Using molten salts to probe outer-coordination sphere effects on lanthanide(<scp>iii</scp>)/(<scp>ii</scp>) electron-transfer reactions

MacInnes, Molly M.; Jones, Zachary R.; Li, Bo; Anderson, Nickolas H.; Batista, Enrique R.; DiMucci, Ida M.; Eiroa-Lledo, Cecilia; Knope, Karah E.; Livshits, Maksim Y.; Kozimor, Stosh A.; Mocko, Veronika; Pace, Kristen A.; Rocha, Francisca R.; Stein, Benjamin W.; Wacker, Jennifer N.; Yang, Ping

doi:10.1039/d1dt02708e

Cited by 11 publications

(24 citation statements)

References 81 publications

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“…This trend follows the order of increasing polarization strength of the cation and suggests that the more polarizing cations are stabilizing the lower valence U III state. A similar trend has been observed in high-temperature molten salt experiments in which Ln II were preferentially stabilized by the addition of highly polarizing cations such as Li + . Thus, there is clearly a dependence of the redox stability of dissolved anionic uranium complexes on the RTIL cation.…”

Section: Resultssupporting

confidence: 75%

“…The detected stabilization of lower oxidation states by more polarizing molten salt cations has been similarly observed for lanthanide complexes in high-temperature chloride salts by MacInnes et al, who described the source of the phenomenon to be charge transfer over extended Ln−Cl−M n+ networks (where M n+ indicates a Group I or II metal cation). 14 The more polarizing solvent cations remove electron density from the metal center to stabilize lower oxidation states that are comparatively "electron-rich." The stabilization is then reflected as an increase in the thermodynamic potential for reduction of the higher oxidation state, as is observed in the experimental U and Np couples in RTIL chlorides from this work.…”

Section: ■ Discussionmentioning

confidence: 99%

“…A similar trend has been observed in high-temperature molten salt experiments in which Ln II were preferentially stabilized by the addition of highly polarizing cations such as Li + . 14 Thus, there is clearly a dependence of the redox stability of dissolved anionic uranium complexes on the RTIL cation.…”

Section: Relative Polarization Strength Of Rtil Structuresmentioning

confidence: 99%

“…This indicates an influence of the molten salt or RTIL cation structure on the stability of dissolved complexes and has been observed for anionic lanthanide complexes in high-temperature chloride molten salts, as well. 14 Moreover, these studies have demonstrated that the UCl 6 n− and NpCl 6 n− species are both optically active and electroactive, demonstrating a simple route for studying their complexation and electron-transfer chemistry in RTILs.…”

Section: ■ Introductionmentioning

confidence: 98%

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Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Unger

Jensen

2023

Inorg. Chem.

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To expand the breadth of knowledge of actinide chemistry in molten chloride salts, chloride room-temperature ionic liquids (RTILs) were used to probe the influence of RTIL cation on second-sphere coordination for anionic complexes of uranium and neptunium. Six chloride RTILs were studied to represent a range of cation polarizing strength, size, and charge density to correlate changes in the complex geometry and redox behaviors. Optical spectroscopy indicated that actinides were dissolved at equilibrium as octahedral AnCl 6 2− (An = U, Np) as is observed in comparable high-temperature molten chloride salts. These anionic metal complexes were sensitive to the RTIL cation polarizing strength and hydrogen bond donating strength and displayed varying levels of fine structure and hypersensitive transition splitting depending on the degree of perturbation to the complex's coordination symmetry. Furthermore, voltammetry experiments on the redox-active complexes indicated a stabilizing effect on lower valence actinide oxidation states by more polarizing RTIL cations whereby the measured E 1/2 potentials for both U(IV/III) and Np(IV/III) couples shifted positively by about 600 mV across the different systems. These results indicate that more polarizing RTIL cations inductively remove electron density from the actinide metal center over An−Cl−Cation bond networks to stabilize electron-deficient oxidation states. Electron-transfer kinetics were generally much slower than in molten chloride systems, partially due to lower temperatures and higher viscosities in the working systems and showed diffusion coefficients of 1.8 × 10 −8 to 6.4 × 10 −8 cm 2 s −1 for U IV and 4.4 × 10 −8 to 8.3 × 10 −8 cm 2 s −1 for Np IV . We also detect a one-electron oxidation of Np IV that we have attributed to the formation of Np V as NpCl 6− . Overall, we observe a coordination environment for the anionic actinide complexes that is susceptible to even small changes in RTIL cation properties.

