[NaCu(2,4-HPdc)(2,4-Pdc)] Mixed Metal-Organic Framework as a Heterogeneous Catalyst

Dramatic Enhancement of Rare-Earth Metal-Organic Framework Stability Via Metal Cluster Fluorination

Rare-earth polynuclear metal-organic frameworks (RE-MOFs) have demonstrated high durability for caustic acid gas adsorption and separation based on gas adsorption to the metal clusters. The metal clusters in the RE-MOFs traditionally contain RE metals bound by μ₃-OH groups connected via organic linkers. Recent studies have suggested that these hydroxyl groups could be replaced by fluorine atoms during synthesis that includes a fluorine-containing modulator. Here, a combined modeling and experimental study was undertaken to elucidate the role of metal cluster fluorination on the thermodynamic stability, structure, and gas adsorption properties of RE-MOFs. Through systematic density-functional theory calculations, fluorinated clusters were found to be thermodynamically more stable than hydroxylated clusters by up to 8-16 kJ/mol per atom for 100% fluorination. The extent of fluorination in the metal clusters was validated through a ¹⁹F NMR characterization of 2,5-dihydroxyterepthalic acid (Y-DOBDC) MOF synthesized with a fluorine-containing modulator. ¹⁹F magic-angle spinning NMR identified two primary peaks in the isotropic chemical shift (δ_iso) spectra located at -64.2 and -69.6 ppm, matching calculated ¹⁹F NMR δ_iso peaks at -63.0 and -70.0 ppm for fluorinated systems. Calculations also indicate that fluorination of the Y-DOBDC MOF had negligible effects on the acid gas (SO₂, NO₂, H₂O) binding energies, which decreased by only ∼4 kJ/mol for the 100% fluorinated structure relative to the hydroxylated structure. Additionally, fluorination did not change the relative gas binding strengths (SO₂ > H₂O > NO₂). Therefore, for the first time the presence of fluorine in the metal clusters was found to significantly stabilize RE-MOFs without changing their acid-gas adsorption properties.

Single-Crystal-to-Single-Crystal Transformation of Two Copper(II) Metal-Organic Frameworks Modulated by Auxiliary Ligands

The single-crystal-to-single-crystal (SCSC) transformations of metal-organic frameworks (MOFs) are fascinating because we can directly observe the change of the crystal structure during the transformation process. It also greatly helps to understand the delicate interaction between the guest molecules and the skeleton framework and therefore fosters a deep understanding of gas storage and separation within the frameworks. Herein, we report two novel MOFs, [Cu₈(BCB)₄(μ₃-OH)₂(μ₃-O)(H₂O)₈(Py)₅]·16DMF·52H₂O (1) and [Cu₃(BCB)₂(Py)₆]·DMF·11H₂O (2) (Py = pyridine; DMF = N,N'-dimethylformamide), which were constructed through the self-assembly of Cu²⁺ and 4,4',4″-benzenetricarbonyltribenzoic acid (H₃BCB) by a solvothermal reaction. Although the structure and coordination patterns of compound 1 are pretty different from those of 2, the two Cu-MOFs were prepared from identical ligands and similar reaction conditions. Interestingly, compound 1 will change to 2 wholly and gradually after the addition of a certain amount of Py with a small amount of dilute hydrochloric acid. This conversion represents a scarce example of SCSC transformation involving transition-metal-based MOFs. Moreover, with its microporous nature, compound 2 shows large carbon dioxide (CO₂) uptake capability and good selectivity for CO₂/N₂ separation. Furthermore, both compounds 1 and 2 could be used as excellent heterogeneous catalysts toward the cyanosilylation reaction under solvent-free conditions.

A new zeolitic lithium aluminum imidazolate framework

An aliovalent mixed-metal framework DUT-174 [LiAl(2-methylimidazolate)4]n, isostructural to ZIF-8, was synthesized from lithium aluminum hydride (LiAlH4) and 2-methylimidazole (2-mImH) through dehydrogenation. Lithium and aluminum cations acting as alternating framework nodes are coordinated tetrahedrally by (2-mIm)-. DUT-174 has a high specific surface area of 1149 m2 g-1 and CO2 uptake of 11.57 mmol g-1 at 195 K.

