Intramolecular ligand-ligand interactions in mixed ligand complexes

Abstract

Intramolecular ligand-ligand interactions in the ternary Cu(II)- and Pd(II)- amino acid systems have been investigated by synthetic, spectroscopic and potentiometric methods. Various ternary Cu(II)- and Pd(II)- complexes containing amino acids with charged and/or polar side groups were isolated as crystals. Electrostatic interactions between oppositely charged side groups of coordinated acidic and basic amino acids and a hydrogen bond in the histidine-containing complexes between the COO − group of histidinate and the OH or the CONH 2 group of asparaginate, threoninate, etc. were inferred and substantiated by magnitude anomaly of circular dichroism spectra in the d-d region and rotamer populations of coordinated amino acids calculated from NMR coupling constants. Geometric isomerism due to the steric requirements for the intramolecular interactions was indicated by the IR spectra of diastereomeric complexes and optical resolution of racemic amino acids via isolation of ternary complexes with ligand-ligand interactions. The stability constants determined in water and 20% dioxane-water at 25 °C and I = 0.1(KNO 3) showed no significant stability difference attributable to the interactions.