SYNTHESIS, PROPERTIES, AND STRUCTURE OF A STABLE COBALT(III) ALKYL PEROXIDE COMPLEX AND ITS ROLE IN THE OXIDATION OF CYCLOHEXANE

Three cobalt(II) complexes of Py(3)PH(2) (H's are the dissociable amid e H's), a strong-field ligand with two peptide groups, have been synth esized. They are [Co(Py(3)P)(H2O)]ClO4 . H2O (6), [Co(Py(3)P)(OH)] (7) , and [Co(Py(3)P)(OO(t)Bu)].2CH2Cl2 (8). Complex 6 crystallizes in the monoclinic space group C2 with a = 22.695(6) Angstrom, b = 10.284(2) Angstrom, c = 10.908(3) Angstrom, alpha = 90 degrees, beta = 112.17(2) degrees, gamma = 90 degrees, V = 2357.7(10) Angstrom(3) and Z = 4. Its structure has been refined to R = 3.91% on the basis of 1844 I > 2 si gma(I) data. Complex 8 crystallizes in the monoclinic space group P2(1 )/n with a = 16.720(4) Angstrom, b = 10.641(3) Angstrom, c = 16.776(3) Angstrom, alpha = 90 degrees, beta = 99.76(2)degrees, gamma = 90 degr ees, V = 2941.5(12) Angstrom(3), and Z = 4. The structure of 8 has bee n refined to R = 6.37% on the basis of 4776 I > 2 sigma(I) reflections . In all three complexes, the doubly deprotonated Py(3)P(2-) ligand bi nds the cobalt(III) center in a pentadentate fashion with five nitroge ns 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 rev ersible, and the pK(a) of the coordinated water molecule in 6 is 7. Co mplex 8 is the first example of a structurally characterized Co(III) a lkyl peroxide complex that contains two deprotonated amido groups bond ed to the metal center. Like a few alkyl peroxide complexes of tervale nt cobalt, 8 oxidizes alkanes upon thermal decomposition. When cyclohe xane is used as the substrate, cyclohexanol, cyclohexanone, and cycloh exyl chloride are the products. Complex 7 is the intermediate in the f ormation of 8 and is also the thermal decomposition product of 8. In s ingle turnover oxidation of cyclohexane by 8 at the optimum temperatur e of 80 degrees C, a maximum yield of 59% of the oxidized products is obtained. The mechanism of cyclohexane oxidation by 8 involves exclusi ve homolytic scission of the O-O bond in 8. The (t)BuO(.) radicals gen erated in such a process abstract an ii atom from cyclohexane to affor d cyclohexyl radicals, which in turn react with dioxygen and produce c yclohexanol and cyclohexanone presumably via a Russell-type terminatio n reaction. The oxidation of cyclohexane by 8 can be either stoichiome tric or catalytic. In the presence of excess TBHP, 8 affords more oxid ized products, indicating multiple turnovers.