Secondary Linker Research Articles

Mixed-Linker Zr-Metal-Organic Framework with Improved Lewis Acidic Sites for CO2 Fixation Reaction Catalysis.

Applying the mixed-linker strategy in synthesizing metal-organic frameworks (MOFs) has drawn considerable attention as a heterogeneous catalyst owing to their easy synthesis and different functional ligands in their frameworks. Following this strategy, we have developed a mixed linker Zr(IV)-based MOF, [Zr6O4(OH)4(FUM)n(PZDC-NO2)6-n] (PZDC-NO2 = 4-nitro-3,5-pyrazoledicarboxylic acid, FUM = fumaric acid) denoted as MOF-801(PZDC-NO2) synthesized via this strategy which possess an electron-withdrawing group (-NO2) on secondary linkers. The MOF-801(PZDC-NO2) has been fully characterized via various analyses, such as Fourier transform infrared, powder X-ray diffraction, 13C/1H nuclear magnetic resonance, XPS, TGA, and N2 adsorption/desorption, SEM, EDX, etc. By considering the concurrent existence of acid-base active sites and the synergistic role of these sites, this mixed-linker MOF was used as a catalyst for the cycloaddition reaction of CO2 and epoxides under mild without-solvent conditions. MOF-801(PZDC-NO2) displays significant catalytic performance by producing the highest catalytic conversion of epoxide to cyclic carbonate (93%) with a turnover number of 130.7 in 8 h reaction time and 100 °C temperature under low-pressure CO2 pressure. The mixed-linker Zr-MOF exhibits exceptional stability and reusability, maintaining its structure and functionality after consecutive cycles of utilization. Finally, the reaction mechanism was further investigated by density functional theory calculations. The total energy of the reactants, intermediates, and products involved in the process.

A hybrid linker-MOF fibrous composite for efficient diclofenac removal and self-cleaning

The treatment of emerging contaminants is essential but challenging. Herein, a novel MOF/PAN fibrous composite (TCPP@UiO-66/PAN) was synthesized and used as a self-cleaning membrane for the efficient removal of a typical emerging contaminant, diclofenac sodium (DF). The introduction of the secondary linker tetra(4-carboxyphenyl)porphyrin (TCPP) served to expand the pores and enhance its photocatalytic activity. TCPP@UiO-66/PAN shows higher adsorption capacity and initial adsorption rate than UiO-66/PAN. The maximum adsorption capacity of DF on TCPP@UiO-66/PAN reached 202 mg/g by Langmuir model. The DF adsorption mechanism involves the π-π/anion-π interaction, Lewis acid-base interaction, and hydrogen bonding. Based on the dynamic filtration experiment, TCPP@UiO-66/PAN exhibited a remarkable treatment capacity for DF-contaminated water, measuring 3.75 × 104kg/kg. Given its fibrous membrane structure, TCPP@UiO-66/PAN could be easily separated from water and reused. In-situ photo-regeneration emerged as a highly effective method for regenerating TCPP@UiO-66/PAN after reaching adsorption saturation, with over 95 % DF degradation achieved under sunlight irradiation. The decomposition of DF primarily resulted from the generation of h+and1O2. Additionally, TCPP@UiO-66/PAN exhibited notable antibacterial properties when exposed to visible light.In vitrocytotoxicity assessments further confirmed the safety of these MOF/PAN composites. Combining the advantages of hybrid-linker strategy and fibrous composite, this work provided new insight into constructing MOF-based functional material for wastewater purification.

Stepwise Assembly of Quinary Multivariate Metal-Organic Frameworks via Diversified Linker Exchange and Installation.

Multivariate MOFs (MTV-MOFs) constructed from multiple components with atomistic precision hold the promise for many fascinating developments in both fundamental sciences and applications. Sequential linker installation can be an effective method to introduce different functional linkers into an MOF that contains coordinatively unsaturated metal sites. However, in many cases, these linkers must be installed according to a specific sequence and the complete synthetic flexibility and freedom is yet to be realized. Here, we rationally decreased the size of the primary ligand used in NPF-300, a Zr-MOF with scu topology (NPF = Nebraska Porous Framework), and synthesized its isostructure, NPF-320. NPF-320 exhibits optimized pocket sizes which allow for the post-synthetic installation of three secondary linkers in all six permuted sequences via both linker exchange and installation, forming a final quinary MTV-MOF via single-crystal-to-single-crystal transformation. With the functionalization of the linkers from the quinary MOF system, one will be able to construct MTV-MOFs not only with variable porosity but also with unprecedented complexity and encoded synthetic sequence information. The utility of sequential linker installation was further demonstrated by the construction of a donor-acceptor pair-based energy transfer system.

