No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Metal-Organic Framework Anchored with a Lewis Pair as a New Paradigm for Catalysis
Abstract
Lewis pair (LP) chemistry has shown broad applications in the catalysis field. However, one significant challenge has been recognized as the instability for most homogeneous LP catalysts upon recycling, thus inevitably leading to dramatic loss in catalytic activity. Additionally, current heterogeneous LP catalysts suffer from low surface area, which largely limits their catalytic efficiency, thereby restricting their potential applications. In this work, we report the successful introduction of LPs, classical and frustrated, into a metal-organic framework (MOF) that features high surface and ordered pore structure via a stepwise anchoring strategy. Not only can the LP be stabilized by the strong coordination interaction between the LP and MOF, but the resultant MOF-LP also demonstrates excellent catalysis performance with interesting size and steric selectivity. Given the broad applicability of LPs, our work therefore paves a way for advancing MOF-LP as a new paradigm for catalysis.
... [88][89][90][91][92][93] However, only in 2018 the Ma's group achieved experimentally the introduction of FLPs into MOF in a semi-immobilized manner. 73 The Cr-MIL-101 was selected as an adequate MOF platform due to its large open porosity and the presence of Cr open metal sites. 94 The selected classical FLP comprises BCF as the LA and 1,4diazabicycl[2.2.2]octane (DABCO) as a potent LB. ...
... 72 The FLP anchored in the MOF was B(C 6 F 5 ) 2 (Mes)/DABCO following the same strategy. 73 Interestingly, this immobilized FLP could activate H 2 to form MIL-101(Cr)-FLP-H 2 at room temperature and hence, form an ammonium hydridoborate salt, [CH(CH 2 CH 2 ) 3 NH] [HB(C 6 F 5 ) 2 (Mes)]. Powder XRD was used to confirm the retention of the structural integrity of MIL-101(Cr)-FLP-H 2 , its surface area being of 1120 m 2 g À1 as obtained by N 2 sorption studies. ...
This review provides an overview of FLP chemistry in the field of CO 2 hydrogenation, whichcovers both experimental and computational aspects, while ranging from homogeneous catalysis to recent heterogenisation strategies in porous solids.
... In this regard, FLPs with bulky substituents will be used to mutual quenching of a pair of LP moieties within the channels. LP can be grafted successfully into a MOF ( Figure 1) by anchoring the LP first into the open metal sites within the MOF through coordination interaction followed by the introducing the corresponding acid moiety of the LP [25] . Given the strong coordination interaction, it is anticipated that the LP would be stabilized within the MOF yet accessible to substrates of interest. ...
... Illustration of anchoring strategy of Lewis pair on MIL-101 MOF [25] . ...
... [a] Prof. A. Dhakshinamoorthy Scheme 8. Proposed mechanism for the reduction of imines by pinacolateborane catalyzed by MIL-101(Cr)-FLP. Reproduced with permission from Ref. [70] ...
... Ma and coworkers have also adopted a similar strategy to prepare MIL-101(Cr)-FLP by treating MIL-101(Cr) with B(C 6 F 5 ) 3 / DABCO pair. [70] The catalytic performance of MIL-101(Cr)-FLP was efficiently employed as a heterogeneous catalyst for imine reduction with HBpin (Scheme 8) and the hydrogenation of alkylidene malonates with H 2 . ...
This article highlights novel prospects for metal–organic frameworks (MOFs) in heterogeneous catalysis as having frustrated Lewis acid‐base pairs (FLPs) or as bifunctional acid‐base solid catalysts able to activate molecular hydrogen. Starting from the extensive application MOFs as Lewis acid and Lewis base catalysts, this article uses catalytic hydrogenation to briefly summarize the efforts made to heterogenize boron and amine in MOFs to mimic molecular FLP systems. The core of this concept is based on recent findings which demonstrate the ability of two commonly used MOFs, namely UiO‐66 and MIL‐101, to catalyze the selective hydrogenation of polar double X=Y bonds at moderate H2 pressures below 10 bar. The influence of electron‐donating, the withdrawal of substituents on the linker, and the aniline poisoning effect highlight the significance of Lewis acid sites, while density‐functional theory calculations indicate the heterolytic H−H bond cleavage at the MOF metal oxo clusters. It is expected that this new perspective on MOFs as solid FLP systems will spur further research to explore and define the potential of dual sites in the catalytic activation of small molecules.
... [24][25][26][27][28][29][30][31][32][33] This is particularly important in FLP catalysts, where the intrinsic activity of the catalytic pair is governed by the spatial separation between the Lewis acid and base. [5,6,10,[34][35][36] Anchoring FLPs in supramolecular assemblies is a new and growing area of interest, with a full gamut of strategies ranging from metal-organic frameworks (MOFs) [37] and zeolites [38] through to polymers, [39] macrocycles [40] and cages. The structural and chemical diversity on show has led to some exciting and innovative results and in this review we will highlight the key areas that are under development and the areas that we consider ripe for future exploitation. ...
Frustrated Lewis pairs (FLPs) have rapidly become one of the key metal‐free catalysts for a variety of chemical transformations. Embedding these catalysts within a supramolecular assembly can offer improvements to factors such as recyclability and selectivity. In this review we discuss advances in this area, covering key supramolecular assemblies such as metal organic frameworks (MOFs), covalent organic frameworks (COFs), polymers and macrocycles.
... [34][35][36][37][38][39] In this vein, many research groups have developed a series of FLP-functionalized metal-organic frameworks (MOFs) (FLP-MOFs) through sequentially dynamic installation of Lewis base and Lewis acid into the prototypical MOF or de novo synthesis, which display efficient catalytic activity and good recyclability. [35][36][40][41][42][43][44][45] Among these works, it should be noted that the distance between LA and LB is of great importance for the catalytic efficiency which can be achieved through regulating steric hindrance of LA and LB or immobilizing them in specifically spatial location of the MOF matrix. For instance, Deng and co-workers reported a series of FLP MOFs with both LA and LB fixed in the MOF skeleton through geometry restriction, wherein the distance and location of LA and LB can be precisely regulated, thus providing a platform to investigate influences from the electrostatic effect and distance to the catalytic performance of FLP. ...
Converting CO2 to high‐value fine chemicals represents one of the most promising approaches to combat global warming and subsequently achieve a sustainable carbon cycle. Herein, we contribute an organoboron functionalized ultra‐thin metal‐organic nanosheet (MON), termed TCPB‐Zr‐NS, featuring an abundance of exposed Lewis acidic B and formate sites, which can effectively promote CO2 conversion upon the addition of Lewis basic o‐phenylenediamines. Compared with the prototypical 3D analogue TCPB‐Zr‐3D, the resultant TCPB‐Zr‐NS showcases dramatically improved catalytic activity for the cyclization of o‐phenylenediamine as a result of the highly exposed active sites and efficient substrates/products diffusion. Strikingly, the incorporation of Lewis acidic B sites into ultra‐thin Zr‐based MON (Zr‐MON) not only promotes the highly efficient CO2 conversion, but also enhances the recyclability/durability of catalysts. Additionally, the underlying catalytic mechanism has been well established by the comprehensive experiments and theoretical calculations, unveiling a formate‐assisted frustrated Lewis pairs (FLP) mediated catalytic pathway. This work opens up a new avenue to heterogeneous FLP‐based catalysts for small molecule activation and beyond.
... 3 Heterogeneous FLP catalysts have typically been synthesized through stepwise grafting of Lewis acid and base or "ship-in-bottle" generation of molecular FLPs on/in porous supports. [3][4][5][6] Potential challenges mainly lie in three aspects: (1) the selection of suitable porous supports with specific surface group and/or matched pore size; (2) synthetic routes to stabilize molecular FLPs on/in porous supports; (3) occupation of porous channels that hinders the access of active sites during catalysis. It is, therefore, of vital significance to develop a facile strategy to construct heterogeneous FLPs with open channels and high stability. ...
