Flexible Metal-Organic Frameworks: Recent Advances and Potential Applications

Flexible metal-organic frameworks (MOFs) receive much attention owing to their attractive properties that originate from their flexibility and dynamic behavior, and show great potential applications in many fields. Here, recent progress in the discovery, understanding, and property investigations of flexible MOFs are reviewed, and the examples of their potential applications in storage and separation, sensing, and guest capture and release are presented to highlight the developing trends in flexible MOFs.

Metal-Organic Frameworks Incorporating Various Alkoxy Pendant Groups: Hollow Tubular Morphologies, X-ray Single-Crystal Structures, and Selective Carbon Dioxide Adsorption Properties

Eight porous metal-organic frameworks (MOFs) incorporating various alkoxy pendant groups (-OC(n)H(2n+1); n=1-8) in the channels have been synthesized. All MOFs have macrosized, hollow, single-crystal morphologies independent of the type of alkoxy pendant groups. The X-ray single-crystal structures indicate that the MOFs have doubly interpenetrated 3D frameworks that generate clover-shaped 1D channels, the window sizes of which reduce as the length of the alkoxy pendant groups increases from -OCH3 to -OC8H17. The hollow axes of the tubular crystals are parallel to the 1D channels of the MOFs, which suggests that the hollow crystal is grown by the unidirectional addition of pillar ligands to the 2D network. Among the activated MOFs, only the MOFs with methoxy and ethoxy pendant groups show selective CO2 adsorption over N2 and CH4, whereas those with longer alkoxy pendant groups barely adsorb CO2 at room temperature, which implies that the pendant doors of the MOFs should have an appropriate length for selective CO2 capture.

Heterometallic cluster-based indium-organic frameworks

With the help of the ligand-oriented method, we have successfully embedded independent copper-based units into the indium-organic framework system for the first time, in which the Cu4I4 clusters and In3O(CO2)6 clusters coexist. This heterometallic cluster-based framework has a large porosity with extra-open channels along the c-axis, and its sorption capacity has also been investigated.

Tetrazine chromophore-based metal-organic frameworks with unusual configurations: synthetic, structural, theoretical, fluorescent, and nonlinear optical studies

Three unusual three-dimensional (3D) tetrazine chromophore-based metal-organic frameworks (MOFs) {(Et4 N)[WS4 Cu3 (CN)2 (4,4'-pytz)0.5 ]}n (1), {[MoS4 Cu4 (CN)2 (4,4'-pytz)2 ]⋅CH2 Cl2 }n (2), and {[WS4 Cu3 (4,4'-pytz)3 ]⋅[N(CN)2 ]}n (3; 4,4'-pytz=3,6-bis(4-pyridyl)tetrazine) have been synthesized and characterized by using FTIR and UV/Vis spectroscopy, elemental analysis, powder X-ray diffraction, gel permeation chromatography, steady-state fluorescence, and thermogravimetric analysis; their identities were confirmed by single-crystal X-ray diffraction studies. MOF 1 possesses the first five-connected M/S/Cu (M=Mo, W) framework with an unusual 3D (4(4) ⋅6(6) ) topology constructed from T-shaped [WS4 Cu3 ](+) clusters as nodes and single CN(-) /4,4'-pytz bridges as linkers. MOF 2 features a novel 3D MOF structure with (4(20) ⋅6(8) ) topology, in which the bridging 4,4'-pytz ligands exhibit unique distorted arch structures. MOF 3 displays the first 3D MOF structure based on flywheel-shaped [WS4 Cu3 ](+) clusters with a non-interpenetrating honeycomb-like framework and a heavily distorted "ACS" topology. Steady-state fluorescence studies of 1-3 reveal significant fluorescence emissions. The nonlinear optical (NLO) properties of 1-3 were investigated by using a Z-scan technique with 5 ns pulses at λ=532 nm. The Z-scan experimental results show that the π-delocalizable tetrazine-based 4,4'-pytz ligands contribute to the strong third-order NLO properties exhibited by 1-3. Time-dependent density functional theory studies afforded insight into the electronic transitions and spectral characterization of these functionalized NLO molecular materials.

Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr(VI) reduction

Porous metal-organic frameworks (MOFs) have been arousing a great interest in exploring the application of MOFs as photocatalyst in environment remediation. In this work, two different MOFs, Ti-benzenedicarboxylate (MIL-125(Ti)) and amino-functionalized Ti-benzenedicarboxylate (NH2-MIL-125(Ti)) were successfully synthesized via a facile solvothermal method. The MIL-125(Ti) and NH2-MIL-125(Ti) were well characterized by XRD, SEM, XPS, N2 adsorption-desorption measurements, thermogravimetric analysis and UV-vis diffuse reflectance spectra (DRS). It is revealed that the NH2-MIL-125(Ti) has well crystalline lattice, large surface area and mesoporous structure, chemical and thermal stability, and enhanced visible-light absorption up to 520 nm, which was associated with the chromophore (amino group) in the organic linker. Compared with MIL-125(Ti), NH2-MIL-125(Ti) exhibited more efficient photocatalytic activity for Cr(VI) reduction from aqueous solution under visible-light irradiation. The addition of hole scavenger, the hole scavenger concentration and the pH value of the reaction solution played important roles in the photo-catalytic reduction of Cr(VI). The presence of Ti(3+)-Ti(4+) intervalence electron transfer was the main reason for photo-excited electrons transportation from titanium-oxo clusters to Cr(VI), facilitating the Cr(VI) reduction under the acid condition. It was demonstrated that amino-functionalized Ti(IV)-based MOFs could be promising visible-light photocatalysts for the treatment of Cr(VI)-contained wastewater.

Synthesis, structure and thermal study of a new 3-aminopyrazine-2-carboxylate based zinc(II) coordination polymer

A two-dimensional coordination polymer [Zn(L)2]n (1) (L = 3-aminopyrazine-2-carboxylate) synthesized from the solvothermal reaction of zinc(II) salt with 3-aminopyrazine-2-carboxylic acid (HL) is described. Compound 1 has been characterized by single X-ray diffraction, IR spectra and thermogravimetric analyses. Crystal structure analysis reveals that each hexacoordinated zinc(II) center adopts a distorted octahedral geometry occupied by three Ocarboxylate and three Npyrazine atoms. The L– ligand binds the metal cation by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. A three-dimensional supramolecular associate in the crystal lattice of 1 has been stabilized by a number of non-covalent interactions. The IR spectroscopic and TGA properties are investigated in this work. Topological analysis of the two-dimensional network has been also discussed.

Spectroscopic and crystallographic investigations of novel BODIPY-derived metal-organic frameworks

To explore new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-derived metal-organic frameworks (MOFs), we employed 2,6-dicarboxyl-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (H2L) as a ligand to successfully synthesize five coordination polymers, namely, {[Zn2(L)2(bpp)]·2H2O·2EtOH}n (1), {[Cd2(L)2(bpp)]·2H2O·EtOH}n (2), {[Cd2(L)(bpe)3(NO3)2]·2H2O·DMF·EtOH}n (3), {[Cd(L)(bpe)0.5(DMF)(H2O)]}n (4), and {[Cd(L)(bpe)0.5]·1.5H2O·DMF}n (5) (bpp = 1,3-bi(4-pyridyl)propane, bpe = 1,2-bi(4-pyridyl)ethane). Except for two 2D-layer coordination polymers 3 and 4, the rest samples exhibit 3D metal-organic frameworks with certain pore sizes, especially MOFs 1 and 5. Spectroscopic and crystallographic investigations demonstrate that the absorption and emission energies of the BODIPY chromophores are sensitive to the coordination modes. Moreover, in case 2, the transition metal centers coordinated with the dicarboxylate ligands L(2-) are capable of forming the two BODIPY units in coplanar arrangements (θ = 37.9°), simultaneously suppressing the uncommon J-dimer absorption band centered at 705 nm with a long tail into the near-infrared region at room temperature. On the other hand, in comparison with the ligand H2L, the emission of monomer-like BODIPY in case 3 is enhanced in the solid state by a considerably long distance between the parallel BODIPY planes (about 14.0 Å).

Coordination polymers from a highly flexible alkyldiamine-derived ligand: structure, magnetism and gas adsorption studies

Studies into a series of coordination polymers from a new diamine polycarboxylate ligand reveal an interplay between flexibility and material properties.

Coordination assemblies of Zn(NO3)2 with 4-pyr-poly-2-ene and polycarboxylates: structural diversification and photoluminescence properties

Reactions of Zn(NO₃)₂·6H₂O with 4-pyr-poly-2-ene and five polycarboxylates afforded a series of coordination polymers with different architectures. Their thermal and photoluminescence properties were also investigated.

A sensor for formaldehyde detection: luminescent metal–organic framework [Zn2(H2L)(2,2′-bpy)2(H2O)]n

The comparison between the two systems shows that the encapsulation of formaldehyde can affect the luminescence behavior of [Zn₂(H₂L)(2,2′-bpy)₂(H₂O)]_n.

