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Enantioselectivities in Electron-Transfer and Excited State Quenching Reactions of a Chiral Ruthenium Complex Possessing a Helical Structure
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Abstract
The outer-sphere electron-transfer reactions between diastereomers of Ru(menbpy)3•+ (menbpy = 4,4‘-di{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2‘-bipyridine) and enantiomers of Co(acac)3 and Co(edta)- have been studied by pulse radiolysis. Δ-Ru(menbpy)3•+ rapidly reduces Co(acac)3 in 85% EtOH/H2O (1 mM NaH2PO4) with second-order rate constants of (2.1 ± 0.1) × 107 and (7.8 ± 0.2) × 106 M-1 s-1 for the Δ- and Λ-Co(acac)3 enantiomers, respectively, and an enantioselectivity factor (ketΔ/ketΛ) of 2.7. Λ-Ru(menbpy)3•+ preferentially reduces Λ-Co(acac)3 with an enantioselectivity factor (ketΔ/ketΛ) of 0.8. Activation volume data (ΔV‡) suggest that the association between the Δ−Δ isomers in the encounter complex allows closer interaction of the metal centers than between the other isomer combinations. The value of ketΔ/ketΛ for the reaction of Δ- and Λ-Co(edta)- with Δ-Ru(menbpy)3•+ is 1.2. Electron-transfer reactions of seven racemic Ru(L)3•+ (L = substituted phenanthroline) complexes with Co(acac)3 were also studied and gave rate constants of ≈1.5 × 109 M-1 s-1. The quenching of photoexcited *Ru(menbpy)32+ by Co(acac)3 and Co(edta)- exhibits small stereoselectivity: For Co(acac)3 in 95 and 85% EtOH/H2O the enantioselectivity factor is 1.2 and 1.1, respectively, barely outside the experimental error. For all other cases the selectivity was unity within the experimental error of the measurement. The quenching rate constants were ≈1 × 108 and 1.1 × 109 M-1 s-1 for Co(acac)3 and Co(edta)-, respectively. Quenching reactions of seven racemic ruthenium(II) phenanthroline complexes with Co(acac)3 were also studied and found to be faster than those of Ru(menbpy)32+ by only a factor of ≈3 despite an increase in the driving force of ≈0.5 eV for electron-transfer quenching. The quenching of *Ru(menbpy)32+ by Co(acac)3 is dominated by an energy-transfer mechanism. This conclusion is supported by the magnitude of the quenching rate constants compared with the rate constants for reduction by Ru(menbpy)3•+, the effect of driving-force changes on the quenching rate constant, the low quantum yield of Co(II) products observed in the CW photolysis, and the lack of long-lived products observed in the flash photolysis experiments. The factors responsible for the selectivity exhibited in the CW photolysis studies of Ru(menbpy)32+ with Co(acac)3 are discussed.
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- Published In Issue July 22, 1999
- Received April 1, 1999
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