Lewis Basic Nitrogen Research Articles

Synergistic Linker and Linkage of Covalent Organic Frameworks for Enhancing Gold Capture.

The tunable pore walls and skeletons render covalent organic frameworks (COFs) as promising absorbents for gold (Au) ion. However, most of these COFs suffered from low surface areas hindering binding sites exposed and weak binding interaction resulting in sluggish kinetic performance. In this study, COFs have been constructed with synergistic linker and linkage for high-efficiency Au capture. The designed COFs (PYTA-PZDH-COF and PYTA-BPDH-COF) with pyrazine or bipyridine as linkers showed high surface areas of 1692 and 2076 m2 g‒1, providing high exposed surface areas for Au capture. In addition, the Lewis basic nitrogen atoms from the linkers and linkages are easily hydronium, which enabled to fast trap Au via coulomb force. The PYTA-PZDH-COF and PYTA-BPDH-COF showed maximum Au capture capacities of 2314 and 1810mg g-1, higher than other reported COFs. More importantly, PYTA-PZDH-COF are capable of rapid adsorption kinetics with achieving 95% of maximum binding capacity in 10min. The theoretical calculation revealed that the nitrogen atoms in linkers and linkages from both COFs are simultaneously hydronium, and then the protonated PYTA-PZDH-COF are more easily binding the AuCl4 ‒, further accelerating the binding process. This study gives the a new insight to design COFs for ion capture.

Photoactivated Pyridine Directed Fluorination through Hydrogen Atom Transfer.

We have established hydrogen atom transfer (HAT) as the key player in a directed, photopromoted fluorination of pyridylic groups. The Lewis basic pyridyl nitrogen directs amine radical dication propagated HAT and Selectfluor fluorination of various ortho substituents in a highly regioselective manner with little to no side product formation. A variety of pyridines and quinolines were employed to showcase the directing capability of the nitrogen atom. Additionally, both experimental and computational data are provided that illuminate how this mechanism differs from and complements prior work in the area.

Dual-Ligand Catalyst for the Nondirected C-H Olefination of Heteroarenes.

Pd(II)-catalyzed nondirected C-H functionalization of heteroarenes is a significant challenge for the following reasons: poor reactivity of electron-deficient heterocycles and the unproductive coordination of Lewis basic nitrogen atoms. Existing methodologies using palladium catalysis often employ a large excess of heterocycle substrates to overcome these hurdles. Despite recent advances in nondirected functionalization of arenes that allow them to be used as limiting reagents, the reaction conditions are incompatible with electron-deficient heteroarenes. Herein we report a dual-ligand catalyst that enables Pd(II)-catalyzed nondirected C-H olefination of heteroarenes without using a large excess of substrate. In general, the use of 1-2 equiv of substrates was sufficient to obtain synthetically useful yields. The reactivity was rationalized by the synergy between two types of ligands: a bidentate pyridine-pyridone ligand promotes C-H cleavage; the monodentate heterocycle substrate acts as a second ligand to form a cationic Pd(II) complex that has high affinity for arenes. The proposed dual-ligand cooperation is supported by a combination of X-ray, kinetics, and control experiments.

Site‐Selective C−H Alkenylation of N‐Heteroarenes by Ligand‐Directed Co/Al and Co/Mg Cooperative Catalysis**

AbstractWe report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site‐selective C−H alkenylation of nitrogen‐containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2‐a]pyridine derivatives at the C−H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt‐diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C−H activation step, the relevance of which to the rate‐limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand‐to‐ligand hydrogen transfer is the common mechanism of C−H activation.

Site-Selective C-H Alkenylation of N-Heteroarenes by Ligand-Directed Co/Al and Co/Mg Cooperative Catalysis.

We report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site-selective C-H alkenylation of nitrogen-containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2-a]pyridine derivatives at the C-H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt-diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C-H activation step, the relevance of which to the rate-limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand-to-ligand hydrogen transfer is the common mechanism of C-H activation.

In-situ growth of zeolitic imidazolate framework-8 on polypyrrole nanotubes for highly efficient and reversible capture of radioiodine

Effective capture of radioiodine derived from nuclear reactor accidents and spent fuel reprocessing is of critical significance for the sustainable development of nuclear energy. Herein, we reported the successful synthesis of core-shell PPyNT@ZIF-8 (PZ) nanocomposites with ZIF-8 nanocrystals decorating polypyrrole nanotubes (PPyNT) through a facile, self-assembly procedure. Taking advantage of abundant electron-rich nitrogen atoms of polypyrrole skeleton as well as the high specific surface areas and porosities, these PZ nanocomposites had potential application in radioiodine capture. The maximum uptake capacity of as-synthesized PZ nanocomposites for vapor iodine reached up to 603.8 wt% within 5 h. Additionally, PZ nanocomposites exhibited excellent recycle abilities, as the uptake capacity of PZ-3 still remained 381.7 wt% (63.2% of the initial capacity) after five capture-release cycles. In iodine/cyclohexane solution, the maximum iodine adsorption capacity was 837.9 mg g-1 and all of the loaded iodine could be effectively released in ethanol solution. The excellent iodine capture performance was primarily attributed to the strong affinity between the Lewis acidic iodine molecules and the Lewis basic nitrogen atoms to form polyiodide ions. Considering their excellent iodine adsorption capacity and superior stability, PZ nanocomposites could be employed to effectively eliminate radioactive iodine.

Enantioselective Synthesis of trans-Disubstituted Cyclopropyltrifluoroborate Building Blocks through Ru-Catalyzed Cyclopropanation

Enantioselective synthesis of trans-1,2-disubstituted cyclopropanes containing Lewis basic nitrogen functionalities remains a challenge to metal-catalyzed cyclopropanation reactions. Herein, we reported an alternative strategy to access similar products by a sequence of enantioselective cyclopropanation of potassium vinyltrifluoroborate, followed by cross-coupling of the cyclopropyl boronate product. The cyclopropanation process was readily performed in a 50 g scale from commercially available starting materials. An interesting enrichment of product enantiomeric purity by simple crystallization is also noted.

Post-synthesis from Lewis acid-base interaction: an alternative way to generate light and harvest triplet excitons.

The changes in absorption and emission of fluorescent materials with the introduction of Lewis acids have been frequently observed due to either physical or chemical interactions. In this mini-review, we elaborate how Lewis acids adjust the optical properties and the bandgap of luminescent materials by simple coordination reactions. It is common that fluorescent materials containing Lewis basic nitrogen heterocycles are more likely to provide the feasible band gap modulation. The essence of such phenomenon originates from Lewis acid–base coordination and adducts, which highly depends on the electron-accepting property of the Lewis acids. This intermolecular mechanism, considered as post-synthesis of new luminescent compounds offers promising applications in sensing and electroluminescence by manipulating the frontier molecular orbital energy levels of organic conjugated materials, simply based on Lewis acid–base chemistry.

Hierarchically mesoporous imidazole-functionalized covalent triazine framework: An efficient metal- and halogen-free heterogeneous catalyst towards the cycloaddition of CO2 with epoxides

The conversion of CO2 into value-added cyclic carbonates over metal- and halogen-free heterogeneous catalyst has attracted much attention but remains a huge challenge. Herein, a covalent triazine framework (CTF) with hierarchical microstructure was synthesized from cyanuric chloride and 1,4-phenylenediamine by a simple solvothermal approach compared to traditional ionothermal trimerization methods. While functionalizing the CTF framework with imidazole (IM) groups, the resultant CTF-IM exhibits excellent catalytic activity for the synthesis of cyclic carbonate from CO2 and epoxides under metal-, halogen- and solvent-free conditions. Under optimal reaction conditions, the representative epoxide, epichlorohydrin, afforded the corresponding cyclic carbonate in almost quantitative yield (94.6%), which is 5.4 times than that obtained over the parent CTF. The Lewis acidic C2-H and Lewis basic nitrogen atom play vital roles in the activation of epoxide and CO2, respectively, which help the cycloaddition reaction to proceed faster. In addition, CTF-IM has good chemical stability and recyclability, as well as could be applicable to a wide range of substituted epoxides.

Immobilization of Lewis Basic Nitrogen Sites into a Chemically Stable Metal-Organic Framework for Benchmark Water-Sorption-Driven Heat Allocations.

Developing efficient and stable water adsorbents for adsorption‐driven heat transfer technology still remains a challenge due to the lack of efficient strategies to enhance low‐pressure water uptakes. The authors herein demonstrate that the immobilization of Lewis basic nitrogen sites into metal–organic frameworks (MOFs) can improve water uptake and target benchmark coefficient of performances (COPs) for cooling and heating. They present the water sorption properties of a chemically stable MOF (termed as Zr‐adip), designed by incorporating hydrophilic nitrogen sites into the adsorbent MIP‐200. Zr‐adip exhibits S‐shaped sorption isotherms with an extremely high water uptake of 0.43 g g−1 at 303 K and P/P 0 = 0.25, higher than MIP‐200 (0.39 g g−1), KMF‐1 (0.39 g g−1) and MOF‐303 (0.38 g g−1). Theoretical calculations reveal that the incorporated N sites can serve as secondary adsorption sites to moderately interact with water, providing more binding sites to strengthen the water binding affinity. Zr‐adip achieves exceptionally high COPs of 0.79 (cooling) and 1.75 (heating) with a low driving temperature of 70 °C, outperforming MIP‐200 (0.78 and 1.53) and KMF‐1 (0.75 and 1.74). Combined with its ultrahigh stability, excellent cycling performance, and easy regeneration, Zr‐adip represents one of the best water adsorbents for adsorption‐driven cooling and heating.

