Efficient and Practical Transfer Hydrogenation of Ketones Catalyzed by a Simple Bidentate Mn−NHC Complex

Catalytic reductions of carbonyl‐containing compounds are highly important for the safe, sustainable, and economical production of alcohols. Herein, we report on the efficient transfer hydrogenation of ketones catalyzed by a highly potent Mn(I)−NHC complex. Mn−NHC 1 is practical at metal concentrations as low as 75 ppm, thus approaching loadings more conventionally reserved for noble metal based systems. With these low Mn concentrations, catalyst deactivation is found to be highly temperature dependent and becomes especially prominent at increased reaction temperature. Ultimately, understanding of deactivation pathways could help close the activity/stability‐gap with Ru and Ir catalysts towards the practical implementation of sustainable earth‐abundant Mn‐complexes

Роль релігійних цінностей у процесі формування права

У контексті з’ясування основних пріоритетів розвитку права розглядається проблема ролі релігійних цінностей як основи розвитку права. Показано взаємодію релігії і права в умовах мінливої динаміки демократизації українського суспільства, обгрунтовано пріоритетні моральні принципи для побудови фундаменту, на якому грунтується і завдяки якому розвивається право. Ключові слова: релігійні цінності, цінності права, мораль.В контексте определения основных приоритетов развития права рассматривается проблема роли религиозных ценностей как базиса развития права. Показано взаимодействие религии и права в условиях изменчивой динамики демократизации украинского общества, обоснованы приоритетные моральные принципы для строительства фундамента, на котором основывается и благодаря которому развивается право. Ключевые слова: религиозные ценности, ценности права, мораль.It is reviewed the issue of rule of religious values as a basis and grounds for law evolution in the context of definition of its main priorities. It is shown the correlation between the religion and the law in circumstances of possible dynamic of Ukrainian society democratization. It is обосновано the priority moral principles for developing pivot as a ground of law evolution. Key words: «religion values», «values of law», «moral»

«Національна згода» як об’єкт філософської рефлексії

У статті розглядається концепт «національної згоди» як певний різновид комунікації між владою та громадськістю. Виділяються основні типи соціальної згоди: «згода» як легітимізація влади з боку населення, «згода» як певний різновид ідентичності («національна згода»), «згода» як консенсус стосовно проведення тієї чи іншої політики.The author reviews a concept of «national consent» as a variety of communication between power and community. The basic types of social consent are determined: consent as legitimization of power on the part of society, consent as a definite variety of identity («national consent»), consent as a consensus in politics

An unprecedented phosphinine with significant P(π)-donor properties

A hitherto unprecedented electronic situation has been observed for a substituted, pyridyl-functionalized phosphinine. In contrast to previous studies, this compound shows considerable π-donor properties as the result of the rather strong +M effect of the CH3S-substituent, changing the electronic properties of this low-coordinate and aromatic phosphorus heterocycle substantially

