Nature of equilibrium shifts in racemic praseodymium(iii) tris(2,2′-oxydiacetate) induced by interaction with chiral probes

Abstract

The effects of L-proline and its derivatives N-acetyl-L-proline, L-proline methyl ester, and L-proline benzyl ester on the equilibrium shift in the racemic Pr(ODA)₃³⁻ complex, in which ODA is 2,2′-oxydiacetate, have been studied by means of circular dichroism (CD) and ¹H NMR spectroscopy. While N-acetyl-L-proline did not induce any optical activity in Pr(ODA)₃³⁻, the addition of L-proline, L-proline methyl ester, and L-proline benzyl ester led to the appearance of the Pfeiffer effect in this lanthanide complex. The signs of the induced CD signals at 484 nm which correspond to the ³P₀ ← ³H₄ transition implied that L-proline interacts more favourably with the Λ-enantiomer of the Pr(ODA)₃³⁻, while L-proline methyl ester and L-proline benzyl ester preferably interact with the Δ-enantiomer of the same complex. The ¹H NMR studies suggested that L-proline and its esters adopt different conformations upon interaction with Pr(ODA)₃³⁻. The dependence of the absorption dissymmetry factor, g_abs, on the concentration of the L-proline indicated that the Pfeiffer effect in this lanthanide complex is best described by the associated model. Measurements in the solution of various pH, ionic strength, and dielectric constant suggested that the chiral discriminatory interaction is a combination of electrostatic and hydrophobic forces, while the hydrogen bonding effects are less important.