A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by cis-[VIV(O)(Cl/F)(N4)]+ Species Mimicking the Active Center of Metal-Enzymes

Abstract

Reaction of V^IVOCl₂ with the nonplanar tetradentate N₄ bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula cis-[V^IV(O)(Cl)(N₄)]Cl. Sequential treatment of the two nonmethylated N₄ oxidovanadium(IV) compounds with KF and NaClO₄ resulted in the isolation of the species with the general formula cis-[V^IV(O)(F)(N₄)]ClO₄. In marked contrast, the methylated N₄ oxidovanadium(IV) derivatives are inert toward KF reaction due to steric hindrance, as evidenced by EPR and theoretical calculations. The oxidovanadium(IV) compounds were characterized by single-crystal X-ray structure analysis, cw EPR spectroscopy, and magnetic susceptibility. The crystallographic characterization showed that the vanadium compounds have a highly distorted octahedral coordination environment and the d(V^IV-F) = 1.834(1) Å is the shortest to be reported for (oxido)(fluorido)vanadium(IV) compounds. The experimental EPR parameters of the V^IVO²⁺ species deviate from the ones calculated by the empirical additivity relationship and can be attributed to the axial donor atom trans to the oxido group and the distorted V^IV coordination environment. The vanadium compounds act as catalysts toward alkane oxidation by aqueous H₂O₂ with moderate ΤΟΝ up to 293 and product yields of up to 29% (based on alkane); the vanadium(IV) is oxidized to vanadium(V), and the ligands remain bound to the vanadium atom during the catalysis, as determined by ⁵¹V and ¹H NMR spectroscopies. The cw X-band EPR studies proved that the mechanism of the catalytic reaction is through hydroxyl radicals. The chloride substitution reaction in the cis-[V^IV(O)(Cl)(N₄)]⁺ species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.