Imidazolate Linkers Research Articles

Coordination Behavior of Bis-Imidazole and Various Carboxylate Ligands towards Zn(II) and Cd(II) Ions: Synthesis, Structure, and Photoluminescence Study

Four coordination polymers (CPs) based on bis-imidazole ligands (1,2-bimb and 1,2-bmimb), namely, {[Zn(1,2-bimb)(2,5-dtpa)] H2O}n (1), {[Cd2(1,2-bimb)2(5-hipa)2] 2H2O} (2), {Zn2(1,2-bimb)(L)(CH3COO) DMF·2H2O}n (3) and {Cd(1,2-bmimb)(3-npa)}n (4), have been synthesized by solvothermal reactions (1,2-bimb = 1,2-bis((1H-imidazol-1-yl)methyl)benzene, 1,2-bmimb = 1,2-bis((2-methyl-1H-imidazol-1-yl)methyl)benzene, 2,5-H2dtpa = 2,5-diaminoterephthalic acid, 5-H2hipa = 5-hydroxyisophthalic acid, H3L= 3,3′,3′′-(2,4,6-trioxo-1,3,5-triazinane-1,3,5-triyl)tripropanoic acid, 3-H2npa = 3-nitrophthalic acid) and structurally verified by single-crystal X-ray diffraction analyses and further characterized by powder X-ray diffraction (PXRD), elemental analyses and infrared spectroscopy (IR). Complex 1 and 2 show a dinuclear 2D layered structure. Complex 4 exhibits a two-dimensional network consisting of [Cd(3-npa)]n and [Cd(1,2-bmimb)]n chains. Both 1,2 and 4 display a 4-connected sql topology sheet, which can be further expanded into a 3D supramolecular network through π···π interaction between layers. Complex 3 features a 3D (3,6)-connected {42·6}·{44·610·8}-3,6T24 topology structure consisting of 2D bilayers. Structural comparison reveals that it is not only the substituents at different positions of ancillary ligands and the primary bis(imidazole) linkers that play crucial roles in the control of the final structures. Besides, the photoluminescence properties of 1–4 have been investigated in the solid state at room temperature.

Solvothermal syntheses, structures, and magnetic properties of three cobalt coordination polymers constructed from naphthalene-1,4-dicarboxylic acid and bis(imidazole) linkers

Three new Co(II) coordination polymers with formulas of {[Co2(L1)(1,4-NDC)2]·3H2O}n (1), [Co3(L2)2(HCOO)2(1,4-NDC)2]n (2) and [Co2(L2)(μ3-OH)(1,4-NDC)1.5]n (3) (1,4-H2NDC = Naphthalene-1,4-dicarboxylic acid, L1 = di(1H-imidazol-1-yl)methane, L2 = 1,4-di(1H-imidazol-1-yl)benzene) were solvothermal synthesized from 1,4-H2NDC with the aid of three different length-controllable auxiliary ligands and fully characterized. Their structures are determined by single-crystal X-ray diffraction, IR spectra, elemental analysis, powder X-ray diffraction and thermogravimetric analysis. Complexes 1 and 3 display 3D framework structures, corresponding to a 6-connected (412·63) net, a 8-connected (424·5·63) net, respectively. However, it is noteworthy that the complex 1 displays a 2-fold interpenetrating framework structure, complex 3 possesses a self-interpenetrating framework structure. Complex 2 displays 2D 4-connected undulating plane net structure. Moreover, magnetic studies indicate antiferromagnetic interactions between the Co(II) ions in the four complexes.

Hierarchical FAU/ZIF-8 Hybrid Materials as Highly Efficient Acid-Base Catalysts for Aldol Condensation.

The composite of hierarchical faujasite nanosheets and zeolitic imidazolate framework-8 (Hie-FAU-ZIF-8) has been successfully prepared via a stepwise deposition of ZIF-8 on modified zeolite surfaces. Compared to the direct deposition of metal organic frameworks (MOFs) on zeolite surfaces, ZIF-8 nanospheres were selectively attached to the external surfaces of the MOF ligand-grafted FAU crystals because of the enhancing interaction between the zeolite and MOF in the composite. In addition, the degree of surface functionalization can be greatly enhanced because of the presence of hierarchical structures. This behavior leads to an increase in the deposited MOF content, improving the hydrophobic properties of the zeolite surfaces. Interestingly, the designed hierarchical composite exhibits outstanding catalytic properties as an acid-base catalyst for the aldol condensation of 5-hydroxymethylfurfural with acetone. Compared to the isolated FAU and ZIF-8, a high yield of the product, 4-[5-(hydroxymethyl)furan-2-yl]but-3-en-2-one (67%), can be observed in the composite because of the synergistic effect between the Na+-stabilized zeolite framework and the imidazolate linkers bearing basic nitrogen functions. This opens up interesting perspectives for the development of new organic and inorganic hybrid materials as heterogeneous acid-base catalysts.

