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Title: Hydrocarbon Oxidation by β-Halogenated Dioxoruthenium(VI) Porphyrin Complexes : effect of reduction potential (RuVI/V) and CH Bond-Dissociation Energy on rate constants
Authors: Che, CM
Zhan, JL
Zhang, R
Huang, JS
Lai, TS
Tsui, WM
Zhou, XG
Zhou, ZY
Zhu, NY
Chang, CK
Issue Date: 2005
Source: Chemistry - a European journal, 2005, v. 11, no. 23, p. 7040-7053
Abstract: β-Halogenated dioxoruthenium(VI) porphyrin complexes [RuVI(F28-tpp)O2] [F28-tpp=2,3,7,8,12,13, 17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2−)] and [RuVI(β-Br8-tmp)O2] [β-Br8-tmp=2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(2,4,6-trimethylphenyl)porphyrinato(2−)] were prepared from reactions of [RuII(por)(CO)] [por=porphyrinato(2−)] with m-chloroperoxybenzoic acid in CH2Cl2. Reactions of [RuVI(por)O2] with excess PPh3 in CH2Cl2 gave [RuII(F20-tpp)(PPh3)2] [F20-tpp=5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2−)] and [RuII(F28-tpp)(PPh3)2]. The structures of [RuII(por)(CO)(H2O)] and [RuII(por)(PPh3)2] (por=F20-tpp, F28-tpp) were determined by X-ray crystallography, revealing the effect of β-fluorination of the porphyrin ligand on the coordination of axial ligands to ruthenium atom. The X-ray crystal structure of [RuVI(F20-tpp)O2] shows a RuO bond length of 1.718(3) Å. Electrochemical reduction of [RuVI(por)O2] (RuVI to RuV) is irreversible or quasi-reversible, with the Ep,c(RuVI/V) spanning −0.31 to −1.15 V versus Cp2Fe+/0. Kinetic studies were performed for the reactions of various [RuVI(por)O2], including [RuVI(F28-tpp)O2] and [RuVI(β-Br8-tmp)O2], with para-substituted styrenes p-XC6H4CHCH2 (X=H, F, Cl, Me, MeO), cis- and trans-β-methylstyrene, cyclohexene, norbornene, ethylbenzene, cumene, 9,10-dihydroanthracene, xanthene, and fluorene. The second-order rate constants (k2) obtained for the hydrocarbon oxidations by [RuVI(F28-tpp)O2] are up to 28-fold larger than by [RuVI(F20-tpp)O2]. Dual-parameter Hammett correlation implies that the styrene oxidation by [RuVI(F28-tpp)O2] should involve rate-limiting generation of a benzylic radical intermediate, and the spin delocalization effect is more important than the polar effect. The k2 values for the oxidation of styrene and ethylbenzene by [RuVI(por)O2] increase with Ep,c(RuVI/V), and there is a linear correlation between log k2 and Ep,c(RuVI/V). The small slope (≈2 V−1) of the log k2 versus Ep,c(RuVI/V) plot suggests that the extent of charge transfer is small in the rate-determining step of the hydrocarbon oxidations. The rate constants correlate well with the CH bond dissociation energies, in favor of a hydrogen-atom abstraction mechanism.
Keywords: Electrochemistry
Structure elucidation
Publisher: Wiley-VCH
Journal: Chemistry - a European journal 
ISSN: 0947-6539
EISSN: 1521-3765
DOI: 10.1002/chem.200500814
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