Introduction of functionality, selection of topology, and enhancement of gas adsorption in multivariate metal-organic framework-177

Understanding Gas Storage in Cuboctahedral Porous Coordination Cages

Porous molecular solids are promising materials for gas storage and gas separation applications. However, given the relative dearth of structural information concerning these materials, additional studies are vital for further understanding their properties and developing design parameters for their optimization. Here, we examine a series of isostructural cuboctahedral, paddlewheel-based coordination cages, M₂₄(^tBu-bdc)₂₄ (M = Cr, Mo, Ru; ^tBu-bdc^2- = 5-tert-butylisophthalate), for high-pressure methane storage. As the decrease in crystallinity upon activation of these porous molecular materials precludes diffraction studies, we turn to a related class of pillared coordination cage-based metal-organic frameworks, M₂₄(Me-bdc)₂₄(dabco)₆ (M = Fe, Co; Me-bdc^2- = 5-methylisophthalate; dabco = 1,4-diazabicyclo[2.2.2]octane) for neutron diffraction studies. The five porous materials display BET surface areas from 1057-1937 m²/g and total methane uptake capacities of up to 143 cm³(STP)/cm³. Both the porous cages and cage-based frameworks display methane adsorption enthalpies of -15 to -22 kJ/mol. Also supported by molecular modeling, neutron diffraction studies indicate that the triangular windows of the cage are favorable methane adsorption sites with CD₄-arene interactions between 3.7 and 4.1 Å. At both low and high loadings, two additional methane adsorption sites on the exterior surface of the cage are apparent for a total of 56 adsorption sites per cage. These results show that M₂₄L₂₄ cages are competent gas storage materials and further adsorption sites may be optimized by judicious ligand functionalization to control extracage pore space.

Unraveling the Arrangement of Al and Fe within the Framework Explains the Magnetism of Mixed-Metal MIL-100(Al,Fe)

Properties of mixed-metal MOFs depend on the distribution of different metals within their frameworks. Determination of this distribution is often very challenging. Using an example of aluminum- and iron-containing MIL-100, we demonstrate that ²⁷Al NMR spectroscopy, when combined with first-principles calculations and magnetic, X-band electron paramagnetic resonance, Fe K-edge extended X-ray absorption fine structure, and Mössbauer measurements, enables one to accurately determine the arrangement of Al and Fe within the metal trimers, which are the basic building units of MIL-100. In this particular material, the incorporation of Fe and Al on the framework metal sites is random. Crucial for deciphering the arrangement is detecting NMR signals, shifted because of the strong hyperfine interaction between the ²⁷Al nuclei and the unpaired electronic spins of Fe³⁺ ions, assigning the shifted signals aided by first-principles calculations of hyperfine couplings, and quantitatively evaluating the NMR intensities and the measured effective magnetic moment.

Metal Organic Frameworks (MOFs) and ultrasound: A review

Metal-organic frameworks (MOFs) have received a lot of attention due to their unique properties and abundant functionalities. Permanent porosity and high surface area are just a few traits that have made them attractive to researchers. They can be prepared as task-specific materials by exploiting the functional group variety and tuning their size and geometry. The main purpose of this review is to present an alternative method of preparing MOF crystals and underline the advantages of ultrasound assisted (sonochemical) synthesis. State of the art ultrasound assisted techniques for the preparation of MOFs in nanoscale are presented. Optimization of morphology and particle size is highlighted throughout this work, as we discuss the effects of various factors, such as energy input, reagent concentration, adequate solvents, reaction time and more.

