STEREOSELECTIVITY IN REACTIONS OF METAL-COMPLEXES PART XXII - KINETICS AND MECHANISM OF ELECTRON-TRANSFER REACTIONS BETWEEN RAC-[CO(L)X](-2,6-DIYL) BIS(METHYLENE)]BIS[AMINO ACID] - X=PYRIDINE, IMIDAZOLE, 1-METHYLIMIDAZOLE AND PYRAZINE) AND OPTICALLY-ACTIVE IRON(II) COMPLEXES() (L=[(PYRIDINE)

Chiral Co-III complexes of the pentadentate ligands alamp and promp wi th the monodentate ligands pyridine, imidazole, l-methylimidazole or p yrazine at the sixth coordination site have been reduced by optically active Fe-II complexes. From kinetic and stereoselectivity data it is concluded that, with all the monodentate ligands except pyrazine, the electron transfer takes place through a bridging carboxylate group coo rdinated to the Co-III center. According to the steric interactions be tween the two reagents either a combined -O-/C=O attack involving both oxygen atoms of the bridging carboxylate group or a C=O attack on the carbonyl oxygen is proposed. In the former case kinetic stereoselecti vity values k(Delta Lambda)/k(Lambda Lambda) varying from 1.2 to 2.3 a re observed, whereas, for the latter, stereoselectivity is absent. Fur thermore, the reaction rates for the non-selective systems are about t en times slower than would be expected on the basis of the potential d ifference between the two redox centers. In the case of the Co-III com plex with pyrazine at the sixth coordination site, pyrazine bridging a ppears to be just as efficient as carboxylate bridging. (C) 1998 Elsev ier Science S.A. All rights reserved.