Construction and application of base-stable MOFs: a critical review

Metal-organic frameworks (MOFs) are a new class of porous crystalline materials constructed from organic ligands and metal ions/clusters. Owing to their unique advantages, they have attracted more and more attention in recent years and numerous studies have revealed their great potential in various applications. Many important applications of MOFs inevitably involve harsh alkaline operational environments. To achieve high performance and long cycling life in these applications, high stability of MOFs against bases is necessary. Therefore, the construction of base-stable MOFs has become a critical research direction in the MOF field. This review gives a historic summary of the development of base-stable MOFs in the last few years. The key factors that can determine the robustness of MOFs under basic conditions are analyzed. We also demonstrate the exciting achievements that have been made by utilizing base-stable MOFs in different applications. In the end, we discuss major challenges for the further development of base-stable MOFs. Some possible methods to address these problems are presented.

New Ln-MOFs based on mixed organic ligands: synthesis, structure and efficient luminescence sensing of the Hg2+ ions in aqueous solutions

In view of Hg²⁺ ion sensing by luminescence, a series of new, phenanthroline-decorated 3D lanthanide metal organic frameworks (Ln-MOFs) valorising an original combination of four different lanthanides and two organic ligands, i.e. thiobis(4-methylene-benzoic acid) (H₂tmba) and 1,10-phenanthroline (phen), have been successfully synthesized, namely {[Ln₄(tmba)₆(phen)₄]·m(H₂O)(phen)}_n [Ln = Ce, m = 3 (1); Pr, m = 1 (2); Eu, m = 3 (3); and Tb, m = 3 (4)]. Compounds 1-4 were characterised by single-crystal X-ray diffraction, elemental and thermogravimetric analyses, and powder X-ray diffraction. The luminescence properties of complexes 3 and 4 were thoroughly investigated. It is herein proved that compound 3 sensitively and selectively acts as an excellent luminescent probe for the detection of Hg²⁺ ions in waters, with a detection limit of 1.00 μM. As additional assets, 3 displays superb stability over a wide pH range (3-12) of the aqueous media, as well as convenient recycling after completion of the detection experiments. The rationale for the observed luminescence quenching effect of mercury might be a strong interaction arising between Hg²⁺ ions and the carboxylate oxygen atoms of the tmba^2- ligand. The results open new perspectives for applications in environmental remediation.

Microflowers formed by complexation-driven self-assembly between palladium(ii) and bis-theophyllines: immortal catalyst for C-C cross-coupling reactions

The Pd catalyst for Suzuki–Miyaura or the other C–C coupling reactions is one of the central tools in organic synthesis related to medicine, agricultural chemicals and advanced materials. However, recycling palladium is a bottleneck for developing the extreme potential of Pd in chemistry. Herein, we established a new heterogeneous Pd catalytic system in which the catalyst is a nanopetal-gathered flower-like microsphere self-assembled from PdCl₂ and alkyl-linked bis-theophyllines. The microflowers catalyzed quantitatively the reaction of aryl bromides and phenylboronic acid in aqueous media at room temperature. It was found that the reaction proceeds better in an air atmosphere than in nitrogen gas even though the Pd(ii) species employed was lowered to 0.001 mol% in the substance. Very interestingly, the microflowers could be recycled 20 times without deactivation in the C–C coupling reaction between bromobenzene and phenylboronic acid in the presence of sodium chloride. We found that the sodium chloride added played an important role in maintaining the morphology of microflowers and preventing the formation of metallic Pd particles.

Bis-theophylline-palladium complex exhibit high catalytic activity in the C–C coupling reaction with excellent recyclability in the presence of NaCl.

Pd(ii)-Based polyoxometalate polymers as highly efficient heterogeneous catalysts for Suzuki–Miyaura reactions

Five Pd(ii)/polyoxometalate catalysts with a unique atomic arrangement and high Pd loading were synthesized under solvothermal conditions, and their structures were determined by single crystal X-ray diffraction.

Striking dual functionality of a novel Pd@Eu-MOF nanocatalyst in C(sp2)-C(sp2) bond-forming and CO2 fixation reactions

Pd nanoparticles were immobilized on a highly porous, hydrothermally stable Eu-MOF via solution impregnation and H2 reduction to yield a novel Pd@Eu-MOF nanocatalyst. This composite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS). Unprecedentedly, the Pd@Eu-MOF nanocatalyst could be applied with excellent results in two strikingly different, mechanistically distinct, reactions i.e., Suzuki-Miyaura cross-coupling and cycloaddition of CO2 to a range of epoxides. Under the best reaction conditions, 98-99% yields have been attained in both catalytic processes. Moreover, in either case the heterogeneous catalyst was easily recovered and efficiently reused for more than four cycles, indicating its high stability and reproducibility. PXRD, TEM and XPS measurements on the recycled catalyst confirmed that it maintained its original structure and morphology; no Pd NP agglomeration was observed.

Engineering Metal-Organic Framework Catalysts for C-C and C-X Coupling Reactions: Advances in Reticular Approaches from 2014-2018

Metal-organic frameworks (MOFs) are a class of crystalline porous materials that have been actively used for several industrial and synthetic applications. MOFs are spatially and geometrically extrapolated coordination polymers with intriguing properties such as tunable porosity and dimensionality. In terms of their catalytic efficiency, MOFs combine the easy recoverability of heterogeneous catalysts with the increased selectivity of biological catalysts. It is therefore not surprising that a lot of work on optimizing MOF catalysts for organic transformations has been carried out over the past decade. In this review, recent developments in MOF catalysis are summarized, with special attention being paid to C-C, C-N, and C-O coupling reactions. The influence of pore size, pore environment, and load on catalytic activity is described. Post-synthetic stabilization techniques and host-guest interactions in caged MOF scaffolds are detailed. Mechanistic aspects pertaining to the use of MOFs in asymmetric heterogeneous catalysis are highlighted and categorized.

Efficient Suzuki⁻Miyaura C-C Cross-Couplings Induced by Novel Heterodinuclear Pd-bpydc-Ln Scaffolds

An easy access to a series of previously unreported heterodinuclear Pd-Ln compounds, Pd-bpydc-La, Pd-bpydc-Ce and Pd-bpydc-Nd (bpydc = 2,2'-bipyridine-5,5'-dicarboxylate) has been developed. The Pd-Ln hybrid networks were effectively applied as catalysts in Suzuki⁻Miyaura C-C cross-coupling reactions of 4-bromoanisole and 4-bromobenzonitrile with phenylboronic acid, under mild conditions. A systematic investigation revealed Pd-bpydc-Nd as the most active catalyst. In all cases, reaction yields varied with the base, catalyst loading and substantially augmented with temperature (from 30 to 60 °C). Substituent effects were operative when changing from 4-bromoanisole to 4-bromobenzonitrile. The key role played by the lanthanides, aromatic substrate and base, in modulating the Pd-catalytic cycle has been highlighted. Importantly, the new catalysts proved to be stable in air and vs. functionalities and are quite efficient in Suzuki⁻Miyaura carbon-carbon bond formation conducted in protic solvents.