Mixed-ligand Metal-organic Framework Research Articles

Mixed‐Ligand Metal‐Organic Frameworks: Synthesis and Characterization of New MOFs Containing Pyridine‐2,6‐dimethanolate and Benzene‐1,4‐dicarboxylate Ligands

AbstractThe development of synthetic approaches towards new mixed‐ligand metal organic frameworks (MOFs) has been attracting considerable attention recently as they display fascinating properties due to the synergistic effect between the different ligands. Herein, the initial combination of pyridine‐2,6‐dimethanol (H2pdm) with benzene‐1,4‐dicarboxylic acid (H2bdc) has provided access to three new mixed‐ligand MOFs, [M3(bdc)2(Hpdm)2]n (MII=ZnII, 1; MnII, 2) and [Mn3(bdc)3(H2pdm)2]n (3). 1–3 are the first MOFs containing H2pdm in its neutral or anionic form. 1 and 2 are isostructural 2D MOFs with a square lattice (sql) framework topology, while 3 is a 3D MOF possessing a primitive cubic (pcu) topology. Dc magnetic susceptibility measurements for 3 demonstrate antiferromagnetic interactions between the MnII ions which is a combination of intra‐ and intertrimer exchange pathways.

Mixed-Ligand Metal-Organic Frameworks for All-in-One Theranostics with Controlled Drug Delivery and Enhanced Photodynamic Therapy.

Mixed-ligand metal-organic frameworks (MOFs) multiply the properties and improve the versatility of conventional MOFs for theranostic applications. A tumor targeting and tumoral microenvironment-responsive system is significant for specific and efficient cancer theranostics. Herein, we report a kind of versatile mixed-porphyrin ligand MOF as a multifunctional matrix for multimodality-imaging-guided synergistic therapy. Tetrakis(4-carboxyphenyl)porphyrin (TCPP) shows the properties of fluorescence (FL) and photodynamic therapy (PDT), while Mn-TCPP owns magically the properties of T1-weighted magnetic resonance (MR) imaging and photothermal conversion for photothermal imaging and photothermal therapy (PTT). Because of the same coordination capacity and mode of TCPP and Mn-TCPP to Zr4+ ions, MOFs with adjustable ligand ratios were easily prepared. The mixed-ligand MOFs exhibited a high drug loading capacity for 10-hydroxycamptothecin (HCPT, 65%). After modification with hyaluronic acid (HA) through a disulfide bond (-S-S-), the MOF-S-S-HA composites possess enhanced PDT and tumor-targeted redox-responsive drug release properties due to the -S-S- bond. Thus, excellent fluorescence, MR, and photothermal trimodality imaging, redox-responsive drug release, and enhanced PDT/PTT are integrated together in the mixed-ligand MOFs as "all-in-one" theranostic agents.

Pore-Nanospace Engineering of Mixed-Ligand Metal–Organic Frameworks for High Adsorption of Hydrofluorocarbons and Hydrochlorofluorocarbons

The isostructural mixed-ligand metal–organic frameworks (MOFs), LIFM-66, 66/67-mix, and 67, have been constructed via organic-linker modification and variation to introduce different portions of functional groups into the pore-nanospaces with tunable surface areas. Adsorption measurements of hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) with these MOFs reveal a record high uptake of R134a (1.09–1.14 g g–1) and an ultrahigh uptake of R22 (0.85–0.96 g g–1) under ordinary conditions of room temperature and atmospheric pressure. Noteworthily, the adsorption performance of greenhouse gases CO2, R134a, and R22 can be finely regulated by introduced methyl groups, which has been elucidated by molecular modeling and transient breakthrough simulations/experiments, demonstrating an effective protocol of pore-nanospace engineering through rational design of mixed-ligand MOFs for HFC and HCFC capture, sequestration, and reclamation.

