Liquid palladium for high-turnover carbon-carbon bond formation

Carbon-carbon (C─C) bond formation is a key step in diverse chemical processes and requires high-performance catalysts to enable energy-efficient technologies. Here, we present liquid Pd catalysts, formed by dissolving Pd in liquid Ga, for high-turnover C─C coupling reactions. The liquid Pd catalyst achieved a turnover frequency of 2.5 × 108 hour-1 for a model coupling reaction at 70°C, surpassing all reported Pd catalysts by 1000-fold. Our results show that Pd atoms in the Ga matrix are liquid-like, exhibiting unique electronic and interfacial properties that substantially lower the energy barrier and enhance reaction kinetics. The system retained full activity over five cycles and showed no Pd leaching, highlighting the transformative potential of liquid-phase metals to advance high-throughput and sustainable C─C bond-forming strategies.

Antibacterial Surfaces Prepared through Electropolymerization of N-Heterocyclic Carbene Complexes: A Pivotal Role of the Metal

N-Heterocyclic carbene (NHC) complexes are known to have antibacterial properties in solutions. However, these complexes have never been immobilized on solid supports to prepare antibacterial surfaces. Here, we tackled this lack and succeeded in immobilizing these NHC complexes on gold surfaces by electropolymerization. For this, we synthesized a series of various NHC complexes of different low-valent transition metals (M = Ag(I), Au(I), Rh(I), Ru(II), Cu(I)) bearing a pyrrole function at the five-membered carbenic cycle. We measured the antibacterial properties of these complexes against two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and two Gram-positive bacteria (Staphylococcus aureus and Listeria innocua) by determining their minimum inhibitory concentration (MIC) values. All NHC complexes presented interesting antibacterial properties that are metal-dependent. The silver-NHC complex showed higher antibacterial activity against Gram-negative bacteria (MIC = 16 μg·mL-1) than against Gram-positive bacteria (MIC = 32 μg·mL-1) and was poorly efficient against L. innocua. All other metal-NHC complexes were more efficient against Gram-positive bacteria, with MIC values in the range 4-16 μg·mL-1. These NHC complexes were then electropolymerized on gold substrates using their pyrrole function. Efficient incorporation of these NHC species into polypyrrole (PPy) films was confirmed by X-ray photoelectron spectroscopy (XPS) measurements with metal contents ranging from 0.8% (Cu) to 12.3% (Ag). Scanning electron microscopy (SEM) and profilometry measurements ascertain that the homogeneity, structure, and thickness of the films depend on the metal. The antibacterial activities of the polypyrrole films were then determined by the halo inhibition method. A very good match between the antibacterial properties of the films and those of the monomers with Ag(I), Au(I), and Rh(I) complexes was found. For the other complexes, the metallic content was too low to obtain interesting antibacterial properties. The cytotoxicity of the films was finally evaluated on normal human dermal fibroblasts (NHDF). Our study reveals a strong impact of the doping anions of polypyrrole on cell viability.

4-Halomethyl-Substituted Imidazolium Salts: A Versatile Platform for the Synthesis of Functionalized NHC Precursors

N,N'-Diarylimidazolium salts containing haloalkyl functional groups that are reactive with various nucleophiles are considered to be promising reagents for the preparation of functionalized N-heterocyclic carbene (NHC) ligands, which are in demand in catalysis, materials science, and biomedical research. Recently, 4-chloromethyl-functionalized N,N'-diarylimidazolium salts became readily available via the condensation of N,N'-diaryl-2-methyl-1,4-diaza-1,3-butadienes with ethyl orthoformate and Me3SiCl, but these compounds were found to have insufficient reactivity in reactions with many nucleophiles. These chloromethyl salts were studied as precursors in the synthesis of bromo- and iodomethyl-functionalized imidazolium salts by halide anion exchange. The 4-ICH2-functionalized products were found to be unstable, whereas a series of novel 4-bromomethyl functionalized N,N'-diarylimidazolium salts were obtained in good yields. These bromomethyl-functionalized imidazolium salts were found to be significantly more reactive towards various N, O and S nucleophiles than the chloromethyl counterparts and enabled the preparation of previously inaccessible heteroatom-functionalized imidazolium salts, some of which were successfully used as NHC proligands in the preparation of Pd/NHC and Au/NHC complexes.

