Kinetic Experiments Research Articles

Steering diffusion selectivity of chemical isomers within aligned nanochannels of metal-organic framework thin film

The movement of molecules (i.e. diffusion) within angstrom-scale pores of porous materials such as metal-organic frameworks (MOFs) and zeolites is influenced by multiple complex factors that can be challenging to assess and manipulate. Nevertheless, understanding and controlling this diffusion phenomenon is crucial for advancing energy-economic membrane-based chemical separation technologies, as well as for heterogeneous catalysis and sensing applications. Through precise assessment of the factors influencing diffusion within a porous metal-organic framework (MOF) thin film, we have developed a chemical strategy to manipulate and reverse chemical isomer diffusion selectivity. In the process of cognizing the molecular diffusion within oriented, angstrom-scale channels of MOF thin film, we have unveiled a dynamic chemical interaction between the adsorbate (chemical isomers) and the MOF using a combination of kinetic mass uptake experiments and molecular simulation. Leveraging the dynamic chemical interactions, we have reversed the haloalkane (positional) isomer diffusion selectivity, forging a chemical pathway to elevate the overall efficacy of membrane-based chemical separation and selective catalytic reactions.

Tuning the Properties of Rigidified Acyclic DEDPA2- Derivatives for Application in PET Using Copper-64.

We present a detailed investigation of the coordination chemistry toward [natCu/64Cu]copper of a series of H2DEDPA derivatives (H2DEDPA = 6,6'-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid) containing cyclohexyl (H2CHXDEDPA), cyclopentyl (H2CpDEDPA) or cyclobutyl (H2CBuDEDPA) spacers. Furthermore, we also developed a strategy that allowed the synthesis of a H2CBuDEDPA analogue containing an additional NHBoc group at the cyclobutyl ring, which can be used for conjugation to targeting units. The X-ray structures of the Cu(II) complexes evidence distorted octahedral coordination around the metal ion in all cases. Cyclic voltammetry experiments (0.15 M NaCl) evidence quasi-reversible reduction waves associated with the reduction of Cu(II) to Cu(I). The complexes show a high thermodynamic stability, with log KCuL values of 25.11(1), 22.18(1) and 20.19(1) for the complexes of CHXDEDPA2-, CpDEDPA2- and CBuDEDPA2-, respectively (25 °C, 1 M NaCl). Dissociation kinetics experiments reveal that both the spontaneous- and proton-assisted pathways operate at physiological pH. Quantitative labeling with 64CuCl2 was observed at 0.1 nmol for CHXDEDPA2- and CpDEDPA2-, 0.025 nmol for CBuDEDPA2- and 1 nmol for CBuDEDPA-NHBoc2-, with no significant differences observed at 15, 30, and 60 min. The radio-complexes are stable in PBS over a period of 24 h.

Effects of dissolved organic matter from different sources on ritonavir photolysis

With the misuse of antiviral drugs, the residual levels of ritonavir (RTV) in aquatic environments continue to increase, potentially posing threats to ecosystems and human health. However, the current understanding of the photochemical behavior of RTV in water, especially the mechanism by which dissolved organic matter (DOM) from different sources affects the indirect photolysis of RTV, remains limited. This study systematically investigated the effects of DOM from different sources (including sludge, algae, dustfall, and soil, namely SL-DOM, AL-DOM, DF-DOM, and SO-DOM, respectively) on the photodegradation of RTV for the first time. DOM exhibited a dual role in RTV degradation, with SL-DOM and AL-DOM accelerating the degradation process, while DF-DOM and SO-DOM inhibited it. Direct photolysis accounted for 40–53% of the overall photodegradation, underscoring its significant contribution to the degradation process. Quenching and competitive kinetics experiments revealed that 3DOM⁎ is the dominant contributor to the indirect photolysis of RTV. Exogenous DOM (DF-DOM, SO-DOM) exhibited higher generation rate and steady-state concentraiton of 3DOM⁎, while endogenous DOM (SL-DOM, AL-DOM) exhibited higher quantum yields of 3DOM⁎ and reactivity, leading to distinct mechanisms for the indirect photodegradation of RTV. This study explored the effects of DOM from different sources on the photodegradation of RTV, providing important insights into how DOM affects the photochemical behavior and ecological risk of RTV. It also provides a reference for exploring the photochemical behavior of other drugs.

