Steric and hydrophobic effects on stereoselectivity in ternary cobalt(III) complexes of amino acid derivatives

Abstract

The preparation, separation, and characterization of mixed ligand cobalt(III) complexes containing either the new asymmetric linear tetradentate ligand, ethylenebis-(S)-phenylalanine ((SS)-EBPhe), or known asymmetric linear tetradentate ligand, ethylenebis-(S)-valine ((SS)-EBV), and an aromatic or aliphatic diamine (dam) respectively are reported. The absolute configuration of these complexes are assigned by considering the electronic absorption, circular dichroism, infrared, 1H and 13C NMR spectra, and the known configuration of the ligands. The optically active tetradentate ligands coordinate stereospecifically because only Δ diastereoisomers are formed. The factors determining stereoselectivity in these complexes are mainly the steric and intramolecular hydrophobic ligand-ligand interactions. These intramolecular hydrophobic interactions have been confirmed by 1H NMR shift measurements in water.