Syntheses, Structures, and Reactivities of Cobalt(III)−Alkylperoxo Complexes and Their Role in Stoichiometric and Catalytic Oxidation of Hydrocarbons

Abstract

Although Co(III)−alkyl peroxo species have often been implicated as intermediates in industrial oxidation of hydrocarbons with cobalt catalysts, examples of discrete [LCoIII−OOR] complexes and studies on their oxidizing capacities have been scarce. In this work, twelve such complexes with two different ligands, L, and various primary, secondary, and tertiary R groups have been synthesized, and seven of them have been characterized by X-ray crystallography. The dianion (L2-) of the two ligands N,N-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide (Py3PH2, 1) and N,N-bis[2-(1-pyrazolyl)ethyl]pyridine-2,6-dicarboxamide (PyPz2PH2, 2) bind Co(III) centers in pentadentate fashion with two deprotonated carboxamido nitrogens in addition to three pyridine or one pyridine and two pyrazole nitrogens to afford complexes of the type [LCoIII(H2O)] and [LCoIII(OH)]. Reactions of the [LCoIII(OH)] complexes with ROOH in aprotic solvents of low polarity readily afford the [LCoIII−OOR] complexes in high yields. This report includes syntheses of [Co(Py3P)(OOR)] complexes with R = tBu (7, tBu = CMe3), Cm (8, Cm = CMe2Ph), CMe2CH2Ph (9), Cy (10, Cy = c-C6H11), iPr (11, iPr = CHMe2) or nPr (12, nPr = CH2CH2CH3), and [Co(PyPz2P)(OOR)] complexes with R = tBu (13), Cm (14), CMe2CH2Ph (15), Cy (16), iPr (17) or nPr (18). The structures of 8−12 and 16 have been established by X-ray crystallography. Complexes 10 and 16 are the first examples of structurally characterized compounds containing the [Co−OOCy] unit, proposed as a key intermediate in cobalt-catalyzed oxidation of cyclohexane. The metric parameters of 7−12 and 16 have been compared with those of other reported [LCoIII−OOR] complexes. When these [LCoIII−OOR] complexes are warmed (60−80 °C) in dichloromethane in the presence of cyclohexane (CyH), cyclohexanol (CyOH) and cyclohexanone (CyO) are obtained in good yields. Studies on such reactions (referred to as stoichiometric oxidations) indicate that homolysis of the O−O bond in the [LCoIII−OOR] complexes generates RO• radicals in the reaction mixtures which are the actual agents for alkane oxidation. [LCo−O•], the other product of homolysis, does not promote any oxidation. A mechanism for alkane oxidation by [LCoIII−OOR] complexes has been proposed on the basis of the kinetic isotope effect (KIE) value (5 at 80 °C), the requirement of dioxygen for oxidation, the dependence of yields on the stability of the RO• radicals, and the distribution of products with different substrates. Both L and R modulate the capacity for alkane oxidation of the [LCoIII−OOR] complexes. The extent of oxidation is noticeably higher in solvents of low polarity, while the presence of water invariably lowers the yields of the oxidized products. Since [LCoIII−OOR] complexes are converted into the [LCoIII(OH)] complexes at the end of single turnover in stoichiometric oxidation reactions, it is possible to convert these systems into catalytic ones by the addition of excess ROOH to the reaction mixtures. The catalytic oxidation reactions proceed at respectable speed at moderate temperatures and involve [LCoIII−OOR] species as a key intermediate. Turnover numbers over 100 and ∼10% conversion of CyH to CyOH and CyO within 4 h have been noted in most catalytic oxidations. The same catalyst can be used for the oxidation of many substrates. The results of the present work indicate that [LCoIII−OOR] complexes can promote oxidation of hydrocarbons under mild conditions and are viable intermediates in the catalytic oxidation of hydrocarbons with ROOH in the presence of cobalt catalysts.