Application of the Pfeiffer effect to the assignment of absolute configurations to octahedral metal complexes of the type cis-[M(diamine)2(X)(Y)]n+ and related complexes

Article abstract of DOI:10.1021/ic50218a021

The Pfeiffer effect of [Cr(ox)₃]^3- (ox = oxalate dianion) was examined by using as a chiral environment substance optically stable metal complexes of the type cis-[M(diamine)₂(X)(Y)]ⁿ⁺ (M = Co³⁺ or Cr³⁺, diamine = ethylenediamine (en) or trimethylenediamine (tn), and X and/or Y = anionic monodentate ligand) and their related complexes in dioxane-water mixtures. It was found that the chiral equilibrium of [Cr(ox)₃]^3- is always displaced in favor of its Δ enantiomer in the presence of Λ enantiomers of these complexes added as an environment substance. This was interpreted to mean that Λ-cis-[M(diamine)₂(X)(Y)]ⁿ⁺ complexes associate more preferably with Δ-[Cr(ox)₃]^3- than with its antipode, the inversion rate of Δ-[Cr(ox)₃]^3- being thereby retarded relative to that of the antipode. On the basis of this finding, it was proposed that the absolute configuration of cis-[M(en or tn)₂(X)(Y)]ⁿ⁺ and of the related complexes is inferred from the direction of the equilibrium shift in racemic [Cr(ox)₃]^3- induced by these chiral complexes. An association model was also proposed for the stereoselective interaction between cis-[M(diamine)₂(X)(Y)]ⁿ⁺ and [Cr(ox)₃]^3-.