Metal-organic framework-based magnetic dispersive micro-solid-phase extraction for the gas chromatography–mass spectrometry determination of polycyclic aromatic compounds in water samples

A magnetic hybrid material based on the use of the mixed-ligand Metal-Organic Framework (MOF) PUM198 is proposed for the magnetic dispersive micro solid-phase extraction (MD-μSPE) of the 16 polycyclic aromatic hydrocarbons (PAHs) included in the US-EPA priority pollutants list. PUM198 is a thermally robust MOF characterized by a doubly interpenetrated microporous framework in which Zn2+ ions and carboxylate groups define 2D planes that are pillared by a bis-pyridine-bis amide ligand containing a biphenyl scaffold. PUM198 revealed to be ideal to adsorb PAHs efficiently through non-covalent interactions. A Plackett-Burman Design followed by a Central Composite Design and the multicriteria method of the desirability functions were applied to find the optimal conditions for the extraction of the investigated PAHs, resulting in a reduced solvent consumption, i.e., 50 μL of solvent per extraction for 5 mL of sample, approximatively 3–20 times lower than those reported in previous studies, thus satisfying the principles of green analytical chemistry. Method validation proved the reliability of the method for the determination of PAHs at trace level, obtaining detection limits in the 6.7–27 ng/L range, good precision with RSDs% lower than 19% and recovery rates in the 99 (±13)–126 (±8)% range near the quantitation limit. Finally, the applicability of the method was demonstrated by analyzing underground water samples taken from contaminated sites

Design and Multiple Applications of Mixed-Ligand Metal-Organic Frameworks with Dual Emission.

Herein, we revealed the factors that affect the emission in mixed-ligand metal-organic frameworks (MOFs) with the combination of terephthalic acid (BDC), 2-aminoterephthalic acid (BDC-NH2), and 2,5-dihydroxylterephthalic acid [BDC-(OH)2] as models. The -NH2 and -(OH)2 groups change the π-conjugation and luminescence behaviors than BDC, so the ligands show different optical behaviors. The Zn2+ ion with a 3d10 full electronic structure shows little effect on the emission of the ligand and is selected as the metal node. We found that the emission of BDC is weak and incompatible to that of BDC-NH2, so only the emission of BDC-NH2 was observed in the BDC/BDC-NH2-MOF. Crosstalk occurs between the emissions from BDC and BDC-(OH)2 for the single emission from BDC/BDC-(OH)2-MOFs, even different ratios are selected. The MOFs prepared with BDC-NH2 and BDC-(OH)2 show dual emission at 450 and 550 nm, while the relative intensity was easily tuned with the ligand ratio and excitation wavelength. Thus, abundant optical behaviors and extensive applications were realized, including but not limited to (1) dual emission from single MOFs, (2) tunable color from blue to yellow with the excitation from 290 to 370 nm for information encryption and decryption, (3) white emission obtained under an excitation of 330 nm, and (4) response of -NH2 groups to HCHO and Fe3+ ions for ratiometric fluorescence sensing and visual detection. This work revealed the factors that affect the emission in mixed-ligand MOFs, studied their optical behaviors, and realized different applications with single MOFs.

Selective ligand removal to improve accessibility of active sites in hierarchical MOFs for heterogeneous photocatalysis

Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies.

Mixed-Ligand Metal-Organic Frameworks with Coordinatively Unsaturated Co(Ii) and Ni(Ii) Sites for Regenerable O2-Selective Adsorption Over N2

Mixed-Ligand Metal-Organic Frameworks with Coordinatively Unsaturated Co(Ii) and Ni(Ii) Sites for Regenerable O2-Selective Adsorption Over N2

In situ time-resolved monitoring of mixed-ligand metal–organic framework mechanosynthesis

The mechanism of mixed-ligand metal–organic framework (MOF) formation, and the possible role of intermediate single-ligand metal complexes during mechanosynthesis, are explored for the first time.

Preparation and application of bimetallic mixed ligand MOF photocatalytic materials

A series of metal-organic framework (MOF) materials have been synthesized by the solvothermal method using trimellitic acid and terephthalic acid to react with nickel and cobalt metal salts for photodegradation of methylene blue (MB) under irradiation with a xenon light source. ML-3, a tight-flower structure synthesized on the basis of solid blocks structure NC-3, is a bimetallic mixed-ligand MOF material with a specific ratio, which has excellent degradation performance against MB. The degradation efficiency of NC-3 is 80.6% in 120 min, while the degradation efficiency of ML-3 can reach 97.8% in 120 min. The radical trapping experiments and Mott-Schottky analysis were carried out to reveal the photocatalytic degradation mechanism. The result showed that h+ and·O2– were the main active substances; while the redox potential of MB (1.77 eV) was lower than the valence band (VB) potential of ML-3 (2.01 eV), and h+ could directly oxidize MB. Therefore, the as-synthesized materials in this study are effective wastewater treatment agents and have a promising future in environmental treatment.

