Asp-193 and Glu-218 of subunit II are involved in the Mn2+-binding of Paracoccus denitrificans cytochrome c oxidase

AbstractCytochrome c oxidase contains a binding site for a non-redox-active metal at the interface of subunits I and II, usually a magnesium ion. In Paracoccus denitrificans oxidase, typically 20% may be replaced by manganese, using standard growth media. Site-directed mutants were constructed in subunit II (D193N and E218Q), and the isolated enzymes analyzed by total-reflection X-ray fluorescence spectrometry and EPR. Both mutants show a strong reduction of the manganese stoichiometry and a diminished electron transfer activity, demonstrating that D193 and E218 are involved in the binding of a manganese/magnesium ion in this site

Binding of dinitrogen to an iron-sulfur-carbon site

© 2015 Macmillan Publishers Limited. All rights reserved. Nitrogenases are the enzymes by which certain microorganisms convert atmospheric dinitrogen (N2) to ammonia, thereby providing essential nitrogen atoms for higher organisms. The most common nitrogenases reduce atmospheric N2 at the FeMo cofactor, a sulfur-rich iron-molybdenum cluster (FeMoco). The central iron sites that are coordinated to sulfur and carbon atoms in FeMoco have been proposed to be the substrate binding sites, on the basis of kinetic and spectroscopic studies. In the resting state, the central iron sites each have bonds to three sulfur atoms and one carbon atom. Addition of electrons to the resting state causes the FeMoco to react with N2, but the geometry and bonding environment of N2 -bound species remain unknown. Here we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe-S bond and binds N2. The product is the first synthetic Fe-N2 complex in which iron has bonds to sulfur and carbon atoms, providing a model for N2 coordination in the FeMoco. Our results demonstrate that breaking an Fe-S bond is a chemically reasonable route to N2 binding in the FeMoco, and show structural and spectroscopic details for weakened N2 on a sulfur-rich iron site

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C−H Bond Oxidation Reactions

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV=O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV=O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).Peer Reviewe

mechanistic promiscuity in hydrogen atom abstraction reactions

In addition to oxometal [Mn+[double bond, length as m-dash]O] and imidometal [Mn+[double bond, length as m-dash]NR] units, transient metal–iodosylarene [M(n−2)+–O[double bond, length as m-dash]IPh] and metal–iminoiodane [M(n−2)+–N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible “second oxidant” responsible for the oxo and imido group transfer reactivity. Although a few metal–iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal–iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal–iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent

Identification of a spin-coupled Mo(III) in the nitrogenase iron-molybdenum cofactor

International audienceNitrogenase is a complex enzyme that catalyzes the formation of ammonia utilizing a MoFe7S9C cluster. The presence of a central carbon atom was recently revealed, finally completing the atomic level description of the active site. However, important prerequisites for understanding the mechanism - the total charge, metal oxidation states and electronic structure are unknown. Herein we present high-energy resolution fluorescence detected Mo K-edge X-ray absorption spectroscopy of nitrogenase. Comparison to FeMo model complexes of known oxidation state indicates that the Mo in the FeMo cofactor of nitrogenase is best described as Mo(III), in contrast to the universally accepted Mo(IV) assignment. The oxidation state assignment is supported by theoretical calculations, which reveal the presence of an unusual spin-coupled Mo(III) site. Although so far Mo(III) was not reported to occur in biology the suggestion raises interesting parallels with the known homogenous Mo catalysts for N-2 reduction, where a Mo(III) compound is the N-2-binding species. It also requires a reassignment of the Fe oxidation states in the cofacto

Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform

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Enhanced Fe-centered redox flexibility in Fe–Ti heterobimetallic complexes

Previously, we reported the synthesis of Ti[N(o-(NCH2P(iPr)2)C6H4)3] and the Fe–Ti complex, FeTi[N(o-(NCH2P(iPr)2)C6H4)3], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc+), respectively. Two isostructural redox members, [FeTiL]+ and [FeTiL]− (2ox and 2red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]+ complex (1ox). The solid-state structures of the [FeTiL]+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and 57Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%)

Synthetic ferripyrophyllite: preparation, characterization and catalytic application

[EN] Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5](2-) entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.The authors gratefully acknowledge the following people for support with analytical measurements and data analysis: Hans-Josef Bongard (SEM-EDX), Silvia Palm (EDX bulk), Adrian Schluter (TEM), Norbert Pfander (STEM), Jan Ternieden and Jan Nicolas Buscher (XRD and XPS), Prof. Dr Osamu Terasaki and Dr Yanghang Ma (3D electron diffraction tomography: failed due to the poor crystallinity and stability under strong beam irradiation), Dr Nicolas Duyckaerts (NH3-TPD measurements), Kai Jeske (GC gas analysis), Dr Yuxiao Ding (ATR-IR) and Dr Zhengwen Cao (titration). The authors also would like to thank Prof. Dr Robert Schlogl, Dr Thomas Lunkenbein, Fabian Pienkoss and Dr Gaetano Calvaruso for helpful and enthusiastic discussions, as well as Niklas Fuhrmann and Lars Winkel for technical support. The studies were carried out as part of our activities in the Cluster of Excellence "Tailor-Made Fuels from Biomass" (EXC 236) and "The Fuel Science Center" funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy -Exzellenzcluster 2186 "The Fuel Science Center" ID: 390919832. Open Access funding provided by the Max Planck Society.Qiao, Y.; Theyssen, N.; Spliethoff, B.; Folke, J.; Weidenthaler, C.; Schmidt, W.; Prieto González, G.... (2021). Synthetic ferripyrophyllite: preparation, characterization and catalytic application. Dalton Transactions. 50(3):850-857. https://doi.org/10.1039/d0dt03125aS85085750

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant".

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the "oxidant" as well as "amino" group donor. It activates the high-spin Fe(II) (St = 2) catalyst [Fe(acac)2(H2O)2] (1) to generate a high-spin (St = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with St = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)2-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N-O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = -524 cm-1) to an iminyl radical, [Fe(III)(acac)2-NH·], giving St = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N-O bond cleavage to generate the active iron-nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe-N bond which, together with the unpaired electron density along the Fe-N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis