Highly Active Gas Phase Organometallic Catalysis Supported Within Metal-Organic Framework Pores

Low-Valent Metals in Metal-Organic Frameworks Via Post-Synthetic Modification

Metal-organic frameworks (MOFs) provide uniquely tunable, periodic platforms for site-isolation of reactive low-valent metal complexes of relevance in modern catalysis, adsorptive applications, and fundamental structural studies. Strategies for integrating such species in MOFs include post-synthetic metalation, encapsulation and direct synthesis using low-valent organometallic complexes as building blocks. These approaches have each proven effective in enhancing catalytic activity, modulating product distributions (i.e., by improving catalytic selectivity), and providing valuable mechanistic insights. In this minireview, we explore these different strategies, as applied to isolate low-valent species within MOFs, with a particular focus on examples that leverage the unique crystallinity, permanent porosity and chemical mutability of MOFs to achieve deep structural insights that lead to new paradigms in the field of hybrid catalysis.

Heterogenization of molecular catalysts within porous solids: the case of Ni-catalyzed ethylene oligomerization from zeolites to metal-organic frameworks

The last decade has seen a tremendous expansion of the field of heterogenized molecular catalysis, especially with the growing interest in metal-organic frameworks and related porous hybrid solids. With successful achievements in the transfer from molecular homogeneous catalysis to heterogenized processes come the necessary discussions on methodologies used and a critical assessment on the advantages of heterogenizing molecular catalysis. Here we use the example of nickel-catalyzed ethylene oligomerization, a reaction of both fundamental and applied interest, to review heterogenization methodologies of well-defined molecular catalysts within porous solids while addressing the biases in the comparison between original molecular systems and heterogenized counterparts.

Studying manganese carbonyl photochemistry in a permanently porous metal-organic framework

Mn(diimine)(CO)3X (X = halide) complexes are critical components of chromophores, photo- and electrocatalysts, and photoactive CO-releasing molecules (photoCORMs). While these entities have been incorporated into metal-organic frameworks (MOFs), a detailed understanding of the photochemical and chemical processes that occur in a permanently porous support is lacking. Here we site-isolate and study the photochemistry of a Mn(diimine)(CO)3Br moiety anchored within a permanently porous MOF support, allowing for not only the photo-liberation of CO from the metal but also its escape from the MOF crystals. In addition, the high crystallinity and structural flexibility of the MOF allows crystallographic snapshots of the photolysis products to be obtained. We report these photo-crystallographic studies in the presence of coordinating solvents, THF and acetonitrile, showing the changing coordination environment of the Mn species as CO loss proceeds. Using time resolved experiments, we report complementary spectroscopic studies of the photolysis chemistry and characterize the final photolysis product as a possible Mn(ii) entity. These studies inform the chemistry that occurs in MOF-based photoCORMs and where these moieties are employed as catalysts.

Evaluation of the catalytic activity of Zn-MOF-74 for the alcoholysis of cyclohexene oxide

In the present work, nanocrystalline Zn-MOF-74 is shown to be a heterogeneous catalyst for the acid-catalyzed ring-opening alcoholysis of cyclohexene oxide. The results corroborated that accessible open metal sites within the material are critical conditions (Zn(ii) Lewis acid sites) for this reaction. Zn-MOF-74 was tested at three different temperatures (30, 40, and 50 °C) for the alcoholysis reaction. Furthermore, the cyclohexene oxide conversion was 94% in less than two days. A comparison of the catalytic activity with different crystal sizes of Zn-MOF-74 and the homogenous phase, zinc acetate, was conducted. Zn-MOF-74 exhibited excellent catalytic cyclability for three cycles without losing its activity. The material showed chemical stability by retaining its crystalline structure after the reaction and cyclability process.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn₂(SDBA)(3-ATZ)₂]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

