Acid pKa Dependence in O–O Bond Heterolysis of a Nonheme FeIII–OOH Intermediate To Form a Potent FeV═O Oxidant with Heme Compound I-Like Reactivity

Selective hydroxylation of benzene to phenol via CuII(μ-O˙)CuII intermediate using a nonsymmetric dicopper catalyst

The one-step oxidation of benzene to phenol represents a significant and promising advancement in modern industries focused on the production of high-value-added chemical products. Nevertheless, challenges persist in achieving sufficient catalytic selectivity and preventing over-oxidation. Inspired by copper enzymes, we present a nonsymmetric dicopper complex ([CuII2(TPMAN)(μ-OH)(H2O)]3+, 1) for the selective oxidation of benzene to phenol. Utilizing H2O2 as the oxidant, complex 1 demonstrates remarkable catalytic activity (a TON of 14 000 within 29 hours) and selectivity exceeding 97%, comparable to the finest homogeneous catalyst derived from first-row transition metals. It is noteworthy that the significant substituent effect, alongside a negligible kinetic isotope effect (KIE = 1.05), radical trapping experiments, and an inconsistent standard selectivity test of the ˙OH radicals, all contradict the conventional Fenton mechanism and rebound pathway. Theoretical investigations indicate that the active CuII(μ-O˙)CuII-OH species generated through the cleavage of the O-O bond in the CuII(μ-1,1-OOH)CuI intermediate facilitates the hydroxylation of benzene via an electrophilic attack mechanism. The nonsymmetric coordination geometry is crucial in activating H2O2 and in the process of O-O bond cleavage.

The [(Bn-tpen)FeII]2+ Complex as a Catalyst for the Oxidation of Cyclohexene and Limonene with Dioxygen

[(Bn-tpen)FeII(MeCN)](ClO4)2, containing the pentadentate Bn-tpen-N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane ligand, was studied in the oxygenation of cyclohexene and limonene using low-pressure dioxygen (0.2 atm air or 1 atm pure O2) in acetonitrile. 2-Cyclohexen-1-one and 2-cyclohexen-1-ol are the main products of cyclohexene oxidations, with cyclohexene oxide as a minor product. Limonene is oxidized to limonene oxide, carvone, and carveol. Other oxidation products such as perillaldehyde and perillyl alcohol are found in trace amounts. This catalyst is slightly less active than the previously reported [(N4Py)FeII(MeCN)](ClO4)2 (N4Py-N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). Based on cyclic voltammetry experiments, it is postulated that [(Bn-tpen)FeIV=O]2+ is the active species. The induction period of approx. 3 h during cyclohexene oxygenation is probably caused by deactivation of the reactive Fe(IV)=O species by the parent Fe(II) complex. Equimolar mixtures of Fe(II) salt and the ligand (in situ-formed catalyst) gave catalytic performance similar to that of the synthesized catalyst.

Proton-assisted activation of a MnIII-OOH for aromatic C-H hydroxylation through a putative [MnVO] species

Adding HClO4 to [(BnTPEN)MnIII-OO]+ in MeOH generates a short-lived MnIII-OOH species, which converts to a putative MnVO species. The potent MnVO species in MeCN oxidizes the pendant phenyl ring of the ligand in an intramolecular fashion. The addition of benzene causes the formation of (BnTPEN)MnIII-phenolate. These findings suggest that high valent Mn species have the potential to catalyze challenging aromatic hydroxylation reactions.