Topology and porosity control of metal-organic frameworks through linker functionalization

Tetratopic organic linkers have been extensively used in Zr-based metal-organic frameworks (MOFs) where diverse topologies have been observed. Achieving meticulous control over the topologies to tune the pore sizes and shapes of the resulting materials, however, remains a great challenge. Herein, by introducing substituents to the backbone of tetratopic linkers to affect the linker conformation, phase-pure Zr-MOFs with different topologies and porosity were successfully obtained under the same synthetic conditions. The conversion of CO2 to valuable cyclic carbonates is a promising route for the mitigation of the greenhouse gas. Owing to the presence of substrate accessible Lewis acidic Zr(iv) sites in the 8-connected Zr6 nodes, the Zr-MOFs in this study have been investigated as heterogenous acid catalysts for CO2 cycloaddition to styrene oxide. The MOFs exhibited drastically different catalytic activities depending on their distinct pore structures. Compared to previously reported MOF materials, a superior catalytic activity was observed with the mesoporous NU-1008, giving an almost 100% conversion under mild conditions.

Fluorine-Phenanthroimidazole Porous Organic Polymer: Efficient Microwave Synthesis and Photocatalytic Activity

A porous polymer containing a fluorophenylphenanthroimidazole core was easily prepared via one-pot Suzuki-Miyaura cross-coupling reactions under microwave heating. These new metal-free polymers have demonstrated heterogeneous photocatalytic activity toward aza-Henry reaction with reasonable recyclability. Their preparation require a minimal workup to build porous networks with control over the apparent surface area and pore volume from suitable molecular building blocks containing 2-(1 H-phenanthro[9,10- d]imidazol-2-yl)-3,5-difluorophenol (PhIm-2F), as rigid and multitopic node, which afforded a conjugated porous polymer (CPP-PhIm-2F). A series of fluorinated ligands have shown their capability in the preparation of soluble and supported cationic Ru(bpy)₂(F-phenanthroimidazole) complexes by reaction with Ru(bpy)₂Cl₂ and demonstrating a beneficial effect of two fluorine atoms on the photocatalytic effect.

Amino modified metal-organic frameworks as pH-responsive nanoplatforms for safe delivery of camptothecin

MIL-100(Fe) and MIL-101(Fe) metal-organic frameworks (MOFs) are excellent vehicles for drug delivery systems (DDSs) due to their high biocompatibility and stability in physiological fluids, as well as their pore diameter in the mesoporous range. Although they are appropriate for the internal diffusion of 20-(S)-camptothecin (CPT), a strongly cytotoxic molecule with excellent antitumor activity, no stable delivery system has been proposed so far for this drug based in MOFs. We here present novel DDSs based in amine functionalized MIL-100(Fe) and MIL-101(Fe) nanoMOFs with covalently bonded CPT. These CPT nanoplatforms are able to incorporate almost 20% of this molecule and show high stability at physiological pH, with no non-specific release. Based on their surface charge, some of these CPT loaded nanoMOFs present improved cell internalization in in vitro experiments. Moreover, a strong response to acid pH is observed, with up to four fold drug discharge at pH 5, which boost intracellular release by endosomolytic activity. These novel DDSs will help to achieve safe delivery of the very cytotoxic CPT, allowing to reduce the therapeutic dose and minimizing drug secondary effects.

Synthesis and Characterization of Zn-Carboxylate Metal–Organic Frameworks Containing Triazatruxene Ligands

Reactions between triazatruxene-based tricarboxylate ligands, H3tat-R, and zinc nitrate under solvothermal conditions afforded new metal–organic frameworks (MOFs) with the general formula [Zn3(tat-R)2(H2O)2], MUF-tat-R (R=a hydrocarbon substituent on the triazatruxene nitrogen atoms). Single-crystal X-ray diffraction analysis revealed that these frameworks are 3D networks with a (10,3)-a topology. Linear trinuclear zinc clusters are connected to tat ligands to form chiral channels that accommodate the substituents on the tat ligands. MUF-tat and MUF-tat-benzyl crystallize in a cubic crystal system whereas MUF-tat-butyl and MUF-tat-hexyl are tetragonal. MUF-tat-benzyl retains its porosity on activation, which was confirmed by gas adsorption studies.

Rational design, crystal structures and sensing properties of a series of luminescent MOFs based on a flexible tetracarboxylate ligand and N-donor ligands

Six Zn/Cd MOFs were synthesized and characterized. 1–3 are 1D ring chains. 4–5 show 2D bilayer network. 6 possesses a 3D network structure. In particular, 2 demonstrates highly selective and sensitive luminescent sensor towards acetone and Fe³⁺ ion.

