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Electronic, Conjugation, and Confinement Effects on Structure, Redox, and Catalytic Behavior of Oxido-Vanadium(IV) and -(V) Chiral Schiff Base Complexes

Mandal, Moon, Nagaraju, Vemu, Karunakar, Galla V., Sarma, Bipul, Borah, Biraj Jyoti, Bania, Kusum K.
The Journal of Physical Chemistry C 2015 v.119 no.52 pp. 28854-28870
2-naphthol, catalysts, catalytic activity, enantioselectivity, epoxidation reactions, ligands, oxidation, oxygen, physical chemistry, redox potential, schiff bases, solvents, styrene, temperature, vanadium
Change in oxophilicity, redox, and catalytic behavior with the modification of ligand system was observed for four oxido-vanadium Schiff base complexes. Mononuclear V(IV) complexes were obtained with methyl-substituted Schiff base ligand and one having higher conjugation. Electronic and conjugation effects reduce the oxophilicity of vanadium and led to mononuclear (LVᴵⱽO, L = Schiff base ligand) species. However, μ-oxido bridged dinuclear [OVⱽOL]₂ complexes were formed with Schiff base ligands without such electronic effect under identical condition. Such structural variation also changed the electronic and redox properties of the metal complexes and brought about substantial differences in the catalytic activities of the vanadium Schiff base complexes. Confinement effect imparted by the zeolite-Y framework on the metal complexes synthesized inside the cavity by the “ship in a bottle synthesis” method was also found to alter the catalytic behavior of the metal complexes. All of the mononuclear and μ-oxido bridged dinuclear vanadium Schiff base complexes were found to give moderate to high enantioselectivity in epoxidation of styrene and oxidative coupling of 2-naphthol. Zeolite-Y encapsulated complexes however were found to show better catalytic activity than the mononuclear homogeneous catalyst but less than the dinuclear species. Catalytic activities of the complexes were found to depend on rate of oxygen flow, temperature, solvent, and the amount of catalyst. Change in redox potentials due to electronic, conjugation, and confinement effect highly influenced the catalytic behavior of the metal complexes. The lower was the oxidation potential, the higher was the catalytic activity. The best results for epoxidation of styrene were observed at −15 °C, while a maximum amount of BINOL was achieved at 0 °C. All of the chiral catalysts were found to be more effective in epoxidation of styrene. As compared to the homogeneous chiral catalyst, zeolite-Y embedded chiral catalysts were found to be advantageous in terms of recyclability, easy separation, and in some cases higher enantioselectivity.