Mechanism and Origins of Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis

The Ni-catalyzed alkylboration of endocyclic olefins is a stereo- and regioselective approach for the synthesis of boron-containing compounds. We report a detailed density functional theory (DFT) study to elucidate the mechanism and origins of the stereo-, chemo-, and regioselectivity of alkylboration of endocyclic olefins enabled by nickel catalysis. The alkylboration proceeds via the migratory insertion of alkenes, β-H elimination of the Ni(II) complex, subsequent migratory insertion leading to a new Ni(II) complex, combined with an alkyl radical, and reductive eliminations. The electronic effects of the endocyclic olefins synergistically control the regioselectivity toward the C1- and C2-position boration. In C1-position boration, a more electron-deficient carbon atom tends to combine with an electron-rich –Bpin group and leads to C1-position boration products. The stereoselectivity is influenced by the solvent effect, and the interaction between the substrate and Ni-catalyzed groups, the low-polarity solvent 1,4-dioxane, and a favorable steric hindrance effect result in the cis-alkylboration product. Chemoselectivity toward 1,3-alkylboration results from the steric hindrance effects of the –Bpin group

Superior Reversible Hydrogen Storage Properties and Mechanism of LiBH₄–MgH₂–Al Doped with NbF₅ Additive

LiBH₄ is one of the most potential candidates for hydrogen storage materials among several sorts of complex borohydrides. Utilizing reactive hydride composites on LiBH₄ could destabilize the thermodynamics and improve dehydrogenation behaviors, such as the excellent reversibility of LiBH₄–MgH₂ and the fast dehydrogenation of LiBH₄–Al. The strategy of combining both outstanding effects of MgH₂ and Al to form LiBH₄–MgH₂–Al system has been proposed. However, reduction of hydrogen capacity during cycles has not been solved for the LiBH₄–MgH₂–Al system, which is considered as the principal problem. In this work, we investigated the reversible hydrogen storage performance and reaction mechanism of LiBH₄–MgH₂–Al doped with/without NbF₅ additive. It can be found that the dehydrogenation of 4LiBH₄–MgH₂–Al can release about 9.0 wt % H₂ quickly without incubation period, compared with 2LiBH₄–MgH₂. Moreover, it is the first time to achieve completely reversible hydrogen desorption property of LiBH₄–MgH₂–Al by doping with NbF₅ and dehydrogenating under hydrogen back pressure in experiment. Microstructure analysis shows that the formation of Mg–Al alloys could result in the formation of Li₂B₁₂H₁₂ and subsequently lead to the capacity degradation. With the additive NbF₅, it shows a totally different pathway and a significant inhibition effect on the alloying between Mg and Al, leading to an improved de/rehydrogenation behavior without the byproduct Li₂B₁₂H₁₂. Meanwhile, NbF₅ could be hydrogenated into NbH₂ and react with element B to form NbB₂, promoting the reaction between Mg/Al metals and B element to form MgAlB₄. On the other hand, those niobium compounds could facilitate the products MgAlB₄ and LiH to be fully rehydrogenated into LiBH₄, MgH₂, and Al, which contributes to the complete reversibility of LiBH₄–MgH₂–Al. A better understanding of the capacity fade mechanism of LiBH₄–MgH₂–Al system and the effects of additives might promote further development of high-capacity hydrogen storage materials

Machine Learning-Assisted Screening of Cu-Based Trimetallic Catalysts for Electrochemical Conversion of CO₂ to CO

The electrochemical reduction of carbon dioxide to useful chemicals and fuels is a new strategy to utilize large amounts of carbon dioxide. However, the lack of efficient catalysts has hindered the development of this technology. Herein, a machine learning (ML)-assisted screening model is developed to explore efficient trimetallic electrocatalysts for the CO2 reduction reaction by combining with density functional theory (DFT) and electrochemical experiments. The group of doped elements in the periodic table is the most important descriptor of Cu-based trimetallic electrocatalysts and the support vector regression algorithm has the best predictive performance. Based on ML predictions, the overpotential of PdPt@Cu is successfully predicted to be 0.11 V, and it shows the best electrocatalytic performance for the CO2 reduction reaction (CO2RR). DFT calculation results show that CO2 → COOH* is the potential-limiting step of CO2RR-to-CO for PdPt@Cu and its overpotential is 0.09 V, which is consistent with the ML-predicted results. The electrochemical experiments show that the Faraday efficiency of CO is 82.12% at −0.8 V vs RHE for PdPt@Cu. After 12 h of electrolysis in the H-cell, the catalyst still maintains good catalytic performance. This work provides an efficient method for screening catalysts

AuPd Nanoparticles Anchored on Nitrogen-Decorated Carbon Nanosheets with Highly Efficient and Selective Catalysis for the Dehydrogenation of Formic Acid

