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

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 (k et Δ/k et Λ) of 2.7. Λ-Ru(menbpy)3 •+ preferentially reduces Λ-Co(acac)3 with an enantioselectivity factor (k et Δ/k et Λ) 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 k et Δ/k et Λ 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)3 2+ 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)3 2+ 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)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 •+, 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.

Reduction of Cobalt and Iron Phthalocyanines and the Role of the Reduced Species in Catalyzed Photoreduction of CO2

“…Since the lifetime of the TP radical anion (TP •- ), formed from the reductive quenching of the excited state with TEA, is longer in MeCN than in THF, subsequent flash photolysis experiments were carried out in MeCN. The details of the laser flash photolysis system are as described before …”

Design and Enantioresolution of Homochiral Fe(II)–Pd(II) Coordination Cages from Stereolabile Metalloligands: Stereochemical Stability and Enantioselective Separation

“…Supramolecular chirality in coordination chemistry is plentiful because of not only chiral information brought about by chiral ligands in the coordination sphere, but also because of enormous stereogenic metal-centers generated by a large variety of stereoconfigurations from versatile coordination geometry. − However, the synthesis and predetermination of chiral-at-metal complexes from achiral ligands is very cumbersome as many possible stereoisomers have to be separated and notorious racemization or epimerization phenomena occur for stereolabile metal-centers. − As a keystone of coordination chemistry, the D 3 -symmetry tris-chelate-M complexes of octahedral geometry with chirality that simply originated from propeller-like arrangement of bidentate ligands are vigorously studied in chiral induction/recognition, − enantioseparation, − asymmetric catalysis, − and pharmaceuticals. , Nevertheless, in contrast to the kinetically inert stereogenic metal-centers like in Ru­(phen) 3 2+ , the chiral complexes of first-row transition metals, which are abundant and give easy access to synthesis, are often highly liable to racemize, hampering their application toward enantioseparation, catalysis, and pharmaceuticals.…”

Solvent control on the electron transfer reaction between CoIII(en)2Br(L)2+–Fe(CN) (L = aryl amines) by regression relationships: the PXRD and electrochemical investigations

“…So, it is clear that interplay of factors namely, solvent caging, H‐bonding specific solvation, and tuning of a ligand will facilitate product formation. The lack of significant amount of ET reaction at low x 2 is ascribed to the fast back reaction of the Co III ….Fe II ion pair within the solvent cage to transform into reactants 44. Meyerstein et al 45.…”