Carboxylation of Terminal Alkynes with Carbon Dioxide Catalyzed by Poly(N-Heterocyclic Carbene)-Supported Silver Nanoparticles

Palladium-catalyzed regioselective decarboxylative hydroarylation of alkynyl carboxylic acids with arylboronic acids

The synthesis of (deuterated) 1,1-disubstituted alkenes via Pd-catalyzed decarboxylative hydroarylation of alkynyl carboxylic acids with arylboronic acids has been developed. The reaction features excellent regioselectivity, a broad substrate scope and gram-scale synthetic ability and offers a general synthetic method to synthesize 1,1-dideuterio olefins. Preliminary mechanism investigations indicate that 1,1-disubstituted alkenes are formed by hydroarylation of terminal alkynes generated by in situ decarboxylation of alkynyl carboxylic acids.

Monomeric and Polymeric Mesoionic N-Heterocyclic Carbene-Tethered Silver Nanoparticles: Synthesis, Stability, and Catalytic Activity

In recent years, N-heterocyclic carbenes (NHCs) have garnered significant attention as promising alternatives to thiols to stabilize metallic nanoparticles and planar surfaces. While most studies thus far have focused on NHC-functionalized gold nanoparticles (AuNPs), as an ideal platform to investigate the role of NHCs in stabilizing such nanoparticles, their ability to protect more unstable coinage metal nanoparticles, such as silver nanoparticles (AgNPs), has been largely overlooked. This is despite the fact that AgNPs possess a much more sensitive optical response that, upon their enhanced stability, can broaden their scope of application in various fields, including nanomedicine and catalysis. In this study, the synthesis and use of monomeric and polymeric mesoionic NHC-Ag(I) complexes as precursors to mono- and multidentate NHC-tethered AgNPs are reported. The polymeric analog was obtained by first synthesizing a polymer, containing 1,2,3-triazole repeat units, employing the copper-catalyzed alkyne-azide cycloaddition click polymerization of monomers containing diazide- and dialkyne functional groups. Subsequent quaternization of the triazole moieties and Ag insertion yielded the target NHC-Ag-containing polymer. Using this polymer as well as its monomeric analog as substrates, AgNPs with either catenated networks of NHCs or monomeric NHCs were fabricated by their reduction using borane-tert-butylamine complex. Our stability studies demonstrate that while monomeric NHCs impart some degree of stability to AgNPs, particularly at elevated temperatures in aqueous as well as organic medium, their polymeric analogs further enhance their stability in acidic environment (pH = 2) and against glutathione (3 mM), as an example of a biologically relevant thiol, in aqueous media. To highlight the application of these NHC-functionalized AgNPs in catalysis, we explore the aqueous phase reduction of methyl orange and 4-nitrophenol.

Nitrogen-doped mesoporous carbon single crystal-based Ag nanoparticles for boosting mild CO2 conversion with terminal alkynes

Fabrication of efficient heterogeneous catalysts with high turnover frequency (TOF) is intriguing for rapid and scalable CO₂ conversion under mild conditions, but it still faces some challenges due to use of some bulky and irregular supports causing inaccessible inner pores and insufficient utilization of active sites. Herein, using a unique nitrogen-doped mesoporous single-crystal carbon (named IRFC) as a host for loading Ag nanoparticles for the first time, a series of Ag/IRFC catalysts with high TOF (8.7-22.3 h^-1) were facilely prepared by a novel "impregnation and in-situ reduction" strategy. The neat morphology and high porosity of IRFC with abundant N species, providing homogeneous surface, adequate space and anchoring sites for Ag immobilization, greatly facilitated the formation of highly-distributed ultrasmall Ag nanoparticles (2.3 nm). Meanwhile, smooth and short diffusion pathways were inherited from the ordered mesopores and small particle sizes of IRFC. Owing to these unparalleled structural features, the Ag/IRFC catalysts exhibited excellent catalytic activity, stability, and generality for mild CO₂ conversion even under diluted conditions. This work not only presents a novel catalyst for mild CO₂ conversion, but also brings some inspirations to designing highly efficient catalysts using well-shaped supporting nanomaterials for direct utilization of low-concentration CO₂, such as flue gas.

Carboxylation of Alkenes and Alkynes Using CO2 as a Reagent: An Overview

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CO2 fixation reactions are of paramount interest both from economical and environmental perspectives. As an abundant, non-toxic, and renewable C1 feedstock, CO2 can be utilized for the synthesis of fuels and commodity chemicals under elevated reaction conditions. The major challenge in the CO2 utilization reactions is its chemical inertness due to high thermodynamic stability and kinetic barrier. The carboxylation of unsaturated hydrocarbons with CO2 is an important transformation as it forms high-value reaction products having industrial as well as medicinal importance. This mini-review is mainly focused on the recent developments in the homogeneously and heterogeneously catalyzed carboxylation of alkenes and alkynes by using carbon dioxide as a reagent. We have highlighted various types of carboxylation reactions of alkenes and alkynes involving different catalytic systems, which comprise mainly C-H bond activation, hydrocarboxylation, carbocarboxylation, heterocarboxylation, and ring-closing carboxylation, including visible-light assisted synthesis processes. The mechanistic pathways of these carboxylation reactions have been described. Moreover, challenges and future perspectives of these carboxylation reactions are discussed.

Silver nanomaterials: synthesis and (electro/photo) catalytic applications

In view of their unique characteristics and properties, silver nanomaterials (Ag NMs) have been used not only in the field of nanomedicine but also for diverse advanced catalytic technologies. In this comprehensive review, light is shed on general synthetic approaches encompassing chemical reduction, sonochemical, microwave, and thermal treatment among the preparative methods for the syntheses of Ag-based NMs and their catalytic applications. Additionally, some of the latest innovative approaches such as continuous flow integrated with MW and other benign approaches have been emphasized that ultimately pave the way for sustainability. Moreover, the potential applications of emerging Ag NMs, including sub nanomaterials and single atoms, in the field of liquid-phase catalysis, photocatalysis, and electrocatalysis as well as a positive role of Ag NMs in catalytic reactions are meticulously summarized. The scientific interest in the synthesis and applications of Ag NMs lies in the integrated benefits of their catalytic activity, selectivity, stability, and recovery. Therefore, the rise and journey of Ag NM-based catalysts will inspire a new generation of chemists to tailor and design robust catalysts that can effectively tackle major environmental challenges and help to replace noble metals in advanced catalytic applications. This overview concludes by providing future perspectives on the research into Ag NMs in the arena of electrocatalysis and photocatalysis.

Silver-catalysed C–H bond activation: a recent review

Transition metal catalysed C–H activations are efficient, simple, mild, cost-effective and stereoselective, and many of them are environmentally sustainable transformations.

Covalent Triazine Frameworks Obtained from Nitrile Monomers for Sustainable CO2 Catalysis

Carbon dioxide catalytic conversion (i. e., CO₂ catalysis) is considered as one of the most promising technologies to control CO₂ emissions, which is of great significance to build a sustainable society with green low-carbon cycle. In view of its thermodynamic stability and kinetic inertness, CO₂ selective activation is still desired. Nowadays, the traditional strategy is to selectively capture and efficiently convert atmospheric CO₂ into high value-added chemicals and fuels. Covalent triazine frameworks (CTFs) as a newly emerging and attractive kind of porous organic polymer (POP) have drawn worldwide attention among heterogeneous catalysis because of their nitrogen-rich porous structures and exceptional physicochemical stabilities. In this Minireview, the focus was mainly placed on the structural design and synthesis of CTFs and their applications in CO₂ catalysis including CO₂ cycloaddition, CO₂ carboxylation, CO₂ hydrogenation, CO₂ photoreduction, and CO₂ electroreduction. By discussing the structure-property relationship, valuable guidance from a sustainable perspective may be provided for developing precisely designed CTFs with high performance and excellent industrial application prospects in sustainable CO₂ catalysis.

Development of Magnesium Oxide-Silver Hybrid Nanocatalysts for Synergistic Carbon Dioxide Activation to Afford Esters and Heterocycles at Ambient Pressure

Multi-metallic hybrid nancatalysts consisting of a porous metal oxide host and metal satellite guests serve as a scaffold for multi-step transformations of divergent and energy-challenging substrates. Here we have developed a 3D porous MgO framework (Lewis basic host) with Ag⁰ nanoparticles (noble metal guest) for ambient pressure activation and insertion of CO₂ into unsaturated alkyne substrates. The hybrid MgO@Ag-x (x = 2, 5, 7, 8 at % Ag) catalysts are synthesized by impregnating Ag⁺ ions in porous MgO cubes followed by reduction using NaBH₄. Morphological (SEM, TEM, EDX mapping) and structural (PXRD, XPS) characterization reveal that the micron-sized hybrid cubes derive from self-assembly of ~100 nm (edge length) MgO cubes decorated with ~ 5 to 25 nm Ag⁰ NPs. Detailed XPS analysis illustrates Ag⁰ is present in two forms, <10 nm NPs and ~25 nm aggregates. The MgO@Ag-7 catalyst is effective for inserting CO₂ into aryl alkynes followed by S_N2 coupling with allylic chlorides to afford a wide range of ester and lactone heterocycles in excellent yields (61-93%) and with low E-factor (2.8). The proposed mechanism suggests a CO₂ capture and substrate assembly role for 3D porous MgO while Ag⁰ performs the key activation of alkyne and CO₂ insertion steps. The catalyst is recyclable (5x) with no significant loss of product yield. Overall, these results demonstrate viable approaches to hybrid catalyst development for challenging conversions such as CO₂ utilization in a green and sustainable manner.

One-Pot Synthesis of α,β-Unsaturated γ-Lactones and Lactams via a Sequential trans-Hydroalumination and Catalytic Carboxylation of Propargyl Alcohols and Amines with Carbon Dioxide

A one-pot, sequential process that combines a trans-selective hydroalumination of propargyl alcohols and amines with a copper- or silver-catalyzed carboxylation reaction using carbon dioxide, followed by an acid-mediated intramolecular condensation step, led to the formation of a wide range of α,β-unsaturated γ-butyrolactones and lactams.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Size-Controlled Growth of Silver Nanoparticles onto Functionalized Ordered Mesoporous Polymers for Efficient CO2 Upgrading

Highly dispersed metallic silver nanoparticles (AgNPs) are promising heterogeneous catalysts for carboxylative coupling of terminal alkynes with CO₂ under mild conditions. Yet, their size-controlled synthesis is very challenging because of the high surface energy. Here, we prepared a series of amino-functionalized ordered mesoporous polymers as hosts for anchoring AgNPs. Control experiments and computations showed that electron-rich amines were confined in mesochannels with varying electron density and steric hindrance, creating "localized active zones (LAZ)" to control the growth of AgNPs. The particle size of AgNPs grows along with the increased volume of LAZ around nitrogen species. We also revealed that the catalytic activity of Ag-based catalysts is size-dependent and increases with decreasing particle size. Building on these findings, we report a facile one-pot synthesis strategy for preparing an amine-incorporated ordered mesoporous polymer (NOMP) with a high specific surface area, small LAZ volume, and uniform amine sites with controllable loading. These features result in the formation of ultrasmall and monodispersed Ag nanoparticles. Remarkably, Ag@NOMP gave a quantitative target yield under the conditions of 1 atm CO₂ pressure and 50 °C, showing superior catalytic activity in CO₂ carboxylation compared to other mesoporous analogues.

Tuning the catalytic activity and selectivity of water-soluble bimetallic RuPt nanoparticles by modifying their surface metal distribution

Bimetallic ruthenium-platinum nanoparticles (RuPt NPs) of different surface distributions and stabilized by using a sulfonated N-heterocyclic carbene ligand (1-(2,6-diisopropylphenyl)-3-(3-potassium sulfonatopropyl)-imidazol-2-ylidene) were prepared from Ru(COD)(COT) (COD = cyclooctadiene and COT = cyclooctatriene), and platinum precursors having various decomposition rates (Pt(NBE)₃, NBE = norbornene, Pt(CH₃)₂(COD) and Pt₂(DBA)₃, DBA = dibenzylideneacetone). Structural and surface studies by FT-IR and solid-state MAS NMR, using carbon monoxide as a probe molecule, revealed the presence of different structures and surface compositions for different nanoparticles of similar sizes, which principally depend on the decomposition rate of the organometallic precursors used during the synthesis. Specifically, the slower the decomposition rate of the platinum precursor, the higher the number of Pt atoms at the NP surface. The different bimetallic RuPt NPs, as well as their monometallic equivalents (Pt and Ru NPs), were used in isotopic H/D exchange through C-H activation on l-lysine. Interestingly, the activity and selectivity of the direct C-H deuteration were dependent on the NP surface composition at the α position but not on that at the ε position. Chemical shift perturbation (CSP) experiments revealed that the difference in reactivity at the α position is due to a Pt-carboxylate interaction, which hinders the H/D exchange.

