Mesoporous Copper/Manganese Oxide Catalyzed Coupling of Alkynes: Evidence for Synergistic Cooperative Catalysis

Matching Bidentate Ligand Anchoring: an Accurate Control Strategy for Stable Single-Atom/ZIF Nanocatalysts

Improving metal loading and controlling the coordination environment is nontrivial and challenging for single-atom catalysts (SACs), which have the greatest atomic efficiency and largest number of interface sites. In this study, a matching bidentate ligand (MBL) anchoring strategy is designed for the construction of CuN₄ SACs with tunable coordination environments (Cu loading range from 0.4 to15.4 wt.%). The obtained Cu SA/ZIF and Cu SA/ZIF* (0.4 wt.%) (ZIF and ZIF* = Zeolitic imidazolate framework with Matching bidentate N-ligands) nanocomposites exhibit superior performance in homo-coupling of phenyl acetylene under light irradiation (TON = 580, selectivity > 99%), which is 22 times higher than that of Cu SA/NC-800 (NC = N-doped porous carbon). Experiments and density functional theory calculations confirmed that the specific Cu five-membered ring formed using the MBL anchoring strategy is the key to the immobilization of isolated Cu atoms. This strategy provides a basis for the construction of M SA/MOF, which has the potential to narrow the gap between experimental and theoretical catalysis, as further confirmed by the successful preparation of Fe SA/ZIF and Ni SA/ZIF.

In Situ Synthesis of CuN4 /Mesoporous N-Doped Carbon for Selective Oxidative Crosscoupling of Terminal Alkynes under Mild Conditions

The 1,3-conjugated diynes are an important class of chemical intermediates, and the selective crosscoupling of terminal alkynes is an efficient chemical process for manufacturing asymmetrical 1,3-conjugated diynes. However, it often occurs in homogenous conditions and costs a lot for reaction treatment. Herein, a copper catalyzed strategy is used to synthesize highly ordered mesoporous nitrogen-doped carbon material (OMNC), and the copper species is in situ transformed into the copper single-atom site with four nitrogen coordination (CuN₄ ). These features make the CuN₄ /OMNC catalyst efficient for selective oxidative crosscoupling of terminal alkynes, and a wide range of asymmetrical and symmetrical 1,3-diynes (26 examples) under mild conditions (40 °C) and low substrates ratio (1.3). Density functional theory (DFT) calculations reveal that the aryl-alkyl crosscoupling has the lowest energy barrier on the CuN₄ site, which can explain the high selectivity. In addition, the catalyst can be separated and reused by simply centrifugation or filtration. This work can open a facile avenue for constructing single-atom loaded mesoporous materials to bridge homogeneous and heterogeneous catalysis.

C-H Bond Activation Mechanism by a Pd(II)-(μ-O)-Au(0) Structure Unique to Heterogeneous Catalysts

We focused on identifying a catalytic active site structure at the atomic level and elucidating the mechanism at the elementary reaction level of liquid-phase organic reactions with a heterogeneous catalyst. In this study, we experimentally and computationally investigated efficient C-H bond activation for the selective aerobic α,β-dehydrogenation of saturated ketones by using a Pd-Au bimetallic nanoparticle catalyst supported on CeO₂ (Pd/Au/CeO₂) as a case study. Detailed characterization of the catalyst with various observation methods revealed that bimetallic nanoparticles formed on the CeO₂ support with an average size of about 2.5 nm and comprised a Au nanoparticle core and PdO nanospecies dispersed on the core. The formation mechanism of the nanoparticles was clarified through using several CeO₂-supported controlled catalysts. Activity tests and detailed characterizations demonstrated that the dehydrogenation activity increased with the coordination numbers of Pd-O species in the presence of Au(0) species. Such experimental evidence suggests that a Pd(II)-(μ-O)-Au(0) structure is the true active site for this reaction. Based on density functional theory calculations using a suitable Pd₁O₂Au₁₂ cluster model with the Pd(II)-(μ-O)-Au(0) structure, we propose a C-H bond activation mechanism via concerted catalysis in which the Pd atom acts as a Lewis acid and the adjacent μ-oxo species acts as a Brønsted base simultaneously. The calculated results reproduced the experimental results for the selective formation of 2-cyclohexen-1-one from cyclohexanone without forming phenol, the regioselectivity of the reaction, the turnover-limiting step, and the activation energy.