Advances in surface-modified nanometal-organic frameworks for drug delivery

Nanometal-organic frameworks (NMOFs) are porous network structures composed of metal ions or metal clusters through self-assembly. NMOFs have been considered as a promising nano-drug delivery system due to their unique properties such as pore and flexible structures, large specific surface areas, surface modifiability, non-toxic and degradable properties. However, NMOFs face a series complex environment during in vivo delivery. Therefore, surface functionalization of NMOFs is vital to ensure that the structure of NMOFs remain stable during delivery, and can overcome physiological barriers to deliver drugs more accurately to specific sites, and achieve controllable release. In this review, the first part summarizes the physiological barriers that NMOFs faced during drug delivery after intravenous injection and oral administration. The second part summarizes the current main ways to load drugs into NMOFs, mainly including pore adsorption, surface attachment, formation of covalent/coordination bonds between drug molecules and NMOFs, and in situ encapsulation. The third part is the main review part of this paper, which summarizes the surface modification methods of NMOFs used in recent years to overcome the physiological barriers and achieve effective drug delivery and disease therapy, which are mainly divided into physical modifications and chemical modifications. Finally, the full text is summarized and prospected, with the hope to provide ideas for the future development of NMOFs as drug delivery.

Conceptual and Practical Aspects of Metal-Organic Frameworks for Solid-Gas Reactions

The presence of site-isolated and well-defined metal sites has enabled the use of metal-organic frameworks (MOFs) as catalysts that can be rationally modulated. Because MOFs can be addressed and manipulated through molecular synthetic pathways, they are chemically similar to molecular catalysts. They are, nevertheless, solid-state materials and therefore can be thought of as privileged solid molecular catalysts that excel in applications involving gas-phase reactions. This contrasts with homogeneous catalysts, which are overwhelmingly used in the solution phase. Herein, we review theories dictating gas phase reactivity within porous solids and discuss key catalytic gas-solid reactions. We further treat theoretical aspects of diffusion within confined pores, the enrichment of adsorbates, the types of solvation spheres that a MOF might impart on adsorbates, definitions of acidity/basicity in the absence of solvent, the stabilization of reactive intermediates, and the generation and characterization of defect sites. The key catalytic reactions we discuss broadly include reductive reactions (olefin hydrogenation, semihydrogenation, and selective catalytic reduction), oxidative reactions (oxygenation of hydrocarbons, oxidative dehydrogenation, and carbon monoxide oxidation), and C-C bond forming reactions (olefin dimerization/polymerization, isomerization, and carbonylation reactions).

3D nanocrystalline metal-organic framework materials for the improved output performance of triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) based on contact electrification and electrostatic induction can effectively convert low-frequency mechanical energy into electrical energy and has attracted considerable attention. However, the low current output performance seriously hinders the wide application of TENGs. Herein, a 3D nanocrystalline metal-organic framework (Cd-MOF) with a specific structure and morphology was reasonably designed as a high-performance triboelectric positive electrode material. The triboelectric test results showed that the maximum instantaneous short-circuit current of Cd-MT was 55.32 μA and the stable output performance maintained a long-term continuous operation for 10 000 s. The peak values of the charge density and electric power density were 102.39 μC m^-2 and 2451.04 mW m^-2, respectively. In addition, the Cd-MT could quickly fully charge commercial capacitors and light a large number of LED lamps. This work provides a new idea for the development and design of functional MOF triboelectric materials.

Switchable Metal Sites in Metal-Organic Framework MFM-300(Sc): Lewis Acid Catalysis Driven by Metal-Hemilabile Linker Bond Dynamics

Uncommon reversible guest-induced metal-hemilabile linker bond dynamics in MOF MFM-300(Sc) was unraveled to switch on/switch off catalytic open metal sites. The catalytic activity of this MOF with non-permanent open metal sites was demonstrated using a model Strecker hydrocyanation reaction as a proof-of-concept. Conclusively, the catalytic activity was evidenced to be fully reversible, preserving the conversion performance and structure integrity of MFM-300(Sc) over multiple cycles. These experimental findings were corroborated by quantum-calculations that revealed a reaction mechanism driven by the Sc-open metal sites. This discovery paves the way towards the design of new effective and easily regenerable heterogeneous MOF catalysts integrating switchable metal sites.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)₂-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time viaoperando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·g_cat^–1·h^–1) under ambient conditions. These performance indicators make UiO-66(COOH)₂-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)₂-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)₂-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)₂-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

Gas sensing based on metal-organic frameworks: Concepts, functions, and developments

Effective detection of pollutant gases is vital for protection of natural environment and human health. There is an increasing demand for sensing devices that are equipped with high sensitivity, fast response/recovery speed, and remarkable selectivity. Particularly, attention is given to the designability of sensing materials with porous structures. Among diverse kinds of porous materials, metal-organic frameworks (MOFs) exhibit high porosity, high degree of crystallinity and exceptional chemical activity. Their strong host-guest interactions with guest molecules facilitate the application of MOFs in adsorption, catalysis and sensing systems. In particular, the tailorable framework/composition and potential for post-synthetic modification of MOFs endow them with widely promising application in gas sensing devices. In this review, we outlined the fundamental aspects and applications of MOFs for gas sensors, and discussed various techniques of monitoring gases based on MOFs as functional materials. Insights and perspectives for further challenges faced by MOFs are discussed in the end.

