Multielectron transfer process for molecular conversions: Oxidative polymerization with vanadium catalysts

Multielectron transfer plays an important role in many chemical reactions. A collection of studies on metal complexes which exhibit one-step multielectron transfer processes and on chemical reactions based on multielectron transfer systems is presented. Emphasis is placed on the role of multielectron transfer process as essential prerequisites for some molecular conversion systems such as the reduction of O₂ and the oxidation of H₂O. As an important example of molecular conversion, oxidative polymerization of aromatic compounds through two-electron transfer is also reviewed.     

Phthalocyanines and related macrocycles for multi-electron transfer in catalysis,photochemistry and photoelectrochemistry

At different phthalocyanines and related macrocycles it is shown that one-step, multi-electron transfer and one-step, multi-change of oxidation states occur. At first, the catalytic oxidations of thiols and sulfide in the presence of different Co(II)phthalocyanines are discussed. Thiolates are oxidized to disulfides via a two-electron transfer whereas the reduction of O₂ occurs via a two- or four-electron transfer to H₂O₂ or H₂O. Zn(II) and Al(III)phthalocyanines are efficient sensitizers for the conversion of triplet to singlet dioxygen under illumination with visible light. In the presence of thiolates or sulfides an efficient photo-oxidation to sulfonic acids or sulfate is observed. The oxidation state of sulfur changes from ?2 to +4 or +6, respectively. This process of singlet oxygen reactions finds application in the photodynamic therapy of cancer. The unsubstituted zinc(II)-phthalocyanine as p-type molecular semiconductor can efficienfly reduce O₂ in photoelectrochemical experiments whereas zinc(II)phthalocyanines with electronwithdrawing groups as n-type conductors are active in the photoelectrochemical oxidation of thiols. All processes include multi-electron transfer. The electrocatalytic reduction of CO₂ is investigated at electrodes modified with Co(II)phthalocyanine. In particular, the phthalocyanine in a polyvinylpyridine membrane is active, so the CO₂ is reduced to CO by multi-electron transfer. In addition, two photon excitations of a Mg(II)phthalocyanine are presented and some examples are reviewed.     

Multi-electron transfer in photoelectrochemical processes

Simultaneous bidirectional forward and backward electron transfers take place on a light-exited semiconductor particle, even at the same geometric site. The potentials of the electron pathways are different, giving rise to two independent molecular conversion reactions. This type of multi-electron transfer reactions is overviewed and the stepwise unidirectional multi-electron transfer on the excited semiconductor particle is also described.     

Influence of the Operative Conditions on the Characteristics of Poly(D,L-lactide-co-glycolide) Synthesized in Supercritical Carbon Dioxide

Summary: The co-polymerizations of D,L -lactide and glycolide in supercritical carbon dioxide (scCO₂) using zinc (II) ethylhexanoate (ZnOct₂) as catalyst and methanol as initiator have been investigated. The influence of stirring rate (N), temperature (T), and mass carbon dioxide (m_CO2) on molecular weight distribution (MWD); co-polymer composition; and conversion has been studied by means an experimental factorial design. The stirring rate has the greatest influence on conversion. Due to the heterogeneous nature of the process the mass transfer enhancement, that the grater turbulence produces, favors greatly the incorporation of monomers into the polymer phase. An important decrease of molecular weight is observed independently of reaction conditions for high conversion values because some thermal degradation or rearrangement reactions are taking place. The influence of the initiator, methanol, on the molecular weight has been also studied. Methanol acts as an effective chain transfer agent initiating more growing chains than expected, what also contributes to get low molecular weights.     

Acid/base-regulated reversible electron transfer disproportionation of N–N linked bicarbazole and biacridine derivatives

Regulation of electron transfer on organic substances by external stimuli is a fundamental issue in science and technology, which affects organic materials, chemical synthesis, and biological metabolism. Nevertheless, acid/base-responsive organic materials that exhibit reversible electron transfer have not been well studied and developed, owing to the difficulty in inventing a mechanism to associate acid/base stimuli and electron transfer. We discovered a new phenomenon in which N–N linked bicarbazole (BC) and tetramethylbiacridine (TBA) derivatives undergo electron transfer disproportionation by acid stimulus, forming their stable radical cations and reduced species. The reaction occurs through a biradical intermediate generated by the acid-triggered N–N bond cleavage reaction of BC or TBA, which acts as a two electron acceptor to undergo electron transfer reactions with two equivalents of BC or TBA. In addition, in the case of TBA the disproportionation reaction is highly reversible through neutralization with NEt₃, which recovers TBA through back electron transfer and N–N bond formation reactions. This highly reversible electron transfer reaction is possible due to the association between the acid stimulus and electron transfer via the acid-regulated N–N bond cleavage/formation reactions which provide an efficient switching mechanism, the ability of the organic molecules to act as multi-electron donors and acceptors, the extraordinary stability of the radical species, the highly selective reactivity, and the balance of the redox potentials. This discovery provides new design concepts for acid/base-regulated organic electron transfer systems, chemical reagents, or organic materials.     

