Europium(II/III) coordination chemistry toward applications

Europium is an f-block metal with two easily accessible oxidation states (+2 and +3) that have vastly different magnetic and optical properties from each other. These properties are tunable using coordination chemistry and are useful in a variety of applications, including magnetic resonance imaging, luminescence, and catalysis. This review describes important aspects of coordination chemistry of Eu from the Allen Research Group and others, how ligand design has tuned the properties of Eu ions, and how those properties are relevant to specific applications. The review begins with an introduction to the coordination chemistry of divalent and trivalent Eu followed by examples of how the coordination chemistry of Eu has made contributions to magnetic resonance imaging, luminescence, catalysis, and separations. The article concludes with a brief outlook on future opportunities in the field.

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.

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.

Rational Design of High-Relaxivity EuII -Based Contrast Agents for Magnetic Resonance Imaging of Low-Oxygen Environments

Metal-based contrast agents for magnetic resonance imaging present a promising avenue to image hypoxia. EuII -based contrast agents have a unique biologically relevant redox couple, EuII/III , that distinguishes this metal for use in hypoxia imaging. To that end, we investigated a strategy to enhance the contrast-enhancing capabilities of EuII -based cryptates in magnetic resonance imaging by controlling the rotational dynamics. Two dimetallic, EuII -containing cryptates were synthesized to test the efficacy of rigid versus flexible coupling strategies. A flexible strategy to dimerization led to a modest (114 %) increase in contrast enhancement per Eu ion (60 MHz, 298 K), but a rigid linking strategy led to an excellent (186 %) increase in contrast enhancement despite this compound's having the smaller molecular mass of the two dimetallic complexes. We envision the rigid linking strategy to be useful in the future design of potent EuII -based contrast agents for magnetic resonance imaging.

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

The electrochemical properties of U(III)-in-crypt (crypt = 2.2.2-cryptand) were examined in dimethylformamide (DMF) and acetonitrile (MeCN) to determine the oxidative stability offered by crypt as a ligand. Cyclic voltammetry revealed a U(III)/U(IV) irreversible oxidation at E_PA= -0.49 V (vs Fe(C₅H₅)₂^+/0) in DMF and at E_PA= -0.31 V (vs Fe(C₅H₅)₂^+/0) in MeCN. The electrochemistry of U(III)-in-crypt complexes in the presence of water was also examined. These studies are supported by crystallographically characterized examples of U(III)-in-crypt complexes as DMF, MeCN, and water adducts.

Synthesis of LnII -in-Cryptand Complexes by Chemical Reduction of LnIII -in-Cryptand Precursors: Isolation of a NdII -in-Cryptand Complex

Lanthanide triflates have been used to incorporate Nd^III and Sm^III ions into the 2.2.2-cryptand ligand (crypt) to explore their reductive chemistry. The Ln(OTf)₃ complexes (Ln=Nd, Sm; OTf=SO₃ CF₃ ) react with crypt in THF to form the THF-soluble complexes [Ln^III (crypt)(OTf)₂ ][OTf] with two triflates bound to the metal encapsulated in the crypt. Reduction of these Ln^III -in-crypt complexes using KC₈ in THF forms the neutral Ln^II -in-crypt triflate complexes [Ln^II (crypt)(OTf)₂ ]. DFT calculations on [Nd^II (crypt)]²⁺ ], the first Nd^II cryptand complex, assign a 4f⁴ electron configuration to this ion.

Spectroscopic and Electrochemical Trends in Divalent Lanthanides through Modulation of Coordination Environment

Due to the importance of both visible-light luminescence and lanthanides in modern society, the influence of the ligand environment on complexes of Yb^II were studied and compared with analogous complexes of Eu^II. Four ligands with systematically varied electronic and steric characteristics were used to probe the coordination environment and electronic and redox properties of the corresponding Yb^II-containing complexes. Strong-field nitrogenous donors gave rise to bathochromic shifts, leading to visible-light absorption by Yb^II. Trends in properties across the series of Yb^II-containing complexes were compared to trends reported for the analogous Eu^II-containing complexes, revealing the translatability of coordination environment effects across the divalent lanthanide series. These studies provide valuable information regarding the behavior of small and medium-sized divalent lanthanides outside of the solid state.

