CuII complexes bearing the 2,2,2-tris(1-pyrazolyl)ethanol or 2,2,2-tris(1-pyrazolyl)ethyl methanesulfonate scorpionates. X-Ray structural characterization and application in the mild catalytic peroxidative oxidation of cyclohexane

Abstract

Reactions between CuCl₂ and the functionalized scorpionate 2,2,2-tris(1-pyrazolyl)ethanol HOCH₂C(pz)₃1 (pz = pyrazolyl) or 2,2,2-tris(1-pyrazolyl)ethyl methanesulfonate CH₃SO₂OCH₂C(pz)₃2 yield the corresponding water soluble Cu^II complexes [CuCl₂{HOCH₂C(pz)₃}] 3 or [CuCl₂{CH₃SO₂OCH₂C(pz)₃}]₂4. In 3 the scorpionate ligand shows the typical N,N,N-coordination mode, whereas in the dinuclear complex 4 it binds the metal as a bidentate species. Compounds 1-4 have been characterized by IR, far-IR, elemental analysis and (for 2-4) single crystal X-ray diffraction. The new scorpionate complexes 3 and 4 are shown to act as catalyst precursors for the peroxidative oxidation of cyclohexane to cyclohexanol (main product) and cyclohexanone, under mild conditions (at room temperature and using an aqueous solution of H₂O₂) reaching TON values up to 186 in NCMe/H₂O. Their hydrosolubility allows them to operate also in pure aqueous media (without any organic solvent, although less effectively), a rare feature of significance towards a green alkane oxidation process.