Atomically dispersed palladium catalyses Suzuki-Miyaura reactions under phosphine-free conditions

Structure-activity relationships in the development of single atom catalysts for sustainable organic transformations

Single atom catalysts (SACs), which can provide the combined benefits of homogeneous and heterogeneous catalysts, are a revolutionary concept in the field of material research. The highly exposed catalytic surfaces, unsaturated sites, as well as unique structural and electronic properties of SACs have the potential to catalyze numerous reactions with unmatched efficiency and durability when stabilized on a suitable support. In this review, we have provided an intuitive insight into the strategies adopted in the last 5 years for morphology control of SACs to know about its impact on metal-support interaction and various organic transformations with special reference to metal oxides, alloys, metal-organic frameworks (MOFs) and carbon-based supports. This review also includes a brief description of unparalleled potentials of SACs and the recent advances in the catalysis of industrially important organic transformations, with special emphasis on the C-C cross-coupling reaction, biomass conversion, hydrogenation, oxidation and click chemistry. This unprecedented and unique perspective will highlight the interactions occurring within SACs that are responsible for their high catalytic efficiency, which will potentially benefit various organic transformations. We have also suggested plausible synergy of various other concepts such as defect engineering and piezocatalysis with SACs, which can provide a new direction to sustainable chemistry. A good understanding of the different types of metal-support interactions will help researchers develop morphology-controlled SACs with tunable properties and establish mechanisms for their exceptional catalytic behaviour in industrially important organic transformations.

Tailoring catalysis at the atomic level: trends and breakthroughs in single atom catalysts for organic transformation reactions

The utilization of precise materials in heterogeneous catalysis will provide various new possibilities for developing superior catalysts to tackle worldwide energy and environmental issues. In recent years, single atom catalysts (SACs) with excellent atom utilization and isolated active sites have progressed dramatically as a thriving sector of catalysis research. Additionally, SACs bridge the gap between homogeneous and heterogeneous catalysts and overcome the limitations of both categories. Current research on SACs is highly oriented towards the organic synthesis of high-significance molecules with promising potential for large-scale applicability and industrialization. In this context, this review aims to comprehensively analyze the state-of-the-art research in the synthesis of SACs and analyze their structural, electronic, and geometric properties. Moreover, the unprecedented catalytic performance of the SACs towards various organic transformation reactions is succinctly summarized with recent reports. Further, a detailed summary of the current state of the research field of SACs in organic transformation is discussed. Finally, a critical analysis of the existing challenges in this emerging field of SACs and the possible countermeasures are provided. We believe that SACs have the potential to profoundly alter the chemical industry, pushing the boundaries of catalysis in new and undiscovered territory.

A Ligand-Free Approach towards Coumarin Analogs via Natural Deep Eutectic Solvent-Mediated Suzuki-Miyaura Coupling

A ligand-free approach for the Suzuki-Miyaura cross coupling reaction using Natural Deep Eutectic Solvents (NaDES) towards coumarin analogs is described. A model reaction between the synthetically prepared 3-(4-acetyloxy-phenyl)-6-bromo-4-methyl-coumarin (3b) and phenylboronic acid was performed in five different NaDES as well as in pure glycerol, using two inorganic bases and palladium catalysts. The reaction proceeded smoothly in Choline Chloride/Glycerol (ChCl/Gly) and Betaine/Glycerol (Bet/Gly) NaDES at 90 °C in 24 h, affording the desired product in high yields up to 95%. The combination of K2CO3, Pd(OAc)2 and ChCl/Gly NaDES provided optimum yields and high purity of the desired compounds, while the solvent was successfully recycled and reused up to two times. The developed methodology is applicable to boronic acids bearing various substituents. The formation of palladium nanoparticles in the reaction mixture was observed, and the size of the nanoparticles was associated with the reaction yield. In addition, in all the glycerol-based NaDES, an effective removal of the acetyl group of the acetyloxy-coumarin analogs was observed; thus, it is noteworthy that the Suzuki-Miyaura coupling and the deacetylation reaction were achieved in one pot. The ten novel coumarin derivatives synthesized were structurally characterized using 1D and 2D NMR spectroscopy and were tested for their cytotoxicity against the A431 squamous cancer cell line, presenting significant activity.

