Lanthanoid Shift Reagents

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Lanthanoid shift reagents are compounds that are used in NMR spectroscopy to shift signals.

Structure and reactivity

general structure

Lanthanoid shift reagents are complexes consisting of a lanthanoid and three β- diketonates . Different substituted diketonates are used. Fluorinated variants are particularly popular.

To induce a shift, the complex must be paramagnetic. In the common complexes of the form LnL 3 , however, not all lanthanides can be used, since lanthanum and lutetium are diamagnetic in the + III oxidation state. Another exception is Gd (III) , whose seven f electrons are distributed isotropically in the five f orbitals and therefore do not lead to paramagnetism. When choosing the lanthanide, it is important not only to consider how large the induced shift is, but also whether a significant peak broadening occurs. In practice, europium complexes to induce a low-field shift and praseodymium complexes to induce a high-field shift have therefore prevailed.

For a successful use in NMR spectroscopy, the contact and pseudo contact shifts are particularly important. Contact shifts occur when the Lewis acidic lanthanoid complex interacts with atoms that have lone pairs of electrons and are therefore Lewis basic. Contact shifts often arise on O, N, S or P atoms. Conclusions about the structure of the analyte can be drawn from the intensity of the contact shift. While the interaction in contact shifts takes place via bonds, the pseudo-contact shift, a dipolar interaction, occurs through space. This interaction is therefore not dependent on Lewis basic centers and is also pronounced with H, C and F atoms.

Chiral Shift Reagents

Eu (hfc) 3

In addition to the achiral lanthanoid shift reagents, there are also enantiomerically pure reagents. The mode of action is analogous to that of the achiral variants, with the difference that the signals are split for enantiomers. With sufficient separation of the signals of the enantiomers, the enantiomeric ratio can be determined by integration and conclusions can be drawn about the absolute configuration. The splitting of the signals of the enantiomers is based on the fact that diastereomeric aggregates are formed which - in contrast to enantiomers - can be differentiated in NMR. The aggregate formation is mostly based on an interaction between Lewis acid and Lewis base. The best known optically active shift reagent is Eu (hfc) 3 , which is based on enantiomerically pure camphor .

use

The handling of the shift reagents is simple. The lanthanoid complexes used can be mixed with the analyte directly in the NMR tube. However, it must be carried out as free of water as possible, since the shift reagents will otherwise hydrolyze. Non-complexing substances are used as solvents, such as. B. CCl 4 , CDCl 3 and CD 2 Cl 2 . The most common use is in 1 H spectra, but lanthanoid shift reagents are also used in the spectroscopy of other nuclei. While 1 H, 13 C and 19 F only experience shifts due to dipolar interaction , contact shifts can also be observed in spectra of 14 N, 15 N, 17 O or 31 P.

history

First chiral shift reagent

The fundamental discovery that paramagnetic substances can induce a shift in NMR spectra was made by Taube in 1960. He studied water and its interaction with ions using 17 O-NMR spectroscopy. In 1969, Eu (dmp) 3 was used by Hinckley in the NMR spectroscopic investigation of cholesterol . This marks the beginning of the use of the lanthanoid shift reagents. In 1970 the first enantiomerically pure shift reagents for the investigation of chiral compounds followed. Whitesides and Lewis synthesized a europium (III) complex bearing three camphor- based ligands. Rondeau and Sievers achieved a significant further development of the shift reagents. In 1971 they first used a fluorinated shift reagent. Compounds such as Eu (fod) 3 are characterized by better solubility and higher Lewis acidity. The improved properties of the fluorinated shift reagents also inspired research in the field of chiral shift reagents, so that as early as 1971 an enantiomerically pure fluorinated shift reagent was synthesized and investigated by Fraser, Petit and Saunders. This reagent is known under the name Eu (hfc) 3 .

Due to the ever better resolution of the NMR spectrometer, pure shift reagents are rarely used. The high point was the 1970s and 1980s. Chiral shift reagents are still used for structure elucidation and checking optical purity.

Individual evidence

  1. ^ Carlos FGC Geraldes: Lanthanides: Shift Reagents . In: Encyclopedia of Inorganic and Bioinorganic Chemistry . John Wiley & Sons, Ltd, Chichester, UK 2012, ISBN 978-1-119-95143-8 , pp. eibc2050 , doi : 10.1002 / 9781119951438.eibc2050 .
  2. ↑ a b 8-TECH-7 Lanthanide Induced Shifts (LIS). Retrieved February 5, 2020 .
  3. ^ BD Flockhart, J. Jonas: Lanthanide Shift Reagents in Nuclear Magnetic Resonance Spectroscopy . In: Critical Reviews in Analytical Chemistry . tape 6 , no. 1 , January 1976, p. 69-130 , doi : 10.1080 / 10408347608542690 .
  4. ^ Anthony F. Cockerill, Geoffrey LO Davies, Raymond C. Harden, David M. Rackham: Lanthanide Shift Reagents in Nuclear Magnetic Resonance Spectroscopy . In: Chemical Reviews . tape 73 , no. 6 December 1973, pp. 553-588 , doi : 10.1021 / cr60286a001 .
  5. ↑ Jasper A. Jackson, Joe F. Lemons, Henry Taube: Nuclear Magnetic Resonance Studies on Hydration of Cations . In: The Journal of Chemical Physics . tape 32 , no. 2 , February 1960, p. 553-555 , doi : 10.1063 / 1.1730733 .
  6. ^ Conrad C. Hinckley: Paramagnetic Shifts in Solutions of Cholesterol and the Dipyridine Adduct of Trisdipivalomethanatoeuropium (III). A shift reagent . In: Journal of the American Chemical Society . tape 91 , no. August 18 , 1969, p. 5160-5162 , doi : 10.1021 / ja01046a038 .
  7. ↑ George M. Whitesides, Daniel William. Lewis: Tris [3- (tert-butylhydroxymethylene) -d-camphorato] europium (III). A reagent for determining enantiomeric purity . In: Journal of the American Chemical Society . tape 92 , no. 23 November 1970, p. 6979-6980 , doi : 10.1021 / ja00726a049 .
  8. ^ Robert E. Sievers, Roger E. Rondeau: New Superior Paramagnetic Shift Reagents for Nuclear Magnetic Resonance Spectral Clarification . In: Journal of the American Chemical Society . tape 93 , no. 6 , March 1971, p. 1522-1524 , doi : 10.1021 / ja00735a049 .
  9. ^ Robert R. Fraser, Michel A. Petit, John K. Saunders: Determination of enantiomeric purity by an optically active nuclear magnetic resonance shift reagent of wide applicability . In: Journal of the Chemical Society D: Chemical Communications . No. 22 , 1971, p. 1450 , doi : 10.1039 / c29710001450 .