Hydrogen atom abstraction by metal-oxo and metal-superoxo complexes: Kinetics and thermodynamics

The superoxochromium complex CraqOO2+ abstracts a hydrogen atom from CMe3CHO in acidic aqueous solution with k = 0.16 M-1 s(-1). This rate constant is only similar to 10(2) times smaller than that for the reaction of CraqO2+ with the same aldehyde, k = 23 M-1 s(-1), in contrast to the much greater reactivity difference between alkoxyl and alkylperoxyl radicals, k(t-BuO)/k(t-BuOO) approximate to 10(6) The, absolute rate constants for hydrogen atom abstraction from a common reagent by metal-ore and -superoxo species and the corresponding organic oxygen-centered radicals, RO. and ROO., can now be compared for the first time: k(BuO) (9 x 10(7) M-1 s(-1)) > k(CrO)(23) greater than or equal to k(BuOO)(8) > k(CrOO) (0.16) The reactivity of individual species is explained by the energetics of the O-H bands in ROH, ROOH, CraqOH2+, and CraqOOH2+.