Modulating the Aggregation States of a Pd6L4 Cage for Selectivity Flipping during the Stereo‐Divergent Semi‐Hydrogenation of Alkynes

An enzyme‐mimicking catalytic system has been established using a singular palladium‐based octahedral cage as the supramolecular reactor, deftly unlocking off‐on‐off selectivity in the semi‐hydrogenation of alkynes. Water serves as a critical regulator, modulating the catalyst states, reaction rates, and endpoints. The choice of solvent system influences the activity of host‐guest binding and the reaction types of homogeneous and heterogeneous catalysis, effectively modifying the reaction steps involved in the Z→E isomerization during the semi‐hydrogenation of alkynes. Kinetic and inhibition experiments indicate that the catalyst mimics the binding and activation characteristics of enzymes towards substrates, enabling selective transformations within the confined enzyme‐mimicking environment. The utility of this switchable cage‐confined catalysis has been demonstrated in the synthesis and modification of complex biologically active molecules with controllable E/Z selectivity. This work sheds light on the design and control of artificial supramolecular counterparts of enzymes, offering fundamental insights into the factors influencing the activity and catalytic selectivity of biological macromolecules.

Modulating the Aggregation States of a Pd6L4 Cage for Selectivity Flipping during the Stereo-Divergent Semi-Hydrogenation of Alkynes.

An enzyme-mimicking catalytic system has been established using a singular palladium-based octahedral cage as the supramolecular reactor, deftly unlocking off-on-off selectivity in the semi-hydrogenation of alkynes. Water serves as a critical regulator, modulating the catalyst states, reaction rates, and endpoints. The choice of solvent system influences the activity of host-guest binding and the reaction types of homogeneous and heterogeneous catalysis, effectively modifying the reaction steps involved in the Z→E isomerization during the semi-hydrogenation of alkynes. Kinetic and inhibition experiments indicate that the catalyst mimics the binding and activation characteristics of enzymes towards substrates, enabling selective transformations within the confined enzyme-mimicking environment. The utility of this switchable cage-confined catalysis has been demonstrated in the synthesis and modification of complex biologically active molecules with controllable E/Z selectivity. This work sheds light on the design and control of artificial supramolecular counterparts of enzymes, offering fundamental insights into the factors influencing the activity and catalytic selectivity of biological macromolecules.

Hierarchically-porous resin for the adsorption of organic matter in raffinate produced by solvent extraction from the vanadium industry

Solvent extraction wastewater contains high concentrations of organic pollutants and metal ions, making it difficult to treat. This paper presents a method of using hierarchically-porous adsorption resin to recover organics from raffinate and regenerate the resin, addressing secondary pollution in extraction processes. The chosen resin, WS6108, removes over 95 % of organics from raffinate, with high metal ion concentrations enhancing its adsorption efficiency. The article conducted an optimization experiment and mechanism study on WS6108 resin for adsorbing organic matter in vanadium raffinate (OIVR), followed by desorption and recycling tests. Results showed WS6108 resin achieved a 96.7 % adsorption efficiency at a solid–liquid ratio of 24 g/L after optimization. The chosen methanol desorbent achieves over 99 % desorption efficiency for the WS6108 cycle under specific conditions. The resin’s performance remains stable for up to 45 cycles. Adsorption isotherms and kinetic experiments indicate that OIVR adsorption by WS6108 resin is endothermic, non-spontaneous and entropy increasing. Moreover, the adsorption kinetics of OIVR by WS6108 resin followed a pseudo-second-order model, based on the activation energy, liquid film diffusion was considered the main rate-controlling step. A column experiment was conducted to verify that WS6108 resin can be used in real vanadium raffinate to remove organics. Density functional theory (DFT) revealed that strong hydrogen bonding, π-π stacking and excellent adsorption diffusion behavior drive OIVR adsorption. This study aims to increase the use of adsorption to treat oily wastewater generated by solvent extraction.

