A Tale of Copper Coordination Frameworks: Controlled Single‐Crystal‐to‐Single‐Crystal Transformations and Their Catalytic CH Bond Activation Properties

Transformation of a copper-based metal-organic polyhedron into a mixed linker MOF for CO2 capture

A new mixed linker metal-organic framework (MOF) has been synthesized from a copper-based metal-organic polyhedron (MOP-1) and 2,2'-bipyridine (2,2'-bipy). The CuMOF-Bipy with a formula of [Cu₂(2,2'-bpy)₂(m-BDC)₂]_n is comprised of a binuclear Cu(II) node coordinated to 2,2'-bipy, and isophthalic acid (m-BDC), which bridges to neighboring nodes. The crystal structure of CuMOF-Bipy consists of a stacked two-dimensional framework with the sql topology. CuMOF-Bipy was characterized by single-crystal X-ray diffraction (SC-XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and CO₂ sorption. CuMOF-Bipy was shown to have one-dimensional sinusoidal channels that allow diffusion of CO₂ but not N₂.

Sequential Solid-State Transformations Involving Consecutive Rearrangements of Secondary Building Units in a Metal-Organic Framework (MOF)

Solid-state transformations in metal-organic frameworks (MOFs) are important and have led to the creation of new MOF structures. Solid-state transformations from interpenetrated to non-interpenetrated networks involving rearrangement of secondary building units (SBUs) in a single-crystal-to-single-crystal (SCSC) fashion have not been explored to date. Herein, we report the sequential, thermally stimulated solid-state transformations in a barium-organic framework (UPC-600). The two-fold interpenetrated framework of UPC-600 is converted at 373 K into UPC-601, a non-interpenetrated framework. This proceeds in a SCSC fashion and involves the rearrangement of two proximate rod-shaped SBUs in different nets to generate a new rod-shaped SBU. At 473 K, a continuous solid-state transformation involving a second rearrangement occurred, UPC-601 converted into UPC-602 by the rearrangement of the 1D rod-shaped SBU to a 2D layer SBU. This is the first example of such a thermally driven stepwise transformation involving simultaneous cleavage and regeneration of multiple bonds.

Structural Engineering of Low-Dimensional Metal-Organic Frameworks: Synthesis, Properties, and Applications

Low-dimensional metal-organic frameworks (LD MOFs) have attracted increasing attention in recent years, which successfully combine the unique properties of MOFs, e.g., large surface area, tailorable structure, and uniform cavity, with the distinctive physical and chemical properties of LD nanomaterials, e.g., high aspect ratio, abundant accessible active sites, and flexibility. Significant progress has been made in the morphological and structural regulation of LD MOFs in recent years. It is still of great significance to further explore the synthetic principles and dimensional-dependent properties of LD MOFs. In this review, recent progress in the synthesis of LD MOF-based materials and their applications are summarized, with an emphasis on the distinctive advantages of LD MOFs over their bulk counterparties. First, the unique physical and chemical properties of LD MOF-based materials are briefly introduced. Synthetic strategies of various LD MOFs, including 1D MOFs, 2D MOFs, and LD MOF-based composites, as well as their derivatives, are then summarized. Furthermore, the potential applications of LD MOF-based materials in catalysis, energy storage, gas adsorption and separation, and sensing are introduced. Finally, challenges and opportunities of this fascinating research field are proposed.

Irreversible solvent-assisted structural transformation in 3D metal-organic frameworks: Structural modification and enhanced iodine-adsorption properties

In this work, we demonstrate a solvent-assisted structural transformation between two 3D metal-organic frameworks (MOFs) ([Zn₄(α-bptc)₂(H₂O)₃]_n (1) → {[Zn₂(α-bptc)(H₂O)₄]·(pra)}_n (2)) (α-H₄bptc = 2,3,3',4'-biphenyl tetra-carboxylic acid and pra = pyridin-2-amine) at room temperature by immersing complex 1 in a mother solution. The structural transformation involves not only solvent exchange but also the cleavage and formation of coordination bonds, which is confirmed by infrared spectroscopy, single-crystal X-ray diffraction analysis, powder X-ray diffraction patterns, and thermogravimetric analysis. Structural analyses revealed that significant modifications occurred during the transformation including the changes in lattice parameters, unit cell volume, space group, coordination number, secondary building units, and topological type. In the case of drastic structural transitions, significant changes in properties were also observed. Complex 2 displayed the interesting uptake and release of iodine with the changes in visible color, UV and fluorescence spectra, and fully reversible I2 uptake of 8.5 mg g-1, which further suggested about its future application as iodine absorbing material.