Chemical reactivity of cation-exchanged zeolites

Zeolites modified with metal cations have been extensively studied during the last two decades because of their wide application in different technologically important fields such as catalysis, adsorption and gas separation. Contrary to the well-understood mechanisms of chemical reactions catalyzed by Brønsted acid sites in the hydrogen forms of zeolites, the nature of chemical reactivity, and related, the structure of the metal-containing ions in cation-exchanged zeolites remains the subject of intense debate. In this thesis, the chemical properties of zeolites modified with hard Lewis acids such as alkaline- and alkali-earth cations (Chapters 2 – 4) and with soft Lewis acids such as Zn-, Cd- and Ga-cations (Chapters 5 – 9) are discussed. Special attention is paid to the mechanism of chemical transformations promoted by such exchangeable species and, accordingly, their role in these processes. Low-silica zeolites modified with alkaline- and alkali-earth cations are rather inert materials. However, it has been experimentally found that they can efficiently promote photo-oxidation of unsaturated hydrocarbons with molecular oxygen. The details of this reactivity are not clear. Chapter 2 reports DFT calculations on the initial charge-transfer step for alkene photo-oxidation in zeolite Y modified with alkali-earth cations (Mg, Ca, and Sr). The photo-oxidation of 2,3-dimethyl-2-butene (DMB) with O2 has been used as a model reaction. It is predicted that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of the charge-transfer state, while the relative orientation and the distance between the adsorbed alkene and oxygen molecules are the critical factors. A high density and specific location of the exchangeable cations in the zeolite matrix determines a specific confinement of the adsorbed reagents in a suitable "pre-transition state" configuration. The optimum configuration of co-adsorbed DMB and O2 molecules is identified for CaY zeolite. A significantly lower activity of SrY and MgY in the photooxidation of 2,3-dimethyl-2-butene-2 in comparison with that of CaY is predicted. Another interesting property of low-silica zeolites modified with alkaline cations is their ability to promote N2O4 disproportionation under very mild conditions. Chapter 3 presents periodic DFT calculations on N2O4 disproportionation in Na-, K-, and Rb-exchanged lowsilica zeolite X. The disproportionation reaction results in rather polar NO+···NO3 – species, which are effectively stabilized by the cage of cation-exchanged zeolite. NO+ binds to the basic framework oxygens, and NO3 – anion coordinates to the exchangeable cations. Although the binding energy of NO+ ion to the zeolite is influenced by the basicity of the framework, the theoretical results show that the overall disproportionation reaction is mainly controlled by the interactions between the negatively charged nitro group and the extra-framework cations. The role of the interaction between the nitrosonium cation and basic sites of the zeolite is only of minor importance. The function of the microporous matrix is to facilitate the charge separation in a fashion similar to that of a polar solvent. It is concluded that steric properties of the zeolite cage, the cooperative effect of the extraframework cations as well as their mobility induced by adsorption are essential to form the optimum configuration of the active site for N2O4 disproportionation. Chapter 4 reports a combined infrared spectroscopic and computational study of light alkane adsorption to alkali-earth exchanged zeolite Y. Although these materials do not catalyze C–H or C–C bond cleavage, they can be successfully used as model adsorbents to investigate the factors influencing structural and electronic properties of the resulting adsorption complexes. The experimental IR spectra of the C–H stretching vibrations of the adsorbed hydrocarbons differ strongly for MgY and CaY zeolites. On the basis of ab initio MP2 and DFT calculations it is found that different geometries of the light alkane adsorption complexes are realized depending on the cation in the adsorption site. Topological analysis of the electron density distribution function in the framework of quantum theory of atoms in molecules is applied to investigate the bonding of the adsorption complexes. It is found that numerous van der Waals bonds between the H atoms of the alkane and basic oxygens of the zeolite are formed, when a hydrocarbon coordinates to Mg2+ ions. These intermolecular contacts significantly contribute to the overall adsorption energy, whereas they play only an indirect role in the adsorption of light alkanes on CaY. On the other hand, in the case of CaY the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation dominates the adsorption energy. It is concluded that the dominance of a particular type of intermolecular interactions is dependent on the properties of the adsorption site. The type of intermolecular interactions determines the final conformation of light alkanes adsorbed to the cation-exchanged zeolite Y. From the results in Chapters 2 – 4 an interesting effect is noted: although the smaller exchangeable cations are expected to bind molecules stronger and exhibit higher reactivity as compared to their larger counterparts because of the increased hardness of such cations, the calculations indicate that the properties of the metal ions stabilized in the zeolite matrix do not follow these trends. Indeed, when stabilized at zeolitic cation site, the larger ions are significantly coordinatively unsaturated. This leads to an enhancement of the adsorption properties of the larger cations in spite of their expected lower Lewis acidity. Molecular and dissociative adsorption of light alkanes on the more reactive high-silica zeolite ZSM-5 modified with zinc and cadmium is investigated in Chapter 5. Adsorption of ethane on coordinatively unsaturated soft Lewis acid sites (Zn2+ and Cd2+) in ZSM-5 zeolite results in stronger changes of the geometry and charge parameters of the adsorbed molecules as compared to the case of adsorption on MgY and CaY. It is found that the degree of the effective shielding of the exchangeable cations by the surrounding oxygen ions is an important factor that influences the perturbations of molecularly adsorbed ethane. The C2H6 binding energy does not apparently depend on the type of the cation (Zn or Cd), whereas the nature of the charge compensation of the cations is important. On the other hand, heterolytic dissociative adsorption is mainly controlled