Symmetry-Guided Synthesis of N,N'-Bicarbazole and Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks: Light Harvesting and Energy Transfer.

In the past decades, many attempts have been made to mimic the energy transfer (EnT) in photosynthesis, a key process occurring in nature that is of fundamental significance in solar fuels and sustainable energy. Metal-organic frameworks (MOFs), an emerging class of porous crystalline materials self-assembled from organic linkers and metal or metal cluster nodes, offer an ideal platform for the exploration of directional EnT phenomena. However, placing energy donor and acceptor moieties within the same framework with an atomistic precision appears to be a major synthesis challenge. In this work, we report the design and synthesis of a highly porous and photoactive N,N'-bicarbazole- and porphyrin-based mixed-ligand MOF, namely, NPF-500-H2TCPP (NPF = Nebraska porous framework; H2TCPP = meso-tetrakis(4-carboxyphenyl)porphyrin), where the secondary ligand H2TCPP is incorporated precisely through the open metal sites of the equatorial plane of the octahedron cage resulting from the underlying (4,8) connected network of NPF-500. The efficient EnT process from N,N'-bicarbazole to porphyrin in NPF-500-H2TCPP was captured by time-resolved spectroscopy and exemplified by photocatalytic oxidation of thioanisole. These results demonstrate not only the capability of NPF-500 as the scaffold to precisely arrange the donor-acceptor assembly for the EnT process but also the potential to directly utilize the EnT process for photocatalytic applications.

Role of a 3D Structure in Energy Transfer in Mixed-Ligand Metal–Organic Frameworks

Role of a 3D Structure in Energy Transfer in Mixed-Ligand Metal–Organic Frameworks

Metal organic framework as “turn-on” fluorescent sensor for Zr(IV) ions and selective adsorbent for organic dyes

A new cobalt-based mixed ligand metal–organic framework (MOF), [Co2(L1)(obc)2].DMF (MOF1), (where L1 = 2,7-bis (3-(1H-imidazole-1-yl)propyl)benzo-[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone and H2obc = 4,4-oxybisbenzoic acid) was prepared solvothermally that simultaneously exhibits sensing and adsorption capability. The single-crystal X-ray structural analysis revealed that MOF1 exhibits an interpenetrated three-dimensional framework, of pcu topology, comprising classic paddlewheel-type units. The synthesized MOF1 was found to be luminescent with an emission/excitation peak at 402/275 nm that is enhanced by Zr4+ ions due to absorbance-caused enhancement (ACE). The fluorescence studies displayed the highly selective and sensitive behavior of MOF1 towards Zr4+ ions through the turn-on fluorescence effect with a short response time and detection limit of 0.067 μM. Furthermore, MOF1 was successfully applied for the treatment of commercial dyes Reactive Black 5 and Orange G with adsorption capacity of 33.79 and 22.32 mg/g, respectively, thus showing rapid adsorption for these dyes.

Dual Intrareticular Oxidation of Mixed-Ligand Metal-Organic Frameworks for Stepwise Electrochemiluminescence.

This work presents a mixed-ligand metal-organic framework (m-MOF) integrated with two ligands, one as a luminophore and the other as a coreactant, on one metal node for self-enhanced electrochemiluminescence (ECL). Both 9,10-di(p-carboxyphenyl)anthracene (DPA) and 1,4-diazabicyclo[2.2.2]octane (D-H2) ligands can be oxidized, generating the cation radicals DPA+• and D-H2+•, respectively. The latter can be deprotonated to form the neutral radical (D-H•) and then react with DPA+• to produce excited DPA* for ECL emission without exogenous coreactants. As a result of the incorporation into the MOF framework and the intrareticular charge transfer between the two ligands, the ECL intensity of the m-MOF was increased 26.5-fold compared with that of the mixture of DPA and D-H2 in aqueous solution. Moreover, with the process of second oxidation of D-H2, stepwise ECL emission was observed as a result of local excitation in the DPA unit, which was identified through density functional theory calculations. Overall, the implementation of the mixed-ligand approach, which combines the luminophore and coreactant as linkers in reticular materials, enriches the fundamentals and applications of ECL systems.

