Utilizing Autoxidation of Solvents To Promote the Formation of Uranyl

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Utilizing autoxidation of solvents to promote the formation of uranyl peroxide materials. Ashini S. Jayasinghe, Lindsey C. Applegate, Daniel K. Unruh, Jeremy Hutton, and Tori Z. Forbes Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01735 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Crystal Growth & Design

For submission to Crystal Growth and Design

Utilizing autoxidation of solvents to promote the formation of uranyl peroxide materials. Ashini S. Jayasinghe, Lindsey C. Applegate, Daniel K. Unruh, Jeremy Hutton, and Tori Z. Forbes* Department of Chemistry, University of Iowa, Iowa City, IA 52242 * corresponding author: [email protected]

Abstract Uranyl peroxide compounds are important to the nuclear fuel cycle and are typically formed from the direct addition of hydrogen peroxide or through photochemical pathways that utilize a sacrificial solvent. In this study, we explore the use of THF as a peroxide forming solvent that could be used to promote the crystallization of uranyl peroxide materials. Compound 1 (Li(H2O)2(UO2)(C7H3NO4)(C4H5NO4) ∙ 2 H2O) was originally crystallized after aging for 10 days at room temperature and was characterized by X-ray diffraction, elemental analysis, Raman spectroscopy, and UV/Vis spectroscopy. Exposure to ambient light for one month led to the complete transformation of 1 to compound 2 ((C5NH6)2[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 4 H2O). Oxidation of the THF molecule was proposed as the major mechanism for the formation of the secondary uranyl peroxide compound, which was accelerated in the presence of the UO22+ cation and light. Presence of THF hydrogen peroxide was confirmed by spectral (NMR and Raman) analysis of the mother liquor. Compound 3 ((C4N2H12)[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 2.75 H2O) formed in the presence of transition metal cations and additional experiments indicated that photoexcitation was not necessary to create free peroxide in this system. Autooxidation of the THF molecule is the most likely mechanism for peroxide formation and indicates that judicial choice of solvent can lead to the formation of new uranyl peroxide materials. 1 ACS Paragon Plus Environment

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Introduction Uranyl peroxide complexes and related materials were largely ignored by the actinide chemistry community until their relevance to the nuclear fuel cycle began to emerge almost 20 years ago.1 Detailed investigations for this system began with the structural characterization of the mineral, studtite [(UO2)(O2)(H2O)2]∙ 2 H2O, which is the only known naturally-occurring peroxide phase.2-6 Thermodynamic investigations indicate that surprisingly low concentrations of H2O2 (1.1 x 10-14 M) can induce the formation of studtite on the surface of uranium dioxide and the mineral is stable over geologic time scales.2 Studtite could have been dismissed as a rare mineral phase, but it was concurrently identified as a major alteration product on the surface of spent nuclear fuel in contact with water for 1.5 years.7 Uranyl peroxide solids have also been identified to exist within yellowcake formed during the initial stage of production, solidifying their importance throughout the entire fuel cycle.8-11 Studtite was the only uranyl peroxide phase of interest until the Burns research discovered of the first uranyl peroxide nanoclusters in 2005, which ultimately resulted in a cascade of novel compounds with unique structural topologies.12 The first reported uranyl nanocluster contained 24, 28, and 32 uranyl cations linked through peroxide or hydroxide bridges to form nanoscale capsules with fullerene-like topologies. Since this original discovery, a large family of uranyl peroxide compounds has been crystallized that range from single monomeric species to clusters containing 124 U6+ atoms.12-16 The topological variations within these nanoclusters are due to the formation of four, five, six and eight-member rings built from [UO2(O2)(OH)2]4- or [UO2(O2)3]4- units.17 Presence of other bridging ligands, including pyrophosphate and oxalate, results in additional structural diversity and the formation of core-shell structures.17 Counterions in solution, particularly alkali- and alkali-earth cations, also impart controls on the structural topologies and stability of individual clusters in solution.18-23 Initial discovery of these clusters focused on their 2 ACS Paragon Plus Environment