Cd(ii) coordination polymers constructed from bis(pyridyl) ligands with an asymmetric spacer in chelating mode and diverse organic dicarboxylates: syntheses, structural evolutions and properties

Thirteen Cd(ii) complexes with diverse chain, (4,4), (6,3), (6³), (6⁵·8), 2-periodic (6,3) layers, and diverse 3D vma, irl, cds, eca nets have been synthesized.

Microporous Metal-Organic Framework with Dual Functionalities for Efficient Separation of Acetylene from Light Hydrocarbon Mixtures

Separating acetylene from light hydrocarbon mixtures like ethylene is a very important process for downstream industrial applications. Herein, we report a new MOF [CuL2(SiF6)] (UTSA-220, L = (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine) with dual functionalities featuring optimal pore size with strong binding sites for acetylene. UTSA-220 exhibits apparently higher uptake capacity for C2H2 than those for other light hydrocarbons. The potential of this material for trace C2H2 removal from C2H4 has also been demonstrated by a dynamic breakthrough experiment performed with C2H2/C2H4 (1/99 v/v) under simulated industrial conditions. According to the dispersion-corrected density functional theory (DFT-D) simulation, SiF6 2- and azine moieties serve as preferential binding sites for C2H2, indicating the feasibility of the dual functionalities incorporated in UTSA-220 for adsorbent-based C2H2 separations.

The chemistry of multi-component and hierarchical framework compounds

This review is expected to provide a library of multi-component hierarchically porous compounds, which shall guide the state-of-the-art design of future porous materials with unprecedented tunability, synergism and precision.

Impact of the functionalization onto structure transformation and gas adsorption of MIL-68(In)

A series of functionalization -NH2, -Br and -NO2 has been performed on MIL-68(In) material in order to improve the porosity features of the pristine material. The functional groups grafted onto the ligand and the molar ratios of the ingredient indicate a profound influence on product formation. With the incremental amount of metal source, product structures undergo the transformation from MIL-68 to MIL-53 or QMOF-2. The situation is different depending on the variation of the ligands. Gas (N2, Ar, H2 and CO2) adsorption-desorption isotherms were systematically investigated to explore the impact of the functionalization on the porous prototypical framework. Comparison of adsorption behaviour of N2 and Ar indicates that the polar molecule exhibits striking interaction to N2 molecule, which has a considerable quadrupole moment. Therefore, as a probe molecule, Ar with no quadrupole moment is more suitable to characterize the surface area with the polar groups. Meanwhile, Ar adsorption result confirms that the negative influence on the surface area stems from the size of the substituting groups. The uptake of H2 and CO2 indicates that the introduction of appropriate polar organic groups can effectively enhance the adsorption enthalpy of relative gases and improve the gas adsorption capacity apparently at low pressure. The introduction of -NO2 is in favour of improving the H2 adsorption capacity, while the grafted -NH2 groups can most effectively enhance the CO2 adsorption capacity.

Enabling Homochirality and Hydrothermal Stability in Zn4O-Based Porous Crystals

The [Zn₄O]⁶⁺ cluster is well-known to form the archetypal MOF-5 topology with dicarboxylate ligands. Here we report two new materials (CPM-300 and -301) that show dramatic alteration of topological and chemical behaviors of [Zn₄O]⁶⁺ clusters. In CPM-300, [Zn₄O]⁶⁺ untypically forms the MIL-88/MOF-235 type framework with a small pentane-ring-based chiral dicarboxylate. In contrast, in CPM-301, when mediated by [Zn₉(btz)₁₂]⁶⁺ clusters (btz = benzotriazolate), the MOF-5 topology is regenerated with the same chiral ligand, albeit with alternating [Zn₄O]⁶⁺ and [Zn₉(btz)₁₂]⁶⁺ clusters. Importantly, both CPM-300 and CPM-301 are homochiral, hydrothermally stable in boiling water and alcohol, and thermally stable to 440 °C or higher. It is concluded that small methyl groups on the chiral ligand is sufficiently powerful to shield [Zn₄O]⁶⁺ clusters from degradation by water, even at high temperatures. These results reveal a promising platform for the development of a new class of cluster-based homochiral and hydrothermally stable porous materials.