B←N Coordination: From Chemistry to Organic Photovoltaic Materials

Coordination between boron Lewis acid and nitrogen Lewis base (B←N) is a classically chemical issue that has been known for several decades. Recently, this B←N chemistry developed into a new stage toward electronic materials, e.g., adjusting optoelectronic properties of N‐based conjugated molecules via adding B Lewis acids, synthesizing B←N‐bridged conjugated units, and further constructing B←N‐embedded organic photovoltaic (OPV) materials. Herein, these progresses are systematically summarized starting from certain chemical issues concerning B←N coordination, e.g., Lewis acidity of typical B Lewis acids and their interactions with N‐contained conjugated molecules. Then, some fused and ladder‐type B←N‐bridged units are reviewed in terms of the relationship between molecular structures and optoelectronic properties. Finally, the recently flourished B←N‐embedded OPV materials are summarized. The prospect of OPV materials concerning B←N coordination is also proposed, hoping to attract more attention to this filed and motivate its rolling rapidly.

Modulation of CO2 adsorption in novel pillar-layered MOFs based on carboxylate-pyrazole flexible linker.

Metal-organic frameworks (MOFs) have attracted significant attention as sorbents due to their high surface area, tunable pore volume and pore size, coordinatively unsaturated metal sites, and ability to install desired functional groups by post-synthetic modification. Herein, we report three new MOFs with pillar-paddlewheel structures that have been synthesized solvothermally from the mixture of the carboxylate-pyrazole flexible linker (H2L), 4,4-bipyridine (BPY)/triethylenediamine (DABCO), and Zn(ii)/Cu(ii) ions. The MOFs obtained, namely [ZnII(L)BPY], [CuII(L)BPY], and [CuII(L)DABCO], exhibit two-fold interpenetration and dinuclear paddle-wheel nodes. The Zn(ii)/Cu(ii) cations are coordinated by two equatorial L linkers that result in two-dimensional sheets which in turn are pillared by BPY or DABCO in the perpendicular direction to obtain a neutral three-dimensional framework that shows one-dimensional square channels. The three pillar-layered MOFs were characterized as microporous materials showing high crystalline stability after activation at 120 °C and CO2 adsorption. All MOFs contain uncoordinated Lewis basic pyrazole nitrogen atoms in the framework which have an affinity toward CO2 and hence could potentially serve as CO2 adsorption material. The CO2 uptake capacity was initially enhanced by replacing Zn with Cu and then replacing the pillar, going from BPY to DABCO. Overall, all the MOFs exhibit low isosteric heat (Qst) of adsorption which signifies an advantage due to the energy required for the adsorption and regeneration processes.

Four anionic Ln-MOFs for remarkable separation of C2H2–CH4/CO2–CH4 and highly sensitive sensing of nitrobenzene

Ln-MOFs with open metal active sites and Lewis basic nitrogen atoms show highly sensitive sensing of nitrobenzene and remarkable separations of C2H2–CH4/CO2–CH4.

Highly stable Zn-MOF with Lewis basic nitrogen sites for selective sensing of Fe3+ and Cr2O7 2− ions in aqueous systems

A luminescent Zn(II)-based MOF, [Zn(dptz)(BDC)(H2O)]n [1, dptz = 3,6-di(1H-pyrazol-4-yl)-1,2,4,5-tetrazine, H2BDC = terephthalic acid], with Lewis basic nitrogen sites has been synthesized by a solvothermal method. Using ligands with higher pK a value, complex 1 exhibits excellent solvent and pH stabilities in the pH range 2–12. 1 can serve as a highly sensitive and efficient luminescent probe for detecting Fe3+ and Cr2O7 2− ions in aqueous systems. The values of K SV toward Fe3+ and Cr2O7 2− were estimated to be 1.61 × 103 M−1 and 1.26 × 104 M−1, respectively. The possible sensing mechanism has been discussed. All these results suggest 1 is a promising dual-functional luminescent sensor for the detection of Fe3+ and Cr2O7 2− ions in aqueous solution.

A Highly Stable Triazole-Functionalized Metal-Organic Framework Integrated with Exposed Metal Sites for Selective CO2 Capture and Conversion.

