CO Intermediate-Assisted Dynamic Cu Sintering During Electrocatalytic CO2 Reduction on Cu-N-C Catalysts.

The electrochemical CO2 reduction reaction (eCO2RR) to multicarbon products has been widely recognized for Cu-based catalysts. However, the structural changes in Cu-based catalysts during the eCO2RR pose challenges to achieving an in-depth understanding of the structure-activity relationship, thereby limiting catalyst development. Herein, we employ constant-potential density functional theory calculations to investigate the sintering process of Cu single atoms of Cu-N-C single-atom catalysts into clusters under eCO2RR conditions. Systematic constant-potential ab initio molecular dynamics simulations revealed that the leaching of Cu-(CO)x moieties and subsequent agglomeration into clusters can be facilitated by synergistic adsorption of H and eCO2RR intermediates (e.g., CO). Increasing the Cu2+ concentration or the applied potential can efficiently suppress Cu sintering. Both microkinetic simulations and experimental results further confirm that sintered Cu clusters play a crucial role in generating C2 products. These findings provide significant insights into the dynamic evolution of Cu-based catalysts and the origin of their activity toward C2 products during the eCO2RR.

MoS2 Hollow Multishelled Nanospheres Doped Fe Single Atoms Capable of Fast Phase Transformation for Fast-charging Na-ion Batteries.

Low Na+ and electron diffusion kinetics severely restrain the rate capability of MoS2 as anode for sodium-ion batteries (SIBs). Slow phase transitions between 2H and 1T, and from NaxMoS2 to Mo and Na2S as well as the volume change during cycling, induce a poor cycling stability. Herein, an original Fe single atom doped MoS2 hollow multishelled structure (HoMS) is designed for the first time to address the above challenges. The Fe single atom in MoS2 promotes the electron transfer, companying with shortened charge diffusion path from unique HoMS, thereby achieving excellent rate capability. The strong adsorption with Na+ and self-catalysis of Fe single atom facilitates the reversible conversion between 2H and 1T, and from NaxMoS2 to Mo and Na2S. Moreover, the buffering effect of HoMS on volume change during cycling improves the cyclic stability. Consequently, the Fe single atom doped MoS2 quadruple-shelled sphere exhibits a high specific capacity of 213.3 mAh g-1 at an ultrahigh current density of 30 A g-1, which is superior to previously-reported results. Even at 5 A g-1, 259.4 mAh g-1 (83.68 %) was reserved after 500 cycles. Such elaborate catalytic site decorated HoMS is also promising to realize other "fast-charging" high-energy-density rechargeable batteries.

Lewis Acid-Induced Reversible Disproportionation of TEMPO Enables Aqueous Aluminum Radical Batteries.

Nitroxide radicals, such as 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO), are typical organic electrode materials featuring high redox potentials and fast electrochemical kinetics and have been widely used as cathode materials in multivalent metal-ion batteries. However, TEMPO and its derivatives have not been used in emerging rechargeable aluminum-ion batteries (AIBs) due to the known disproportionation and possible degradation of nitroxide radicals in acidic conditions. In this study, the (electro)chemical behavior of TEMPO is examined in organic and aqueous Lewis acid electrolytes. Through in situ (electro)chemical characterizations and theoretical computation, we reveal for the first time an irreversible disproportionation of TEMPO in organic Al(OTf)3 electrolytes that can be steered to a reversible process when switching to an aqueous media. In the latter case, a fast hydrolysis and ligand exchange between [Al(OTf)3TEMPO]- anion and water enable the overall reversible electrochemical redox reaction of TEMPO. These findings lead to the first design of radical polymer aqueous AIBs that are fire-retardant and air-stable, delivering a stable voltage output of 1.25 V and a capacity of 110 mAh g-1 over 800 cycles with 0.028% loss per cycle. This work demonstrates the promise of using nonconjugated organic electroactive materials for cost-effective and safe AIBs that currently rely on conjugated organic molecules.

Reversible Transformation and Distribution Determination of Diverse Pt Single-Atom Species.

