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Developments in chemistry, materials science and biology have been fuelled by our search for structure–property relationships in matter at different levels of organization. Transformations in chemical synthesis and living systems predominantly take place in solution, such that many efforts have focused on studying nanoscale systems in the liquid phase. These studies have largely relied on spectroscopic data, the assignment of which can often be ambiguous. By contrast, electron microscopy can be used to directly visualize chemical systems and processes with up to atomic resolution. Electron microscopy is most amenable to studying solid samples and, until recently, to study a liquid phase, one had to remove solvent and lose important structural information. Over the past decade, however, liquid-phase electron microscopy has revolutionized direct mechanistic studies of reactions in liquid media. Scanning electron microscopy and (scanning) transmission electron microscopy of liquid samples have enabled breakthroughs in nanoparticle chemistry, soft-matter science, catalysis, electrochemistry, battery research and biochemistry. In this Review, we discuss the utility of liquid-phase electron microscopy for studying chemical reaction mechanisms in liquid systems.

Magnetic stir bars are routinely used by every chemist doing synthetic or catalytic transformations in solution. Each bar lasts for months or years, as the regular PTFE (polytetrafluoroethylene) coating is believed to be highly durable, inert, and resistant to multiple washings and cleanings. By using electron microscopy, we found out quite unexpectedly that the surface of magnetic stir bars is susceptible to microscale destruction and forms various types of defects. These microscopic defects effectively trap and accumulate trace amounts of active components from reaction mixtures, most notably metal species. Trapped in surface defects, the impurities escape elimination by washing and cleaning, thus remaining on the surface. FE-SEM/EDX analysis shows that the surface of used stir bars is littered with contaminants representing a variety of metals (Pd, Pt, Au, Fe, Co, Cr, etc.). ESI-MS monitoring corroborates the transfer of the trace metal species to reaction mixtures, while chemical tests indicate their significant catalytic activity. A theoretical DFT study reveals a remarkably high binding energy of metal atoms to the PTFE surface, especially in cases of local mechanical disruption or chemical influence. A plausible mechanism of PTFE surface contamination is suggested, and the results show that metal contamination of reusable polymer-coated labware is greatly underestimated. The present study suggests that corresponding control experiments with an unused stir bar (to avoid misinterpretations due to the influence of contamination of magnetic stir bars) are a "must do" for reporting high-performance catalytic reactions, reactions with low catalyst loadings, metal-catalyst-free reactions, and mechanistic studies.

The mercury test is a rapid and widely used method for distinguishing truly homogeneous molecular catalysis from nanoparticle metal catalysis. In the current work, using various M ⁰ and M^II complexes of palladium and platinum that are often used in homogeneous catalysis as examples, we demonstrated that the mercury test is generally inadequate as a method for distinguishing between homogeneous and cluster/nanoparticle catalysis mechanisms for the following reasons: (i) the general and facile reactivity of both molecular M⁰ and M^II complexes toward metallic mercury and (ii) the very high and often unpredictable dependence of the test results on the operational conditions and the inability to develop universal quantitatively defined operational parameters. Two main types or mercury-induced transformations, the cleavage of M⁰ complexes and the oxidative–reductive transmetalation of M^II complexes, including a reaction of highly popular M^II/NHC complexes, were elucidated using NMR, ESI-MS, and EDXRF techniques. A mechanistic picture of the reactions involving metal complexes was revealed with mercury, and representative metal species were isolated and characterized. Even in an attempt to not overstate the results, one must note that the use of the mercury tests often leads to inaccurate conclusions and complicates the mechanistic studies of these catalytic systems. As a general concept, distinguishing reaction mechanisms (homogeneous vs cluster/nanoparticle) by using catalyst poisoning requires careful rethinking in the case of dynamic catalytic systems.

Reversible leaching of palladium nanoparticles occurs in a variety of catalytic reactions including cross-couplings, amination, the Heck reaction, etc. It is complemented by capturing of soluble palladium species on the surface of nanoparticles and de novo formation of nanoparticles from Pd precatalysts. We report here a detailed computational study of leaching/capture pathways and analysis of related stabilization energies. We demonstrate the validity of the "cocktail-of-species" model for the description of Pd catalysts in ArX oxidative addition-dependent reactions. Three pools of Pd species were evaluated, including (1) the pool of catalytically active Pd nanoparticles with a high concentration of surface defects, (2) the pool of monomeric and oligomeric L[ArPdX] _nL species, and (3) the pool of irreversibly deactivated Pd. Stabilization by ArX oxidative addition, coordination of base species, and binding of X− anions (derived from salt additives) were found to be crucial for "cocktail"-type systems, and the corresponding reaction energies were estimated. An inherent process of ArX homocoupling, leading to the formation of Pd halides that require re-activation, was considered as well. The pool of irreversibly deactivated Pd comprises nanoparticles with (1 1 1) and (1 0 0) facets and Pd in the bulk form. The study is based on DFT modeling and specifies the role of Pd nanoparticles in (quasi )homogeneous coupling reactions involving ArX reagents.

The paramount progress in the field of organic–inorganic hybrid nanomaterials was stimulated by numerous applications in chemistry, physics, life sciences, medicine, and technology. Currently, in the field of hybrid materials, researchers may choose either to mimic complex natural materials or to compete with nature by constructing new artificial materials. The deep mechanistic understanding and structural insight achieved in recent years will guide a new wave in the design of hybrid materials at the atomic and molecular levels.

N‐Heterocyclic carbene ligands (NHC) are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M‐NHC binding in this metal‐ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M‐NHC bond (rather than its stabilization) could be more important for high‐performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal‐NHC binding and [PdX ₃]^–[NHC‐R]⁺ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC‐R]⁺ azolium cation is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well‐defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC‐free anionic Pd in polar media via either Ph‐NHC coupling or reversible H‐NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

Calcium carbide, a stable solid compound composed of two atoms of carbon and one of calcium, has proven its effectiveness in chemical synthesis, due to the safety and convenience of handling the C≡C acetylenic units. The areas of CaC ₂ application are very diverse, and the development of calcium‐mediated approaches resolves several important challenges. This Review aims to discuss the laboratory chemistry of calcium carbide, and to go beyond its frontiers to organic synthesis, life sciences, materials and construction, carbon dioxide capturing, alloy manufacturing, and agriculture. The recyclability of calcium carbide and the availability of large‐scale industrial production facilities, as well as the future possibility of fossil‐resource‐independent manufacturing, position this compound as a key chemical platform for sustainable development. Easy regeneration and reuse of the carbide highlight calcium‐based sustainable chemical technologies as promising instruments for total carbon recycling.

Bring on the subs! Biorefining will be realized by using two different approaches: the production of new biobased molecular targets or sustainable access to traditional base and commodity chemicals. Awakening of 5‐hydroxymethylfurfural (HMF) can be expected with different probabilities, depending on the approach chosen to create a sustainable future.

Oxidative esterification of biomass-derived 5-(hydroxymethyl) furfural (HMF) and furfural and their derivatives has been performed using a simple MnO 2/NaCN system. The developed method allows the selective one-pot transformation of HMF to dimethyl furan-2,5-dicarboxylate (FDME) in 83% isolated yield without the formation of a free acid. Simplification of FDME production provides the missing link for manufacturing sustainable value-added materials from biomass. Addition of water to the oxidative system allows fine-tuning of reaction selectivity to obtain the previously difficult-to-access pure methyl 5-(hydroxylmethyl)furan-2-carboxylate in one step directly from the unprotected HMF without chromatographic separation.

An efficient two-step procedure to get synthetically useful sulfur-functionalized dienes is evaluated. The overall transformation can be classified as an atom-economic hydrothiolation of alkynes followed by elimination of water at the dehydration step. Taking the alkynes hydrofunctionalization reaction as a representative example, critical analysis from the point of view of quantitative green metrics was carried out and key stumbling blocks in the area of atom-economic transformations were discussed. Ecological acceptability of the whole process was assessed by thorough examination of the yields and careful adjustment of the synthetic conditions, considering the opportunities for waste minimization. Careful optimization of the reaction conditions was followed by selection of environmentally friendly protocols for accessing pure product. Green metrics of synthetic procedures as well as different isolation techniques (column chromatography, dry column chromatography, extraction, and distillation) were comparatively analyzed to afford minimization of waste and improve efficiency. For the first time, quantitative green metrics and life cycle assessment were applied and optimized for a very popular atom-economic functionalization process.

Low chemical stability and high oxygen content limits utilization of bio‐based platform chemical 5‐(hydroxymethyl)furfural (HMF) in biofuels development. In this work, Lewis‐acid‐catalyzed conversion of renewable 6‐deoxy sugars leading to formation of more stable 5‐methylfurfural (MF) is carried out with high selectivity. Besides its higher stability, MF is a deoxygenated analogue of HMF with increased C:O ratio. A highly selective synthesis of the innovative liquid biofuel 2,5‐dimethylfuran starting from MF under mild conditions is described. Superior synthetic utility of MF against HMF in benzoin and aldol condensation reactions leading to long‐chain alkane precursors is demonstrated.