Mimicking the Reactivity of LPMOs with a Mononuclear Cu Complex

Lytic polysaccharide monooxygenases (LPMOs) are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C-H bonds in polysaccharides using O2 or H2O2 as oxidants (monooxygenase/peroxygenase). In the absence of C-H substrate, LPMOs reduce O2 to H2O2 (oxidase) and H2O2 to H2O (peroxidase) using proton/electron donors. This rich oxidative reactivity is promoted by a mononuclear Cu center in which some of the amino acid residues surrounding the metal might can accept and donate protons and/or electrons during O2 and H2O2 reduction. Herein, we utilize a podal ligand containing H-bond/proton donors (LH2) to analyze the reactivity of mononuclear Cu species towards O2 and H2O2. [(LH2)CuI]1+ (1), [(LH2)CuII]2+ (2), [(LH-)CuII]1+ (3), [(LH2)CuII(OH)]1+ (4), and [(LH2)CuII(OOH)]1+ (5) were synthesized and characterized by structural and spectroscopic means. Complex 1 reacts with O2 to produce 5, which releases H2O2 to generate 3, suggesting that O2 is used by LPMOs to generate H2O2. The reaction of 1 with H2O2 produces 4 and hydroxyl radical, which reacts with C-H substrates in a Fenton-like fashion. Complex 3, which generate 1 via a reversible protonation/reduction, binds H2O and H2O2 to produce 4 and 5, respectively, a mechanism that could be used by LPMOs to control oxidative reactivity.

Efficient Inhibition of Deep Conversion of Partial Oxidation Products in C-H Bonds' Functionalization Utilizing O2 via Relay Catalysis of Dual Metalloporphyrins on Surface of Hybrid Silica Possessing Capacity for Product Exclusion

To inhibit the deep conversion of partial oxidation products (POX-products) in C-H bonds' functionalization utilizing O2, 5-(4-(chloromethyl)phenyl)-10,15,20-tris(perfluorophenyl)porphyrin cobalt(II) and 5-(4-(chloromethyl)phenyl)-10,15,20-tris(perfluorophenyl)porphyrin copper(II) were immobilized on the surface of hybrid silica to conduct relay catalysis on the surface. Fluorocarbons with low polarity and heterogeneous catalysis were devised to decrease the convenient accessibility of polar POX-products to catalytic centers on the lower polar surface. Relay catalysis between Co and Cu was designed to utilize the oxidation intermediates alkyl hydroperoxides to transform more C-H bonds. Systematic characterizations were conducted to investigate the structure of catalytic materials and confirm their successful syntheses. Applied to C-H bond oxidation, not only deep conversion of POX-products was inhibited but also substrate conversion and POX-product selectivity were improved simultaneously. For cyclohexane oxidation, conversion was improved from 3.87% to 5.27% with selectivity from 84.8% to 92.3%, which was mainly attributed to the relay catalysis on the surface excluding products. The effects of the catalytic materials, product exclusion, relay catalysis, kinetic study, substrate scope, and reaction mechanism were also investigated. To our knowledge, a practical and novel strategy was presented to inhibit the deep conversion of POX-products and to achieve efficient and accurate oxidative functionalization of hydrocarbons. Also, a valuable protocol was provided to avoid over-reaction in other chemical transformations requiring high selectivity.

Development of Catalytic Site-Selective C-H Oxidation

Direct C-H bond oxygenation is a strong and useful tool for the construction of oxygen functional groups. After Chen and White's pioneering works, various non-heme-type iron and manganese complexes were introduced, leading to strong development in this area. However, for this method to become a truly useful tool for synthetic organic chemistry, it is necessary to make further efforts to improve site-selectivity, and catalyst durability. Recently, we found that non-heme-type ruthenium complex cis-1 presents efficient catalysis in C(sp3 )-H oxygenation under acidic conditions. cis-1-catalysed C-H oxygenation can oxidize various substrates including highly complex natural compounds using hypervalent iodine reagents as a terminal oxidant. Moreover, the catalyst system can use almost stoichiometric water molecules as the oxygen source through reversible hydrolysis of PhI(OCOR)2 . It is a strong tool for producing isotopic-oxygen-labelled compounds. Moreover, the environmentally friendly hydrogen peroxide can be used as a terminal oxidant under acidic conditions.