Microporous mixed-ligand metal–organic framework with fluorine-decorated pores for efficient C2H2/C2H4 separation

The selective separation of C2H2 from C2H4 is a very important process for downstream industrial production, and porous metal-organic frameworks (MOFs) have shown great potential for such an application due to their tunable pore surroundings and adjustable building blocks. In this study, a new microporous MOF formulated as {(Et2NH2)[In(TBOT)(2,3-FDA)](DEF)(H2O)2}n (1, H3TBOT ​= ​4,4′,4’’-(((2,4,6-trimethylbenzene-1,3,5-triyl)tris(oxy))tris(methylene))tribenzoic acid, 2,3-HFDA ​= ​2,3-difluorobenzoic acid and DEF ​= ​N,N-diethylformamide) with 2-fold interpenetrated network and 1D fluorine-decorated pores has been prepared via using an acid-acid mixed-ligand strategy. The activated complex 1 (named as 1a) demonstrates apparently higher C2H2 uptake than C2H4 around room temperature, resulting in its relative high C2H2/C2H4 (v:v ​= ​1:99) IAST (Ideal Adsorbed Solution Theory) selectivity of 10.3 ​at 298 ​K and 1 ​bar. Furthermore, the GCMC (grand canonical Monte Carlo) simulation results indicate that the 2,3-FDA moieties serve as preferential binding sites for C2H2 via the multipoint F2⋯H–CC–H⋯F2 interactions, implying the feasibility of the mixed-ligand approach for building porous MOFs with the targeted applications.

Structural Dynamics and Adsorption Properties of the Breathing Microporous Aliphatic Metal-Organic Framework.

A mixed-ligand metal-organic framework [Zn2(chdc)2(dabco)]·2NMP (chdc2- = trans-1,4-cyclohexanedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane; NMP = N-methylpyrrolidone) was synthesized under solvothermal conditions. This coordination compound demonstrates a guest-driven framework breathing due to a conformational change between e,e-chdc and a,a-chdc forms of the linkers with a reversible restoration of crystallinity. Both the local and longer-range coordination environment of the metal centers were extensively studied by electron paramagnetic resonance on a Cu(II)-doped compound. This approach allowed the detailed investigation of the ligand structural conformations and the framework structural dynamics, supported by an X-ray diffraction method. Carbon dioxide and methane adsorption measurements as well as vapor sorption of benzene and cyclohexane at 298 K of the activated compound were studied. While adsorption of small gas molecules, such as CO2, CH4, and N2, is moderate and does not induce the phase transition, the multistepped character of C6H6 and C6H12 adsorption isotherms characterize the breathing nature of [Zn2(chdc)2(dabco)]. The uptake of benzene from the vapor phase reaches 125 mL·g-1 at 298 K, which surpasses most of benzene uptake values reported for microporous metal-organic frameworks.

Large Differences in Carbon Dioxide and Water Sorption Capabilities in a System of Closely Related Isoreticular Cd(II)-based Mixed-Ligand Metal-Organic Frameworks.