Porous frustrated Lewis pairs (FLPs) have been demonstrated as cutting‐edge heterogeneous catalysts in acidic–basic catalysis, which, yet faces great challenges in synthesis. Herein, we propose a one‐step de novo assembly strategy for constructing porous FLPs by capping monodentate ionic liquids (ILs) on metal clusters during the synthesis of metal–organic frameworks (MOFs). The capping of ILs makes adjacent metal sites on MOFs transforming to coordinatively‐unsaturated metal Lewis acids, producing FLPs containing Lewis basic Cl⁻ ions from ILs and metal sites. The distribution of ILs on the framework of MOFs endows the catalysts with large surface areas (up to 2125 m² g⁻¹), open channels and good stability. The porous FLPs behave unprecedented activity for atmospheric CO2 activation and coupling with epoxides (25–40°C, 1 bar, without cocatalyst). This de novo assembly strategy exhibits good universality for synthesizing porous FLPs from different kinds of ILs, paving a facile approach for designing high‐efficiency acidic–basic catalysts.
Metal–organic frameworks (MOFs) composed of infinite metal units exhibit enhanced electron transport and charge migration capabilities compared to discrete metal units. Herein, three underdeveloped isomorphic MOFs featuring 3D infinite zinc units are designed and synthesized. The Lewis acidity and photocatalytic activity of these MOFs are fine‐tuned through atomic‐level engineering of nitrogen atoms of ligands and the resultant change of charge transfer modes. These MOFs are promising catalysts in the photocatalytic monooxygenation of sulfenamides with molecular oxygen. Mechanistic investigations suggest that the uneven charge distribution and large dipole moment at the pyridine center of 1‐PTB and the infinite Zn unit frameworks of degenerate energy levels are key to its excellent photocatalytic activity.
This review is focused on ‘frustrated Lewis pair’ (FLP) hydrogenations. Following a discussion of the conceptual discovery and initial efforts to exploit the finding for metal-free hydrogenations, the scope of substrates that are susceptible to FLP hydrogenations are considered. The further advancement of FLPs to enantioselective reductions are discussed. Applications of the concept in transfer hydrogenation, and the reductions of carbon dioxide and main group substrates are also presented. Finally, the utility of the concept of FLPs in several heterogeneous catalysis systems is considered. The review concludes with an outlook for the future of FLP reductions.
the discovery of reversible hydrogenation using metal-free phosphonate species in 2006 marked the official advent of frustrated Lewis pair (FLP) chemistry. This break through revolutionized homogeneous catalysis approaches and paved the way for innovative catalytic strategies. The unique reactivity of FLPsis attributed to the Lewis base (LB) and Lewis acid (LA) sites either in spatial separation or in equilibrium, which actively react with withmolecules. Since 2010, heterogeneous FLP catalysts have gained increasing attention for their ability to enhance catalytic performance through tailored surface designs and improved recyclability, making them promising for industrial applications. Over the past 5years, our group has focused on investigating and strategically modifying various types of solid catalysts with FLPs that are unique from classic solid FLPs. We have explored systematic characterization techniques to unravel the underlying mechanisms between the active sites and reactants. Additionally, we have demonstrated the critical role of catalysts’ intrinsic electronic and geometric properties in promoting FLP formation and stimulating synergistic effects. The characterization of FLP catalysts has been greatly enhanced by the use of advanced techniques such as synchrotron X-ray diffraction, neutron powder diffraction, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure, elemental mapping in scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, diffuse-reflectance infrared Fourier transform spectroscopy and solid-state nuclear magnetic resonance spectroscopy. These techniques have provided deeper insights into the structural and electronic properties of FLP systems for the future design of catalysts. Understanding electron distribution in the overlapping orbitals of LA and LB pairs is essential for inducing FLPs in operando in heterogeneous catalysts through target electron reallocation by external stimuli. For instance, in silicoaluminophosphate-type zeolites with weak orbital overlap, the adsorption of polar gas molecules leads to heterolytic cleavage of the Alδ+−Oδ− bond, creating unquenched LA−LB pairs. In a Ru-doped metal−organic framework, the Ru−N bond can be polarized through metal−ligand charge transfer under light, forming Ru+−N− pairs. This activation of FLP sites from the framework represents a groundbreaking innovation that expands the catalytic potential of existing materials. For catalysts already employing FLP chemistry to dynamically generate products from substrates, a complete mechanistic interpretation requires a thorough examination of the surface electronic properties and the surrounding environment. The hydrogen spillover ability on the Ru-doped FLP surfaces improves conversion efficiency by suppressing hydrogen poisoning at metal sites. In situ H2−H2O conditions enable the production of organic chemicals with excellent activity and selectivity by creating new bifunctional sites via FLP chemistry. By highlighting the novel FLP systems featuring FLPinduction and synergistic effects and the selection of advanced characterization techniques to elucidate reaction mechanisms, we hope that this Account will offer innovative strategies for designing and characterizing FLP chemistry in heterogeneous catalysts to the research community.
Converting CO2 to high‐value fine chemicals represents one of the most promising approaches to combat global warming and subsequently achieve a sustainable carbon cycle. Herein, we contribute an organoboron functionalized ultra‐thin metal‐organic nanosheet (MON), termed TCPB‐Zr‐NS, featuring an abundance of exposed Lewis acidic B and formate sites, which can effectively promote CO2 conversion upon the addition of Lewis basic o‐phenylenediamines. Compared with the prototypical 3D analogue TCPB‐Zr‐3D, the resultant TCPB‐Zr‐NS showcases dramatically improved catalytic activity for the cyclization of o‐phenylenediamine as a result of the highly exposed active sites and efficient substrates/products diffusion. Strikingly, the incorporation of Lewis acidic B sites into ultra‐thin Zr‐based MON (Zr‐MON) not only promotes the highly efficient CO2 conversion, but also enhances the recyclability/durability of catalysts. Additionally, the underlying catalytic mechanism has been well established by the comprehensive experiments and theoretical calculations, unveiling a formate‐assisted frustrated Lewis pairs (FLP) mediated catalytic pathway. This work opens up a new avenue to heterogeneous FLP‐based catalysts for small molecule activation and beyond.
The reusable and separable surface frustrated Lewis pairs (SFLPs) open up a novel approach to efficient small‐molecule activation and conversion in heterogeneous catalysis. However, SFLPs have only been reported on limited systems due to the difficulty in the design and synthesis process. The inherent Lewis pairs on various solid materials offer promising opportunities for finding natural SFLPs, providing a straightforward and efficient strategy to overcome the current limitations. In this concept, we retrospect the concept of natural SFLPs proposed on wurtzite crystal surfaces and identify other natural SFLPs that probably exist on solid materials, including reduced oxide surfaces, corrugated graphene, and perovskite quantum dots. Having focused on the reactivity of natural SFLPs in small‐molecule activation, we discuss the current challenges, propose possible research directions, and highlight potential applications of natural SFLPs in heterogeneous catalysis.
In 2019, the IUPAC started a quest to select the most interesting emerging technologies in the chemical sciences [1]. Now, this established initiative continues year after year—adding ideas to a list of innovations with an enormous potential to transform fields as diverse as materials science, energy, healthcare, agriculture and computing, among others [2]. Overall, the IUPAC “Top Ten Emerging Technologies in Chemistry” align with the United Nations’ Sustainable Development Goals, in a quest to secure a sustainable future and pave the way to a circular economy [3]. This new list delves into new materials, unexplored physical phenomena, and creative solutions to global challenges, including prevalent diseases and the still ongoing energy and fuel crisis. As in the first “Top Ten” paper, the technologies hover over a broad range of readiness—from laboratory discoveries to commercial realities, hence “emerging.” But all of them, carefully curated by a panel of experts nominated by IUPAC, are equally exciting. Read on.
The construction of heterogeneous frustrated Lewis pairs (FLPs) catalysts is crucial for realizing highly efficient and recyclable pyridines catalytic hydrogenation. In this work, we prepared a recyclable heterogenous FLPs catalyst CMP-BF with conjugated microporous polymer CMP-ethynyl as the support via self-catalyzed 1,1-carboboration reaction with commercial Lewis acid B(C6F5)3. The as-synthesized CMP-BF demonstrates superior heterogenous catalytic hydrogenation performance (conversion>99%), and considerable stability (84% conversion after three cycles) in recyclable hydrogenation of 2,6-phenylpyridine. This work provides insights into the fabrication and catalytic application of recyclable heterogenous FLP catalysts.
Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal–organic frameworks collected in a “top-down” fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.
Multifunctional catalysts in organic synthesis are highly attractive, particularly in the construction of complex molecules. Here, we report the first example of heterogeneous Cu-MOFs catalyst with dual functions of photoredox and cross-coupling. IMU-108 can be synthesized on a gram scale from Cu(NO 3 ) 2 ·3H 2 O and 5-mercaptoisophthalic acid precursors by a simple reflux process. X-ray diffraction results revealed that IMU-108 is constructed from the in-situ bridged S-S bond between the zero-dimension metal-organic polyhedra (MOPs). The inter-polyhedras S-S bonds not only create a rigid geometric structure, but also allow IMU-108 to be bench-stable, while still maintaining excellent catalysis. We demonstrate the capacity of IMU-108 in heterogeneous photoredox-catalyzed cross-coupling of styrenes, α-carbonyl alkyl and bromides with boronic acids to yield various substituted 1,1-diarylalkanes in a single step. The versatility of this method enables late-stage functionalization of complex molecules without the need for de novo synthesis. Moreover, IMU-108 exhibits good recyclability, maintaining its catalytic activity over six consecutive reaction cycles.
The surface frustrated Lewis pairs (SFLPs) open up new opportunities for substituting noble metals in the activation and conversion of stable molecules. However, the applications of SFLPs on a larger scale are impeded by the complex construction process, low surface density, and sensitivity to the reaction environment. Herein, wurtzite‐structured crystals such as GaN, ZnO, and AlP are found for developing natural, dense, and stable SFLPs. It is revealed that the SFLPs can naturally exist on the (100) and (110) surfaces of wurtzite‐structured crystals. All the surface cations and anions serve as the Lewis acid and Lewis base in SFLPs, respectively, contributing to the surface density of SFLPs as high as 7.26 × 1014 cm‐2. Ab initio molecular dynamics simulations indicate that the SFLPs can keep stable under high temperatures and the reaction atmospheres of CO and H2O. Moreover, outstanding performance for activating the given small molecules is achieved on these natural SFLPs, which originates from the optimal orbital overlap between SFLPs and small molecules. Overall, these findings not only provide a simple method to obtain dense and stable SFLPs but also unfold the nature of SFLPs toward the facile activation of small molecules.
The surface frustrated Lewis pairs (SFLPs) open up new opportunities for substituting noble metals in the activation and conversion of stable molecules. However, the applications of SFLPs on a larger scale are impeded by the complex construction process, low surface density, and sensitivity to the reaction environment. Herein, wurtzite‐structured crystals such as GaN, ZnO, and AlP are found for developing natural, dense, and stable SFLPs. It is revealed that the SFLPs can naturally exist on the (100) and (110) surfaces of wurtzite‐structured crystals. All the surface cations and anions serve as the Lewis acid and Lewis base in SFLPs, respectively, contributing to the surface density of SFLPs as high as 7.26×10¹⁴ cm⁻². Ab initio molecular dynamics simulations indicate that the SFLPs can keep stable under high temperatures and the reaction atmospheres of CO and H2O. Moreover, outstanding performance for activating the given small molecules is achieved on these natural SFLPs, which originates from the optimal orbital overlap between SFLPs and small molecules. Overall, these findings not only provide a simple method to obtain dense and stable SFLPs but also unfold the nature of SFLPs toward the facile activation of small molecules.
Urea is one of the most important artificial nitrogen fertilizers in the agricultural economy and can provide essential nitrogen for plant growth. However, the industrial production of urea is very...
The ever-increasing landscape of heterogeneous catalysis, pure and applied, utilizes many different catalysts. Academic insights along with many industrial adaptations paved the way for the growth. In designing a catalyst, it is desirable to have a priori knowledge of what structure needs to be targeted to help in achieving the goal. When focusing on catalysis, one needs to cope with a vast corpus of knowledge and information. The overwhelming desire to exploit catalysis toward commercial ends is irresistible. In today’s world, one of the requirements of developing a new catalyst is to address the environmental concerns. The well-established heterogeneous catalysts have microporous structures (<25 Å), which find use in many industrial processes. The metal–organic framework (MOF) compounds, being pursued vigorously during the last two decades, have similar microporosity with well-defined pores and channels. The MOFs possess large surface area and assemble to delicate structural and compositional variations either during the preparation or through postsynthetic modifications (PSMs). The MOFs, in fact, offer excellent scope as simple Lewis acidic, Brönsted acidic, Lewis basic, and more importantly bifunctional (acidic as well as basic) agents for carrying out catalysis. The many advances that happened over the years in biology helped in the design of many good biocatalysts. The tools and techniques (advanced preparative approaches coupled with computational insights), on the other hand, have helped in generating interesting and good inorganic catalysts. In this review, the recent advances in bifunctional catalysis employing MOFs are presented. In doing so, we have concentrated on the developments that happened during the past decade or so.
An ethynyl-modified interpenetrated MOF material with lvt topology, [Cu2(BTEB)(NMF)2]·NMF·8H2O (compound 1, H4BTEB = 4,4',4'',4'''-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, NMF = N-Methylformamide), was successfully synthesized by using an alkynyl-functionalized H4BTEB organic ligand under solvothermal conditions. Structural analysis shows that compound 1, consisting of a tetradentate carboxylic acid ligand and classical [Cu2(CO2)4] paddle-wheel structure building units, has a rare 4-connected lvt topology with dual interpenetrating structure, which can improve the framework stability, as well as the gas adsorption capacity and selectivity due to the restricted pore channel. According to the study of gas adsorption performance, compound 1 with a larger surface area, boasts a superior adsorption capacity for small gas molecules. Also, ideal adsorption solution theory (IAST) computational simulation shows that compound 1 has good gas adsorption selectivity for C3H8/CH4, indicating its potential application in gas separation.
The establishment of active sites as the frustrated Lewis pair (FLP) has recently attracted much attention ranging from homogeneous to heterogeneous systems in the field of catalysis. Their unquenched reactivity of Lewis acid and base pairs in close proximity that are unable to form stable adducts has been shown to activate small molecules such as dihydrogen heterolytically. Herein, we show that grafted Ru metal-organic framework-based catalysts prepared via N-containing linkers are rather catalytically inactive for H2 activation despite the application of elevated temperatures. However, upon light illumination, charge polarization of the anchored Ru bipyridine complex can form a transient Lewis acid-base pair, Ru+-N- via metal-to-ligand charge transfer, as confirmed by time-dependent density functional theory (TDDFT) calculations to carry out effective H2-D2 exchange. FTIR and 2-D NMR endorse the formation of such reactive intermediate(s) upon light irradiation.
Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)‐based metal–organic framework (MOF) by introducing coordination‐defect metal nodes (LA) and surface‐basic POM with abundant oxygen (LB). The well‐defined and unique spatial conformation of the defective POM‐based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H 2 . This FLP catalyst can heterolytically dissociate H 2 into active H δ− , thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect‐free POM‐based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi‐site catalyst to achieve specific activation of target substrate for synergistic catalysis.
The development of efficient heterogeneous catalysts with multiselectivity (e.g., enantio- and chemoselectivity) has long been sought after but with limited progress being made so far. To achieve enantio- and chemoselectivity in a heterogeneous system, as inspired by enzymes, we illustrate herein an approach of creating an enzyme-mimic region (EMR) within the nanospace of a metal-organic framework (MOF) as exemplified in the context of incorporating a chiral frustrated Lewis pair (CFLP) into a MOF with a tailored pore environment. Due to the high density of the EMR featuring the active site of CFLP and auxiliary sites of the hydroxyl group/open metal site within the vicinity of CFLP, the resultant EMR@MOF demonstrated excellent catalysis performance in heterogeneous hydrogenation of α,β-unsaturated imines to afford chiral β-unsaturated amines with high yields and high enantio- and chemoselectivity. The role of the hydroxyl group/open metal site in regulating chemoselectivity was proved by the observation of a catalyst-substrate interaction experimentally, which was also rationalized by computational results. This work not only contributes a MOF as a new platform for multiselective catalysis but also opens a promising avenue to develop heterogeneous catalysts with multiselectivity for challenging yet important transformations.
Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)‐based metal–organic framework (MOF) by introducing coordination‐defect metal nodes (LA) and surface‐basic POM with abundant oxygen (LB). The well‐defined and unique spatial conformation of the defective POM‐based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H2. This FLP catalyst can heterolytically dissociate H2 into active Hδ−, thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect‐free POM‐based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi‐site catalyst to achieve specific activation of target substrate for synergistic catalysis.
Hydrogenated nitrogen heterocyclic compounds play a critical role in the pharmaceutical, polymer, and agrochemical industries. Recent studies on partial hydrogenation of nitrogen heterocyclic compounds have focused on costly and toxic precious metal catalysts. As an important class of main-group catalysts, frustrated Lewis pairs (FLPs) have been widely applied in catalytic hydrogenation reactions. In principle, the combination of FLPs and metal-organic framework (MOF) is anticipated to efficiently enhance the recyclability performance of FLPs; however, the previously studied MOF-FLPs showed low reactivity in the hydrogenation of N-heterocycles compounds. Herein, we offer a novel P/B type MOF-FLP catalyst that was achieved via a solvent-assisted linker incorporation approach to boost catalytic hydrogenation reactions. Using hydrogen gas under moderate pressure, the proposed P/B type MOF-FLP can serve as a highly efficient heterogeneous catalyst for selective hydrogenation of quinoline and indole to tetrahydroquinoline and indoline-type drug compounds in high yield and excellent recyclability.
Photoinduced metal–organic framework (MOF) enabled heterogeneous thiol catalysis has been achieved for the first time. MOF Zr‐TPDCS‐1 , consisting of Zr 6 ‐clusters and TPDCS linkers (TPDCS=3,3′′,5,5′′‐tetramercapto[1,1′:4′,1′′‐terphenyl]‐4,4′′‐dicarboxylate), effectively catalyzed the borylation, silylation, phosphorylation, and thiolation of organic molecules. Upon irradiation, the fast electron transfer from TPDCS to Zr 6 ‐cluster is believed to facilitate the formation of the thiyl radical, a hydrogen atom transfer catalyst, which competently abstracts the hydrogen from borane, silane, phosphine, or thiol for generating the corresponding element radical to engender the chemical transformations. The elaborate control experiments evidenced the generation of thiyl radicals in MOF and illustrated a radical reaction pathway. The gram‐scale reaction worked well, and the product was conveniently separated via centrifugation and vacuum with a turnover number (TON) of ≈3880, highlighting the practical application potential of heterogeneous thiyl‐radical catalysis.
Photoinduced metal–organic framework (MOF) enabled heterogeneous thiol catalysis has been achieved for the first time. MOF Zr‐TPDCS‐1, consisting of Zr6‐clusters and TPDCS linkers (TPDCS=3,3′′,5,5′′‐tetramercapto[1,1′:4′,1′′‐terphenyl]‐4,4′′‐dicarboxylate), effectively catalyzed the borylation, silylation, phosphorylation, and thiolation of organic molecules. Upon irradiation, the fast electron transfer from TPDCS to Zr6‐cluster is believed to facilitate the formation of the thiyl radical, a hydrogen atom transfer catalyst, which competently abstracts the hydrogen from borane, silane, phosphine, or thiol for generating the corresponding element radical to engender the chemical transformations. The elaborate control experiments evidenced the generation of thiyl radicals in MOF and illustrated a radical reaction pathway. The gram‐scale reaction worked well, and the product was conveniently separated via centrifugation and vacuum with a turnover number (TON) of ≈3880, highlighting the practical application potential of heterogeneous thiyl‐radical catalysis.
Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.
Owing to the limited availability of natural sources, the widespread demand of the flavouring, perfume and pharmaceutical industries for unsaturated alcohols is met by producing them from α,β-unsaturated aldehydes, through the selective hydrogenation of the carbon-oxygen group (in preference to the carbon-carbon group). However, developing effective catalysts for this transformation is challenging, because hydrogenation of the carbon-carbon group is thermodynamically favoured. This difficulty is particularly relevant for one major category of heterogeneous catalyst: metal nanoparticles supported on metal oxides. These systems are generally incapable of significantly enhancing the selectivity towards thermodynamically unfavoured reactions, because only the edges of nanoparticles that are in direct contact with the metal-oxide support possess selective catalytic properties; most of the exposed nanoparticle surfaces do not. This has inspired the use of metal-organic frameworks (MOFs) to encapsulate metal nanoparticles within their layers or inside their channels, to influence the activity of the entire nanoparticle surface while maintaining efficient reactant and product transport owing to the porous nature of the material. Here we show that MOFs can also serve as effective selectivity regulators for the hydrogenation of α,β-unsaturated aldehydes. Sandwiching platinum nanoparticles between an inner core and an outer shell composed of an MOF with metal nodes of Fe³⁺, Cr³⁺ or both (known as MIL-101; refs 19, 20, 21) results in stable catalysts that convert a range of α,β-unsaturated aldehydes with high efficiency and with significantly enhanced selectivity towards unsaturated alcohols. Calculations reveal that preferential interaction of MOF metal sites with the carbon-oxygen rather than the carbon-carbon group renders hydrogenation of the former by the embedded platinum nanoparticles a thermodynamically favoured reaction. We anticipate that our basic design strategy will allow the development of other selective heterogeneous catalysts for important yet challenging transformations.
Improving the efficiency of electron-hole separation and charge-carrier utilization plays a central role in photocatalysis. Herein, Pt nanoparticles of ca. 3 nm are incorporated inside or supported on a representative metal-organic framework (MOF), UiO-66-NH2 , denoted as Pt@UiO-66-NH2 and Pt/UiO-66-NH2 , respectively, for photocatalytic hydrogen production via water splitting. Compared with the pristine MOF, both Pt-decorated MOF nanocomposites exhibit significantly improved yet distinctly different hydrogen-production activities, highlighting that the photocatalytic efficiency strongly correlates with the Pt location relative to the MOF. The Pt@UiO-66-NH2 greatly shortens the electron-transport distance, which favors the electron-hole separation and thereby yields much higher efficiency than Pt/UiO-66-NH2 . The involved mechanism has been further unveiled by means of ultrafast transient absorption and photoluminescence spectroscopy.
This review summarizes the use of metal-organic frameworks (MOFs) as a versatile supramolecular platform to develop heterogeneous catalysts for a variety of organic reactions, especially for liquid-phase reactions. Following a background introduction about catalytic relevance to various metal-organic materials, crystal engineering of MOFs, characterization and evaluation methods of MOF catalysis, we categorize catalytic MOFs based on the types of active sites, including coordinatively unsaturated metal sites (CUMs), metalloligands, functional organic sites (FOS), as well as metal nanoparticles (MNPs) embedded in the cavities. Throughout the review, we emphasize the incidental or deliberate formation of active sites, the stability, heterogeneity and shape/size selectivity for MOF catalysis. Finally, we briefly introduce their relevance into photo- and biomimetic catalysis, and compare MOFs with other typical porous solids such as zeolites and mesoporous silica with regard to their different attributes, and provide our view on future trends and developments in MOF-based catalysis.
The rational integration of multiple functional components into a composite material could result in enhanced activity tailored for specific applications. Herein, imidazolium-based poly(ionic liquid)s (denoted as polyILs) have been confined into the metal-organic framework (MOF) material MIL-101 via in situ polymerization of encapsulated monomers. The resultant composite polyILs@MIL-101 exhibits good CO2 capture capability that is beneficial to catalyze the cycloaddition of CO2 with epoxides to form cyclic carbonates at sub-atmospheric pressure in the absence of any co-catalyst. The significantly enhanced activity of polyILs@MIL-101 compared to either MIL-101 or polyILs is attributed to the synergistic effect among the good CO2 enrichment capacity, the Lewis acid sites in the MOF as well as the Lewis base sites in the polyILs.
