Mixed-ligand Metal-organic Framework Research Articles

Heterobimetallic Synergism in Triple-Redox 2D Framework for Largely Boosted Water Oxidation and Flanked Carboxylic-Acid-Triggered Unconventional Tandem Catalysis.

A fish-bone-shaped and thermochemically stable 2D metal-organic framework (MOF) with multimodal active center-decked pore-wall is devised. Redox-active [Co2(COO)4] node and thiazolo[5,4-d]thiazole functionalization benefit this mixed-ligand MOF exhibiting electrochemical water oxidation with 375mV overpotential at 10mA cm-2 current density and 78mV per dec Tafel slope in alkaline medium. Pair of oppositely oriented carboxylic acids aids postmetalation with transition metal ions to engineer heterobimetallic materials. Notably, overpotential of Ni2+ grafted triple-redox composite reduces to 270mV with twofold declined Tafel slope than the parent MOF, ranking among the best-reported values, and outperforming majority of related catalysts. Significantly, turnover frequency and charge transfer resistance display 35.5 and 1.4-fold upsurge, respectively, with much uplifted chronopotentiometric stability and increase active surface area owing to synergistic Co(II)-Ni(II) coupling. The simultaneous presence of ─COOH and nitrogen-rich moieties renders this hydrogen-bonded MOF as acid-base synergistic catalyst for recyclable deacetalization-Knoevenagel reaction with >99% product yield under solvent-free mild condition. Besides control experiments, unique role of ─COOH as hydrogen-bond donor site in substrate activation is validated from comparing the performances of molecular-shearing approach-derived structurally similar unfunctionalized MOF, and the heterobimetallic composite. To the best of tandem Knoevenagel condensation, larger-sized acetal exhibits poor yield of α,β-unsaturated dicyanides, and demonstrates pore-fitting-mediated size-selectivity.

Proton-induced switching of excitation-wavelength-dependent emission based on mixed-ligand metal–organic frameworks

Switchable Ex-De emission of ML-MOFs has been realized via a proton-controlled ET process, enabling the design of high-level anti-counterfeiting applications.

A mixed-ligand Co metal-organic framework and its carbon composites as excellent electrocatalysts for the oxygen evolution reaction in green-energy devices.

Oxygen evolution reaction (OER) electrocatalysts are frequently made from noble metal-based oxides like ruthenium/iridium oxides. However, because of their scarcity and high price, researchers are now focusing on creating innovative OER catalysts based on affordable transition metals that have improved electrical conductivity and accessibility to active sites. Metal-organic frameworks (MOFs), a unique class of inorganic materials with excellent physical and chemical properties, have witnessed significant progress in promising green energy systems. In this work, a novel mixed-ligand metal-organic framework [Co(μ-1κN,2κN'-BDP)(μ3-1κoo',2κo''2κo'''-BTC)]n·nH2O (BDP = boron-dipyrromethene or BODIPY; BTC = benzene tricarboxylate) denoted as CoBDPMOF has been synthesized, and its composites with different carbon materials have been designed. Compared to the pristine MOF, the composites showed enhanced electrocatalytic activity toward the oxygen evolution reaction (OER) in alkaline media. In addition, the CoBDPMOF with activated carbon showed the highest OER performance with a low Tafel slope (82 mV dec-1) and the highest j600 (59.8 mA cm-2), outperforming noble metal IrO2, the OER benchmark electrocatalyst. This study presents new insights into the design and application of CoBDPMOF-based materials for energy conversion and suggests promising avenues for further research and development in electrocatalysis.

Electron Donor Coordinated Metal-Organic Framework to Enhance Photoelectrochemical Performance.

To enhance photoelectrochemical (PEC) performance, additional electron donor/acceptor is generally required to inhibit the electron-hole recombination. However, the enhancement is limited due to the long-distance diffusion. Herein a self-supplying electron strategy is designed for PEC enhancement by coordinating an electron donor 1.4-diazabicyclo[2.2.2]octane (Dabco) in metal-organic framework (MOF). The intrareticular photoelectron transfer mechanism in mixed-ligand MOF (m-MOF) is experimentally revealed and verified by density functional theory calculations. The presence of Dabco efficiently inhibits the electron-hole recombination due to the self-supplying electrons and longer electron lifetime in the framework, and thus leads to 23.2-fold enhancement of photocurrent. As proof of concept, a simple PEC method is constructed with the designed m-MOF to demonstrate its application in sensitive bioanalysis. This work provides a new avenue for improving the PEC performance of nanomaterials.