Nanozyme sensor array based on manganese dioxide for the distinction between multiple amyloid β peptides and their dynamic aggregation process

The determination of the amyloid β (Aβ) peptide and its aggregation intermediates helps to understand the pathological mechanism of Alzheimer's disease (AD) caused by toxic amyloid fragments. Because of the transient and heterogeneous properties of Aβ aggregates, it is very difficult to dynamically detect Aβ and its aggregation intermediates. Herein, we successfully constructed a two-dimensional manganese dioxide (MnO₂) nanozyme sensor array by modulating the peroxidase-mimicking activity using various Aβ species and accurately distinguished among six types of Aβ within 1 h through linear discriminant analysis (LDA), with a dynamic detection range of 0.01-500 nmol/L and a detection limit of 0.44 pmol/L. Subsequently, 30 unknown blind samples were used to verify the practicability of the sensor array, and all unknown samples were identified with 100% accuracy. It is worth noting that the sensor array successfully distinguished healthy individuals from AD patients using clinical blood samples. This study provides a convenient and reliable nanozyme biosensing system for detecting Aβ species and their related aggregation processes.

Copper nanocatalysts applied in coupling reactions: a mechanistic insight

Copper-based nanocatalysts have seen great interest for use in synthetic applications since the early 20th century, as evidenced by the exponential number of contributions reported (since 2000, more than 48 000 works published out of about 81 300 since 1900; results from SciFinder using "copper nanocatalysts in organic synthesis" as keywords). These huge efforts are mainly based on two key aspects: (i) copper is an Earth-abundant metal with low toxicity, leading to inexpensive and eco-friendly catalytic materials; and (ii) copper can stabilize different oxidation states (0 to +3) for molecular and nanoparticle-based systems, which promotes different types of metal-reagent interactions. This chemical versatility allows different pathways, involving radical or ionic copper-based intermediates. Thus, copper-based nanoparticles have become convenient catalysts, in particular for couplings (both homo- and hetero-couplings), transformations that are involved in a remarkable number of processes affording organic compounds, which find interest in different fields (medicinal chemistry, natural products, drugs, materials, etc.). Clearly, this richness in reactivity makes understanding the mechanisms more complex. The present review focuses on the analysis of reported contributions using monometallic copper-based nanoparticles as catalytic precursors applied in coupling reactions, paying attention to those shedding light on the reaction mechanism.

Metal Nanoparticles as Sustainable Tools for C-N Bond Formation via C-H Activation

The design of highly active metal nanoparticles to be employed as efficient heterogeneous catalysts is a key tool for the construction of complex organic molecules and the minimization of their environmental costs. The formation of novel C-N bonds via C-H activation is an effective atom-economical strategy to access high value materials in pharmaceuticals, polymers, and natural product production. In this contribution, the literature of the last ten years on the use of metal nanoparticles in the processes involving direct C-N bond formation will be discussed. Where possible, a discussion on the role and influence of the support used for the immobilization and/or the metal chosen is reported. Particular attention was given to the description of the experiments performed to elucidate the active mechanism.

C-H Amidation and Amination of Arenes and Heteroarenes with Amide and Amine using Cu-MnO as a Reusable Catalyst under Mild Conditions

An atom-economical and efficient route for the direct amidation and amination of aryl C-H bonds using our synthesized recyclable heterogeneous Cu-MnO catalyst is reported here. The direct C-H amidation was carried out using a simple amide without any preactivated coupling partner, and simple air was used as the sole oxidant. The reaction proceeds very smoothly with a broad range of substrates containing numerous functional groups in very good to excellent yields. Direct C-H aminations with a secondary amine were carried out under base-, ligand-, and external oxidant-free conditions in very good to excellent yields in very mild conditions. Both the amidation and amination can be scaled up on a gram scale with similar yields. The major advantage is that our catalyst is recyclable and reused several times without any significant loss of reactivity.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln³⁺ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu³⁺ , Tb³⁺ ) and Cu²⁺ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

