Diastereoselective Synthesis of Pyranoquinolines on Zirconium-Containing UiO-66 Metal-Organic Frameworks

Catalysis of a Diels-Alder Reaction between Azachalcones and Cyclopentadiene by a Recyclable Copper(II)-PEIP Metal-Organic Framework

Metal-organic frameworks (MOFs) have attracted considerable interest as emerging heterogeneous catalysts for organic transformations of synthetic utility. Herein, a Lewis-acidic MOF, {[Cu3(PEIP)2(5-NH2-mBDC)(DMF)]·7DMF}∞, denoted as Cu(ΙΙ)-PEIP, has been synthesized via a one-pot process and deployed as an efficient heterogeneous catalyst for a Diels-Alder cycloaddition. Specifically, the [4 + 2] cycloaddition of 13 substituted azachalcone dienophiles with cyclopentadiene has been investigated. MOF-catalyzed reaction conditions were optimized, leading to the selection of water as the solvent, in the presence of 10% mol sodium dodecyl sulfate (SDS) to address substrate solubility. The Cu(II)-PEIP catalyst showed excellent activity under these green and mild conditions, exhibiting comparable or, in some cases, superior efficiency to a homogeneous catalyst often employed in Diels-Alder reactions, namely, Cu(OTf)2. The nature of the azachalcone substituent played a significant role in the reactivity of the dienophiles, with electron-withdrawing (EW) substituents enhancing conversion and electron-donating (ED) ones exhibiting the opposite effect. Coordinating substituents appeared to enhance the endo selectivity. Importantly, the Cu(II)-PEIP catalyst can be readily isolated from the reaction mixture and recycled up to four times without any significant reduction in conversion or selectivity.

Influence of 1-Butene Adsorption on the Dimerization Activity of Single Metal Cations on UiO-66 Nodes

Grafting metal cations to missing linker defect sites in zirconium-based metal-organic frameworks, such as UiO-66, produces a uniquely well-defined and homotopic catalytically active site. We present here the synthesis and characterization of a group of UiO-66-supported metal catalysts, M-UiO-66 (M = Ni, Co, Cu, and Cr), for the catalytic dimerization of alkenes. The hydrogen-deuterium exchange via deuterium oxide adsorption followed by infrared spectroscopy showed that the last molecular water ligand desorbs from the sites after evacuation at 300 °C leading to M(OH)-UiO-66 structures. Adsorption of 1-butene is studied using calorimetry and density functional theory techniques to characterize the interactions of the alkene with metal cation sites that are found active for alkene oligomerization. For the most active Ni-UiO-66, the removal of molecular water from the active site significantly increases the 1-butene adsorption enthalpy and almost doubles the catalytic activity for 1-butene dimerization in comparison to the presence of water ligands. Other M-UiO-66 (M = Co, Cu, and Cr) exhibit 1-3 orders of magnitude lower catalytic activities compared to Ni-UiO-66. The catalytic activities correlate linearly with the Gibbs free energy of 1-butene adsorption. Density functional theory calculations probing the Cossee-Arlman mechanism for all metals support the differences in activity, providing a molecular level understanding of the metal site as the active center for 1-butene dimerization.

Catalytic synthesis of new pyrazolo [3,4-b] pyridine via a cooperative vinylogous anomeric-based oxidation

In this study, a novel nano-magnetic metal–organic frameworks based on Fe3O4 namely Fe3O4@MIL-101(Cr)-N(CH2PO3)2 was synthesized and fully characterized. The prepared sample was used as catalyst in the synthesis of pyrazolo [3,4-b] pyridines as convenient medicine by condensation reaction of aldehydes, 5-(1H-Indol-3-yl)- 2H-pyrazol-3-ylamine and 3-(cyanoacetyl)indole via a CVABO. The products were obtained with high yields at 100 °C and under solvent-free conditions.

Construction of Hyaluronic Acid-Covered Hierarchically Porous MIL-nanoMOF for Loading and Controlled Release of Doxorubicin