Degradation study of δ-hexachlorocyclohexane by iron sulfide nanoparticles: elucidation of reaction pathway using compound specific isotope analysis and pH variation

pH influences the reactivity of iron (II) minerals towards halogenated pollutants like hexachlorocyclohexanes (HCHs). To explore these incompletely understood interactions, we investigated the carbon isotope fractionation of the δ-HCH isomer during dehalogenation by iron sulfide at pHs spanning a pH range across slightly acidic to alkaline domains (5.8 to 9.6). The δ-1,3,4,5,6-pentachlorocyclohex-1-ene (δ-PCCH) was the intermediate degradation product, while benzene, monochlorobenzene (MCB), but especially 1,2-dichlorobenzene (1,2-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), were the main degradation products of δ-HCH. These degradation products suggested dehydrochlorination as the main degradation pathway of δ-HCH by iron sulfide. Different kinetic experiments indicate that the rate constants (ka) during dechlorination of δ-HCH by iron sulfide rose with pH: 0.003 d-1 (pH 5.8) < 0.034 d-1 (pH 8) < 0.085 (pH 9.3) < 0.286 d-1 (pH 9.6). Upon Rayleigh model calculations, an enrichment factor (εC) of -7.8 ± 1.0 ‰ was calculated for δ-HCH dehalogenation by FeS at pH 8.0. This suggests an apparent kinetic isotope effect (AKIEC) value of 1.049 ± 0.006 for dehydrohalogenation. The magnitude of the isotope effect from this paper furthermore supports dehydrohalogenation and opens the possibility to study the degradation of HCHs by iron (II) minerals containing FeS as mackinawite in oxygen-deprived environments.

Efficient activation of peroxymonosulfate by 3D blocky sponge-supported spinel ferrite nanocomposites for convenient removal of aniline

Aniline, a toxic aromatic amine prevalent in printing and dyeing effluents, has garnered significant attention due to its potential risks to human health and the environment. In this study, 3D blocky melamine sponge-supported spinel ferrite nanocomposites (MFe2O4@MS) were synthesized to activate peroxymonosulfate (PMS) for the removal of aniline from printing and dyeing wastewater. The CoFe2O4@MS-PMS achieved complete removal of aniline within 10 min across a pH range of 5 to 11, significantly outperforming the CuFe2O4@MS-PMS and ZnFe2O4@MS-PMS systems. Electron paramagnetic resonance (EPR) and probe-based kinetic model experiments certified the formation of sulfate radicals (•SO4−), hydroxyl radicals (•OH), and singlet oxygen (1O2) during the process, where •SO4− was identified as the principal reactive species responsible for aniline degradation. The Langmuir-Hinshelwood model demonstrated that the melamine sponge framework significantly improved the dispersion and adsorption capabilities of MFe2O4 nanoparticles. This enhancement led to localized enrichment of aniline and PMS on the surface. Furthermore, the removal of aniline by CoFe2O4@MS-PMS performed satisfactorily in five-cycle experiments and in actual printing and dyeing wastewater. This research offers new insights into the design and synthesis of environmentally friendly and highly active PMS catalysts for aniline degradation in industrial wastewater.

Atmospheric Degradation of CH2═C(CH3)C(O)O[CH2]4CH3 by •OH Radicals: Reactivity, POCP, and Carbonyl Formation.

Relative rate studies of the gas-phase reaction of amyl methacrylate, CH2═C(CH3)C(O)O[CH2]4CH3, with •OH radicals were performed at (298 ± 2) K and 1000 mbar. The experiments were conducted in an atmospheric Pyrex chamber coupled to in situ Fourier transform infrared spectroscopy (FTIR). The rate coefficient obtained from the average of several experiments was kAMMA+•OH = (8.10 ± 1.98) × 10-11 cm3 molecule-1 s-1. Additionally, product studies were conducted under conditions similar to those of the kinetic experiments by using in situ FTIR spectroscopy. Pentanal, butanal, and hydroxyacetone were identified as the main reaction products. The initial pathway for the degradation of amyl methacrylate with •OH radicals occurs via addition of •OH to the >C═C< bond or hydrogen abstraction from the alkyl chain of the ester. The likelihood of hydrogen atom abstraction is 25%, while the addition of hydroxyl radicals to the double bond occurs with a probability of 75%. Based on these outcomes, a degradation mechanism is postulated. Furthermore, the atmospheric implications of the studied reaction were evaluated by estimating the tropospheric lifetime of amyl methacrylate toward •OH radicals as τOH = 3.43 h. Additionally, the Photochemical Ozone Creation Potential (POCP) of 84 was calculated for the reaction studied. Carbonyl compounds found as reaction products can exert a substantial influence on both air quality and public health.

