A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by cis -[V IV (O)(Cl/F)(N 4 )] + Species Mimicking the Active Center of Metal-Enzymes.

Reaction of V IV OCl 2 with the nonplanar tetradentate N 4 bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula cis -[V IV (O)(Cl)(N 4 )]Cl. Sequential treatment of the two nonmethylated N 4 oxidovanadium(IV) compounds with KF and NaClO 4 resulted in the isolation of the species with the general formula cis -[V IV (O)(F)(N 4 )]ClO 4 . In marked contrast, the methylated N 4 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 IV O 2+ 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 H 2 O 2 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 51 V and 1 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 cis -[V IV (O)(Cl)(N 4 )] + species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.