Enantioselectivities in Electron-Transfer and Excited State Quenching Reactions of a Chiral Ruthenium Complex Possessing a Helical Structure

摘要

The outer-sphere electron-transfer reactions between diastereomers of Ru(menbpy)_3~(centre dot +) (menbpy = 4,4'-di-{(1R, 2S, 5R)-(-)-menthoxycarbony1}-2,2' -bipyridine) and enantiomers of Co(acac)_3 and Co(edta)~- have been studied by pulse radiolysis. #DELTA#-Ru(menbpy)_3~(centre dot +) rapidly reduces Co(acac)_3 in 85% EtOH/H_2O (1 mM NaH_2-PO_4) 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~(centre dot +) preferentially reduces #LAMBDA#-Co(acac)_3 with an enantioselectivity factor (k_(et)~#DELTA#/k_(et)~#LAMBDA#) of 0.8. Activation volume data (#DELTA#V~(++)) suggest that the association between the #DELTA#-#DELTA# isomers in the encounter complex allows closer interaction of the metal centers than between the other isomer combinations. The value of k_(et)~#DELTA#/k_(et)~(LAMBDA) for the reaction of #DELTA#- and #LAMBDA#-Co(edta)~- with #DELTA#-Ru(menbpy)_3~(centre dot +) is 1.2. Electron-transfer reactions of seven racemic Ru(L)_3~(centre dot +) (L = substituted phenanthroline) complexes with Co(acac)_3 were also studied and gave rate constants of approx= 1.5 X 10~9 M~(-1) s~(-1). The quenching of photoexcited ~*Ru(menbpy)_3~(2+) by Co(acac)_3 and Co(edta)~- exhibits small stereoseletivity: For Co(acac)_3 in 95 and 85% EtOH/H_2O 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 apporx= 1 X 10~8 and 1.1 X 10~9 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)_3~(2+) by only a factor of approx= 3 despite an increase in the driving force of approx= 0.5 eV for electron-transfer quenching. The quenching of ~*Ru(menbpy)_3~(2+) 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~(centre dot +),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)_3~(2+) with Co(acac)_3 are discussed.