COMPARATIVE KINETIC-STUDY OF OXYGEN ATOM TRANSFER-REACTIONS OF DIPEROXO AND MONOPEROXO COMPLEXES OF OXOVANADIUM(V) IN AQUEOUS-SOLUTION

The oxygen atom transfer reactions of peroxo and diperoxo complexes of oxovanadium(V) with (en)2Co(SCH2CH2NH2)2+have been studied. At 25 °C and I = 0.10 M the second-order rate constants (M−1s−1) are as follows: VO(O2)(O2H)(C2H4NCO2)−, 87 ◅ 3; VO(O2)2(C5H4NCO2)2−, 13 ◅ 2; VO(O2)2(C2O4)3−, 12 (pH 5.1 and 20.6 °C); VO(O2)2(H2O)2−, 11; VO(O2){C5H3N-(CO2)2}(H2O)−, ≤(1−3) X 10−3. The observed rates for VO(O2)(H2O)4 + were very low, but a rate constant could not be assigned. A value pKa= 4.41 ± 0.02 for VO(O2)(O2H)(C5H4NCO2)− was determined by potentiometrie titration. The much greater reactivity of the oxo diperoxo complexes relative to the oxo peroxo species confirms an earlier prediction based on studies with peroxo complexes of other d° transition-metal ions. A survey of available crystal structures reveals that a significant lengthening of the (O-O)peroxo and one of the asymmetric M-Operoxo bonds occurs in the oxo diperoxo complexes. Both of these features are consistent with their greater reactivity as oxygen atom transfer and inner-sphere, one-electron oxidizing reagents. However, it is unclear why Mo(VI) and W(VI) are so much more effective than V(V) at activating peroxide in the oxo diperoxo complexes. © 1990, American Chemical Society. All rights reserved.