A Theoretical Study of the Inner-Sphere Disproportionation Reaction Mechanism of the Pentavalent Actinyl Ions

Abstract

The inner-sphere mechanisms of the disproportionation reactions of U(V), Np(V), and Pu(V) ions have been studied using a quantum mechanical approach. The U(V) disproportionation proceeds via the formation of a dimer (a cation-cation complex) followed by two successive protonations at the axial oxygens of the donor uranyl ion. Bond lengths and spin multiplicities indicate that electron transfer occurs after the first protonation. A solvent water molecule then breaks the complex into solvated U(OH)2(2+) and UO2(2+) ions. Pu(V) behaves similarly, but Np(V) appears not to follow this path. The observations from quantum modeling are consistent with existing experimental data on actinyl(V) disproportionation reactions.

Computational Redox Potential Predictions: Applications to Inorganic and Organic Aqueous Complexes, and Complexes Adsorbed to Mineral Surfaces

“…This disproportionation process takes place via the formation of a cation-cation intermediate and followed by protonation steps as computationally proposed by Steele et al [17]. Eventually this lead to the formation of actinyl(VI) and An(IV) species.…”

“…E(η = 0) = E Oxd (16) E(η = 1) = E Red (17) where η is the coupling or integration parameter, E Oxd is the energy of the oxidized species (where η = 0) and E Red is the energy of the reduced species (where η = 1). The derivative of the energy of the redox reaction with respect to the integration parameter, η can be written in terms of the vertical energy gap, where the vertical energy gap is defined as the difference between the E Oxd and E Red terms.…”

Reduction of Pentavalent Uranyl to U(IV) Facilitated by Oxo Functionalization

“…9 Nonetheless, this is a key mechanistic step in uranyl reduction, and its exploration would help validate the proposed mechanisms for U(IV) formation. 4,5 Herein we report that reVersible reduction of pentavalent uranyl to U(IV) can be achieved by functionalization of the two oxo ligands of the uranyl moiety.…”

Single‐Electron Uranyl Reduction by a Rare‐Earth Cation

“…[7,8] Usually the [UO 2 ] + cation decomposes by disproportionation, which is also a poorly understood process, but is important in the precipitation of uranium salts out of aqueous environments. [9,10] The disproportionation is suggested, by analogy with the transuranic metal oxo Lewis base behavior, to involve the formation of cation-cation interactions (CCIs) [11,12] in which the oxo groups ligate to adjacent actinyl centers forming diamond (A) or T-shaped (B) dimers or clusters which can then allow the transfer of protons and electrons between metals, such as in C. [13] We reported that the use of rigid, Pacman-shaped macrocycles can allow the isolation of heterobimetallic uranyltransition metal complexes that form an oxo interaction with the transition metal in the solid state and solution, [14] and how the inclusion of more than one metal cation alongside the uranyl cation led to isolable, stable pentavalent uranyl complexes with a covalently functionalized oxo group. [15] More recently, pentavalent uranyl complexes with Group 1 cation oxo-coordination, [16] and a doubly silylated complex [17] have been isolated.…”