Recyclable and Reusable Heteroleptic Nickel Catalyst Immobilized on Metal-Organic Framework for Suzuki-Miyaura Coupling

Metal-organic frameworks (MOFs) provide highly versatile platforms to stabilize molecular catalysts that are not readily accessible under homogeneous conditions, thus enabling access to a new set of catalytic materials. Herein, we describe a recyclable and highly active nickel catalyst immobilized on MOF for Suzuki-Miyaura coupling reaction, which operates under mild conditions. This mixed ligand catalyst forms from the combination of 1 equiv of MOF-immobilized ligand, 1 equiv of nickel source, and 1 equiv of PPh₃. The nature of the catalyst was verified through a series of analytical tests and catalysis experiments. The immobilized catalyst was reusable for at least up to 7 cycles without decrease in the yield of the coupled product. We also verified that this reaction does not work under homogeneous conditions and that the reaction is truly heterogeneous through "hot filtration" experiments. We identified that the reaction is first order in arylborane concentration and negative order in arylbromide concentration through the effect of substrate concentrations on the initial rate. This informed us to conduct the catalysis under slow addition of the arylbromide and reduce the catalyst loading to 1% from 3%, without detriment to the yield or rate of the reaction. The catalyst gave good to excellent isolated yields with a range of functionalities, including heterocycles on aryl bromide with widely varying electronic properties.

Recent progress in the electron paramagnetic resonance study of polymers

This review article provides an overview of the contemporary research based on a tailor-made technique to understand the paramagnetic behavior of different polymer classes.

TEMPO-Appended Metal-Organic Frameworks as Highly Active, Selective, and Reusable Catalysts for Mild Aerobic Oxidation of Alcohols

Metal-organic frameworks (MOFs) decorated with stable organic radicals are highly promising materials for redox catalysis. Unfortunately however, the synthesis of chemically robust MOFs typically requires harsh solvothermal conditions, which are not compatible with organic radicals. Here, we describe the synthesis of two isoreticular families of stable, mixed component, zirconium MOFs with UiO-66 and UiO-67 structures and controlled amounts of covalently attached TEMPO radicals. The materials were obtained using a relatively low-temperature, HCl-modulated de novo method developed by Hupp and Farha and shown to contain large amounts of missing cluster defects, forming nanodomains of the reo phase with 8-connected clusters. In the extreme case of homoleptic UiO-67-TEMPO(100%), the material exists as an almost pure reo phase. Large voids due to missing clusters and linkers allowed these materials to accommodate up to 2 times more of bulky TEMPO substituents than theoretically predicted for the idealized structures and proved to be beneficial for catalytic activity. The TEMPO-appended MOFs were shown to be highly active and recyclable catalysts for selective aerobic oxidation of a broad range of primary and secondary alcohols under exceptionally mild conditions (room temperature, atmospheric pressure of air). The influence of various parameters, including the pore size and TEMPO content, on the catalytic activity was also comprehensively investigated.

A combined continuous wave electron paramagnetic resonance and DFT calculations of copper-doped 3∞[Cd0.98Cu0.02(prz-trz-ia)] metal–organic framework

Successful combination of EPR and DFT was applied for investigation the novel heteronuclear MOF compounds.

Post‐synthetic Modification of MOFs

Post‐synthetic modification is increasingly recognised as an important and versatile tool in the preparation of functionalised metal organic frameworks (MOFs). The process involves one or more reactions on a pre‐formed MOF, and it can be used to prepare MOFs that are not accessible by direct combination of metal and linker. This review explores the methods and strategies that have been developed for post‐synthetically modifying MOFs, concentrating on four classes of reaction: covalent transformations of the linker, coordination of a metal centre to a linker, modification of the inorganic part of the MOF and exchange of counter‐ions. Examples of the use of the modified MOFs are given, with a focus on their utility in catalysis.

An alternative pathway for the synthesis of isocyanato- and urea-functionalised metal-organic frameworks

We have developed a generic two-step post-functionalisation technique for transforming amino-functionalised MOFs into their isocyanate analogues. The first part of the synthetic pathway consists in the conversion of the amino moieties into azido groups. Next, the thermal activation of these azido groups leads to nitrene species that can react with carbon monoxide to yield the desired products. As a proof of concept, this method was applied to the highly stable Al-MIL-53-NH2 and to the acid-sensitive In-MIL-68-NH2. The resulting nitrene species were highly reactive, with side reactions dominating initially. This issue was overcome through the use of a mixed-linker strategy applied during the MOF synthesis that decreased the nitrene radical density within the pore, thereby permitting In-MIL-68-NH2 to be converted into its isocyanate analogue with 100% selectivity. To illustrate the potential of this method for grafting a wide library of potentially active organic groups inside MOFs, amines were condensed onto isocyanato MOFs to form urea analogues.

Thermal response, catalytic activity, and color change of the first hybrid vanadate containing Bpe guest molecules

Four isomorphic compounds with formula [{Co2(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe, CoBpe 1; [{CoNi(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe, CoNiBpe 2; [{Co0.6Ni1.4(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe, NiCoBpe 3; and [{Ni2(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe, NiBpe 4, have been obtained by hydrothermal synthesis. The crystal structures of CoBpe 1 and NiBpe 4 were determined by single-crystal X-ray diffraction (XRD). The Rietveld refinement of CoNiBpe 2 and NiCoBpe 3 XRD patterns confirms that those are isomorphic. The compounds crystallize in the P1̅ space group, exhibiting a crystal structure constructed from inorganic layers pillared by Bpe ligands. The crystal structure contains intralayer and interlayer channels, in which the crystallization water molecules and Bpe guest molecules, respectively, are located. The solvent molecules establish a hydrogen bonding network with the coordinated water molecules. Thermodiffractometric and thermogravimetric studies showed that the loss of crystallization and coordinated water molecules takes place at different temperatures, giving rise to crystal structure transformations that involve important reduction of the interlayer distance, and strong reduction of crystallinity. The IR, Raman, and UV-vis spectra of the as-synthesized and heated compounds confirm that the structural building blocks and octahedral coordination environment of the metal centers are maintained after the structural transformations. The color change and reversibility of the water molecules uptake/removal were tested showing that the initial color is not completely recovered when the compounds are heated at temperatures higher than 200 °C. The thermal evolution of the magnetic susceptibility indicates one-dimensional antiferromagnetic coupling of the metal centers at high temperatures. For NiCoBpe 3 and NiBpe 4 compounds magnetic ordering is established at low temperatures, as can be judged by the maxima observed in the magnetic susceptibilities. CoNiBpe 2 was proved as catalyst being active for cyanosilylation reactions of aldehydes.