Understanding and Controlling Reactivity Patterns of Pd1@C3N4-Catalyzed Suzuki-Miyaura Couplings

Using heterogeneous single-atom catalysts (SACs) in the Suzuki-Miyaura coupling (SMC) has promising economic and environmental benefits over traditionally applied palladium complexes. However, limited mechanistic understanding hinders progress in their design and implementation. Our study provides critical insights into the working principles of Pd1@C3N4, a promising SAC for the SMC. We demonstrate that the base, ligand, and solvent play pivotal roles in facilitating interface formation with reaction media, activating Pd centers, and modulating competing reaction pathways. Controlling the previously overlooked interplay between base strength, reagent solubility, and surface wetting is essential for mitigating mass transfer limitations in the triphasic reaction system and promoting catalyst reusability. Optimum conditions for Pd1@C3N4 require polar solvents, intermediate base strength, and increased water content. Our investigations reveal that high selectivity requires minimizing competitive coordination of bases and phosphine ligands to the Pd centers, to avoid homocoupling and alternative reductive elimination mechanisms giving rise to phosphonium side-products. Furthermore, in situ XAS investigations probing electronic structures and coordination environments of Pd sites further rationalize the base and ligand coordination, confirming and expanding upon previous computational hypotheses for Pd1@C3N4. This understanding allows for designing a more selective ligand-free reaction pathway using the solvent and base to modulate the chemical environment of the active sites. We highlight the importance of environment-compatible surface tension, the creation of coordinatively available active sites, and the stabilization of partially reduced Pd centers, emphasizing the importance of mechanistic studies to advance the design of SACs in organic liquid phase reactions.

Atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized β-O-4 segments

This work presents an innovative approach focusing on fine-tuning the coordination environment of atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized lignin model compounds. By meticulously regulating the Co-N coordination environment, the activity of these catalysts in the hydrogenolysis and reductive amination reactions was effectively controlled. Notably, our study demonstrates that, in contrast to cobalt nanoparticle catalysts, atomically dispersed cobalt catalysts exhibit precise control of the sequence of hydrogenolysis and reductive amination reactions. Particularly, the CoN3 catalyst with a triple Co-N coordination number achieved a remarkable 94% yield in the synthesis of primary benzylamine. To our knowledge, there is no previous documentation of the synthesis of primary benzylamines from lignin dimer model compounds. Our study highlights a promising one-pot route for sustainable production of nitrogen-containing aromatic chemicals from lignin.

Droplet-Based Microfluidics Reveals Insights into Cross-Coupling Mechanisms over Single-Atom Heterogeneous Catalysts

Single-atom heterogeneous catalysts (SACs) hold promise as sustainable alternatives to metal complexes in organic transformations. However, their working structure and dynamics remain poorly understood, hindering advances in their design. Exploiting the unique features of droplet-based microfluidics, we present the first in-situ assessment of a palladium SAC based on exfoliated carbon nitride in Suzuki-Miyaura cross-coupling using X-ray absorption spectroscopy. Our results confirm a surface-catalyzed mechanism, revealing the distinct electronic structure of active Pd centers compared to homogeneous systems, and providing insights into the stabilizing role of ligands and bases. This study establishes a valuable framework for advancing mechanistic understanding of organic syntheses catalyzed by SACs.

Surface-Modified Pd/CeO2 Single-Atom Catalyst Shows Increased Activity for Suzuki Cross-Coupling

Single-atom catalysts (SACs) comprise catalytically active atoms dispersed on supports; they combine the high activity and site uniformity of homogeneous catalysts with the ease of separability of heterogeneous catalysts. However, SACs lack fine control over the active site, provided by ligands in homogeneous catalysts. In this work, we demonstrate that modification of the support with an organic monolayer is a viable approach to improving the catalytic performance. The addition of catechol-type monolayers to a Pd/CeO2 SAC increases its catalytic activity for Suzuki cross-coupling, a central reaction in the synthesis of fine chemicals and pharmaceuticals. Kinetic trials reveal that the coating reduces the activation energy from 49 ± 9 to 22 ± 5 kJ/mol and produces a 4-fold rate enhancement at 25 °C, an effect we attribute to π-π interactions between the reactant and the catechol coating. Further development of this approach could vastly increase the utility of SACs in organic synthesis.

