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タイトル: | Formation and characterization of a reactive chromium(V)-oxo complex: Mechanistic insight into hydrogen-atom transfer reactions |
著者: | Kotani, Hiroaki Kaida, Suzue Ishizuka, Tomoya Sakaguchi, Miyuki Ogura, Takashi Shiota, Yoshihito Yoshizawa, Kazunari Kojima, Takahiko |
著者名の別形: | 吉澤, 一成 |
発行日: | Feb-2015 |
出版者: | Royal Society of Chemistry |
誌名: | Chemical Science |
巻: | 6 |
開始ページ: | 945 |
終了ページ: | 955 |
抄録: | A mononuclear Cr(V)-oxo complex, [CrV(O)(6-COO--tpa)](BF4)2 (1; 6-COO--tpa = N, N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2, 2′-bipyridine). The reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e--oxidized products in moderate yields as determined from 1H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA. |
著作権等: | © The Royal Society of Chemistry 2015. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. |
URI: | http://hdl.handle.net/2433/216548 |
DOI(出版社版): | 10.1039/c4sc02285h |
PubMed ID: | 29560181 |
出現コレクション: | 学術雑誌掲載論文等 |
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