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 (μ-peroxo)diiron(III) intermediate P gives rise to the diiron(IV) intermediate Q with an Fe2 (μ–O)2 diamond core, which oxidizes methane to methanol. As a model for this conversion, (μ–oxo) diiron(III) complex 1 ([FeIII 2 (μ–O)(μ–O2 H3 )(L)2 ]3+ , L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) has been treated consecutively with one eq of H2 O2 and one eq of HClO4 to form 3 ([FeIV 2 (μ–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 atm /z 1,399, corresponding to the {[Fe2 O3 L2 H](OTf)2 }+ ion in which 1 oxygen atom derives from 1 and the other two originate from H2 O2 . Mössbauer 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 Å and an Fe–Fe distance of 3.32 Å. Taken together, the spectroscopic data point to an HO-FeIV -O-FeIV = O core for 2. Protonation of 2 results in the loss of H2 O and the formation of 3. Isotope labeling experiments show that the [FeIV 2 (μ–O)2 ] core of 3 can incorporate both oxygen atoms from H2 O2 . 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 [FeIV 2 (μ–O)2 ] core.