Controllable deposition of dispersed Pd nanoparticles on ZnO for Suzuki-Miyaura cross-coupling reactions

Palladium nanoparticles find extensive applications in catalysis in both homogeneously and heterogeneously catalyzed processes. Supporting metal nanoparticles enhances their stability as compared to their unsupported counterparts. The role of catalytic support is increasingly recognized as crucial in determining the behaviour of these materials. However, controlling the deposition and anchoring of palladium nanoparticles remains a significant challenge. This contribution discusses the preparation of straight lines of palladium particles on zinc oxide by wet impregnation. This phenomenon is attributed to the highly stepped morphology of the employed ZnO that created steric anchoring sites to stabilize the metal particles. Palladium-based catalysts were evaluated for the valuable Suzuki-Miyaura cross-coupling reaction. The dispersed Pd/ZnO catalyst achieved a conversion rate of 86% with 100% selectivity, remarkably superior to that of the Pd/Al2O3 and Pd/TiO2 counterparts.

Single-Atom Catalysis in Organic Synthesis

Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Highly Efficient Fabrication of Kilogram-Scale Palladium Single-Atom Catalysts for the Suzuki-Miyaura Cross-Coupling Reaction

Palladium single-atom catalysts (SACs) have caught great attention owing to their maximal atom utilization and outstanding activity for the Suzuki-Miyaura cross-coupling reaction. However, a facile manufacturing method for kilogram-scale synthesis of noble metal SACs with high productivity is still in demand. This study reports on the synthesis of SACs by direct ball milling of commercial metal oxides and nitrate precursors with a productivity of ∼100%. The as-prepared Pd₁/FeO_x SACs show high catalytic performance (the TOFs are 7844 h^-1), high stability, and general applicability for the Suzuki-Miyaura cross-coupling reaction under mild conditions. More encouragingly, kilogram-scale Pd₁/FeO_x SACs can be synthesized in one batch by this approach, endowing great potential for industrial applications. Furthermore, the preparation of Pt, Ru, and Rh SACs is also successfully carried out via the ball milling method, demonstrating favorable applicability. Our findings illustrate exciting chances presented by the highly efficient synthesis of SACs for the formation of C-C bonds.

Assessing the environmental benefit of palladium-based single-atom heterogeneous catalysts for Sonogashira coupling

The Pd-Cu catalysed Sonogashira coupling of terminal alkynes and aryl halides is a cornerstone synthetic strategy for C-C bond formation. Homogeneous organometallic systems conventionally applied are typically not reusable and require efficient downstream Pd removal steps for product purification, making it challenging to fully recover the precious metal. A holistic cradle-to-gate life cycle assessment (LCA) unveils that process footprint can be improved up to two orders of magnitude from repeated catalyst reuse. New classes of heterogeneous catalysts based on isolated metal atoms (single-atom catalysts, SACs) demonstrate promising potential to synergise the benefits of solid and molecular catalysts for efficient Pd utilisation. Here we show that using Pd atoms anchored on nitrogen-doped carbon permits full recovery of the metal and reuse of the catalyst over multiple cycles. A hybrid process using the Pd-SAC with a homogeneous CuI cocatalyst is more effective than a fully heterogeneous analogue based on a bimetallic Pd-Cu SAC, which deactivates severely due to copper leaching. In some scenarios, the LCA-based metrics demonstrate the footprint of the hybrid homogeneous-heterogeneous catalytic process is leaner than the purely homogeneous counterpart already upon single reuse. Combining LCA with experimental evaluation will be a useful guide to the implementation of solid, reusable catalysts for sustainable organic transformations.

The use of metal-organic frameworks as heterogeneous catalysts

This article presents an overview of some of the available research studies of MOFs as catalysts. Catalytic studies of magnetic iron oxide nanoparticles with modified surfaces, MOFs with precious metals such as palladium, platinum, and silver, with zirconium, hafnium, copper, alkaline earth metals, lanthanides are generalized. The studies of the catalytic activity of micro- and mesoporous MOF structures are described.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Ni single atoms on carbon nitride for visible-light-promoted full heterogeneous dual catalysis

Visible-light-driven organic transformations are of great interest in synthesizing valuable fine chemicals under mild conditions. The merger of heterogeneous photocatalysts and transition metal catalysts has recently drawn much attention due to its versatility for organic transformations. However, these semi-heterogenous systems suffered several drawbacks, such as transition metal agglomeration on the heterogeneous surface, hindering further applications. Here, we introduce heterogeneous single Ni atoms supported on carbon nitride (NiSAC/CN) for visible-light-driven C-N functionalization with a broad substrate scope. Compared to a semi-heterogeneous system, high activity and stability were observed due to metal-support interactions. Furthermore, through systematic experimental mechanistic studies, we demonstrate that the stabilized single Ni atoms on CN effectively change their redox states, leading to a complete photoredox cycle for C-N coupling.

Single-Atom Catalysis: From Simple Reactions to the Synthesis of Complex Molecules

To date, the scope of single-atom catalysts (SAC) in liquid-phase transformations is rather limited owing to stability issues and the inability to activate complex substances. This calls for a better design of the catalyst support that can provide a dynamic coordination environment needed for catalytic action, and yet retain robustness against leaching or aggregation. In addition, the chemical orthogonality of SAC is useful for designing tandem or multicomponent reactions, in which side reactions common to metal nanoparticles are suppressed. In this review, the intrinsic mechanism will be highlighted that controls reaction efficiency and selectivity in SAC-catalyzed pathways, as well as the structural dynamism of SAC under complex liquid-phase conditions. These mechanistic insights are helpful for the development of next-generation SAC systems for the synthesis of high-value pharmaceuticals through late-stage functionalization, sequential and multicomponent strategies.

Advances in single-atom catalysts: Design, synthesis and environmental applications

Over the past few years, single-atom catalysts (SACs) on the horizon have driven rapid and extensive scientific advances in heterogeneous catalysis. Nevertheless, large-scale applications of SACs in the environment have been hindered by the problematic synthesis of catalysts, because the atomic-scale materials with high activation energy are easy to form nanoclusters and nanoparticles in the synthesis stage. The catalytic stability and catalytic activity of SACs in the treatment of complex environmental pollutants also need to be further researched. Herein, the review is built on a comprehensive discussion of the design and synthesis strategies of SACs. The shortcomings of traditional methods and the improvement from different angles like defect regulation are analyzed. Furthermore, the reaction mechanism of SACs in different reactions was summarized, and the environmental applications of SACs, such as wastewater treatment, carbon dioxide reduction, nitrogen reduction, hydrogen evolution, NO_x reduction and oxidation, volatile organic compounds removing and environmental monitoring are exemplified to deeply evaluate the prospects and challenges of SACs in the field of environmental protection.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Selective hydroxylation of aryl iodides to produce phenols under mild conditions using a supported copper catalyst

Owing to the high activity and low-cost, copper-based catalysts are promising candidates for transforming aromatic halides to yield phenols. In this work, we report the selective hydroxylation of aromatic iodides to produce phenols using an atomically dispersed copper catalyst (Cu-ZnO-ZrO2) under mild reaction conditions. The reactions were conducted without the use of additional organic ligands, and the protection of an inert atmosphere environment is not required. The catalyst can be easily prepared, scalable, and is very efficient for a wide range of substrates. The catalytic reactions can be carried out with only 1.24 mol% Cu loading, which shows great potential in mass production.

Pt1–O4 as active sites boosting CO oxidation via a non-classical Mars–van Krevelen mechanism

A new strategy to increase the total catalytic activity of SACs with low-loading noble metal for practical applications has been developed via fabricating super active SACs.

Si-thiol supported atomic-scale palladium as efficient and recyclable catalyst for Suzuki coupling reaction

Atomic-scale catalysts leverage the advantages of both heterogeneous catalysts for their stability and reusability and homogeneous catalysts for their isolated active sites. Here, a palladium catalyst supported by Si-thiol, a commercially available mercaptopropyl-modified and TMS-passivated amorphous silica, was synthesized and characterized by SEM,TEM, aberration-corrected STEM-HAADF, XRD, FT-IR and XPS. Statistical analysis revealed that the catalytic Pd species predominantly consisted of intermediate sized nanoparticles (<2 nm), small amounts of essentially isolated atoms (ca. 0.1 nm), and limited amounts of somewhat larger nanoparticles (<5 nm). The nanoscale atomic clusters dominated the reactivity and served as the key active sites for Suzuki coupling. The outcomes of the reaction were greatly affected by the choice of solvents, and Pd/Si-thiol was demonstrated to be reusable for more than three times without a noticeable loss of catalytic activity. [Formula: see text].