Phantom Reactivity in Organic and Catalytic Reactions as a Consequence of Microscale Destruction and Contamination-Trapping Effects of Magnetic Stir Bars

Pentsak E.O., Eremin D.B., Gordeev E.G., Ananikov V.P., ACS Catal., 2019, 9, 3070-3081.
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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.

Pd and Pt Catalyst Poisoning in the Study of Reaction Mechanisms: What Does the Mercury Test Mean for Catalysis?

Chernyshev V.M., Astakhov A.V., Chikunov I.E., Tyurin R.V., Eremin D.B., Ranny G.S., Khrustalev V.N., Ananikov V.P., ACS Catal., 2019, 9, 2984-2995.
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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 MII 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 MII 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 MII complexes, including a reaction of highly popular MII/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.

Exploring the performance of nanostructured reagents with organic-group-defined morphology in cross-coupling reaction

Kashin A. S., Degtyareva E. S., Eremin D. B., Ananikov V. P., Nat. Commun., 2018, 9, 2936.
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The great impact of the nanoscale organization of reactive species on their performance in chemical transformations creates the possibility of fine-tuning of reaction parameters by modulating the nano-level properties. This methodology is extensively applied for the catalysts development whereas nanostructured reactants represent the practically unexplored area. Here we report the palladium- and copper-catalyzed cross-coupling reaction involving nano-structured nickel thiolate particles as reagents. On the basis of experimental findings we propose the cooperative effect of nano-level and molecular-level properties on their reactivity. The high degree of ordering, small particles size, and electron donating properties of the substituents favor the product formation. Reactant particles evolution in the reaction is visualized directly by dynamic liquid-phase electron microscopy including recording of video movies. Mechanism of the reaction in liquid phase is established using on-line mass spectrometry measurements. Together the findings provide new opportunities for organic chemical transformations design and for mechanistic studies.

Revealing the Unusual Role of Bases in Activation/Deactivation of Catalytic Systems: O–NHC Coupling in M/NHC Catalysis

Chernyshev V. M., Khazipov O. V., Shevchenko M. A., Chernenko A. Yu., Astakhov A. V., Eremin D. B., Pasyukov D. V., Kashin A. S., Ananikov V. P., Chem. Sci., 2018, 9, 5564-5577.
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Numerous reactions are catalyzed by complexes of metals (M) with N-heterocyclic carbene (NHC) ligands, typically in the presence of oxygen bases, which significantly shape the performance. It is generally accepted that bases are required for either substrate activation (exemplified by transmetallation in the Suzuki cross-coupling), or HX capture (e.g. in a variety of C–C and C-heteroatom couplings, the Heck reaction, C–H functionalization, heterocyclizations, etc.). This study gives insights into the behavior of M(II)/NHC (M = Pd, Pt, Ni) complexes in solution under the action of bases conventionally engaged in catalysis (KOH, NaOH, t-BuOK, Cs2CO3, K2CO3, etc.). A previously unaddressed transformation of M(II)/NHC complexes under conditions of typical base-mediated M/NHC catalyzed reactions is disclosed. Pd(II) and Pt(II) complexes widely used in catalysis react with the bases to give M(0) species and 2(5)-oxo-substituted azoles via an O–NHC coupling mechanism. Ni(NHC)2X2 complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O–NHC coupling to give azolones and metallic nickel under the action of t-BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M(II) into "ligandless" M(0) species. This demonstrates that the disclosed base-mediated O–NHC coupling reaction is integrated into the catalytic M/NHC systems and can define the mechanism of catalysis (molecular M/NHC vs. "NHC-free" cocktail-type catalysis). A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including 18O labeling experiments.

“Solvent-in-Salt” Systems for Design of New Materials in Chemistry, Biology and Energy Research

Azov V. A., Egorova K. S., Seitkalieva M. M., Kashin A. S., Ananikov V. P., Chem. Soc. Rev., 2018, 47, 1250-1284 .
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Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

3D Printing with Biobased PEF for Carbon Neutral Manufacturing

Kucherov F.A., Gordeev E.G., Kashin A.S., Ananikov V. P., Angew. Chem. Int. Ed., 2017, 56, 15931 – 15935 .
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We demonstrate the utility of 100% biomass-derived poly(ethylene-2,5-furandicarboxylate) (PEF) as an efficient material for Fused Deposition Modeling (FDM) 3D printing. A complete cycle from cellulose to printed object has been performed. PEF-printed objects created in the present study demonstrated higher chemical resistance than objects printed with commonly available materials (ABS, PLA, PETG). The studied PEF polymer has shown key advantages for 3D printing: optimal adhesion, thermoplasticity, lack of delamination and low heat shrinkage. The high thermal stability of PEF and relatively low temperature that are necessary for extrusion are optimal for recycling printed objects and minimizing waste. Several successive cycles of 3D-printing and recycling were successfully demonstrated. The suggested approach for extending additive manufacturing to carbon neutral materials opens a new direction in the field of sustainable development.

