Mononuclear Nonheme Iron(III)‐Iodosylarene and High‐Valent Iron‐Oxo Complexes in Olefin Epoxidation Reactions

High‐spin iron(III)‐iodosylarene complexes are highly reactive in the epoxidation of olefins, in which epoxides are formed as the major products with high stereospecificity and enantioselectivity. The reactivity of the iron(III)‐iodosylarene intermediates is much greater than that of the corresponding iron(IV)‐oxo complex in these reactions. The iron(III)‐iodosylarene species—not high‐valent iron(IV)‐oxo and iron(V)‐oxo species—are also shown to be the active oxidants in catalytic olefin epoxidation reactions. The present results are discussed in light of the long‐standing controversy on the one oxidant versus multiple oxidants hypothesis in oxidation reactions.     

Two‐Color Valence‐to‐Core X‐ray Emission Spectroscopy Tracks Cofactor Protonation State in a Class I Ribonucleotide Reductase

Proton transfer reactions are of central importance to a wide variety of biochemical processes, though determining proton location and monitoring proton transfers in biological systems is often extremely challenging. Herein, we use two‐color valence‐to‐core X‐ray emission spectroscopy (VtC XES) to identify protonation events across three oxidation states of the O₂‐activating, radical‐initiating manganese–iron heterodinuclear cofactor in a class I‐c ribonucleotide reductase. This is the first application of VtC XES to an enzyme intermediate and the first simultaneous measurement of two‐color VtC spectra. In contrast to more conventional methods of assessing protonation state, VtC XES is a more direct probe applicable to a wide range of metalloenzyme systems. These data, coupled to insight provided by DFT calculations, allow the inorganic cores of the Mn^IVFe^IV and Mn^IVFe^III states of the enzyme to be assigned as Mn^IV(μ‐O)₂Fe^IV and Mn^IV(μ‐O)(μ‐OH)Fe^III, respectively.     

Mononuclear Nonheme High‐Spin (S=2) versus Intermediate‐Spin (S=1) Iron(IV)–Oxo Complexes in Oxidation Reactions

Mononuclear nonheme high‐spin (S=2) iron(IV)–oxo species have been identified as the key intermediates responsible for the C?H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C?H bond activation of hydrocarbons by a synthetic mononuclear nonheme high‐spin (S=2) iron(IV)–oxo complex occurs through an oxygen non‐rebound mechanism, as previously demonstrated in the C?H bond activation by nonheme intermediate (S=1) iron(IV)–oxo complexes. We also report that C?H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high‐spin (HS) and intermediate‐spin (IS) iron(IV)–oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)–oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.     

X‐Ray Emission Spectroscopy: A Spectroscopic Measure for the Determination of NO Oxidation States in Fe–NO Complexes

Extensive study of the electronic structure of Fe‐NO complexes using a variety of spectroscopic methods was attempted to understand how iron controls the binding and release of nitric oxide. The comparable energy levels of NO π* orbitals and Fe 3d orbitals complicate the bonding interaction within Fe? NO complexes and puzzle the quantitative assignment of NO oxidation state. Enemark–Feltham notation, {Fe(NO)_x}ⁿ, was devised to circumvent this puzzle. This 40‐year puzzle is revisited using valence‐to‐core X‐ray emission spectroscopy (V2C XES) in combination with computational study. DFT calculation establishes a linear relationship between ΔE_σ2s*‐σ2p of NO and its oxidation state. V2C Fe XES study of Fe? NO complexes reveals the ΔE_σ2s*‐σ2p of NO derived from NO σ_2s*/σ_2p→Fe_1s transitions and determines NO oxidation state in Fe? NO complexes. Quantitative assignment of NO oxidation state will correlate the feasible redox process of nitric oxide and Fe‐nitrosylation biology.     

Spin Crossover in Nitrito‐Myoglobin as Revealed by Resonance Raman Spectroscopy

The myoglobin (Mb) heme Fe‐O‐N=O and heme Fe‐O‐N=O/2‐nitrovinyl species have been characterized by resonance Raman spectroscopy. In the heme Fe‐O‐N=O species, the bound nitrite ligand is removed by solvent exchange, thus reforming metmyoglobin (metMb). The high‐spin heme Fe‐O‐N=O unit is converted into a low‐spin heme Fe‐O‐N=O/2‐nitrovinyl species that can be reversibly switched between a low‐ and a high‐spin state without removing the bound nitrite ligand, as observed in the case of the heme Fe‐O‐N=O species. This spin‐state change is likely to be accompanied by a general structural rearrangement in the protein‐binding pocket. This example is the first of a globin protein that can reversibly change its metal spin state through an internal perturbation. These findings provide a basis for understanding the structure–function relationship of the spin cross found in other metalloenzymes and Fe^III–porphyrin complexes.     

A Thorough Investigation of the Active Titanium Species in TS‐1 Zeolite by In Situ UV Resonance Raman Spectroscopy

A thorough investigation of the active titanium species in TS‐1 zeolite was conducted by in situ UV resonance Raman spectroscopy combined with UV/Vis diffuse reflectance spectroscopy, DFT calculations, and epoxidation experiments. A new titanium species was identified with a characteristic Raman band at 695 cm^?1 when excited at the 266 nm laser line. It is shown that the newly found titanium species is active in the epoxidation reactions in addition to the tetrahedrally coordinated titanium species. However, the acidity of the new titanium species could catalyze the ring‐opening reactions of the epoxy products. It results in a lower selectivity toward the epoxy products relative to that of the tetrahedrally coordinated titanium species. The side reaction can be suppressed by the addition of a weak basic reagent.     