Metallic Nanoparticles Assimilation within Metal-Organic Framework Monolith

A facile and versatile method is reported for the inclusion of gold nanoparticles (AuNPs) within a monolithic metal-organic framework (HKUST-1 MOF). Simple addition of stabilized colloidal AuNPs solution to the reaction mixture at the early stages of the formation of the MOF monolith resulted in quantitative uptake of the AuNPs within the MOF matrix. Several characterization techniques including solution and solid UV-vis spectroscopy, TEM, and XRD indicated the successful immobilization of the AuNPs. Controllable loading of AuNPs was also demonstrated, where gas sorption measurements indicated the maintained microporosity of the AuNPs-containing monoliths. This methodology has wide potential applications in demanding technologies, including sensing and catalysis, where monolithic materials of controllable physicochemical properties can be readily accessible through pore size and guest selectivity of the host MOF matrix controlling access of guest molecules to immobilized AuNPs.

Open-tubular capillary electrochromatographic determination of ten sulfonamides in tap water and milk by a metal-organic framework-coated capillary column

In this study, a metal-organic framework (MOF), [Mn(cam)(bpy)], was synthesized and characterized by thermogravimetric analysis, scanning electron microscopy, and Fourier transform infrared spectrometry. An open-tubular capillary column was fabricated from [Mn(cam)(bpy)] via the amide coupling method. Ten types of sulfonamides were separated through the fabricated capillary column, which showed a good limits of detection (<0.07 μg/mL) and linear ranges (1-100 or 5-100 μg/mL) with a high correlation coefficients (R² > 0.9987). The intra-day, inter-day and column-to-column relative standard deviations (RSDs) in the migration times ranged from 0.44 to 4.87%, and the peak area RSDs ranged from 0.80 to 7.28%. The developed capillary electrochromatography method can be successfully utilized for the determination of sulfonamides in tap water and milk samples.

Amine-functionalized Zn(ii) MOF as an efficient multifunctional catalyst for CO2 utilization and sulfoxidation reaction

Herein, a zinc(ii)-based 3D mixed ligand metal organic framework (MOF) was synthesized via versatile routes including green mechanochemical synthesis. The MOF {[Zn(ATA)(L)·H2O]}n (ZnMOF-1-NH2) has been characterized by various physico-chemical techniques, including SCXRD, and composed of the bipyridyl-based Schiff base (E)-N'-(pyridin-4-ylmethylene)isonicotinohydrazide (L) and 2-aminoterephthalic acid (H2ATA) ligands as linkers. The MOF material has been explored as a multifunctional heterogeneous catalyst for the cycloaddition of alkyl and aryl epoxides with CO2 and sulfoxidation reactions of aryl sulfides. The influence of various reaction parameters is examined to optimize the performance of the catalytic reactions. It is found that solvent-free catalytic reaction conditions offer good catalytic conversion in the case of cyclic carbonates, and for sulfoxide, good conversion and selectivity are achieved in the presence of DCM as a solvent medium under ambient reaction conditions. The chemical and thermal stability of the catalyst are excellent and it is active for up to four catalytic cycles without significant loss in activity. Furthermore, based on the catalytic activity and structural evidence, a plausible mechanism for both catalytic reactions is proposed.

Liquid phase blending of metal-organic frameworks

The liquid and glass states of metal-organic frameworks (MOFs) have recently become of interest due to the potential for liquid-phase separations and ion transport, alongside the fundamental nature of the latter as a new, fourth category of melt-quenched glass. Here we show that the MOF liquid state can be blended with another MOF component, resulting in a domain structured MOF glass with a single, tailorable glass transition. Intra-domain connectivity and short range order is confirmed by nuclear magnetic resonance spectroscopy and pair distribution function measurements. The interfacial binding between MOF domains in the glass state is evidenced by electron tomography, and the relationship between domain size and T_g investigated. Nanoindentation experiments are also performed to place this new class of MOF materials into context with organic blends and inorganic alloys.

