Insights into the P-to-Q conversion in the catalytic cycle of methane monooxygenase from a synthetic model system

For the catalytic cycle of soluble methane monooxygenase (sMMO), it has been proposed that cleavage of the O-O bond in the (mu-peroxo)diiron(III) intermediate P gives rise to the diiron(IV) intermediate Q with an Fe-2(mu-O)(2) diamond core, which oxidizes methane to methanol. As a model for this conversion, (mu-oxo)-diiron(III) complex 1 ([Fe-2(III)(mu-O)(mu-O2H3)(L)(2)](3+), L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) has been treated consecutively with one eq of H2O2 and one eq of HClO4 to form 3 ([Fe-2(IV)(mu-O)(2)(L)(2)](4+)). In the course of this reaction a new species, 2, can be observed before the protonation step; 2 gives rise to a cationic peak cluster by ESI-MS at m/z 1,399, corresponding to the {[Fe2O3L2H](OTf)(2)}(+) ion in which 1 oxygen atom derives from 1 and the other two originate from H2O2. Mossbauer studies of 2 reveal the presence of two distinct, exchange coupled iron(IV) centers, and EXAFS fits indicate a short Fe-O bond at 1.66 angstrom and an Fe-Fe distance of 3.32 angstrom. Taken together, the spectroscopic data point to an HO-Fe-IV-O-Fe-IV = O core for 2. Protonation of 2 results in the loss of H2O and the formation of 3. Isotope labeling experiments show that the [Fe-2(IV)(mu-O)(2)] core of 3 can incorporate both oxygen atoms from H2O2. The reactions described here serve as the only biomimetic precedent for the conversion of intermediates P to Q in the sMMO reaction cycle and shed light on how a peroxodiiron(III) unit can transform into an [Fe-2(IV)(mu-O)(2)] core.