The porous nano-sized metal-organic framework (nanoMOF) and its proper surface modification could greatly promote the drug loading capability and introduce biocompatibility, biodegradability, and targeting functions into nano-drug delivery systems. Herein, the HACD@ADA-PA/MIL-101_NH₂ (Fe)-P nanoparticle was successfully fabricated through supramolecular and coordination interactions from three building blocks, including hierarchically porous MIL-101_NH₂ (Fe)-P nanoMOF, phosphite-modified adamantane (ADA-PA), and β-cyclodextrin (β-CD)-modified hyaluronic acid (HACD). The obtained HACD@ADA-PA/MIL-101_NH₂ (Fe)-P nanoparticle was nano-sized and highly stable in physiological fluids. The porous structure of HACD@ADA-PA/MIL-101_NH₂ (Fe)-P nanoparticle could effectively load the commercial chemotherapeutic drug doxorubicin (DOX) with an encapsulation rate of 41.20 % and a loading rate of 48.84 %. The obtained drug-loaded HACD@ADA-PA/MIL-101_NH₂ (Fe)-P@DOX nanoparticle was pH-sensitive and relatively stable at neutral condition (pH 7.2) but could release DOX in a controlled way in subacid solution at pH 5.7. The simulated in vitro DOX release experiment signified that the HACD@ADA-PA/MIL-101_NH₂ (Fe)-P@DOX nanoparticle could realize the controlled release of DOX in tumor issues.

Tandem synthesis of tetrahydroquinolines and identification of the reaction network by operando NMR

Bifunctional MOF supported Pd nanoparticles for the one-pot tandem synthesis of substituted tetrahydroquinolines were developed, and operando high-pressure MAS-NMR were performed to understand the complex reaction network.

Glycyrrhetinic acid modified MOFs for the treatment of liver cancer

Liver cancer remains a major cause of cancer-related death across the globe. Nano medicines have emerged as promising candidates to improve liver cancer chemotherapy. In this study, a glycyrrhetinic acid (GA) modified metal-organic framework-based drug delivery system (GA-MOFs) was developed to enhance the liver targeting ability of 5-FU. The physicochemical properties of GA-MOFs regarding particle size, size distribution and morphology were evaluated. The results showed that the obtained 5-FU@GA-MOFs had an octahedral structure, a uniform particle size distribution, and a diameter of ∼200 nm. In vitro release experiments demonstrated that 5-FU@GA-MOFs exhibited a pH-dependent release pattern. MTT assays indicated that 5-FU-loaded GA-MOFs showed greater cytotoxicity towards HepG2 cells when compared to 5-FU alone at the same dose. In vivo tissue distribution demonstrated that the 5-FU@GA-MOFs significantly increased the accumulation of 5-FU in the liver. In vivo imaging analysis further manifested the liver targeting ability of GA-MOFs. Taken together, these results suggested that GA-modified MOFs showed promising potential as liver-targeting nanocarriers for the delivery of anti-tumor drugs.

Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Brønsted Acidity

The Lewis/Brønsted acidity and catalytic properties of UiO-66-type metal-organic frameworks are studied in the context of tunable acid catalysts based on the presence of linker defects that create coordinatively unsaturated Zr⁴⁺ centers. Fourier transform infrared spectroscopy of adsorbed CO and direct pH measurements are employed to characterize hydrated and dehydrated UiO-66 containing different number of Zr⁴⁺ sites associated with defects. These sites can strongly polarize coordinated water molecules, which induces Brønsted acidity in the hydrated material. Upon dehydration of the solid, the coordinated water molecules are removed, and the underlying coordinatively unsaturated Zr⁴⁺ cations become exposed and available as Lewis acid sites. Herein we show, for various acid-catalyzed reactions, how it is possible to shift from a Brønsted acid to a Lewis acid catalyst by simply controlling the hydration degree of the solid. This control adds a new dimension to the design and engineering of MOFs for catalytic applications.

Organic synthesis of high added value molecules with MOF catalysts

Recent examples of organic synthesis of fine chemicals and pharmaceuticals in confined spaces of MOFs are highlighted and compared with silica-based ordered porous solids, such as zeolites or mesoporous (organo)silica. These heterogeneous catalysts offer the possibility of stabilizing the desired transition states and/or intermediates during organic transformations of functional groups and (C-C/C-N) bond forming steps towards the desired functional high added value molecular scaffolds. A short introduction on zeolites, mesoporous silica and metal-organic frameworks is followed by relevant applications in which confined active sites in the pores promote single or multi-step organic synthesis of industrially relevant molecules. A critical discussion on the catalytic performances of the different types of hybrid inorganic-organic catalysts in the synthesis of O- and N-containing acyclic and heterocyclic molecules has been presented.