Understanding Active Species in Catalytic Transformations: from Molecular Catalysis to Nanoparticles, Leaching, “Cocktails” of Catalysts and Dynamic Systems

Eremin D.B., Ananikov V.P., Coord. Chem. Rev., 2017, 346, 2-19.
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In the present review, we consider the transformations of molecular catalysts, leaching, aggregation and various interconversions of metal complexes, clusters and nanoparticles that occur during catalytic processes. The role of catalyst evolution and the mechanistic picture of "cocktail"-type systems are considered from the perspective of the development of a new generation of efficient, selective and re-usable catalysts for synthetic applications. Rational catalyst development and the improvement of catalyst performance cannot be achieved without an understanding of the dynamic nature of catalytic systems.

Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and Medicine

Egorova K.S., Gordeev E.G., Ananikov V.P., Chem. Rev., 2017, 117, 7132–7189.
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Ionic liquids are remarkable chemical compounds, which find applications in many areas of modern science. Because of their highly tunable nature and exceptional properties, ionic liquids have become essential players in the fields of synthesis and catalysis, extraction, electrochemistry, analytics, biotechnology, etc. Apart from physical and chemical features of ionic liquids, their high biological activity has been attracting significant attention from biochemists, ecologists, and medical scientists. This Review is dedicated to biological activities of ionic liquids, with a special emphasis on their potential employment in pharmaceutics and medicine. The accumulated data on the biological activity of ionic liquids, including their antimicrobial and cytotoxic properties, are discussed in view of possible applications in drug synthesis and drug delivery systems. Dedicated attention is given to a novel active pharmaceutical ingredient-ionic liquid (API-IL) concept, which suggests using traditional drugs in the form of ionic liquid species. The main aim of this Review is to attract a broad audience of chemical, biological, and medical scientists to study advantages of ionic liquid pharmaceutics. Overall, the discussed data highlight the importance of the research direction defined as "Ioliomics", studies of ions in liquids in modern chemistry, biology, and medicine.

Which Metals are Green for Catalysis? Comparison of the Toxicities of Ni, Cu, Fe, Pd, Pt, Rh, and Au Salts

Egorova K.S., Ananikov V. P., Angew. Chem. Int. Ed., 2016, 55, 12150-12162.
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Environmental profiles for the selected metals were compiled on the basis of available data on their biological activities. Analysis of the profiles suggests that the concept of toxic heavy metals and safe nontoxic alternatives based on lighter metals should be re-evaluated. Comparison of the toxicological data indicates that palladium, platinum, and gold compounds, often considered heavy and toxic, may in fact be not so dangerous, whereas complexes of nickel and copper, typically assumed to be green and sustainable alternatives, may possess significant toxicities, which is also greatly affected by the solubility in water and biological fluids. It appears that the development of new catalysts and novel applications should not rely on the existing assumptions concerning toxicity/nontoxicity. Overall, the available experimental data seem insufficient for accurate evaluation of biological activity of these metals and its modulation by the ligands. Without dedicated experimental measurements for particular metal/ligand frameworks, toxicity should not be used as a "selling point" when describing new catalysts.

Critical Influence of 5-HMF Aging and Decomposition on the Utility of Biomass Conversion in Organic Synthesis

Galkin K.I., Khokhlova E.A., Romashov L.V., Zalesskiy S.S., Kachala V.V., Burykina J.V., Ananikov V. P., Angew. Chem. Int. Ed., 2016, 55, 8338-8342.
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Spectral studies revealed the presence of a specific arrangement of 5-hydroxymethylfurfural (5-HMF) molecules in solution as a result of a hydrogen–bonding network, and this arrangement readily facilitates the aging of 5-HMF. Deterioration of the quality of this platform chemical limits its practical applications, especially in synthesis/pharma areas. The model drug Ranitidine (Zantac®) was synthesized with only 15 % yield starting from 5-HMF which was isolated and stored as an oil after a biomass conversion process. In contrast, a much higher yield of 65 % was obtained by using 5-HMF isolated in crystalline state from an optimized biomass conversion process. The molecular mechanisms responsible for 5-HMF decomposition in solution were established by NMR and ESI-MS studies. A highly selective synthesis of a 5-HMF derivative from glucose was achieved using a protecting group at O(6) position.

