Oxidation of Hydrocarbons with H2O2/O2 Catalyzed by Osmium Complexes in Acetonitrile

Ten osmium complexes containing polypyridine and cyclometalated ligands have been studied as catalysts for the oxidation of alkanes. The complexes effectively catalyze the oxygenation of alkanes (cyclohexane, n-heptane, methylcyclohexane, hydrogen peroxide in acetonitrile solution in air at 60°C in the presence of silver salts as the co-catalysts. Alkanes are oxidized mainly to alkyl hydroperoxides, which are easily reduced by PPh₃ to the corresponding alcohols. The oxygenation of cyclohexane catalyzed by osmium complexes used at a low concentration (2.5 × 10^–4 M) gave the maximum yield of oxidation products of 35% in the case of complex (ttpy)₂OsCl₃ (ttpy = 4′-(p-tolyl)-2,2′:6′,2′′-terpyridine). The measurement of regio- and bond selectivity parameters for the oxidation of linear and branched alkanes, kinetic analysis of the dependence of the initial rate of cyclohexane oxidation on concentration alkane and analysis of the competitive oxidation of cyclohexane and n-hexane indicates that oxidation induces the interaction of the osmyl-oxo complex with the substrate. The structure of a new cyclometalated complex [(η⁶-cymene)Os(bq)(MeCN)](PF₆) (bqH = 7,8-benzoquinoline) was determined by SC-XRA.