Abstract:The reactions of FeCl2 center dot 2H(2)O and 2,2,2-tris(1-pyrazolyl) ethanol HOCH2C(pz)(3) (1) (5), depending on the experimental conditions. Compounds 1-5 were isolated as air-stable crystalline solids and fully characterized, including (1-4) by single-crystal X-ray diffraction analyses. The latter technique revealed strong intermolecular H-bonds involving the OH group of the scorpionate 2 and 3 giving rise to 1D chains which, in 3, are further expanded to a 2D network with intercalated infinite and almost plane chains of H-interacting water molecules. In 4, intermolecular pi center dot center dot center dot pi interactions involving the pyrazolyl rings are relevant. Complexes 2-5 display a high solubility in water (S-25 degrees C ca. 10-12 mg mL(-1)), a favourable feature towards their application as catalysts (or catalyst precursors) for the peroxidative oxidation of cyclo-hexane to cyclohexanol and cyclohexanone, with aqueous H2O2/MeCN, at room temperature (TON values up to ca. 385). (C) 2011 Elsevier B. V. All rights reserved.

Reactions between CuCl2 and the functionalized scorpionate 2,2,2-tris(1-pyrazolyl)ethanol HOCH2C(pz)3 1 (pz = pyrazolyl) or 2,2,2-tris(1-pyrazolyl)ethyl methanesulfonate CH3SO2OCH2C(pz)3 2 yield the corresponding water soluble CuII complexes [CuCl2{HOCH2C(pz)3}] 3 or [CuCl2{CH3SO2OCH2C(pz)3}]2 4. 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.

The V-scorpionate complexes [VCl 3 {HC(pz) 3 }] (pz = pyrazolyl) and [VCl 3 {SO 3 C(pz) 3 }] catalyze cyclohexane oxidation with dioxygen, to cyclohexanol (the main product) and cyclohexanone, under solvent-free conditions. [VCl 3 {HC(pz) 3 }] provides the best activity (13 % conversion into the ketone and alcohol, with high selectivity, at the O 2 pressure of 15 atm, at 140 °C, 18 h reaction time). The reaction is further promoted (to 15 % conversion) by pyrazinecarboxylic acid (PCA). The use of C-or O-radical traps supports the involvement of a free-radical reaction mechanism. Several reaction parameters have been varied in a systematic study, directed toward optimization of the process.

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