Visualization of Vibrational Modes in Real Space by Tip‐Enhanced Non‐Resonant Raman Spectroscopy

We present a general theory to model the spatially resolved non‐resonant Raman images of molecules. It is predicted that the vibrational motions of different Raman modes can be fully visualized in real space by tip‐enhanced non‐resonant Raman scattering. As an example, the non‐resonant Raman images of water clusters were simulated by combining the new theory and first‐principles calculations. Each individual normal mode gives rise its own distinct Raman image, which resembles the expected vibrational motions of the atoms very well. The characteristics of intermolecular vibrations in supermolecules could also be identified. The effects of the spatial distribution of the plasmon as well as nonlinear scattering processes were also addressed. Our study not only suggests a feasible approach to spatially visualize vibrational modes, but also provides new insights in the field of nonlinear plasmonic spectroscopy.     

Mimicking Class I b Mn2‐Ribonucleotide Reductase: A MnII2 Complex and Its Reaction with Superoxide

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn₂) active site in class I b RNRs that requires superoxide anion (O₂^.?), rather than dioxygen (O₂), to access a high‐valent Mn₂ oxidant. Complex 1 ([Mn₂(O₂CCH₃)(N‐Et‐HPTB)](ClO₄)₂, 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 O₂^.? at ?40 °C resulting in the formation of a metastable species ( 2 ). 2 displayed electronic absorption features (λ_max=460, 610 nm) typical of a Mn‐peroxide species and a 29‐line EPR signal typical of a Mn^IIMn^III entity. Mn K‐edge X‐ray absorption near‐edge spectroscopy (XANES) suggested a formal oxidation state change of Mn^II₂ in 1 to Mn^IIMn^III for 2 . Electrospray ionisation mass spectrometry (ESI‐MS) suggested 2 to be a Mn^IIMn^III‐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 O₂^.? activation at class I b Mn₂ RNRs.     

Vanadium Speciation by XANES Spectroscopy: A Three‐Dimensional Approach

A library of X‐ray absorption near‐edge structure (XANES) spectroscopic data for V^V, V^IV and V^III complexes with a broad range of biologically relevant ligand has been used to demonstrate that three‐dimensional plots of key XANES parameters (pre‐edge and edge energies; pre‐edge and white line intensities) can be used for the prediction of V oxidation states and coordination numbers in biological or environmental matrices. The reliability of the technique has been demonstrated by re‐analysis of the published XANES data for a V^V‐dependent bromoperoxidase.     

Active Intermediates in Copper Nitrite Reductase Reactions Probed by a Cryotrapping‐Electron Paramagnetic Resonance Approach

Redox active metalloenzymes catalyse a range of biochemical processes essential for life. However, due to their complex reaction mechanisms, and often, their poor optical signals, detailed mechanistic understandings of them are limited. Here, we develop a cryoreduction approach coupled to electron paramagnetic resonance measurements to study electron transfer between the copper centers in the copper nitrite reductase (CuNiR) family of enzymes. Unlike alternative methods used to study electron transfer reactions, the cryoreduction approach presented here allows observation of the redox state of both metal centers, a direct read‐out of electron transfer, determines the presence of the substrate/product in the active site and shows the importance of protein motion in inter‐copper electron transfer catalyzed by CuNiRs. Cryoreduction‐EPR is broadly applicable for the study of electron transfer in other redox enzymes and paves the way to explore transient states in multiple redox‐center containing proteins (homo and hetero metal ions).     

A Synthetic High‐Spin Oxoiron(IV) Complex: Generation,Spectroscopic Characterization,and Reactivity

High versus low : The high‐yield generation of a synthetic high‐spin oxoiron(IV) complex, [Fe^IV(O)(TMG₃tren)]²⁺ (see picture, TMG₃tren = 1,1,1‐tris{2‐[N2‐(1,1,3,3‐tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG₃tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S=2 ground state is favored.

     

Deciphering Iron‐Dependent Activity in Oxygen Evolution Catalyzed by Nickel–Iron Layered Double Hydroxide

Nickel iron oxyhydroxide is the benchmark catalyst for the oxygen evolution reaction (OER) in alkaline medium. Whereas the presence of Fe ions is essential to the high activity, the functions of Fe are currently under debate. Using oxygen isotope labeling and operando Raman spectroscopic experiments, we obtain turnover frequencies (TOFs) of both Ni and Fe sites for a series of Ni and NiFe layered double hydroxides (LDHs), which are structurally defined samples of the corresponding oxyhydroxides. The Fe sites have TOFs 20–200 times higher than the Ni sites such that at an Fe content of 4.7 % and above the Fe sites dominate the catalysis. Higher Fe contents lead to larger structural disorder of the NiOOH host. A volcano‐type correlation was found between the TOFs of Fe sites and the structural disorder of NiOOH. Our work elucidates the origin of the Fe‐dependent activity of NiFe LDH, and suggests structural ordering as a strategy to improve OER catalysts.