Water-Tolerant DUT-Series Metal-Organic Frameworks: A Theoretical-Experimental Study for the Chemical Fixation of CO2 and Catalytic Transfer Hydrogenation of Ethyl Levulinate to γ-Valerolactone

A series of highly thermally and hydrolytically stable porous solids with intriguing properties of zirconium- and hafnium-based metal-organic frameworks (MOFs) [Dresden University of Technology (DUT) series] was synthesized. The DUT MOFs were found to be effective catalysts for both epoxide-CO₂ cycloaddition reactions and the catalytic transfer hydrogenation (CTH) of ethyl levulinate (EL). In particular, 12-connected DUT-52(Zr) showed higher catalytic activity than eight- and six-connected catalysts in the synthesis of cyclic carbonates as well as in the production of γ-valerolactone (GVL). The secondary building unit connectivity, coexistence of a moderate number of acidic and basic sites, Brunauer-Emmett-Teller surface area, and combined effects of the pores of the MOFs seem to influence the catalytic activity. The reaction mechanism for the DUT-52(Zr)-mediated cycloaddition reaction of CO₂ and the CTH reactions were investigated in detail by using periodic density functional theory calculations. To the best of our knowledge, this is the first detailed computational study for the formation of GVL from EL by using MOF as the catalyst. In addition, grand canonical Monte Carlo simulations predicted the strong interaction of CO₂ molecules with the DUT-52(Zr) framework. Remarkably, the DUT-series catalysts possess extraordinary tolerance toward water. Further, DUT-52(Zr) is recyclable and is an efficient catalyst for cycloaddition and CTH reactions for at least five uses without obvious reductions in the activity or structural integrity.

Amino modified metal-organic frameworks as pH-responsive nanoplatforms for safe delivery of camptothecin

MIL-100(Fe) and MIL-101(Fe) metal-organic frameworks (MOFs) are excellent vehicles for drug delivery systems (DDSs) due to their high biocompatibility and stability in physiological fluids, as well as their pore diameter in the mesoporous range. Although they are appropriate for the internal diffusion of 20-(S)-camptothecin (CPT), a strongly cytotoxic molecule with excellent antitumor activity, no stable delivery system has been proposed so far for this drug based in MOFs. We here present novel DDSs based in amine functionalized MIL-100(Fe) and MIL-101(Fe) nanoMOFs with covalently bonded CPT. These CPT nanoplatforms are able to incorporate almost 20% of this molecule and show high stability at physiological pH, with no non-specific release. Based on their surface charge, some of these CPT loaded nanoMOFs present improved cell internalization in in vitro experiments. Moreover, a strong response to acid pH is observed, with up to four fold drug discharge at pH 5, which boost intracellular release by endosomolytic activity. These novel DDSs will help to achieve safe delivery of the very cytotoxic CPT, allowing to reduce the therapeutic dose and minimizing drug secondary effects.

Network topology and cavity confinement-controlled diastereoselectivity in cyclopropanation reactions catalyzed by porphyrin-based MOFs

In this work, we show that the stereoselectivity of a reaction can be controlled by directing groups of substrates, by network topology and by local cavity confinement of metal–organic framework (MOF) catalysts.

Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls over Hafnium-Based Metal-Organic Frameworks

A series of highly crystalline, porous, hafnium-based metal-organic frameworks (Hf-MOFs) have been shown to catalyze the transfer hydrogenation reaction of levulinic ester to produce γ-valerolactone by using isopropanol as a hydrogen donor. The results are compared with their zirconium-based counterparts. The role of the metal center in Hf-MOFs has been identified and reaction parameters optimized. NMR studies using isotopically labeled isopropanol provide evidence that the transfer hydrogenation occurs through a direct intermolecular hydrogen transfer route. The catalyst, Hf-MOF-808, can be recycled several times with only a minor decrease in catalytic activity. The generality of the procedure has been demonstrated by accomplishing the transformation with aldehydes, ketones, and α,β-unsaturated carbonyl compounds. The combination of Hf-MOF-808 with the Brønsted-acidic Al-Beta zeolite gives the four-step one-pot transformation of furfural to γ-valerolactone in good yield of 75 %.

Catalytic properties of pristine and defect-engineered Zr-MOF-808 metal organic frameworks

Defect-engineered Zr-MOF-808 are superior catalysts for Meerwein–Ponndorf–Verley reduction of (bulky) carbonyls.

Operando study of palladium nanoparticles inside UiO-67 MOF for catalytic hydrogenation of hydrocarbons

Formation of Pd nanoparticles inside UiO-67 MOF was monitored by in situ X-ray absorption and diffraction.

Metal-organic and covalent organic frameworks as single-site catalysts

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

The duality of UiO-67-Pt MOFs: connecting treatment conditions and encapsulated Pt species by operando XAS

XAS study of Pt-functionalized UiO-67 MOFs shows that 2 types of catalytically active sites can be formed in MOF cavities isolated Pt-complexes and Pt nanoparticles.