Reaction landscape of a mononuclear MnIII-hydroxo complex with hydrogen peroxide

Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq2Me)]+ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [MnIIIMnIV(μ-O)2(dpaq2Me)2]+, the MnIII-hydroperoxo complex [MnIII(OOH)(dpaq2Me)]+, and the MnIII-peroxo complex [MnIII(O2)(dpaq2Me)]. The formation of the MnIII-hydroperoxo species from the reaction of a MnIII-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.

Efficient and Selective Oxygenation of Cycloalkanes and Alkyl Aromatics with Oxygen through Synergistic Catalysis of Bimetallic Active Centers in Two-Dimensional Metal-Organic Frameworks Based on Metalloporphyrins

Confined catalytic realms and synergistic catalysis sites were constructed using bimetallic active centers in two-dimensional metal-organic frameworks (MOFs) to achieve highly selective oxygenation of cycloalkanes and alkyl aromatics with oxygen towards partly oxygenated products. Every necessary characterization was carried out for all the two-dimensional MOFs. The selective oxygenation of cycloalkanes and alkyl aromatics with oxygen was accomplished with exceptional catalytic performance using two-dimensional MOF Co-TCPPNi as a catalyst. Employing Co-TCPPNi as a catalyst, both the conversion and selectivity were improved for all the hydrocarbons investigated. Less disordered autoxidation at mild conditions, inhibited free-radical diffusion by confined catalytic realms, and synergistic C-H bond oxygenation catalyzed by second metal center Ni employing oxygenation intermediate R-OOH as oxidant were the factors for the satisfying result of Co-TCPPNi as a catalyst. When homogeneous metalloporphyrin T(4-COOCH3)PPCo was replaced by Co-TCPPNi, the conversion in cyclohexane oxygenation was enhanced from 4.4% to 5.6%, and the selectivity of partly oxygenated products increased from 85.4% to 92.9%. The synergistic catalytic mechanisms were studied using EPR research, and a catalysis model was obtained for the oxygenation of C-H bonds with O2. This research offered a novel and essential reference for both the efficient and selective oxygenation of C-H bonds and other key chemical reactions involving free radicals.

Catalytic oxidation properties of an acid-resistant cross-bridged cyclen Fe(II) complex. Influence of the rigid donor backbone and protonation on the reactivity

The catalytic properties of an iron complex bearing a pentadentate cross-bridged ligand backbone are reported. With H₂O₂ as an oxidant, it displays moderate conversions in epoxidation and alkane hydroxylation and satisfactory ones in aromatic hydroxylation. Upon addition of an acid to the reaction medium, a significant enhancement in aromatic and alkene oxidation is observed. Spectroscopic analyses showed that accumulation of the expected Fe^III(OOH) intermediate is limited under these conditions, unless an acid is added to the mixture. This is ascribed to the inertness induced by the cross-bridged ligand backbone, which is partly reduced under acidic conditions.

Aromatic and aliphatic hydrocarbon hydroxylation via a formally NiIVO oxidant

The reaction of (NMe4)2[NiII(LPh)(OAc)] (1[OAc], LPh = 2,2',2''-nitrilo-tris-(N-phenylacetamide); OAc = acetate) with 3-chloroperoxybenzoic acid (m-CPBA) resulted in the formation of a self-hydroxylated NiIII-phenolate complex, 2, where one of the phenyl groups of LPh underwent hydroxylation. 2 was characterised by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a previously characterised NiII-phenolate complex, 3. We postulate that self-hydroxylation was mediated by a formally NiIVO oxidant, formed from the reaction of 1[OAc] with m-CPBA, which undergoes electrophilic aromatic substitution to yield 2. This is supported by an analysis of the kinetic and thermodynamic properties of the reaction of 1[OAc] with m-CPBA. Addition of exogenous hydrocarbon substrates intercepted the self-hydroxylation process, producing hydroxylated products, providing further support for the formally NiIVO entity. This study demonstrates that the reaction between NiII salts and m-CPBA can lead to potent metal-based oxidants, in contrast to recent studies demonstrating carboxyl radical is a radical free-chain reaction initiator in NiII/m-CPBA hydrocarbon oxidation catalysis.

