Assessing the effect of ligand proximity on chiroptical and other properties in cobalt(III) model, tyrosine-like complexes

A series of new cobalt(III) octahedral complexes of the general formula Na[Co(ohb-aa)(2)] (ohb-aa = N-(o-hydroxybenzyl)amino acid anion); amino acid = glycine, (S)-alpha-alanine, alpha-aminoisobutyric acid, (S)-valine, (S)-norvaline and (S)-leucine) were prepared by the charcoal catalyzed reaction of the appropriate ligand with [Co(NH3)(6)](3+) in alkaline aqueous solution. Complexes obtained have, regardless of the amino acid used, the same facial all-trans symmetry (CoN2O4 chromophore) with the vicinal effects as the entire source of the optical activity. C-13 NMR spectra reveal that the leucine derivative has, due to the steric reasons, a different ground state structure. Absorption maxima reflect a positive inductive effect from the amino acid side chain carbon atoms. Complexes of the ligands bearing electrophobic alkyl groups exhibit more negative E(1/2) in comparison with the glycine derivative, reduction of which occurs at a more positive potential. Reduction potentials do not correlate with cobalt(III) Lewis acidity modulated by ligands.