Templated synthesis of zirconium(IV)-based metal-organic layers (MOLs) with accessible chelating sites

Metal-organic layers (MOLs) are of great interest in heterogeneous catalysis, particularly materials that can accommodate extraneous metal centres. Here, we demonstrate a two-step preorganisation/delamination synthetic strategy using CuI as a template to prepare Zr-based MOLs with accessible 'syn' bis-pyrazolyl chelating sites (named UAM-2·ns) that are poised for quantitative post-synthetic metalation with late transition metals.

Coordination modulated on-off switching of flexibility in a metal-organic framework

Stimuli-responsive metal-organic frameworks (MOFs) exhibit dynamic, and typically reversible, structural changes upon exposure to external stimuli. This process often induces drastic changes in their adsorption properties. Herein, we present a stimuli-responsive MOF, 1·[CuCl], that shows temperature dependent switching from a rigid to flexible phase. This conversion is associated with a dramatic reversible change in the gas adsorption properties, from Type-I to S-shaped isotherms. The structural transition is facilitated by a novel mechanism that involves both a change in coordination number (3 to 2) and geometry (trigonal planar to linear) of the post-synthetically added Cu(i) ion. This process serves to 'unlock' the framework rigidity imposed by metal chelation of the bis-pyrazolyl groups and realises the intrinsic flexibility of the organic link.

Heterogeneous Isomerization for Stereoselective Alkyne Hydrogenation to trans-Alkene Mediated by Frustrated Hydrogen Atoms

Stereoselective production of alkenes from the alkyne hydrogenation plays a crucial role in the chemical industry. However, for heterogeneous metal catalysts, the olefins in cis-configuration are usually dominant in the products due to the most important and common Horiuti-Polanyi mechanism involved over the metal surface. In this work, through combined theoretical and experimental investigations, we demonstrate a novel isomerization mechanism mediated by the frustrated hydrogen atoms via the H₂ dissociation at the defect on solid surface, which can lead to the switch in selectivity from the cis-configuration to trans-configuration without overhydrogenation. The defective Rh₂S₃ with exposing facet of (110) exhibits outstanding performance as a heterogeneous metal catalyst for stereoselective production of trans-olefins. With the frustrated hydrogen atoms at spatially separated high-valence Rh sites, the isolated hydrogen mediated cis-to-trans isomerization of olefins can be effectively conducted and the overhydrogenation can be completely inhibited. Furthermore, the bifunctional Rh-S/Pd nanosheets have been synthesized through the surface modification of Pd nanosheets with rhodium and sulfide. With the selective semihydrogenation of alkynes into cis-olefins catalyzed by the small surface PdS_x ensembles, the bifunctional Rh-S/Pd nanosheets exhibit excellent activity and stereoselectivity in the one-pot alkyne hydrogenation into trans-olefin, which surpasses the most reported homogeneous and heterogeneous catalysts.

Single-Crystal-to-Single-Crystal Transformations of Metal-Organic-Framework-Supported, Site-Isolated Trigonal-Planar Cu(I) Complexes with Labile Ligands

Transition-metal complexes bearing labile ligands can be difficult to isolate and study in solution because of unwanted dinucleation or ligand substitution reactions. Metal-organic frameworks (MOFs) provide a unique matrix that allows site isolation and stabilization of well-defined transition-metal complexes that may be of importance as moieties for gas adsorption or catalysis. Herein we report the development of an in situ anion metathesis strategy that facilitates the postsynthetic modification of Cu(I) complexes appended to a porous, crystalline MOF. By exchange of coordinated chloride for weakly coordinating anions in the presence of carbon monoxide (CO) or ethylene, a series of labile MOF-appended Cu(I) complexes featuring CO or ethylene ligands are prepared and structurally characterized using X-ray crystallography. These complexes have an uncommon trigonal planar geometry because of the absence of coordinating solvents. The porous host framework allows small and moderately sized molecules to access the isolated Cu(I) sites and displace the "place-holder" CO ligand, mirroring the ligand-exchange processes involved in Cu-centered catalysis.