ESI and tandem MS for mechanistic studies with high-valent transition metal species

The analysis of high-valent metal species has been in the focus of research for over 20 years. Mass spectrometry (MS) represents a technique routinely used for their characterization, in particular...

Room Temperature Aerobic Peroxidation of Organic Substrates Catalyzed by Cobalt(III) Alkylperoxo Complexes

Room temperature aerobic oxidation of hydrocarbons is highly desirable and remains a great challenge. Here we report a series of highly electrophilic cobalt(III) alkylperoxo complexes, Co^III(qpy)OOR supported by a planar tetradentate quaterpyridine ligand that can directly abstract H atoms from hydrocarbons (R'H) at ambient conditions (Co^III(qpy)OOR + R'H → Co^II(qpy) + R'^• + ROOH). The resulting alkyl radical (R'^•) reacts rapidly with O₂ to form alkylperoxy radical (R'OO^•), which is efficiently scavenged by Co^II(qpy) to give Co^III(qpy)OOR' (Co^II(qpy) + R'OO^• → Co^III(qpy)OOR'). This unique reactivity enables Co^III(qpy)OOR to function as efficient catalysts for aerobic peroxidation of hydrocarbons (R'H + O₂ → R'OOH) under 1 atm air and at room temperature.

Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center

A mononuclear nonheme cobalt(ii) complex, [(TMG3tren)CoII(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMG3tren)CoIV(O)]2+-(Sc(OTf)3)n (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)3. Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(ii) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.

The most reactive iron and manganese complexes with N-pentadentate ligands for dioxygen activation—synthesis, characteristics, applications

The iron and manganese complexes that activate oxygen atom play multiple role in technologically relevant reactions as well as in biological transformations, in which exist in different redox states. Among them, high-valent oxo intermediate seems to be the most important one. Iron, and/or manganese-based processes have found application in many areas, starting from catalysis and sustainable technologies, through DNA oxidative cleavage, to new substances useful in chemotherapeutic drugs. This review is not only the latest detailed list of uses of homogeneous N-pentadentate iron and manganese catalysts for syntheses of valuable molecules with huge applications in green technologies, but also a kind of "a cookbook", collecting "recipes" for the discussed complexes, in which the sources necessary to obtain a full characterization of the compounds are presented. Following the catalytic activity of metalloenzymes, and taking into account the ubiquity of iron and manganese salts, which in combination with properly designed ligands may show similarity to natural systems, the discussed complexes can find application as new anti-cancer drugs. Also, owing to ability of oxygen atom to exchange in reaction with H₂O, they can be successfully applied in photodriven reactions of water oxidation, as well as in chemically regenerated fuel cells as a redox catalyst.

Enhancing Chemo- and Stereoselectivity in C-H Bond Oxygenation with H2O2 by Nonheme High-Spin Iron Catalysts: The Role of Lewis Acid and Multimetal Centers

Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H₂O₂) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H₂O₂ and Sc³⁺ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc³⁺ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.

Degradation pathways of a highly active iron(iii) tetra-NHC epoxidation catalyst

Elucidation of different decomposition pathways of a highly active tetradentate iron–NHC epoxidation catalyst reveals direct carbene oxidation to be the decisive cause of catalyst degradation.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

Bio-inspired Nonheme Iron Oxidation Catalysis: Involvement of Oxoiron(V) Oxidants in Cleaving Strong C-H Bonds

Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C-H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron-oxygen intermediates like iron(III)-hydroperoxo and high-valent iron-oxo species have been trapped and identified in investigations of these bio-inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio-inspired nonheme iron systems to form the high-valent iron-oxo intermediates.