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

Maurya, M. R. M. ; Kumar, U. K. ; Avecilla, F. ; Henriques, R. T. ; Kusnetsov, M. L. K. ; Pessoa, J. ; Correia, I.

Pure and Applied Chemistry Vol. 81, Nº 7, pp. 1279 - 1296, July, 2009.

ISSN (print): 0033-4545
ISSN (online):

Scimago Journal Ranking: 1,19 (in 2009)

Digital Object Identifier: 10.1351/PAC-CON-08-09-07

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)(mu-O)(2)center dot 2(CH(3))(2)NCHO, where the ligand is the "half" Schiff base formed by pyridoxal and 1S,2S-diaminocyclohexane. The dinuclear species shows a OV(V)(mu-O)(2)V(V)O 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 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.