Dual role tertiary amines in photocatalytic cyclizations: towards sustainable syntheses of 1,3-dinitrogen heterocycles

A novel photocatalytic cascade has been engineered for the transformation of diketimines into 1,3-dinitrogen heterocycles, ingeniously repurposing methylamine a typically utilized sacrificial additive as a dual-function reagent, serving simultaneously as an electron donor and a methyl source. This innovation champions atom economy and underscores a commitment to sustainable chemical synthesis.

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

CO2-Triggered Hierarchical-Pore UiO-66-Based Pickering Emulsions for Efficient and Recyclable Suzuki-Miyaura Cross-Coupling in Biphasic Systems

Hierarchical-pore metal-organic frameworks (H-MOFs) are considered to be emerging stabilizers for Pickering emulsion formation because of their hierarchically arranged pores, tailorable structures, and ultrahigh surface areas. However, stimulus-triggered Pickering emulsions built by H-MOFs have been seldom presented to date despite their great significance in diverse applications. Herein, by grafting Pd(OAc)2 on the hierarchical-pore zirconium MOF UiO-66, namely, H-UiO-66, with the aid of 1-alkyl-3-methylimidazolium 2-cyanopyrrolide salts ([CnMIM][2-CN-Pyr], n = 4, 6, and 8), a series of Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 have been developed and utilized as emulsifiers for constructing CO2-switching Pickering emulsions. It was found that Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 was able to stabilize the n-hexane-water mixture to form a Pickering emulsion even at an amount of 0.5 wt %. Upon alternate addition of CO2 and N2 at normal pressure, Pickering emulsions could be smartly converted between demulsification and re-emulsification. Through combining varieties of spectroscopic techniques, the mechanism of the switchable phase transformation lay in the acid-base reaction of ionic liquids with CO2 on H-UiO-66 and the creation of more hydrophilic salts, which reduced the wettability of the emulsifier and destabilized the emulsion. As an example of application, the stimulus-triggered Pickering emulsion was employed as a palladium-catalyzed Suzuki-Miyaura cross-coupling microreactor to achieve the combination of chemical reactions, isolation of products, and recovery of catalysts.

Towards "homeopathic" palladium-catalysed alkoxycarbonylation of aliphatic and aromatic olefins

Palladium-catalysed alkoxycarbonylation of alkenes allows for atom-efficient synthesis of esters from easily available alkenes in an industrially viable manner. One of the major costs associated with this process is the consumption of the catalyst system. Hence, for economic and ecologic reasons it is desirable to minimize the amount of metal and ligands wherever possible. Herein, we report "a homeopathic" palladium-catalysed alkoxycarbonylation of olefins under comparably mild conditions. The key to success is the homemade ligand LIKATphos providing good to excellent yields of ester products with catalyst turnover numbers in the range of 10⁶.

(CAAC)Pd(py) Catalysts Disproportionate to Pd(CAAC)2

Palladium complexes with one N-heterocyclic carbene (NHC) and a pyridine ancillary ligand are powerful cross-coupling precatalysts. Herein, we report such complexes with a cyclic (alkyl)(amino)carbene (CAAC) ligand replacing the NHC. We find that the alleged reduced form, (CAAC)Pd(py), disproportionates to the (CAAC)2Pd0 complex and palladium nanoparticles. This notwithstanding, they are potent catalysts in the Buchwald-Hartwig amination with aryl chlorides under mild conditions (60 °C). In the presence of dioxygen, these complexes catalyze the formation of diazenes from anilines. The catalytic activities of the NHC- and CAAC-supported palladium(0) and palladium(II) complexes are similar in the cross-coupling reaction, yet the CAAC complexes are superior for diazene formation.