by the basicity of the proton-accepting oxygen-site (O-site) and the steric properties of the dissociation products, which determine their stability. As a result, no apparent correlation between the perturbations of the adsorbed molecules and their heterolytic dissociation is observed. Chapters 6 to 8 report cluster DFT calculations of the various potential reaction paths of catalytic dehydrogenation of light alkanes over zinc- and gallium-exchanged high-silica zeolites. The mechanism of the catalytic reaction and the most probable active site are identified. In addition, an attempt is made to understand the factors, which determine the catalytic activity of different intrazeolite cationic species as well as the preference for a particular reaction path. The theoretical results form a basis for interpreting the experimental catalytic data. Catalytic dehydrogenation of ethane over various zinc species in Zn/ZSM-5 zeolite is investigated in Chapter 6. It is shown that isolated Zn2+ stabilized at the cation sites with distantly placed anionic [AlO2]– framework units are the most probable active species. A novel mechanism of ethane dehydrogenation is proposed. It involves decomposition of the products of dissociative ethane adsorption (Z–Zn2+-C2H5 –···H+Z–) via one-step desorption of ethylene and hydrogen. This path is strongly favored for the isolated Zn2+ sites as compared to the conventional mechanism involving consecutive desorption of the dehydrogenation products. Similar to the initial heterolytic C-H bond cleavage, the basicity of the O-sites is a determinative factor for the particular reaction mechanism. In the case of Ga-exchanged ZSM-5 zeolite (Chapter 7), univalent gallium cations are the most probable active sites for reduced catalysts. Hydrogenated extra-framework species decompose rapidly toward Ga+ cations during the catalytic reaction. Initial oxidative addition of C2H6 to Ga+, which has been observed experimentally before, is shown to proceed via an indirect route involving heterolytic C-H cleavage over the Lewis acid-base pair formed by the Ga+ cation and a framework oxygen anion. The direct route is strongly disfavored due to the electronic properties of univalent gallium. C2H4 and H2 desorption in one step closes the catalytic cycle. Although this reaction is reminiscent to that proposed for Zn/ZSM-5, it strongly differs in nature and is controlled by the properties of the Ga site. It has been observed experimentally that the catalytic activity of ZSM-5 zeolite predominantly containing Ga+ ions can be remarkably enhanced after selective oxidation with N2O. The higher activity of the resulting material has been attributed to formation of extra-framework GaO+ ions. However, a detailed investigation of various possible reaction paths over isolated gallyl ions in ZSM-5 zeolite (Chapter 8) shows that ethane interacts with these species stoichiometrically, because of the extremely low stability of these sites. Indeed, the unfavorable tridentate coordination of gallium along with the high basicity of the extra-framework terminal oxygen ion in GaO+ leads to a rapid heterolytic dissociation of C2H6 molecules. The resulting products are very stable, and the closure of the catalytic cycle is not likely to occur. It is concluded that the isolated gallyl ions cannot be considered as catalytically active sites for light alkane dehydrogenation. The very low stability of GaO+ species, on the other hand, can cause their oligomerization in the zeolite micropores, resulting in formation of various multinuclear cationic gallium-oxide clusters (Chapter 9). Periodic DFT calculations show that formation of cyclic Ga2O2 2+ dimers is strongly favored independently of the aluminum distribution in the high-silica zeolite. Moreover, oligomers with a higher degree of aggregation can be in principle formed in oxidized Ga-exchanged zeolites. The zeolite lattice plays the role of a chelating ligand which stabilizes the (GaO)n cationic cluster. Parallels between conventional coordination chemistry and chemistry of high-silica zeolites modified with gallium are drawn. It is shown that the location and stability of such cationic clusters is mainly controlled by the favorable geometrical environment of the Ga3+ ions, while the effect of the direct interaction with the framework anionic sites which compensates for the positive charge of the extra-framework species is less important. In spite of higher stability, binuclear sites are shown to be active for alkane activation. The lower basicity of the extra-framework oxygen ions provides a path for the closure of the catalytic cycle. However, these sites still tend to reduce upon light alkane dehydrogenation via water desorption, resulting in formation of less reactive reduced Ga-species. The mechanistic insight provided by the quantum-chemical calculations suggests that the reduction path can be suppressed by addition of water to the hydrocarbon feed. This would lead to an increased steady-state concentration of reactive oxygenated Ga-species in the catalyst. The experimental catalytic tests (Chapter 9) indeed show significant enhancement of the dehydrogenation activity of Ga+ sites in ZSM-5 upon water co-feeding. Continuous addition of water is required to maintain a high steady-state concentration of the reactive oxygenated extra-framework species in the zeolite and leads to high and stable activity of the catalyst. Thus, it is shown that the reactivity of low-silica zeolites modified with rather inert alkaline- and alkali-earth cations derives mainly from the properties of the confined space of the zeolite cages. The high density and the specific arrangement of the exchangeable cations in the microporous matrix lead to optimum configuration of the adsorbed reagents and consequently to their chemical activation. On the other hand, the active sites in highsilica zeolites modified with softer Zn, Cd, and Ga cations are rather local and usually directly involved in catalytic transformations of the reagents. The chemical reactivity of these system derives from the properties of the Lewis acid-base conjugate pair, which in turn are controlled by the topology of the zeolite cation site accommodating the extraframework species as well as by the type of charge compensation and the nature of the cation. Both the Lewis acidity of the extra-framework cationic species and the properties of the conjugate basic sites are important for their activity. An optimum must be found in the Lewis acid-base properties of the zeolite active site to achieve a high catalytic activity