C-H Bond Carboxylation with Carbon Dioxide

Carbon dioxide is a nontoxic, renewable, and abundant C₁ source, whereas C-H bond functionalization represents one of the most important approaches to the construction of carbon-carbon bonds and carbon-heteroatom bonds in an atom- and step-economical manner. Combining the chemical transformation of CO₂ with C-H bond functionalization is of great importance in the synthesis of carboxylic acids and their derivatives. The contents of this Review are organized according to the type of C-H bond involved in carboxylation. The primary types of C-H bonds are as follows: C(sp)-H bonds of terminal alkynes, C(sp² )-H bonds of (hetero)arenes, vinylic C(sp² )-H bonds, the ipso-C(sp² )-H bonds of the diazo group, aldehyde C(sp² )-H bonds, α-C(sp³ )-H bonds of the carbonyl group, γ-C(sp³ )-H bonds of the carbonyl group, C(sp³ )-H bonds adjacent to nitrogen atoms, C(sp³ )-H bonds of o-alkyl phenyl ketones, allylic C(sp³ )-H bonds, C(sp³ )-H bonds of methane, and C(sp³ )-H bonds of halogenated aliphatic hydrocarbons. In addition, multicomponent reactions, tandem reactions, and key theoretical studies related to the carboxylation of C-H bonds are briefly summarized. Transition-metal-free, organocatalytic, electrochemical, and light-driven methods are highlighted.

Selective assembly of N1- and N2-alkylated 1,2,3-triazoles via copper-catalyzed decarboxylative cycloaddition of alkynyl carboxylic acids with ethers and azidotrimethylsilane

An efficient and regiocontrolled method for the synthesis of various N1- and N2 alkylated 1,2,3-triazoles via copper-catalyzed decarboxylative cycloaddition of alkynyl carboxylic acids with ethers and azidotrimethylsilane.

Ultrafine Ag Nanoparticles Encapsulated by Covalent Triazine Framework Nanosheets for CO2 Conversion

This paper describes the fabrication of covalent triazine framework nanosheet-encapsulated Ag nanoparticles (Ag⁰@CTFN) via a simple combination of the ultrasonic exfoliation and solution infiltration method. The as-prepared Ag⁰@CTFN displays an order layered-sheet structure with abundant micropores and mesopores, whereas ultrafine Ag nanoparticles are confined and stabilized in their interlayers through the interaction between N sites of triazine units and Ag nanoparticles. Considering that the Ag⁰@CTFN possesses the merits of high nitrogen, low density, and abundant basic sites, it was thus believed to have enough abilities to adsorb and activate CO₂ in the CO₂ conversion and catalysis. Importantly, the Ag⁰@CTFN, as a heterogeneous catalyst, showed highly catalytic activity in the carboxylation of various alkynes with CO₂ at ambient pressure and low temperature. This catalyst also exhibited good functional group tolerance and excellent stability without any significant loss of its activity after six recycles. This work not only achieves valuable and novel composite material but also provides the first application of covalent triazine framework nanosheets in chemical conversion of CO₂, opening a new field in preparing recyclable heterogeneous catalysts to accelerate the utilization of CO₂.

Novel Covalent Triazine Framework for High-Performance CO2 Capture and Alkyne Carboxylation Reaction

Carbon dioxide capture and conversion have attracted extreme enthusiasm from the scientific community owing to global warming and environmental problems. However, conversion of CO₂ under atmospheric pressure is of great challenge because of the inertness of CO₂. Herein, we present a novel covalent triazine framework (CTF-DCE) prepared via ZnCl₂-catalyzed ionothermal trimerization reaction of di(4-cyanophenyl)ethyne, which displays a high Brunauer-Emmett-Teller surface area of 1355 m² g^-1 and an excellent CO₂ capture capacity of 191 mg/g at 273 K/1 bar. More importantly, silver species can be successfully fixed on the CTF matrix to produce a stable CTF-DCE-Ag heterogeneous catalyst for outstanding catalysis in the terminal alkyne carboxylation reactions under atmospheric pressure. CTF-DCE-Ag exhibited over sixfold higher turnover numbers than Ag@MIL-101. The recyclability test of the CTF-DCE-Ag catalyst demonstrated a great potential application in various environmental and energy-related applications.

Direct Monitoring of Molecular Events at the Surface: One-Step Access to Flexibly Stable Colloidal Ag Nanoparticles

Design, control, and direct characterization of surface properties are prerequisites to all the practical applications of nanoparticles. Since stable and homogeneous colloidal conditions are required for most applications, the amenability of nanoparticles to in-solution processing must also be addressed. Herein, we demonstrate that solution ¹H NMR spectroscopy is an effective tool for direct monitoring of the production of Ag nanoparticles. The production consists of two stages, complexation and reduction, which are both clearly observed by ¹H NMR spectroscopy. Design and synthesis of a series of amphiphilic amines have led to the one-step production of "flexibly" stable colloidal Ag nanoparticles, which form clear stable brown solutions in a wide range of solvents.