In single-atom catalysts (SACs), the complexity of the support anchoring sites creates a vast diversity of single-atom species with varied coordination environments. To date, the quantitative distribution of these diverse single-atom species in a given SAC has remained elusive. Recently, CeO2-supported metal SACs have been extensively studied by modulating their local environments via numerous synthetic strategies. However, owing to the absence of a quantitative description, unraveling the site-specific reactivity and regulating their transformation remain challenging. Here, we show that two distinct Pt/CeO2 SACs can be reversibly generated by oxidative and nonoxidative dispersions, which contain varied Pt1On-Ceδ+ single-atom species despite similar Pt charge states and coordination numbers. By means of Raman spectroscopy and computational studies, we semiquantitatively reveal the distribution of diverse Pt1On-Ceδ+ species in each specific SACs. Remarkably, the minority species of Pt1O4-Ce3+-Ov accounting for only 14.2% affords the highest site-specific reactivity for low-temperature CO oxidation among the other abundant counterparts, i.e., Pt1O4-Ce4+ and Pt1O6-Ce4+. The second nearest oxygen vacancy (Ov) not only acts synergistically with the nearby active metal sites to lower the reaction barrier but also facilitates the dynamic transformation from six-coordinated to four-coordinated sites during cyclic nonoxidative and oxidative dispersions. This work elucidates the quantitative distribution and dynamic transformation of varied single-atom species in a given SAC, offering a more intrinsic descriptor and quantitative measure to depict the inhomogeneity of SACs.

Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand.

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Ribulose 1,5-bisphosphate carboxylase/oxygenase activates O2 by electron transfer.

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the cornerstone of atmospheric CO2 fixation by the biosphere. It catalyzes the addition of CO2 onto enolized ribulose 1,5-bisphosphate (RuBP), producing 3-phosphoglycerate which is then converted to sugars. The major problem of this reaction is competitive O2 addition, which forms a phosphorylated product (2-phosphoglycolate) that must be recycled by a series of biochemical reactions (photorespiratory metabolism). However, the way the enzyme activates O2 is still unknown. Here, we used isotope effects (with 2H, 25Mg, and 18O) to monitor O2 activation and assess the influence of outer sphere atoms, in two Rubisco forms of contrasted O2/CO2 selectivity. Neither the Rubisco form nor the use of solvent D2O and deuterated RuBP changed the 16O/18O isotope effect of O2 addition, in clear contrast with the 12C/13C isotope effect of CO2 addition. Furthermore, substitution of light magnesium (24Mg) by heavy, nuclear magnetic 25Mg had no effect on O2 addition. Therefore, outer sphere protons have no influence on the reaction and direct radical chemistry (intersystem crossing with triplet O2) does not seem to be involved in O2 activation. Computations indicate that the reduction potential of enolized RuBP (near 0.49 V) is compatible with superoxide (O2•-) production, must be insensitive to deuteration, and yields a predicted 16O/18O isotope effect and energy barrier close to observed values. Overall, O2 undergoes single electron transfer to form short-lived superoxide, which then recombines to form a peroxide intermediate.

Two new iridoid glycosides from Hedyotis diffusa.

Two new iridoid glycosides, named productasperulosidic acid butyl ester (1) and E-6-O-3-hydroxy-p-methoxycinnamoyl scandoside methyl ester (2), along with nine known ones (3-11), were isolated from Hedyotis diffusa Willd. The structures of them were elucidated by extensive 1D, 2D NMR and HR-ESI-MS spectral data. Compounds 1-11 showed no significant cytotoxic activity against HeLa cells.

Photoactive Metal Carbonyl Complexes Bearing N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Viability as Photoredox Catalysts.

This report details the synthesis and characterization of a small family of previously unreported, structurally related chromium, molybdenum, tungsten, manganese, and iron complexes bearing N-heterocyclic carbene and carbonyl supporting ligands. These complexes have the general form [ML(CO)3X] or [ML(CO)3], where X = CO or Br and L = 1-phenyl-3-(2-pyridyl)imidazolin-2-ylidene. Where possible, the solid-state, spectroscopic, electrochemical, and photophysical properties of these molecules were studied using a combination of experiment and theory. Photophysical studies reveal that decarbonylation occurs when these complexes are exposed to ultraviolet light, with the CO ligand being replaced with a labile acetonitrile solvent molecule. To obtain insights into the potential utility, scope, and applications of these complexes in visible-light-mediated photoredox catalysis, their capacity to facilitate a range of photoinduced reactions via the reductive or oxidative functionalization of organic molecules was investigated. These chromium, molybdenum, and manganese catalysts efficiently facilitated atom-transfer radical addition processes. In light of their photolability, these types of catalysts may potentially allow for the development of photoinduced reactions involving less conventional inner-sphere electron-transfer pathways.

