Praseodymium chelates

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

The square antiprismatic or distorted square antiprismatic geometries are found in various inorganic and chelated complexes of the lanthanides and antinides. It has been suggested that the tetrafluorides of cerium (98) praseodymium (99),... 

Europium(III), and particularly ytterbium(III) shift reagents, induce downfield proton resonance shifts while the praseodymium(III) analogs cause upfield shifts. Lanthanide chelates of fluorinated /3-diketonates are more soluble in organic solvents, and they form more stable association complexes with donor molecules, than do LSRs with nonfluorinated ligands. Thus Eu(fod)3 is the preferred achiral LSR for weak nucleophiles89. 

For expanding NMR spectra of aqueous sample solutions, lanthanide salts such as the chlorides, nitrates, or perchlorates of europium and praseodymium are used [103]. In organic solvents, tris-(/ -diketonato)-europium(III) chelates (1) are usually applied [103]. Chiral europium(III) chelates such as tris-(3-tm-butylhydroxymethylene)-D-camphora-to-europium(III) (2) separate the signals of enantiomers in the NMR spectra of racemic solutions [103],... 

Diphosphazane dioxide complexes of lanthanides have potential application in the solvent extraction separation of lanthanides. Reaction of lanthanide nitrate with X2P(0)NPr )-P(0)X2(L-L) yields the bis chelate complexes Ln(NC>3)(L-L)2. The structure of the praseodymium complex has been determined by X-ray diffraction and the space group is P32- There are two independent molecules in the unit cell which differ in orientation of the phenyl group. The metal ion is ten-coordinated [264]. 

The chelates can be purified, and mixtures of the complexes can be separated by fractional sublimation and distillation. In the gas phase, in solution, and in the solid state, Tb(thd)3 emits a brilliant green fluorescence when irradiated at 3660 A. with an ultraviolet lamp. Fluorescence is also exhibited by Eu(thd)3, Dy(thd)3, and Sm(thd)3. The praseodymium complex is thermally stable in the gas phase when heated for prolonged periods of time. Vapor pressure measurements on this complex showed no increase in pressure when the sample was heated at 250° for 6 hours. Thermogravimetric analyses and discussions of trends in volatility of the rare-earth-thd chelates have been published. 

The enantiomers of camphorcarboxylic and sulfonic acids are used for resolution of enantiomers from racemic chiral amines and alcohols via diastereomeric salts and esters, respectively. Europium(III)- and praseodymium(III)-chelates of hydroxy-methylenecamphor derivatives are suitable chiral shift reagents for the determination of enantiomeric purity by integration of NMR spectra, because they exchange ligands with enantiomeric substrates such as alcohols and amines, thus forming diastereomeric chelates characterized by different spectra. 

In the NMR investigation of solutions, the effects of paramagnetic substances in solution on the proton resonance peaks of the solvent may be of particular significance. Certain europium(III) and praseodymium(III) chelates, for example, behave as Lewis acids towards donor solvents such as alcohols, ethers, esters and ketones. As a result of this acid-base interaction the proton resonance peaks undergo shifts which favour, among others, the resolution of the PMR spectrum. 

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