Tetradentate amido azo Schiff base Cu(II), Ni(II) and Pd(II) complexes: Synthesis, characterization, spectral properties, and applications to catalysis in C–C coupling and oxidation reaction

New Schiff base ligands (H₂L¹ & H₂L²) were synthesized by the reaction of salicylaldehyde with (2-((2-aminophenyl)diazenyl)-N-alkylaniline), and were used for the preparation of complexes with Cu(II), Ni(II) and Pd(II) metal ions. The structural features of the synthesized compounds were examined by UV–Vis, IR and ¹H NMR spectroscopy. The crystal structures of Schiff base (H₂L¹) and two metal complexes Cu(L¹) and Ni(L²) were determined by single crystal X-ray diffraction. The studies revealed that the synthesized Schiff bases existed as tetradentate (N,N,N,O) ligands and bonded to the metal ions through the donor atoms of the amido nitrogen, azo nitrogen, azomethine nitrogen and phenolic oxygen atoms. The redox property of Cu(L¹) and Ni(L¹) and emission behavior of ligand H₂L¹ and complexes Cu(L¹) and Ni(L¹) were examined. The complex Cu(L¹) shows excellent catalytic activity towards oxidation of benzyl alcohol to benzyldehyde (under solvent-free condition) using H₂O₂ as the oxidant. Complex Pd(L¹) acts as highly efficient catalyst in the Suzuki–Miyaura cross-coupling reaction of various aryl halides with phenyl boronic acid to produce the corresponding biaryls with high yields under mild reaction conditions. Both the catalysts Cu(L¹) and Pd(L¹) were easily recovered by simple chromatographic separation and reused for next catalytic cycle.