A metal-organic framework (MOF)-based catalyst, chromium hydroxide/MIL-101(Cr), was prepared by a one-pot synthesis method. The combination of chromium hydroxide particles on and within Lewis acidic MIL-101 accomplishes highly selective conversion of glucose to fructose in the presence of ethanol, matching the performance of optimized Sn-containing Lewis acidic zeolites. Differently from zeolites, NMR studies with isotopically labeled molecules demonstrate that isomerization of glucose to fructose on this catalyst, proceeds predominantly via a proton transfer mechanism.
The development of catalysts capable of fast, robust C-H bond amination under mild conditions is an unrealized goal despite substantial progress in the field of C-H activation in recent years. Here we describe a Mn-based metal-organic framework (CPF-5) that promotes the direct amination of C-H bonds with exceptional activity. CPF-5 is capable of functionalizing C-H bonds in an intermolecular fashion with unrivaled catalytic stability producing an unrivaled >105 turnovers.
Metal-organic frameworks (MOFs) are a class of promising materials for diverse catalysis, but they are usually not directly employed for oxygen evolution electrocatalysis. Most reports focus on using MOFs as templates to in-situ create efficient electrocatalysts via annealing. Herein, we prepared a series of Fe/Ni/-based trimetallic MOFs (Fe/Ni/Co(Mn)-Material of Institute Lavoisier (MIL-53)) by solvothermal synthesis, which can be directly adopted as electrocatalysts. It was revealed that the Fe/Ni/Co(Mn)-MIL-53 exhibited volcano-type oxygen evolution reaction (OER) activity as a function of compositions. The optimized Fe/Ni2.4/Co0.4-MIL-53 can reach the current density of 20 mA*cm-2 at low overpotential of 236 mV with small Tafel slope of 52.2 mV*dec-1. The OER performance of these MOFs can be further enhanced by directly being grown on nickel foam (NF), in which the optimized Fe/Ni/Mn0.4-MIL-53/NF exhibits low overpotential of 290 mV at 500 mA*cm-2 as well as high durability.
Metal-organic frameworks (MOFs) are a class of promising materials for diverse catalysis, but they are usually not directly employed for oxygen evolution electrocatalysis. Most reports focus on using MOFs as templates to in-situ create efficient electrocatalysts via annealing. Herein, we prepared a series of Fe/Ni/-based trimetallic MOFs (Fe/Ni/Co(Mn)-Material of Institute Lavoisier (MIL-53)) by solvothermal synthesis, which can be directly adopted as electrocatalysts. It was revealed that the Fe/Ni/Co(Mn)-MIL-53 exhibited volcano-type oxygen evolution reaction (OER) activity as a function of compositions. The optimized Fe/Ni2.4/Co0.4-MIL-53 can reach the current density of 20 mA*cm-2 at low overpotential of 236 mV with small Tafel slope of 52.2 mV*dec-1. The OER performance of these MOFs can be further enhanced by directly being grown on nickel foam (NF), in which the optimized Fe/Ni/Mn0.4-MIL-53/NF exhibits low overpotential of 290 mV at 500 mA*cm-2 as well as high durability.
Nitrones are key intermediates in organic synthesis and the pharmaceutical industry. The heterogeneous synthesis of nitrones with multifunctional catalysts is extremely attractive but rarely explored. Herein, we report ultrasmall platinum nanoclusters (Pt NCs) encapsulated in amine-functionalized UiO-66-NH₂ (Pt@UiO-66-NH₂) as a multifunctional catalyst in the one-pot tandem synthesis of nitrones. By virtue of the cooperative interplay among the selective hydrogenation activity provided by the ultrasmall Pt NCs and Lewis acidity/basicity/nanoconfinement endowed by UiO-66-NH₂, Pt@UiO-66-NH₂ exhibits remarkable activity and selectivity, in comparison to Pt/carbon, Pt@UiO-66, and Pd@UiO-66-NH₂. Pt@UiO-66-NH₂ also outperforms Pt nanoparticles supported on the external surface of the same MOF (Pt/UiO-66-NH₂). To the best of our knowledge, this work demonstrates the first examples of one-pot synthesis of nitrones using recyclable multifunctional heterogeneous catalysts.
Nitrones are key intermediates in organic synthesis and the pharmaceutical industry. The heterogeneous synthesis of nitrones with multifunctional catalysts is extremely attractive but rarely explored. Herein, we report ultrasmall platinum nanoclusters (Pt NCs) encapsulated in amine-functionalized UiO-66-NH₂ (Pt@UiO-66-NH₂) as a multifunctional catalyst in the one-pot tandem synthesis of nitrones. By virtue of the cooperative interplay among the selective hydrogenation activity provided by the ultrasmall Pt NCs and Lewis acidity/basicity/nanoconfinement endowed by UiO-66-NH₂, Pt@UiO-66-NH₂ exhibits remarkable activity and selectivity, in comparison to Pt/carbon, Pt@UiO-66, and Pd@UiO-66-NH₂. Pt@UiO-66-NH₂ also outperforms Pt nanoparticles supported on the external surface of the same MOF (Pt/UiO-66-NH₂).
The past decade has seen the subject of transition metal-free catalytic hydrogenation develop incredibly rapidly, transforming from a largely hypothetical possibility to a well-established field that can be applied to the reduction of a diverse variety of functional groups under mild conditions. This remarkable change is principally attributable to the development of so-called ‘frustrated Lewis pairs’: unquenched combinations of bulky Lewis acids and bases whose dual reactivity can be exploited for the facile activation of otherwise inert chemical bonds. While a number of comprehensive reviews into frustrated Lewis pair chemistry have been published in recent years, this tutorial review aims to provide a focused guide to the development of efficient FLP hydrogenation catalysts, through identification and consideration of the key factors that govern their effectiveness. Following discussion of these factors, their importance will be illustrated using a case study from our own research, namely the development of FLP protocols for successful hydrogenation of aldehydes and ketones, and for related moisture-tolerant hydrogenation.
The search for versatile heterogeneous catalysts with multiple active sites for a broad of asymmetric transformations has long been of great interest, but it remains a formidable synthetic challenge. Here we demonstrate that multivariate metal-organic frameworks (MTV-MOFs) can be used as an excellent platform to engineer heterogeneous catalysts featuring multiple and cooperative active sites. An isostructural series of twofold interpenetrated MTV-MOFs that contain up to three different chiral metallosalen catalysts were constructed and used as efficient and recyclable heterogeneous catalysts for a variety of asymmetric sequential alkene epoxidation/epoxide ring-opening reactions. Interpenetration of the frameworks brings metallosalen units adjacent to each other, allowing cooperative activation, which results in improved efficiency and enantioselectivity over the sum of the individual parts. The fact that manipulation of molecular catalysts in MTV-MOFs can control the activi-ties and selectivities would facilitate the design of novel multifunctional materials for enantioselective processes.
We show that an enzyme maintains its biological function under a wider range of conditions after being embedded in metal-organic framework (MOF) microcrystals via a de novo approach. This enhanced stability arises from confinement of the enzyme molecules in the mesoporous cavities in the MOFs, which reduces the structural mobility of enzyme molecules. We embedded catalase (CAT) into zeolitic imidazolate frameworks (ZIF-90 and ZIF-8), and then exposed both embedded CAT and free CAT to a denaturing reagent (i.e., urea) and high temperatures (i.e., 80 °C). The embedded CAT maintains its biological function in the decomposition of hydrogen peroxide even when exposed to 6 M urea and 80 °C, with apparent rate constants kobs (s(-1)) of 1.30 × 10(-3) and 1.05 × 10(-3), respectively, while free CAT shows undetectable activity. A fluorescence spectroscopy study shows that the structural conformation of the embedded CAT changes less under these denaturing conditions than free CAT. Furthermore, we tested CAT that was loosely confined in mesoporous silica (i.e., siliceous mesocellular foam; MCF), and the results show that the tight confinement provided by the de novo approach in MOFs is critical for reducing the structural change and maintaining enzyme activity.
