Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Zhou, De-Ling; Tinembart, O.; Scheffold, Rolf; Walder, Lorenz

doi:10.1002/hlca.19900730820

Cited by 36 publications

(40 citation statements)

References 55 publications

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“…Zhou et al (44) showed that electron-withdrawing groups at the Co-bound C-atom shift the reduction potential of the corrinoid to more positive values. So, it is likely that adducts like vinyl-and ethynylcobalamin are more difficult to reduce (44), thus slowing down or even blocking the regeneration of the reactive Co(I) species.…”

Section: Resultsmentioning

confidence: 99%

Corrinoid-Mediated Reduction of Tetrachloroethene, Trichloroethene, and Trichlorofluoroethene in Homogeneous Aqueous Solution: Reaction Kinetics and Reaction Mechanisms

Glod

Angst

Holliger

et al. 1996

Environ. Sci. Technol.

172

235

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It is shown that in homogeneous aqueous solution containing titanium(III) citrate or titanium(III)-NTA as bulk electron donor, cobalamin, cobinamide, and cobamide are effective electron transfer mediators for the reduction of tetrachloroethene (PCE), trichloroethene (TCE), and trichlorofluoroethene (TCFE). For a given chlorinated ethene, the reaction rate varied only slightly with pH and type of corrinoid present and was about 5 and 50 times faster for PCE as compared to TCFE and TCE, respectively. Evidence is presented that the first and rate-limiting step of the reduction of PCE, TCE, and TCFE by super-reduced corrinoids is a dissociative one-electron transfer yielding the corresponding vinyl radicals. Furthermore, the elimination of a chloride radical from the 1,1-dichlorovinyl radical yielding chloroacetylene and subsequently acetylene is proposed to account for the direct formation of acetylene out of TCE. Finally, it is demonstrated that at higher reduction potentials the corrinoid mediators may be blocked by the formation of addition products.

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Section: Resultsmentioning

confidence: 99%

Corrinoid-Mediated Reduction of Tetrachloroethene, Trichloroethene, and Trichlorofluoroethene in Homogeneous Aqueous Solution: Reaction Kinetics and Reaction Mechanisms

Glod

Angst

Holliger

et al. 1996

Environ. Sci. Technol.

172

235

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“…Concerning the activation of the C–Co bond by electrolysis, Scheffold et al studied the reduction potential of alkylcobalamine and alkylcobyrinates using single-scan voltammetry by varying the axial ligand (namely the R group of R-Co III complexes) . Accordingly, a correlation was established between the reduction potential of R-Co III and the p K a of RH, which helped in the visualization of intermediates formed during the light-induced insertion of an olefin into the C–Co bond of the R-Co III .…”

Section: Vitamin B12-type Organocobalt(iii)mentioning

confidence: 99%

Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries

2019

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Organocobalt(III) complexes (R-Co III ), defined as cobalt complexes featuring a carbon−cobalt bond, are largely used to produce carbon-centered radicals by homolytic cleavage of their C−Co bond under mild conditions. They are key compounds in cutting-edge developments in the fields of organic chemistry, biochemistry, medical research, radical reactions, and organometallic chemistry. This is the first Review of the use of R-Co III in both organic and polymer chemistries. Although pioneering works in organic synthesis have largely contributed to the implementation of R-Co III in polymer design, the two fields have evolved independently, with many breakthroughs on both sides. The main motivation of this Review is to confront both fields to stimulate cross-fertilization. It notably describes the most important synthetic pathways for R-Co III , the influence of the ligand structure and the environment of the complex on the C−Co bond strength, the modes of formation of the radicals, and the most relevant R-Co III -promoted radical reactions, with a focus on the main reaction mechanisms. CONTENTS 6.4. Polymerizations 6938 7. Conclusions 6945 Author Information 6946 Corresponding Author 6946 ORCID 6946 Notes 6946 Biographies 6946 Acknowledgments 6946 References 6946

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“…133 The reactivity of corrin-based and other Co I complexes in nucleophilic substitutions and additions has been studied intensely, particularly due to the biologic relevance of vitamin B12. [133][134][135][136][137] In the present case it is reasonable to assume that Co I could add to the iminium ion as a suitable electrophile, as schematically represented by equation (11).…”

Section: Phase II -Intermediate Decay Of Co Imentioning

confidence: 99%

Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution

et al. 2014

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The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of fac-[Re(py)(CO)3bipy](+) (py = pyridine, bipy = 2,2'-bipyridine) as photosensitizer, [Co(TPY-OH)(OH2)](2+) (TPY-OH = 2-bis(2-pyridyl)(hydroxy)methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co(II)-WRC. Protonation of Co(I) yields Co(III)H which is reduced in the presence of excess Co(I). Co(II)H releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co(I), which is explained by a nucleophilic addition of Co(I) to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations.

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Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Cited by 36 publications

References 55 publications

Corrinoid-Mediated Reduction of Tetrachloroethene, Trichloroethene, and Trichlorofluoroethene in Homogeneous Aqueous Solution: Reaction Kinetics and Reaction Mechanisms

Corrinoid-Mediated Reduction of Tetrachloroethene, Trichloroethene, and Trichlorofluoroethene in Homogeneous Aqueous Solution: Reaction Kinetics and Reaction Mechanisms

Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries

Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution

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