Tetradentate N2O2 type Nickel(II) Schiff base complexes derived from meso-1,2-diphenyle-1,2-ethylenediamine: synthesis, characterization, crystal structures, electrochemistry, and catalytic studies

A series of Ni(II) complexes of salen type Schiff base ligands were synthesized and characterized. The ligands were synthesized from the condensation of meso-1,2-diphenyle-1,2-ethylenediamine with salicylaldehyde (H2L1), 5-bromosalicylaldehyde (H2L2), 5-bromo-3-nitrosalicylaldehyde (H2L3), and 2-hydroxyacetophenone (H2L4). The complexes were characterized by means of (HNMR)-H-1, IR, and UV-Vis spectroscopy and elemental analysis. Crystal structures of NiL3 and NiL4 were also determined by x-ray crystallography. Electrochemistry of the complexes was studied by means of cyclic voltammetry. Catalytic performance of the complexes was studied in the oxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant. Various factors including solvent type, reaction temperature, time, catalyst amount, and substrate to oxidant ratio were optimized. Solvent free oxidation of cyclooctene with these catalysts was also studied. Increased catalytic activity and higher epoxide selectivity was achieved in solvent-free conditions. NiL4 with more electron donating substituents on the ligand was the most efficient oxidation catalyst.