Shift reagent ligands

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

Germanium Porphyrins. The electrosynthesis of a-bonded mono-alkyl or mono-aryl germanium porphyrins involves the conversion of (P)Ge(R)2 to (P)Ge(R)X where X is an anionic ligand. A standard Grignard reaction between (P)Ge(Cl)2 and RMgX leads to the o-bonded bis-alkyl or bis-aryl complexes, (P)Gp(R)2. These complexes were initially synthesized as lH IWR shift reagents(28,29). However, almost no reactivity of these species was reported until several years later when it was shown that the... 

In addition to these systematic studies of lanthanide sulfoxide complexes, with variation in both sulfoxide and anion, other more isolated reports are available. Lanthanide isothiocyanate complexes of the cyclic sulfoxides thioxane oxide (490) and tetramethylene sulfoxide (493) have been synthesized and complexes of the unusual potentially chelating ligand 2-(ethylsulfinyl)pyridine-V-oxide (63) described. Detailed studies of the solvation of lanthanide-shift reagents by Me2SO have also appeared (178,179). 

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. 

An important class of chiral, nonfluorinated lanthanide shift reagents contain acetylacetonate ligands in which the pendant methyl groups are replaced by chiral residues, e.g.. europium(III) tris[(7 ,l )-dicampholylmethanate] [Eu(dcm)3, see Table 1 and Figure 9]89. 

In 1975, it was reported that while lanthanide shift reagents could not be used directly to simplify the NMR spectra of alkenes, when coupled with silver salts substantial shifts could be induced.232 Since then, a number of studies have reported the use of both chiral and achiral lanthanide(III)-silver(I) binuclear shift reagents,233-237 where the ligands were generally fluorinated /3-diketones. 

There are several useful shift reagents, usually of europium, in which the organic ligands are chiral. An example is 1 ... 

Fig. 5.56. Some lanthanide-chelating ligands used as pseudocontact shift reagents.

The chirality of metal helicates can be demonstrated experimentally by X-ray crystal structure determinations and in solution by NMR spectroscopy. Addition of chiral shift reagents such as [Eu(tfc)3] (tfc = 3-(trifluoromethylhydroxymethylene)-(+)-camphorato) to selected helicates results in the splitting of some of the ligand signals as a consequence of the formation of diastereomeric complexes with the shift reagent. Such splitting is not observed for the free ligands, which are achiral. 

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