Supported nickel-rhenium catalysts for selective hydrogenation of methyl esters to alcohols

The addition of Re to Ni on TiO2 yields efficient catalysts for the hydrogenation of acids and esters to alcohols under mild conditions. Rhenium promotes the formation of atomically dispersed and sub-nanometre-sized bimetallic species interacting strongly with the oxide support

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane—a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small‐molecule organometallic mechano‐responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft‐matter systems for which global characterization techniques fall short of providing molecular‐level insight

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane—a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small‐molecule organometallic mechano‐responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft‐matter systems for which global characterization techniques fall short of providing molecular‐level insight

Electric Double Layer Effect on the Outer-Sphere Benzyl Halides Electro-Reduction Mechanism

Electrocatalytic reduction of organic halides and subsequent carboxylation are promising methods for the valorization of CO2 as a C1 source in synthetic organic chemistry. The reaction mechanism underlying the selectivity and reduction mechanism of benzyl halides is highly dependent on the nature of the electrode material as well as the processes, composition, and structure of the liquid phase at the electrode−solution interface. Herein, we present a computational study on the influence of the electric double layer (EDL) on the activation of benzyl halides at different applied potentials over the Au (111) cathode. Using a multiscale modeling approach, we demonstrate that, under realistic electrocatalytic conditions, the formation of a dense EDL over the cathode hampers the diffusion of benzyl halides toward the electrode surface. A combination of classical molecular dynamics simulations and density functional theory calculations reveals the most favorable benzyl halide electro-carboxylation pathway over the EDL that does not require direct substrate adsorption to the cathode surface. The dense EDL promotes the dissociative reduction of the benzyl halides via the outer-sphere electron transfer from the cathode surface to the electrolyte. Such a reduction mechanism results in a benzyl radical intermediate, which is then converted to benzyl anions in the EDL via an additional electron transfer step

Influence of pore topology on synthesis and reactivity of Sn-modified zeolite catalysts for carbohydrate conversions

A range of Sn-modified MWW, MFI, MOR and Beta zeolites were prepared by a post-synthetic Sn functionalization method and their catalytic properties for sugar conversions were evaluated. The focus of this work was to understand the effect of micropore dimensions and additional mesoporosity on the Sn incorporation and on the catalytic properties. The post-synthetic approach, which involves acid-dealumination of the parent zeolite followed by SnCl4 grafting, is highly efficient for the selective incorporation of lattice Sn sites in wide-pore Beta and MOR zeolites. The modification of the medium-pore MWW and MFI is impaired by the more difficult dealumination and hence the lower efficiency of the Sn incorporation. Hierarchical structuring of the zeolites allows the increase of the Sn loading in the final zeolites. The catalytic properties were assessed in the isomerization and retro-aldolization reactions of glucose and the conversion of 1,3-dihydroxyacetone to methyl lactate. The catalytic results depend strongly on the structural and topological properties of the catalysts as well as on the reactant. Glucose isomerization carried out at a relatively low temperature is mainly limited by strong adsorption of carbohydrates to the active sites. This explains why zeolite nanostructuring had little effect on the catalyst activity, which instead depends mainly on the zeolite topology and the nature of the reactive Sn centers. The influence of pore size is most pronounced for Sn-MWW and Sn-MFI zeolites which are inactive in glucose-to-fructose isomerization, but perform in the higher-temperature retro-aldolization of carbohydrates with an activity similar to that of Sn-Beta. Because of the limited accessibility of the Sn sites inside the 1D MOR pore system, Sn-MOR catalysts were only moderately active in all probe reactions considered