2-Azidomethyl-5-ethynylfuran, a new ambivalent compound with both azide and alkyne moieties that can be used as a self-clickable monomer, is synthesized starting directly from renewable biomass. The reactivity of the azide group linked to furfural is tested via the efficient preparation of a broad range of furfural-containing triazoles in good to excellent yields using a 'green' copper(I)-catalyzed azide–alkyne cycloaddition procedure. Access to new bio-based chemicals and oligomeric materials via a click-chemistry approach is also demonstrated using this bio-derived building block.

It has recently been shown that palladium-catalyzed reactions with N-heterocyclic carbene (NHC) ligands involve R–NHC coupling accompanied by transformation of the molecular catalytic system into the nanoscale catalytic system. An important question appeared in this regard is whether such a change in the catalytic system is irreversible. More specifically, is the reverse nano-to-molecular transformation possible? In view of the paramount significance of this question to the area of catalyst design, we studied the capability of 2-substituted azolium salts to undergo the breakage of C–C bond and exchange substituents on the carbene carbon with corresponding aryl halides in the presence of Pd nanoparticles. The study provides important experimental evidence of possibility of the reversible R–NHC coupling. The observed behavior indicates that the nanosized metal species are capable of reverse transition to molecular species. Such an option, known for phosphine ligands, was previously unexplored for NHC ligands. The present study for the first time demonstrates bidirectional dynamic transitions between the molecular and nanostructured states in Pd/NHC systems. As a unique feature, surprisingly small activation barriers ( <18 kcal/mol) and noticeable thermodynamic driving force (−5 to −7 kcal/mol) were calculated for c–c bond oxidative addition to pd(0) centers in the studied system. the first example of nhc-mediated pd leaching from metal nanoparticles to solution was observed and formation of pd/nhc complex in solution was detected by esi-ms.

A well‐established oxidative addition of organic halides (R‐X) to N‐heterocyclic carbene (NHC) complexes of palladium(0) leads to formation of (NHC)(R)Pd ^II(X)L species, the key intermediates in a large variety of synthetically useful cross‐coupling reactions. Typical consideration of the cross‐coupling catalytic cycle is based on the assumption of intrinsic stability of these species, where the subsequent steps involve coordination of the second reacting partner. Thus, high stability of the intermediate (NHC)(R)Pd^II(X)L species is usually taken for granted. In the present study it is discussed that such intermediates are prone to non‐classical R‐NHC intramolecular coupling process (R = Me, Ph, Vinyl, Ethynyl) that results in removal of NHC ligand and generation of another type of Pd catalytic system. DFT calculations (BP86, TPSS, PBE1PBE, B3LYP, M06, wB97X‐D) clearly show that outcome of R‐NHC coupling process is not only determined by chemical nature of the organic substituent R, but also strongly depends on the type of solvent. The reaction is most favorable in polar solvents, whereas the non‐polar solvents render the products less stable.

R–NHC coupling was previously considered as a process of degradation of M/NHC species, however recent studies have pointed out that it may be responsible for generation of catalytically active NHC-free complexes or/and metallic nanoparticles. Therefore, a detailed and systematic study of R-NHC coupling for various carbene ligands is an important topic. In the present article this process has been studied for reactive aryl iodide coupling partners by a combination of quantum chemical calculations and continuous reaction monitoring via pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS). DFT calculations revealed strong tendency of (NHC)Pd(Ph)(I)DMF complexes bearing various N-heterocyclic carbene ligands (NHC) to undergo Ph–NHC coupling. Calculated energy barriers of these reactions lie in the range of 17.9 – 25.1 kcal/mol. Ph–NHC coupling is thermodynamically more favorable for the complexes containing unsaturated NHC ligands with bulky substituents. NBO analysis has suggested that the process of Ph–NHC formation is similar for different NHC ligands. In order to confirm theoretical studies, a series of ESI-MS reaction monitoring experiments was performed for (NHC)Pd(I)2(Py) and (NHC)Pd(Cl)(η3-1-Ph-C3H4) complexes interacting with iodobenzene, where Ph–NHC coupling products were observed in all cases. As a direct experimental evidence, formation of colloidal Pd-containing nanoparticles was observed in situ for different Pd/NHC complexes in the studied reaction mixtures.

The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M⁰/M^II catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, MII/MIVcatalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R–NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC–R]⁺ cations. In this study, the relative stabilities of (NHC)M^II(R)(X)L and (NHC)M^IV(R)(X)₃L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R–NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R–NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M^II and M^IV), while it is thermodynamically and kinetically more facile for the MIV complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R–NHC coupling for the MIVcomplexes were significantly stronger than that for the MII complexes. In particular, for the (NHC)₂M^IV(Ph)(Br)₃ complexes, Ph–NHC coupling was facilitated dramatically from Pt (ΔG = −36.9 kcal mol⁻¹, ΔG^≠ = 37.5 kcal mol⁻¹) to Pd (ΔG = −61.5 kcal mol⁻¹, ΔG^≠ = 18.3 kcal mol⁻¹) and Ni (ΔG = −80.2 kcal mol⁻¹, ΔG^≠ = 4.7 kcal mol⁻¹). For the MIIoxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R–NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the MIV oxidation state of the metal as the (NHC)₂M^IV(R)(X)₃ complexes were kinetically (4.3–15.9 kcal mol⁻¹) and thermodynamically (8.0–23.2 kcal mol⁻¹) significantly less stable than the (NHC)M^IV(R)(X)₃L (L = pyridine) complexes. For the Ni^IV and Pd^IV complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC–X]⁺ cation (X–NHC coupling) or reductive elimination of the X–X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising M^IV intermediates.

The article provides the first example of metal-catalyzed aryl disulfide addition to unsubstituted acetylene. The use of inexpensive Ni(acac)2 precatalyst with phosphine ligands results in competitive formation of Z-1,2-bis(arylthio)ethenes and Z,Z-1,4-bis(arylthio)buta-1,3-dienes. The process with the PPhCy2 as a ligand results in selective formation of diene molecular skeletons. Replacement of PPhCy2 with the PPh3 switches the reaction toward formation of alkenes. The use of substituted phenyl disulfides does not affect the selectivity and allows obtaining alkenes or dienes in good to high yields. Mechanistic investigations reveal major differences on the catalyst activation stage depending on the nature of phosphine ligand. Key novel point is to carry out video-monitoring of catalyst evolution with electron microscopy, which revealed the dynamic nature of the catalytic system and showed that the ligand played a prominent role in formation of the catalytically active phase. For PPh3, the development of catalytically active species proceeds through nickel thiolate [Ni(SAr)2]n formation, which renders the system heterogeneous. In contrast to PPh3, the PPhCy2 ligand promotes direct activation of the catalyst in its molecular form without disturbing the homogeneous state of the system.

A highly‐efficient catalytic system for hydrodebenzylation reaction is described. The cleavage of O‐benzyl and N‐benzyl protecting groups was performed using an uncommonly low palladium loading (0.02–0.3 mol%; TON up to 5000) in a relatively short reaction time. The approach was used for a variety of substrates including pharmaceutically important precursors, and gram‐scale deprotection reaction was shown. Transfer conditions together with easy‐to‐make Pd/C catalyst are the key features of this debenzylation scheme.

Solvent‐free reactions belong to a very attractive area of organic chemistry. The solvent‐free Suzuki‐Miyaura coupling is of special importance due to the problem of catalyst leaching in the presence of a solvent. This study investigates the course of reaction of solid aryl halides with arylboronic acids in the absence of a solvent and without any liquid additives. For the first time, a number of important conditions for performing a solid‐state Suzuki‐Miyaura reaction were analyzed in details. The results indicate a prominent role of water, which is formed as a by‐product in the side reaction of arylboronic acid trimerization. Electron microscopy study revealed surprising changes occurring within the reaction mixture during the reaction and indicated the formation of spherical nano‐sized particles containing the reaction product. Catalyst recycling was easily performed in the developed system and the product was isolated by sublimation, thus providing a possibility to completely avoid the use of solvents at all stages.

