Synthesis, characterization, and immobilization of nickel(II) tetradentate Schiff-base complexes on clay as heterogeneous catalysts for the oxidation of cyclooctene

Nickel(II) tetradentate Schiff-base complexes of N,N'-(bis(pyridin-2-yl)formylidene) ethane-1,2-diamine (L1), N,N'-(bis(pyridin-2-yl)formylidene)cyclohexane-1,2-diamine (L2), N,N'-(bis(pyridin-2-yl)formylidene)benzene-1,2-diamine (L3), N,N'-(bis(pyridin-2-yl)formylidene)meso-stilben-1,2-diamine (L4), and N,N'-(bis(pyridin-2-yl)formylidene)propane-1,3-diamine (L5) were synthesized, characterized, and immobilized on sodium montmorillonite. The complexes were characterized by IR spectroscopy, diffuse reflectance spectra (DRS), and atomic absorption spectroscopy (AAS). IR and DRS data of the heterogeneous catalysts show that the Ni(II) complexes were physically entrapped within the sodium montmorillonite clay. The supported complexes show good catalytic activity for the epoxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxygen source in acetonitrile. The Ni-catalyzed oxidation proceeds with 62.3% selectivity for epoxidation with 69% conversion for supported [Ni(L5)].