Abstract
Many metalloporphyrins are catalysts for the oxidation of organic substrates using oxidants such as hydrogen peroxide or oxygen. They have been much studied, but the factors that influence their reaction with oxidant within the initial (~1 s) period are poorly understood and have been studied in this work. The research used the stopped-flow technique to obtain the first direct measurement of the equilibrium constant K for the important complexation of hydrogen peroxide to tetrakis(pentafluorophenyl)porphyrin iron(III) chloride Fe-1. In addition it provided direct measurement of rate constants for key steps in the mechanism. Specifically, a complex MeOH:FeIII:H2O2 (388 nm) is produced from the initial reaction in MeOH of Fe-1 (404 nm) with H2O2 (log K 2. 39), then this undergoes 0-0 bond cleavage, being quickly converted to FeIV=0 (408 nm) (kf 4. 4 s-1). This species itself oxidizes (is reduced by) excess H2O2 (kr 54. 3 M-1S-1) to regenerate the MeOH:FeIII:H2O2 complex, setting up an equilibrium between this complex and FeIV=O. As the proportion of DCM increases the value of log K falls, although in 3:1 MeOH-DCM changes are small (log K 1. 39, kf = 7 s-1, kr = 20 M-1s-1). In all cases after ~ 1 s decomposition (bleaching) is observed. Addition of an oxidizable substrate HNQ gives a MeOH:FeIII:HNQ complex (log K 3. 27 and 4. 29 in MeOH and 3:1 MeOH-DCM, respectively), but under the reaction conditions used (mM H2O2 vs. uM HNQ) is displaced by H2O2 to give MeOH:FeIII:H2O2, and the reaction proceeded as in the HNQ-free case. Finally, when oxidizable substrate HNQ was added to a preformed mixture of MeOH:FeIII:H2O2 and FeIV=O, it had little effect and reaction kinetics (kf, kr) on the spectra. This was interpreted to mean that FeIV=O does not react with HNQ, and more speculatively, that the often-proposed FeV=O or por+• -FeIV=O may not be involved, at least under these conditions.