A new triazole-functionalized tetracarboxylic acid ligand (H4 L) has been synthesized and utilized for the fabrication of a 3D ZnII organic framework with a Zn4 (-COO)6 cluster as the secondary building unit. The framework exhibits very good thermal stability and consists of dual functionalities of exposed Lewis acidic metal sites and accessible nitrogen-donor Lewis basic sites. The Lewis basic nitrogen sites in the framework serve as CO2 binding sites for highly selective CO2 capture and the presence of exposed Lewis acidic metal sites in the framework make it an efficient heterogeneous catalyst for the chemical fixation of CO2 into value-added cyclic carbonates under ambient conditions.

Progress towards metal-free radical alkylations of quinones under mild conditions

A new method for the radical alkylation of quinones is reported. Lewis basic nitrogen additives increase the efficacy of quinone alkylations from carboxylic acids using catalytic AgNO3 and Selectfluor as a mild oxidant. Electrochemical data suggests that certain Lewis basic additives are capable of directly reducing Selectfluor through a single-electron transfer, presumably via a charge-transfer complex. This process yields intermediates capable of promoting oxidative decarboxylation of alkyl carboxylic acids without an added metal initiator. Using this strategy, we have demonstrated progress towards a metal-free C–H quinone alkylation reaction that proceeds at room temperature under mild conditions.

A water-stable fcu-MOF material with exposed amino groups for the multi-functional separation of small molecules

The energy-efficient purification of methane from C2-hydrocarbons is of great significance for the upgrading of natural gas. So does the capture of carbon dioxide for remission of greenhouse effect. It is well established that some functional sites, such as open metals sites, Lewis basic nitrogen sites and fluorine groups, have shown significantly enhanced affinity toward more polarizable molecules. Thus, a water-stable Eu3+-based fcu-metal-organic framework (MOF) (compound 1 ) with amino functional groups has been successfully constructed through a reticular chemistry approach. As a result, the activated compound 1 exhibits moderately high uptakes of C2-hydrocarbons, but a less obvious adsorption of CH4 at the same conditions. Among them, the adsorption capacity of C2H2 is up to 143.6 cm3 cm−3 and a relatively high selectivity of C2H2/CH4 (107.7) is obtained at near room temperature. Moreover, compound 1 is also validated as an exceptional adsorbent for CO2 capture, with the fairly high capacity of CO2 (92.6 cm3 cm−3) and CO2/N2 selectivity (151.7) at ambient conditions. The excellent performance of compound 1 is mainly driven by the exposed amino functional groups within the contracted pores. Such effect thus leads to the achievement of dual-functional platform for methane purification and carbon dioxide capture. Furthermore, compound 1 features a satisfactory water stability, which is confirmed by the powder X-ray diffraction (PXRD) analysis and the retest of porosity after being soaked in water.

Zirconium-Nitrogen Intermolecular Frustrated Lewis Pairs.

A series of intermolecular transition metal frustrated Lewis pairs (FLPs) based on zirconocene alkoxide complexes ([Cp2Zr(OMes)]+ 1 or ([Cp*2Zr(OMes)]+ 2) with nitrogen Lewis bases (NEt3, NEtiPr2, pyridine, 2-methylpyridine, 2,6-lutidine) are reported. The interaction between Zr and N depends on the specific derivatives used, in general more sterically encumbered pairs leading to a more frustrated interaction; however, DOSY NMR spectroscopy reveals these interactions to be dynamic in nature. The pairs undergo typical FLP-type reactivity with D2, CO2, THF, and PhCCD. The catalytic dehydrocoupling of Me2NH·BH3 is also reported. Comparisons can be made with previous work employing phosphines as Lewis bases suggesting that hard-hard or hard-soft acid-base considerations are of little importance compared to the more prominent roles of steric bulk and basicity.

Synthesis, structure and palladium coordination of ambiphilic, pyridine- and phosphine-tethered N-boryl imine ligands.

We describe here the synthesis and structural characterization of two new classes of ambiphilic, N-boryl imine ligands, wherein boron is associated with a Lewis basic imine nitrogen. These ligands can be easily generated in two steps from the corresponding pyridinyl- and phosphinyl-tethered aldehydes. 11B NMR analysis suggests the association of the Lewis acidic boron to either the pyridine unit or via intermolecular acid/base interactions with the imine. Both of these ligands can coordinate to palladium, and their structures were confirmed by X-ray crystallography.

Enhancing light hydrocarbon storage and separation through introducing Lewis basic nitrogen sites within a carboxylate-decorated copper–organic framework

A novel copper–organic framework (NEM-4) with functional groups exhibits both outstanding C1–C3 hydrocarbon uptakes and selectivities under ambient conditions.