Synthesis and structural characterization of iron complexes with 2,2,2-tris(1-pyrazolyl)ethanol ligands: Application in the peroxidative oxidation of cyclohexane under mild conditions

The reactions of FeCl2.2H2O and 2,2,2-tris(1-pyrazolyl)ethanol HOCH2C(pz)3 (1) (pz = pyrazolyl) afford [Fe{HOCH2C(pz)3}2][FeCl4]Cl (2), [Fe{HOCH2C(pz)3}2]2[Fe2OCl6](Cl)2·4H2O (3·4H2O), [Fe{HOCH2C(pz)3}2][FeCl{HOCH2C(pz)3}(H2O)2]2(Cl)4 (4) or [Fe{HOCH2C(pz)3}2]Cl2 (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 π⋯π interactions involving the pyrazolyl rings are relevant. Complexes 2–5 display a high solubility in water (S25 °Cca. 10–12 mg mL−1), a favourable feature towards their application as catalysts (or catalyst precursors) for the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone, with aqueous H2O2/MeCN, at room temperature (TON values up to ca. 385).