Construction, Structural Diversity and Properties of Five Coordination Polymers Based on 5-Nitroisophthalate and Bis(imidazole) Linkers

Five coordination polymers, namely, [Cd(µ3–5-nip)(µ-obix)]n (1), [Co(µ3–5-nip)(µ-obix)]n (2), [Zn(µ−5-nip)(µ-obix)]n (3 and 4) and [Cd(µ−5-nip)(µ-bisobix)]n (5) (5-nip: 5-nitroisophthalate, obix: 1,2-bis(imidazol-1ylmethyl)benzene, bisobix: 1,2-bis(2-isopropylimidazol-1ylmethyl)benzene) were hydrothermally synthesized and characterized by IR spectroscopy, elemental analysis, single crystal and powder X-ray diffraction and thermal analysis (TG/DTA). X-ray results showed that the complexes displayed structural diversity depending on metal ions and conformations of bis(imidazole) linkers. Complexes 1 and 2 were 1D structures and obix ligand displayed cis-conformation. Complexes 3 and 4 exhibited 2D and 3D structures with same components depending on obix conformation. In complex 5, 3D+3D→3D interpenetrated structure was obtained with dia topology when bisobix having sterically hindered groups on imidazole rings was used. Moreover, thermal, photoluminescence and optical properties of the complexes were also investigated.

Quantitative Predictions of Molecular Diffusion in Binary Mixed-Linker Zeolitic Imidazolate Frameworks Using Molecular Simulations

Experimental studies have shown that adsorbate diffusion in zeolitic imidazolate frameworks (ZIFs) can be tuned by incorporating two different imidazolate linkers in the ZIF crystals. We demonstrat...

Energetic Performances of ZIF-8 Derivatives: Impact of the Substitution (Me, Cl, or Br) on Imidazolate Linker

The energetic performances of three isomorphic ZIF materials, i.e., ZIF-8_CH3 (Basolite® Z1200), ZIF-8_Cl, and ZIF-8_Br of SOD topology are studied with high-pressure intrusion-extrusion experiments using water and aqueous electrolyte solutions (KCl 4 M and LiCl 20 M) as non-wetting liquids. This work represents an important progress in the field of energetic applications, as the “ZIF-8_Cl–LiCl 20 M” system exhibits a spring behavior with a stored energy of 77 J.g-1. To the best of our knowledge, this is the highest value for the stored energy obtained using intrusion-extrusion experiments. Experimental results reveal that the intrusion pressure increases with the addition of electrolytes. The systems evolve from a bumper to a shock-absorber or spring behavior with the decrease in the strength of the interactions between the cation of the salt and the imidazolate linker. This explains the bumper or rather the shock-absorber behavior observed for the “ZIF-8_Br-KCl 4 M” and “ZIF-8_CH3-LiCl 20 M” systems com...

Crystal Growth of ZIF-8, ZIF-67, and Their Mixed-Metal Derivatives.

A facile method to produce zeolitic imidazolate frameworks (ZIF-8, ZIF-67, and solid-solution ZIFs (mixed Co and Zn)) is reported. ZIF crystals are produced via a reaction-diffusion framework (RDF) by diffusing an outer solution at a relatively high concentration of the 2-methyl imidazole linker (HmIm) into an agar gel matrix containing the metal ions (zinc(II) and/or cobalt(II)) at room temperature. Accordingly, a propagating supersaturation wave, initiated at the interface between the outer solution and the gel matrix, leads to a precipitation front with a gradient of crystal sizes ranging between 100 nm and 55 μm along the reaction tube. While the precipitation fronts of ZIF-8 and ZIF-67 travel the same distance for the same initial conditions, ZIF-8 crystals therein are consistently smaller than the ZIF-67 crystals due to the disparity of their rate of nucleation and growth. The effects of the temperature, the concentration of the reagents, and the thickness of the gel matrix on the growth of the ZIF crystals are investigated. We also show that by using RDF we can envisage the formation mechanism of the ZIF crystals, which consists of the aggregation of ZIF nanospheres to form the ZIF-8 dodecahedrons. Moreover, using RDF, the formation of a solid-solution ZIF via the incorporation of Co(II) and Zn(II) cations within the same framework is achieved in a controlled manner. Finally, we demonstrate that doping ZIF-8 by Co(II) enhances the photodegradation of methylene blue dye under visible light irradiationin the absence of hydrogen peroxide.