Palladium-Catalyzed Cyclization Coupling with Cyclobutanone-Derived N-Tosylhydrazones: Synthesis of Benzofuran-3-Cyclobutylidenes and Spirocyclobutanes

A palladium-catalyzed cyclization coupling of iodoarene-tethered alkynes with cyclobutanone-derived N-tosylhydrazones is reported, providing a convenient and efficient approach to benzofuran-3-cyclobutylidenes. On this basis, spirocyclobutanes can be generated smoothly in an efficient cascade manner by the addition of dienophiles. Good yields and scalability are demonstrated. Sequential intramolecular carbopalladation, palladium-carbene migratory insertion, δ-hydride elimination, and cycloaddition processes are involved.

A Sustainable Approach for Graphene Oxide-supported Metal N-Heterocyclic Carbenes Catalysts

Sustainable noble metal-N-heterocyclic carbenes (NHC's) are a topic of arising concern in both the chemical industry and the academic community due to a growing consciousness of environmental pollution and scarcity. Recovering and reusing homogeneous catalysts from the reaction mixture requires a tremendous amount of capital investment in the chemical manufacturing industry. Heterogeneous catalysts are proved to have better functional groups tolerance; however, catalysts support largely influences the active catalyst sites to affect catalyst efficiency and selectivity. Thus the, choice of catalyst supports plays an almost decisive role in this emerging area of catalysis research. Graphene oxide (GO)/reduced graphene oxide (rGO) support has a potential growth in heterogeneous catalysis owing to their commercial availability, considerably larger surface area, inert towards chemical transformations, and easy surface functionalization to attached metal complexes via covalent and non-covalent aromatic π-conjugates. To take advantage of two independently well-established research areas of noble metal-N-heterocyclic carbenes and GO/rGO support via covalent or non-covalent interactions approach would offer novel heterogeneous complexes with improved catalytic efficiency without sacrificing product selectivity. This unique concept of marrying metal-N-heterocyclic carbenes with GO/rGO support has potential growth in the chemical and pharmaceutical industry, however, limited examples are reported in the literature. In this perspective, a comprehensive summary of metal-NHC synthesis on GO/rGO support and synthetic strategies to graft M-NHC onto GO/rGO surface, catalytic efficiency, for the catalytic transformation are critically reviewed. Furthermore, a plausible mechanism for non-covalent grafting methodology is summarized to direct readers to give a better understanding of M-NHC@rGO complexes. This would also allow the designing of engineered catalysts for unexplored catalytic applications.

Metamorphic oxygen-evolving molecular Ru and Ir catalysts

Today sustainable and clean energy conversion strategies are based on sunlight and the use of water as a source of protons and electrons, in a similar manner as it happens in Photosystem II. To achieve this, the charge separation state induced by light has to be capable of oxidising water by 4 protons and 4 electrons and generating molecular oxygen. This oxidation occurs by the intermediacy of a catalyst capable of finding low-energy pathways via proton-coupled electron transfer steps. The high energy involved in the thermodynamics of water oxidation reaction, coupled with its mechanistic complexity, is responsible for the difficulty of discovering efficient and oxidatively robust molecules capable of achieving such a challenging task. A significant number of Ru coordination complexes have been identified as water oxidation catalysts (WOCs) and are among the best understood from a mechanistic perspective. In this review, we describe the catalytic performance of these complexes and focus our attention on the factors that influence their performance during catalysis, especially in cases where a detailed mechanistic investigation has been carried out. The collective information extracted from all the catalysts studied allows one to identify the key features that govern the complex chemistry associated with the catalytic water oxidation reaction. This includes the stability of trans-O-Ru-O groups, the change in coordination number from CN6 to CN7 at Ru high oxidation states, the ligand flexibility, the capacity to undergo intramolecular proton transfer, the bond strain, the axial ligand substitution, and supramolecular effects. Overall, combining all this information generates a coherent view of this complex chemistry.

A Review on Generation and Reactivity of the N-Heterocyclic Carbene-Bound Alkynyl Acyl Azolium Intermediates

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has emerged as an effective intermediate to access functionalized cyclic molecular skeleton until very recently. In this review, we summarized the generation and reactivity of the NHC-bound alkynyl acyl azolium intermediates, which covers the efforts and advances in the synthesis of achiral and axially chiral cyclic scaffolds via the NHC-bound alkynyl acyl azolium intermediates. In particular, the mechanism related to this intermediate is discussed in detail.

