Iron and Copper Active Sites in Zeolites and Their Correlation to Metalloenzymes

Low Reorganization Energy for Electron Self-Exchange by a Formally Copper(III,II) Redox Couple

The rate constant for electron self-exchange (k₁₁) between LCuOH and [LCuOH]^- (L = bis-2,6-(2,6-diisopropylphenyl)carboximidopyridine) was determined using the Marcus cross relation. This work involved measurement of the rate of the cross-reaction between [Bu₄N][LCuOH] and [Fc][BAr₄^F] (Fc⁺ = ferrocenium; BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) by stopped-flow methods at -88 °C in CH₂Cl₂ and measurement of the equilibrium constant for the redox process by UV-vis titrations under the same conditions. A value of k₁₁ = 3 × 10⁴ M^-1 s^-1 (-88 °C) led to estimation of a value 9 × 10⁶ M^-1 s^-1 at 25 °C, which is among the highest values known for copper redox couples. Further Marcus analysis enabled determination of a low reorganization energy, λ = 0.95 ± 0.17 eV, attributed to minimal structural variation between the redox partners. In addition, the reaction entropy (ΔS°) associated with the LCuOH/[LCuOH]^- self-exchange was determined from the temperature dependence of the redox potentials, and found to be dependent upon ionic strength. Comparisons to other Cu redox systems and potential new applications for the formally Cu^III,II system are discussed.

Ligand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond

A superoxide-bridged dicopper(II) complex, [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu^II₂(XYLO)(O₂^2-)]⁺ (2), using ferrocenium (Fc⁺) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me₁₀Fc⁺, a 1 ⇆ 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs Fc^+/0. Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [Cu^II₂(XYLO)(OOH)]²⁺ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pK_a of 3 was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M^-1 s^-1. Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O₂-derived reduced species, such as pK_a, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.

Recent Progress in Direct Conversion of Methane to Methanol Over Copper-Exchanged Zeolites

The conversion of methane into an easily transportable liquid fuel or chemicals has become a highly sought-after goal spurred by the increasing availability of cheap and abundant natural gas. While utilization of methane for the production of syngas and its subsequent conversion via an indirect route is typical, it is cost-intensive, and alternative direct conversion routes have been investigated actively. One of the most promising directions among these is the low-temperature partial oxidation of methane to methanol over a metal-loaded zeolite, which mimics facile enzymatic chemistry of methane oxidation. Thus mono-, bi-, and trinuclear oxide compounds of iron and copper stabilized on ZSM-5 or mordenite, which are structurally analogous to those found in methane monooxygenases, have demonstrated promising catalytic performances. The two major problems of theses metal-loaded zeolites are low yield to methanol and batch-like non-catalytic reaction systems challenging to extend to an industrial scale. In this mini-review, attention was given to the direct methane oxidation to methanol over copper-loaded zeolite systems. A brief introduction on the catalytic methane direct oxidation routes and current status of the applied metal-containing zeolites including the ones with copper ions are given. Next, by analyzing the extensive experimental and theoretical data available, the consensus among the researchers to achieve the target of high methanol yield is discussed in terms of zeolite topology, active species, and reaction parameters. Finally, the recent efforts on continuous methanol production from the direct methane oxidation aiming for an industrial process are summarized.

Single-Sites and Nanoparticles at Tailored Interfaces Prepared via Surface Organometallic Chemistry from Thermolytic Molecular Precursors

