TRANSIENT CHIRAL INDUCTION IN THE REACTIONS OF OPTICALLY-ACTIVE [CO(OX)3]3- WITH [CO(PHEN)3]2+ AND DERIVATIVES

The kinetics of reduction of [Co(ox)3]3- by [Co(phen)3]2+ (ox2- = oxalate(2-), phen = 1,10-phenanthroline) were investigated under pseudo-first-order conditions with an excess of reductant at 25.0-degrees-C and 0.017 M ionic strength. At low [[Co(phen)3]2+] the reaction is first order in both reactants, with a second-order rate constant, k(os) = 15.8 M-1 s-1 (DELTA-H double-ended dagger = 12.1 kJ mol-1, DELTA-S double-ended dagger = -181 J K-1 mol-1). The rate shows limiting first-order behavior at higher [[Co(phen)3]2+], consistent with an outer-sphere mechanism involving a kinetically significant ion pair. The ion-association constant for formation of this ion pair is calculated to be 65 M-1 (DELTA-H-degrees = -20 kJ mol-1, DELTA-S-degrees = -33 J K-1 mol-1), and the limiting first-order electron-transfer rate is 0.24 s-1 (DELTA-H(et) double-ended dagger = 32 kJ mol-1, DELTA-S(et) double-ended dagger = -148 J K-1 mol-1). As [phen] is reduced so that [Co(phen)2]2+ predominates, a second, inner-sphere pathway is revealed. In this pathway, a doubly bridged oxalate group is transferred from [Co(ox)3]3- to [Co(phen)2]2+ with k(is) = 94 M-1 s-1 to give [Co(phen)2(ox)]+ as the initial product. Both outer-sphere and inner-sphere processes show significant overall stereoselectivities with k(os)(DELTA-DELTA)/k(os)(DELTA-LAMBDA) = 1.63 and k(is)(DELTA-DELTA)/k(is)(DELTA-LAMBDA) = 4.4, respectively. Less extensive studies have also been carried out on the reduction of [Co(ox)3]3- by [Co(bpy)3]2+, [Co(4,7-Me2phen)3]2+, and [Co(5,6-Me2phen)3]2+ (bpy = 2,2'-bipyridyl). Information on stereoselectivity in the ion pair formed between [Co(ox)3]3- and [Co(phen)3]2+ was investigated with conductivity studies on the analogues [Rh(ox)3]3- and [Ru(phen)3]2+. At 0.010 M ionic strength and 25.0-degrees-C, there is little difference in ion-association constants for the diastereomeric ion pairs with K0(DELTA-DELTA)/K0(DELTA-LAMBDA) = 1.05. Consequently, it is argued that most of the chiral discrimination in the outer-sphere electron-transfer pathway occurs in the intramolecular electron-transfer step within the ion pair. NMR studies and crystallographic data from related systems suggest that despite the small diastereomeric discrimination, the ion pair shows a significant degree of structure, with metal centers almost-equal-to 6 angstrom apart and with a predominant C3-C3 interaction. The dependence of stereoselectivity on reductant structure is consistent with this model. The self-exchange rate for [Co(bpy)3]3+/2+ was also determined to be 5.7 M-1 s-1 (DELTA-H double-ended dagger = 31.1 kJ mol-1, DELTA-S double-ended dagger = -127 J K-1 mol-1) at 25.0-degrees-C and 0.10 M ionic strength.