Selective diesterase-like activity of bioinspired FeIIINiII and FeIIIZnII complexes and their 3-aminopropyl silica composites: A homo- and heterogeneous catalytic study

In this study, the preparation and characterization of two new mixed-valence heterodinuclear complexes [FeIIINiII(BPPAMFF)(μ-OAc)2(H2O)]ClO4 (1) and [FeIIIZnII(BPPAMFF)(μ-OAc)2(H2O)]ClO4 (2) with the bioinspired ligand 2-[(N-benzyl-N-2-pyridylmethylamine)]-4-methyl-6-[N-(2-pyridylmethyl)aminomethyl)])-4-methyl-6-formylphenol (H2BPPAMFF) are reported. These compounds were immobilized in 3-aminopropyl silica (APS) to afford composites APS-1 and APS-2 successfully. The aldehyde-containing ligand provided a reactive functional group, which could serve as a cross-linking group to bind the complexes to the directly amino-modified SiO2 surface. The complexes’ chemical integrity on the APS inorganic platform were probed by spectroscopical techniques, such as FTIR, UV–Vis and EPR. Potentiometric and spectrophotometric titrations allowed the chemical species present in solution to be rationalized, and identified which of them were potentially active in the hydrolytic cleavage of phosphodiester 2,4-BDNPP. Kinetic studies showed that FeIIINiII species (1 and APS-1) presented higher catalytic efficiency (E = kcat/KM) than FeIIIZnII species (2 and APS-2). Catalytic mechanisms were proposed based on a series of kinetic experiments, in which all the catalysts tested behaved as selective phosphodiesterases. In addition, it was also demonstrated that the hydrolase activity of the immobilized catalytic centers, APS-1 and APS-2, was better than the homogeneous processes, where second coordination sphere effects may be involved in directing and stabilizing the transition state.

Propane Dehydrogenation on PtxZny Active Sites in Silicalite‐1

AbstractThe improvement of Pt‐based catalysts for propane dehydrogenation (PDH) has progressed by recent investigations that have identified Zn as a promising promoter for Pt subnanometer catalysts. It is desirable to gain insights into the structure, stability, and activity of such active sites and the factors that influence them, such as Zn : Pt ratio, Pt coordination and nuclearity. Here, we employ density functional theory and microkinetic simulations to investigate the stability of PtxZny (x=1–3, y=0–3) active sites grafted on silanols of Silicalite‐1 and the PDH activity of Pt. We find that the coordination of a Pt atom to a nest of grafted Zn(II) atoms increases the stability of the Pt1Zny sites, whose activity is similar for y=0–2 and drops dramatically for y&gt;2. We further demonstrate, via linear scaling relations and microkinetic simulations, that the turnover frequency obeys a volcano law as a function of propylene binding strength. The Pt2Zn1 and Pt3Zn1 sites are stable and exhibit activity similar to Pt1Zn2, but only Pt1Zn2 manifests reaction kinetics consistent with experimental data, strongly suggesting the active site composition in the synthesized catalyst samples. The methodology presented here suggests a general strategy for deducing active site information such as composition through simple kinetic experiments.

Selective Monoborylation of Methane by a Mono Bipyridyl‐Nickel(II) Hydride Catalyst