2.2.2-Cryptand as a bidentate ligand in rare-earth metal chemistry

Crystal structures demonstrate that 2.2.2-cryptand can function as a κ²-O,O′ bidentate ligand to rare-earth metal ions.

Facile Encapsulation of Ln(II) Ions into Cryptate Complexes from LnI2(THF)2 Precursors (Ln = Sm, Eu, Yb)

The reactivity of LnI₂(THF)₂ (Ln = Sm, Eu, Yb; THF = tetrahydrofuran) with 2.2.2-cryptand (crypt) was explored to see if these readily accessible precursors could provide new examples of lanthanide-in-crypt complexes. The crystallographically characterized Ln(II)-in-crypt complexes [Ln(crypt)(DMF)₂][I]₂ (Ln = Sm, Eu) and [Yb(crypt)(DMF)][I]₂ (DMF = dimethylformamide) were synthesized by reacting LnI₂(THF)₂ (Ln = Sm, Eu, Yb) with crypt in THF and recrystallizing from DMF. Crystallographic data were also obtained on the Ln(II)-in-crypt (Ln = Sm, Eu) complexes [Ln(crypt)(DMF)₂][BPh₄]₂, which were synthesized by addition of 2 equiv of NaBPh₄ to [Ln(crypt)(DMF)₂][I]₂.

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex

The reaction of SmI₂ with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu₄N][BPh₄], allows for the isolation of [Sm^II(DB30C10)][BPh₄]₂ as bright-red crystals in good yield. Exposure of [Sm(DB30C10)]²⁺ to solvents containing trace water results in the conversion to the dinuclear Sm^III complex, Sm₂(DB30C10)(OH)₂I₄. Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with Sm^II to a twisted conformation with Sm^III. The optical properties of [Sm^II(DB30C10)][BPh₄]₂ exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Examination of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) shows that the ground state of Sm^II in [Sm^II(DB30C10)]²⁺ is a 4f⁶ state with low-lying 4f⁵5d¹ states, where the latter states have been lowered in energy by ∼12 000 cm^-1 with respect to the free ion. The decacoordination of the Sm^II cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of Sm^II.

Synthesis of uranium-in-cryptand complexes

The facile encapsulation of U(iii) and La(iii) by 2.2.2-cryptand (crypt) using simple starting materials is described. Addition of crypt to UI3 and LaCl3 forms the crystallographically-characterizable complexes, [U(crypt)I2]I and [La(crypt)Cl2]Cl. In the presence of water, the U(iii)-aquo adducts, [U(crypt)I(OH2)][I]2 and [U(crypt)I(OH2)][I][BPh4], can be isolated.

Synthesis and reactivity of rare-earth metal phosphaethynolates

Over the course of the last six years, research on the synthesis and reactivity of molecular metal phosphaketenes (M-PCO) has gained increasing attention. However, lanthanide complexes of the heavier group 15 cyanate analogue PCO^- have not been investigated so far. Herein we present exemplar studies on the nature and reactivity of rare-earth phosphaethynolato-complexes using three characteristic representatives of the rare-earth metals: Y, Nd and Sm. Our investigations comprise both +2 and +3 redox states, and one defined amidinate-based ligand set, as well as novel reaction pathways in the presence of the sequestering agents 18-crown-6 and 2,2,2-crypt.

Photophysical characterization of a highly luminescent divalent-europium-containing azacryptate

We report a new luminescent EuII-containing complex. The complex is excited with visible light, leading to emission centered at 447 nm with a lifetime of 1.25 μs. Computational studies suggest that the steric bulk of the ligand is a major factor influencing the wavelength of emission.

Activation of carbon suboxide (C3O2) by U(iii) to form a cyclobutane-1,3-dione ring

[U(η⁵-C₅H₄SiMe₃)₃] reductively couples three C₃O₂ molecules to form a tetranuclear complex with a central cyclobutane-1,3-dione ring, via an intermediate bridging ketene complex.