Stable Pentavalent Uranyl Species and Selective Assembly of a Polymetallic Mixed‐Valent Uranyl Complex by Cation–Cation Interactions

Abstract

Cation-cation interactions are a key feature of actinide chemistry. These interactions can be used in two areas that currently attract great interest, namely the expansion of felement supramolecular chemistry and the enhancement of magnetic interactions in actinide compounds. [1][2][3][4][5][6][7][8][9][10][11][12] Moreover, oligomeric cation-cation species that present mutually coordinated actinyl ions are likely to play a crucial role in nuclear waste reprocessing and in the migration of radioactive actinides in the environment. [1] Cation-cation interactions are known to be important in neptunyl(V) structural chemistry, [13] but are more rarely found in uranyl(VI) compounds because of the lower basicity of the UO 2 2+ oxygen atoms. Dimeric compounds formed through the mutual binding of pentavalent uranyl(V) ions have been proposed as intermediates in the disproportionation of pentavalent uranyl ions to UO 2 2+ and U IV species.[14]As a result, bulky ligands have been used in the past few years to disfavor cation-cation interactions and allow the synthesis of rare UO 2 + complexes, [15][16][17][18][19][20][21] which have been the subject of two recent reviews.[ [8] The presence of antiferromagnetic coupling between the oxo-bridged uranium centers was unambiguously demonstrated for the dimetallic complex but is less evident for the tetrametallic complex. [8] The decomposition of these polymetallic complexes of pentavalent uranyl ions to UO 2 2+ and U IV species starts rapidly after dissolution in organic solvents and is accelerated by traces of water. From these reactivity studies, it occurred to us that the stability of these polymeric systems could possibly be modulated by fine-tuning the electronic and steric properties of the ligand and coordinating cation. Herein, we report the first example of a UO 2 + ···UO 2 + complex that is highly stable in organic solvents and, significantly, is stable toward hydrolysis. We also describe the selective synthesis and the structure of the first mixed-valent UO 2 + ···UO 2 2+ molecular complex, which provides a rare example of functionalization of the U VI =O group. [20,22] By breaking away from the current trend of using steric bulk to prevent dimer formation and the associated disproportionation of UO 2 + complexes, we show that the non-bulky Schiff base ligand salen 2À (N,N'-bis(salicylidene)ethylenediamine) can stabilize pentavalent uranyl ions through the formation of a highly stable cation-cation complex. Moreover, we demonstrate that the resulting tetrameric uranyl(V) complex exhibits unambiguous antiferromagnetic coupling between the uranium centers.The reaction of the recently reported UO 2 + complex [(UO 2 Py 5 )(KI 2 Py 2 )] n (2; Py = pyridine) [23] with salenK 2 in pyridine led to the formation of the complex of pentavalent uranyl 3 as a violet powder that was insoluble in pyridine. The elemental analysis of 3 indicates the presence of a complex of general formula [(UO 2 )(salen)K(Py)]·1.4 KI, which most likely has a solid-state polymeric structure. Compound ...

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.…”

A new uranyl(VI) complex of rigid V-shaped carboxylate ligand: Synthesis, structure and properties

“…Rigid carboxylate ligands were used widely in assembly with transition ions or lanthanides, because the rigidity of ligand is helpful to construct large rings or channels and the complicated coordination modes of carboxyl are good for structural diversity [2]. However, the complexes containing uranium (VI) centers have not been reported extensively [3], though they presented not only the diversity of their structures [4][5][6][7], but also potential applications in ion-exchange, magnetic property, and photo-catalysis [8][9][10]. The structures and properties of complexes formed by rigid carboxylate ligand and uranium ions are worthy of researching.…”

Single‐Electron Uranyl Reduction by a Rare‐Earth Cation

“…[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.…”