Direct Observation of Self-Organized Water-Containing Structures in the Liquid Phase and Their Influence on 5-(Hydroxymethyl)furfural Formation in Ionic Liquids

Kashin A.S., Galkin K.I., Khokhlova E.A., Ananikov V. P., Angew. Chem. Int. Ed., 2016, 55, 2161-2166.
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Water-containing organic solutions are widespread reaction media in organic synthesis and catalysis. This type of liquid multicomponent system has a number of unique properties due to the tendency for water to self-organize in mixtures with other liquids. In spite of key importance, the characterization of these water domains is a challenging task due to their soft and dynamic nature. In the present study, morphology and dynamics of μm-scale and nm-scale water-containing compartments in ionic liquids were directly observed by electron microscopy. A variety of morphologies, including isolated droplets, dense structures, aggregates and 2D meshwork, have been experimentally detected and studied. Using the developed method, the impact of water on the acid‑catalyzed biomass conversion reaction was studied at the microscopic level. The process that produced nanostructured domains in solution led to better yields and higher selectivities compared with reactions involving the bulk system.

Visible Light Mediated Metal-free Thiol–yne Click Reaction

Zalesskiy S.S., Shlapakov N.S., Ananikov V. P., Chem. Sci., 2016, 7, 6740-6745.
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The carbon-sulfur bond formation reaction is of paramount importance for functionalized materials design, as well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first environmentally friendly metal-free photoredox pathway to the thiol–yne click reaction. Using Eosin Y as a cheap and readily available catalyst, C-S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to 60:1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to improve the performance of the catalytic system.

Nature of the Copper-Oxide-Mediated C–S Cross-Coupling Reaction: Leaching of Catalytically Active Species from the Metal Oxide Surface

Panova Yu.S., Kashin A.S., Vorobev M.G., Degtyareva E.S., Ananikov V.P., ACS Catalysis, 2016, 6, 3637 – 3643.
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Copper-oxide-catalyzed cross-coupling reaction is a well-known strategy in heterogeneous catalysis. A large number of applications have been developed, and catalytic cycles have been proposed based on the involvement of the copper oxide surface. In the present work, we have demonstrated that copper(I) and copper(II) oxides served as precursors in the coupling reaction between thiols and aryl halides, while catalytically active species were formed upon unusual leaching from the oxide surface. A powerful cryo-SEM technique has been utilized to characterize the solution-state catalytic system by electron microscopy. A series of different experimental methods were used to reveal the key role of copper thiolate intermediates in the studied catalytic reaction. The present study shows an example of leaching from a metal oxide surface, where the leaching process involved the formation of a metal thiolate and the release of water. A new synthetic approach was developed, and many functionalized sulfides were synthesized with yields of up to 96%, using the copper thiolate catalyst. The study suggests that metal oxides may not act as an innocent material under reaction conditions; rather, they may represent a source of reactive species for solution-state homogeneous catalysis.

Spatial Imaging of Carbon Reactivity Centers in Pd/C Catalytic Systems

Pentsak E. O., Kashin A. S., Polynski M. V., Kvashnina K. O., Glatzel P., Ananikov V. P., Chem. Sci., 2015, 6, 3302-3313.
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Gaining insight into Pd/C catalytic systems aimed at locating reactive centers on carbon surfaces, revealing their properties and estimating the number of reactive centers presents a challenging problem. In the present study state-of-the-art experimental techniques involving ultra high resolution SEM/STEM microscopy (1 Å resolution), high brilliance X-ray absorption spectroscopy and theoretical calculations on truly nanoscale systems were utilized to reveal the role of carbon centers in the formation and nature of Pd/C catalytic materials. Generation of Pd clusters in solution from the easily available Pd 2dba3 precursor and the unique reactivity of the Pd clusters opened an excellent opportunity to develop an efficient procedure for the imaging of a carbon surface. Defect sites and reactivity centers of a carbon surface were mapped in three-dimensional space with high resolution and excellent contrast using a user-friendly nanoscale imaging procedure. The proposed imaging approach takes advantage of the specific interactions of reactive carbon centers with Pd clusters, which allows spatial information about chemical reactivity across the Pd/C system to be obtained using a microscopy technique. Mapping the reactivity centers with Pd markers provided unique information about the reactivity of the graphene layers and showed that >2000 reactive centers can be located per 1 μm2 of the surface area of the carbon material. A computational study at a PBE-D3-GPW level differentiated the relative affinity of the Pd2 species to the reactive centers of graphene. These findings emphasized the spatial complexity of the carbon material at the nanoscale and indicated the importance of the surface defect nature, which exhibited substantial gradients and variations across the surface area. The findings show the crucial role of the structure of the carbon support, which governs the formation of Pd/C systems and their catalytic activity.

