Shape selectivity in outer-sphere electron transfer reactions

Chiral induction has been examined in the four diastereomeric products formed in a series of outer-sphere electron transfer reactions between the oxidants [Co(ox)3]3−, [Co(edta)]−, [Co(gly)(ox)2]2−, C1-cis(N)-[Co(gly)2(ox)]−, [Co(en)(ox)2]−, C2-cis(N)-[Co(gly)2(ox)]− and trans(N)-[Co(gly)2(ox)]− with [Co((RR,SS)-chxn)3]2+ and [Co((R, S)-pn)3]2+ as reductants. The products; [Co((RR,SS)-chxn)3-lel3]3+, [Co((RR,SS)-chxn)3-lel2ob]3+, [Co((RR,SS)-chxn)3-lelob2]3+, [Co((RR,SS)-chxn)3-ob3]3+ and corresponding species for [Co((R, S)-pn)3]3+ show patterns of selectivity which are analyzed in terms of the size and structure of the reactants. The presence of a pseudo-C3 carboxylate face on the oxidant enhances selectivity but the pattern is quite different for those oxidants that contain oxalate as one of their ligands compared with non-oxalate containing species such as [Co(edta)]−. A very simple model is developed in which the reductant employs a limited set of interactions corresponding to the major symmetry axes. The unrestricted reductant has very low aggregate selectvity. Steric and hydrogen bonding patterns in both oxidant and reductant enhance individual interactions resulting in the observed selectivities.