Finely Controlled Stepwise Engineering of Pore Environments and Mechanistic Elucidation of Water-Stable, Flexible 2D Porous Coordination Polymers

Two porous coordination polymers (PCPs) with different topologies (NTU-19: sql and NTU-20: dia) underwent finely controlled, stepwise crystal conversions to yield a common water-stable, flexible 2D framework (NTU-22: kgm). The crystal conversions occurred directly at higher temperature via the 3D intermediate (NTU-21: nbo), which could be observed at lower temperature. The successful isolation of the intermediate product of NTU-21, characterization with in situ PXRD and UV/Vis spectra were combined with DFT calculations to allow an understanding of the dynamic processes at the atomic level. Remarkably, breakthrough experiments demonstrate NTU-22 with integral structural properties allowed significant CO₂ /CH₄ mixture separation.

Single-crystal-to-single-crystal (SCSC) transformation and dissolution–recrystallization structural transformation (DRST) among three new copper(ii) coordination polymers

Three new copper(ii) complexes with single-crystal-to-single-crystal (SCSC) transformation and dissolution–recrystallization structural transformation (DRST) have been synthesized and fully characterized.

A Copper(II)-Paddlewheel Metal-Organic Framework with Exceptional Hydrolytic Stability and Selective Adsorption and Detection Ability of Aniline in Water

Copper(II)-paddlewheel-based metal-organic frameworks (CP-MOFs) represent a unique subclass of MOFs with highly predictable porous structures, facile syntheses, and functional open metal sites. However, the lack of high hydrolytic stability is an obstacle for CP-MOFs in many practical applications. In this work, we report a new CP-MOF, [Cu₄(tdhb)] (BUT-155), which is constructed from a judiciously designed carboxylate ligand with high coordination connectivity (octatopic), abundant hydrophobic substituents (six methyl groups), and substituent constrained geometry (tetrahedral backbone), tdhb^8- [H₈tdhb = 3,3',5,5'-tetrakis(3,5-dicarboxyphenyl)-2,2',4,4',6,6'-hexamethylbiphenyl)]. BUT-155 shows high porosity with a Brunauer-Emmett-Teller surface area of 2070 m²/g. Quite interestingly, this CP-MOF retains its structural integrity after being treated in water for 10 days at room temperature or in boiling water for 24 h. To the best of our knowledge, BUT-155 represents the first CP-MOF that is demonstrated to retain its structural integrity in boiling water. The high hydrolytic stability of BUT-155 allowed us to carry out adsorption studies of water vapor and aqueous organic pollutants on it. Water-vapor adsorption reveals a sigmoidal isotherm and a high uptake (46.7 wt %), which is highly reversible and regenerable. In addition, because of the availability of soft-acid-type open Cu(II) sites, BUT-155 shows a high performance for selective adsorption of soft-base-type aniline over water or phenol, and a naked-eye detectable color change for the MOF sample accompanies this. The adsorption selectivity and high adsorption capacity of aniline in BUT-155 are also well-interpreted by single-crystal structures of the water- and aniline-included phases of BUT-155.

The cumene/O2 system: a very simple tool for the radical chain oxidation of some functional groups

Due to the relative stability of the cumyl radical, cumenes and α-methyl-styrenes are ideally structured to directly harvest the oxidizing reactivity of O2 and initiate radical chain reactions in catalyst-free conditions. In the absence of additional substrates, these processes can lead to acetophenones. In the presence of substrates, the cumene oxidation process can be intercepted in various chain reactions, affording very simple protocols for functional group oxidation.

An efficient combination of Zr-MOF and microwave irradiation in catalytic Lewis acid Friedel-Crafts benzoylation

A zirconium-based metal-organic framework, an effective heterogeneous catalyst, has been developed for the Friedel-Crafts benzoylation of aromatic compounds under microwave irradiation. Constructed by a Zr(iv) cluster and a linker 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene (H2CPEB), the MOF, possessing large pores and high chemical stability, was appropriate for the enhancement of Lewis acid activity under microwave irradiation. The reaction studies demonstrated that the material could give high yields for a few minutes and maintain its reactivity and structure over several cycles.

A direct strategy to synthesize hybrid benzothiazole–carbamate moieties via O-acylation of phenols under metal–organic framework catalysis

Cu-CPO-27 was used as an efficient recyclable catalyst for the synthesis of benzothiazole–carbamate structures.