Enantioselectivities in electron-transfer and excited state quenching reactions of a chiral ruthenium complex possessing a helical structure

The outer-sphere electron-transfer reactions between diastereomers of Ru(me nbpy)3(.+) (menbpy = 4,4'-di {(1R,2S,5R)-(-)-menthoxycarbonyl}2,2'-bipyridi ne) and enantiomers of Co(acac)(3) and Co(edta)(-) have been studied by pul se radiolysis. Delta-Ru(menbpy)3(.+) rapidly reduces Co(acac)(3) in 85% EtO H/H2O (1 mM NaH2PO4) with second-order rate constants of (2.1 +/- 0.1) x 10 (7) and (7.8 +/- 0.2) x 10(6) M-1 s(-1) for the Delta- and Lambda-Co(acac)( 3) enantiomers, respectively, and an enantioselectivity factor (k(et)(Delta )/k(et)(Lambda)) of 2.7. Lambda-Ru(menbpy)(3)(.+) preferentially reduces La mbda-Co(acac)(3) with an enantioselectivity factor (k(et)(Delta)/k(et)(Lamb da)) of 0.8. Activation volume data (Delta V double dagger) suggest that th e association between the Delta-Delta isomers in the encounter complex allo ws' closer interaction of the metal centers than between the other isomer c ombinations. The value of k(et)(Delta)/k(et)(Lambda) for the reaction of De lta- and Lambda-Co(edta)(-) with Delta-Ru(menbpy)(3)(.+) is 1.2. Electron-t ransfer reactions of seven racemic Ru(L)(3)(.+) (L substituted phenanthroli ne) complexes with Co(acac)(3) were also studied and gave rate constants of approximate to 1.5 x 10(9) M-1 s(-1). The quenching of photoexcited *Ru(me nbpy)(3)(2+) by Co(acac)(3) and Co(edta)(-) exhibits small stereoselectivit y: 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 o ther cases the selectivity was unity within the experimental error of the m easurement. The quenching rate constants were approximate to 1 x 10(8) and 1.1 x 10(9) M-1 s(-1) for Co(acac)(3) and Co(edta)(-), respectively. Quench ing reactions of seven racemic ruthenium(II) phenanthroline complexes with Co(acac)(3) were also studied and found to be faster than those of Ru(menbp y)(3)(2+) by only a factor of approximate to 3 despite an increase in the d riving force of approximate to 0.5 eV for electron-transfer quenching. The quenching of *Ru(menbpy)(3)(2+) by Co(acac)(3) is dominated by an energy-tr ansfer mechanism. This conclusion is supported by the magnitude of the quen ching rate constants compared with the rate constants for reduction by Ru(m enbpy)(3)(.+), the effect of driving-force changes on the quenching rate co nstant, the low quantum yield of Co(LT) products observed in the CW photoly sis, and the lack of long-lived products observed in the flash photolysis e xperiments. The factors responsible for the selectivity exhibited in the CW photolysis studies of Ru(menbpy)(3)(2+) with Co(acac)(3) are discussed.