An europium(iii) diglycolamide complex: insights into the coordination chemistry of lanthanides in solvent extraction

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 44; no. 2; pp. 515 - 521
• Main Authors: Antonio, Mark R, McAlister, Daniel R, Horwitz, E. Philip
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 14.01.2015
• Subjects: Atomic beam spectroscopy; bismuth tetrachloride; Cationic; Chemical bonds; coordination chemistry; Coordination compounds; EXAFS; inner-sphere; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; lanthanide; Lanthanides; Ligands; liquid-liquid extraction; outer-sphere; Solvent extraction; Stoichiometry; tetraalkyldiglycolamides
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c4dt01775g

The synthesis, stoichiometry, and structural characterization of a homoleptic, cationic europium( iii ) complex with three neutral tetraalkyldiglycolamide ligands are reported. The tri(bismuth tetrachloride)tris( N , N , N ′, N ′-tetra- n -octyldiglycolamide)Eu salt, [Eu(TODGA) 3 ][(BiCl 4 ) 3 ] obtained from methanol was examined by Eu L 3 -edge X-ray absorption spectroscopy (XAS) to reveal an inner-sphere coordination of Eu 3+ that arises from 9 O atoms and two next-nearest coordination spheres that arise from 6 carbon atoms each. A structural model is proposed in which each TODGA ligand with its O&z.dbd;C a -C b -O-C b -C a &z.dbd;O backbone acts as a tridentate O donor, where the two carbonyl O atoms and the one ether O atom bond to Eu 3+ . Given the structural rigidity of the tridentate coordination motif in [Eu(TODGA) 3 ] 3+ with six 5-membered chelate rings, the six Eu-C a and six Eu-C b interactions are readily resolved in the EXAFS (extended X-ray absorption fine structure) spectrum. The three charge balancing [BiCl 4 ] − anions are beyond the cationic [Eu(TODGA) 3 ] 3+ cluster in an outer sphere environment that is too distant to be detected by XAS. Despite their sizeable length and propensity for entanglement, the four n -octyl groups of each TODGA (for a total of twelve) do not perturb the Eu 3+ coordination environment over that seen from previously reported single-crystal structures of tripositive lanthanide (Ln 3+ ) complexes with tetraalkyldiglycolamide ligands (of the same 1 : 3 metal-to-ligand ratio stoichiometry) but having shorter i -propyl and i -butyl groups. The present results set the foundation for understanding advanced solvent extraction processes for the separation of the minor, tripositive actinides (Am, Cm) from the Ln 3+ ions in terms of the local structure of Eu 3+ in a solid state coordination complex with TODGA. A cationic europium( iii ) complex with three neutral diglycolamide ligands (for R &z.tbd; n -C 8 H 17 ) was prepared as the bismuth tetrachloride salt revealing a 9-O inner-sphere environment with 6 distant carbonyl (C a ) and 6 ether (C b ) carbon atoms in the Eu L 3 -edge EXAFS.