Influence of the Base on Pd@MIL‐101‐NH2(Cr) as Catalyst for the Suzuki–Miyaura Cross‐Coupling Reaction

The chemical stability of metal–organic frameworks (MOFs) is a major factor preventing their use in industrial processes. Herein, it is shown that judicious choice of the base for the Suzuki–Miyaura cross‐coupling reaction can avoid decomposition of the MOF catalyst Pd@MIL‐101‐NH₂(Cr). Four bases were compared for the reaction: K₂CO₃, KF, Cs₂CO₃ and CsF. The carbonates were the most active and achieved excellent yields in shorter reaction times than the fluorides. However, powder XRD and N₂ sorption measurements showed that the MOF catalyst was degraded when carbonates were used but remained crystalline and porous with the fluorides. XANES measurements revealed that the trimeric chromium cluster of Pd@MIL‐101‐NH₂(Cr) is still present in the degraded MOF. In addition, the different countercations of the base significantly affected the catalytic activity of the material. TEM revealed that after several catalytic runs many of the Pd nanoparticles (NPs) had migrated to the external surface of the MOF particles and formed larger aggregates. The Pd NPs were larger after catalysis with caesium bases compared to potassium bases.     

Nano palladium supported on high‐surface‐area metal–organic framework MIL‐101: an efficient catalyst for Sonogashira coupling of aryl and heteroaryl bromides with alkynes

Palladium nanoparticle‐incorporated metal–organic framework MIL‐101 (Pd/MIL‐101) was successfully synthesized and characterized using X‐ray diffraction, nitrogen physisorption, X‐ray photoelectron, UV–visible and infrared spectroscopies, and transmission electron microscopy. The characterization techniques confirmed high porosity and high surface area of MIL‐101 and high stability of nano‐size palladium particles. Pd/MIL‐101 nanocomposite was investigated for the Sonogashira cross‐coupling reaction of aryl and heteroaryl bromides with various alkynes under copper‐free conditions. The reusability of the catalyst was tested for up to four cycles without any significant loss in catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

A Stable Microporous Mixed‐Metal Metal–Organic Framework with Highly Active Cu2+ Sites for Efficient Cross‐Dehydrogenative Coupling Reactions

Two metalloporphyrin octacarboxylates were used to link copper(II) nodes for the formation of two novel porous mixed‐metal metal–organic frameworks (M′MOFs) containing nanopore cages (2.1 nm in diameter) or nanotubular channels (1.5 nm in diameter). The highly active Cu²⁺ sites on the nanotubular surfaces of the stable porous M′MOF ZJU‐22 , stabilized by three‐connected nets, lead to the superior catalytic activity for the cross‐dehydrogenative coupling (CDC) reaction.     

Oxygen‐Promoted Suzuki–Miyaura Reaction for Efficient Construction of Biaryls

As one of the most powerful and versatile methods for the construction of carbon–carbon bonds, the Suzuki–Miyaura cross‐coupling reaction has attracted great attention over the past three decades. In recent years, a huge amount of interest has been focused on the development of ligand‐free Suzuki–Miyaura reaction systems, which have the advantages of low cost, mild reaction conditions, and easy operation. So far, a number of ligand‐free Suzuki–Miyaura reaction systems have been developed by using simple palladium salts, nanopalladium, or supported palladium catalysts. In this account, we will review our recent research on the oxygen‐promoted ligand‐free Suzuki–Miyaura reaction. Interestingly, the oxygen‐promoting effect has been observed in different reaction media, including polyethylene glycol, organic/water mixed solvents and pure water. The oxygen‐promoted reaction systems demonstrate high efficiency for the construction of biaryls.

