Diglycolamide-Functionalized Calix[4]arenes Showing Unusual Complexation of Actinide Ions in Room Temperature Ionic Liquids: Role of Ligand Structure, Radiolytic Stability, Emission Spectroscopy, and Thermodynamic Studies

Abstract

Diglycolamide-functionalized calix[4]arenes (C4DGAs) with varying structural modifications were evaluated for actinide complexation from their extraction behavior toward actinide ions such as UO2(2+), Pu(4+), PuO2(2+), and Am(3+) in the room temperature ionic liquid (RTIL) 1-n-octyl-3-methylimidazolium bis(trifluoromethane)sulfonamide (C8mimNTf2). The formation constants were calculated for Am(3+) which showed a significant role of ligand structure, nature of substituents, and spacer length. Although the alkyl substituents on the amidic nitrogen increase the extraction efficiency of americium at lower acidity because of the inductive effect of the alkyl groups, at higher acidity the steric crowding around the ligating site determines the extraction efficiency. All C4DGAs formed 1:1 complexes with Am(3+) while for the analogous Eu(3+) complexes no inner sphere water molecules were detected and the asymmetry of the metal ligand complex differed from one another as proved by time-resolved laser induced fluorescence spectroscopy (TRLIFS). Thermodynamic studies indicated that the extraction process, predominant by the Am(3+)-C4DGA complexation reaction, is exothermic. The unique role of the medium on Am(3+) complexation with the C4DGA molecules with varying spacer length, L-IV and L-V, was noticed for the first time with a reversal in the trend observed in the RTIL compared to that seen in a nonpolar molecular diluent like n-dodecane. Various factors leading to a more preorganized structure were responsible for favorable metal ion complexation. The solvent systems show promise to be employed for nuclear waste remediation, and sustainability options were evaluated from radiolytic stability as well as stripping studies.

Luminescence investigation on Eu–pillar[5]arene-based diglycolamide (DGA) complexes: Nature of the complex, Judd–Ofelt calculations and effect of ligand structure

“…energy short bond length. The lowest value of Ω 2 obtained for the Eu 3+ aq system [6] can be attributed to the low covalency of the Eu 3+ -O bond. It was observed that when pure aqueous medium is replaced by ethanol/water mixture, the covalency of metal-oxygen bond increases.…”

“…Formation of reverse micelle aggregation by four TODGA molecules was reported to be responsible for unusually high extraction efficiency of trivalent actinides and lanthanides over other oxidation states [2][3]. In view of the above fact, several attempts were reported to study the actinide extraction using tripodal diglycolamide, where three diglycolamide moieties are attached in C pivotal position [4], diglycolamide functionalized calix [4] arenes and their different structural modifications, where diglycolamide moieties are anchored on a calix [4]arene molecular platform [5][6][7]. Recently, pillararene platform tethered with functional groups were reported by us to be a novel class of extractants for Ln/An separation [8].…”

Probing of the local environment and calculation of J.O. parameters for Eu3+ CMPO functionalized pillararene complexes by time resolved fluorescence spectroscopy

“…The Eu 3 þ complexes of CMPO functionalized pillararenes were found to be more symmetric compared to the diglycolamide functionalized pillararenes. Among the various supramolecular ligands, the asymmetric factors for the Eu 3 þ complexes followed the trend of ligands as tripodal diglycolamide [44] oDGA functionalized ionic liquid with NTf 2 − anion [45] o CMPO pillararene LI oCMPO pillararene LII oDGA Calixarene LI [46] oDGA functionalized ionic liquid with PF 6 − anion [45] oTODGA [45] oCMPO pillararene LIII oDGA Calixarene LII [46] oDGA pillararene LI [47] oPyrazine functionalized calixarene L4 [48] oboth side DGA calixarene [49] oDGA pillararene LII [46] o Pyrazine functionalized calixarene L6 [48] oDGA pillararene LIII [47] olower rim DGA calixarene [49]. …”

Understanding the complexation of the Eu3+ ion with TODGA, CMPO, TOPO and DMDBTDMA: Extraction, luminescence and theoretical investigation

“…Since the nature of the species also depends on the medium, the A value for the Eu-TODGA complex in ionic liquid was found to be more than the present dodecane medium [70]. The A value for the Eu-tripodal diglycolamide complex was reported to be 1.52, while those for functionalized calix [4]arenes, pillar [5]arenes and pyrazines were found to be in the range 2.1-2.86, 2.1-2.55 and 1.5-2.51, respectively [71][72][73][74][75][76].…”