of, Ln-18C6-Pic and Ln-EO5-Pic complexes having six O donor atoms, produced rather different coordination complexes even though the acyclic EO5 adopted a ‘cyclic-like'' conformation. The molecular structure of the cyclic complexes were shown conclusively to form a ten-coordinated complexes by six oxygen atoms from 18C6 and four from bidentate picrates. However, the acyclic complexes exhibited a nine-coordinated complexes by six oxygen atoms from EO5 and two from one bidentate and one monodentate picrate. Solid lanthanide complexes with cyclic (18C6) and acyclic (EO5) with picrate as counter anion revealed that La formed 10 coordination complexes for all cyclic and acyclic ligands, while Ce was able to form both 10 and 9 coordination complexes with cyclic and acyclic ligands respectively. Other lanthanide elements formed 10 coordination complexes with cyclic and 9 coordination complexes with acyclic ligands respectively. The stronger bonding of lanthanum (due to the highest coordination number) in the solid state complexes explained the selectivity in the solvent extraction. The percentage of extraction of lanthanum when diaza-18-crown-6 (DD18C6) as extractant was almost 100 % compared to cerium of about 20% while the rest of the lanthanide elements was less than 2% extracted. The variation in the coordination behaviours of lanthanides with donor atoms also explained the difference of retention in the capillary electrophoresis column when cyclic polyether was incorporated as the mobile electrolyte solution.
More abstracts about the Separation and Coordination Chemistry of Lanthanides