Porous polymer networks based on sterically encumbered triphenylphosphine motifs, mimicking the basic sites employed in frustrated Lewis pair chemistry, were synthesized via Yamamoto polymerization and their interactions with the strong Lewis acid B(C6F5)3 probed. The combinations yield semi-immobilized frustrated Lewis pairs, one of which is able to cleave dihydrogen heterolytically at ambient temperature and low hydrogen pressure.
The selectivity control toward aldehyde in the aromatic alcohol oxidation remains a grand challenge using molecular oxygen under mild conditions. In this work, we have designed and synthesized Pt/PCN-224(M) composites by integration of Pt nanocrystals and porphyrinic metal-organic frameworks (MOFs), PCN-224(M). The composites exhibit excellent catalytic performance in the photo-oxidation of aromatic alcohols by 1 atm O2 at ambient temperature, based on synergetic photothermal effect and singlet oxygen production. Additionally, in opposition to the function of Schottky junction, the injection of hot electrons from plasmonic Pt into PCN-224(M) would lower the electron density of Pt surface, which thus is tailorable for the optimized catalytic performance via the competition between Schottky junction and plasmonic effect by altering the light intensity. To the best of our knowledge, this is not only an unprecedented report on singlet oxygen-engaged selective oxidation of aromatic alcohols to aldehydes but also the first report on photothermal effect of MOFs.
Molybdenum(VI) oxide was deposited on the Zr6 node of the mesoporous metal-organic framework NU-1000 via condensed-phase deposition where the MOF is simply submerged in the precursor solution, a process named solvothermal deposition in MOFs (SIM). Exposure to oxygen leads to a monodisperse, porous heterogeneous catalyst, named Mo-SIM, and its structure on the node was elucidated both computationally and spectroscopically. The catalytic activity of Mo-SIM was tested for the epoxidation of cyclohexene. Near-quantitative yields of cyclohexene oxide and the ring-opened 1,2-cyclohexanediol were observed, indicating activity significantly higher than that of molybdenum(VI) oxide powder and comparable to that of a zirconia-supported analogue (Mo-ZrO2) prepared in a similar fashion. Despite the well-known leaching problem of supported molybdenum catalysts (i.e., loss of Mo species thus causes deactivation), Mo-SIM demonstrated no loss in the metal loading before and after catalysis, and no molybdenum was detected in the reaction mixture. In contrast, Mo-ZrO2 led to significant leaching and close to 80 wt % loss of the active species. The stability of Mo-SIM was further confirmed computationally, with density functional theory calculations indicating that the dissociation of the molybdenum(VI) species from the node of NU-1000 is endergonic, corroborating the experimental data for the Mo-SIM material.
Carbon dioxide (CO2 ), as the primary greenhouse gas in the atmosphere, triggers a series of environmental and energy related problems in the world. Therefore, there is an urgent need to develop multiple methods to capture and convert CO2 into useful chemical products, which can significantly improve the environment and promote sustainable development. Over the past several decades, metal-organic frameworks (MOFs) have shown outstanding heterogeneous catalytic activity due in part to their high internal surface area and chemical functionalities. These properties and the ability to synthesize MOF platforms allow experiments to test structure-function relationships for transforming CO2 into useful chemicals. Herein, recent developments are highlighted for MOFs participating as catalysts for the chemical fixation and photochemical reduction of CO2 . Finally, opportunities and challenges facing MOF catalysts are discussed in this ongoing research area.
Synthesis of CF3-containing compounds is of great interest because of their broad use in the phar- maceutical and agrochemical industries. Herein, selec- tive 2,2,2-trifluoroethylation of styrenes was catalyzed by Zr(IV)-based MOFs bearing visible-light photocata- lysts in the form of Ir(III) polypyridyl complexes. When compared to the homogenous Ir(III) catalyst, the MOF- based catalyst suppressed the dimerization of benzyl radicals, thus enhancing the selectivity of the desired hydroxytrifluoroethyl compounds.
Inspired by the zwitterion species generated from the splitting of H2 by frustrated Lewis pairs, we put forward a novel frustrated Lewis pair by the combination of Hδ- and Hδ+ incorporated Lewis acid and base together. Piers’ borane and chiral tert-butylsulfinamide were chosen as the FLP, and a metal-free asymmetric transfer hydrogenation of imines has been realized with high enantioselectivities. Significantly, with ammonia borane as hydrogenation source, a catalytic asymmetric reaction using 10 mol % of Piers’ borane, tert-butylsulfinamide, and pyridine additive, has been successfully achieved to furnish optically active amines in 78-99% yields with 84-95% ees. Experimental and theoretical mechanistic studies reveal an interesting 8-membered ring hydrogen transfer transition state and an expected regeneration of reactive species with ammonia borane. Accordingly, a possible catalytic pathway for this reaction is predicted.
A tetra(carboxylated) PCP pincer ligand has been synthesized as a building block for porous coordination polymers (PCPs). The air- and moisture-stable PCP metalloligands are rigid tetratopic linkers that are geometrically akin to ligands used in the synthesis of robust metal–organic frameworks (MOFs). Here, the design principle is demonstrated by cyclometalation with PdIICl and subsequent use of the metalloligand to prepare a crystalline 3D MOF by direct reaction with CoII ions and structural resolution by single crystal X-ray diffraction. The Pd−Cl groups inside the pores are accessible to post-synthetic modifications that facilitate chemical reactions previously unobserved in MOFs: a Pd−CH3 activated material undergoes rapid insertion of CO2 gas to give Pd−OC(O)CH3 at 1 atm and 298 K. However, since the material is highly selective for the adsorption of CO2 over CO, a Pd−N3 modified version resists CO insertion under the same conditions.
A tetra(carboxylated) PCP pincer ligand has been synthesized as a building block for porous coordination polymers (PCPs). The air- and moisture-stable PCP metalloligands are rigid tetratopic linkers that are geometrically akin to ligands used in the synthesis of robust metal-organic frameworks (MOFs). Here, the design principle is demonstrated by cyclometalation with Pd(II) Cl and subsequent use of the metalloligand to prepare a crystalline 3D MOF by direct reaction with Co(II) ions and structural resolution by single crystal X-ray diffraction. The Pd-Cl groups inside the pores are accessible to post-synthetic modifications that facilitate chemical reactions previously unobserved in MOFs: a Pd-CH3 activated material undergoes rapid insertion of CO2 gas to give Pd-OC(O)CH3 at 1 atm and 298 K. However, since the material is highly selective for the adsorption of CO2 over CO, a Pd-N3 modified version resists CO insertion under the same conditions.
We report the direct synthesis of a solid-phase frustrated Lewis pair (s-FLP) by combining a silica-supported Lewis acid ([triple bond, length as m-dash]SiOB(C6F5)2, s-BCF) with a Lewis base (tri-tert-butylphosphine, (t)Bu3P) to give [[triple bond, length as m-dash]SiOB(C6F5)2][(t)Bu3P]. Reaction of this s-FLP with H2 under mild conditions led to heterolytic H-H bond cleavage and the formation of [[triple bond, length as m-dash]SiOB(H)(C6F5)2][(t)Bu3PH].
Metal−organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.
Immobilized enzymes typically have greater thermal and operational stability than their soluble form. Here we report that for the first time, a nerve agent detoxifying enzyme, organophosphorus acid anhydrolase (OPAA), has been successfully encapsulated into a water-stable zirconium metal-organic framework (MOF). This MOF features a hierarchical mesoporous channel structure and exhibits a 12 wt % loading capacity of OPAA. The thermal and long-term stabilities of OPAA are both significantly enhanced after immobilization.
Improving the efficiency of electron–hole separation and charge-carrier utilization plays a central role in photocatalysis. Herein, Pt nanoparticles of ca. 3 nm are incorporated inside or supported on a representative metal–organic framework (MOF), UiO-66-NH2, denoted as Pt@UiO-66-NH2 and Pt/UiO-66-NH2, respectively, for photocatalytic hydrogen production via water splitting. Compared with the pristine MOF, both Pt-decorated MOF nanocomposites exhibit significantly improved yet distinctly different hydrogen-production activities, highlighting that the photocatalytic efficiency strongly correlates with the Pt location relative to the MOF. The Pt@UiO-66-NH2 greatly shortens the electron-transport distance, which favors the electron–hole separation and thereby yields much higher efficiency than Pt/UiO-66-NH2. The involved mechanism has been further unveiled by means of ultrafast transient absorption and photoluminescence spectroscopy.