Magnesium Pincer Complexes and Their Applications in Catalytic Semihydrogenation of Alkynes and Hydrogenation of Alkenes: Evidence for Metal-Ligand Cooperation

The development of catalysts for environmentally benign organic transformations is a very active area of research. Most of the catalysts reported so far are based on transition-metal complexes. In recent years, examples of catalysis by main-group metal compounds have been reported. Herein, we report a series of magnesium pincer complexes, which were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ via aromatization/dearomatization metal-ligand cooperation was studied. Utilizing the obtained complexes, the unprecedented homogeneous main-group metal catalyzed semihydrogenation of alkynes and hydrogenation of alkenes were demonstrated under base-free conditions, affording Z-alkenes and alkanes as products, respectively, with excellent yields and selectivities. Control experiments and DFT studies reveal the involvement of metal-ligand cooperation in the hydrogenation reactions. This study not only provides a new approach for the semihydrogenation of alkynes and hydrogenation of alkenes catalyzed by magnesium but also offers opportunities for the hydrogenation of other compounds catalyzed by main-group metal complexes.

Proof of Concept: Interface of Recyclable Organogels with Embedded Palladium Nanoparticles Catalyzing Suzuki-Miyaura Coupling in Water at Room Temperature

A sustainable approach for C-C cross-coupling reaction at room temperature in water has been developed to avoid tedious Pd separation, reduce the carbon footprint, and save energy. Another important aspect is the catalyst recycling and easy product separation. α,γ-Hybrid peptides were designed to selectively use as a ligand for C-C cross-coupling catalysts as well as to form organogels. The peptides form antiparallel sheet-like structures in the solid state. The peptide containing m-aminobenzoic acid, glycine, and dimethylamine forms a whitish gel in toluene, and co-gelation with Pd(OAc)2 results in light brown gel, which acts as a biphasic catalyst for Suzuki-Miyaura cross-coupling at room temperature in water by mild shaking. The organic-inorganic hybrid gel was characterized by rheology, field-emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray analyses. On completion of the cross-coupling reaction, the basic aqueous layer (containing products) above the gel can be simply decanted and the intact organic-inorganic hybrid gel can be recycled by topping-up fresh reactants multiple times. The reaction permitted a range of different substitution patterns for aryl and heterocyclic halides with acid or phenol functional groups. Both electron-donating- and electron-withdrawing-substituted substrates exhibited good results for this transformation. The findings inspire toward a holistic green technology for Suzuki-Miyaura coupling reaction and an innovative avenue for catalyst recycling and product isolation.

Ir(III)-catalyzed quadruple C-H activation of N-arylimidazolium and diaryliodonium salts: facile access to polysubstituted imidazo[1,2-f]phenanthridiniums

Herein, N-heterocyclic carbene-directed Ir(III)-catalyzed cascade C-H arylation/annulation of N-arylimidazolium with diaryliodonium salts has been accomplished for the first time via a quadruple C-H activation strategy to construct imidazo[1,2-f]phenanthridinium structures. This protocol overcomes the compatibility of three kinds of different C-H activations with high catalytic efficiency, which allows ortho-unhindered N-arylimidazoliums to undergo a diarylation/annulation reaction, affording a variety of polysubstituted imidazo[1,2-f]phenanthridiniums. Neutral imidazo[1,2-f]phenanthridines are also prepared via a demethylation reaction of imidazo[1,2-f]phenanthridiniums.

Pd-Catalyzed Heteroannulation Using N-Arylureas as a Sterically Undemanding Ligand Platform

We report the development of ureas as sterically undemanding pro-ligands for Pd catalysis. N-Arylureas outperform phosphine ligands for the Pd-catalyzed heteroannulation of N-tosyl-o-bromoanilines and 1,3-dienes, engaging diverse coupling partners for the preparation of 2-subsituted indolines, including sterically demanding substrates that have not previously been tolerated. Experimental and computational studies on model Pd-urea and Pd-ureate complexes are consistent with monodentate binding through the nonsubstituted nitrogen, which is uncommon for metal-ureate complexes.