Substrate-Independent Epitaxial Growth of the Metal-Organic Framework MOF-508a.

Plasmachemical deposition is a substrate-independent method for the conformal surface functionalization of solid substrates. Structurally well-defined pulsed plasma deposited poly(1-allylimidazole) layers provide surface imidazole linker groups for the directed liquid-phase epitaxial (layer-by-layer) growth of metal-organic frameworks (MOFs) at room temperature. For the case of microporous [Zn (benzene-1,4-dicarboxylate)-(4,4'-bipyridine)0.5] (MOF-508), the MOF-508a polymorph containing two interpenetrating crystal lattice frameworks undergoes orientated Volmer-Weber growth and displays CO2 gas capture behavior at atmospheric concentrations in proportion to the number of epitaxially grown MOF-508 layers.

Assembly of a series of coordination polymers built from rigid a tetracarboxylate ligand and flexible bis(imidazole) linker: syntheses, structural diversities, luminescence sensing, and photocatalytic properties

Hydrothermal reactions of aromatic terphenyl-4,2'',5'',4'-tetracarboxylic acid (H4tta) and transitional metal cations in the presence of two flexible N-donor ancillary ligands afforded four novel coordination polymers, namely, {[Zn(tta)0.5(m-bimb)]·H2O}n (1), [Ni2(tta)(m-bimb)2(H2O)]n (2), [Ni(H2tta)(o-bimb)·H2O]n (3), and [Cd(tta)0.5(o-bimb)]n (4), from solvothermal reactions in the presence of bis(imidazole) bridging linkers (m-bimb = 1,3-bis(imidazol-1-ylmethyl)benzene, and o-bimb = 1,2-bis(imidazol-1-ylmethyl)benzene). Their structures were determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses (EA), powder X-ray diffraction (PXRD), and thermogravimetric (TG) analyses. Compound 1 displayed a 2-fold 3D → 3D parallel entangled (4,4)-connected bbf net with the point symbol of (64·82)(66)2. Compound 2 featured a predocumented 3D (4,8)-connected (32·53·6)(34·44·59·611) net. Compound 3 exhibited a 2D 4-connected sql sheet with the point symbol of (44·62), and compound 4 showed a 2D 4-connected (32·62·72) kgm sheet. To our delight, fluorescence measurements showed that compound 1 could selectively and sensitively detect Cr3+ cations and nitrobenzene derivatives (nitrobenzene (NB), p-nitrotoluene (PNT), and p-nitroaniline (PNA)), which suggested that CPs of 1 was a promising bifunctional luminescence sensor material that could sense metal ions and small organic molecules. Moreover, 1 showed excellent photocatalytic activity.

Gas adsorption, magnetic, and fluorescent sensing properties of four coordination polymers based on 1,3,5-tris(4-carbonylphenyloxy)benzene and bis(imidazole) linkers

Four 3D novel coordination polymers (CPs) have been synthesized under solvothermal conditions. Furthermore, gas sorption behaviors revealed that complex 1 has excellent selective separation for CO2/CH4.

Tuning the Swing Effect by Chemical Functionalization of Zeolitic Imidazolate Frameworks.

Many zeolitic imidazolate frameworks (ZIFs) are promising candidates for use in separation technologies. Comprising large cavities interconnected by small windows they can be used, at least in principle, as molecular sieves where molecules smaller than the window size are able to diffuse into the material while larger molecules are rejected. However, "swing effect" or "gate opening" phenomena resulting in an enlargement of the windows have proven to be detrimental. Here, we present the first systematic experimental and computational study of the effect of chemical functionalization of the imidazole linker on the framework dynamics. Using high-pressure (HP) single-crystal X-ray diffraction, density functional theory, and grand canonical Monte Carlo simulations, we show that in the isostructural ZIF-8, ZIF-90, and ZIF-65 functional groups of increasing polarity (-CH3, -CHO, and -NO2) on the imidazole linkers provide control over the degree of rotation and thus the critical window diameter. On application of pressure, the substituted imidazolate rings rotate, resulting in an increase in both pore volume and content. Our results show that the interplay between the guest molecules and the chemical function of the imidazole linker is essential for directing the swing effect in ZIF frameworks and therefore the adsorption performance.