The C4-bridged unsaturated phosphane/borane frustrated Lewis pairs (P/B FLPs) 4 undergo borane induced phosphane addition to a variety of acetylenic esters or ketones to generate heterocyclic ten-membered intermediates that contain pairs of allyl phosphonium/allenic enolate functionalities. These subsequently undergo phospha-Claisen type rear-rangement reactions to give the respective substituted phosphanyl pentadiene products. In two exceptional cases sub-sequent reactions leading to anomalous phospha-Claisen products were found. One example involved carbon-carbon bond activation, the other cyclopropane ring formation. Potential mechanistic schemes leading to these products are discussed. Essential examples were characterized by X-ray diffraction.
Here we present the first example of a single-site main group catalyst stabilized by a metal-organic framework (MOF) for organic transformations. The straightforward metalation of the secondary building units of a Zr-MOF with Me2Mg affords a highly active and reusable solid catalyst for hydroboration of carbonyls and imines and for hydroamination of aminopentenes. Impressively, the Mg-functionalized MOF displayed very high turnover numbers of up to 8.4 × 10(4) for ketone hydroboration and could be reused more than 10 times. MOFs can thus be used to develop novel main group solid catalysts for sustainable chemical synthesis.
Herein we report on the catalytic polymerization of diverse Michael-type monomers with high precision by using simple but highly active combinations of phosphorus containing Lewis bases and organoaluminum compounds. The interacting Lewis pair catalysts enable the control of molecular weight and microstructure of the produced polymers. The reactions show a linear Mn vs. consumption plot thus proving a living type polymerization. The initiation has been investigated by end group analysis with ESI mass spectrometric analysis. With these main-group element Lewis acid base pairs it is not only possible to polymerize sterically demanding, functionalized as well as heteroatom containing monomers, it is for the first time possible to catalytically polymerize extended Michael systems, like 4-vinylpyridine.
Surface wettability of active sites plays a crucial role in the activity and selectivity of catalysts. This report describes modification of surface hydrophobicity of Pd/UiO-66, a composite comprising a metal-organic framework (MOF) and stabilized palladium nanoparticles (NPs), using a simple polydimethylsiloxane (PDMS) coating. The modified catalyst demonstrated significantly improved catalytic efficiency. The approach can be extended to various Pd nanoparticulate catalysts for enhanced activity in reactions involving hydrophobic reactants, as the hydrophobic surface facilitates the enrichment of hydrophobic substrates around the catalytic site. PDMS encapsulation of Pd NPs prevents aggregation of NPs and thus results in superior catalytic recyclability. Additionally, PDMS coating is applicable to a diverse range of catalysts, endowing them with additional selectivity in sieving reactants with different wettability.
We have designed a strategy for postsynthetic installation of the -diketiminate ("NacNac") functionality in a metal-organic framework (MOF) of UiO-topology. Metalation of the NacNac-MOF (I) with earth-abundant metal salts afforded the desired MOF-supported NacNac-M complexes (M= Fe, Cu, and Co) with coordination environments established by detailed EXAFS studies. The NacNac-Fe-MOF catalyst, I•Fe(Me), efficiently catalyzed the challenging intramolecular sp3 C-H amination of a series of alkyl azides to afford -substituted pyrrolidines. The NacNac-Cu-MOF catalyst, I•Cu(THF), was effective in promoting the intermolecular sp3 C-H amination of cyclohexene using unprotected anilines to provide access to secondary amines in excellent selectivity. Finally, the NacNac-Co-MOF catalyst, I•Co(H), was used to catalyze alkene hydrogenation with turnover numbers (TONs) as high as 700,000. All of the NacNac-M-MOF catalysts were more effective than their analogous homogeneous catalysts and could be recycled and reused without a noticeable decrease in yield. The NacNac-MOFs thus provide a novel platform for engineering recyclable earth-abundant element-based single-site solid catalysts for many important organic transformations.
New and active earth-abundant metal catalysts are critically needed to replace precious metal-based catalysts for sustaina-ble production of commodity and fine chemicals. We report here the design of highly robust, active, and reusable cobalt-bipyridine and cobalt-phenanthroline based metal-organic framework (MOF) catalysts for alkene hydrogenation and hydroboration, aldehyde/ketone hydroboration, and arene C-H borylation. In alkene hydrogenation, the MOF catalysts dis-played unprecedentedly high turnover numbers of ~2.5×106 and turnover frequencies of ~1.1×105 h-1. Structural, computational, and spectroscopic studies show that site isolation of the highly reactive (bpy)Co(THF)2 species in the MOFs prevents intermolecular deactivation and stabilizes solution-inaccessible catalysts for broad-scope organic transformations. Compu-tational, spectroscopic, and kinetic evidences further support a hitherto unknown (bpy∙ )CoI(THF)2 ground state that coordi-nates to alkene and dihydrogen, then undergoing sigma-complex assisted metathesis to form (bpy)Co(alkyl)(H). Reductive elimination of alkane followed by alkene binding completes the catalytic cycle. MOFs thus provide a novel platform for discovering new base-metal molecular catalysts and exhibit enormous potential in sustainable chemical catalysis.
A porous metal-organic framework Zr6O4(OH)4(L-PdX)3 (1-X) has been constructed from Pd diphosphinite pincer complexes ([L-PdX](4-) = [(2,6-(OPAr2)2C6H3)PdX](4-), Ar = p-C6H4CO2(-), X = Cl, I). Reaction of 1-X with PhI(O2CCF3)2 facilitates I(-)/CF3CO2(-) ligand exchange to generate 1-TFA and I2 as a soluble byproduct. 1-TFA is an active and recyclable catalyst for transfer hydrogenation of benzaldehydes using formic acid as a hydrogen source. In contrast, the homogeneous analogue (t)Bu(L-PdTFA) is an ineffective catalyst owing to decomposition under the catalytic conditions, highlighting the beneficial effects of complex immobilization.
The effects of surface acidity on the cascade ethanol-to-isobutene conversion were studied using ZnxZryOz catalysts. The ethanol-to-isobutene reaction was found to be limited by the secondary reaction of the key intermediate, acetone, namely the acetone-to-isobutene reaction. Although the catalysts with coexisting Brønsted acidity could catalyze the rate-limiting acetone-to-isobutene reaction, the presence of Brønsted acidity is also detrimental. First, secondary isobutene isomerization is favored, producing a mixture of butene isomers. Second, undesired polymerization and coke formation prevail, leading to rapid catalyst deactivation. Most importantly, both steady-state and kinetic reaction studies as well as FTIR analysis of adsorbed acetone-d6 and D2O unambiguously showed that a highly active and selective nature of balanced Lewis acid-base pairs was masked by the coexisting Brønsted acidity in the aldolization and self-deoxygenation of acetone to isobutene. As a result, ZnxZryOz catalysts with only Lewis acid-base pairs were discovered, on which nearly a theoretical selectivity to isobutene (~88.9%) was successfully achieved, which has never been reported before. Moreover, the absence of Brønsted acidity in such ZnxZryOz catalysts also eliminates the side isobutene isomerization and undesired polymerization/coke reactions, resulting in the production of high purity isobutene with significantly improved catalyst stability (< 2% activity loss after 200 h time-on-stream). This work not only demonstrates a balanced Lewis acid-base pair for the highly active and selective cascade ethanol-to-isobutene reaction, but also sheds light on the rational design of selective and robust acid-base catalyst for C-C coupling via aldolization reaction.
The one-step polycondensation of diamines and diboranes triggered by the in situ deprotonation of the diammonium salts and concomitant reduction of bisboronic acids leads to the assembly of polymer chains through multiple Lewis pairing in their backbone. These new polyboramines are dihydrogen reservoirs that can be used for the hydrogenation of imines and carbonyl compounds. They also display a unique dihydrogen thermal release profile that is a direct consequence of the insertion of the amine-borane linkages in the polymeric backbone.