Theoretical study of chain transfer in anionic polymerization. Transfer by the mechanism of side-group substitution

A new theoretical consideration of chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. It is shown that the kinetic scheme used in theoretical studies reported previously contradicts the widespread views on the chemical mechanism of carbanionic reactions. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer; the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The molecular characteristics of polymer formed in processes with monomer transfer by side-group substitution are determined. At high conversion, the polymer formed in such a process is shown to possess a number-average degree of polymerization, P̄_n, approaching the theoretical value for living polymers, and a P̄_w exceeding it the more the higher the intensity of transfer. Furthermore, it shows a broad molecular weight distribution and a fairly noticeable degree of branching. These results considerably differ from those previously reported.     

Accumulation of Four Electrons on a Terphenyl (Bis)disulfide

The activation of N₂, CO₂ or H₂O to energy-rich products relies on multi-electron transfer reactions, and consequently it seems desirable to understand the basics of light-driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four-electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible-light irradiation using the organic super-electron donor tetrakis(dimethylamino)ethylene, a cascade of light-induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage.     

Theoretical study on photooxidation mechanism of ruthenium complex [Ru(II)‐(bpy)2(TMBiimH2)]2+ with molecular oxygen

Photoinduced reactions of ruthenium complexes with molecular oxygen have attracted a lot of experimental attention; however, the reaction mechanism remains elusive. In this work, we have used the density functional theory method to scrutinize the visible‐light induced photooxidation mechanism of the ruthenium complex [Ru(II)‐(bpy)₂(TMBiimH₂)]²⁺ (bpy: 2, 2‐bipyridine and TMBiimH₂: 4, 5, 4, 5‐tetramethyl‐2, 2‐biimidazole) initiated by the attack of molecular oxygen. The present computational results not only explain very well recent experiments, also provide new mechanistic insights. We found that: (1) the triplet energy transfer process between the triplet molecular oxygen and the metal‐ligand charge transfer triplet state of the ruthenium complex, which leads to singlet molecular oxygen, is thermodynamically favorable; (2) the singlet oxygen addition process to the S₀ ruthenium complex is facile in energy; (3) the chemical transformation from endoperoxide to epidioxetane intermediates can be either two‐ or one‐step reaction (the latter is energetically favored). These findings contribute important mechanistic information to photooxidation reactions of ruthenium complexes with molecular oxygen. © 2016 Wiley Periodicals, Inc.     

Copper fluoride as a low-cost sodium-ion battery cathode with high capacity

Sodium-ion batteries（SIBs） are promising alternatives to lithium-ion batteries（LIBs） for large-scale energy storage considering the abundance and low cost of Na-containing resources. However, the energy density of SIBs has been limited by the typically low specific capacities of traditional intercalation-based cathodes. Metal fluorides, in contrast, can deliver much higher capacities based on multi-electron conversion reactions. Among metal fluorides, CuF₂ presents a theoretical speci...     

Modelling free-radical copolymerization kinetics—evaluation of the pseudo-kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching

The full moment equations and equations using pseudo-kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number- and weight-average molecular weights (M̄_n and M̄_w) were compared with those found using the pseudo-kinetic rate constant method (PKRCM). The results show that the weight-average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ₁^˙ and φ₂^˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄_w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄_w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄_w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo-kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo-kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.     

Evidence for chain transfer in the atom transfer radical polymerization of butyl acrylate

Poly(butyl acrylate) (PBuA) of high molecular weight was synthesized by atom transfer radical polymerization (ATRP) in ethyl acetate. Whereas for low molecular weight polymers, a linear increase of the number‐average molecular weight, M_n, versus conversion and narrow molecular weight distributions indicate the suppression of side reactions, a downward curvature in the plot of M_n versus conversion was observed for high molecular weights (M_n > 50 000). This effect is explained by chain transfer reactions, leading to branched polymers. GPC measurements with a viscosity detector give evidence for the branched structure of high molecular weight polymers obtained in ATRP. In addition, transfer to solvent or monomer is likely to occur.     

