Silver Metal-Organic Framework Derived N-Doped Carbon Nanofibers for CO2 Conversion into ß-Oxopropylcarbamates.

Developing efficient heterogeneous catalysts for chemical fixation of CO2 to produce high-value-added chemicals under mild conditions is highly desired but still challenging. Herein, we first reported an approach to prepare a novel catalyst (Ag@NCNFs), featuring Ag nanoparticles (NPs) embedded within porous nitrogen-doped carbon nanofibers (NCNFs), via growing a Ag metal-organic framework on one-dimensional electrospun nanofibers followed by pyrolysis. Benefiting from the abundant nitrogen species and porous structure, Ag NPs is well dispersed in the obtained Ag@NCNFs. Catalytic studies indicated that Ag@NCNFs exhibited excellent catalytic activity for the three-component coupling reaction of CO2, secondary amines, and propargylic alcohols to generate ß-oxopropylcarbamates under mild conditions with a turnover number (TON) of 16.2, and it can be recycled and reused at least 5 times without an obvious decline in catalytic activity. The reaction mechanism was clearly clarified by FTIR, NMR, 13C isotope labeling, control experiments, and density functional theory calculations. The results suggest that Ag@NCNFs and 1,8-diazabicyclo[5.4.0]undec-7-ene can synergistically activate propargylic alcohol to react with CO2, and then the generated α-alkylidene cyclic carbonate was invaded by secondary amine to produce ß-oxopropylcarbamate. Importantly, to the best of our knowledge, this is the first experimental and theoretical investigation on this reaction.

Highly Efficient Conversion of Aziridines and CO2 Catalyzed by Microporous [Cu12] Nanocages.

The conversion of CO2 as a C1 source into value-added products is an attractive alternative in view of the green synthesis. Among the reported approaches, the cyclization reaction of aziridines with CO2 is of great significance since the generated N-containing cyclic skeletons are extensively found in pharmaceutical chemistry and industrial production. However, a low turnover number (TON) and homogeneous catalysts are often involved in this catalytic system. Herein, one novel copper-organic framework {[Cu2(L4-)(H2O)2]·3DMF·2H2O}n (1) (H4L = 2'-fluoro-[1,1':4',1″-Terphenyl]-3,3″,5,5″-tetracarboxylic acid) assembled by nanosized [Cu12] cages was successfully synthesized and structurally characterized, which exhibits high CO2/N2 selectivity due to the strong interactions between CO2 and open Cu(II) sites and ligands in the framework. Catalytic investigations suggest that 1 as a heterogeneous catalyst can effectively catalyze the cyclization of aziridines with CO2, and the TON can reach a record value of 90.5. Importantly, 1 displays excellent chemical stability, which can be recycled at least five times. The combination explorations of nuclear magnetic resonance (NMR), 13C-isotope labeling experiments, and density functional theory (DFT) clearly uncover the mechanism of this aziridine/CO2 coupling reaction system, in which 1 and tetrabutylammonium bromide (TBAB) can highly activate the substrate molecule, and the synergistic catalytic effect between them can greatly reduce the reaction energy barrier from 51.7 to 36.2 kcal/mol.

Highly Efficient Conversion of Propargylic Alcohols and Propargylic Amines with CO2 Activated by Noble-Metal-Free Catalyst Cu2 O@ZIF-8.

The cyclization reactions of propargylic alcohols and propargylic amines with CO2 are important in industrial applications, but it was a great challenge that non-noble-metal catalysts catalyzed both reactions under mild conditions. Herein, the catalyst Cu2 O@ZIF-8 was prepared by encapsulating Cu2 O nanoparticles into robust ZIF-8, and it can effectively catalyze the cyclization of both propargylic alcohols and propargylic amines with CO2 into valuable α-alkylidene cyclic carbonates and oxazolidinones with turnover numbers (TONs) of 12.1 and 19.6, which can be recycled at least five times. The mechanisms were further uncovered by NMR, FTIR, 13 C isotope-labeling experiments and DFT calculations, in which Cu2 O and DBU can synergistically activate the C≡C bond and the hydroxy/amino group of substrates. Importantly, it is the first example of a noble-metal-free catalyst that can catalyze both propargylic alcohols and propargylic amines with CO2 simultaneously.

Non-Noble-Metal Metal-Organic-Framework-Catalyzed Carboxylative Cyclization of Propargylic Amines with Atmospheric Carbon Dioxide under Ambient Conditions.

