A synthetic precedent for the [Fe^IV₂(μ-O) ₂] diamond core proposed for methane monooxygenase intermediate Q

Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe^IV₂(μ-O) ₂] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc^+/0) has been performed in MeCN at -40° C on a valence-delocalized [Fe ^IIIFe^IV(μ-O)₂(L^b) ₂]³⁺ complex (1b) (E_1/2 = 760 mV versus Fc ^+/0). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with max at 485 nm (9,800 M^-1·cm^-1) and 875 nm (2,200 M ^-1·cm^-1). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with δ = -0.04(1) mm·s^-1 and ΔEQ = 2.09(2) mm·s^-1, parameters typical of an iron(IV) center. The Mössbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe ₂(μ-O)₂] core at 674 cm^-1, downshifting 30 cm^-1 upon ¹⁸O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 Å and an Fe scatterer at 2.73 Å. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [Fe^IV₂(μ-O) ₂(L^b)₂]⁴⁺, the first synthetic complex with an [Fe^IV₂(μ-O)₂] core. A comparison of 2b and its mononuclear analog [Fe^IV(O)(L ^b)(NCMe)]²⁺ (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C - H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.