Stereoselective interactions as manifested by vibrational circular dichroism spectra: the interplay between chiral metal complexes co-adsorbed in a montmorillonite clay

Abstract

Vibrational circular dichroism (VCD) spectroscopy was applied to study intermolecular interactions on the surface of a clay mineral. A montmorillonite clay loaded with two kinds of chiral molecules was prepared. Firstly a cationic ruthenium(II) complex, Δ- or Λ-[Ru(phen)₃]²⁺ (phen = 1,10-phenanthroline), was ion-exchanged into sodium montmorillonite (denoted as [Ru(phen)₃]²⁺/mont). Thereafter a neutral metal(III) complex, Δ- or Λ-[M(acac)₃] (acac = acetylacetonato; M = Co and Cr), was adsorbed spontaneously, when the complex was added to an aqueous dispersion of [Ru(phen)₃]²⁺/mont (denoted as [M(acac)₃]/[Ru(phen)₃]²⁺/mont). VCD measurements were performed on a KBr disk containing ca. 1 wt% of solid [M(acac)₃]/[Ru(phen)₃]²⁺/mont. Both of the chiral components gave clear VCD peaks in the wavelength region of 1200–1800 cm⁻¹. Attention was focussed on the spectral differences between the following two samples: the one containing a pair of co-adsorbed chelates with the same chirality (i.e. Λ-[M(acac)₃]/Λ-[Ru(phen)₃]²⁺/mont or Δ-[M(acac)₃]/Δ-[Ru(phen)₃]²⁺/mont) and the other containing a pair of co-adsorbed chelates with the opposite chirality (i.e. Δ-[M(acac)₃]/Λ-[Ru(phen)₃]²⁺/mont or Λ-[M(acac)₃]/Δ-[Ru(phen)₃]²⁺/mont). Notably a new peak shoulder appeared and the relative peak intensity of the couplet peaks varied, depending on the chirality relation between the co-existing complexes. With the help of theoretical calculations, these results were rationalized in terms of the stereoselective intermolecular interactions.