Hydrocarbon oxidation by β-halogenated dioxoruthenium(vi) porphyrin complexes: Effect of reduction potential (Ru VI/V) and C-H bond-dissociation energy on rate constants

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 = por-phyrinato(2-)] with m-chloroperoxybenzoic acid in CH 2Cl 2. Reactions of [Ru VI(por)O 2] with excess PPh 3 in CH 2Cl 2 gave [Ru II(F 20tpp)(PPh 3) 2] [F 20)-tpp = 5,10,15,20-tetrakis(pentafluo-rophenyl)porphyrinato(2-)] and [Ru II-(F 28-tpp)(PPh 3) 2]. The structures of [Ru II(por)(CO)(H 2O)] 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 Ru=O 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 -1.15V versus Cp 2Fe +/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-X-C 6H 4CH=CH 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 C-H bond dissociation energies, in favor of a hydrogen-atom abstraction mechanism. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.