Pyrene Carboxylate Ligand Based Coordination Polymers for Microwave-Assisted Solvent-Free Cyanosilylation of Aldehydes

The new coordination polymers (CPs) [Zn(μ-1κO¹:1κO²-L)(H₂O)₂]_n·n(H₂O) (1) and [Cd(μ₄-1κO¹O²:2κN:3,4κO³-L)(H₂O)]_n·n(H₂O) (2) are reported, being prepared by the solvothermal reactions of 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H₂L) with Zn(NO₃)₂.6H₂O or Cd(NO₃)₂.4H₂O, respectively. They were synthesized in a basic ethanolic medium or a DMF:H₂O mixture, respectively. These compounds were characterized by single-crystal X-ray diffraction, FTIR spectroscopy, thermogravimetric and elemental analysis. The single-crystal X-ray diffraction analysis revealed that compound 1 is a one dimensional linear coordination polymer, whereas 2 presents a two dimensional network. In both compounds, the coordinating ligand (L^2-) is twisted due to the rotation of the pyrene ring around the CH₂-NH bond. In compound 1, the Zn(II) metal ion has a tetrahedral geometry, whereas, in 2, the dinuclear [Cd₂(COO)₂] moiety acts as a secondary building unit and the Cd(II) ion possesses a distorted octahedral geometry. Recently, several CPs have been explored for the cyanosilylation reaction under conventional conditions, but microwave-assisted cyanosilylation of aldehydes catalyzed by CPs has not yet been well studied. Thus, we have tested the solvent-free microwave-assisted cyanosilylation reactions of different aldehydes, with trimethylsilyl cyanide, using our synthesized compounds, which behave as highly active heterogeneous catalysts. The coordination polymer 1 is more effective than 2, conceivably due to the higher Lewis acidity of the Zn(II) than the Cd(II) center and to a higher accessibility of the metal centers in the former framework. We have also checked the heterogeneity and recyclability of these coordination polymers, showing that they remain active at least after four recyclings.

Double role of metalloporphyrins in catalytic bioinspired supramolecular metal-organic frameworks (SMOFs)

Heterogeneous catalysts are of great interest in many industrial processes for environmental reasons and, during recent years, a great effort has been devoted to obtain metal-organic frameworks (MOFs) with improved catalytic behaviour. Few supramolecular metal-organic frameworks (SMOFs) are stable under ambient conditions and those with anchored catalysts exhibit favourable properties. However, this paper presents an innovative approach that consists of using metal nodes as both structural synthons and catalysts. Regarding the latter, metalloporphyrins are suitable candidates to play both roles simultaneously. In fact, there are a number of papers that report coordination compounds based on metalloporphyrins exhibiting these features. Thus, the aim of this bioinspired work was to obtain stable SMOFs (at room temperature) based on metallo-porphyrins and explore their catalytic activity. This work reports the environmentally friendly microwave-assisted synthesis and characterization of the compound [H(bipy)]2[(MnTPPS)(H2O)2]·2bipy·14H2O (TPPS = meso-tetra-phenyl-porphine-4,4',4'',4'''-tetra-sulfonic acid and bipy = 4,4'-bi-pyridine). This compound is the first example of an MnTPPS-based SMOF, as far as we are aware, and has been structurally and thermally characterized through single-crystal X-ray diffraction, IR spectroscopy, thermogravimetry and transmission electron microscopy. Additionally, this work explores not only the catalytic activity of this compound but also of the compounds μ-O-[FeTCPP]2·16DMF and [CoTPPS0.5(bipy)(H2O)2]·6H2O. The structural features of these supra-molecular materials, with accessible networks and high thermal stability, are responsible for their excellent behaviour as heterogeneous catalysts for different oxidation, condensation (aldol and Knoevenagel) and one-pot cascade reactions.

Highly thermally stable heterogeneous catalysts: study of 0D and 3D porphyrinic MOFs

Porphyrin-based MOFs as heterogeneous catalysts show high catalytic activity, recyclability and selectivity towards alkene oxidation.

Mixed-metal or mixed-linker metal organic frameworks as heterogeneous catalysts

This review illustrates the recent developments in heterogeneous catalysis using mixed metal or mixed linker MOFs.