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f-element coordination behavior of minor actinide chelators and impacts of ionizing radiation on actinide systems in aqueous solutions
Rotermund, Brian M.
Rotermund, Brian M.
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2025
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Within this work, the structural trends and spectroscopy characteristics of the hydrophilic f-element chelator tetramethyl diglycolamide (TMDGA) complexes with 4f and 5f elements were studied comprehensively to identify trends and systematic behaviors across the lanthanide (Ln) series and compared to differences and trends observed across the actinide (An) series. Additionally, this in-depth structural study attempts to bridge structural analysis with the field of radiation chemistry, to further develop an understanding of radiation interactions as it pertains to metal ion complexation of chelating ligands.
Crystals of TMDGA metal ion complexes across the lanthanides display consistent tris Ln TMDGA cationic metal centers taking distorted spherical capped square antiprism geometries across the lanthanides and actinides. Early Ln (Ln = La – Sm) TMDGA complexes took the formula, [Ln(TMDGA)3][Ln(NO3)6], featuring a hexanitrato anionic environment. A change in solvent for the crystallization reactions was found to be necessary after samarium in order to produce crystals suitable for single-crystal X-ray diffraction. The mid to late lanthanides (Ln = Eu – Yb) displayed a divergence from the anionic environment, taking multiple coordination environments with the generic formula, [Ln(TMDGA)3][Ln(NO3)5(H2O)]1−x[Ln(NO3)4(H2O)2]x(NO3)1+x. These structures displayed a trend across the series increasingly favoring a less sterically crowded coordination environment around the anionic metal ion complex, which is displayed through the decreasing favorability of nitrate coordination. The Lu structure displayed another coordination change, displaying a further favorability towards decreasing nitrate coordination, taking the formula, [Lu(TMDGA)3]2[Lu(NO3)4(H2O)2]0.75[Lu(NO3)5(H2O)]1.25(NO3)2.75·H2O. Significant asymmetric bonding in the lutetium system indicated that the steric crowding around the metal was almost extreme enough to cause a decrease in coordination number around the lutetium metal ion from 10- to 9-coordinate.
Early transuranic actinides (An = Pu, Am) displayed isostructural behavior to that of the early lanthanides, taking the formula [An(TMDGA)3][An(NO3)6]. Coordination of the plutonium complex in the tetravalent state afforded a homoleptic plutonium complex. A significant drop in metal–oxygen bond lengths, indicative of an increase in oxidation state were observed, as well as a change in the local symmetry around the plutonium TMDGA metal ion complex. However, although the changing reaction conditions observed in the lanthanides helped to inform the necessary reaction conditions for the mid-actinide reactions, berkelium and californium displayed deviations from the structural patterns observed in the lanthanides. Berkelium, taking the formula [Bk(TMDGA)3]2(NO3)6, displayed a notable deviation in anionic behavior in these TMDGA systems, as the nitrate ion are no longer coordinating with a metal center, and instead, are situated around the outer sphere of the Bk(TMDGA)3 complex. Similarly, the nitrate anions in the californium complex did not display any coordination with a californium metal ion, instead, the four nitrate ions orienting themselves around an ammonium via hydrogen bonding interactions.
Crystallization reactions were conducted with TMDGA using tetraphenylborate as the counter ion, taking the formula [M(TMDGA)3](BPh4)3 (M = Nd, Eu, Am, Bk, Cf). Although the coordination habits in the metal ion TMDGA complex were similar, geometry calculations revealed some subtle differences in the coordination environment. The close proximity of the large tetraphenylborate counterions to the coordinated TMDGA ligands imposed bending and torsion of the ligand as it was complexed. This bending resulted in a decrease in symmetry from C4v to Cs and a notable increase in complexity of the absorption spectra.
Through the development of this work, it was unveiled that there was a serious lack in structural analysis of a popular aqueous phase chelating ligand, triethylenediaminepentaacetic acid (DTPA). Successful synthesis of neodymium and americium DTPA, holding the formula [C(NH2)3]4[M(DTPA)]2, consisted of two americium DTPA complexes, connected together via a carboxylic acid group to afford a bimetallic complex. Solvent voids consisting of water molecules were modeled between the bimetallic complexes. When placed under pressure, the americium DTPA crystal displayed little response to pressure, atypical of most actinide coordination complexes, especially in those with softer electron donating atoms such as nitrogen in DTPA. Contractions in the metal–nitrogen bond lengths indicated a slight preference for the actinide complex over the neodymium complex.
The radiation kinetics of berkelium and californium with common nitric acid reactive radical species (eaq−, H•, NO3•−, and •OH) were investigated to study their readiness to undergo various redox reactions within nuclear reprocessing technologies. Despite these reactive radiolysis species not holding sufficient redox potentials, berkelium and californium readily react to transiently undergo these oxidation state changes. In all cases, berkelium displayed increased reactivity over that to the equivalent californium reaction. Additional measurements with californium and the dichloride radical anion (Cl2•−) and the sulfate radical anion (SO4•−) were undertaken to explore the reactivity of potential radical anions present in some lesser used separation and purification processes. The dichloride radical anion was found to have little to no reactivity, showing that this reaction holds negligible impact on processes in the presence of chloride. The SO4•−, however, displayed significant reactivity over that of any of the aforementioned oxidizing radical species and faster than its reaction between that of other trivalent actinides, indicating that the sulfate radical anion displays promise for the selective oxidation of californium.
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