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Section: Resultssupporting

confidence: 75%

Section: ■ Discussionmentioning

confidence: 99%

Section: Relative Polarization Strength Of Rtil Structuresmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 98%

See 2 more Smart Citations

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Unger

Jensen

2023

Inorg. Chem.

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“…It is well understood that there are a number of factors that govern speciation, including the metal ion oxidation state, hydrolysis and condensation, identity of the complexing ligands, and solution conditions (e.g., pH, ionic strength, and temperature) . In addition to these considerations, there is growing evidence that outer sphere (OS) interactions exhibit considerable effects on the chemical behavior of metal ions. − …”

Section: Introductionmentioning

confidence: 99%

Th(IV) Bromide Complexes: A Homoleptic Aqua Ion and a Novel Th(H₂O)₄Br₄ Structural Unit

Blanes-Díaz

Stewart

Vasiliu

et al. 2022

Crystal Growth & Design

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The synthesis and structural chemistry of two tetravalent thorium compounds precipitated from acidic bromide solutions are described. One of the phases, [Th(H2O)10]Br4 (1), has been previously reported. The other phase, [Th(H2O)4Br4](HPy·Br)2 (2), is a novel compound and exhibits Th–Br coordination. While complexes with such Th–Br bonding have been observed in the solid state, most have been isolated from nonaqueous solutions. Notably, the two compounds crystallize from HBr(aq) solutions containing pyridinium (HPy+) counterions; in the absence of HPy+, only 1 is observed. The reaction energies of stepwise bromide addition were predicted using electronic structure theory. These calculations highlighted an overall thermodynamic driving force for Th–Br complexation up to the addition of four bromide ions. Electrostatic potential (ESP) surfaces were calculated to better understand the role of HPy+ in the isolation of 2. Consistent with our previous work, the ESP surfaces point to the importance of the pK a of the organic cation in determining the distribution of the Br– ligands about the Th metal center.

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Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

Kravchuk,

Augustine,

Rajapaksha

et al. 2024

Chemistry A European J

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Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O4(OH)2]3‐ during ozonolysis of aqueous MOH (M = Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co‐precipitating counter‐cation Li+/Na+/K+. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O2(OH)4]3‐ with the hydroxide radical to form [Np(VI)O2(OH)4]2‐, followed by an additional oxidation with HO● to create [Np(VII)O4(OH)2]3‐.

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Using molten salts to probe outer-coordination sphere effects on lanthanide(iii)/(ii) electron-transfer reactions

Abstract: Molten salt matrices were used to evaluate outer-coordination sphere effects on lanthanide redox chemistry. Results were rationalized by correlating the polarization power of the outer-sphere cation with shifts in the Ln3+/Ln2+ reduction potentials.

Cited by 11 publications

References 81 publications

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Th(IV) Bromide Complexes: A Homoleptic Aqua Ion and a Novel Th(H₂O)₄Br₄ Structural Unit

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

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Using molten salts to probe outer-coordination sphere effects on lanthanide(iii)/(ii) electron-transfer reactions

Abstract: Molten salt matrices were used to evaluate outer-coordination sphere effects on lanthanide redox chemistry. Results were rationalized by correlating the polarization power of the outer-sphere cation with shifts in the Ln3+/Ln2+ reduction potentials.

Cited by 11 publications

References 81 publications

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids

Th(IV) Bromide Complexes: A Homoleptic Aqua Ion and a Novel Th(H2O)4Br4 Structural Unit

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

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Th(IV) Bromide Complexes: A Homoleptic Aqua Ion and a Novel Th(H₂O)₄Br₄ Structural Unit