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unique chemistry, but additional studies found that these soluble uranyl peroxide phases may be important to the nuclear fuel cycle because they enhance the corrosion of UO2 solids placed in alkaline solutions24,

25or

seawater.26 The robustness and prevalence of the uranyl peroxide

nanocapsules in solution has also inspired cluster-based separations technologies27 and nanoscale compositional tuning for fuel fabrication processes.24 Synthetic uranyl peroxide phases are generally created through the direct addition of hydrogen peroxide to the system, but there are alternative chemical and physical pathways that generate peroxide in-situ. The uranyl peroxide clusters and synthetic studtite are formed through the addition of 30% hydrogen peroxide to aqueous solution in the presence of varying amounts of base and counterions.2, 17, 25 Formation of uranyl peroxide within ore bodies or on the surface of spent nuclear fuel requires the generation of peroxide in-situ; thus, a number of other mechanisms have been investigated and reported in the literature. Hydrothermal treatment of uranyl solutions in the presence of organic ligands produces peroxide phases, although the reported yields for the solid compounds are low.28 Photocatalysis can also generate uranyl peroxide materials in the presence of an organic substrate. During this process, the organic ligands or co-solvents are used as a substrate by which excited U(VI)O22+ cation abstracts a hydrogen atom to create a radical. These radicals react with the O2 gas present in the solution to create O2- in-situ and bind with soluble U(VI)O22+ cations in solution to create uranyl peroxide complexes.29-32 This photocatalytic process may occur even after the formation of uranyl hydrolysis products, as previously described for both uranyl pyridine33 and pyrrolidone34 systems. More recently, a uranyl peroxide phase was also identified by Kirkegaard et al. in a hydrated uranyl fluoride solids and experiments in the absence of light suggested a non-photocatalytic mechanism.35 Non-photocatalytic pathways have been proposed in previous studies as Kubatko et al.2 postulated that naturally-occurring studtite

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forms through the alpha-radiolysis of water. Interaction between water molecules and ionizing radiation leads to the creation of H, O, and OH radicals that can form peroxide in-situ.6, 36 The uranyl fluoride system explored by Kirkegard et al. used depleted uranium; thus, the radiationlevels and timescale do not align with peroxide generation through alpha-radiolysis.35 This recent study indicates that additional mechanisms for in-situ peroxide generation have not yet been delineated and have implications in the solid-state transformation of uranyl compounds and the formation of novel materials. During our synthetic efforts to develop novel metal-organic compounds, we discovered the complete transformation of a uranyl carboxylate coordination polymer (1; (Li(H2O)2(UO2) (C7H3NO4)(C4H5NO4) ∙ 2 H2O) into a uranyl peroxide coordination complex (2; (C5NH6)2[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 4 H2O) in the presence of light.

Additional

experiments in a related system revealed that a similar uranyl peroxide coordination complex (3; (C4N2H12)[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 2.75 H2O) could be formed in the absence of light. Herein, we describe the synthesis and chemical characterization of these uranyl peroxide materials and probe the mechanism of peroxide generation within this system. We propose that catalytic insitu peroxide generation from the co-solvent may be a new mechanism for peroxide formation and could be used in future efforts to design novel uranyl peroxide materials.

Experimental Methods Synthesis All solutions were prepared using Millipore water (18.2 MΩ) and chemicals purchased were used directly without further purification. Caution: (UO2)(NO3)2·6H2O contains radioactive 238U,

which is an α emitter, and like all radioactive materials must be handled with care. These