Partially disordered nano-porous metallic oxide engineering: surface morphology controllability and multiple scattering properties

Random multiple light scattering in disordered photonics leads to interesting and unexpected physical phenomena. Here, we demonstrate two types of partially disordered nano-porous metallic oxide materials: disordered grating nano-pores and two-dimensional disordered nano-tubes, which are produced just with one-step anodic oxidation. The relations among the processing parameters, morphology properties and multiple scattering characteristics are investigated. The surface morphology controllability can be achieved by simply changing the processing direct voltages, leading to different scattering properties. The probabilistic model of partially disordered nano-porous metallic oxide is constructed according to the nano-structure characteristics of oxide, and the rigorous coupled wave analysis is utilized for optical field simulation to exhibit the theoretical multiple scattering properties. Futhermore, the experimental scattering fields are measured and are analysed by statistical method. The research focuses on the disorder caused by one-step oxidation, which is distinct from previous studies that introducing disorder into periodic materials, and would open up new prospects for sensing, bionics and structural color.

Facile Fabrication of the Cu-N-C Catalyst with Atomically Dispersed Unsaturated Cu-N2 Active Sites for Highly Efficient and Selective Glaser-Hay Coupling

M-N-C catalysts have attracted considerable attention in the fields of energy storage and conversion as well as catalysis over the past decades. However, the current synthetic strategies for fabricating M-N-C catalysts via high-temperature pyrolysis unavoidably lead to a structural heterogeneity with the presence of a mixture of species including atomically dispersed M-N_x moieties and inorganic metal-containing particles, which not only decreases the atomic utilization but also clouds the accurate understanding of the nature of the catalytically active sites. Herein, we first report a straightforward and cost-effective preparation strategy for fabricating a Cu-N-C catalyst with atomically dispersed and coordinately unsaturated Cu-N₂ moieties on hierarchically N-doped porous carbon (Cu₁/NC-800) without formation of any metal-containing phases. Cu₁/NC-800 exhibits outstanding catalytic performance for Glaser-Hay coupling of terminal alkynes under mild and sustainable conditions, which surpass those of the state-of-the-art catalysts. A broad set of (un)symmetrical aryl-aryl, aryl-alkyl, and alkyl-alkyl 1,3-diynes were selectively synthesized in high yields with good tolerance of various functional groups. More importantly, the Cu₁/NC-800 could be easily reused with good maintenance of the activity and atomic dispersion of Cu in the structure. Experimental results and theoretical calculations reveal that the low N coordination number of single-atom Cu sites in Cu-N₂ exhibit a preferential adsorption to terminal alkyne; meanwhile, the adjacent pyridinic N sites on the carbon matrix facilitate the deprotonation of the adsorbed alkyne to generate the key intermediate Cu^δ-acetylide species, thus synergistically boosting the reaction. Therefore, this work not only provides an alternative facile synthetic strategy for fabricating atomically dispersed M-N-C catalysts but also represents a significant advance for accessing (un)symmetrical 1,3-diynes from Glaser-Hay coupling.

Tuning of cross-Glaser products mediated by substrate-catalyst polymeric backbone interactions

Tuning of cross-Glaser products using different polymeric backbones supported by copper oxide nano-catalysts has been demonstrated by tweaking the substrate-catalyst interactions under greener conditions. Further, highly reactive magnetically separable and recyclable catalyst with scalability is demonstrated.

Recent Advances in the Application of Nanometal Catalysts for Glaser Coupling

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Synthesis of symmetrical 1,3-diynes from terminal alkynes through an oxidative process is generally called Glaser coupling. The classic Glaser coupling is catalyzed by copper salts under an atmosphere of molecular oxygen as an oxidant. Over the past years, different metal catalysts and oxidants were successfully used in this atom economical C-C coupling reaction. Moreover, several procedures for the preparation of unsymmetrical 1,3-diynes by coupling two different alkyne substrates have been developed. In this review, we will highlight the usefulness of transition metal nanoparticles as efficient catalysts in homo- and hetero-coupling of alkynes by hoping that it will be beneficial to the development of novel and extremely efficient catalytic systems for this fast-growing and important reaction.

