Oxidation of hydrocarbons and alcohols with peroxides catalyzed by new π-cymene osmium complexes

Abstract

Two new osmium(II) complexes containing π-coordinated p-cymene (cym) ligands, [(η6-p-cym)Os(bipy)(MeCN)](PF6)2 (1) and (η6-p-cym)Os(py)Cl2 (2) where bipy is 2,2′-bipyridine, have been synthesized. Both complexes efficiently catalyze oxygenation of alkanes (cyclohexane, n-octane, methylcyclohexane, cis- and trans-isomers of 1,2-dimethylcyclohexane), benzene and 1-phenylethanol with hydrogen peroxide in acetonitrile solution in air at 60 °C. Alkanes are oxidized predominantly to alkyl hydroperoxides which can be easily reduced by PPh3 to afford corresponding alcohols. In the cyclohexane oxygenation catalyzed by complex 2 used in a very low concentration (2 × 10−7 M) turnover number (TON) attained 83,000 after 24 h; initial turnover frequency (TOF) was 3400 h−1. The ESI-MS spectrum of the reaction mixture demonstrated the absence of the initial dication [(η6-p-cym)Os(bipy)(MeCN)]2+ which indicates that complex 1 is only a pre-catalyst. Benzene and 1-phenylethanol were transformed into phenol and acetophenone, respectively. The regio- and bond-selectivity parameters measured for the oxidation of linear and branched alkanes and a kinetic analysis of the dependence of initial cyclohexane oxidation rate on concentration of the alkane indicates that the reaction proceeds via attack of hydroxyl radicals at a substrate. The half order of the reaction rate in respect to osmium concentration [Os] can be explained by the assumption that an osmium compound Os introduced into the reaction mixture is rapidly dimerized Os + Os ⇌ D with the equilibrium constant K1. The catalytic activity of the system is due to a monomeric osmium species. The monomer Os forms an adduct with a pyridine molecule (constant K9): Os + py ⇌ Os·py. The interaction with H2O2 affords hydroxyl radicals with rate constant k10. The obtained kinetic data allowed us to estimate values K1/K92 = 520 M and k10 = 1 M−1 s−1.