We report the synthesis of two isoreticular, mixed-ligand metal-organic frameworks (MOFs), [Cd(μ2-mia)(μ2-bpe)]n·n(DMF)0.5·n(H2O)0.5 (1) and [Cd(μ2-nia)(μ2-bpee)]n·nDMF (2), where mia = 5-methoxyisophthalate, nia = 5-nitroisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene, and DMF = N,N'-dimethylformamide. Variable-temperature powder X-ray diffraction studies confirmed that both MOFs remain crystalline with activation at high temperatures. Variable-temperature single-crystal X-ray diffraction studies were performed on 1, 2, and a previously published, isoreticular structure, [Cd(μ2-nia)(μ2-bpe)1.5]n·nDMF0.84 (3). These studies show that upon desolvation that monocrystallinity is retained to significantly higher temperatures for 2 and 3 when compared to 1 for which only a partially desolvated crystal structure could be obtained. Carbon dioxide sorption is negligible for 1 at 195 and 298 K, while it is higher for 2 than 3 at 298 K and reversed at 195 K. Water vapor sorption increases in the order 1, 2, and 3. On the contrary, water liquid sorption was significantly higher for 1 when compared to 2. The variable-temperature structures of the (partially) desolvated forms of 1, 2, and 3 give some insight into the reasons for the remarkably different gas, vapor, and liquid sorption properties.

Enhanced Photocatalytic Performance of Nanosized Mixed-Ligand Metal–Organic Frameworks through Sequential Energy and Electron Transfer Process

Effective sequestration of harmful organic pollutants from wastewater has been a persistent concern in the interest of environmental and ecological protection from pollution and hazards. Currently, common water treatment technologies such as adsorption, coagulation, and membranes are expensive and not greatly effective. A new class of organic and inorganic composite metal-organic frameworks (MOFs) has emerged as an essential class of materials for numerous applications, including photocatalytic degradation of organic pollutants. Herein, we present a nanosize mixed-ligand MOF (nMLM) which was successfully synthesized by reacting a Zr metal source with a mixture of pyrene and porphyrin building units and further utilized as photocatalyst in the photodegradation of rhodamine B (RhB). The nMLM MOF showed excellent photocatalytic efficiency, which was due to the complementary absorption and sequential energy and electron transfer properties of its building blocks, pyrene and porphyrin. We also propose herein a possible mechanism of the photocatalytic function of the material.

Correction to Mixed-Ligand Metal-Organic Frameworks and Heteroleptic Coordination Cages as Multifunctional Scaffolds-A Comparison.

Correction to Mixed-Ligand Metal-Organic Frameworks and Heteroleptic Coordination Cages as Multifunctional Scaffolds-A Comparison.

Microwave-assisted continuous-flow synthesis of mixed-ligand UiO-66(Zr) frameworks and their application to toluene adsorption

Mixed-ligand metal–organic frameworks (MIXMOFs) containing two or more organic linkers that exhibit functionalities can show improved selective gas adsorption and separation properties with respect to those of the conventional MOFs. In this study, the mixed-ligand UiO–66(Zr) framework was rapidly prepared by employing a microwave-assisted continuous-flow method, which can be easily scaled up for efficient mass production. A microfluidic syringe pump system was used to introduce the precursor solutions of zirconium metal salt and ligand mixture (terephthalic acid and 2-amino terephthalic acid) into a polytetrafluoroethylene tubular reactor kept under microwave irradiation. MIXMOFs were obtained within 10min, a much shorter time than the reaction time required by the conventional heating method. The prepared samples were characterized by N2 porosimetry, field-emission scanning electron microscopy, Fourier transform infrared spectrometry, thermogravimetric, and powder X-ray diffraction analyses to elucidate the effects of the ligand ratios. Furthermore, the toluene adsorption–desorption tests were conducted using the synthesized mixed-ligand UiO–66(Zr) adsorbents at different temperatures.

Structural analysis of and selective CO2 adsorption in mixed-ligand hydroxamate-based metal-organic frameworks.

Two mixed-ligand metal-organic frameworks, [Zn2(BDHA)0.5(INA)3] (MOF-1: H2BDHA = benzene-1,4-dihydroxamic acid; HINA = isonicotinic acid) and [Co2(BDHA)0.5(INA)3(DMF)] (MOF-2), were solvothermally synthesized and fully characterized by single-crystal X-ray crystallography as well as N2, H2, and CO2 gas-sorption measurements. The results constitute the first detailed analysis of the bonding environment around the hydroxamates in such MOFs, which are simultaneously decorated with Lewis-basic sites from the hydroxamate moieties and metal sites predisposed for coordinative unsaturation. MOF-2 shows a desirably selective adsorption of CO2 relative to N2.