Ytterbium(III) chloride

Ytterbium, a Lanthanide series element, was discovered in 1878 byMarignac who named the element after a town (Ytterby) in Sweden.^[1] The first synthesis of YbCl₃ did not appear in literature until 1946 by Hoogschagen.^[2] YbCl₃ is now a readily available source of Yb⁺³ ions, which is why the study of YbCl₃ is chemically relevant.

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Chemical Properties

The valence electron configuration of Yb⁺³ (from YbCl₃) is 4f¹³5s²5p⁶, which has crucial implications in chemical behavior of Yb⁺³. Also, the size of Yb⁺³ governs it’s catalytic behavior and biological applications. For example, while both Ce⁺³ and Yb⁺³ have a single unpairedf electron, Ce⁺³ is much larger than Yb⁺³ because Lanthanides become much smaller with increasing effective nuclear charge sincef electrons are not well shielded, compared tod electrons.^[1] This behavior is known as the Lanthanide contraction. The small size of Yb⁺³ produces fast catalytic behavior and an atomic radius (0.99 Ǻ) comparable to many important biological ions.^[1]

The thermodynamic properties tabulated herein were difficult to obtain since many of the measurements were made in the gas phase where YbCl₃ can exist as ([YbCl₆]^-3)^[3] or Yb₂Cl₆.^[4] The Yb₂Cl₆ species was detected by electron impact (EI) mass spectrometry as (Yb₂Cl₅⁺).^[4] Additional complications in obtaining experimental data arise from the myriad of low-lying f-d andf-f electronic transitions.^[5] Despite these issues, the thermodynamic properties of YbCl₃ have been obtained and the C_3V symmetry group has been assigned based upon the four active infrared vibrations.^[5]

Preparation

YbCl₃ is prepared from Yb₂O₃ with either carbon tetrachloride^[6], or hot hydrochloric acid.^[7]

In practice there are better ways to prepare YbCl₃ for lab use. The aqueous HCl/ammonium chloride route is low tech but very effective. Alternatively hydrated YbCl3 may be dehydrated using a variety of reagents, particularly trimethylsilyl chloride. Other methods have been published including reacting the finely powdered metal with mercuric chloride at high temperature in a sealed tube. A variety of routes to solvated YbCl₃ have been reported including reaction of the metal with various halocarbons in the present of a donor solvent such as THF, or dehydration of the hydrated chloride using trimethylsilyl or thionyl chloride, again in a solvent such as THF.

Uses

Catalysis

YbCl₃, with a single unpairedf electron, acts as a Lewis acid in order to fill the 4f orbital. The Lewis acidic nature of YbCl₃ allows YbCl₃ to coordinate (usually as [YbCl₂]⁺) in transition states to catalyze alkylation reactions, such as the Aldol reaction^[8] and the Pictet-Spengler reaction.^[9]

The Aldol reaction is a versitle reaction in synthetic organic chemistry. YbCl₃ serves a Lewis acid catalyst which aids the Pd(0) catalyzed decarboxylative Aldol reaction between a ketone enolate and an aldehyde. Transition statesA andB show the coordination method of the ytterbium salt as a Lewis acid.^[8] For the depicted decarboxylative Aldol reaction with R = tert-butyl and
R’ = -(CH₂)₂Ph, the reaction yields show YbCl₃ is an effective Lewis acid catalyst:

The Pictet-Spengler reaction produces a valuable tetrahydro-β-carboline ring system which can be later used for synthetically prepared indole alkaloids.^[9] The Lewis acid catalyzed reaction with YbCl₃ gave excellent yields and reduced reaction times from four days to 24 hours.^[9]

The small size of Yb³⁺ provides fast catalysis, but at the cost of selectivity. For example, the mono-acetylation ofmeso-1,2-diols is fastest (2 h) with YbCl₃, but chemoselectivity for the mono-acetylated product is low (50%) compared to CeCl₃ (23 h, 85%).^[10]

Biology

YbCl₃ is a NMR shift reagent that produces different resonances with nuclei in contact with the YbCl₃ versus those nuclei not in contact with the shift reagent.^[11] Generally paramagnetic species, such as a (Lanthanide)⁺³ ion, are desirable shift reagents.^[11] Membrane biology has been greatly influenced by YbCl₃, where³⁹K⁺ and²³Na⁺ ion movement is critical in establishing electrochemical gradients.^[11] Nerve signaling is a fundamental aspect of life that may be probed with YbCl₃ using NMR techniques. YbCl₃ may also be used as a calcium ion probe, in a fashion similar to a sodium ion probe.^[12]

YbCl₃ is also used to track digestion in animals. Certain additives to swine feed, such as probiotics, may be added to either solid feed or drinking liquids. YbCl₃ travels with the solid food and therefore helps determine which food phase is ideal to incorporate the food additive.^[13] The YbCl₃ concentration is quantified from ICP (inductively coupled plasma) mass spectrometry to within 0.0009 μg/mL.^[1] YbCl₃ concentration versus time yields the flow rate of solid particulates in the animal’s digestion. The animal is not harmed by the YbCl₃ since YbCl₃ is simply excreted in fecal matter and no change in body weight, organ weight, or hematocrit levels has been observed in mice.^[12]

The catalytic nature of YbCl₃ also has an application in DNA microarrays, or so called DNA “chips”.^[14] YbCl₃ led to a 50-80 fold increase in flourescein incorporation into target DNA, which could revolutionize infectious disease detection (such as a rapid test for tuberculosis).^[14]

References

1. ^ ^{a b c d} Evans, C.H. Biochemistry of the Lanthanides; Plenum: New York, 1990.
2. ^ Hoogschagen, J. Pysica 1946, 11(6), 513-517.
3. ^ Gau, W.J.; Sun, I.W. J. Electrochem. Soc. 1996, 143(1), 170-174.
4. ^ ^{a b} Chervonnyi, A.D.; Chervonnaya, N.A. Russ. J. Inorg. Chem. (Engl. Transl.) 2004, 49(12), 1889-1897.
5. ^ ^{a b} Zasorin, E.Z. Russ. J. Phys. Chem. (Engl. Transl.) 1988, 62(4), 441-447.
6. ^ Goryushkin, V.F.; Zalymova, S.A.; Poshevneva, A.I.Russ. J. Inorg. Chem. (Engl. Transl.) 1990, 35(12), 1749-1752.
7. ^ Jörg, S.; Seifert, H.J. Termochim. Acta 1998, 318, 29-37.
8. ^ ^{a b c} Lou, S.; Westbrook, J.A.; Schaus, S.E. J. Am. Chem. Soc. 2004, 126, 11440-11441.
9. ^ ^{a b c} Srinivasan, N.; Ganesan, A. Chem. Commun. 2003, 916-917.
10. ^ Clarke, P.A. Tetrahedron Lett. 2002, 43, 4761-4763.
11. ^ ^{a b c} Hayer, M.K.; Riddell, F.G. Inorg. Chim. Acta 1984, 92, L37-L39.
12. ^ ^{a b} Shinohara, A.; Chiba, M.; Inaba, Y. J. Alloys Cmpds. 2006, 408-412, 405-408.
13. ^ Ohashi, Y.; Umesaki, Y.; Ushida, K. Int. J. Food Micro. 2004, 96, 61-66.
14. ^ ^{a b} Browne, K.A. J. Am. Chem. Soc. 2002, 124, 7950-7962.