STEREOSELECTIVITY IN REACTIONS OF METAL-COMPLEXES PART XXI - KINETICSAND MECHANISM OF ELECTRON-TRANSFER BETWEEN 2O](-[(PYRIDINE-2,6-DIYL)-BIS(METHYLENE)]BIS[AMINO ACID]) AND OPTICALLY-ACTIVE IRON(II) COMPLEXES() (L=N,N')

The kinetics of electron-transfer reactions between chiral Co-III and optically active Fe-II complexes has been studied using circular dichr oism (CD) spectroscopy. The ligands used all have the same basic struc ture of N,N'-[(pyridine-2,6-diyl)bis(methylene)]bis[amino acid]. Accor ding to the substitution pattern on this linear pentadentate framework , the complex formation with inert Co-III and Fe-II is considered to b e stereospecific. The substituents were systematically varied in an ef fort to provide evidence for the intimate mechanism of the electron tr ansfer. Observed kinetic stereoselectivities of the reduction of the a qua-Co-III complexes varied from 1.7 to 2.4, and, with one exception, heterochiral couples of the complexes reacted faster. Considering expe rimental data and models, an inner-sphere mechanism is proposed, with a transition state where the carboxylate groups on the inert Co-III co mplex act as the bridge. The question of bridging between the two meta l centers by the carboxylate group is discussed. (C) 1998 Elsevier Sci ence S.A. All rights reserved.