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experiments were conducted by trained personnel in a licensed research facility with special precautions taken toward the handling, monitoring, and disposal of radioactive materials. Li(H2O)2(UO2)(C7H3NO4)(C4H5NO4) ∙ 2 H2O (1): A 0.2 M stock solution of 2,6-pyridine dicarboxylate was generated by adding 0.334 g of the acid form of the ligand to 10.0 mL of water. Base (1.0 M LiOH) was added to deprotonate the carboxylate groups and promote dissolution. A 1.0 mL aliquot of the 2,6-pyridine dicarboxylate stock solution was combined with 1.0 mL of a 0.2 M uranyl nitrate (Flynn Scientific, Inc.) solution in a 20-mL glass scintillation vial. THF (2.0 mL) was also added to the vial and then the solution was thoroughly mixed. A 1.0 mL aliquot of a 0.2 M iminodiacetic acid solution was subsequently added and the pH adjusted to 4.4 using pyridine. The solution was also layered with 5.0 mL acetonitrile to aid in crystallization. After approximately 10 days, yellow acicular single crystals were formed with 75% yield based upon U. Crystals remaining in the mother liquor for a period of 1-2 months, changed color from yellow to orange (2; (C5NH6)2[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 4H2O) and formed in similar yields. Initial syntheses were performed on the benchtop in the presence of ambient light (benchtop and sunlight) and a portion of subsequent experiments was stored in a drawer to minimize photochemical reactions. (C4N2H12)[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]∙ 2.75 H2O (3): A 0.2 M stock solution 2,6pyridine dicarboxylic acid (Sigma-Aldrich) was prepared by adding 0.3342 g of 2,6-PDC to 10.0 mL of water and dissolved in solution using a 1.0 M piperazine solution. A 1.0 mL aliquot of the 2,6-PDC solution was added to a scintillation vial and mixed with 0.5 mL of a 0.2 M solution containing a metal salt (NiCl2, CoCl2, Zn(NO3)2, La(NO3)3, Al(NO3)3 and CrCl3). Uranyl nitrate (0.5 mL of 0.2 M solution) and three mL of THF was added to the solution and thoroughly homogenized using a vortex mixer. Vials were covered with perforated parafilm and allowed to



slowly evaporate at room temperature. After approximately two weeks, small, orange-yellow plates were formed in 30% yield based upon U. Experimental conditions were also varied in this synthesis. Light levels were changed by storing the vials in a different location (benchtop or drawer) and solvents (acetonitrile, ethyl ether, and DMSO) were varied. Peroxide formation conditions were also tested by separately adding 0.25 mL of FeCl2 and lowering the pH to 2.

Single crystal X-ray diffraction experiments High-quality single crystals were isolated from the mother liquor, coated in Infinium oil and mounted on a Nonius Kappa CCD single-crystal X-ray diffractometer equipped with Mo Kα radiation (λ = 0.7107 Å) and a low-temperature cryostat. Data were collected at 100 K with the Nonius Collect software package,37 and peak intensities were corrected for Lorentz, polarization, and background effects using the Bruker APEX II software.38 An empirical absorption correction was applied using the program SADABS, and the structure solution was determined by intrinsic phasing methods and refined on the basis of F2 for all unique data using the SHELXTL version 5 series of programs.39 U atoms were located by direct methods, and the O, N, and C atom positions were identified in the difference Fourier maps calculated following refinement of the partialstructure models. Hydrogen atom positions associated with the organic molecules were fixed using a riding model. Table 1 provides select crystallographic parameters for compounds 1-3. Intermolecular π-π stacking distances and angles within the crystal structures were analyzed using the Mercury software.40, 41 Additional information on bond lengths and angles can be found in Table S1 in the supporting information section and the crystallographic information files can be found on the Cambridge Structural Database by requesting numbers 1880171-1880173.


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Table 1. Select crystallographic parameters for compound 1-3. Formula Formula weight Space group a b c Alpha Beta Gamma Volume Z ρcalc (g/cm3) µ mm-1 F(000) Crystal size ( µm ) θ range for data collection (°) Limiting indices

Compound 1

Compound 2

Compound 3

Li(H2O)2 (UO2)(C7NO4H3)(C4NO4H6) ∙ 2 H2O

(C5NH6)2(UO2)2(μ-O2)(H2O)2(C7NO4H3)2 ∙ 4 H2O

(C4N2H12)(UO2)2(μ-O2)(H2O)2(C7NO4H3)2 ∙ 2.75 H2O

646.23 P21/c 6.956(2) 27.668(7) 9.961(2) 90 108.088(5) 90 1822.2(8) 4 2.356 8.985 1216 0.08 x 0.06 x 0.04 2.607 to 26.941

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Utilizing Autoxidation of Solvents To Promote the Formation of Uranyl

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