Nucleophilic addition of .alpha.-hydroxyalkyl radicals to polypyridyl complexes of chromium(III), cobalt(III), rhodium(III), and ruthenium(II) in aqueous solution

Venturi, M.; Emmi, Salvatore S.; Fuochi, P. G.; Mulazzani, Quinto G.

doi:10.1021/j100454a011

Cited by 27 publications

(10 citation statements)

References 8 publications

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“…The dose-corrected absorbance changes from the pulse-radiolysis experiments in N 2 O-saturated EtOH/H 2 O agree well with the difference spectrum in acetonitrile, indicating the formation of the ligand-centered radical Ru(menbpy) 3 •+ (Figure ). Although the CH 3 C • HOH radical does not reduce Ru(bpy) 3 2+ at pH 7 ( E ° ≈ −1.3 V vs NHE), − the reduction potential of Ru(menbpy) 3 2+/+ is significantly more positive, −0.90 V vs NHE in water. CH 3 C • HOH (−1.25 V at pH 7) reduces Ru(menbpy) 3 2+ with a rateconstant of (8.2 ± 0.5) × 10 8 M -1 s -1 .…”

Section: Resultsmentioning

confidence: 94%

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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Section: Resultsmentioning

confidence: 94%

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 rate constant for this reaction is 7.5 ¥ 10 10 dm 3 mol -1 s -1 . 30 The spectra in Fig. 10 may be compared with the spectra obtained in spectroelectrochemical experiments (Fig.…”

Section: Reaction Of E -mentioning

confidence: 99%

Synthesis, characterization and pulse radiolysis of cobalt(ii) complexes of 2-picolinate and polypyridyl ligands

Kawade

Kumbhar

Naik³

et al. 2010

Dalton Trans.

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Complexes of the type [Co(pic)(2)(LL)], where pic = picolinate; LL = (H(2)O)(2) (1), 2,2'-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3) and dipyrido[3,2-a:2',3'-c]phenazine (dppz) (4) have been synthesized and characterized by elemental analysis, IR, UV-visible and (1)H-NMR spectroscopy. Complex 2 crystallized from a mixture of chloroform and methanol in orthorhombic crystal system, space group Iba2. Complex 3 crystallizes from chloroform in monoclinic crystal system, space group P12(1)/a1 and in a mixture of water and methanol in triclinic crystal system, space group P1. Co(II)/Co(III) oxidation potentials have been determined by cyclic voltammetry and the concomitant spectral changes measured by spectroelectrochemistry. The reactions of one electron reducing (e(-)(aq), (CH(3))(2)*COH and CO(2)(*-)) and one electron oxidizing ( OH) radicals with the above metal complexes have been studied by pulse radiolysis. The rate constants for the reaction of e(-)(aq) and *OH radicals with the complexes are of the order of 10(10) and 10(9) dm(3) mol(-1) s(-1), respectively. The reaction of hydrated electron (e(-)(aq)) with the complexes results in the formation of a ligand radical anion species which decays by either protonation of ligand or reduction of metal by intramolecular electron transfer resulting in cobalt(I) species. The reaction of *OH radical forms the OH-adducts of the ligand.

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“…It has been observed that adduct formation with cobalt(II1)-amine complexes and alcohol radicals is possible. 29 We suggest that adduct formation of the isopropyl radical with the p-superoxo complex leads to the formation of an isomer of the peroxo complex 0 I \! /O\-' I / / \ /"r\ "i instead of the isomer which has lower molar absorptivity.15 Alternatively, part of the reduction proceeds through the cobalt(II1) center which leads to the intramolecular isomer formation as shown 0 I Isopropyl radical reduces p-p-nitrobenzoato-di-phydroxo-bis(tetraminecobalt(II1)) with a rate constant of 2 X lo9 M-' s-l, and it is shown in that study that the reduction occurs initially at the bridging aromatic ring.22 Later on, the reduced ligand transfers the electron at a slower rate to reduce the cobalt(II1) center.…”

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confidence: 95%

“…The redox behavior of the 2-propanol radical with polypyridyl transition metal complexes also shows behavior different from the C H 2 0 H and CH,CHOH radicals. 29 It was suggested that the radical forms an adduct with the ligands in an initial step before reduction takes place. It has been observed that adduct formation with cobalt(II1)-amine complexes and alcohol radicals is possible.…”

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confidence: 99%

Redox reactions of dioxygen complexes of cobalt(III) with organic radicals studied by pulse radiolysis

Natarajan¹,

Raghavan²

1981

J. Phys. Chem.

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Reduction processes of p-superoxo complexes of cobalt(III), [(NH3)4Co(p02,pNH2)C~(NH3)4]4+, 1, [ (ed2Co-(p02,pNH2)Co(en)2]4+, 2 (en = ethylenediamine), [ (CN)&O(~O~)CO(CN)~]~, 3, with the formate radical, the alkyl hydroxy radicals CH20H, CH3CHOH, (CH3)&OH, and (CH3)COH and the solvated electron were investigated. Reduction of these complexes occurs at the dioxygen center and the products of the reduction are determined to be the corresponding p-peroxo complexes. The reduction of complexes 2 and 3 by the solvated electron is studied and the bimolecular rate constant for the reduction of 2 is higher by two orders of magnitude than that for the reduction of 3. The same behavior is observed for the reaction of these p-superoxo complexes with the formate radical, indicating that the effect is due to the presence of the 4+ charge in 1 and 2 and the 5-charge in 3. The isopropyl radical reacts with the p-superoxo cobalt(II1) compelxes faster than .CH20H and C2H40H radicals. The product observed for the reduction of the p-superoxo amine complexes by the isopropyl radical appears to be an isomer of the peroxo complex. It is proposed that the reaction of the isopropyl radical with the superoxo complexes proceeds in part by involving the metal center. IntroductionReversible and irreversible uptake of oxygen by transition metal complexes has been known for some time.3 Oxygenation reactions of transition metal complexes are believed to model the uptake of oxygen in biological syst e m~.~~ Irreversible uptake of oxygen by metal ions results in the formation of superoxo and peroxo complexes with different structures and reactivities.6 The reactions of the oxygen-coordinated complexes have been actively investigated in recent years in order to understand the more complex processes occurring in living cellsS7 Amine complexes of cobalt(I1) and cobalt(II1) ions are known to coordinate with oxygen to give the superoxo (02-) or peroxo (OZ2-) complexes. Reactions of oxygen with cobalt(I1) complexes to give a 1:l adduct are fast at room temperature, and the 1:l adducts formed dimerize to give the peroxo c~mplexes.~ Structural6 and spectroscopic8 studies of these complexes suggest that the coordinated dioxygen exists perdominantly in the superoxo or peroxo form. Electron transfer reactions of the oxo-bridged complexes of cobalt(II1) have been studied with a variety of reducing

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Nucleophilic addition of .alpha.-hydroxyalkyl radicals to polypyridyl complexes of chromium(III), cobalt(III), rhodium(III), and ruthenium(II) in aqueous solution

Cited by 27 publications

References 8 publications

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

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

Synthesis, characterization and pulse radiolysis of cobalt(ii) complexes of 2-picolinate and polypyridyl ligands

Redox reactions of dioxygen complexes of cobalt(III) with organic radicals studied by pulse radiolysis

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