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

Abstract

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 Eu II , Yb II , Sm II , and U III and hexanuclear clusters of La III and Ce III to study the impact of tris[2-(2-methoxyethoxy)ethyl]amine, a flexible acyclic analogue of the extensively studied 2.2.2cryptand, 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 Eu II 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-(2methoxyethoxy)ethyl]amine also deprotonates water molecules coordinated to trivalent triflate salts of f-block metal ions, enabling the isolation of hexanuclear clusters containing either La III or Ce III . The ligand was also found to bind more tightly to Yb II and U III 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 Cf II , Np III , and Pu III . 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.Article pubs.acs.org/IC

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

“…S3 †) and the reported acyclic U III TDA-1. 26 The differences in the optical properties of U III 1I 3 and U III 2I 3 in solution, specifically the large differences in shifts of color producing bands, indicate an influence of the change of ligand denticity from octadentate to heptadentate on 5f-6d orbital energy gaps.…”

“…Additional support for this suspicion is in our recent report that the flexible counterpart of 2.2.2-cryptand, tris[2-(2-methoxyethoxy)ethyl]amine (TDA-1), forms U IIIcontaining complexes with smaller coordination numbers (nine) compared to all reported 2.2.2-cryptates (with coordination numbers of ten). 26 This report of an acyclic ligand implies that trivalent uranium can be encapsulated by cryptands with smaller denticities than that of 2.2.2-cryptand. Therefore, based on the studies of Eu III cryptand chemistry and U III chemistry with acyclic TDA-1, we hypothesized that 2.2.1-cryptand is a better match for U III than 2.2.2-cryptand.…”

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…With softer lanthanide(II) ions (Sm, Eu, and Yb) and an array of different counteranions, a huge variety of coordination modes and geometries can be realized, including monomers and dimers, neutral species, and separated ion pairs. 38 Alongside this, La III , Ce III , and U III were successfully encapsulated in this new framework, and ultraviolet−visiblenear-infrared absorption and luminescence spectroscopy of these, along with the X-ray photoelectron spectroscopy data of the La III and Ce III complexes, adds valuable insight into the coordination chemistry and photophysics of these ions across diverse environments. While La III cations in isolation are somewhat spectroscopically uninteresting, at least at visible wavelengths of light, due to the [Xe] closed-shell electron configuration, the solution chemistry of La III is often comparable to that of Ce III and U III , due to similarities in their ionic radii, and so it acts as a useful diamagnetic surrogate for the exploration of new ligand frameworks for members at the start of the 4f and 5f rows.…”

“…Allen and co-workers described their work on tris­[2-(2-methoxyethoxy)­ethyl]­amine, an acyclic analogue to 2.2.2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]­hexacosane; or 2.2.2-crypt henceforth) designed to tame large and small rare-earth and actinide ions. With softer lanthanide­(II) ions (Sm, Eu, and Yb) and an array of different counteranions, a huge variety of coordination modes and geometries can be realized, including monomers and dimers, neutral species, and separated ion pairs . Alongside this, La III , Ce III , and U III were successfully encapsulated in this new framework, and ultraviolet–visible-near-infrared absorption and luminescence spectroscopy of these, along with the X-ray photoelectron spectroscopy data of the La III and Ce III complexes, adds valuable insight into the coordination chemistry and photophysics of these ions across diverse environments.…”