ChemInform Abstract: Ligand Design for Enhanced Stereoselectivity in the Electron-Transfer Reactions Between Nickel(IV) Bis(oxime-imine) Complexes and (Co(edta))2-.

ChemInform Abstract

A number of derivatives (Ib)-(Ig) of the bis(oxime-imine) (Ia) (prepared in general by published methods) are used as ligands for Ni complexes. The neutral Ni(II) complexes of these ligands can be oxidized to the corresponding Ni(III) and Ni(IV) derivatives, the effect of changing ligand structure on the reduction potentials of the complexes is small. Self-exchange rate constants for the Ni(IV)/Ni(III) change are estimated from the rates of the cross-reactions between the Ni(III) complex ions of (Ic) and (If) and the chiral Ni(IV) complexes of (Id) and (Ig). The kinetics and mechanisms of reduction of the Ni(IV) complex ions of (Ic) and (If) and the title Co complex are also investigated. The reactions are biphasic. The initial rapid reduction of the Ni(IV) compound to give a Ni(III) transient is outer-sphere in nature. Binding of the chiral ligand systems to the Ni ions is deduced from CD studies to be stereospecific giving the Λ absolute configuration around the metal in all cases. The oxidation of the Co(II) complex ion by the chiral Ni(IV) complexes of (Ig), (Id), (Ib), and (Ie) is stereoselective, and in all cases the preferred isomer of the Co(III) complex is Δ with enantiomeric excesses of 7, 21, 22, and 46% for the Ni complexes in the order given above.