Application of a Mixed-Ligand Metal–Organic Framework in Photocatalytic CO2 Reduction, Antibacterial Activity and Dye Adsorption

In this paper, a known mixed-ligand MOF {[Co2(TZMB)2(1,4-bib)0.5(H2O)2]·(H2O)2}n (compound 1) was reproduced, and its potential application potential was explored. It was found that compound 1 had high photocatalytic activity for CO2 reduction. After 12 h of illumination, the formation rate of CO, which is the product of CO2 reduction by compound 1, reached 3012.5 μmol/g/h. At the same time, compound 1 has a good antibacterial effect on Staphylococcus aureus (S. aureus), Escherichia coli (E. coli) and Candida albicans (C. albicans), which has potential research value in the medical field. In addition, compound 1 can effectively remove Congo Red from aqueous solutions and achieve the separation of Congo red from mixed dye solutions.

Large Degree of Variation in Sorption Properties within a System of Isoreticular, Mixed-Ligand Ni- and Co-Based 2-Periodic Metal–Organic Frameworks

Two Ni-based and two Co-based mixed-ligand metal–organic frameworks (MOFs), which are all isoreticular with each other, are shown to have a large variation in nitrogen, carbon dioxide, hydrogen, and water vapor sorption properties, despite their similar structures. The MOFs have 1,2-bis(4-pyridyl)ethane (bpe) as the common ligand with the second ligand being either 2-nitroterephthalate (2-nta) or 2-bromoterephthalate (2-bta) for the Ni-based MOFs or 2-nta or 2-aminoterephthalate (2-ata) for the Co-based MOFs. The four MOFs have the formula [Ni(2-nta)(bpe)(H2O)2]n·2(DMA)n (1), [Ni(2-bta)(bpe)(H2O)2]n·2.8(DMF)n (2), [Co(2-nta)(bpe)(H2O)2]n·2(DMA)n (3), and [Co(2-ata)(bpe)(H2O)2]n·3(DMA)n·(H2O)n (4), respectively, where DMA = N,N′-dimethylacetamide and DMF = N,N′-dimethylformamide. Variable-temperature powder X-ray diffraction (VT-PXRD) studies indicate that the activated MOF structures are different from those of the as-synthesized MOFs, showing that the 2-periodic layers are dynamic upon desolvation. For the same metal ion, the MOF involving the 2-nta ligand has higher sorption, while sorption for the Ni-based MOFs is higher than for the Co-based MOFs. MOF 1 has the highest sorption within the isoreticular series and also has the highest sorption (the highest for water vapor) compared to other MOFs with the same or related ligands.

Solid-phase extraction of personal care products using mixed-ligand metal–organic framework combined with spectrophotometric determination

The adsorption of personal care products (diclofenac sodium and chlorhexidine digluconate) on a mixed-ligand metal–organic framework based on a complex of nickel terephthalate with 2,2'-bipyridine was investigated by solid-phase extraction in combination with spectrophotometric analysis.

Two-Dimensional Mixed-Ligand Metal–Organic Framework Constructed from Bridging Bidentate V-Shaped Ligands

A two-dimensional and bifunctional pillar-layered metal–organic framework (MOF)—with the molecular formula [Zn(cba)(bpdb)]·DMF (2DTMU-1), H2cba = 4,4′-methylenedibenzoic acid, bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene—was obtained via the reaction of zinc(II) nitrate with H2cba as the carboxylate linker and bpdb as the N-donor pillar. 2DTMU-1 is based on a binuclear paddlewheel Zn(II) unit complexed by four bridging bidentate (dicarboxylate) V-shaped ligands, which combine to from H2cba; this tetragonal array, which is connected by bpdb with a bridging azine group, presents a pore size of 18 × 12 Å2.

Structure and Properties of a Mixed-Ligand Co-MOF That Was Synthesized in Situ from a Single Imidazole–Pyridyl–Tetrazole Trifunctional Ligand

A mixed-ligand Co-MOF (SCNU-Z7) based on a new imidazole–pyridyl–tetrazole trifunctional ligand, 3-(imidazol-1-yl)-5-(tetrazol-5-yl) pyridine (HITP), was constructed using a solvothermal reaction. Single-crystal X-ray diffraction analysis revealed that in addition to HITP, the 5,5′-bis-(2H-tetrazol-5-yl)-[3,3′]bipyridinyl (H2BTBP) ligand obtained by the in situ reaction of HITP is also incorporated into the framework, thus generating a mixed-ligand metal–organic framework (MOF). SCNU-Z7 has a 3D porous framework with 1D channels along the c axis based on tetranuclear secondary building blocks and has a new highly (3,4,10)-connected network topology with the symbol (416·620·89)(43)2(44·62). SCNU-Z7 exhibits high stability in organic solvents but instability in water. Gas adsorption studies indicated that SCNU-Z7 can absorb CO2 more than N2. Magnetic studies indicate the existence of dominant antiferromagnetic interactions between Co(II) atoms. Due to porosity, Co(II) ions, nitrogen-rich conjugated aromatic rings, and uncoordinated nitrogen atoms in its framework, SCNU-Z7 can capture iodine in vapor with a capacity of 2.7 g/g. The strong interactions between the framework and the iodine molecules cause the amorphization of the framework, and approximately 60% of the adsorbed iodine molecules can be stably stored in the framework. This study provides a new possible route for obtaining mixed-ligand MOFs and provides a new example for stably capturing iodine in MOFs via amorphization.

