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

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 H2O2. 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 V-51 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: [(VO)-O-V{pyr(S,S-chen)}](2)(mu-O)(2)center dot 2(CH3)(2)NCHO, where the ligand is the "half" Schiff base formed by pyridoxal and 1S,2S-diaminocyclohexane. The dinuclear species shows a OVV(mu-O)(2)(VO)-O-V unit with tridentate ligands and two mu-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 H2O2 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.