Highly fluorescent selectivity of two Cd(II) coordination networks with active sites for nitroaromatic compounds and Fe3+ and Hg2+

Based on 5,5′-(1,3,6,8-tetraoxobenzo[lmn][3,8] phenanthrolin-2-7-diyl)bis-1,3- benzenedicarboxylic acid (H4BIPA-TC), two novel 2D coordination polymers, formulated as {[Cd(BIPA-TC)0.5(1,4-bib)(H2O)]·(H2O)3.5}n (1) and {[Cd2(BIPA-TC)(4,4′-bibp)(H2O)]·(H2O)}n (2) have been solvothermally synthesized with the help of two bis(imidazole) linkers, 1,4-bib and 4,4′-bibp(1,4-bib = 1,4-bis(benzimidazo-1-ly)benzene and 4,4′-bibp = 4,4′-bis(imidazolyl)biphenyl). Single-crystal X-ray diffraction analysis shows that complex 1 is a (4,4)-connected bilayer network structure with the point symbol of (46·48)2(42·64), which further expand into an interestingly 2D + 2D → 3D interweaved framework. Complex 2 possesses a (4,6)-connected network with two kinds of {Cd2(COO)2} SBUs with the point symbol of (32·42·52)(34·44·54·63), which further pack into a 3D supramolecular architecture through hydrogen bonds. Furthermore, the luminescent recognition properties of 1 and 2 for small organic molecules, nitroaromatic compounds, and metal cations have been explored systematically.

Zapping a ZIF amps up gas separation

Exposing a membrane made of the metal-organic framework ZIF-8 to an external electric field transforms the material from one with a modest ability to separate propene from propane to a good separator of these gases. The finding, from a team led by Leibniz University Hannover scientists Jurgen Caro and Alexander Knebel, could be broadly applicable to different classes of framework materials that are similar to ZIF-8 (Science 2017, DOI: 10.1126/science.aal2456). ZIF-8 is a soft MOF crystal with zinc-coordinated imidazolate linker units that move in a manner described as “breathing.” Applying an external electric field of 500 V/mm to ZIF-8 reduces this motion and causes the material’s lattice to polarize, prompting a transition in its crystal structure. The zapped ZIF-8 has a 0.36-nm pore aperture, which is only slightly larger than the 0.34-nm opening in the unzapped material. However, because the linkers move less when electrified, the MOF’s ability to separate

Bifunctional ZIF-78 heterogeneous catalyst with dual Lewis acidic and basic sites for carbon dioxide fixation via cyclic carbonate synthesis

Zeolitic imidazole framework-78 (ZIF-78) hexagonal prisms, a combination of Zn metal and imidazolate linkers, were successfully synthesized using a facile hydrothermal process. The as-prepared ZIF-78 hexagonal prisms possessed a bifunctional property with dual Lewis acidic (Zn atom) and basic sites (-NH groups of the imidazolate linkers). This bifunctional property allowed the ZIF-78 crystals to be a potential heterogeneous catalyst in the CO2/propylene oxide (PO) cycloaddition reaction for the synthesis of propylene carbonate (PC). To the best of our knowledge, this is the first work to use microporous ZIF-78 crystals as a catalyst for the CO2/PO cycloaddition reaction without the addition of any co-catalyst or solvent. The effects of reaction temperature, pressure and reaction time on the PO conversion, PC selectivity and yield were also investigated in this work. The highest PC yield of 54% was obtained at a reaction temperature of 150°C, pressure of 10kg/cm2 and reaction time of 15h. Moreover, a possible mechanism of the cycloaddition reaction was also proposed in this study.

Aqueous-System-Enabled Spray-Drying Technique for the Synthesis of Hollow Polycrystalline ZIF-8 MOF Particles.

Zeolitic imidazolate framework-8 shares the same topology with sodalite zeolite but consists of Zn nodes bridged by imidazolate linkers to form a neutral open-framework structure. ZIF-8 has been recognized as a unique molecular sieving material with a flexible framework enabling interesting “gate-opening” functionality. Controlling the crystal size and shape is crucial for regulating the structural flexibilities and mass transport properties. The present study demonstrates that an aqueous-system-enabled spray-drying process enables the shape engineering of ZIF-8 with a hollow polycrystalline structure. It is notable that our synthesis route produces an amorphous zinc complex compound, which possesses a continuous random network partially with crystalline fillers, after spray drying followed by an amorphous-to-crystal transition via activation treatment using polar organic solvents. The size of primary ZIF-8 crystals consisting of secondary polycrystals depends on the kind of the organic solvent. The macro-/microscopic structures of hollow polycrystalline ZIF-8 significantly structurally enhanced the adsorption capacity and uptake rate. The large-scale, rapid production and enhanced adsorption performances make this continuous method a very promising candidate for industrial applications and shaping of MOF.

