THE STEREOCHEMISTRY AND REACTIVITY OF METAL-SCHIFF BASE COMPLEXES .7. CONTRIBUTION OF HYDROPHOBIC INTERLIGAND INTERACTION TO CHIRAL RECOGNITION OF PHENYLALANINATE AND TRYPTOPHANATE WITH (1R,2R)-N,N'-DISALICYLIDENE-1,2-CYCLOHEXANEDIAMINECOBALT(III) COMPLEX

The stability constants, K1, of Δ-β2-diastereomers of mixed ligand cobalt(III) complexes with a chiral quadridentate Schiff base (sal-(R,R)-chxn), derived from salicylaldehyde and (R,R)-1,2-cyclohexanediamine, and D- or L-amino acidate (aa-=gly, ala, val, leu, thr, phe, trp, pro, asp, asn, and glu) were determined spectrophotometrically in water-methanol (2:3 by volume) containing acetate buffer (0.3 mol dm-3) at 22°C: trans-[Co{sal-(R,R)-chxn}(H2O)2++aa-(K1)⇆Δ-β2-[Co{sal- (R,R,)-chxn}(aa)]. The K1 values range from 5.6X106 to 1.2X109 mol-1 dm3 and obey a linear free energy relationship except for D-phe, D-trp, D-asp, D-asn, and D-pro. In the cases of D-phe, D-trp, D-asp, and D-asn, their stability constants are 5-30 times as high as those for the corresponding L-aa. Extraordinary stabilization of the D-phe and D-trp complexes is discussed in terms of the interligand stacking of the aromatic rings between the Schiff base ligand and a side chain of amino acidate on the basis of conformational analysis and 1H NMR spectra.