Guest Editorial: Solar Energy Utilization

Guest Editorial: Solar Energy Utilization

Mixed-ligand metal-organic frameworks as an effective photocatalyst for selective oxidation reaction.

By incorporating tetrakis(4-carboxyphenyl)porphyrin and bis(3,5-dicarboxyphenyl)pyridine into one single metal-organic framework (MOF), a multifunctional mixed-ligand Zn-MIX with large pores was obtained. Under visible-light irradiation, Zn-MIX exhibits high photocatalytic activity for the oxidation of amines and sulfides.

A mixed-ligand Co(II) MOF synthesized from a single organic ligand to capture iodine and methyl iodide vapour.

Mixed-ligand metal-organic frameworks (MOFs) are usually synthesized from two or more organic ligands as initial reactants, and MOFs synthesized from one organic ligand precursor through partial in situ reactions remain very limited. Herein, by introducing an imidazole-tetrazole bifunctional ligand, 5-(4-imidazol-1-yl-phenyl)-2H-tetrazole (HIPT), as a single ligand and performing in situ hydrolysis of the tetrazolium group, a mixed-ligand Co(II)-MOF based on HIPT and 4-imidazol-1-yl-benzoic acid (HIBA), [Co2(μ3-O)(IPT)(IBA)]·x solvent (Co-IPT-IBA), was constructed and applied to capture I2 and methyl iodide vapours. Single crystal structural analyses reveal that Co-IPT-IBA exhibits a 3D porous framework with 1D channels based on the relatively few reported ribbon-like rod SBUs. The nitrogen adsorption-desorption isotherms indicate that the BET surface area of Co-IPT-IBA is 168.5 m2 g-1 and it possesses both micropores and mesopores. Due to its porosity, nitrogen-rich conjugated aromatic rings, and Co(II) ions, Co-IPT-IBA was applied to capture iodine molecules in vapour and exhibited an adsorption capacity of 2.88 g g-1. By combining the IR, Raman, XPS and grand canonical Monte Carlo (GCMC) simulation results, it was deduced that the tetrazole ring, coordination water molecules, and the redox potential of Co3+/Co2+ facilitate iodine capture. The presence of mesopores was also responsible for the high iodine adsorption capacity. In addition, Co-IPT-IBA showed the ability to capture methyl iodide in vapours with a moderate capacity of 625 mg g-1. The transformation of crystalline Co-IPT-IBA to amorphous MOFs may be due to the methylation reaction. This work represents a relatively rare example of methyl iodide adsorption by MOFs.

Mixed-ligand metal–organic frameworks with coordinatively un-saturated Co(II) and Ni(II) sites for regenerable O2-selective ad-sorption over N2

For air separation, the separation method relying on adsorbents has attracted wide attention as a low energy consumption process, and the development of O2-selective adsorbents is of great significance. Metal-organic frameworks with coordinatively unsaturated transition metal sites have great potential in O2-selective adsorption, but most of them cannot be completely regenerated at ambient temperature or can only be effective at very low temperatures. Herein, we report two mixed-ligand metal–organic framework adsorbents [M(AIP)(BPY)0.5·H2O]n·2nH2O with coordinatively unsaturated cobalt(II) and nickel(II) sites, which can preferentially adsorb O2 versus N2, and their IAST O2/N2 selectivities are both significantly greater than 1 at 25 °C. These materials exhibited excellent stability and had no loss of adsorption capacities even after immersion in water for 7 days. More importantly, O2 adsorption–desorption cycle experiments showed that these adsorbents can be completely regenerated at ambient temperature after adsorbing O2, and the breakthrough experiments further confirmed their dynamic O2/N2 separation potential and regeneration ability. The results of theoretical calculations suggested that the interactions between adsorbents and O2 are stronger than those of N2, and they have relatively obvious differences in the N2 and O2 interaction energies. This work provides inspirations for searching for metal–organic frameworks that can selectively adsorb O2 at ambient temperature.

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