MCM-41 supported manganese and ruthenium catalysts for the oxidation of hydrocarbons with tert-butyl hydroperoxide and hydrogen peroxide

Abstract

Several novel MCM-41 supported Mn (III) complexes with the nitrogen donor ligands 1,11-bis(2-pyridyl)-2,6,10,-triazaundec-1,10-diene [MCM-41-Mn-(NNNN)] and 1,11-bis(2-pyridyl)-2,6,10,-triazaundecane [MCM-41-Mn-(NHNH)] and their Schiff base analogue [MCM-41-Mn-(NNOO)] have been prepared. At similar surface coverage, the activity of [MCM-41-Mn-(NNNN)] and [MCM-41-Mn-(NHNH)] in the oxidation of cyclohexene were found to be higher than that of [MCM-41-Mn-(NNOO)]. Extended X-ray Absorption Fine Structure (EXAFS) analysis indicated that at high surface coverage isolated catalyst sites were maintained in [MCM-41-Mn-(NNNN)] and [MCM-41-Mn-(NHNH)], whereas in [MCM-41-Mn-(NNOO)] phenoxy-bridged oligomers were found on the mesoporous silica support. MCM-41-Mn-NNNN is an effective catalyst for alkene oxidation with t-BuOOH. With the exception of cyclohexene, oxidation of cycloalkenes gave epoxides as the major product. For phenyl substituted alkenes, oxidation products resulting from C=C double bond cleavage were formed predominantly. A ruthenium diaquo complex with 5,5'-bis(trifluoromethyl)-2,2'-bipyridine (cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ has been synthesized. ¹H NMR spectroscopic and X-ray crystallographic studies indicated that the two 5,5'-(CF₃)₂-bpy ligands are in the cis configuration. Electrochemical studies of cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ indicated that the oxidation of RuII to RuVI is a proton coupled-electron transfer reaction and a disproportionation mechanism is proposed to account for the oxidation of RuIII to RuIV. Introduction of trifluoromethyl substituents on the bipyridine ligand shifts the reduction potential of the ruthenium complex towards the anodic side. cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ is a robust electrocatalyst for alcohol oxidation with high current efficiency (87 %). Both cis-[RuIV(5,5'-(CF₃)₂-bpy)₂(H₂O)(O)]²⁺ and cis-[RuVI(5,5'-(CF₃)₂-bpy)₂(O)₂]²⁺ are active towards benzyl alcohol oxidation in 0.1 M CF₃COOH. The reactivities of cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ towards hydrocarbon oxidation with t-BuOOH and H₂O₂ were studied. cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ was found to be an active catalyst for the oxidation of alkenes to their corresponding epoxides and aldehydes. The results of spectroscopic studies suggested that an oxoruthenium species was the active intermediate when H₂O₂ was used as oxidant. With t-BuOOH, a radical mechanism involving t-BuOO● as the active species is more likely. In the oxidation of cis-stilbene, the formation of a mixture of cis-epoxide and trans-epoxide indicated that the epoxidation reaction involved a nonconcerted mechanism. By consideration of the spin delocalization and polar effects (dual-parameter Hammett correlation), the rate constants for the oxidation of para-substituted styrenes by cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ with H₂O₂ can be linearly correlated with the carboradical substituent constants (sp) and the spin delocalization substituent constants (sjj) . This implied the involvement of a benzylic radical intermediate in the oxidation of styrene by cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)(O)]²⁺. The value of 0.57 for Pp/Pjj (Pp = coefficient of electron-pair donating ability, Pjj = coefficient of spin delocalizing ability) suggested that both polar and spin delocalization effects are important in the cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ mediated alkene oxidation with H₂O₂. For the oxidation of benzyl alcohol, a linear dual-parameter Hammett correlation (r = 0.99, Pp/Pjj = 0.47) and a small primary kinetic isotope value (kH/kD = 4.4) were observed suggesting that the oxidation of benzyl alcohol involved the abstraction of an α-hydrogen atom by an oxoruthenium species during catalysis. cis-[RuII(5,5'-(CF₃)₂-bpy)₂(H₂O)₂]²⁺ was immobilized onto mesoporous MCM-41 molecular sieves by an organic linker with -NH₂ group. The supported ruthenium complex catalyzed the oxidation of alkenes with H₂O₂ as oxidant. The catalytic activity of the immobilized ruthenium complex was found to depend on the complex loading. Oxidation of cycloalkenes (e.g. cyclooctene and norbomene) resulted in the formation of the corresponding cycloalkene oxides. Aromatic alkenes (e.g. cis/trans-stilbene and styrene) were converted to their corresponding epoxides with aldehydes as the side product. The yield and turnover number were found to be higher than the homogeneous counterpart. The supported ruthenium catalyst displayed size selectivity in the oxidation of (+)-limonene in which the terminal C=C double bond (vs. internal trisubstituted C=C double bond) was more readily oxidized under heterogeneous reaction conditions. After a 12 hr reaction, the supported ruthenium catalyst retained about 53 % of its initial activity. The loss of activity could be attributed to both catalyst leaching and deactivation.