Stereoselective molecular recognition of enantiomeric cobalt(III) complexes by novel photosensitizers of helical ruthenium(II) complexes

Ohkubo, Katsutoshi; Hamada, Tomoyuki; Ishida, Hideyuki; Fukushima, Mitsuhiro; Watanabe, Megumi

doi:10.1016/0304-5102(93)e0341-d

Cited by 6 publications

(8 citation statements)

References 12 publications

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“…Both of these compounds were known to form a reversible monolayer at an air−water interface. , [Ru(phen) 2 (dC18-bpy)] 2+ is fluorescent when it is illuminated by light at 450 nm, while [Ru(acac) 2 (acacC12)] is nonfluorescent and acts as an electron acceptor from excited [Ru(phen) 2 (dC18-bpy)] 2+ . The previous photophysical studies in a homogeneous solution have confirmed that the photoinduced electron transfer really takes place from an excited polypyridyl−Ru(II) complex to an acetylacetonato−Ru(III) complex …”

Section: Resultsmentioning

confidence: 76%

Preparation of the Langmuir−Blodgett Film of a Clay−Alkylammonium Adduct and Its Use as a Barrier for Interlayer Photoinduced Electron Transfer

et al. 2000

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We report on the photophysical application of the multilayer films of an ion-exchange adduct of a clay (synthetic saponite) and an alkylammonium cation (trimethylstearylammonium) as prepared by the Langmuir-Blodgett method. For preparing a film, the adduct was dispersed in chloroform and spread over the surface of pure water. A surface pressure-area isotherm was obtained at 25.0 °C with complete reproducibility. The critical surface area, Ac, was defined as an area where the linear portion of the isotherm was extrapolated to zero surface pressure. From the dependence of Ac on the spread amount of the adduct, it was concluded that the adduct formed a film of a single clay layer at an air-water interface. The floating film was transferred onto a hydrophilic glass substrate or a calcium fluoride plate as a Z-type multilayer film. The film was characterized by X-ray diffraction, infrared absorption spectra, and atomic force microscopy images. The prepared clay film was revealed to act as an efficient barrier for the occurrence of photoinduced electron transfer from an amphiphilic polypyridyl-Ru(II) complex (electron donor) to an amphiphilic acetylacetonato-Ru(III) complex (electron acceptor).

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Section: Resultsmentioning

confidence: 76%

Preparation of the Langmuir−Blodgett Film of a Clay−Alkylammonium Adduct and Its Use as a Barrier for Interlayer Photoinduced Electron Transfer

et al. 2000

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“…Electron-transfer quenching should be sensitive to changes in driving force. The reduction potentials of Ru(menbpy) 3 n + are −0.90, +1.55, and −0.45 V for E 0(2+/+) , E 0(3+/2+) , and E 0(3+/ * 2+) , respectively, − and the reduction potential of Co(acac) 3 is ≈−0.34 V . Thus, the free-energy changes, Δ G °, for the thermal electron-transfer reaction between Ru(menbpy) 3 •+ and Co(acac) 3 and the photochemical electron-transfer reaction between *Ru(menbpy) 3 2+ and Co(acac) 3 are −0.56 and −0.11 eV, respectively.…”

Section: Discussionmentioning

confidence: 97%

“…Electrontransfer quenching should be sensitive to changes in driving force. The reduction potentials of Ru(menbpy) 3 n+ are -0.90, +1.55, and -0.45 V for E 0(2+/+) , E 0(3+/2+) , and E 0(3+/ * 2+) , respectively, [16][17][18] and the reduction potential of Co(acac) 3 is ≈-0.34 V. 37 Thus, the free-energy changes, ∆G°, for the thermal electron-transfer reaction between Ru(menbpy) 3 •+ and Co(acac) 3 and the photochemical electron-transfer reaction between *Ru(menbpy) 3 2+ and Co(acac) 3 are -0.56 and -0.11 eV, respectively. The relative rates of the Ru(menbpy) 3 •+ and * Ru(menbpy) 3 2+ reactions are surprising in light of these ∆G°v alues; despite the much larger driving force for the Ru-(menbpy) 3 •+ reaction, the quenching of the *Ru(II) excited state is 1 order of magnitude faster.…”

Section: Discussionmentioning

confidence: 98%

“…Enantioselectivity factors as large as 92 have been reported in the photocatalytic reduction of Co(acac) 3 (acac = acetylacetonate) by a chiral ruthenium(II) complex Δ-Ru(menbpy) 3 2+ (menbpy = 4,4‘-di{(1 R ,2 S ,5 R )-(−)-menthoxycarbonyl}-2,2‘-bipyridine), − shown in Figure .…”

Section: Introductionmentioning

confidence: 95%

“…Energy-transfer reactions between lanthanide(III) complexes and a series of ruthenium(II) or cobalt(III) complexes have shown quenching rates that differ by up to a factor of 4 between enantiomers. [8][9][10][11][12] Enantioselectivity factors as large as 92 have been reported in the photocatalytic reduction of Co(acac) 3 (acac ) acetylacetonate) by a chiral ruthenium(II) complex ∆-Ru(menbpy) 3 2+ (menbpy ) 4,4′-di{(1R,2S,5R)-(-)-menthoxycarbonyl}-2,2′bipyridine), [13][14][15][16][17][18] shown in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 1999

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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.

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Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

et al. 2021

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Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

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Stereoselective molecular recognition of enantiomeric cobalt(III) complexes by novel photosensitizers of helical ruthenium(II) complexes

Cited by 6 publications

References 12 publications

Preparation of the Langmuir−Blodgett Film of a Clay−Alkylammonium Adduct and Its Use as a Barrier for Interlayer Photoinduced Electron Transfer

Preparation of the Langmuir−Blodgett Film of a Clay−Alkylammonium Adduct and Its Use as a Barrier for Interlayer Photoinduced Electron Transfer

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

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

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