A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by -[V(O)(Cl/F)(N)] Species Mimicking the Active Center of Metal-Enzymes.

Reaction of VOCl with the nonplanar tetradentate N bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula -[V(O)(Cl)(N)]Cl. Sequential treatment of the two nonmethylated N oxidovanadium(IV) compounds with KF and NaClO resulted in the isolation of the species with the general formula -[V(O)(F)(N)]ClO. In marked contrast, the methylated N 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 (V-F) = 1.834(1) Å is the shortest to be reported for (oxido)(fluorido)vanadium(IV) compounds. The experimental EPR parameters of the VO 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 coordination environment. The vanadium compounds act as catalysts toward alkane oxidation by aqueous HO 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 V and H 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 -[V(O)(Cl)(N)] species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.