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

Abstract

Abstract 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 benzo[ b ]thiophene is the preferred site of complexation with the silver reagent. On complexation with binuclear shift reagents (lanthanide = Eu, Pr, Yb) the 1 H NMR shifts were shown to result from several mechanisms. Better insight into the precise location of the reagent is obtained in 13 C NMR by the use of the binuclear relaxation reagent Ag(tfa)‐Gd(fod) 3 .