Single‐Electron Uranyl Reduction by a Rare‐Earth Cation

Abstract

Unlike their transition-metal analogues, the oxo groups of the uranyl dication, [UO 2 ] 2+ , which has a linear geometry and short, strong UÀO bonds are commonly considered inert. [1] Very little Lewis base character has been demonstrated for the uranyl oxo groups, [2,3] which makes them poor models for the heavier, highly radioactive transuranic actinyl cations such as neptunyl [NpO 2 ] n+ (n = 1, 2). [4,5] The heavier actinyls are important components in nuclear waste and demonstrate oxo basicity that can give rise to poorly understood cluster formation and problems in nuclear waste PUREX separation processes.[6] However, it has been shown recently that the more Lewis basic, pentavalent uranyl cation, [UO 2 ] + , can be stabilized indefinitely using suitable equatorial-binding ligands and anaerobic conditions. [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. Here, we report the first uranyl-4f metal interaction, prepared by either standard, or sterically induced reduction procedures, and demonstrate strong magnetic coupling between the 4f and 5f electrons.The reaction between the divalent samarium silylamide [Sm(THF) 2 {N(SiMe 3 ) 2 } 2 ] and the uranyl Pacman complex [UO 2 (py)(H 2 L)] (1) in pyridine resulted in the deposition of the new uranyl-samarium complex [UO 2 Sm(py) 2 (L)] 2 (2) as a very poorly soluble, thermally stable, red crystalline powder in good yield (Scheme 1), and containing crystals suitable for single-crystal X-ray diffraction studies (Figure 1).The 1 H NMR spectrum of 2 in [D 5 ]pyridine reveals the presence of paramagnetically shifted resonances between d = 12.4 and À21.5 ppm, the number and integrals of which are consistent with the retention of a wedged, Pacman structure in solution of C s symmetry. In the solid state, the molecular structure shows that 2 is dimeric (Figure 1) and the unit cell contains two similar molecules. Focusing on one molecule of 2, both the uranium and samarium centers are sevencoordinate w...

Oxo-Functionalization and Reduction of the Uranyl Ion through Lanthanide-Element Bond Homolysis: Synthetic, Structural, and Bonding Analysis of a Series of Singly Reduced Uranyl–Rare Earth 5f1-4fn Complexes

“…The X-ray crystallographic analyses for the series of compounds 2-Ln showed that they are all dimers in the solid state with the asymmetric unit containing two similar molecules of diamond-shaped UO 2 dimers in most cases ( Figure 1). They are isostructural to [{UO 2 Ln(py) 2 (L)} 2 ] (M = Sm, Y), both of which were previously communicated by us, 19 in which both the uranium and lanthanide centers are seven-coordinate with approximate pentagonal bipyramidal geometry but with a greater distortion at the lanthanide. Four equatorial N atoms from the N 4 -donor set of the macrocycle contribute to each metal coordination sphere.…”

“…19 First, we fitted the d.c. magnetic susceptibility measured for 3-Li at different values of the applied magnetic field by diagonalizing the Zeeman Hamiltonian…”

Oxo Group Protonation and Silylation of Pentavalent Uranyl Pacman Complexes

“…[1,2] For example, new routes to the singly reduced, pentavalent uranyl have been developed and have facilitated fascinating new Lewis acidbase interactions between the oxo groups and alkali metals, [3,4] boranes, [5][6][7] transition metals, [8] rare earth metals, [9] and other uranyl cations. [10] In some cases it is evident that oxo group functionalization can modify the U VI / U V reduction potential such that reduction is facilitated.…”

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The chemical reactivity of the oxo group of the uranyl dication, [UO 2 ] 2+ , the most prevalent aqueous and environmental form of uranium and by far the most chemically inert, has been transformed in recent years. [1,2] For example, new routes to the singly reduced, pentavalent uranyl have been developed and have facilitated fascinating new Lewis acidbase interactions between the oxo groups and alkali metals, [3, 4] boranes, [5][6][7] transition metals, [8] rare earth metals, [9] and other uranyl cations. [10] In some cases it is evident that oxo group functionalization can modify the U VI / U V reduction potential such that reduction is facilitated.

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Spectroscopic and magnetic investigations of actinide‐based nanomagnets

“…Pentavalent uranium seems to retain its Ising ground state, and one U V -Sm III complex displays a very strong superexchange coupling between the 5f 1 and the 4f 5 centers, which may sound surprising because of the largely reduced radial extent of the 4f shell (with respect to 5f and especially 3d) but might perhaps be explained considering the relative instability of the pentavalent state of uranium and the close divalent state of samarium. [18] Conversely, the dimeric (U V -Dy III ) 2 complex exhibits a butterfly-shaped hysteresis cycle, behaving as a SMM up to 3 K. [19] The , so that the dominant exchange interaction, albeit antiferromagnetic, actually favors a "parallel" alignment of their two spins. [20] The 5f 3 Configuration: U III , Np…”