Equilibrium studies of the axial addition of chiral bases to chiral metal Schiff base complexes

The syntheses of new chiral 1,2- diphenylethylene-bridged nickel(II) and methylcobalt(III) Schiff base complexes are described. The equilibrium constants for the axial addition of the chiral bases (R)- and (S)-1-phenylethylamine to the enantiomerically pure metal complexes have been determined by means of spectrophotometric titrations. Comparison of the constants for the cyclohexanediyl-bridged complexes (type 1) with those for the more bulky 1,2-diphenylethylene-bridged complexes (type 2) reveals that chiral recognition is only observed in the case of type-2 complexes.