Lanthanide shift reagents silver complexes

Lanthanide shift reagents silver complexes, 806 Leaving group effect platinum complexes substitution reactions, 494 Leucine aminopeptidase zinc, 1005 Ligases zinc, 1002... 

The inability of normal shift reagents to complex with olefinic double bonds has been overcome by the use of a mixture of silver heptafluorobutyrate and europium or praeseodymium heptafluoro-octanedionate, the silver interacting with the olefin and the lanthanide with the carboxylate group. (5) The shifts observed are, however, rather small, as expected in view of the distance of the lanthanide from the hydrogens. 

Finally, binuclear lanthanide(III)-silver(I) shift reagents are noteworthy. These form complexes with olefins, aromatic rings, halogenated saturated hydrocarbons, and phosphines. Due to the lack of polar groups, these functionalities do not give significant LIS with common mononuclear LSR. Applications of this binuclear technique have been reviewed261 for example, the Z- and E-isomers of 2-octene can be differentiated. 

Recently the use of mixed lanthanide silver complexes as NMR shift reagents has been assessed. In the original experiments232,233 silver(I) carboxylates such as Ag02CCF3 and Ag02CC3F7 were used, although later studies showed that larger shifts could be induced with silver /3-diketonates (see Section 54.1.4.4). 

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. 

The lanthanide tetrakisOS-diketonate) complexes are effective NMR shift reagents for organic salts (Wenzel and Zaia, 1985, 1987). The active [R( S-diketonate)4] species are formed in situ by reaction between an oiganic salt and Ag[R(jS-diketonate)4]. For instance when organic ammonium halide Q+X is added to a solution of Ag[R(fod)4] in chloroform, the silver halide AgX precipitates from the solution and Q[R(fod)4] is formed ... 

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