Highly efficient hydroboration of alkynes catalyzed by porous copper-organic framework under mild conditions

Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

Emerging Mixed-Valence Porous Materials

The development and applications of porous materials have been revolutionized over the past decades. To fine-tune their fascinating behaviors such as charge-transport, magnetic, and catalytic properties, one of the most effective strategies is incorporating mixed valency into the structures. This Concept provides recent progress on emerging crystalline porous materials, namely, metal-organic frameworks, metal-organic polyhedra, covalent organic frameworks, and hydrogen-bonded organic frameworks, where their mixed valency is either intrinsic or postsynthetically induced via an external stimulus. We further highlight the investigation of mixed valency and potential applications, which sheds light on the design and exploitation of mixed-valence crystalline porous materials.

UiO-67 MOF-Encapsulated NHC-Based Single-Site Copper Catalyst and Its Application in Regioselective Borylation of Terminal Alkynes

N-Heterocyclic carbenes (NHCs) act as versatile ligand backbones due to their strong σ-donation and π-acceptance properties. However, the encapsulation of NHC-coinage metal complexes in a metal-organic framework (MOF) to utilize them in organic catalysis is rare. In this work, an NHC-coordinated CuBr (NHC = Bn2Im; 1,3-dibenzyl-imidazol-2-ylidene) complex was encapsulated in UiO-67 MOF ((Bn2Im)2CuBr@UiO-67) and further utilized toward the regioselective protoboration of terminal alkynes. (Bn2Im)2CuBr@UiO-67 was found to show superior catalytic performance in aiding the protoboration of terminal alkynes, with a very high turnover frequency (TOF) of 14333.3 h-1, much higher than those of many other reported copper-based heterogeneous catalysts. Our catalyst also retained excellent catalytic efficiency for up to five cycles for the above-mentioned process. The newly synthesized (Bn2Im)2CuBr@UiO-67 and the recovered catalyst post-catalysis were characterized using various analytical techniques, including powder X-ray diffraction (PXRD), IR spectroscopy, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).

Defect-engineering improves the activity of Metal-Organic frameworks for catalyzing hydroboration of Alkynes: A combination of experimental investigation and Density functional theory calculations

Defect-engineered metal-organic frameworks (DEMOFs) are emerging advanced materials. The construction of DEMOFs is of great significance; however, DEMOF-based catalysis remains unexplored. (E)-vinylboronates, an important building block for asymmetric synthesis, can be synthesized via the hydroboration of alkynes. However, the lack of high-performance catalysts considerably hinders their synthesis. Herein, a series of DEHKUST-1 (HKUST = Hong Kong University of Science and Technology) (Da-f) catalysts with missing occupation of linkers at Cu nodes were designed by partially replacing benzene-1,3,5-tricarboxylate (H3BTC) with defective connectors of pyridine-3,5-dicarboxylate (PYDC) to efficiently promote the hydroboration of alkynes. Results showed that the Dd containing 0.8 doping ratio of PYDC exhibited remarkable catalytic activity than the defect-free HKUST-1. This originated from the improved accessibility for reactants towards the Lewis acid active Cu sites of DEHKUST-1 due to the presence of plenty of rooms next to the Cu sites and enhanced coordination ability in such 'defective' HKUST-1. Dd had high selectivity (>99 %) and yield (>96 %) for (E)-vinylboronates and extensive functional group compatibility for terminal alkynes. Density functional theory (DFT) calculations were performed to elucidate the mechanism of hydroboration. Compared with that of defect-free HKUST-1, the low energy barrier of DEHKUST-1 can be attributed to the lower coordination number of Cu sites and enhanced accessibility of Cu active sites towards reagents.

Heterovalent Metal Pair Sites on Metal-Organic Framework Ordered Macropores for Multimolecular Co-Activation

The precise design of catalytic metal centers with multiple chemical states to facilitate sophisticated reactions involving multimolecular activation is highly desirable but challenging. Herein, we report an ordered macroporous catalyst with heterovalent metal pair (HMP) sites comprising CuII-CuI on the basis of a microporous metal-organic framework (MOF) system. This macroporous HMP catalyst with proximity heterovalent dual copper sites, whose distance is controlled to ∼2.6 Å, on macropore surface exhibits a co-activation behavior of ethanol at CuII and alkyne at CuI, and avoids microporous restriction, thereby promoting additive-free alkyne hydroboration reaction. The desired yield enhances dramatically compared with the pristine MOF and ordered macroporous MOF both with solely isovalent CuII-CuII sites. Density functional theory calculations reveal that the Cu-HMP sites can stabilize the Bpin-CuII-CuI-alkyne intermediate and facilitate C-B bond formation, resulting in a smooth alkyne hydroboration process. This work provides new perspectives to design multimolecular activation catalysts for sophisticated matter transformations.