Ultrasound analysis of the effect of second delivery on pelvic floor function in Chinese women.

In our study, patients who had a second delivery were categorised into the following 4 groups. Pelvic floor ultrasound data were compared during the 6th week after the second delivery. The incidence of cystoceles was highest in group A and lowest in group D. In addition, groups A and B had a higher rate of rectoceles or perineum descent. Similarly, the areas of the levator hiatus were higher in Groups A and B during Valsalva manoeuvres. The area of the levator hiatus from the resting state to the Valsalva manoeuvre effect had the greatest change in Group A. A comparison of the PR thickening rates among the four groups did not reveal significant differences. All second delivery methods can cause varying degrees of pelvic organ prolapse and decreased pelvic floor function; however, vaginal delivery as the second delivery mode may have a more significant effect in Chinese women.Impact StatementWhat is already known on this subject? Different modes of delivery have significantly different effects on female pelvic floor function. Pregnancy beyond 35 weeks of gestation has an effect on female pelvic floor function, irrespective of the mode of delivery.What do the results of this study add? This study analysed the impact of different delivery modes on Chinese female pelvic floor function. Parous women who underwent different modes of second delivery all demonstrated different degrees of pelvic organ prolapse, as well as pelvic floor function decline.What are the implications of these findings for clinical practice and/or further research? Our study will provide basic research of Chinese female pelvic floor function after a second delivery, which will be of clinical significance around the world, as well as in China. China will keep promoting further delivery as the aging population is increasing. If the developing countries want to promote the second delivery around the women, they have basic research and data to instruct the females.

Enantioselective oxa-Diels-Alder Sequences of Dendralenes.

Diene-transmissive hetero-Diels-Alder sequences involving carbonyl dienophiles are reported for the first time. High enantioselectivities are achieved in the reaction of phenylglyoxal with a broad range of dendralene structures, through the optimization of a Pd2+ catalyst system. The initial catalyst-controlled enantioselective oxa-Diels-Alder (ODA) cycloaddition to a [3]dendralene generates a dihydropyran carrying a semicyclic diene. This participates in a subsequent catalyst or substrate-controlled Diels-Alder reaction to generate sp3 -rich fused polycyclic systems containing both heterocycles and carbocycles. Computational investigations reveal a concerted asynchronous mechanism. π-Complexation of a diene C=C bond to Pd2+ occurs in both the pre-transition state (TS) complex and in cycloaddition TSs, controlling stereoselectivity. A formal enantioselective [4+2]cycloaddition of a CO2 dienophile is demonstrated.

Luciferase-free Luciferin Electrochemiluminescence.

Luciferin is one of Nature's most widespread luminophores, and enzymes that catalyze luciferin luminescence are the basis of successful commercial "glow" assays for gene expression and metabolic ATP formation. Herein we report an electrochemical method to promote firefly's luciferin luminescence in the absence of its natural biocatalyst-luciferase. We have gained experimental and computational insights on the mechanism of the enzyme-free luciferin electrochemiluminescence, demonstrated its spectral tuning from green to red by means of electrolyte engineering, proven that the colour change does not require, as still debated, a keto/enol isomerization of the light emitter, and gained evidence of the electrostatic-assisted stabilization of the charge-transfer excited state by double layer electric fields. Luciferin's electrochemiluminescence, as well as the in situ generation of fluorescent oxyluciferin, are applied towards an optical measurement of diffusion coefficients.

What's in an Atom? A Comparison of Carbon and Silicon-Centred Amidinium⋠⋠⋠Carboxylate Frameworks*.

Despite their apparent similarity, framework materials based on tetraphenylmethane and tetraphenylsilane building blocks often have quite different structures and topologies. Herein, we describe a new silicon tetraamidinium compound and use it to prepare crystalline hydrogen bonded frameworks with carboxylate anions in water. The silicon-containing frameworks are compared with those prepared from the analogous carbon tetraamidinium: when biphenyldicarboxylate or tetrakis(4-carboxyphenyl)methane anions were used similar channel-containing networks are observed for both the silicon and carbon tetraamidinium. When terephthalate or bicarbonate anions were used, different products form. Insights into possible reasons for the different products are provided by a survey of the Cambridge Structural Database and quantum chemical calculations, both of which indicate that, contrary to expectations, tetraphenylsilane derivatives have less geometrical flexibility than tetraphenylmethane derivatives, that is, they are less able to distort away from ideal tetrahedral bond angles.