Formic acid (FA), a sustainable and safe hydrogen storage vector, has the advantages of nontoxicity, high hydrogen content (4.4 wt %) and low cost. However, the dehydrogenation of formic acid at near room temperature remains a big challenge in terms of favorable hydrogen release rate and CO-absence hydrogen production. Herein, a series of nitrogen-decorated carbon nanosheets (n-CNS) supported AuPd nanoparticles (NPs) were designed and employed as efficient catalysts to dehydrogenate FA for the first time. The catalyst AuPd NPs supported on n-CNS synthesized at hydrothermal temperature of 160 °C (AuPd/n-CNS-T_h-160) exhibits excellent catalytic activity toward the dehydrogenation of FA compared with AuPd/g-carbon nitride (AuPd/g-C₃N₄) and commercial Pd/C catalysts, reaching an initial turnover frequency (TOF) of 527 h^–1, 100% hydrogen generation and selectivity at room temperature (25 °C), while the TOF achieves even 1896 h^–1 at 60 °C. The enhanced catalytic performance can be attributed to the coordinated effect from Au–Pd alloying and the doped nitrogen atoms on carbon nanosheets. It is the first time to systematically probe the promoting mechanism of nitrogen on FA dehydrogenation. It is also illustrated that the promoting mechanism of N atoms on carbon nanosheets results from its nitrogen-bonding configuration, specifically, the ratio between graphitic N and pyridinic N. The high ratio of graphitic N to pyridinic N can modify the distribution of electron density and minimize the size of the metal nanoparticles, thereby greatly enhances the catalytic effect. The present study, by varying the catalysts’ composition and regulating the active material to boost the catalytic performance, provides a general pathway to further enhance the efficiency of hydrogen generation strongly depending on the properties of the support

ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH 2 and synergetic catalysis mechanism

© 2018 Hydrogen Energy Publications LLC Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH 2 , yet this catalysis could be enhanced further by supporting NPs on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently doped into MgH 2 to remarkably improve its hydrogen storage properties. The MgH 2 -Co@CNTs starts to obviously release hydrogen at 267.8 °C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 °C within 15 min. For absorption, the MgH 2 -Co@CNTs uptakes 6.15 wt% H 2 at 250 °C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH 2 -Co@CNTs can be well preserved during the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based on XRD, TEM and EDS results, the catalytic mechanism of MgH 2 -Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg 2 Co to Mg 2 CoH 5 , and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg 2 Co/Mg 2 CoH 5 can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH 2 . Meanwhile, the carbon pieces coated on the surfaces of MgH 2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH 2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH 2 -Co@CNTs system

Immunohistochemistry of γH2AX expression in embryos.

<p>Changes in γ-H2AX expression in embryos fertilized by A: fresh sperm and B: sperm treated with H₂O₂. PI = propidium iodide staining.</p

Heterobimetallic Dinuclear Lanthanide Alkoxide Complexes as Acid–Base Difunctional Catalysts for Transesterification

A practical lanthanide­(III)-catalyzed transesterification of carboxylic esters, weakly reactive carbonates, and much less-reactive ethyl silicate with primary and secondary alcohols was developed. Heterobimetallic dinuclear lanthanide alkoxide complexes [Ln₂Na₈{(OCH₂CH₂NMe₂)}₁₂(OH)₂] (Ln = Nd (<b>I</b>), Sm (<b>II</b>), and Yb (<b>III</b>)) were used as highly active catalysts for this reaction. The mild reaction conditions enabled the transesterification of various substrates to proceed in good to high yield. Efficient activation of transesterification may be endowed by the above complexes as cooperative acid–base difunctional catalysts, which is proposed to be responsible for the higher reactivity in comparison with simple acid/base catalysts

Fluorescence intensity of γH2AX in nuclei among fresh control, H₂O₂ and cryopreserved sperm (A) and quantification (B).

<p>Data are mean±SD, *<i>P</i><0.05 compared with 0.1 mM H₂O₂. DAPI = staining of nuclei.</p

Funnel plot of the association between<i>STK39</i> rs3754777 variant and hypertension.

<p>Funnel plot of the association between<i>STK39</i> rs3754777 variant and hypertension.</p

Characteristics of the studies included in the meta-analysis.

a<p>And/or current under antihypertensive treatment.</p>b<p>In the lower third of the distribution of population blood pressures.</p>*<p>1, age; 2, sex; 3, BMI; 4, heart rate; 5, interaction variables; 6, follow-up years; 7,△-BMI; 8, type 2 diabetes; 9, smoking; 10, alcohol drinking; 11, creatinine; 12,triglyceride; 13, high density lipoprotein; 14,total cholesterol.</p