Heterogeneous catalysts are complex by nature, making particularly difficult to assess the structure of their active sites. Such complexity is inherited in part from their mode of preparation, which typically involves coprecipitation or impregnation of metal salts in aqueous solution, and the associated complex surface chemistries. In this context, surface organometallic chemistry (SOMC) has emerged as a powerful approach to generate well-defined surface species, where the metal sites are introduced by grafting tailored molecular precursors. When combined with thermolytic molecular precursors (TMPs) that can lose their organic moieties quite readily upon thermal treatment, SOMC provides access to supported isolated metal sites with defined oxidation state and nuclearity inherited from the precursor. The resulting surface species bear unusual coordination imposed by the surface that provides them high reactivity in comparison with their molecular precursor. In addition, these molecularly defined species bare strong resemblance with the active sites of supported metal oxides. However, they typically contain a higher proportion of active sites making structure-activity relationship possible. They thus constitute ideal models for this important class of industrial catalysts that are used in numerous applications such as olefin epoxidation (Shell process), olefin metathesis (triolefin process), ethylene polymerization (Phillips catalysts), or propane dehydrogenation (Catofin and related processes). This SOMC/TMP approach can thus provide detailed information about the structure of active sites in industrial catalysts, their mode of initiation and deactivation, as well as the role of the support and specific thermal treatment on the final activity of the catalysts. Nonetheless, these structurally characterized surface sites still exhibit heterogeneous environments borrowed from the support itself, that explain the intrinsic complexity of heterogeneous catalysis. Furthermore, SOMC/TMP can also be used to generate and investigate supported metal nanoparticles. Starting from the well-defined isolated sites, that also contain adjacent surface OH groups, one can graft a second metal and then generate after treatment under hydrogen small and narrowly dispersed alloys or nanoparticles with tailored interfaces that can show improved catalytic performances and are amiable to detailed structure-activity relationships. This approach is illustrated by two case studies: (1) formation of supported copper nanoparticles at tailored interfaces that contain isolated metal sites for the selective hydrogenation of carbon dioxide to methanol, allowing for a detailed understanding of the role of dopants and supports in heterogeneous catalysis, and (2) preparation of highly selective and productive propane dehydrogenation catalysts based on silica-supported Pt _xGa _y alloy. Overall, this Account shows how the combination of SOMC and TMP helps to generate catalysts, particularly suited for elucidating structural characterization of active sites at a molecular-level which in turn enables structure-activity relationship to be drawn. Such detailed information obtained on well-defined catalysts can then be used to understand complex effects observed in industrial catalysts (effects of supports, additives, dopants, etc.), and to extract information that can then be used to improve them in a more rational way.

Aerobic oxygenation catalyzed by first row transition metal complexes coordinated by tetradentate mono-carbon bridged bis-phenanthroline ligands: intra- versus intermolecular carbon-hydrogen bond activation

Commonly, iron(ii) and copper(i) complexes bind dioxygen (O2) and then activate O2 through a reductive reaction pathway. There is, however, significant interest in low temperature oxygenation with O2 without the use of a sacrificial reductant. Here, earth-abundant metal complexes (FeII, CoII, NiII and CuII) coordinated by two different tetra-dentate mono-carbon bridged bis-phenanthroline ligands, (1,10-Phen)2-2,2'-CR1R2, where R1 = n-butyl and R2 = n-butyl or H were synthesized. The structures all showed the expected metal complexation in the equatorial plane by the bridged bis-phenanthroline ligands. For R1 = n-butyl; R2 = H, where the ligand has a tertiary carbon bridging group, fast intramolecular oxygenation occurred at the pseudobenzylic position. Depending on the transition metal the main products formed were oxygen bridged dimers of the metal complexes (Co and Fe) or metal complexes with a carbonyl moiety at the bridging pseudobenzylic position as a result of C-R1 bond cleavage (Ni and Cu). The different product assemblages are explained by different reaction pathways that are metal specific. For quaternary carbon bridged ligands, R1 = R2 = n-butyl, the complexes catalytically activated C-H bonds of cyclohexene under catalytic conditions, showing higher effective turnover numbers at low catalyst loading. The reactivity observed is commensurate with a room temperature autooxidation reaction although the initiation of the free radical reaction is metal specific. In general labelling studies with 18O2, UV-vis and EPR spectroscopy as well as cyclic voltammetry measurements led to a conclusion that the reaction pathways involved both C-H bond activation and O2 activation.

Particulate methane monooxygenase contains only mononuclear copper centers

Bacteria that oxidize methane to methanol are central to mitigating emissions of methane, a potent greenhouse gas. The nature of the copper active site in the primary metabolic enzyme of these bacteria, particulate methane monooxygenase (pMMO), has been controversial owing to seemingly contradictory biochemical, spectroscopic, and crystallographic results. We present biochemical and electron paramagnetic resonance spectroscopic characterization most consistent with two monocopper sites within pMMO: one in the soluble PmoB subunit at the previously assigned active site (CuB) and one ~2 nanometers away in the membrane-bound PmoC subunit (CuC). On the basis of these results, we propose that a monocopper site is able to catalyze methane oxidation in pMMO.