Nickel: The "Spirited Horse" of Transition Metal Catalysis

Ananikov V.P., ACS Catal., 2015, 5, 1964-1971.
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In recent years, the emergence of nickel catalysis and the development of many remarkable synthetic applications have been observed. The key advantages of nickel catalysts include: a) efficient catalysis and the ability to initiate transformations involving usually unreactive substrates; b) the accessibility of Ni0/NiI/NiII/NiIII oxidation states and radical pathways; c) new reactivity patterns beyond the traditional framework of metal catalysts; d) the facile activation of unsaturated molecules and a variety of transformations involving multiple bonds; and e) opportunities in photocatalytic applications and dual photocatalysis. The present viewpoint briefly summarizes the fundamental aspects of nickel chemistry and highlights promising directions of catalyst development.

Pd-NHC Catalytic System for the Efficient Atom-Economic Synthesis of Vinyl Sulfides from Tertiary, Secondary, or Primary Thiols

Degtyareva E.S., Burykina Yu.V., Fakhrutdinov A.N., Gordeev E.G., Khrustalev V.N., Ananikov V.P., ACS Catal., 2015, 5, 7208–7213.
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Vinyl sulfides represent an important class of compounds in organic chemistry and materials science. Atom-economic addition of thiols to the triple bond of alkynes provides an excellent opportunity for environmentally friendly processes. We have found that well-known and readily available Pd-NHC complex (IMes)Pd(acac)Cl is an efficient catalyst for alkyne hydrothiolation. The reported technique provides a general one-pot approach for the selective preparation of Markovnikov-type vinyl sulfides starting from tertiary, secondary, or primary aliphatic thiols, as well as benzylic and aromatic thiols. In all the studied cases, the products were formed in excellent selectivity and good yields.

Noninnocent Nature of Carbon Support in Metal/Carbon Catalysts: Etching/Pitting vs Nanotube Growth under Microwave Irradiation

Pentsak E.O., Gordeev E.G., Ananikov V.P., ACS Catal., 2014, 4, 3806-3814.
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Microwave irradiation of Ni, Co, Cu, Ag, and Pt metal salts supported on graphite and charcoal revealed a series of carbon surface modification processes that varied depending on the conditions used (inert atmosphere, vacuum, or air) and the nature of metal salt. Carbon materials, routinely used to prepare supported metal catalysts and traditionally considered to be innocent on this stage, were found to actively change under the studied conditions: etching and pitting of the carbon surface by metal particles as well as growth of carbon nanotubes were experimentally observed by FE-SEM analysis. Catalyst preparation under microwave irradiation led to the formation of complex metal/carbon structures with significant changes in carbon morphology. These findings are of great value in developing an understanding of how M/C catalysts form and evolve and will help to design a new generation of efficient and stable catalysts. The energy surfaces of carbon support modification processes were studied with theoretical calculations at the density functional level. The energy surface of the multistage process of carbon nanotube formation from an etched graphene sheet was calculated for various types of carbon centers. These calculations indicated that interconversion of graphene layers and single wall carbon nanotubes is possible when cycloparaphenylene rings act as building units.

Miniaturization of NMR Systems: Desktop Spectrometers, Microcoil Spectroscopy, and "NMR on a Chip" for Chemistry, Biochemistry, and Industry

Zalesskiy S.S., Danieli E., Blümich B., Ananikov V. P., Chem. Rev., 2014, 114, 5641-5694.
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Efficient General Procedure To Access a Diversity of Gold(0) Particles and Gold(I) Phosphine Complexes from a Simple HAuCl4 Source. Localization of Homogeneous/Heterogeneous System's Interface and Field-Emission Scanning Electron Microscopy Study

Zalesskiy S. S., Sedykh A. E., Kashin A. S., Ananikov V. P., J. Am. Chem. Soc., 2013, 135, 3550-3559.
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Soluble gold precatalysts, aimed for homogeneous catalysis, under certain conditions may form nanoparticles, which dramatically change the mechanism and initiate different chemistry. The present study addresses the question of designing gold catalysts, taking into account possible interconversions and contamination at the homogeneous/heterogeneous system's interface. It was revealed that accurate localization of boundary experimental conditions for formation of molecular gold complexes in solution versus nucleation and growth of gold particles opens new opportunities for well-known gold chemistry. Within the developed concept, a series of practical procedures was created for efficient synthesis of soluble gold complexes with various phosphine ligands (R3P)AuCl (90–99% yield) and for preparation of different types of gold materials. The effect of the ligand on the particles growth in solution has been observed and characterized with high-resolution field-emission scanning electron microscopy (FE-SEM) study. Two unique types of nanostructured gold materials were prepared: hierarchical agglomerates and gold mirror composed of ultrafine smoothly shaped particles.