Vanadium complexes with N-heterocyclic vinylidene ligands

The preparation and the structural characterization of vanadium complexes with terminal and bridging N-heterocyclic vinylidene ligands is reported. The synthesis of the complexes was enabled by utilization of diazoolefins as ligand precursors. Structural data and theoretical results show that N-heterocyclic vinylidenes can act as 6e^- donor ligands, leading to strong metal-carbon interactions.

Supramolecular Immobilization of Adamantyl and Carboxylate Modified N-Heterocyclic Carbene Ligand on Cucurbituril Substrates

Herein, the design, synthesis, supramolecular interactions and structural analysis of a novel bidentate carboxylate chelating N-heterocylic carbene (NHC) ligand is presented. The NHC structure was modified to strategically incorporate adamantyl moiety for the formation of a supramolecular complex with host molecules such as cucurbiturils. The adamantyl modified NHC ligand could potentially be used in recoverable homogeneous catalysts when Immobilized on a solid support via host–guest chemistry. As a versatile precursor, NHC ligand (8) was synthesized and characterized by ¹H-NMR, ¹³C-NMR, FTIR, single crystal x-ray crystallography and elemental analysis. A proof-of-principle non-covalent immobilization of the NHC ligand (8) with a Cucurbit[7]uril (CB7) host was demonstrated using ¹H-NMR titration.

Palladium-Catalyzed Carbene Coupling Reactions of Cyclobutanone N-Sulfonylhydrazones

Described herein are the palladium-catalyzed cross-coupling reactions of cyclobutanone-derived N-sulfonylhydrazones with aryl or benzyl halides, suggesting that the metal carbene process and β-hydride elimination can smoothly occur in strained ring systems. Structurally diversified products including cyclobutenes, methylenecyclobutanes, and conjugated dienes are selectively afforded in good to excellent yields. Preliminary success in asymmetric carbene coupling reactions in strained ring systems has been achieved, providing a promising route for the synthesis of enantioenriched four-membered-ring molecules.

Platinum-Catalyzed Alkene Hydrosilylation: Solvent-Free Process Development from Batch to a Membrane-Integrated Continuous Process

The integration of a membrane separation protocol with the platinum-catalyzed hydrosilylation of olefins is investigated. The catalytic reaction is first optimized in batch where [Pt(IPr*)(dms)Cl₂ ] (IPr*=1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazol-2-ylidene, dms=dimethyl sulfide) demonstrates superior activity compared to the less sterically encumbered [Pt(SIPr)(dms)Cl₂ ] (SIPr=1,3-bis(2,6-diisopropylphenyl)imidazolidine) congener. Filtration conditions are identified in membrane screening experiments. Hydrosilylation of 1-octene catalyzed by [Pt(IPr*)(dms)Cl₂ ] is conducted in continuous mode and the platinum catalyst is separated efficiently over the commercially available Borsig oNF-2 membrane, all under solvent-free conditions. An advantage of this process is that both reaction and separation are coupled in a single step. Moreover, at the end of the process the intact catalyst was recovered in 80 % yield as an off-white solid without any further purification.

Unveiling the catalytic nature of palladium-N-heterocyclic carbene catalysts in the α-alkylation of ketones with primary alcohols

We report herein the synthesis of four new Pd-PEPPSI complexes with backbone-modified N-heterocyclic carbene (NHC) ligands and their application as catalysts in the α-alkylation of ketones with primary alcohols using a borrowing hydrogen process and tandem Suzuki-Miyaura coupling/α-alkylation reactions. Among the synthesized Pd-PEPPSI complexes, complex 2c having 4-methoxyphenyl groups at the 4,5-positions and 4-methoxybenzyl substituents on the N-atoms of imidazole exhibited the highest catalytic activity in the α-alkylation of ketones with primary alcohols (18 examples) with yields reaching up to 95%. Additionally, complex 2c was demonstrated to be an effective catalyst for the tandem Suzuki-Miyaura-coupling/α-alkylation of ketones to give biaryl ketones with high yields. The heterogeneous nature of the present catalytic system was verified by mercury poisoning and hot filtration experiments. Moreover, the formation of NHC-stabilized Pd(0) nanoparticles during the α-alkylation reactions was identified by advanced analytical techniques.