Cu(i)-O2 oxidation reactions in a fluorinated all-O-donor ligand environment

Investigation of Cu-O2 oxidation reactivity is important in biological and anthropogenic chemistry. Zeolites are one of the most promising Cu/O based oxidation catalysts for development of industrial-scale CH4 to CH3OH conversion. Their oxidation mechanisms are not well understood, however, highlighting the importance of the investigation of molecular Cu(i)-O2 reactivity with O-donor complexes. Herein, we give an overview of the synthesis, structural properties, and O2 reactivity of three different series of O-donor fluorinated Cu(i) alkoxides: K[Cu(OR)2], [(Ph3P)Cu(μ-OR)2Cu(PPh3)], and K[(R3P)Cu(pinF)], in which OR = fluorinated monodentate alkoxide ligands and pinF = perfluoropinacolate. This breadth allowed for the exploration of the influence of the denticity of the ligand, coordination number, the presence of phosphine, and KF/O interactions on their O2 reactivity. KF/O interactions were required to activate O2 in the monodentate-ligand-only family, whereas these connections did not affect O2 activation in the bidentate complexes, potentially due to the presence of phosphine. Both families formed trisanionic, trinuclear cores of the form {Cu3(μ3-O)2}3-. Intramolecular and intermolecular substrate oxidation were also explored and found to be influenced by the fluorinated ligand. Namely, {Cu3(μ3-O)2}3- from K[Cu(OR)2] could perform both monooxygenase reactivity and oxidase catalysis, whereas those from K[(R3P)Cu(pinF)] could only perform oxidase catalysis.

Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO₂] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ^XArOH (X = para substituent NO₂, CF₃, Cl, H, Me, ^tBu, OMe, or NMe₂) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M^-1 s^-1) by second-order kinetics to yield TEMPO^• and [LCuOOH]^- on the basis of electron paramagnetic resonance spectroscopy, the production of H₂O₂ upon treatment with protic acid, and independent preparation from reaction of [NBu₄][LCuOH] with H₂O₂ ( K_eq = 0.022 ± 0.007 for the reverse reaction). The reactions with ^XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K_a, E_1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO₂, CF₃, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe₂ (data for X = H, Me, ^tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO₂]⁺ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.

Revisiting the Synthesis and Nucleophilic Reactivity of an Anionic Copper Superoxide Complex

The addition of 1 equiv of KO₂ and Kryptofix222 (Krypt) in CH₃CN to a solution of LCu(CH₃CN) [L = N, N'-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at -80 °C yielded [K(Krypt)][LCuO₂], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO₂] reacts with wet 2-PPA to form [LCuOH]^-, which then deprotonates 2-PPA to yield the copper(II) enolate complex [LCu(OC═C(Me)Ph)]^-. Acetophenone was observed upon workup of this complex or mixtures of KO₂ and 2-PPA alone, in support of an alternative mechanism(s) to the one proposed previously involving an initial nucleophilic attack at the carbonyl group of 2-PPA.

Consequences of exchange-site heterogeneity and dynamics on the UV-visible spectrum of Cu-exchanged SSZ-13