Electrostatic Switching of Stereoselectivity in Aldol Reactions.

Density functional theory (DFT) has been employed in predicting the enantioselectivity of the aldol reaction between acetone and p-nitrobenzaldehyde catalyzed by proline and its derivatives Me2bdc-Pro (bdc = 1,4-benzenedicarboxylate) and Me2bpdc-Pro (bpdc = 4,4'-biphenyldicarboxylate). For each catalyst, our computationally predicted values at the M062X/6-31+G(d) level of theory with the SMD solvent model are in excellent agreement with experimental results reported in the literature. Electron-donating and electron-withdrawing groups (viz., SO3-, NMe2, SO3H, and NMe3+) were installed at the C4 position of the proline-based catalysts to study the impact of electrostatic effects on stereoselectivity. The electron-donating groups decrease and even invert the enantioselectivity, while the electron-withdrawing ones increase it. Enantiomeric excesses in the range of 49-71 and 59-68% are predicted for Me2bdc-Pro and Me2bpdc-Pro catalysts with the electron-withdrawing SO3H and NMe3+ installed respectively, values much higher than those of the corresponding unmodified catalysts. More interestingly, enantiomeric excesses decrease and, in the case of SO3-, are even inverted in favor of the other enantiomer when the electron-donating groups are installed. These results highlight the importance of electrostatic effects, and polar effects more generally, in optimal organocatalyst design for stereoselective C-C bond-forming reactions.

Atom Transfer Radical Polymerization-Inspired Room Temperature (sp3)C-N Coupling.

A simple nonphotochemical procedure is reported for Cu(I)-catalyzed C-N coupling of aliphatic halides with amines and amides. The process is loosely based on the Goldberg reaction but takes place readily at room temperature. It uses Cu(I)Br, a commonly used and inexpensive atom transfer radical polymerization precatalyst, along with the cheap ligand N,N,N',N″,N″-pentamethyldiethylenetriamine, to activate the R-X bond of the substrate via inner-sphere electron transfer. The procedure brings about productive C-N bond formation between a range of alkyl halide substrates with heterocyclic aromatic amines and amides. The mechanism of the coupling step, which was elucidated through application of computational methods, proceeds via a unique Cu(I) → Cu(II) → Cu(III) → Cu(I) catalytic cycle, involving (a) inner-sphere electron transfer from Cu(I) to the alkyl halide to generate the alkyl radical; (b) successive coordination of the N-nucleophile and the radical to Cu(II); and finally reductive elimination. In the absence of a nucleophile, debrominative homocoupling of the alkyl halide occurs. Control experiments rule out SN-type mechanisms for C-N bond formation.

Room Temperature Hydrolysis of Benzamidines and Benzamidiniums in Weakly Basic Water.

Benzamidinium compounds have found widespread use in both medicinal and supramolecular chemistry. In this work, we show that benzamidiniums hydrolyze at room temperature in aqueous base to give the corresponding primary amide. This reaction has a half-life of 300 days for unsubstituted benzamidinium at pH 9, but is relatively rapid at higher pH's (e.g., t1/2 = 6 days at pH 11 and 15 h at pH 13). Quantum chemistry combined with first-principles kinetic modeling can reproduce these trends and explain them in terms of the dominant pathway being initiated by attack of HO- on benzamidine. Incorporation of the amidinium motif into a hydrogen bonded framework offers a substantial protective effect against hydrolysis.

Selective Bond Cleavage in RAFT Agents Promoted by Low-Energy Electron Attachment.

Radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) has been successfully applied to generate polymers of well-defined architecture. For RAFT polymerization a source of radicals is required. Recent work has demonstrated that for minimal side-reactions and high spatio-temporal control these should be formed directly from the RAFT agent or macroRAFT agent (usually carbonothiosulfanyl compounds) thermally, photochemically or by electrochemical reduction. In this work, we investigated low-energy electron attachment to a common RAFT agent (cyanomethyl benzodithioate), and, for comparison, a simple carbonothioylsulfanyl compound (dimethyl trithiocarbonate, DMTTC) in the gas phase by means of mass spectrometry as well as quantum chemical calculations. We observe for both compounds that specific cleavage of the C-S bond is induced upon low-energy electron attachment at electron energies close to zero eV. This applies even in the case of a poor homolytic leaving group (. CH3 in DMTTC). All other dissociation reactions found at higher electron energies are much less abundant. The present results show a high control of the chemical reactions induced by electron attachment.

