Synthesis, Properties, and Structure of a Stable Cobalt(III) Alkyl Peroxide Complex and Its Role in the Oxidation of Cyclohexane

Abstract

Three cobalt(III) complexes of Py3PH2 (H's are the dissociable amide H's), a strong-field ligand with two peptide groups, have been synthesized. They are [Co(Py3P)(H2O)]ClO4·H2O (6), [Co(Py3P)(OH)] (7), and [Co(Py3P)(OOtBu)]·2CH2Cl2 (8). Complex 6 crystallizes in the monoclinic space group C2 with a = 22.695(6) Å, b = 10.284(2) Å, c = 10.908(3) Å, α = 90°, β = 112.17(2)°, γ = 90°, V = 2357.7(10) Å3, and Z = 4. Its structure has been refined to R = 3.91% on the basis of 1844 I > 2σ(I) data. Complex 8 crystallizes in the monoclinic space group P21/n with a = 16.720(4) Å, b = 10.641(3) Å, c = 16.776(3) Å, α = 90°, β = 99.76(2)°, γ = 90°, V = 2941.5(12) Å3, and Z = 4. The structure of 8 has been refined to R = 6.37% on the basis of 4776 I > 2σ(I) reflections. In all three complexes, the doubly deprotonated Py3P2- ligand binds the cobalt(III) center in a pentadentate fashion with five nitrogens situated in two deprotonated amido groups and three pyridine rings. The aqua complex 6 can easily be converted into the hydroxo complex 7 by the addition of 1 equiv of base. The transformation 6 ↔ 7 is reversible, and the pKa of the coordinated water molecule in 6 is 7. Complex 8 is the first example of a structurally characterized Co(III) alkyl peroxide complex that contains two deprotonated amido groups bonded to the metal center. Like a few alkyl peroxide complexes of tervalent cobalt, 8 oxidizes alkanes upon thermal decomposition. When cyclohexane is used as the substrate, cyclohexanol, cyclohexanone, and cyclohexyl chloride are the products. Complex 7 is the intermediate in the formation of 8 and is also the thermal decomposition product of 8. In single turnover oxidation of cyclohexane by 8 at the optimum temperature of 80 °C, a maximum yield of 59% of the oxidized products is obtained. The mechanism of cyclohexane oxidation by 8 involves exclusive homolytic scission of the O−O bond in 8. The tBuO• radicals generated in such a process abstract an H atom from cyclohexane to afford cyclohexyl radicals, which in turn react with dioxygen and produce cyclohexanol and cyclohexanone presumably via a Russell-type termination reaction. The oxidation of cyclohexane by 8 can be either stoichiometric or catalytic. In the presence of excess TBHP, 8 affords more oxidized products, indicating multiple turnovers.