Optimization of Cu catalysts for nitrophenol reduction, click reaction and alkyne coupling

Earth-abundant nanocatalysts are actively searched to replace expensive noble metal catalysts for a number of essential processes.

Selective C–C coupling of terminal alkynes under an air atmosphere without base over Cu–NX–C catalysts

Highly dispersed copper nanoparticles supported on mesoporous nitrogenated carbon were synthesized and exhibited superior catalytic activity towards aerobic oxidative coupling of terminal alkynes.

Surface redox characters and synergetic catalytic properties of macroporous ceria-zirconia solid solutions

Macroporous CeO₂-ZrO₂ (CZ) solid solutions with gradually changing ceria content were prepared through the EISA method. Pore sizes of the samples are about 100 nm-1 μm and pore walls are 100 nm-1.5 μm. The surface and near surface reduction bands of Ce⁴⁺ below 600 °C were maximized for the Ce_0.5Zr_0.5O₂ sample (C5) according to the quantitative de-convolution to the acquired TPR curves. The area percentage of the O^2-2p⁶ → Ce³⁺3d⁹4f² electronic transition band on XPS spectra, which related to the concentration of the Ce³⁺, was found to be a function of the ceria content. The oxygen storage capacity showed a positive relationship with the chemical compositions. Redox reactions below 600 °C play a key role in determining the reduction performances of ceria based TWCs. Three-way catalytic performances of the Pd + Rh + Pt /C5 sample showed an ignition temperature for CO and NOx at about 240 °C, and finished before 300 °C. The ignition of C₃H₈ started at 270 °C while finished at differed samples. The maximum catalytic efficiencies of CO, NOx, and C₃H₈ on C5 sample were revealed to 100%, 98%, and 97%, respectively. The performances showed that porous CZ solid solutions are suitable for high performance catalytic applications.

Lithium promoted mesoporous manganese oxide catalyzed oxidation of allyl ethers

Herein we report the first example of the catalytic aerobic partial oxidation of allyl ether to its acrylate ester derivative. Many partial oxidations often need an expensive oxidant such as peroxides or other species to drive such reactions. In addition, selective generation of esters using porous catalysts has been elusive. This reaction is catalyzed by a Li ion promoted mesoporous manganese oxide (meso-Mn₂O₃) under mild conditions with no precious metals, a reusable heterogeneous catalyst, and easy isolation. This process is very attractive for the oxidation of allyl ethers. We report on the catalytic activity, selectivity, and scope of the reaction. In the best cases presented, almost complete conversion of allyl ether with near complete chemo-selectivity towards acrylate ester derivatives is observed. Based on results from controlled experiments, we propose a possible reaction mechanism for the case in which N-hydroxyphthalimide (NHPI) is used in combination with trichloroacetonitrile (CCl₃CN).

Acrylics and acrylates have become important building blocks for the chemical industry, but their efficient synthesis remains a challenge. Here, the authors report the first example of the catalytic aerobic partial oxidation of allyl ether to its acrylate ester derivative using a Li ion promoted mesoporous manganese oxide under mild conditions.

A walk around the application of nanocatalysts for cross-dehydrogenative coupling of C–H bonds

Cross-dehydrogenative coupling reactions between two unmodified C–H bonds are one of the most attractive and fundamental strategies for the construction of C–C bonds. As these reactions avoid pre-functionalization and de-functionalization of the substrates, they are cleaner, safer, and faster than traditional cross-coupling reactions. After the introduction of the modern area of cross-dehydrogenative coupling in 2003, many efforts have been devoted to the development of more efficient and selective catalytic systems for these appealing reactions. Among the different types of catalytic systems that have been investigated, nanostructured metal catalysts are highly attractive in view of their high catalytic performance, easy separability and good reusability. The purpose of this review is to focus on the application of nanocatalysts for cross-dehydrogenative coupling of C–H bonds with particular emphasis on the mechanistic aspects of the reactions. Specifically, we have structured this review based on the type of C–C bonds. Thus, the review is divided into six major sections: (i) C(sp³)–C(sp³) coupling; (ii) C(sp³)–C(sp²) coupling; (iii) C(sp³)–C(sp) coupling; (iv) C(sp²)–C(sp²) coupling; (v) C(sp²)–C(sp) coupling; and (vi) C(sp)–C(sp) coupling.