We report the development of an earth‐abundant metal catalyst for methane C‒H borylation. The post‐synthetic metalation of bipyridine‐functionalized zirconium metal−organic framework (MOF) with NiBr2, followed by treatment of NaEt3BH affords MOF‐supported monomeric bipyridyl‐nickel(II) dihydride species via active site isolation. The heterogeneous and recyclable nickel catalyst selectively borylates methane at 200 ºC using pinacolborane (HBpin) to afford CH3Bpin in 61% yield with a turnover number (TON) up to 1388. The confinement of the active NiH2‐species within the uniformly porous MOF allows selective monoborylation of methane via shape‐selective catalysis by preventing the formation of sterically encumbered overborylated products. Unlike MOF‐Ni catalyst, its homogeneous control is almost inactive in methane borylation due to its intermolecular decomposition. Our mechanistic investigation, including spectroscopic, kinetic, and control experiments, as well as DFT calculations, revealed that stabilizing mononuclear bipyridyl‐nickel dihydride and diboryl species by MOF is crucial for achieving efficient methane borylation via turnover‐limiting σ‐bond metathesis. This work shows promise in designing MOF‐based abundant metal catalysts for the chemoselective functionalization of methane and other inert molecules into valuable chemicals.

Selective Monoborylation of Methane by a Mono Bipyridyl-Nickel(II)Hydride Catalyst.

We report the development of an earth-abundant metal catalyst for methane C‒H borylation. The post-synthetic metalation of bipyridine-functionalized zirconium metal-organic framework (MOF) with NiBr2, followed by treatment of NaEt3BH affords MOF-supported monomeric bipyridyl-nickel(II) dihydride species via active site isolation. The heterogeneous and recyclable nickel catalyst selectively borylates methane at 200 ºC using pinacolborane (HBpin) to afford CH3Bpin in 61% yield with a turnover number (TON) up to 1388. The confinement of the active NiH2-species within the uniformly porous MOF allows selective monoborylation of methane via shape-selective catalysis by preventing the formation of sterically encumbered overborylated products. Unlike MOF-Ni catalyst, its homogeneous control is almost inactive in methane borylation due to its intermolecular decomposition. Our mechanistic investigation, including spectroscopic, kinetic, and control experiments, as well as DFT calculations, revealed that stabilizing mononuclear bipyridyl-nickel dihydride and diboryl species by MOF is crucial for achieving efficient methane borylation via turnover-limiting σ-bond metathesis. This work shows promise in designing MOF-based abundant metal catalysts for the chemoselective functionalization of methane and other inert molecules into valuable chemicals.

Exploring hydrogen isotope fractionation in lipid biomolecules of freshwater algae: implications for ecological and paleoenvironmental studies

ABSTRACT Understanding the stable hydrogen isotope (δ 2H) composition and fractionation in lipid biomolecules of primary producers, such as terrestrial and aquatic plants, is crucial for deciphering past environmental conditions, as well as applying compound-specific stable isotope analysis for the study of metabolic and ecological processes. We conducted a new tracer experiment to explore the δ 2H composition of algal fatty acid biomarkers, focusing on freshwater algae, which form the base of aquatic food webs. We selected a range of algal species widely found in freshwater ecosystems and cultivated them under controlled conditions. First, we added 2H2O to ambient water as a tracer to investigate the net hydrogen isotope fractionation during algal lipid synthesis at isotopic equilibrium, which is particularly informative for paleo-geochemical studies. Then, we conducted kinetic experiments to quantify the time needed for algal fatty acids to achieve isotopic steady-state conditions in response to the change in ambient water δ 2H values. Our findings revealed substantial variability in hydrogen isotope fractionation among different algal taxa and various fatty acids. Based on taxa, different fatty acids exhibited faster integration of water hydrogen than others, but they were not necessarily in the order of the biosynthetic pathway. This experiment underscores the complexity of hydrogen isotope fractionation and the requirement for controlled laboratory studies to properly apply compound-specific stable H isotope analysis techniques in ecological and paleo-environmental studies.

Heteropolyacids combined with Y zeolite as superior solid acid catalysts to accelerate lactic acid esterification

