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

beta-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)(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)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 beta-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 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)(beta-Br(8)-tmp)O(2)], with para-substituted styrenes p-X-C(6)H(4)CH=CH(2) (X=H, F, Cl, Me, MeO), cis- and trans-beta-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 (approximately 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.

The (Porphyrin)ruthenium‐Catalyzed Aziridination of Olefins Using Aryl Azides as Nitrogen Sources

“…All starting materials were commercial products and were used as received unless otherwise reported. Aryl azides, [67][68][69] meso-tetraphenylporphyrins, [70,71] β-substituted meso-tetraphenylporphyrins [72,73] and their ruthenium complexes, [54,74,75] were synthesized according to methods reported in the literature or by using minor modifications of them. [Ru(OEP)(CO)] (Aldrich) was used as received.…”

The [Ru(CO)(porphyrin)]‐Catalyzed Synthesis of N‐Aryl‐2‐vinylaziridines

“…4-Nitrophenyl azide, [55] 4-chlorophenyl azide, [55] 4-bromophenyl azide, [55] 4-methoxyphenyl azide, [55] 4-cyanophenyl azide, [55] and 3,5-bis(trifluoromethyl)-phenyl azide [55] were synthesized by methods reported in the literature. The complexes [Ru(CO)(porphyrin)] [porphyrin = TPP, [56] 4-(CF 3 )TPP, [56] 3,5-(CF 3 ) 2 TPP, [57] 4-nBuTPP, [56] TMOP, [56] TMP, [56] 2,6-(Cl) 2 TPP] [56] (TPP = dianion of tetraphenylporphyrin) were synthesized by the literature methods. [Ru(CO)(OEP)] (Aldrich) was used as received.…”

Fluorinated Five‐Membered Nitrogen‐Containing Heterocycles

“…87 In another example, a ruthenium complex functioned as a potential methane functionalization catalyst by virtue of the ability of 88 Additional studies of the use of ruthenium complexes of fluorinated porphyrins as oxidation catalysis have been reported. 89 Partially fluorinated porphyrins were made by a similar Lindsey cyclization. Condensation of benzaldehyde with mixtures of 3,4-difluoropyrrole and pyrrole gave mixtures of di-, tetra-, and hexafluoro meso-tetraphenylporphyrin with fluorine atoms in pairs per pyrrole.…”