Mechanism of Coordination-site Exchange Reaction of the Cobalt(III) Complexes Containing Multidentate Ligands

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The isomers of the [Co(edda)(en)]^+, [Co(edda)(gly)], [Co(edda)(beta-ala)], and [Co(eddp)(en)]^+ complexes were prepared and optically resolved. The stereochemistry of these complexes was studied on the basis of absorption, CD, ^1H-NMR and ^<13>C-NMR data and partially on the basis of X-ray analysis. Two beta isomers, beta(e-1) and beta(e-2), of [Co(eddp)(en)]^+ were comformational diastereomers arising from a difference of the chirality around one of the asymmetric N atoms in a coordinated eddp ligand.Coordination-site exchange (isomerization and racemization) reactions of the complexes described above were studied in a basic aqueous solution. In the [Co(edda)(en)]<@D1+@>D1 system, LAMBDA(R)-beta isomer isomerized to LAMBDA(S,S)-alpha isomer with an optical purity (o.p.) of ca. 45%. In the reactions of three isomers of [Co(edda)(gly)], LAMBDA(R)-beta-fac(O) isomerized to LAMBDA(R)-betamer(O) (o.p.:ca.100%) and LAMBDA(S,S)-alpha-mer(O) (o.p.:ca. 33%), LAMBDA(R)-beta-mer(O) to LAMBDA(R) … More -beta-fac(O) (o.p.:ca.65%) and LAMBDA(S,S)-alpha-mer(O) (o.p.: ca.40%), and LAMBDA(S,S)-alpha-mer(O) to LAMBDA(R)-beta-fac(O) (o.p.:ca.50%) and LAMBDA(R)-beta-mer(O) (o.p.:ca.65%). The isomerization behavior of the [Co(edda)(beta-ala)] system was similar to that of the [Co(edda)(gly)] system except for the low stereoselectivity of the beta-fac(O) * beta-mer(O) change. In the [Co(eddp)(en)]<@D1+@>D1 system, the beta(e-1) (] SY.tautm.[) beta(E-2) change was an inversion reaction at the asymmetric sec-N atom in the in-plane beta-alaninate ring of the coordinated eddp, and the rate was slow. The alpha (] SY.tautm.[) beta(e-1,or2) change (coordination-site exchange reaction) was stereoselective: LAMBDA(R,R)-alpha * LAMBDA(S,S)-beta(e-1) (o.p.:ca.40%), LAMBDA(R,R)-alpha * LAMBDA(S,R)-beta(e-2) (o.p.:ca.45%). These results were explained on the basis of a Co-O bond rupture mechanism: the coordination-site exchange between the sec-N and O atoms of the edda or eddp ligand occurred predominantly; consequently, the asymmetry at the sec-N center was inverted. Less