     

Pd Nanocubes@ZIF‐8: Integration of Plasmon‐Driven Photothermal Conversion with a Metal–Organic Framework for Efficient and Selective Catalysis

Composite nanomaterials usually possess synergetic properties resulting from the respective components and can be used for a wide range of applications. In this work, a Pd nanocubes@ZIF‐8 composite material has been rationally fabricated by encapsulation of the Pd nanocubes in ZIF‐8, a common metal–organic framework (MOF). This composite was used for the efficient and selective catalytic hydrogenation of olefins at room temperature under 1 atm H₂ and light irradiation, and benefits from plasmonic photothermal effects of the Pd nanocube cores while the ZIF‐8 shell plays multiple roles; it accelerates the reaction by H₂ enrichment, acts as a “molecular sieve” for olefins with specific sizes, and stabilizes the Pd cores. Remarkably, the catalytic efficiency of a reaction under 60 mW cm^?2 full‐spectrum or 100 mW cm^?2 visible‐light irradiation at room temperature turned out to be comparable to that of a process driven by heating at 50 °C. Furthermore, the catalyst remained stable and could be easily recycled. To the best of our knowledge, this work represents the first combination of the photothermal effects of metal nanocrystals with the favorable properties of MOFs for efficient and selective catalysis.     

Supported palladium nanoparticle‐catalysed Suzuki–Miyaura cross‐coupling approach for synthesis of aminoarylbenzosuberene analogues from natural precursor

A semi‐synthetic method has been developed for the synthesis of aminoarylbenzosuberenes (AABs) from naturally occurring himachalenes, an isomeric mixture of sesquiterpenes present in Cedrus deodara oil. Polymer‐stabilized Pd(0) nanoparticle‐catalysed Suzuki–Miyaura cross‐coupling reaction of aminovinyl bromide‐substituted benzosuberenes has been adopted for AAB synthesis. The catalyst performed well with different amine substituents, and was recycled up to five times. The synthesis of such arylated benzosuberene class of compounds from natural precursors following semi‐synthetic approaches could provide an attractive alternative method with reduced number of steps.     

The Origin of Shape Sensitivity in Palladium‐Catalyzed Suzuki–Miyaura Cross Coupling Reactions

The shape sensitivity of Pd catalysts in Suzuki–Miyaura coupling reactions is studied using nanocrystals enclosed by well‐defined surface facets. The catalytic performance of Pd nanocrystals with cubic, cuboctahedral and octahedral morphologies are compared. Superior catalytic reactivity is observed for Pd NCs with {100} surface facets compared to {111} facets. The origin of the enhanced reactivity associated with a cubic morphology is related to the leaching susceptibility of the nanocrystals. Molecular oxygen plays a key role in facilitating the leaching of Pd atoms from the surface of the nanocrystals. The interaction of O₂ with Pd is itself facet‐dependent, which in turn gives rise to more efficient leaching from {100} facets, compared to {111} facets under the reaction conditions.     

PEPPSI‐Type Palladium Complexes Containing Basic 1,2,3‐Triazolylidene Ligands and Their Role in Suzuki–Miyaura Catalysis

A series of PEPPSI‐type palladium(II) complexes was synthesized that contain 3‐chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki–Miyaura cross‐coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol‐2‐ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N‐heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.     

Iron Metal–Organic Frameworks MIL‐88B and NH2‐MIL‐88B for the Loading and Delivery of the Gasotransmitter Carbon Monoxide

Crystals of MIL‐88B‐Fe and NH₂‐MIL‐88B‐Fe were prepared by a new rapid microwave‐assisted solvothermal method. High‐purity, spindle‐shaped crystals of MIL‐88B‐Fe with a length of about 2 μm and a diameter of 1 μm and needle‐shaped crystals of NH₂‐MIL‐88B‐Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as‐prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier‐transform infrared (UHV‐FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as‐prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL‐88B‐Fe with t_1/2=38 min and from crystals of NH₂‐MIL‐88B‐Fe with t_1/2=76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL‐88B‐Fe and NH₂‐MIL‐88B‐Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide.     

Palladium Nanoparticles Supported on a Porous Organic Polymer: An Efficient Catalyst for Suzuki‐Miyaura and Sonogashira Coupling Reactions

A new porous organic polymer (POP) with high thermal stability and large surface area has been synthesized and applied in the preparation of Pd/POP catalyst. Pd/POP was characterized by XRD, TGA, SEM and TEM. The catalyst consists of highly dispersed palladium nanoparticles of 0.9–4 nm size on POP with a large surface area of 650 m²/g. It presents high catalytic activity for Suzuki‐Miyaura and Sonogashira reactions. The catalyst was reusable for three to five times without significant loss of activity.     

