Tetradentate Schiff base ligands and their complexes: Synthesis, structural characterization, thermal, electrochemical and alkane oxidation

Three Schiff base ligands (H2L1–H2L3) with N2O2 donor sites were synthesized by condensation of 1,5-diaminonapthalene with benzaldehyde derivatives. A series of Cu(II), Co(II), Ni(II), Mn(II) and Cr(III) complexes were prepared and characterized by spectroscopic and analytical methods. Thermal, electrochemical and alkane oxidation reactions of the ligands and their metal complexes were investigated. Extensive application of 1D (1H, 13C NMR) and 2D (COSY, HETCOR, HMBC and TOSCY) NMR techniques were used to characterize the structures of the ligands and establish the 1H and 13C resonance assignments of the three ligands. Ligands H2L1 and H2L3 were obtained as single crystals from THF solution and characterized by X-ray diffraction. Both molecules are centrosymmetric and asymmetric unit contains one half of the molecule. Catalytic alkane oxidation reactions with the transition metal complexes investigated using cyclohexane and cyclooctane as substrates. The Cu(II) and Cr(III) complexes showed good catalytic activity in the oxidation of cyclohexane and cyclooctane to desired oxidized products. Electrochemical and thermal properties of the compounds were also investigated.

Graphical abstractThe catalytic oxidation of alkanes with 30% H2O2 under mild reaction conditions is especially an interesting topic, because direct functionalization of inactivated CH bonds in saturated hydrocarbons usually requires drastic reaction conditions, such as high pressure and high temperature. Catalytic oxidation of cyclohexane and cyclooctane under microwave irradiation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of the transition metal catalyst, MII/MIII(Salen) by ligand method. ► Catalysts characterized by FTIR, UV–Vis, magnetic susceptibility, mass spectroscopy. ► Catalytic activation of the metal complexes with H2O2 for oxidation of cyclohexane and cyclooctane. ► X-ray structure analysis of the Schiff base ligands.