The coupling reaction of propargylic amines and carbon dioxide (CO2) to synthesize 2-oxazolidinones is an important reaction in industrial production, and yet harsh reaction conditions and noble-metal catalysts are often required to achieve high product yields. Herein, one novel noble-metal-free three-dimensional framework, [Mg3Cu2I2(IN)4(HCOO)2(DEF)4]n (1), assembled by magnesium and copper clusters was synthesized and applied to this reaction. Compound 1 displays excellent solvent stability. Importantly, 1, acting as heterogeneous catalyst, can highly catalyze the cyclization of propargylic amines with CO2 under atmospheric pressure at room temperature, which can be recycled at least five times without an obvious decrease of the catalytic activity. NMR spectroscopy, coupled with 13C-isotope- and deuterium-labeling experiments, clearly clarifies the mechanism of this catalytic system: CO2 was successfully captured and converted to the product of 2-oxazolidinones, the C≡C bond of propargylic amines can be effectively activated by 1, and proton transfer was involved in the reaction process. Density functional theory calculations are further conducted to uncover the reaction path and the crucial role of compound 1 during the reaction.

A Porous Copper-Organic Framework Assembled by [Cu12] Nanocages: Highly Efficient CO2 Capture and Chemical Fixation and Theoretical DFT Calculations.

A new porous copper-organic framework assembled from 12-nuclear [Cu12] nanocages {[Cu2(L4-)(H2O)2]·4DMA·2H2O}n (1) (H4L = 5,5'-(butane-1,4-diyl)-bis(oxy)-diisophthalic acid) was successfully prepared and structurally characterized. Compound 1 feathering of a 3D framework with two types of 1D nanotubular channels and a large specific surface area can effectively enrich various harmful dyes. Additionally, due to the carbon dioxide (CO2) interactions with open Cu(II) sites and the electron-rich ether oxygen atoms of ligand in 1, it exhibits a highly selective CO2 uptake. Interestingly, 1 can effectively catalyze the cycloaddition reaction of CO2 with various epoxides under mild conditions, which is ascribed to the Lewis acid Cu(II) sites in the framework of 1. Importantly, 1 acting as a heterogeneous catalyst can be recycled at least 10 times without an obvious loss of catalytic activity, and the CO2 cycloaddition mechanism was further uncovered by density functional theory (DFT) calculations. This study can greatly enrich the MOF catalysts system of CO2 conversion and also provide a valuable guidance for the design of efficient MOFs catalysts.

A more effective catalysis of the CO2 fixation with aziridines: computational screening of metal-substituted HKUST-1.

A vital issue for the fixation and conversion of CO2 into useful chemical products is to find effective catalysts. In this work, in order to develop more effective and diverse catalysts, we implemented the first computational screening study (at M06-2X//B3LYP level) on the cycloaddition of CO2 with aziridines under eighteen metal-substituted HKUST-1 MOFs and tetrabutylammonium bromide (TBAB) as a co-catalyst. For all considered catalytic systems, the ring-opening of aziridine is calculated to be the rate-determining step. Up to 11 M-HKUST-1 systems, i.e., Rh (31.87 kcal mol-1), Y (31.02), Sc (30.50), V (30.02), Tc (29.90), Cd (29.80), Ti (29.32), Mn (29.05), Zn (28.29), Fe (27.85) and Zr (25.09), possess lower ring-opening barrier heights than the original Cu-HKUST-1 (32.90), indicative of their superior catalytic ability to the original Cu-HKUST-1 in theory. With the lowest ring-opening barrier, Zr-HKUST-1 is strongly advocated for future synthetic and catalytic studies.

CAl3X (X = B/Al/Ga/In/Tl) with 16 valence electrons: can planar tetracoordinate carbon be stable?

As a perpetual chemical curiosity, planar tetracoordinate carbon (ptC) that violates the traditional tetrahedral carbon (thC) has made enormous achievements. In particular, the 18-valence-electron (18ve) counting rule has been found to be very effective in predicting ptC structures, as in CX42- (X = Al/Ga/In/Tl). By contrast, the corresponding neutral CX4 with 16ve each takes the thC form like methane. Herein, we report a mono-substituted neutral 16ve-CAl3X (X = Al/Ga/In/Tl). Our theoretical results showed that the competition between thC and ptC can be well tuned upon variation of X, and for X = In and Tl, the ptC structure becomes isoenergetic to and even more stable than thC, respectively. Thus, a low-lying ptC can be achieved in the 16ve-CAl3X set without acquiring additional electrons. This unintuitive result can be ascribed to the increased energetic preference of the ionic sub-structure [CAl3-]X+ from X = Al to Tl. We thus predict the first penta-atomic ptC species with 16ve, and the ionic strategy presented in this work is expected to promote novel designs of ptC molecules.