Three-phase electrochemistry for green ethylene production

The electrochemical conversion of greenhouse gases (mainly CO₂ and CH₄) into ethylene has attracted worldwide attention. Compared with thermal cracking and dehydrogenation ethylene production processes, electrochemical ethylene production is an energy-saving and environmentally friendly process with high atom and energy economies. Great efforts have been made in enhancing the performance of electrochemical CO_x reduction and alkane dehydrogenation reactions in recent years. The complicated interactions between gas reactants, electrolytes, and catalysts force the three-phase interface mass transfer process an important issue in determining the electrochemical activity and product selectivity. Herein, we summarize the recent progresses on electrochemical ethylene production. Special attention has been paid to the principles for the design of gas–liquid–solid and gas–solid–solid three-phase interfaces and their influence on the electrochemical CO_x reduction and alkane dehydrogenation reactions. The comprehensive understanding of those different ethylene production reactions together from the perspective of the three-phase interface-related mass transfer process would provide new insights into the design of advanced electrochemical cells for green ethylene production.     

Integrating proton coupled electron transfer (PCET) and excited states

In many of the chemical steps in photosynthesis and artificial photosynthesis, proton coupled electron transfer (PCET) plays an essential role. An important issue is how excited state reactivity can be integrated with PCET to carry out solar fuel reactions such as water splitting into hydrogen and oxygen or water reduction of CO₂ to methanol or hydrocarbons. The principles behind PCET and concerted electron–proton transfer (EPT) pathways are reasonably well understood. In Photosystem II antenna light absorption is followed by sensitization of chlorophyll P₆₈₀ and electron transfer quenching to give P₆₈₀⁺. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn₄ cluster, through an intervening tyrosine–histidine pair, Y_Z. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e^?/4H⁺ from the OEC with oxygen evolution. The key elements in photosynthesis and artificial photosynthesis – light absorption, excited state energy and electron transfer, electron transfer activation of multiple-electron, multiple-proton catalysis – can also be assembled in dye sensitized photoelectrochemical synthesis cells (DS-PEC). In this approach, molecular or nanoscale assemblies are incorporated at separate electrodes for coupled, light driven oxidation and reduction. Separate excited state electron transfer followed by proton transfer can be combined in single semi-concerted steps (photo-EPT) by photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H-bonded electron transfer donors or acceptors. In these assemblies, photochemically induced electron and proton transfer occur in a single, semi-concerted event to give high-energy, redox active intermediates.     

Photoirradiated Fe-Mediated AGET ATRP of Methyl Methacrylate in the Presence of Alcohol

In this study, photoirradiated Fe-mediated AGET (activators generated by electron transfer) atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was investigated at ambient temperature in N,N-dimethylformamide (DMF) using carbon tetrachloride as initiator, FeCl₃·6H₂O/bipyridine (Bpy) as catalyst complex, and alcohol as reducing agent. Linear semi-logarithmic plot of conversion vs. time was obtained from the kinetic experiments, and the number-average molecular weight increased linearly with monomer conversion and corresponded to the theoretic values with molecular weight distributions (M_w/M_n) as low as 1.25, which agreed with the character of controlled/living polymerization. The kinds of alcohol played an important role in photoirradiated Fe-mediated AGET ATRP of MMA. The living characteristics were demonstrated through chain extension experiment. The obtained polymer was characterized by proton nuclear magnetic resonance (NMR) and gel permeation chromatography.     

Non-covalent Interactions and Charge Transfer between Propene and Neutral Yttrium-Doped and Pure Gold Clusters

The dopant and size-dependent propene adsorption on neutral gold (Au_n) and yttrium-doped gold (Au_n−1Y) clusters in the n=5–15 size range are investigated, combining mass spectrometry and gas phase reactions in a low-pressure collision cell and density functional theory calculations. The adsorption energies, extracted from the experimental data using an RRKM analysis, show a similar size dependence as the quantum chemical results and are in the range of ≈0.6–1.2 eV. Yttrium doping significantly alters the propene adsorption energies for n=5, 12 and 13. Chemical bonding and energy decomposition analysis showed that there is no covalent bond between the cluster and propene, and that charge transfer and other non-covalent interactions are dominant. The natural charges, Wiberg bond indices, and the importance of charge transfer all support an electron donation/back-donation mechanism for the adsorption. Yttrium plays a significant role not only in the propene binding energy, but also in the chemical bonding in the cluster-propene adduct. Propene preferentially binds to yttrium in small clusters (n<10), and to a gold atom at larger sizes. Besides charge transfer, relaxation also plays an important role, illustrating the non-local effect of the yttrium dopant. It is shown that the frontier molecular orbitals of the clusters determine the chemical bonding, in line with the molecular-like electronic structure of metal clusters.     

Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure

Results of chemical kinetics modeling in methane subjected to the microwave plasma at atmospheric pressure are presented in this paper. The reaction mechanism is based on the methane oxidation model without reactions involving nitrogen and oxygen. For the numerical calculations 0D and 1D models were created. 0D model uses Calorimetric Bomb Reactor whereas 1D model is constructed either as Plug Flow Reactor or as a chain of Plug Flow Reactor and Calorimetric Bomb Reactor. Both models explain experimental results and show the most important reactions responsible for the methane conversion and production of H₂, C₂H₂, C₂H₄ and C₂H₆ detected in the experiment. Main conclusion is that the chemical reactions in our experiment proceed by a thermal process and the products can be defined by considering thermodynamic equilibrium. Temperature characterizing the methane pyrolysis is 1,500–2,000 K, but plasma temperature is in the range of 4,000–5,700 K, which means that methane pyrolysis process is occurring outside the plasma region in the swirl gas flowing around the plasma.     

Hierarchical Microtubular Covalent Organic Frameworks Achieved by COF-to-COF Transformation

Pore environment and aggregated structure play a vital role in determining the properties of porous materials, especially regarding the mass transfer. Reticular chemistry imparts covalent organic frameworks (COFs) with well-aligned micro/mesopores, yet constructing hierarchical architectures remains a great challenge. Herein, we reported a COF-to-COF transformation methodology to prepare microtubular COFs. In this process, the C₃-symmetric guanidine units decomposed into C₂-symmetric hydrazine units, leading to the crystal transformation of COFs. Moreover, the aggregated structure and conversion degree varied with the reaction time, where the hollow tubular aggregates composed of mixed COF crystals could be obtained. Such hierarchical architecture leads to enhanced mass transfer properties, as proved by the adsorption measurement and chemical catalytic reactions. This self-template strategy was successfully applied to another four COFs with different building units.     

Ab initio molecular dynamics study of prebiotic production processes of organic compounds at meteorite impacts on ocean

Recent experiments concerning prebiotic materials syntheses suggest that the iron-bearing meteorite impacts on ocean during Late Heavy Bombardment provided abundant organic compounds associated with biomolecules such as amino acids and nucleobases. However, the molecular mechanism of a series of chemical reactions to produce such compounds is not well understood. In this study, we simulate the shock compression state of a meteorite impact for a model system composed of CO₂, H₂O, and metallic iron slab by ab initio molecular dynamics combined with multiscale shock technique, and clarify possible elementary reaction processes up to production of organic compounds. The reactions included not only pathways similar to the Fischer–Tropsch process known as an important hydrocarbon synthesis in many planetary processes but also those resulting in production of a carboxylic acid. It is also found that bicarbonate ions formed from CO₂ and H₂O participated in some forms in most of these observed elementary reaction processes. These findings would deepen the understanding of the full range of chemical reactions that could occur in the meteorite impact events. © 2018 Wiley Periodicals, Inc.     

Structure�Creactivity relationships in inorganic electrochemistry

Relationships between structure and electron transfer reactivity underlie many important electrochemical applications and provide fundamental insight to chemical and biological processes. The vast array of experimental techniques developed during the latter half of the twentieth century helped greatly to foster progress in this area, and the advent of powerful computational techniques such as density functional theory promises even more far-reaching developments. It is evident that molecular composition, geometric and electronic structure, and changes in these features influence the thermodynamics and kinetics of transition metal electron transfer reactions in predictable and understandable ways. Several examples drawn from the author??s research program to illustrate this premise include the influence of sulfur versus oxygen donation on molybdenum-centered electron transfer, reactions in which a change in metal atom spin state accompanies electron transfer, and concomitant multi-electron transfer and metal?Cmetal bond cleavage in binuclear, ligand-bridged complexes.     

Single-stage catalytic conversion of ethylene to butadiene

It has been found that butadiene is formed in the process of ethylene conversion over an aluminosilicate catalyst. Data have been obtained on the total conversion of C₂H₄ and product selectivities in the temperature interval from 873 to 973 K. Probable chemical reactions are suggested for this process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 4, pp. 369–371, July–August, 1993.