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. [21] In some of the reported UO 2 + systems, the ligand bulk does not prevent cation-cation interactions and results in decomposition. However, only two examples of UO 2 + ···UO 2 + intermediate complexes have been reported to date: the tetrameric [UO 2 (dbm) 2 ] 4 [K 4 (CH 3 CN) 4 ] (1) and the dimeric [{UO 2 (dbm) 2 K(18C6)} 2 ] (dbm À = dibenzoylmethanate, 18C6 = [18]crown-6) complexes. [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 proper-ties 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 recentl...

Cation–Cation Complexes of Pentavalent Uranyl: From Disproportionation Intermediates to Stable Clusters

“…Synthesis and solution structure: Three tetranuclear complexes of pentavalent uranyl were prepared by using the Schiff base ligands salen 2À , acacen 2À , and salophen 2À , which have different geometric and electronic characteristics. The (2) was prepared in 70 % yield as previously reported [35] from the reaction of the uranyl(V) precursor 1 with one equivalent of salenK 2 in the presence of [18]crown-6 ether in pyridine. We have previously reported that the reaction of 1 with salenK 2 in the absence of [18]C-6 yields [35] an insoluble compound of formula [UO 2 A C H T U N G T R E N N U N G (salen)K(py)]·1.4 KI, probably of polymeric structure, which can be dissolved in pyridine solutions containing [18]C-6 to afford 2.…”

“…The stable UO 2 [18]C-6)(py)] (2) ([18]C-6 = [18]crown-6) complex can be prepared from the reaction of [(UO 2 py 5 )-A C H T U N G T R E N N U N G (KI 2 py 2 )] n (1) with the Schiff base ligand salen 2À (H 2 salen = N,N'-ethylene-bis(salicylideneimine)). [35] Complex 2 is stable with respect to the disproportionation reaction even in the presence of stoichiometric amounts of water, but can be easily oxidized to yield a 3 U V /1 U VI species providing a route to mixed-valent assemblies. [35] Cation-cation interactions are an important structural feature in the chemistry of neptunyl(V) and plutonyl(V) [36][37][38][39][40][41][42][43] , which is likely to be responsible for the formation of insoluble aggregates and complex mixtures in nuclear waste, resulting in technological problems in the reA C H T U N G T R E N N U N G processing and storage of nuclear waste.…”

“…[35] Complex 2 is stable with respect to the disproportionation reaction even in the presence of stoichiometric amounts of water, but can be easily oxidized to yield a 3 U V /1 U VI species providing a route to mixed-valent assemblies. [35] Cation-cation interactions are an important structural feature in the chemistry of neptunyl(V) and plutonyl(V) [36][37][38][39][40][41][42][43] , which is likely to be responsible for the formation of insoluble aggregates and complex mixtures in nuclear waste, resulting in technological problems in the reA C H T U N G T R E N N U N G processing and storage of nuclear waste. [1,44] The anticipated role of cationcation complexes as intermediates in the disproportionation of pentavalent uranyl seemed to rule out the possibility of developing such chemistry for U V .…”

New Reactivity of the Uranyl(VI) Ion

“…Recently a series of mixed-valent [UO 2 ] 2 + ···A C H T U N G T R E N N U N G [UO 2 ] + compounds have been structurally characterised and these can exist as dimers, trimers or tetramers. [67] A bimetallic and trimetallic species supported by the [CH(Ph 2 PNSiMe 3 ) 2 ] À ligand also displays CCIs ( Figure 5). [66] The bond lengths allow the unambiguous assignment of the oxidation states and the presence of a T-shaped CCI that is stable in solution for months.…”

“…The bond lengths suggest a localised U VI U V 3 structure. [67] A bimetallic and trimetallic species supported by the [CH(Ph 2 PNSiMe 3 ) 2 ] À ligand also displays CCIs ( Figure 5). [24] Examples of the heavier actinyl(V) (Np, [68] Pu, [69] Am [70] ) forming CCIs with [UO 2 ] 2 + are known, and these persist in solution.…”