Electron-Transfer Oxidation of Coenzyme B<sub>12</sub>Model Compounds and Facile Cleavage of the Cobalt(IV)−Carbon Bond via Charge-Transfer Complexes with Bases. A Negative Temperature Dependence of the Rates
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- Kei Ohkubo
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
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- Shunichi Fukuzumi
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
書誌事項
- 公開日
- 2005-01-20
- DOI
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- 10.1021/jp0453008
- 公開者
- American Chemical Society (ACS)
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説明
The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2Co(III)(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH)2Co(III)(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2Co(IV)(Me)(py)]+, which were well identified by the ESR spectra. The reorganization energy (lambda) for the electron-transfer oxidation of (DH)2Co(Me)(py) was determined from the ESR line broadening of [(DH)2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The lambda value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2Co(IV)(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.
収録刊行物
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- The Journal of Physical Chemistry A
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The Journal of Physical Chemistry A 109 (6), 1105-1113, 2005-01-20
American Chemical Society (ACS)
