Spin State Tunes Oxygen Atom Transfer towards FeIV O Formation in FeII Complexes

Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [Fe^II (CH₃ CN)₂ L](SbF₆ )₂ ([1](SbF₆ )₂ and [2](SbF₆ )₂ ) and [Fe^II (CF₃ SO₃ )₂ L] ([1](OTf)₂ and [2](OTf)₂ (1, L=^Me,H Pytacn; 2, L=^nP,H Pytacn; ^R,R' Pytacn=1-[(6-R'-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu₄ NIO₄ to form the corresponding [Fe^IV (O)(CH₃ CN)L]²⁺ (3, L=^Me,H Pytacn; 4, L=^nP,H Pytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl^- as entering ligand, which indicates that formation of [Fe^IV (O)(CH₃ CN)L]²⁺ is kinetically controlled by substitution in the starting complex to form [Fe^II (IO₄ )(CH₃ CN)L]⁺ intermediates that are converted rapidly to oxo complexes 3 and 4. The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species.

CeIV - and HClO4 -Promoted Assembly of an Fe2IV (μ-O)2 Diamond Core from its Monomeric FeIV =O Precursor at Room Temperature

Diiron(IV)-oxo species are proposed to effect the cleavage of strong C-H bonds by nonheme diiron enzymes such as soluble methane monooxygenase (sMMO) and fatty acid desaturases. However, synthetic mimics of such diiron(IV) oxidants are rare. Herein we report the reaction of (TPA*)FeII (1) (TPA*=tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) in CH3 CN with 4 equiv CAN and 200 equiv HClO4 at 20 °C to form a complex with an [FeIV2 (μ-O)2 ]4+ core. CAN and HClO4 play essential roles in this unprecedented transformation, in which the comproportionation of FeIII -O-CeIV and FeIV =O/Ce4+ species is proposed to be involved in the assembly of the [FeIV2 (μ-O)2 ]4+ core.

A High-Valent Manganese(IV)-Oxo-Cerium(IV) Complex and Its Enhanced Oxidizing Reactivity

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce⁴⁺ ion, [(dpaq)Mn^IV (O)]⁺ -Ce⁴⁺ (1-Ce⁴⁺ ), was synthesized by reacting [(dpaq)Mn^III (OH)]⁺ (2) with cerium ammonium nitrate (CAN). 1-Ce⁴⁺ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI-MS, resonance Raman, XANES, and EXAFS, showing an Mn-O bond distance of 1.69 Å with a resonance Raman band at 675 cm^-1 . Electron-transfer and oxygen atom transfer reactivities of 1-Ce⁴⁺ were found to be greater than those of Mn^IV (O) intermediates binding redox-inactive metal ions (1-Mⁿ⁺ ). This study reports the first example of a redox-active Ce⁴⁺ ion-bound Mn^IV -oxo complex and its spectroscopic characterization and chemical properties.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

Water (H₂ O) is the most environmentally benign reductant and is oxidized to evolve dioxygen (O₂ )-the greenest oxidant-in photosystem II. This Minireview focuses on photocatalytic oxygenation of substrates with H₂ O as an oxygen source and O₂ as an oxidant. Metal complexes can be oxidized by two molecules of one-electron oxidants with H₂ O to produce high-valent metal-oxo complexes, which act as active oxidants for oxygenating organic substrates. When an appropriate oxidant is employed for the substrate oxidation, the reduced oxidant can be oxidized by dioxygen to regenerate the oxidant when water and dioxygen are used as an oxygen source and an oxidant, respectively. Photoinduced electron transfer from a substrate (S) to the excited state of complex [(L)M^III ]⁺ produces a substrate radical cation (S^.+ ), accompanied by the regeneration of [(L)M^II ]. S^.+ then reacts with H₂ O to produce an OH adduct radical that is oxidized by [(L)M^III ]⁺ to yield an oxygenated product (SO), in which the oxygen atom originates from H₂ O, accompanied by regeneration of [(L)M^II ]. Photocatalytic oxidation of H₂ O by O₂ to produce H₂ O₂ is combined with the catalytic oxygenation of substrates with H₂ O₂ to produce the oxygenated products, in which the oxygen atom originates from O₂ at the beginning but later from water. This Minireview provides a promising strategy for oxygenation of substrates by using H₂ O as an oxygen source and O₂ as the greenest oxidant.

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non-Heme Oxoiron(IV) Complex that Makes it a Better Oxidant

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [Fe^IV (O)(L)]²⁺ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k₂ ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.

Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

Naphthalene oxidation with metal-oxygen intermediates is a difficult reaction in environmental and biological chemistry. Herein, we report that a MnIV bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as ESI-MS, UV/Vis, and EPR analysis, X-ray diffraction, and XAS, can be employed for the oxidation of naphthalene in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV bis(hydroxo) complex gave a one-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme MnIV bis(hydroxo) species as well as MnIV oxo analogues. Kinetic studies, including kinetic isotope effect analysis, suggest that the naphthalene oxidation occurs through a rate-determining electron transfer process.

Formation of Mono Oxo Molybdenum(IV) PNP Pincer Complexes: Interplay between Water and Molecular Oxygen

The synthesis of cationic mono oxo MoIV PNP pincer complexes of the type [Mo(PNPMe-iPr)(O)X]+ (X = I, Br) from [Mo(PNPMe-iPr)(CO)X2] is described. These compounds are coordinatively unsaturated and feature a strong Mo≡O triple bond. The formation of these complexes proceeds via cationic 14e intermediates [Mo(PNPMe-iPr)(CO)X]+ and requires both molecular oxygen and water. ESI MS measurements with 18O labeled water (H2 18O) and molecular oxygen (18O2) indicates that water plays a crucial role in the formation of the Mo≡O bond. A plausible mechanism based on DFT calculations is provided. The X-ray structure of [Mo(PNPMe-iPr)(O)I]SbF6 is presented.

Facile and Reversible Formation of Iron(III)-Oxo-Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV (NH₄ )₂ (NO₃ )₆ is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III -O-Ce^IV (OH₂ )(NO₃ )₄ ]⁺ (3), a complex obtained from the reaction of [(N4Py)Fe^II (NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV =O]²⁺ (2) with Ce^III (NO₃ )₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 Fe^IV in 2 to S=5/2 in 3, which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe^IV =O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.

Identification and spectroscopic characterization of nonheme iron(III) hypochlorite intermediates

Fe(III)-hypohalite complexes have been implicated in a wide range of important enzyme-catalyzed halogenation reactions including the biosynthesis of natural products and antibiotics and post-translational modification of proteins. The absence of spectroscopic data on such species precludes their identification. Herein, we report the generation and spectroscopic characterization of nonheme Fe(III)-hypohalite intermediates of possible relevance to iron halogenases. We show that Fe(III)-OCl polypyridylamine complexes can be sufficiently stable at room temperature to be characterized by UV/Vis absorption, resonance Raman and EPR spectroscopies, and cryo-ESIMS. DFT methods rationalize the pathways to the formation of the Fe(III)-OCl, and ultimately Fe(IV)=O, species and provide indirect evidence for a short-lived Fe(II)-OCl intermediate. The species observed and the pathways involved offer insight into and, importantly, a spectroscopic database for the investigation of iron halogenases.

Identification and Spectroscopic Characterization of Nonheme Iron(III) Hypochlorite Intermediates

FeIII-hypohalite complexes have been implicated in a wide range of important enzyme-catalyzed halogenation reactions including the biosynthesis of natural products and antibiotics and post-translational modification of proteins. The absence of spectroscopic data on such species precludes their identification. Herein, we report the generation and spectroscopic characterization of nonheme FeIII-hypohalite intermediates of possible relevance to iron halogenases. We show that FeIII-OCl polypyridylamine complexes can be sufficiently stable at room temperature to be characterized by UV/Vis absorption, resonance Raman and EPR spectroscopies, and cryo-ESIMS. DFT methods rationalize the pathways to the formation of the FeIII-OCl, and ultimately FeIV=O, species and provide indirect evidence for a short-lived FeII-OCl intermediate. The species observed and the pathways involved offer insight into and, importantly, a spectroscopic database for the investigation of iron halogenases.

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

This feature article summarises recent developments in homogeneous C–H bond oxygenation catalysed by molecular iron complexes.

Influence of ligand architecture on oxidation reactions by high-valent nonheme manganese oxo complexes using water as a source of oxygen

Mononuclear nonheme Mn(IV)=O complexes with two isomers of a bispidine ligand have been synthesized and characterized by various spectroscopies and density functional theory (DFT). The Mn(IV)=O complexes show reactivity in oxidation reactions (hydrogen-atom abstraction and sulfoxidation). Interestingly, one of the isomers (L(1) ) is significantly more reactive than the other (L(2) ), while in the corresponding Fe(IV)=O based oxidation reactions the L(2) -based system was previously found to be more reactive than the L(1) -based catalyst. This inversion of reactivities is discussed on the basis of DFT and molecular mechanics (MM) model calculations, which indicate that the order of reactivities are primarily due to a switch of reaction channels (σ versus π) and concomitant steric effects.