Despite decades of intense research, the core structure of the methane C-H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase (sMMO) remains controversial, with conflicting reports supporting either a ‘diamond’ diiron core structure or an open-core structure. Early EXAFS data assigned a short 2.46 Å Fe-Fe distance to Q (Shu, et al., Science 1997) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo, et al., JACS, 2017). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe-Fe distance, but rather a long 3.4 Å diiron distance, as observed in open-core synthetic model complexes. The previously reported 2.46 Å feature plausibly arises from a background metallic iron contribution from the experimental setup, which is eliminated in HERFD EXAFS due to the increased selectivity. Herein, we have explored the origin of the short diiron feature in partial-fluorescent yield EXAFS measurements and discuss the diagnostic features of background metallic scattering contribution to the EXAFS of dilute biological samples. Lastly, differences in sample preparation and resultant sample inhomogeneity in rapid-freeze quenched samples for EXAFS analysis are discussed. The presented approaches have broad implications for EXAFS studies of all dilute iron-containing samples.

We report the generation and spectroscopic characterization of a heterobimetallic [(TMC)FeIII-O-CrIII(OTf)4] species (1) by O2 bubbling into a mixture of Fe(TMC)(OTf)2 and Cr(OTf)2 in NCCH3. Complex 1 also formed quantitatively by adding Cr(OTf)2 to [FeIV(O)(TMC)(NCCH3)]2+. The proposed O2 activation mechanism involves the trapping by a Cr-O2 adduct by Fe(TMC)(OTf)2.

Nonheme mononuclear hydroxoiron(III) species are important intermediates in biological oxidations, but well-characterized examples of synthetic complexes are scarce due to their instability or tendency to form μ-oxodiiron(III) complexes, which are the thermodynamic sink for such chemistry. Herein, we report the successful stabilization and characterization of a mononuclear hydroxoiron(III) complex, [Fe(OH)(TMC-py)] (3; TMC-py = 1-(pyridyl-2'-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane), which is directly generated from the reaction of [Fe(O)(TMC-py)] (2) with 1,4-cyclohexadiene at -40 °C by H-atom abstraction. Complex 3 exhibits a UV spectrum with a λ at 335 nm (ε ≈ 3500 M cm) and a molecular ion in its electrospray ionization mass spectrum at m/z 555 with an isotope distribution pattern consistent with its formulation. Electron paramagnetic resonance and Mössbauer spectroscopy show 3 to be a high-spin Fe(III) center that is formed in 85% yield. Extended X-ray absorption fine structure analysis reveals an Fe-OH bond distance of 1.84 Å, which is also found in [(TMC-py)Fe-O-Cr(OTf)] (4) obtained from the reaction of 2 with Cr(OTf). The S = 5/2 spin ground state and the 1.84 Å Fe-OH bond distance are supported computationally. Complex 3 reacts with 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) at -40 °C with a second-order rate constant of 7.1 M s and an OH/OD kinetic isotope effect value of 6. On the basis of density functional theory calculations, the reaction between 3 and TEMPOH is classified as a proton-coupled electron transfer as opposed to a hydrogen-atom transfer.

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn ) active site in class I b RNRs that requires superoxide anion (O ), rather than dioxygen (O ), to access a high-valent Mn oxidant. Complex 1 ([Mn (O CCH )(N-Et-HPTB)](ClO ) , N-Et-HPTB=N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O at -40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λ =460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a Mn Mn entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn in 1 to Mn Mn for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn Mn -peroxide complex. 2 was capable of oxidizing ferrocene and weak O-H bonds upon activation with proton donors. Our findings provide support for the postulated mechanism of O activation at class I b Mn RNRs.

Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)-FeIV(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)FeIV(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)FeIV(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)-(TMC)FeIII−Oanti−CrIII(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)FeIV(Oanti)(NCCH3)]-(OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)FeIV(Osyn)(NCCH3)]-(OTf)2, which is formulated as [(TMC)FeIII−Osyn−CrIII(OTf)4(NCCH3)] based on its characterization by UV−vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe−O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of 1 (11-unit pre-edge area and 1.81 Å Fe−O distance) but more closely resemble the values reported for [(TMC)FeIII−Osyn−ScIII(OTf)4(NCCH3)] (3, 32-unit pre-edge area and 1.75 Å Fe−O distance). This comparison suggests that 2 has a square pyramidal iron center like 3, rather than the six-coordinate center deduced for 1. Density functional theory calculations further validate the structures for 1 and 2. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)FeIII−Oanti−FeIIICl3] (4Cl) and [(TMC)FeIII−Osyn−FeIIICl3](OTf) (5). Complexes 1–5 thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.

Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH2(Mn-L)W17F6O55]q−, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O2, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K+, Rb+, and Cs+) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li+ and Na+ did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O2 formation is fastest in the presence of Cs+ and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O2 formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O2 formation.

Nonheme oxoiron(IV) complexes can serve as synthons for generating heterobimetallic oxo-bridged dimetal complexes by reaction with divalent metal complexes. The formation of Fe-O-Cr and Fe-O-Mn complexes is described herein. The latter complexes may serve as models for the Fe-X-Mn active sites of an emerging class of Fe/Mn enzymes represented by the Class 1c ribonucleotide reductase from Chlamydia trachomatis and the R2-like ligand-binding oxidase (R2lox) found in Mycobacterium tuberculosis. These synthetic complexes have been characterized by UV-Vis, resonance Raman, and X-ray absorption spectroscopy, as well as electrospray mass spectrometry. The Fe-O-Cr complexes exhibit a three-band UV-Vis pattern that differs from the simpler features associated with Fe-O-Fe complexes. The positions of these features are modulated by the nature of the supporting polydentate ligand on the iron center, and their bands intensify dramatically in two examples upon the binding of an axial cyanate or thiocyanate ligand trans to the oxo bridge. In contrast, the Fe-O-Mn complexes resemble Fe-O-Fe complexes more closely. Resonance Raman characterization of the Fe-O-M complexes reveals an O-sensitive vibration in the range of 760-890 cm. This feature has been assigned to the asymmetric Fe-O-M stretching mode and correlates reasonably with the Fe-O bond distance determined by EXAFS analysis. The likely binding of an acetate as a bridging ligand to the Fe-O-Mn complex 12 lays the foundation for further efforts to model the heterobimetallic active sites of Fe/Mn enzymes.

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