The speciation and structure of Cu ions and complexes in chabazite (SSZ-13) zeolites, which are relevant catalysts for nitrogen oxide reduction and partial methane oxidation, depend on material composition and reaction environment. Ultraviolet-visible (UV-Vis) spectra of Cu-SSZ-13 zeolites synthesized to contain specific Cu site motifs, together with ab initio molecular dynamics and time-dependent density functional theory calculations, were used to test the ability to relate specific spectroscopic signatures to specific site motifs. Geometrically distinct arrangements of two framework Al atoms in six-membered rings are found to exchange Cu2+ ions that become spectroscopically indistinguishable after accounting for the finite-temperature fluctuations of the Cu coordination environment. Nominally homogeneous single Al exchange sites are found to exchange a heterogeneous mixture of [CuOH]+ monomers, O- and OH-bridged Cu dimers, and larger polynuclear complexes. The UV-Vis spectra of the latter are sensitive to framework Al proximity, to precise ligand environment, and to finite-temperature structural fluctuations, precluding the precise assignment of spectroscopic features to specific Cu structures. In all Cu-SSZ-13 samples, these dimers and larger complexes are reduced by CO to Cu+ sites at 523 K, leaving behind isolated [CuOH]+ sites with a characteristic spectroscopic identity. The various mononuclear and polynuclear Cu2+ species are distinguishable by their different responses to reducing environments, with implications for their relevance to catalytic redox reactions.

Room temperature O transfer from N2O to CO mediated by the nearest Cd(i) ions in MFI zeolite cavities

The dominant oxidation state of cadmium is +ii. Although extensive investigations into the +ii oxidation state have been carried out, the chemistry of CdI is still largely underdeveloped. Here, we report a new functionality of cadmium created by the zeolite lattice: room temperature O transfer from N2O to CO mediated by the nearest monovalent cadmium ions in MFI zeolite. Thermal activation of CdII ion-exchanged MFI zeolite in vacuo affords the diamagnetic [CdI-CdI]2+ species with a short CdI-CdI σ bond (2.67 Å). This species generates two CdI˙ sites under UV irradiation through homolytic cleavage of the CdI-CdI σ bond, and the thus-formed nearest CdI˙ sites abstract an O atom from N2O to generate the [CdII-Ob-CdII]2+ core, where Ob means bridged oxygen. This bridging atomic oxygen species is transferred to CO at room temperature, through which CO oxidation and regeneration of the CdI-CdI σ bond then proceed. This is the first example pertaining to the reversible redox reactivity of the nearest monovalent cadmium ions toward stable small molecules. In situ spectroscopic characterization captured all the intermediates in the reaction processes, and these data allowed us to calibrate the density-functional-theory cluster calculations, by means of which we were able to show that the charge compensation requirement at the nearest two Al sites arrayed circumferentially in the 10-membered ring of MFI zeolite creates such novel functionalities of cadmium. The unprecedented reactivity of CdI and its origin are discussed.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid-Catalyzed Reactions

The organization of Al atoms in the framework of Si-rich zeolites is very important and includes two classes: (i) the Al siting that determines which individual, crystallographically distinguishable framework T sites are occupied by Al atoms and (ii) the Al distribution, which describes the relation of two or more Al atoms in the framework, their distances, and the possibility of neighboring Al atoms to cooperate in the formation of active sites. The organization of Al significantly affects the catalytic properties of Si-rich, zeolite-based catalysts in acid and redox catalysis. Herein, what is known about the organization of Al in the framework of industrially very important pentasil-ring Si-rich zeolites (ZSM-5, beta zeolite, mordenite, ferrierite, MCM-22, and TNU-9), as well as the very promising SSZ-13 Si-rich zeolite with the CHA structure, is summarized.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Unprecedented direct cupric-superoxo conversion to a bis-μ-oxo dicopper(III) complex and resulting oxidative activity

Investigations of small molecule copper-dioxygen chemistry can and have provided fundamental insights into enzymatic processes (e.g., copper metalloenzyme dioxygen binding geometries and their associated spectroscopy and substrate reactivity). Strategically designing copper-binding ligands has allowed for insight into properties that favor specific (di)copper-dioxygen species. Herein, the tetradentate tripodal TMPA-based ligand (TMPA = tris((2-pyridyl)methyl)amine) possessing a methoxy moiety in the 6-pyridyl position on one arm (OCH3TMPA) was investigated. This system allows for a trigonal bipyramidal copper(II) geometry as shown by the UV-vis and EPR spectra of the cupric complex [(OCH3TMPA)CuII(OH2)](ClO4)2. Cyclic voltammetry experiments determined the reduction potential of this copper(II) species to be -0.35 V vs. Fc+/0 in acetonitrile, similar to other TMPA-derivatives bearing sterically bulky 6-pyridyl substituents. The copper-dioxygen reactivity is also analogous to these TMPA-derivatives, affording a bis-μ-oxo dicopper(III) complex, [{(OCH3TMPA)CuIII}2(O2-)2]2+, upon oxygenation of the copper(I) complex [(OCH3TMPA)CuI](B(C6F5)4) at cryogenic temperatures in 2-methyltetrahydrofuran. This highly reactive intermediate is capable of oxidizing phenolic substrates through a net hydrogen atom abstraction. However, after bubbling of the precursor copper(I) complex with dioxygen at very low temperatures (-135 °C), a cupric superoxide species, [(OCH3TMPA)CuII(O2 •-)]+, is initially formed before slowly converting to [{(OCH3TMPA)CuIII}2(O2-)2]2+. This appears to be the first instance of the direct conversion of a cupric superoxide to a bis-μ-oxo dicopper(III) species in copper(I)-dioxygen chemistry.

Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Oxygenase reactivity toward selective partial oxidation of CH₄ to CH₃OH requires an atomic oxygen-radical bound to metal (M-O^•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH₄. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O^• intermediate. However, no experimental data of M-O^• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O^•, its reactivity for CH₄, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH₄ over Zn^II-O^• in the MFI zeolite. One Zn^II-O^• species does perform H-abstraction from CH₄ at room temperature. The resultant CH₃^• species reacts with the other Zn^II-O^• site to form the Zn^II-OCH₃ species. The H₂O-assisted extraction of surface methoxide yields 29 μmol g^-1 of CH₃OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol^-1. On the basis of the findings in gas-phase experiments regarding the CH₄ activation by the free [M-O^•]⁺ species, the remarkable H-abstraction reactivity of the Zn^II-O^• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O^• bond and thereby enables to create effectively the highly reactive M-O^• bond required for low-temperature activation of CH₄. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

Temperature-dependent dynamics of NH3-derived Cu species in the Cu-CHA SCR catalyst

In situ XAS and UV-vis–NIR spectroscopy shed light on Cu-speciation during NH₃ temperature-programmed desorption and surface reaction (TPSR) over a commercial Cu-chabazite deNO_x catalyst, expanding the fundamental knowledge required to unravel the NH₃-SCR mechanism across the whole operation-relevant temperature range.

The influence of zeolite morphology on the conversion of methane to methanol on copper-exchanged omega zeolite (MAZ)

The synthesis conditions and morphology of the zeolite play an enormous role in the direct conversion of methane to methanol.

Direct conversion of methane to methanol with zeolites: towards understanding the role of extra-framework d-block metal and zeolite framework type

This Perspective article highlights the latest advances in the field of direct methane to methanol conversion by zeolites containing first row, extra-framework d-block metals (Mn, Fe, Co, Ni, Cu and Zn).

Methane selective oxidation to methanol by metal-exchanged zeolites: a review of active sites and their reactivity

A review of the recent progress in revealing the structures, formation, and reactivity of the active sites in Fe-, Co-, Ni- and Cu-exchanged zeolites as well as outlooks on future research challenges and opportunities is presented.

Catalytic use of layered materials for fine chemical syntheses

The present work reviews the catalytic use of layered solid materials for fine chemical syntheses with focus on layered double hydroxides, but including other classes of layered compounds of catalytic relevance.

Dual oxidase/oxygenase reactivity and resonance Raman spectra of {Cu3O2} moiety with perfluoro-t-butoxide ligands

A mechanism for the formation of O-donor trinuclear {Cu₃O₂} moiety is reported.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Photoexcited state chemistry of metal-oxygen complexes

Recent advances on the excited state chemistry of metal-oxygen synthetic complexes based on earth-abundant metals such as copper, cobalt, and manganese are reviewed to show a much enhanced reactivity of the photoexcited states as compared with their relative ground states. Mononuclear copper(ii)-superoxide and dinuclear copper(ii)-peroxo complexes underwent copper-oxygen bond cleavage, dioxygen release, and copper(i)/dioxygen rebinding upon photoexcitation at low temperature. Photoirradiation of the cobalt-oxygen compound [(TAML)CoIV(O)]2- (6) (TAML = tetraamidomacrocyclic ligand) at 5 °C yielded a cobalt-oxygen excited state with 0.6(1) ns lifetime, showing a high reactivity in the bimolecular electron-transfer oxidations of m-xylene and anisole. An extremely long-lived excited state was generated upon photoexcitation of a manganese(iv)-oxo complex binding two Sc(OTf)3 molecules, which enabled the hydroxylation of benzene.

Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

Theoretical Overview of Methane Hydroxylation by Copper-Oxygen Species in Enzymatic and Zeolitic Catalysts

As fossil-based energy sources become more depleted and with renewable-energy technologies still in a very early stage of development, the utilization of highly abundant methane as a transitional solution for current energy demands is highly important despite difficulties in transport and storage. Technologies enabling the conversion of methane to liquid/condensable energy carriers that can be easily transported and integrated into the existing chemical infrastructures are therefore essential. Although there commercially exists a two-step gas-to-liquid process involving syngas production, a novel route of methane conversion that can circumvent the high-cost production of syngas should be developed. Among all of the conceptually possible methods for converting methane to methanol, methane hydroxylation (CH₄ + ¹/₂O₂ → CH₃OH) at low temperature seems to be the most viable since it provides a direct route of conversion and allows a much lower operational cost. However, it is hampered by the fact that the complete oxidation to CO₂ is thermodynamically more favored. To overcome this, an effective catalyst that is able to "mildly" oxidize methane and stabilize the resultant methyl radical toward methanol formation is required. Particulate methane monooxygenase (pMMO) and copper-exchanged zeolites are two catalysts known to hydroxylate methane into methanol at low temperature with high selectivity. Having been studied for more than 30 years, these copper-cored catalysts are still relevant topics of discussion since the actual structure of the active sites has not been agreed upon, and thus, the reaction mechanism and factors influencing their reactivity and productivity are yet to be understood. Density functional theory (DFT) has provided us with a powerful computational tool for accomplishing these tasks. This Account presents an overview of the recent progress in the computational elucidation of the catalytic mechanism of methane hydroxylation by mono-, di-, and trinuclear copper sites in pMMO and Cu-exchanged zeolites as well as its correlations to the influencing factors that must be controlled to achieve higher reactivity. First, we briefly introduce the catalytic mechanism of a bare CuO⁺ cation as the simplest copper-oxo system in methane hydroxylation. The system is then extended to the copper-oxo species in pMMO and zeolites, and the radical and nonradical mechanisms are examined. Investigations of the reactivities of mononuclear and dinuclear copper-oxo species in the pMMO active site suggest that the bis(μ-oxo)Cu^IICu^III, (μ-oxo)(μ-hydroxo)Cu^IICu^III, and Cu^IIIO species are important for the catalytic activity of pMMO. In the case of Cu-exchanged zeolites, as the mono(μ-oxo)Cu^IICu^II and tris(μ-oxo)Cu^IICu^IIICu^III active sites have been fully characterized in experiments, here we discuss the effects of zeolite structures on the geometry and reactivity of the active sites.

Spectroscopic Identification of the α-Fe/α-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H Bond

The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N₂O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe₂O₃ being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.

Small-Pore Zeolites: Synthesis and Catalysis

In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si⁴⁺ and Al³⁺), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx removal) or to methanol en route to its conversion into light olefins, while restricting larger molecule entrance and departure that is critical to overall catalyst performance. In total, there are forty-four structurally different small-pore zeolites. Forty-one of these zeolites can be synthesized, and the first synthetic zeolite (KFI, 1948) was in fact a small-pore material. Although the field of 8MR zeolite chemistry has expanded in many directions, the progress in synthesis is framework-specific, leaving insights and generalizations difficult to realize. This review first focuses on the relevant synthesis details of all 8MR zeolites and provides some generalized findings and related insights. Next, catalytic applications where 8MR zeolites either have been commercialized or have dominated investigations are presented, with the aim of providing structure-activity relationships. The review ends with a summary that discusses (i) both synthetic and catalytic progress, (ii) a list of opportunities in the 8MR zeolite field, and (iii) a brief future outlook.