Water-soluble Al(iii), Fe(iii) and Cu(ii) formazanates: synthesis, structure, and applications in alkane and alcohol oxidations

Abstract

A series of water-soluble metal formazanates, [Al₂(OH)₂(H₂O)₈](HL)₂ (1), [{Fe(H₂O)₄}(HL)(H₂L)]·2H₂O (2) and [Cu(en)₂(H₂O)₂][CuL(H₂O)₂]₂ (3), were synthesized by the reaction of sodium 2-(2-((E)-cyano((E)-(2-sulfonatophenyl)diazenyl)methylene)hydrazineyl) benzenesulfonate (Na₂HL) with Al(NO₃)₃·9H₂O, FeCl₃·6H₂O and Cu(NO₃)₂·3H₂O (in the presence of ethylenediamine) in methanol, respectively. The formazan pro-ligand Na₂HL and its complexes 1–3 were characterized by ESI-MS, ¹H/¹³C NMR (for Na₂HL), IR spectroscopy, elemental and X-ray diffraction analyses (for 1–3). According to the X-ray analysis, the formazanate behaves as an anion or displays a mono- or bidentate coordination mode, for the Al, Fe or Cu centre, respectively. All complexes were tested as catalysts for the peroxidative (with H₂O₂) oxidation of cyclohexane and cyclohexanol to the coresponding organic products at room temperature. The complexes 2 and 3 are active homogeneous catalysts for the microwave-assisted peroxidative oxidation with aqueous H₂O₂ of cyclohexane (to cyclohexanol and cyclohexanone) and cyclohexanol to cyclohexanone.