Hydrocarbon Oxidation by β-Halogenated Dioxoruthenium(VI) Porphyrin Complexes: Effect of Reduction Potential (RuVI/V) and CH Bond-Dissociation Energy on Rate Constants
✍ Scribed by Chi-Ming Che; Jun-Long Zhang; Rui Zhang; Jie-Sheng Huang; Tat-Shing Lai; Wai-Man Tsui; Xiang-Ge Zhou; Zhong-Yuan Zhou; Nianyong Zhu; Chi Kwong Chang
- Publisher
- John Wiley and Sons
- Year
- 2005
- Tongue
- English
- Weight
- 383 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
β‐Halogenated dioxoruthenium(VI) porphyrin complexes [Ru^VI^(F~28~‐tpp)O~2~] [F~28~‐tpp=2,3,7,8,12,13, 17,18‐octafluoro‐5,10,15,20‐tetrakis(pentafluorophenyl)porphyrinato(2−)] and [Ru^VI^(β‐Br~8~‐tmp)O~2~] [β‐Br~8~‐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 [Ru^II^(por)(CO)] [por=porphyrinato(2−)] with m‐chloroperoxybenzoic acid in CH~2~Cl~2~. Reactions of [Ru^VI^(por)O~2~] with excess PPh~3~ in CH~2~Cl~2~ gave [Ru^II^(F~20~‐tpp)(PPh~3~)~2~] [F~20~‐tpp=5,10,15,20‐tetrakis(pentafluorophenyl)porphyrinato(2−)] and [Ru^II^(F~28~‐tpp)(PPh~3~)~2~]. The structures of [Ru^II^(por)(CO)(H~2~O)] and [Ru^II^(por)(PPh~3~)~2~] (por=F~20~‐tpp, F~28~‐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 [Ru^VI^(F~20~‐tpp)O~2~] shows a RuO bond length of 1.718(3) Å. Electrochemical reduction of [Ru^VI^(por)O~2~] (Ru^VI^ to Ru^V^) is irreversible or quasi‐reversible, with the E~p,c~(Ru^VI/V^) spanning −0.31 to −1.15 V versus Cp~2~Fe^+/0^. Kinetic studies were performed for the reactions of various [Ru^VI^(por)O~2~], including [Ru^VI^(F~28~‐tpp)O~2~] and [Ru^VI^(β‐Br~8~‐tmp)O~2~], with para‐substituted styrenes p‐XC~6~H~4~CHCH~2~ (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 (k~2~) obtained for the hydrocarbon oxidations by [Ru^VI^(F~28~‐tpp)O~2~] are up to 28‐fold larger than by [Ru^VI^(F~20~‐tpp)O~2~]. Dual‐parameter Hammett correlation implies that the styrene oxidation by [Ru^VI^(F~28~‐tpp)O~2~] should involve rate‐limiting generation of a benzylic radical intermediate, and the spin delocalization effect is more important than the polar effect. The k~2~ values for the oxidation of styrene and ethylbenzene by [Ru^VI^(por)O~2~] increase with E~p,c~(Ru^VI/V^), and there is a linear correlation between log k~2~ and E~p,c~(Ru^VI/V^). The small slope (≈2 V^−1^) of the log k~2~ versus E~p,c~(Ru^VI/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 CH bond dissociation energies, in favor of a hydrogen‐atom abstraction mechanism.