Binuclear lanthanide-silver complexes

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. 

Reagents formed with chiral /3 -diketonate ligands such as Lnftfcfs and Ln(hfc)3 in the presence of Ag(fod) or other silver /3-diketonates are effective for chiral discrimination . Lanthanide chelates with dcm are ineffective in the binuclear complexes, presumably because the dcm ligand is so sterically hindered that it prevents the formation of the quadruple chelate anion. Silver complexes besides Ag(fod) actually form more effective chiral discriminating agents with Pr(tfc)3, Yb(tfc)3 and Yb(hfc)3, but these complexes are not commercially available so have not been used beyond exploratory studies . ... 

The spectra of 69a and 69b exhibited only small nonequivalence in the presence of Yb(hfc)3, whereas addition of Ag(fod) caused larger nonequivalence of methyl and methoxy resonances of the R-groups at 500 MHz . The binuclear dipyrromethene complex with Zn(n) (70) has a helical structure that renders it chiral. This was confirmed by a doubling of certain resonances in the H NMR spectrum of the zinc complex in the presence of Eu(tfc)3/Ag(fod) or Yb(tfc)3/Ag(fod). Splitting of the resonances was not observed in the presence of the lanthanide chelate alone and was only observed on addition of the silver reagent . 

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. 

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