AbstractBACKGROUNDEthyl lactate produced via esterification between lactic acid and ethanol can be used in plastics, polymers and food industries and renowned for its biodegradability and low toxicity. However, lactic acid self‐catalyzed esterification presents a slow reaction process, and acid catalysts are needed to improve the catalysis. Homogeneous acid catalysts like H2SO4 are corrosive, as well as being difficult to be seperated from the reaction media; therefore, advanced heterogeneous catalysts are more ideal. In this study, two kinds of heteropolyacid–zeolite catalysts, involving loading silicotungstic acid and phosphotungstic acid onto Y zeolite, were synthesized, characterized using X‐ray diffraction, NH3 temperature‐programmed desorption and scanning electron microscopy and analyzed for their catalytic efficiency in the catalytic esterfication.RESULTSResults showed that the Keggin structures of heteropolyacids were retained during the preparation process and strong acid sites of Y zeolite were enriched, which made the main contribution to the esterification of lactic acid and ethanol. Furthermore, the initial‐stage reaction rate of esterification was significantly enhanced. During the first hour of esterification, the conversion of lactic acid increased from 18% to 65%. Exp Dec1 index model was employed to determine the activation energies of 60HSiW‐Y and 60HPW‐Y, which exhibit values of 29.3 and 28.2 kJ mol−1, respectively.CONCLUSIONThis study indicates that Keggin structures of heteropolyacids play a key role in the esterification and heteropolyacid–zeolite catalysts can effectively catalyze the esterification. Meanwhile, the results of the kinetic experiments can be of reference significance for large‐scale industrial processes involving esterification. © 2024 Society of Chemical Industry (SCI).

Biogenic fabrication of spinel nickel ferrite imprinted on Bifurcaria bifurcata Macro-Alga activated carbon for the adsorption of ciprofloxacin and metronidazole

The rise in globalization and industrialization has led to an increase in population, increasing improperly treated discharge containing dyes, pharmaceuticals, pesticides etc in the water bodies. This has led to the need to remove these contaminants from water sources. The NiFe@BBAL adsorbent was developed and analyzed using various techniques such as FTIR, XRD, SEM, and XPS. The effectiveness of the NiFe@BBAL in adsorbing Ciprofloxacin (CIP) and metronidazole (MET) in water was tested. The adsorption of CIP and MET, both individually and in mixtures, was studied. The kinetic sorption experiments showed that the adsorption followed a pseudo-second-order model, with a higher R2 of 0.9934 for MET compared to 0.9916 for CIP. This suggests that the rate-limiting step is chemisorption. The primary adsorption mechanisms for both CIP and MET were hydrogen bonding, hydrophobic interactions, and electrostatic interactions, while CIP also involved electrostatic and metal complexation interactions. The NiFe@BBAL effluent showed no toxic effects on bacteria, indicating that no harmful material was leached in the effluent. The NiFe@BBAL demonstrated excellent performance and stability in removing 97.64% CIP and 97.62% MET in single contaminant experiments, and 57.33% CIP and 70.69% MET in cocktail mixture experiments. This can be attributed to the smaller structure of MET compared to CIP, allowing it better access to the active site, leading to higher adsorption. Furthermore, the repeated use of the adsorbent proved to be stable in the removal of CIP and MET over five cycles, demonstrating that the material is sustainable and suitable for large-scale experiments.

Uranium removal from contaminated groundwater using goethite-loaded composite microporous membrane

In this study, we have coupled adsorption and membrane separation for the removal of uranium from contaminated groundwater in environmentally relevant conditions at low energy requirements. This study mainly focuses on elucidating uranium, U(VI), adsorption mechanisms using surface complexation modeling approach in a novel goethite-loaded composite microfiltration membrane (GLM). This experiment involved immobilizing goethite nanorods in a microporous (0.22 μm pore size) poly (vinylidene fluoride) (PVDF) membrane. The effect of varying goethite loading and hydraulic residence time on U(VI) removal was investigated at field-relevant pH (i.e. pH 8.5). U(VI) adsorption (i.e. 4.95 μg·mg−1) was optimum at a goethite loading 1.20 mg·cm−2. The effect of varying hydraulic residence time had no impact on U(VI) removal which was also confirmed via performing batch adsorption kinetic experiments. GLM membrane loaded at 1.2 mg·cm−2 could treat 275 L of U(VI) contaminated water having 200 μg of U L−1 below WHO drinking water limit (i.e. 30 μg of U L−1) with 1 m2 of membrane surface area with a adsorption capacity of 6.12 μg·mg−1. Varying the pH of aqueous solution, containing U(VI) from pH 4.0 to pH 10.0, showed a significant impact on uranium uptake ranging from 0.7 μg·mg−1 to 2.63 μg·mg−1 by the composite membrane. The adsorption mechanism of uranium onto goethite was explained via the formation of bidentate surface complexes using the Surface Complexation Model (SCM). The results of batch pH edge experiments and SCM have been compared with pH experiments performed using GLM. The results of SCM predicted the batch pH edge experiment within a RMSE of 0.055. The trend of U(VI) removal in membrane experiments was observed to be similar to that of batch pH edge experiments and was well predicted SCM model. Our results showed that the novel goethite-loaded membrane has the potential for the effective removal of uranium with a lower specific energy consumption.

