Stabilization of U(III) to Oxidation and Hydrolysis by Encapsulation Using 2.2.2-Cryptand

Expanding the Coordination of f-Block Metals with Tris[2-(2-methoxyethoxy)ethyl]amine: From Molecular Complexes to Cage-like Structures

Low-valent f-block metals have intrinsic luminescence, electrochemical, and magnetic properties that are modulated with ligands, causing the coordination chemistry of these metals to be imperative to generating critical insights needed to impact modern applications. To this end, we synthesized and characterized a series of twenty-seven complexes of f-metal ions including EuII, YbII, SmII, and UIII and hexanuclear clusters of LaIII and CeIII to study the impact of tris[2-(2-methoxyethoxy)ethyl]amine, a flexible acyclic analogue of the extensively studied 2.2.2-cryptand, on the coordination chemistry and photophysical properties of low-valent f-block metals. We demonstrate that the flexibility of the ligand enables luminescence tunability over a greater range than analogous cryptates of EuII in solution. Furthermore, the ligand also displays a variety of binding modes to f-block metals in the solid state that are inaccessible to cryptates of low-valent f-block metals. In addition to serving as a ligand for f-block metals of various sizes and oxidation states, tris[2-(2-methoxyethoxy)ethyl]amine also deprotonates water molecules coordinated to trivalent triflate salts of f-block metal ions, enabling the isolation of hexanuclear clusters containing either LaIII or CeIII. The ligand was also found to bind more tightly to YbII and UIII in the solid state compared to 2.2.2-cryptand, suggesting that it can play a role in the isolation of other low-valent f-block metals such CfII, NpIII, and PuIII. We expect that our findings will inspire applications of tris[2-(2-methoxyethoxy)ethyl]amine in the design of light-emitting diodes and the synthesis of extremely reducing divalent f-block metal complexes that are of interest for a wide range of applications.

Comparison of the structural, electrochemical, and spectroscopic properties of two cryptates of trivalent uranium

We describe a study of the influence of cryptand denticity on the structural, electronic, and electrochemical properties of UIII-containing cryptates. Two cryptands (2.2.2 and 2.2.1) are reported. The cryptand with the smaller denticity leads to negative electrochemical potentials and shorter bond lengths that are consistent with a better fit for UIII than the larger cryptand. These studies provide insight into the rational design of cryptand-based ligands for trivalent uranium.

A trinuclear metallasilsesquioxane of uranium(III)

The silsesquioxane ligand (iBu)7Si7O9(OH)3 (iBuPOSSH3) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U3(iBuPOSS)3]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

Synthesis of Trimethyltriazacyclohexane (Me3tach) Sandwich Complexes of Uranium, Neptunium, and Plutonium Triiodides: (Me3tach)2AnI3

1,3,5-Trimethyl-1,3,5-triazacyclohexane (Me₃tach) readily complexes uranium triiodide to form (Me₃tach)₂UI₃. The complex is soluble in THF and arenes and can function as a source of UI₃ to form organometallic U(III) complexes. When dissolved in pyridine (py), (Me₃tach)₂UI₃ forms (Me₃tach)UI₃(py)₂. A related complex with the larger 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn) ligand, namely (Me₃tacn)UI₃(THF), was synthesized for comparison. Since X-ray quality crystals of (Me₃tach)₂UI₃ can be synthesized in high yield even with small-scale reactions, the system is ideal for extension to transuranium elements. Accordingly, the neptunium and plutonium complexes (Me₃tach)₂NpI₃ and (Me₃tach)₂PuI₃ were synthesized in an analogous manner from NpI₃(THF)₄ and PuI₃(THF)₄, respectively.

2.2.2-Cryptand complexes of neptunium(III) and plutonium(III)

New coordination environments are reported for Np(III) and Pu(III) based on pilot studies of U(III) in 2.2.2-cryptand (crypt). The U(III)-in-crypt complex, [U(crypt)I₂][I], obtained from the reaction between UI₃ and crypt, is treated with Me₃SiOTf (OTf = O₃SCF₃) in benzene to form the [U(crypt)(OTf)₂][OTf] complex. Similarly, the isomorphous Np(III) and Pu(III) complexes were obtained similarly starting from [AnI₃(THF)₄]. All three complexes (1-An; An = U, Np, Pu) contain an encapsulated actinide in a THF-soluble complex. Absorption spectroscopy and DFT calculations are consistent with 5f³ U(III), 5f⁴ Np(III), and 5f⁵ Pu(III) electron configurations.