Syntheses and structures of alkyl peroxo adducts of β-diketonate cobalt(III) complexes and their role in oxidation of hydrocarbons and olefin epoxidation

To assess the oxidizing capacity of [Co(beta-diketonate)(2)(L)OOR] complexes, four such species, namely, [Co(acac)(2)(L)((OOBu)-Bu-t)] (acacH = acetylacetone; L = pyridine (py) (1), 4-methylpyridine (4-Mepy) (2), 1-methylimidazole (1-MeIm) (3)) and [Co(dbm)(2)(py)((OOBu)-Bu-t)] (4, dbmH = dibenzoylmethane), have been synthesized and the structures of 1, 3, and 4 have been determined by X-ray crystallography. Complex 1 crystallizes in the triclinic space group P (1) over bar with a = 9.149(2) Angstrom, b = 9.741(2) Angstrom, c = 13.067(2) Angstrom, alpha = 84.22(2)degrees, beta = 77.89(2)degrees, gamma = 67.83(2)degrees, V = 1054.1(4) Angstrom(3), and Z = 2. Complex 3 crystallizes in the monoclinic space group P2(1)/n with a = 11.341(2) Angstrom, b = 10.485(2) Angstrom, c = 18.863(2) Angstrom, alpha = 90 degrees, beta = 106.230(15)degrees, gamma = 90 degrees, V = 2153.6(8) Angstrom(3), and Z = 4. Complex 4.1.5C(6)H(6) crystallizes in the monoclinic space group P2(1)/c with a = 14.907(5) Angstrom, b = 18.701(5) Angstrom, c = 15.207(4) Angstrom, alpha = 90 degrees, beta = 103.52(2)degrees, gamma = 90 degrees, V = 4122(2) Angstrom(3), and Z = 4. The geometry around the Co(III) center in all four complexes is distorted octahedral, and the two beta-diketonate ligands are cis to each other. The Co-O and O-O bond distances in the Co-(OOBu)-Bu-t moiety fall in the narrow ranges of 1.860(3)-1.879(2) and 1.451(3)-1.466(3) Angstrom, respectively. Although stable in the solid state, these complexes decompose in benzene or halocarbon solutions to afford (BuOO.)-Bu-t and (BuO.)-Bu-t radicals. When alkanes like cyclohexane are present in the reaction mixture, these radicals initiate oxidation of the C-H bond. The oxidizing capacity follows the order 1 > 2 > 3 > 4. Decomposition of the complexes in the presence of cyclohexene and an aldehyde results in selective epoxidation in high yield.