Exploiting Chitin as a Source of Biologically Fixed Nitrogen: Formation and Full Characterization of Small-Molecule Hetero- and Carbocyclic Pyrolysis Products.

Pyrolysis of variously pretreated or untreated samples of chitin (1) and certain congeners at 150-350 °C afforded a range of platform molecules, as exemplified by compounds 4, 5, 6, 8, 12 and 13. All of these products have been fully characterized, including by single-crystal X-ray analysis. Pathways for the formation of them are proposed and theoretical studies of certain aspects of these described.

High-Pressure-Promoted and Facially Selective Diels-Alder Reactions of Enzymatically Derived cis-1,2-Dihydrocatechols and Their Acetonide Derivatives: Enantiodivergent Routes to Homochiral and Polyfunctionalized Bicyclo[2.2.2]octenes.

cis-1,2-Dihydrocatechols 5 (X = Me and Cl), which are available in the homochiral form through the whole-cell biotransformation of toluene and chlorobenzene, respectively, undergo Diels-Alder cycloaddition reactions with a range of electron-deficient dienophiles at 19 kbar (1.9 GPa). The favored products of such reactions are adducts of the general form 7 and that arise through the operation of a contrasteric or syn-addition pathway. In contrast, the acetonide derivatives of metabolites 5 undergo anti-selective addition reactions under the same conditions and so producing adducts of the general form 11. Bicyclo[2.2.2]octenes 7 and 11, which embody carbocyclic frameworks of opposite enantiomeric form, are useful scaffolds for chemical synthesis. Computational studies reveal that syn-adduct formation is kinetically and normally thermodynamically favored over anti-adduct formation when the free diols 5 are involved, but the reverse is so when the corresponding acetonides participate as the 4π-addend. Furthermore, the reactions become more exothermic as pressure increases while, concurrently, the activation barrier diminishes and at 6 GPa (60 kbar) almost vanishes.

Atomistic simulations of the aggregation of small aromatic molecules in homogenous and heterogenous mixtures.

The relatively weak London dispersion forces are the only interactions that could cause aggregation between simple aromatic molecules. The use of molecular dynamics and high-level ab initio computer simulations has been used to describe the aggregation and interactions between molecular systems containing benzene, naphthalene and anthracene. Mixtures containing one type of molecule (homogenous) and more than one type of molecule (heterogenous) were considered. Our results indicate that as molecular weight increases so does the temperature at which aggregation will occur. In all simulations, the mechanism of aggregation is through small clusters coalescing into larger clusters. The structural analysis of the molecules within the clusters reveals that benzene will orient itself in T-shaped and parallel displaced configurations. Molecules of anthracene prefer to orient themselves in a similar manner to a bulk crystal with no T-shaped configuration observed. The aggregation of these aromatic molecules is discussed in the context of astrochemistry with particular reference to the dust formation region around stars.

Electrostatic Switching between SN1 and SN2 Pathways.

A test set 264 nucleophilic substitution reactions was studied via accurate quantum chemical reactions to establish the relative preferences for SN1 versus SN2 mechanisms. In low polar solvents, reactions involving anionic nucleophiles and leaving groups favored SN2 pathways. In contrast, SN1 is preferred for those reactions involving neutral nucleophiles and leaving groups except where the carbocation intermediates are exceptionally unstable. For neutral nucleophiles and anionic leaving groups, SN2 is generally preferred over SN1 except for exceptionally stable carbocation intermediates. On the basis of these studies, candidate reactions for which distinct SN1 or SN2 preferences could be reversed by electric fields were selected. As proof of concept, the SN1/SN2 preferences for the reaction of tBu-triflate with pyridine (SN2 to SN1) and with piperidine (SN1 to SN2) were switched by both charged functional groups and point charges (i.e., electric fields) along the reaction axis, with a positive charge on the nucleophile side favoring SN1 and a negative charge favoring SN2 for these reactions.