Metal(II) Coordination Polymers Derived from Mixed 4-Imidazole Ligands and Carboxylates: Syntheses, Topological Structures, and Properties

Four new metal⁻organic coordination polymers [Cu(L)(mpa)]·3H₂O (1), [Co(L)(mpa)]·H₂O (2), [Zn(L)(mpa)]·H₂O (3), and [Cd(L)(mpa)(H₂O)]·H₂O (4) were synthesized by reactions of the corresponding metal(II) salts based on mixed ligands of 1,4-di(1H-imidazol-4-yl)benzene (L) and 4-methylphthalic acid (H₂mpa), respectively. The structures of the complexes were characterized by elemental analysis, FT-IR spectroscopy, and single-crystal X-ray diffraction. Compound 1 exhibits a binodal 4-connected three dimensional (3D) architecture with (6⁵·8)-CdSO₄ topology, while complexes 2 and 3 are isostructural and have two-dimensional (2D) layer structure with (4, 4) sql topology based on the binuclear metal subunits. Complex 4 has the same 2D layer structure with (4, 4) sql topology as complexes 2 and 3, but the inclined interpenetration of parallel sets of layers result in the formation with 2D + 2D → 3D framework. The activated sample 1 shows selective CO₂ uptake over N₂. The photoluminiscent properties together with quantum yield (QY) and luminescence lifetime are also investigated for complexes 3 and 4 in the solid state at room temperature.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Fluorescent sensing and selective adsorption properties of metal-organic frameworks with mixed tricarboxylate and 1H-imidazol-4-yl-containing ligands

Herein, two metal-organic frameworks (MOFs), [Co₄(μ₃-OH)₂(L)(BTB)₂(H₂O)₃]·5.6H₂O (1) and [Cd₃(L)₂(BTB)₂(μ₂-H₂O)]·7.4H₂O (2), based on 1,3-di(1H-imidazol-4-yl)benzene (L) and 1,3,5-tri(4-carboxyphenyl)benzene (H₃BTB), respectively, have been achieved. Compound 1 is a porous three-dimensional (3D) framework with butterfly-like tetranuclear clusters as 7-connected nodes, and compund 2 is a 3D net with a different topology. Remarkably, compounds 1 and 2 exhibit selective adsorption of CO₂ over N₂ and methyl orange (MO) dye molecules. Magnetic measurements reveal that there are antiferromagnetic interactions within the tetranuclear cluster in 1. Furthermore, 2 was well-dispersed in different solvents, and their luminescent properties were investigated, and the results indicated that 2 could be considered as a potential luminescent probe for the detection of ketone molecules.

High proton conduction behavior in 12-connected 3D porous lanthanide–organic frameworks and their polymer composites

Two novel 12-connected 3D porous lanthanide–organic frameworks have been synthesized by the reaction of bipyridine-carboxylate ligand bpydbH₂ and lanthanide metal ions. Both of them show a high proton conduction behavior as well as their composite membranes.

Carbon flakes based metal organic frameworks for H2, CH4 and CO2 gas storage: a GCMC simulation study

In this study, new metal organic frameworks have been designed by modifying the linker of IRMOF-1 with different carbon flakes (circular, rectangular and rhombus).

Three Cd(ii) coordination polymers constructed from a series of multidentate ligands derived from cyclotriphosphazene: synthesis, structures and luminescence properties

By utilizing three ligands derived from cyclotriphosphazene, three novel cadmium-based coordination polymers have been successfully synthesized and structurally characterized, which exhibit the corresponding photo-chemical properties.

Five transition metal coordination polymers driven by a semirigid trifunctional nicotinic acid ligand: selective adsorption and magnetic properties

Five coordination polymers have been synthesized by a new organic linker containing three distinct types of functional groups together with the mixed 2,2′-bipy or 4,4′-bipy co-ligand, revealing various framework structures and selective gas adsorption and magnetic properties.