Copper(I) and copper(II) metallacycles as catalysts for microwave assisted selective oxidation of cyclohexane

Abstract

New copper(I) and copper(II) pyrazolate compounds have been prepared by similar synthesis procedures. For the 3,5-NO2-pyrazole ligands, the dinuclear cyclic Cu(I) compound [Cu2(µ-N,N-3,5-(NO2)2pz)2(PPh3)2], 1, was the only compound isolated in high yield, while in the case of the 3,5-bis(trifluoromethyl)-pyrazole ligand a spontaneous rearrangement to the hexanuclear cyclic [trans-Cu6(µ-OH)6(µ-3,5-(CF3)2pz)6], 2, was observed. The formation of this latter compound ruled out an anion templating mechanism for the synthesis. X-ray diffraction analysis of 1 revealed that the copper cation is in a trigonal environment and involved in a six-membered Cu2N4 core; while the structural analysis of 2 showed that the metal adopts a square-planar geometry, forms five-membered Cu2N2O rings and ultimately gives rise to a large Cu6(N2O)6 crown-type assembly. Compounds 1 and 2 act as very effective homogeneous catalysts towards the neat microwave (MW) assisted peroxidative (with aq. tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane to cyclohexanol and cyclohexanone (total yield up to 58% in 30min reaction time) via formation of cyclohexyl hydroperoxide (CyOOH) as the primary product.