Cross-dehydrogenative coupling reactions between two unmodified C–H bonds are one of the most attractive and fundamental strategies for the construction of C–C bonds.

Enhanced peroxymonosulfate activation for phenol degradation over MnO2 at pH 3.5-9.0 via Cu(II) substitution

Cu(II) doped mesoporous MnO₂ (Cu-MnO₂) was prepared to establish an intimate functional link between the structure substitution and catalytic peroxymonosulfate (PMS) activation. Based on the characterization of powder X-ray diffraction (XRD), N₂ adsorption-desorption measurement, scanning electron microscope (FE-SEM) and transmission electron microscope (TEM), Cu-MnO₂ had a typical long range ordered mesoporous structure and Cu was successfully introduced in octahedral framework. It exhibited excellent catalytic activity and stability for the phenol degradation by PMS. Phenol was always efficiently degraded over Cu-MnO₂ at a pH range of 3.5-9.0. For example, the reaction rate constant at pH 7.0 was 0.073 min^-1, which was two times higher than that of MnO₂ (0.039 min^-1). Importantly, ¹O₂ was identified as the primary reactive species in Cu-MnO₂/PMS system. X-ray photoelectron spectroscopy (XPS) confirmed that more exposed surface oxygen defects due to Cu doping were responsible to the enhancement of PMS activation for phenol degradation. The results of PMS decomposition and oxygen evolution indicated that surface oxygen defects lower the reaction energy barrier of PMS decomposition by generating ¹O₂ via the energy trapping by oxygen. Finally, the heterogeneous PMS activation mechanism over Cu-MnO₂ was proposed.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

Whereas noble metal compounds have long been central in catalysis, Earth-abundant metal-based catalysts have in the same time remained undeveloped. Yet the efficacy of Earth-abundant metal catalysts was already shown at the very beginning of the 20th century with the Fe-catalyzed Haber-Bosch process of ammonia synthesis and later in the Fischer-Tropsch reaction. Nanoscience has revolutionized the world of catalysis since it was observed that very small Au nanoparticles (NPs) and other noble metal NPs are extraordinarily efficient. Therefore the development of Earth-abundant metals NPs is more recent, but it has appeared necessary due to their "greenness". This review highlights catalysis by NPs of Earth-abundant transition metals that include Mn, Fe, Co, Ni, Cu, early transition metals (Ti, V, Cr, Zr, Nb and W) and their nanocomposites with emphasis on basic principles and literature reported during the last 5 years. A very large spectrum of catalytic reactions has been successfully disclosed, and catalysis has been examined for each metal starting with zero-valent metal NPs followed by oxides and other nanocomposites. The last section highlights the catalytic activities of bi- and trimetallic NPs. Indeed this later family is very promising and simultaneously benefits from increased stability, efficiency and selectivity, compared to monometallic NPs, due to synergistic substrate activation.

OMS-2-Supported Cu Hydroxide-Catalyzed Benzoxazoles Synthesis from Catechols and Amines via Domino Oxidation Process at Room Temperature

In the presence of manganese oxide octahedral molecular sieve (OMS-2) supported copper hydroxide Cu(OH)_x/OMS-2, aerobic synthesis of benzoxazoles from catechols and amines via domino oxidation/cyclization at room temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the efficient oxidation of catechols over Cu(OH)_x/OMS-2 tolerates a variety of substrates, especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl, ester, and even acetyl groups) and heterocycles, which affords functionalized benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol % Cu). The characterization and plausible catalytic mechanism of Cu(OH)_x/OMS-2 are described. The notable features of our catalytic protocol such as the use of air as the benign oxidant and EtOH as the solvent, mild conditions, ease of product separation, being scalable up to the gram level, and superior reusability of catalyst (up to 10 cycles) make it more practical and environmentally friendly for organic synthesis.

Heterogeneous mesoporous manganese oxide catalyst for aerobic and additive-free oxidative aromatization of N-heterocycles

Herein, we report a heterogeneous, aerobic, additive-free and environmentally benign catalytic protocol for oxidative aromatization of saturated nitrogen-heterocycles using a mesoporous manganese oxide material.