B/N Lewis pairs have been discovered to catalyze rapid epimerization of meso-lactide (LA) or LA diastereomers quantitatively into rac-LA. The obtained rac-LA is kinetically polymerized into poly(L-lactide) and optically resolved D-LA, with a high stereoselectivity factor kL/kD of 53 and an ee value of 91% at 50.6% monomer conversion, by newly designed bifunctional chiral catalyst 4 that incorporates three key elements (β-isocupreidine core, thiourea functionality, and chiral BINAM) into a single organic molecule. The epimerization and enantioselective polymerization can be coupled into a one-pot process for transforming meso-LA directly into poly(L-lactide) and D-LA.
The articulation of the notion of "frustrated Lewis pairs" (FLPs), which emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered Lewis acids and bases, has prompted a great deal of recent activity. Perhaps the most remarkable consequence has been the development of FLP catalysts for the hydrogenation of a range of organic substrates. In the past 9 years, the substrate scope has evolved from bulky polar species to include a wide range of unsaturated organic molecules. In addition, effective stereoselective metal-free hydrogenation catalysts have begun to emerge. The mechanism of this activation of H2 has been explored and the nature and range of Lewis acid/base combinations capable of effecting such activation have also expanded to include a variety of non-metal species. The reactivity of FLPs with a variety of other small molecules including olefins, alkynes, and a range of element oxides have also been developed. Although much of this latter chemistry has uncovered unique stoichiometric transformations, metal-free catalytic hydroamination, CO2 reduction chemistry, and applications in polymerization have also been achieved. The concept is also beginning to find applications in bioinorganic and materials chemistry as well as heterogeneous catalysis. This perspective article high-lights many of these developments and discusses the relation-ship between FLPs and established chemistry. Some of the directions and developments that are likely to emerge from FLP chemistry in the future are also presented.
The past 20 years have witnessed rapid development of reticular chemistry, which is concerned with linking molecular building units with strong chemical bonds to form crystalline, extended structures. Given such flexibility in MOF chemistry, it was, indeed, only a matter of time before the first example of a Lewis acidic MOF was reported for catalyzing the cyanosilylation reaction of aldehydes. In successive years, coordinative unsaturation or open metal sites on metal SBUs were made and found to be suitable as Lewis acid sites. In this case, several coordination positions of the metal center are occupied by solvent molecules, which can be removed by heating or evacuation during the activation process without framework collapse. The guest molecules can be acidified after their inclusion to further increase the amount or strength of their Bronsted acidity.
Efficient catalytic reduction of CO2 is critical for the large-scale utilization of this greenhouse gas. We have used density functional electronic structure methods to design a catalyst for producing formic acid from CO2 and H2 via a two-step pathway having low reaction barriers. The catalyst consists of a microporous metal organic framework that is functionalized with Lewis pair moieties. These functional groups are capable of chemically binding CO2 and heterolytically dissociating H2. Our calculations indicate that the porous framework remains stable after functionalization and chemisorption of CO2 and H2. We have identified a low barrier pathway for simultaneous addition of hydridic and protic hydrogens to carbon and oxygen of CO2, respectively, producing a physisorbed HCOOH product in the pore. We find that activating H2 by dissociative adsorption leads to a much lower energy pathway for hydrogenating CO2 than reacting H2 with chemisorbed CO2. Our calculations provide design strategies for efficient catalysts for CO2 reduction.
Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Herein, we demonstrate that the incorporation of an acidic hydrogen-bond-donating squaramide moiety into a porous UiO-67 metal-organic framework (MOF) derivative leads to dramatic acceleration of the biorelevant Friedel-Crafts reaction between indole and β-nitrostyrene. In comparison, it is shown that free squaramide derivatives, not incorporated into MOF architectures, have no catalytic activity. Additionally, using the UiO-67 template, we were able to perform a direct comparison of catalytic activity with that of the less acidic urea-based analogue. This is the first demonstration of the functionalization of a heterogeneous framework with an acidic squaramide derivative.
Silver nanoparticles were successfully supported on the zeolite-type metal–organic framework MIL-101 to yield Ag@MIL-101 by a simple liquid impregnation method. For the first time, the conversion of terminal alkynes into propiolic acids with CO2 was achieved by the use of the Ag@MIL-101 catalysts. Owing to the excellent catalytic activity, the reaction proceeded at atmospheric pressure and low temperature (50 °C). The Ag@MIL-101 porous material is of outstanding bifunctional character as it is capable of simultaneously capturing and converting CO2 with low energy consumption and can be recovered easily by centrifugation.
Silver nanoparticles were successfully supported on the zeolite-type metal–organic framework MIL-101 to yield Ag@MIL-101 by a simple liquid impregnation method. For the first time, the conversion of terminal alkynes into propiolic acids with CO2 was achieved by the use of the Ag@MIL-101 catalysts. Owing to the excellent catalytic activity, the reaction proceeded at atmospheric pressure and low temperature (50 °C). The Ag@MIL-101 porous material is of outstanding bifunctional character as it is capable of simultaneously capturing and converting CO2 with low energy consumption and can be recovered easily by centrifugation.
Two chiral carboxylic acid functionalized micro- and mesoporous metal–organic frameworks (MOFs) are constructed by the stepwise assembly of triple-stranded heptametallic helicates with six carboxylic acid groups. The mesoporous MOF with permanent porosity functions as a host for encapsulation of an enantiopure organic amine catalyst by combining carboxylic acids and chiral amines in situ through acid–base interactions. The organocatalyst-loaded framework is shown to be an efficient and recyclable heterogeneous catalyst for the asymmetric direct aldol reactions with significantly enhanced stereoselectivity in relative to the homogeneous organocatalyst.
Cadmium Sulfide (CdS) quantum dots (<10 nm in size) have been successfully synthesized in situ without any capping agent in a Zn(II) based low-molecular weight metallohydrogel (ZAVA). Pristine ZAVA hydrogel shows blue luminescence but the emission can be tuned upon encapsulation of the CdS quantum dots. Time dependent tunable emission (white-to-yellow-to-orange) of the CdS incubated gel (CdS@ZAVA gel) can be attributed to sluggish growth of the quantum dots inside the gel matrix. Once entrapped, the augmentation of CdS quantum dots can be ceased by converting the gel into xerogel wherein the quantum dots remains embedded in the solid xerogel matrix. Similar size-stabilization of CdS quantum dots can be achieved by means of a unique room temperature conversion of the CdS incubated ZAVA gel to CdS incubated MOF (CdS@ZAVCl) crystals. This in turn arrests the tunability in emission owing to the restriction in the growth of CdS quantum dots inside xerogel and MOF. These CdS embedded MOFs have been utilized as a catalyst for water splitting under visible light.
Hybrid materials of the metal–organic framework (MOF), chromium(III) terephthalate (MIL-101), and phosphotungstic acid (PTA) were synthesized in aqueous media in the absence of hydrofluoric acid. XRD analysis of the MIL101/PTA composites indicates the presence of ordered PTA assemblies residing in both the large cages and small pores of MIL-101, which suggests the formation of previously undocumented structures. The MIL101/PTA structure enables a PTA payload 1.5–2 times higher than previously achieved. The catalytic performance of the MIL101/PTA composites was assessed in the Baeyer condensation of benzaldehyde and 2-naphthol, in the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, and in the epoxidation of caryophyllene by hydrogen peroxide. The catalytic efficiency was demonstrated by the high (over 80–90%) conversion of the reactants under microwave-assisted heating. In four consecutive reaction cycles, the catalyst recovery was in excess of 75%, whereas the product yields were maintained above 92%. The simplicity of preparation, exceptional stability, and reactivity of the novel composites indicate potential in utilization of these catalytic matrices in a multitude of catalytic reactions and engineering processes.
Dual systems combining Zn(C6F5)2 with an organic base (an amine or a phosphine) promote the controlled ring-opening polymerization of lactide and e-caprolactone. The Lewis pairs cooperate to activate the monomers, affording well-defined high-molecular weight cyclic polyesters. Efficient chain-extension gives access to cyclic block copolymers.