Palladium supported on silica–chitosan hybrid material (Pd‐CS@SiO2) for Suzuki–Miyaura and Mizoroki–Heck cross‐coupling reactions

Palladium supported on silica–chitosan hybrid material was prepared and characterized using thermogravimetric and differential thermogravimetric analyses, scanning electron microscopy, and Fourier transform infrared, energy‐dispersive X‐ray and X‐ray photoelectron spectroscopies. The prepared Pd‐CS@SiO₂ catalyst (1 mol%) was used for the Suzuki–Miyaura cross‐coupling reaction of various aryl halides and arylboronic acids in 95% ethanol at 80 °C and the Mizoroki–Heck reaction in dimethylformamide at 110 °C using K₂CO₃ as a base. The developed catalyst is well suitable for the 3R approach (recoverable, robust, recyclable) for cross‐coupling reactions without appreciable loss of its activity. Copyright © 2015 John Wiley & Sons, Ltd.     

The development of phosphinoamine–Pd(II)–imidazole complexes: implications in room‐temperature Suzuki–Miyaura cross‐coupling reaction

The reaction of N‐methylimidazole (N‐MeIm) and N‐butylimidazole (N‐BuIm) with the complexes [PdCl₂(PPh₂py–P,N)] and [PdCl₂(PPh₂Etpy–P,N)] in the presence of NH₄PF₆ under N₂ at room temperature afforded four new cationic Pd(II) complexes [PdCl(PPh₂py–P,N)(N‐MeIm)](PF₆) ( 1 ), [PdCl(PPh₂py–P,N)(N‐BuIm)](PF₆) ( 2 ), [PdCl(PPh₂Etpy–P,N)(N‐MeIm)](PF₆) ( 4 ) and [PdCl(PPh₂Etpy‐P,N)(N‐BuIm)](PF₆) ( 5 ) in good yields, where PPh₂py is 2‐(diphenylphosphino)pyridine and PPh₂Etpy is 2‐{2‐(diphenylphosphino)ethyl}pyridine). The complexes were fully characterized. The catalytic activities of these complexes were investigated for Suzuki–Miyaura cross‐coupling reactions at room temperature. Complex 2 exhibited excellent activity compared to other analogs. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhancing the Water Stability of Al‐MIL‐101‐NH2 via Postsynthetic Modification

The resistance of metal–organic frameworks towards water is a very critical issue concerning their practical use. Recently, it was shown for microporous MOFs that the water stability could be increased by introducing hydrophobic pendant groups. Here, we demonstrate a remarkable stabilisation of the mesoporous MOF Al‐MIL‐101‐NH₂ by postsynthetic modification with phenyl isocyanate. In this process 86 % of the amino groups were converted into phenylurea units. As a consequence, the long‐term stability of Al‐MIL‐101‐URPh in liquid water could be extended beyond a week. In water saturated atmospheres Al‐MIL‐101‐URPh decomposed at least 12‐times slower than the unfunctionalised analogue. To study the underlying processes both materials were characterised by Ar, N₂ and H₂O sorption measurements, powder X‐ray diffraction, thermogravimetric and chemical analysis as well as solid‐state NMR and IR spectroscopy. Postsynthetic modification decreased the BET equivalent surface area from 3363 to 1555 m² g^?1 for Al‐MIL‐101‐URPh and reduced the mean diameters of the mesopores by 0.6 nm without degrading the structure significantly and reducing thermal stability. In spite of similar water uptake capacities, the relative humidity‐dependent uptake of Al‐MIL‐101‐URPh is slowed and occurs at higher relative humidity values. In combination with ¹H‐²⁷Al D ‐HMQC NMR spectroscopy experiments this favours a shielding mechanism of the Al clusters by the pendant phenyl groups and rules out pore blocking.     

Synthesis of a palladium complex bearing 2‐phenylbenzothiazole and its application to Suzuki–Miyaura coupling reaction

The palladacycle complex [L^sPdOAc]₂ bearing 2‐phenyl benzothiazole was synthesized and characterized by NMR and X‐ray crystallography. [L^sPdOAc]₂ was used as a catalyst in the Suzuki–Miyaura cross coupling reaction of 4‐bromotoluene with phenylboronic acid, which resulted in a conversion of >90% with 5 mol% of the Pd complex within 10 min at 60°C.