Mild peroxidative oxidation of cyclohexane catalyzed by mono-, di-, tri-, tetra- and polynuclear copper triethanolamine complexes

The mono-, di-, tri-, tetra- and polynuclear copper(II) triethanolamine (H(3)tea) complexes [Cu(H(2)tea)(N-3)] (1), [Cu-2(H(2)tea)(2)(XC6H4COO)(2)]center dot 2H(2)O (X = 4-H 2a, 4-CH3 2b, 3-Cl 2c), [Cu-3(H(2)tea)(2)-(4-OC6H4COO)(2)(H2O)center dot 4H(2)O (3), [O subset of Cu-4(tea)(4)(BOH)(4)][BF4](2) (4) and [Cu-2(H(2)tea)(2){mu-C6H4(COO)(2)-1,4}(n)] center dot 2n H2O (5), respectively, are highly active and selective catalysts or catalyst precursors for the peroxidative oxidation of cyclohexane, in acetonitrile, to a cyclohexanol and cyclohexanone mixture, by aqueous hydrogen peroxide in acidic medium (liquid biphasic catalysis) at room temperature and atmospheric pressure. The effects on the catalytic activity of various factors, e.g., the relative amounts of cyclohexane, oxidant, catalyst, solvent and nitric acid, reaction time, catalyst recycling and impact of both carbon- and oxygen-centred radical traps (supporting mainly radical mechanisms) were investigated and allowed us to achieve yields and TONs up to ca. 39% and 380, respectively, corresponding to the most active copper systems so far reported for the oxidation of cyclohexane under mild conditions. The catalysts can be reused for recycling and, at least complex 4 maintains almost the same level of activity even after five reaction cycles. The preparation of the new complexes 1, 2b and 2c by self-assembly at room temperature, and their full characterization by IR spectroscopy, FAB-MS+, elemental and X-ray diffraction structural (for I and 2c) analyses are also reported.