Metal–organic frameworks catalyzed C–C and C–heteroatom coupling reactions

This critical review aims to illustrate the recent developments in the field of C–C and C–X (X = O, N) coupling reactions promoted by metal–organic frameworks.

Structure modulation in zinc–ditetrazolate coordination polymers by in situ ligand synthesis

Three new coordination polymers have been constructed by an in situ ditetrazolate-ligand synthesis system involving the sharpless [2 + 3] cycloaddition reaction between IPN and NaN₃ in the presence of Zn²⁺ ions as Lewis-acid catalysts.

The Biginelli reaction under batch and continuous flow conditions: catalysis, mechanism and antitumoral activity

Biginelli scaffolds have been synthesized using two new coordination polymer as catalysts under batch or continuous flow conditions. The mechanism of the reaction and the antitumoral activity of the obtained DHPMs were also evaluated.

A metal–organic framework constructed using a flexible tripodal ligand and tetranuclear copper cluster for sensing small molecules

A porous MOF based on a tetranuclear copper cluster with intracluster antiferromagnetic interactions was synthesized, which reveals highly sensitive and selective sensing of small molecules.

Microwave-assisted large scale synthesis of lanthanide metal–organic frameworks (Ln-MOFs), having a preferred conformation and photoluminescence properties

We have prepared 2 g Ln-MOF on a laboratory scale with high purity using microwave-assisted synthesis by changing the conformation of flexible ligand using Ln, which adopts only the preferred (energetically stable) conformation from green solvents (water and ethanol).

4-(4-Carboxyphenoxy)phthalate-based coordination polymers and their application in sensing nitrobenzene

Structural characterization of 4-(4-carboxyphenoxy)phthalate-based Cd²⁺ and Mn²⁺ coordination polymers are reported, and Cd²⁺ coordination polymer can serve as probe to sense nitrobenzene.

The copper(i) metal azolate framework showing unusual coordination mode for the 1,2,4-triazole derivative and photocatalytic activity

A novel photocatalyst containing a tetranuclear Cu₄ cluster constructed by six 1,2,4-triazole units was reported where two of the six 1,2,4-triazole units show unusual bridging “crevice” coordination mode with their 1- and 2-positioned sp² N-atoms as symmetrically bridging centers.

Three new luminescent Cd(ii)-MOFs by regulating the tetracarboxylate and auxiliary co-ligands, displaying high sensitivity for Fe3+ in aqueous solution

Three new luminescent Cd(ii)-MOFs have been synthesized by regulating the steric effect of tetracarboxylate and auxiliary co-ligands. Notably, 3 displayed high sensitivity for Fe³⁺ in aqueous solution with mixed ions.

A novel [4 + 3] interpenetrated net containing 7-fold interlocking pseudo-helical chains and exceptional catenane-like motifs

The first example of a [4 + 3] 7-fold interpenetrating network was prepared, showing 7-fold interlocking pseudo-helical chains and a unique catenane-like motif with Hopf links.

Porous materials based on metal–nucleobase systems sustained by coordination bonds and base pairing interactions

Two approaches to metal–nucleobase porous materials: coordination bond sustained MOFs and hydrogen bond pairing based SMOFs.

Coordination polymers constructed from a tripodal phosphoryl carboxylate ligand: synthesis, structures and physical properties

Four coordination polymers based on a new tripodal phosphoryl carboxylate H₃TBPPO ligand have been constructed and characterized. Complexes 1–3 show 2-fold interpenetrating flu net, while complex 4 possesses a 4-fold interpenetrating dia network.

Transforming metal–organic frameworks into functional materials

Transformation of metal–organic frameworks as a way to improve properties and design better materials is reported.

Tubular porous coordination polymer for the selective sensing of Cu2+ ions and cyclohexane in mixed suspensions of metal ions via fluorescence quenching

A tubular porous coordination polymer has been developed that can rapidly and selectively sense Cu²⁺ ions via fluorescence quenching in a suspension of mixed metal ions.

Two robust metal–organic frameworks with uncoordinated N atoms for CO2adsorption

Two highly porous metal–organic frameworks are successfully constructed through the assembling of Co/Ni ions and mixed ligands. From the contribution of the uncoordinated tetrazolate N atoms of the ligand the two complexes show high CO₂adsorption capacities, serving as potential CO₂adsorption materials.

Structural and luminescence modulation in 8-hydroxyquinolinate-based coordination polymers by varying the dicarboxylic acid

Using dicarboxylic acids as secondary auxiliary ligands, six coordination polymers are synthesized under solvothermal conditions. These polymers exhibited disparate fluorescence emission bands and lifetimes due to their different metal centers and supramolecular structures.