Investigation of zeolitic imidazolate frameworks using 13C and 15N solid-state NMR spectroscopy

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) with extended three-dimensional networks of transition metal nodes (bridged by rigid imidazolate linkers), with potential applications in gas storage and separation, sensing and controlled delivery of drug molecules. Here, we investigate the use of 13C and 15N solid-state NMR spectroscopy to characterise the local structure and disorder in a variety of single- and dual-linker ZIFs. In most cases, a combination of a basic knowledge of chemical shifts typically observed in solution-state NMR spectroscopy and the use of dipolar dephasing NMR experiments to reveal information about quaternary carbon species are combined to enable spectral assignment. Accurate measurement of the anisotropic components of the chemical shift provided additional information to characterise the local environment and the possibility of trying to understand the relationships between NMR parameters and both local and long-range structure. First-principles calculations on some of the simpler, ordered ZIFs were possible, and provided support for the spectral assignments, while comparison of these model systems to more disordered ZIFs aided interpretation of the more complex spectra obtained. It is shown that 13C and 15N NMR are sufficiently sensitive to detect small changes in the local environment, e.g., functionalisation of the linker, crystallographic inequivalence and changes to the framework topology, while the relative proportion of each linker present can be obtained by comparing relative intensities of resonances corresponding to chemically-similar species in cross polarisation experiments with short contact times. Therefore, multinuclear NMR spectroscopy, and in particular the measurement of both isotropic and anisotropic parameters, offers a useful tool for the structural study of ordered and, in particular, disordered ZIFs.

Molecular Mechanism of Swing Effect in Zeolitic Imidazolate Framework ZIF-8: Continuous Deformation upon Adsorption.

Zeolitic imidazolate framework ZIF-8 displays flexibility of its structure by rotation of its imidazolate linker. This "swing effect" has been widely described in the literature, both experimentally and theoretically, as a bistable system where the linker oscillates between two structures: "open window" and "closed window". By using quantum chemistry calculations and first-principles molecular dynamics simulations, it is shown that the deformation upon adsorption is in fact continuous upon pore loading, with thermodynamics of packing effects being the reason behind stepped adsorption isotherms experimentally observed. Finally, we study a variant of ZIF-8 with a different linker, highlighting the influence of the linker and the balance of microscopic interactions on the framework's flexibility.

Lattice-Gas Modeling of Adsorbate Diffusion in Mixed-Linker Zeolitic Imidazolate Frameworks: Effect of Local Imidazolate Ordering.

The rates of adsorbate diffusion in zeolitic imidazolate frameworks (ZIFs) can be varied by several orders of magnitude by incorporating two different imidazolate linkers in the ZIF crystals. Although some prior measurements of short-range order in these mixed-linker materials have been reported, it is unclear how this short-range order impacts the net diffusion of adsorbates. We introduce a lattice diffusion model that treats diffusion in ZIF-8x-90100-x crystals as a series of activated hops between cages, allowing us to assess the effects of short-range imidazolate order on molecular diffusion.

Principles of Designing Extra-Large Pore Openings and Cages in Zeolitic Imidazolate Frameworks.

We report three design principles for obtaining extra-large pore openings and cages in the metal-organic analogues of inorganic zeolites, zeolitic imidazolate frameworks (ZIFs). Accordingly, we prepared a series of 15 ZIFs, members of which have the largest pore opening (22.5 Å) and the largest cage size (45.8 Å) known for all porous tetrahedral structures. The key parameter allowing us to access these exceptional ZIFs is what we define as the steric index (δ), which is related to the size and shape of the imidazolate linkers employed in the synthesis. The three principles are based on using multiple linkers with specific range and ratios of δ to control the size of rings and cages from small to large, and therefore are universally applicable to all existing ZIFs. The ZIF with the largest cage size (ZIF-412) shows the best selectivity of porous materials tested toward removal of octane and p-xylene from humid air.

Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties-most importantly the linker distributions-and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of 1H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8-90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core-shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.