In recent years, the application of microwave (MW) irradiation has played an increasingly important role in the synthesis and development of high performance nanoscale catalytic systems. However, the interaction of microwave irradiation with solid catalytic materials and nanosized structures remains a poorly studied topic. In this paper we carried out a systematic study of changes in morphology under the influence of microwave irradiation on nanoscale particles of various metals and composite particles, including oxides, carbides, and neat metal systems. All systems were studied in the native solid form without a solvent added. Intensive absorption of microwave radiation was observed for many samples, which in turn resulted in strong heating of the samples and changes in their chemical structure and morphology. A comparison of two very popular catalytic materials—metal particles (M) and supported metal on carbon (M/C) systems—revealed a principal difference in their behavior under microwave irradiation. The presence of carbon support influences the heating mechanism; the interaction of substances with the support during the heating is largely determined by heat transfer from the carbon. Etching of the carbon surface, involving the formation of trenches and pits on the surface of the carbon support, were observed for various types of the investigated nanoparticles.

A catalytic system based on OX-1 metal–organic framework nanosheets is reported, incorporating catalytically active palladium (Pd) species. The Pd@OX-1 guest@host system is rapidly synthesized via a one-step single-pot supramolecular assembly, with the possibility of controlling the Pd loading. The structures of the resulting framework and of the active Pd species before and after catalytic reactions are studied in detail using a wide variety of techniques including synchrotron radiation infrared spectroscopy, inelastic neutron scattering, and X-ray absorption spectroscopy. Crystals of the resulting Pd@OX-1 composite material contain predominantly atomic and small cluster Pd species, which selectively reside on benzene rings of the benzenedicarboxylate (BDC) linkers. The composites are shown to efficiently catalyze the Suzuki coupling and Heck arylation reactions under a variety of conditions. Pd@OX-1 further shows potential to be recycled for at least five cycles of each reaction as well as an ability to recapture active Pd species during both catalytic reactions.

Until recently, chemical derivatives of platinum group metals have not been in a systematic direct contact with living organisms. The situation has changed dramatically due to anthropogenic activity, which has led to significant redistribution of these metals in the biosphere. Millions of modern cars are equipped with automotive catalytic converters, which contain rhodium, palladium and platinum as active elements. Everyday usage of catalytic technologies promotes the propagation of catalyst components in the environment. Nevertheless, we still have not accumulated profound information on possible ecotoxic effects of these metal pollutants. In this study, we report a case of an extraordinarily rapid development of lethal toxicity of a rhodium (III) salt in the terrestrial plants Pisum sativum, Lupinus angustifolius and Cucumis sativus. The growth stage, at which the exposure occurred, had a crucial impact on the toxicity manifestation: at earlier stages, RhCl₃ killed the plants within 24 h. In contrast, the salt was relatively low-toxic in human fibroblasts. We also address phytotoxicity of other common metal pollutants, such as palladium, iron, nickel and copper, together with their cytotoxicity. None of the tested compounds exhibited phytotoxic effects comparable with that of RhCl₃. These results evidence the crucial deficiency in our knowledge on environmental dangers of newly widespread metal pollutants.

A novel methodology for the preparation of trideuterovinyl derivatives of high purity directly from alcohols, thiols, and NH-compounds was developed. Commercially available calcium carbide and D ₂O acted as a D₂-acetylene source, and DMSO-d 6 was used to complete the formation of the D₂C=C(D)–X fragment (X = O, S, N). Polymerization of a selected trideuterovinylated compound showed a very promising potential of these substances in the synthesis of labeled polymeric materials. Biological activity of the synthesized trideuterovinyl derivatives was evaluated and the results indicated a significant increase of cytotoxicity upon deuteration.

Poor stability of 3D printed plastic objects in a number of solvents limits several important applications in engineering, chemistry and biology. Due to layered type of assembling, 3D-printed surfaces possess rather different properties as compared to bulk surfaces made by other methods. Here we study fundamental interactions at the solid-liquid interface and evaluate polymeric materials towards advanced additive manufacturing. A simple and universal stability test was developed for 3D printed parts and applied to a variety of thermoplastics. Specific modes of resistance/destruction were described for different plastics and their compatibility to a representative scope of solvents (aqueous and organic) was evaluated. Classification and characterization of destruction modes for a wide range of conditions (including geometry and 3D printing parameters) were carried out. Key factors of tolerance to solvent media were investigated by electron microscopy. We show that the overall stability and the mode of destruction depend on chemical properties of the polymer and the nature of interactions at the solid-liquid interface. Importantly, stability also depends on the layered microstructure of the sample, which is defined by 3D printing parameters. Developed solvent compatibility charts for a wide range of polymeric materials (ABS, PLA, PLA-Cu, PETG, SBS, Ceramo, HIPS, Primalloy, Photoresin, Nylon, Nylon-C, POM, PE, PP) and solvents represent an important benchmark for practical applications.