1H and 13C NMR studies of binuclear lanthanide(III):—silver(I) shift or relaxation reagents: Competition between several bonding sites in olefinic,aromatic and heteroaromatic compounds

The relative abilities of weak Lewis bases to complex binuclear lanthanide(III)—silver(I) reagents were checked by inter- or intra- molecular competitions. Preferential complexation at one particular site was shown to be determined mainly by the occurrence of well localized π electrons, by the relief of strain effects or by the lack of steric hindrance. In benzofuran and methoxybenzene derivatives oxygen never interacts directly with the silver reagent and plays a role mainly through electronic effects. Unlike oxygen, sulphur in benzob]thiophene is the preferred site of complexation with the silver reagent. On complexation with binuclear shift reagents (lanthanide = Eu, Pr, Yb) the ¹H NMR shifts were shown to result from several mechanisms. Better insight into the precise location of the reagent is obtained in ¹³C NMR by the use of the binuclear relaxation reagent Ag(tfa)-Gd(fod)₃.