Europium(II/III) coordination chemistry toward applications

Abstract

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.

Fig. 1. Energy levels for Eu^III and Eu^II. All levels for each ion are shown directly over the ion label. For clarity, subsets of levels are projected to the right of the full diagram and labeled with term symbols. Projected subsets with black lines represent 4f states, and red lines represent 4f5d states. Values for the levels were obtained from previous reports.

Fig. 2. Selected ligands relevant to Eu-based contrast agents for MRI.

Fig. 3. T ₁-weighted MR image of a multiwell plate of solutions of Eu^II1–3, Eu^II8, Eu^II12–15 and Gd^III1–3, Gd^III8, Gd^III12–15 at ambient temperature. Label a represents phosphate-buffered saline, b denotes GdCl₃, and c represents EuCl₂. Adapted from Corbin et al., copyright 2018, with permission from Elsevier.

Fig. 4. T ₁-weighted in vivo sagittal plane images of a 4T1 tumor injected with Eu^II4. Image (a) is before the injection and (b) is 3 min, (c) is 20 min, and (d) is 120 min post-intratumoral injection. Image (e) is the difference between images (d) and (a). Image (f) is a hematoxylin- and eosin-stained slice of the tumor corresponding to images a–e. Figure (g) is an overlaid image of images (e) and (f). Used with permission of John Wiley & Sons, from A Eu^II-Containing Cryptate as a Redox Sensor in Magnetic Resonance Imaging of Living Tissue, L. A. Ekanger, L. A. Polin, Y. Shen, M. E. Haacke, P. D. Martin, M. J. Allen, Volume 54, Copyright 2015; permission conveyed through Copyright Clearance Center, Inc.

Fig. 5. Cartoon representation of Eu^II17 in lecithin/perfluorocarbon emulsion. The orange represents a perfluorocarbon mixture surrounding Eu^III17, the grey circle, and Eu^II17, the red circle.

Fig. 6. Image (A) is a T₁-weighted image, and image (B) is a CEST difference image. For both (A) and (B), spot 1 represents Eu^III8, spot 2 represents Eu^II8, spot 3 represents Eu^III8 from the oxidation of Eu^II8, and spot 4 represents water. Reprinted with permission from L. A. Ekanger, D. R. Mills, M. M. Ali, L. A. Polin, Y. Shen, E. M. Haacke and M. J. Allen, Inorg. Chem., 2016, 55, 9981. Copyright 2016 American Chemical Society.

Fig. 7. Ligands used in luminescence-based studies.

Fig. 8. Calculated d-orbital splitting of Eu^II1 and Eu^II12. Adapted from Corbin et al., copyright 2018, with permission from Elsevier.

Fig. 9. Emission spectra of Eu^II1 (top) and Eu^II28 (bottom). Color code for 1: red = [Eu(1)(NO₃)₃]I_0.83[NO₃]_0.17; green = Eu(1)(OTf)₂; dark-purple = [Eu(1)I]I; and cyan = Eu(1)(NCS)₂. Color code for 28: blue = Eu28(NO₃)₂; black = Eu28(OTf)₂; purple = [Eu28I(CH₃OH)]I; brown = [Eu28Br(CH₃OH)]Br; and grey = (CH₃OH)28(Eu(μ-Cl)(ZnCl₃)₃). Reprinted with permission from S. S. Bokouende, D. N. Kulasekara, S. A. Worku, C. L. Ward, A. B. Kajjam, J. C. Lutter and M. J. Allen, Inorg. Chem., 2024, 63, 9434. Copyright 2023 American Chemical Society.

Fig. 10. Ligands used for catalysis-based studies.

Fig. 11. Proposed transition state for enantioselective catalyst and substrate binding for Mukaiyama–aldol reaction. Reprinted with permission from Y. Mei, P. Dissanayake and M. J. Allen, J. Am. Chem. Soc., 2010, 132, 12871. Copyright 2010 American Chemical Society.

Fig. 12. Proposed photoredox catalytic mechanism of Eu^II12 for reductive coupling reactions of benzyl chloride.

Fig. 13. Proposed catalytic mechanism of chromophore-based complexes of Eu for organic reduction reactions. Reprinted with permission from M. Tomar, R. Bhimpuria, D. Kocsi, A. Thapper and K. E. Borbas, J. Am. Chem. Soc., 2023, 145, 22555. Copyright 2023 American Chemical Society. Licensed by Creative Commons CC-BY 4.0.

Fig. 14. Ligands used for separations-based studies.

Fig. 15. Benzo-2.2.2-cryptand covalently linked to a solid support, 38, was used to selectively separate Eu from Gd. Reprinted with permission from D. N. Kulasekara, A. B. Kajjam, S. Praneeth, T. M. Dittrich and M. J. Allen, ACS Appl. Mater. Interfaces, 2023, 15, 42037. Copyright 2023 American Chemical Society.

Fig. 16. Metal sorbed onto 39 as plotted as a function of complexation constants of 13 for rare-earth elements at pH 3.3. Reprinted with permissions from Hovey et al., copyright 2021, with permission from Elsevier.

Fig. 17. Sorption at pH 0.9 and 3.3 using 39. Reprinted with permissions from Hovey et al., copyright 2021, with permission from Elsevier.

Elizabeth C. Lewandowski

Colin B. Arban

Morgan P. Deal

Andrea L. Batchev

Matthew J. Allen