Europium(III) Chloride1

EuCl3.6H2O
(anhydrous)

[10025-76-0]  · Cl3Eu  · Europium(III) Chloride  · (MW 258.32) (hexahydrate)

[13759-92-7]  · H12Cl3EuO6  · Europium(III) Chloride  · (MW 366.41)

(member of a family of moderate Lewis acids capable of acting as catalysts in selected oxidative and reductive processes,2-4 with particular utility in systems designed to exploit the chemistry of divalent and zerovalent reagents2,4 and organometallic intermediates5-7)

Alternate Name: europium trichloride.

Physical Data: mp 623 °C (dec); d 4.89 g cm-3.

Solubility: anhydrous salt: sol dilute HCl; hydrolyzes in water to form oxychloride; sol alcohol and acetone; slightly sol in THF. Hydrate: sol water.

Form Supplied in: hydrated white crystalline solid which can be dehydrated to a yellow powder which intensifies in color upon heating; commercially available in both forms; readily recyclable; typically >99.9% pure with regard to total metallic impurities.

Preparative Methods: highest quality material is produced by reacting Eu2O3 at ca. 200 °C with the vapors of a suitable chlorinating agent (CCl4 has been shown to work well).

Handling, Storage, and Precautions: hydrated salt is hygroscopic: keep tightly closed; hydration state may vary slightly; anhydrous salt should be handled and kept under nitrogen or argon; color modification to yellow-green may occur upon long-term storage under inert atmosphere.

Introduction.

The ability of selected lanthanide salts to promote an intriguing array of organic reaction mechanisms has led synthetic chemists to focus on key elements such as cerium,8 lanthanum,9 and samarium.10 The relatively low cost of cerium and lanthanum reagents makes them particularly attractive for synthetic studies, whereas the use of samarium in organic synthesis features the unique chemistry of the SmII species, typically derived in situ or used in solution as the diiodide (see Samarium(II) Iodide).10,11 The other lanthanide species which form stable MII oxidation states, ytterbium and europium, have not been as extensively explored as synthetic organic reagents for a variety of reasons. In the case of europium, the primary factor has been material cost relative to the other lanthanides.

The accessibility of both EuIII and EuII species has not been lost on researchers in various fields. Favorable thermodynamic parameters have been the basis for studies of new energy systems;12 in the mid-1980s, investigation of solar energy conversion determined that both SmCl3 and EuCl3 (hydrated) catalyze the photogeneration of hydrogen in weakly acidic alcohol solutions, forming a variety of organic byproducts by a light-assisted solvent-to-ion single electron transfer.13

Hydroxymethylation.

Adapting hydrogen-generating EuIII/EuII photoredox systems directly to organic photochemical reactions has shown limited synthetic promise. A radical mechanism governing the reaction of a-methylstyrene with a hydrogen atom and a hydroxymethyl radical was proposed for eq 1.14 The formation of EuII ions as the key reactive intermediate for the reduction was confirmed by spectroscopic measurements.

When 1,3-dimethyluracil is used as substrate, efficient regioselective hydroxymethylation takes place via an electron transfer mechanism (eq 2).15 The EuII resulting from the oxidation of the alcohol becomes the primary light-absorbing species, promoting hydrogen generation. This chemistry was deemed pertinent to the photoinduced cross-linking of uracil and amino acids.

Further insights into the scope of this redox system came from a study of the reaction of various alkenes with methanol solutions of europium trichloride. The process has been described as a useful, general hydroxymethylation method.16 The potential of this technique in more elaborate organic synthesis was explored in a study involving hydroxyalkylation and lactone formation of dialkyl malonates, where the europium species serves as photocatalyst and also as a condensation catalyst (eq 3).17 A similar reaction carried out in ethanol produced a more complex array of products at a significantly reduced rate of reaction. Diethyl maleate undergoes photoconversion according to eq 4.

Lewis Acid Catalyst.

More recent literature surveys of organic reactions involving europium trichloride offer little beyond comparative studies of lanthanide chlorides as Lewis acid catalysts in classical organic syntheses such as Friedel-Crafts reactions. The efficiency of a variety of anhydrous lanthanide chloride catalysts relative to Aluminum Chloride in the benzylation of benzene follows the order AlCl3 > DyCl3 > SmCl3 > YbCl3 > EuCl3.18

Transesterification of phosphate diesters is an area in which the catalytic activity of trivalent lanthanide ions appears to exceed that of transition metal ions.19 In the reaction shown in eq 5, the observed order of activity was TbIII > GdIII > YbIII > EuIII > NdIII based on apparent-second-order rate constants. The catalytic activity of EuIII in phosphorus(V) nucleophilic displacement reactions has also been demonstrated in the cleavage of transfer-RNA, where relative rates of cleavage follow the order EuIII > PbII > ZnII > MgII.20 In this case, favorable Lewis acid character rather than redox potential seems to be the dominant factor in metal ion catalyst efficiency.

Overcoming the unfavorable economics of synthetic procedures involving europium species may depend upon the development of creative methods that combine europium's unique redox capabilities with the favorable Lewis acidity exhibited in selected bioinorganic processes. Otherwise, other lanthanide reagents will continue to dominate the attention of synthetic organic chemists.


1. To date, the chemistry of europium trichloride has been of insufficient scope to warrant review as a major contributor to organic synthesis. Inference of its potential in selected chemical transformations can be derived from reviews cited below.2-7
2. Kagan, H. B.; Namy, J. L. T 1986, 42, 6573.
3. Molander, G. A. CRV 1992, 92, 29.
4. Long, J. R. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr.; Eyring, L.; Eds.; North-Holland: New York, 1986; Vol. 8, p 335.
5. Schumann, H.; Genthe, W. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A.; Jr.; Eyring, L., Eds; North-Holland: New York, 1984; Vol. 7, p 446.
6. Schumann, H. AG(E) 1984, 23, 474.
7. Marks, T. J.; Ernst, F. D. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: New York, 1982; Vol. 3, p 173.
8. Luche, J.-L. JACS 1978, 100, 2226.
9. Garlaschelli, L.; Vidari, G. TL 1990, 31, 5815.
10. Kagan, H. B.; Namy, J. L. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr.; Eyring, L., Eds.; North-Holland: New York 1983; Vol. 6, p 525.
11. For an overview of SmI2 chemistry, see Aldrichim. Acta 1989, 22, 40 (it was not until 1994 that anhydrous samarium(II) iodide became commercially available).
12. Martin, D. S.; Long, J. R. Interim Report ERDA Contract No. W-7402-ENG-82, 1975; A Europium-Chlorine-Oxygen-Hydrogen Cycle for Splitting Water.
13. Tennakone, K.; Ketipearachchi, U. S. Chem. Phys. Lett. 1990, 167, 524.
14. Ishida, A.; Toki, S.; Takamuku, S. CL 1985, 893.
15. Ishida, A.; Toki, S.; Takamuku, S. CC 1985, 1481.
16. Ishida, A.; Yamashita, S.; Toki, S.; Takamuku, S. BCJ 1986, 59, 1195.
17. Ishida, A.; Yamashita, S.; Takamuku, S. BCJ 1988, 61, 2229.
18. Mine, N.; Hou, Z.; Maita, M.; Fujiwara, Y.; Taniguchi, H. Rare Earths 1985, 6, 122.
19. Morrow, J. R.; Buttrey, L. A.; Berback, K. A. IC 1992, 31, 16.
20. Rorodorf, B. F.; Kearns, D. R. Biopolymers 1976, 1491.

John R. Long

Aldrich Chemical Company, Milwaukee, WI, USA



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.