Shift reagents paramagnetic lanthanide ions

Bimetallic complexes formed using a lanthanide tris (3-diketonate and silver 3-diketonate are useful shift reagents for soft Lewis bases such as alkenes, aromatics, phosphines, and halogenated compounds.Chiral lanthanide tris (3-diketonates of tfc and hfc are used in forming the binuclear reagents. A variety of silver 3-diketonates have been evaluated, but only the silver chelate of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione [Ag (fod)] is commercially available. The mixture forms a tetrakis chelate anion to which the silver is ion paired ([Ln (P-dik)4 ]Ag" "). In the bimetallic reagent, the silver binds to the soft Lewis base while the paramagnetic lanthanide ion causes perturbations in the chemical shifts that account for the enantiomeric discrimination. 

Crudely approximated, the magnitude of the pseudocontact shift Afipolai, which predominates in lanthanide shift reagents, decreases with increasing distance between the observed nucleus i and the paramagnetic ion ... 

In accordance with the proportionality (3.8 a), the magnitude of the pseudocontact shift, which predominates for lanthanide shift reagents, decreases with the distance of nucleus i from the paramagnetic ion. Thus, C-9 of camphor is shifted more than C-10 on addition of the europium chelate. Often, the crude approximation (3.8 a) is a valuable assignment aid. 

The paramagnetism of many lanthanide ions finds practical application in NMR shift reagents and, increasingly, in MRI contrast agents. EPR (see Electron Paramagnetic Resonance) spectra are only readily obtained from Gd +, with its ground state, as yet there has been little study of the effects of environment upon the spectra. 

Perhaps the most widely recognized use of lanthanide )8-diketonates is as NMR shift reagents. This application takes advantage not only of the intrinsic paramagnetic nature of certain of the lanthanide ions, but also of the Lewis acidity, hydrolytic stability, and high solubility in nonpolar organic solvents of their complexes. This paper describes our recent studies of the use of these unusual chelates as NMR shift reagents. 

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