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

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. © 2022 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.