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

P-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-)I and [Ru-VI(beta-Br-8-tmp)O-2] [beta-Br-8-tmp = 2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(2,4,6-trimethylphenyl)porphyr- inato(2-)] were prepared from reactions of [Ru-II(por)(CO)] [por=porphyrinato(2-)] with ni-chloroperoxybenzoic acid in CH2Cl2. Reactions of [Ru-VI(por)O-2] with excess PPh3 in CH2Cl2 gave [Ru-VI(F-20-tpp)(PPh3)(2)] [F-20-tPP = 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2-)] and [Ru-VI(F-28-tpp)(PPh3)(2)]. The structures of [Ru-VI(por)(CO)(H2O)] and [Ru-VI(por)(PPh3)(2)] (por=F-20-tpp, F-28-tpp) were determined by X-ray crystallography, revealing the effect of P-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) A. 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 Cp2Fe+/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(beta-Br-8-tmp)O-2], with para-substituted styrenes p-X-C6H4CH=CH2 (X = H, F, Cl, Me, MeO), Cis- and trans-P-in ethylstyrene, cyclohexene, norbornene, ethylbenzene, cumene, 9,10-dihydroanthracene, xanthene, and fluorene. ne 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 (approximate to 2 V-1) of the log k(2) versus E-p.c(Ru-VI/V) 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.