| Low-temperature stopped-flow electronic spectroscopy was utilized to resolve the intermediates formed in the reaction of a diiron(II) compound, Fe2(H2Hbamb)2(N-MeIm)2 (H 4HBamb = 2,3-bis(2-hydroxybenzamido)dimethylbutane), 1, with the oxygen-atom donors 2,6-dimethyliodosylbenzene and p-cyanodimethylaniline N-oxide and the mechanistic probe hydroperoxide 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH). Previous studies showed that 1 is capable of catalytically oxidizing cyclohexane to cyclohexanol (300 turnovers) via a pathway involving the heterolytic cleavage of the O-O bond of MPPH (>98% peroxide utilization). We now report intimate details of the formation of the reactive intermediate and its subsequent decay in the absence of substrates. The reaction, which is independent of the nature of the oxidant, proceeds via three consecutive steps assigned as (i) oxygen-atom transfer to one of the iron centers of 1 to form an FeIV=O species, 2, (ii) ligand rearrangement to 3, and (iii) internal collapse of the terminal oxo group to generate a diferric, mu-oxo species, 4. Assignment of the second step as a ligand rearrangement was corroborated by stopped-flow spectroscopic studies of the one-electron oxidation of diferrous 1, which is also known to undergo ligand rearrangement upon the formation of the [Fe II FeIII] mixed-valent complex. The role of the ligand reorganization in the energetic profile for the formation of the catalytically competent intermediate is discussed, along with the potential biological significance of the internal conversion of the active oxidant to the inert, mu-oxo diiron(III) dimer, 4.;Heterolytic O-O bond cleavage of MPPH by 1 results in the formation of intermediate species whose UV/vis spectra and kinetic behavior suggest parallel reactivity to that seen in reactions with oxygen atom donor molecules. Kinetic analysis of the reaction with MPPH indicates that there is a preequilibrium binding step that manifests as saturation behavior in peroxide concentration.;Under cryogenic stopped-flow conditions, addition of p-CN-DMANO to the non-heme mixed-valence (FeII, FeIII), [Fe2(H2Hbamb)2(N-MeIm)2] +, 5 results in the rapid formation of a high-valent species, 6, assigned as a mixed-valence (FeIII, FeIV(O)) compound. Addition of 2,4,6-tri-tert-butylphenol to the reaction mixture affords production of the corresponding phenoxyl radical species resulting from the two-step oxidation of 1.9(1) equivalents of 2,4,6-tri- tert-butylphenol by the high-valent iron species. |