Radical‐driven imine hydrogenation promoted by the cooperativity of nickel and a redox‐active ligand

In the case of N‐alkylation reaction via borrowing hydrogen (BH) methods, imine hydrogenation is one of the critical steps. Mechanistically, the majority of imine hydrogenation, either by dihydrogen gas or by alcohol in case of BH methods, follow two‐electron chemistry, where a metal‐hydride intermediate plays a pivotal role. Herein we demonstrate a completely complementary protocol, where the hydrogenation is governed by a radical pathway. Such a pathway is adopted from the redox active nature of the azophenolate ligand backbone. The facile and reversible 2e‐/2H+, azo/hydrazo redox couple in a nickel catalyst helps in storing the hydrogen from alcohol dehydrogenation. An imine substrate is reduced by one‐electron from the azo motif with the assistance of the nickel center. The reduced imine drives a critical hydrogen atom transfer step from the hydrazo motif to conduct the hydrogenation. A set of kinetics experiments including Eyring analysis, radical inhibition experiment along with computational probation attest for the radical‐promoted hydrogenation mechanism.

Activation and Deactivation of Chirality Transfer in the Superbundles of Sequence-defined Stereoisomers.

Discrete oligomers can be used to precisely evaluate the structure-property relationship and enable unique chiroptical activities, however, the role of stereochemical sequences in chirality transfer is still unclear. Herein, we report the successful synthesis of a series of sequence-defined chiral azobenzene (Azo) oligomers via iterative stepwise chain growth strategy, and the discovery of controllable activation and deactivation of chirality transfer. Sequence-defined stereoisomers with one single chiral (L or D) stereocenter at the α-position, ω-position and middle- (m-) position have completely different self-assembly dynamics and chirality transfer mechanisms. ω-positional stereocenter can effectively command all Azo building blocks to adopt a tilted π-π stacking along the helical superbundles, exhibiting the activation of chirality transfer. However, discrete oligomers with the stereocenter at other positions can only self-assemble into non-helical nanowires, accompanied by the deactivation of chirality transfer. Kinetic experiments as well as chemical simulations indicate that the cooperative supramolecular interaction, including the π-π interaction between repeated Azo units, the intermolecular hydrogen bonding and steric hindrance, are intrinsic driving forces for these differentiations. This study has shed light on the unique self-assembled kinetics in stereosequential oligomers and the mechanism of chirality induction at hierarchical levels.

Activation and Deactivation of Chirality Transfer in the Superbundles of Sequence‐defined Stereoisomers

Discrete oligomers can be used to precisely evaluate the structure‐property relationship and enable unique chiroptical activities, however, the role of stereochemical sequences in chirality transfer is still unclear. Herein, we report the successful synthesis of a series of sequence‐defined chiral azobenzene (Azo) oligomers via iterative stepwise chain growth strategy, and the discovery of controllable activation and deactivation of chirality transfer. Sequence‐defined stereoisomers with one single chiral (L or D) stereocenter at the α‐position, ω‐position and middle‐ (m‐) position have completely different self‐assembly dynamics and chirality transfer mechanisms. ω‐positional stereocenter can effectively command all Azo building blocks to adopt a tilted π‐π stacking along the helical superbundles, exhibiting the activation of chirality transfer. However, discrete oligomers with the stereocenter at other positions can only self‐assemble into non‐helical nanowires, accompanied by the deactivation of chirality transfer. Kinetic experiments as well as chemical simulations indicate that the cooperative supramolecular interaction, including the π‐π interaction between repeated Azo units, the intermolecular hydrogen bonding and steric hindrance, are intrinsic driving forces for these differentiations. This study has shed light on the unique self‐assembled kinetics in stereosequential oligomers and the mechanism of chirality induction at hierarchical levels.