Immobilization of Rh(I) precursor in a porphyrin metal-organic framework - turning on the catalytic activity

Two model porphyrin metal-organic frameworks were used for the incorporation of Rh(i) species by a post-synthetic metallation under mild conditions. As a result, new rhodium MOFs (Rh/MOFs), Rh/PCN-222 and Rh/NU-1102, were synthesized and structurally characterized. To illustrate the potential of this catalytic platform, we use Rh/MOFs as phosphine-free heterogeneous catalysts in the hydrogenation of unsaturated hydrocarbons under mild reaction conditions (30 °C and 1 atm H2). We found that for our Rh/MOFs an activation step is required during the first run of the catalytic process. The presence of Rh-CO moieties allowed us to monitor the activation pathway of the catalyst under a H2 atmosphere, by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). After activation, the catalyst remains highly active during the subsequent catalytic cycles. This simple post-synthetic modification approach presents new possibilities for the utilization of Rh-based catalytic systems with robust porphyrin-based MOFs as supports.

In crystallo organometallic chemistry

X-ray crystallography is an invaluable tool in design and development of organometallic catalysis, but application typically requires species to display sufficiently high solution concentrations and lifetimes for single crystalline samples to be obtained. In crystallo organometallic chemistry relies on chemical reactions that proceed within the single-crystal environment to access crystalline samples of reactive organometallic fragments that are unavailable by alternate means. This highlight describes approaches to in crystallo organometallic chemistry including (a) solid-gas reactions between transition metal complexes in molecular crystals and diffusing small molecules, (b) reactions of organometallic complexes within the extended lattices of metal-organic frameworks (MOFs), and (c) intracrystalline photochemical transformations to generate reactive organometallic fragments. Application of these methods has enabled characterization of catalytically important transient species, including σ-alkane adducts of transition metals, metal alkyl intermediates implicated in metal-catalyzed carbonylations, and reactive M-L multiply bonded species involved in C-H functionalization chemistry. Opportunities and challenges for in crystallo organometallic chemistry are discussed.

Selectivity of Rh⋅⋅⋅H-C Binding in a σ-Alkane Complex Controlled by the Secondary Microenvironment in the Solid State

Single-crystal to single-crystal solid-state molecular organometallic (SMOM) techniques are used for the synthesis and structural characterization of the σ-alkane complex [Rh(tBu2 PCH2 CH2 CH2 PtBu2 )(η2 ,η2 -C7 H12 )][BArF 4 ] (ArF =3,5-(CF3 )2 C6 H3 ), in which the alkane (norbornane) binds through two exo-C-H⋅⋅⋅Rh interactions. In contrast, the bis-cyclohexyl phosphine analogue shows endo-alkane binding. A comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, traces this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ-alkane complex is also a catalyst for solid/gas 1-butene isomerization, and catalyst resting states are identified for this.

Incorporation of homogeneous organometallic catalysts into metal–organic frameworks for advanced heterogenization: a review

The heterogenization of homogeneous organometallic catalysts by incorporation into MOFs using different strategies, MOF selection, OMC selection, and the use of hybrid heterogeneous catalysts OMC@MOFs in catalytic applications are summarized and discussed.

Elucidating pore chemistry within metal–organic frameworks via single crystal X-ray diffraction; from fundamental understanding to application

The application of metal–organic frameworks (MOFs) to diverse chemical sectors is aided by their crystallinity, which permits the use of X-ray crystallography to characterise their pore chemistry and provides invaluable insight into their properties.

A metal-organic framework supported iridium catalyst for the gas phase hydrogenation of ethylene

The mutable structures of metal-organic frameworks (MOFs) allow their use as novel supports for transition metal catalysts. Herein we prepare an iridium bis(ethylene) catalyst bound to the neutral N-donors of a MOF structure and show that the compound is a stable gas phase ethylene hydrogenation catalyst. The data illustrate the need to carefully consider the inner sphere (support) and outer sphere (anion) chemistry.