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Contrasting effects of axial ligands on electron-transfer versus proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes

Title
Contrasting effects of axial ligands on electron-transfer versus proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes
Authors
Fukuzumi S.Kotani H.Suenobu T.Hong S.Lee Y.-M.Nam W.
Ewha Authors
남원우Shunichi Fukuzumi이용민
SCOPUS Author ID
남원우scopus; Shunichi Fukuzumiscopus; 이용민scopusscopus
Issue Date
2010
Journal Title
Chemistry - A European Journal
ISSN
0947-6539JCR Link
Citation
vol. 16, no. 1, pp. 354 - 361
Indexed
SCI; SCIE; SCOPUS WOS scopus
Abstract
The effects of axial ligands on electron-transfer and proton-coupled electron-transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [FeIV(O)(tmc)(X)]n+ (1-X) with various axial ligands, in which tmc is 1,4,8,11-tetramethyl-1,4,8,11- tetraazacyclotetradecane and X is CH3CN (1-NCCH3), CF 3COO- (1-OOCCF3), or N3- (1-N3), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one-electron reduction potentials of 1-X (Ered, V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron-donating axial ligands in the order of 1-NCCH3 (0.39) > 1-OOCCF3 (0.13) > 1-N3 (-0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1-X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (λ) of electron transfer. The λ values decrease in the order of 1-NCCH3 (2.37) > 1-OOCCF3 (2.12) > 1-N3 (1.97 eV). Thus, the electron-transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron-transfer rate in the order of 1-NCCH3 > 1-OOCCF3 > 1-N 3. In sharp contrast to this, the rates of the proton-coupled electron-transfer reactions of 1-X are markedly accelerated in the presence of an acid in the opposite order: 1-NCCH3 < 1-OOCCF3 < 1-N3. Such contrasting effects of the axial ligands on the electron-transfer and proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen-atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri etal. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19181-19186). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
DOI
10.1002/chem.200901163
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자연과학대학 > 화학·나노과학전공 > Journal papers
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