Vanadium-salen and -salan complexes: Characterization and application in oxygen-transfer reactions

Abstract

Salen complexes are a versatile and standard system in oxidation catalysis. Their reduced derivatives, called salan, share their versatility but are still widely unexplored. We report the synthesis of a group of new vanadium-salen and -salan complexes, their characterization and application in the oxidation of simple organic molecules with H 2 O 2 . The ligands are derived from pyridoxal and chiral diamines (1,2-diaminocyclohexane and 1,2-diphenylethylenediamine) and were easily obtained in high yields. The V IV complexes were prepared and characterized in the solid state (Fourier transform infrared, FTIR, and magnetic properties) and in solution by spectroscopic techniques: UV–vis, circular dichroism (CD), electron paramagnetic resonance (EPR), and 51 V NMR, which provide information on the coordination geometry. Single crystals suitable for X-ray diffraction studies were obtained from solutions containing the V IV -pyr( S , S -chan) complex: [V V O{pyr( S , S -chen)}] 2 (μ-O) 2 ·2(CH 3 ) 2 NCHO, where the ligand is the “half” Schiff base formed by pyridoxal and 1 S ,2 S -diaminocyclohexane. The dinuclear species shows a OV V (μ-O) 2 V V O unit with tridentate ligands and two μ-oxo bridges. The V IV complexes of the salan-type ligands oxidize in organic solvents to a V V species, and the process was studied by spectroscopic techniques. The complexes were tested as catalysts in the oxidation of styrene, cyclohexene, and cumene with H 2 O 2 as oxidant. Overall, the V-salan complexes show higher activity than the parent V-salen complexes and are an alternative ligand system for oxidation catalysis.