Solvent-free hydroboration of alkenes and alkynes catalyzed by rhodium-ruthenium nanoparticles on carbon nanotubes

A heterogeneous catalyst consisting of bimetallic rhodium-ruthenium particles immobilized on carbon nanotubes was used in the hydroboration reaction and proved highly effective for a variety of alkenes and alkynes. The reactions were carried out with low catalytic loadings (0.04 mol%), under solvent-free conditions, and at room temperature. In addition, to demonstrate its recyclability, the catalyst was recovered by a simple centrifugation process and reused over 5 consecutive cycles without losing any activity.

A copper nitride nanocube catalyst for highly efficient hydroboration of alkynes

The hydroboration of alkynes with bis(pinacolato)diboron is a useful method for the synthesis of vinyl boronate esters, which are essential intermediates in organic syntheses. Copper catalysts have been used extensively in these reactions. However, previously reported Cu-catalyst systems inevitably require additives and elevated temperatures. Herein, we report, for the first time, a simple and efficient hydroboration of alkynes under additive-free and mild reaction conditions (i.e., at a temperature of 30 °C) using a copper nitride nanocube (Cu₃N NC) catalyst. A wide range of alkynes can be transformed into their corresponding boronate esters. Cu₃N NCs are also applicable in the hydroboration of alkynes with tetrahydroxydiboron to synthesize vinyl boronic acids. Moreover, the Cu₃N NCs were easily separated by simple filtration and could be reused several times without any loss of their original activity. Hence, these highly active and reusable Cu₃N NC catalysts offer an environmentally friendly route for the efficient production of vinyl boronates.

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(L^4-)(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.

Ultrathin Metal-Organic Framework Nanosheets Exhibiting Exceptional Catalytic Activity

Two-dimensional (2D) metal-organic framework nanosheets (MONs) or membranes are classes of periodic, crystalline polymeric materials that may show unprecedented physicochemical properties due to their modular structures, high surface areas, and high aspect ratios. Yet preparing 2D MONs from multiple components and two different types of polymerization reaction remains challenging and less explored. Here, we report the synthesis of MOF films via interfacial polymerization, which involves three active monomers for simultaneous polycondensation and polycoordination taking place in a confined interface. The well-defined lamellar structure of the MOF films allowed feasible and scalable exfoliation to produce free-standing 2D MONs with high aspect ratio up to 2000:1 and ultrathin thickness (∼1.7 nm). The pore structure was revealed by high-resolution TEM images with near-atomic precision. The imide-linkage of MONs provided superior thermal (up to 530 °C) and good chemical stability in the pH range from 3 to 12. More importantly, the MONs exhibited exceptional catalytic activity and superior reusability for the hydroboration reactions of alkynes, in which the turnover frequency (TOF) reached 41734 h^-1, which is 2-4 orders of magnitude greater than that reported for homogeneous and heterogeneous catalysts.

Copper-Catalyzed Hydroboration of Enamides with Bis(pinacolato)diboron: Promising Agents with Antimicrobial Activities

We reported in this study the hydroboration of enamides in methanol at room temperature catalyzed by copper complexes. Under such conditions, a Gram-scale reaction with a high yield was also completed. Hydroboration of 3-methylene, 2-(alkyl and phenylisoindolin-1-one 5 with bis(pinacolato)diboron yields the respective compounds 6 in good yields with high-to-moderate enantioselectivity (58% ee). Furthermore, the antimicrobial properties of the synthesized compounds were tested against four indicator microorganisms: the two Gram-positive bacteria L. monocytogenes ATCC 1911 and S. aureus ATCC 6538, the Gram-negative bacterium S. typhimurium ATCC 14028, and the fungus C. albicans (ATCC 90028). The MIC values of compounds 5-6 range from 0.312 to 2.5 (μg/mL) against L. monocytogenes, from 2.1 to 0.136 (μg/mL) against S. aureus, and from 0.126 to 0.923 (μg/mL) against S. typhimurium.