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

Abstract

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO(2)py(5))(KI(2)py(2))](n) (1) with the Schiff base ligands salen(2-), acacen(2-), and salophen(2-) (H(2)salen = N,N'-ethylene-bis(salicylideneimine), H(2)acacen = N,N'-ethylenebis(acetylacetoneimine), H(2)salophen = N,N'-phenylene-bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen(2-) in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, {[UO(2)(acacen)](4)[μ(8)-](2)[K([18]C-6)(py)](2)} (3) and {[UO(2)(acacen)](4)[μ(8)-]}⋅2 [K([222])(py)] (4), {[UO(2)(salophen)](4)[μ(8)-K](2)[μ(5)-KI](2)[(K([18]C-6)]}⋅2 [K([18]C-6)(thf)(2)]⋅2 I (5), and {[UO(2)(salen)(4)][μ(8)-Rb](2)[Rb([18]C-6)](2)} (9) ([222] = [222]cryptand, py = pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U(V)O(2)(salen)(py)][Cp*(2)Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K(+) and Rb(+), whereas Li(+) leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U(V)O(2)(salen)(py)][Cp*(2)Co] (Cp* = pentamethylcyclopentadienyl) with its U(VI) analogue yields the oxo-functionalized dimer [UO(2)(salen)(py)](2)[Cp*(2)Co] (8). The reaction of the {[UO(2)(salen)(4)][μ(8)-K](2)[K([18]C-6)](2)} tetramer with protons leads to disproportionation to U(IV) and U(VI) species and H(2)O confirming the crucial role of the proton in the U(V) disproportionation.

Heterometallic Fe2II–UV and Ni2II–UV Exchange‐Coupled Single‐Molecule Magnets: Effect of the 3 d Ion on the Magnetic Properties

“…In both complexes, the mean UO bond lengths lie in the range of the values previously observed for uranyl(V) complexes4b,c, 6e, 9–10 (2.06(7) Å in UFe 2 TPA and 2.05(1) Å in UMn 2 TPA 4c). The mean Fe II O yl distance (2.07(7) Å) is longer than that found in the only other heteronuclear uranyl(V)Fe II 2 complex (1.946(4) Å) reported 10d.…”

“…This {U V Mn II 2 } system appears to be an ideal and so far unique candidate for investigating the effects of the nature of the d block ion (through the replacement of the Mn II ion), and of the overall complex architecture (through ligand replacement) on the magnetic behavior of uranium‐based SMMs. However, due to the low stability of the UO 2 + species, the replacement of the Mn II ion is synthetically challenging as it may lead to disproportionation of uranyl(V) 9. Here we report the synthesis of a series of trinuclear 3d–5f {U V M II 2 } (M: Fe, Ni) complexes that were assembled through cation–cation interactions between the UO 2 + group and the respective 3d cations, complexed with different capping ligands.…”

Future Directions for Transuranic Single Molecule Magnets

“…It is likely that antiferromagnetic coupling is also active in Np 4 ; taking into account the strong axial anisotropy of the actinyl group, this would lead to a ground state with anapole-like arrangement of the spins but zero total magnetic moment, as for the U 3 complex discussed in [39]. On the other hand, it has been suggested that the reaction of a pentavalent uranium tetramer analogue to Np 4 with hexavalent uranyl can lead to oxo-group functionalization and to the formation of a heterovalent 3U V -1U VI complex [40]. If this could be achieved for Np 4 , the imbalance could result in a molecule with non-zero magnetic moment, with potential SMM behaviour.…”

New Reactivity of the Uranyl(VI) Ion

“…[65] CCIs are not limited to [UO 2 ] 2 + ···A C H T U N G T R E N N U N G [UO 2 ] 2 + and a number of other cations are found to interact with the uranyl oxygen. [66] In contrast, 2 ] displays a tetrameric core with Tshaped CCIs. In particular the salen (salen = N,N'-ethylenebis(salicylimine)) ligand has been shown to stabilise pentavalent uranyl (vide infra) and the dimeric [Cp* 2 Co]A C H T U N G T R E N N U N G [{UO 2 A C H T U N G T R E N N U N G (salen)(py)} 2 ] compound has been isolated and structurally characterised.…”