Etheno-bridged Azaacene Spiro Dimers.

We present the synthesis of two sets of spiro-connected azaacene dimers. Their geometry and electronic coupling are critically determined by a secondary linker, i. e., an etheno- and an ethano-bridge. The core fragment of the etheno-bridged dimer corresponds to a conformationally locked cis-stilbene. Optoelectronic properties, single crystal X-ray structures and stability with respect to oxidation of the conjugated and non-conjugated dimers are reported and compared. The conjugated dimers exhibit smaller optical gaps and bathochromically shifted absorption maxima, but are prone to unexpected oxygen addition, dearomatizing one of the azaacene substituents.

Substrate-Bound Diarylethene-Based Anisotropic Metal-Organic Framework Films as Photoactuators with a Directed Response.

Molecular machines and responsive materials open a plethora of new opportunities in nanotechnology. We present an oriented crystalline array of diarylethene (DAE)-based photoactuators, arranged in a way to yield an anisotropic response. The DAE units are assembled, together with a secondary linker, into a monolithic surface-mounted metal-organic framework (SURMOF) film. By Infrared (IR) and UV/Vis spectroscopy as well as by synchrotron X-ray diffraction, we show that the light-induced extension changes of the molecular DAE linkers multiply to yield mesoscopic and anisotropic length changes. Due to the special architecture and substrate-bonding of the SURMOF, these length changes are transferred to the macroscopic scale, leading to the bending of a cantilever and performing work. This research shows the potential of assembling light-powered molecules into SURMOFs to yield photoactuators with a directed response, presenting a path to advanced actuators.

The Synthesis and Properties of TIPA-Dominated Porous Metal-Organic Frameworks.

Metal-Organic Frameworks (MOFs) as a class of crystalline materials are constructed using metal nodes and organic spacers. Polydentate N-donor ligands play a mainstay-type role in the construction of metal−organic frameworks, especially cationic MOFs. Highly stable cationic MOFs with high porosity and open channels exhibit distinct advantages, they can act as a powerful ion exchange platform for the capture of toxic heavy-metal oxoanions through a Single-Crystal to Single-Crystal (SC-SC) pattern. Porous luminescent MOFs can act as nano-sized containers to encapsulate guest emitters and construct multi-emitter materials for chemical sensing. This feature article reviews the synthesis and application of porous Metal-Organic Frameworks based on tridentate ligand tris (4-(1H-imidazol-1-yl) phenyl) amine (TIPA) and focuses on design strategies for the synthesis of TIPA-dominated Metal-Organic Frameworks with high porosity and stability. The design strategies are integrated into four types: small organic molecule as auxiliaries, inorganic oxyanion as auxiliaries, small organic molecule as secondary linkers, and metal clusters as nodes. The applications of ratiometric sensing, the adsorption of oxyanions contaminants from water, and small molecule gas storage are summarized. We hope to provide experience and inspiration in the design and construction of highly porous MOFs base on polydentate N-donor ligands.

Precise Assembly and Supramolecular Catalysis of Tetragonal- and Trigonal-Elongated Octahedral Coordination Containers

The construction of distorted or irregular coordination polyhedrons with specific shapes and functionalities is highly challenging. Here, we demonstrate a viable strategy for attaining a severely d...

Structure and phase behavior of polymer-linked colloidal gels

Low-density "equilibrium" gels that consist of a percolated, kinetically arrested network of colloidal particles and are resilient to aging can be fabricated by restricting the number of effective bonds that form between the colloids. Valence-restricted patchy particles have long served as one archetypal example of such materials, but equilibrium gels can also be realized through a synthetically simpler and scalable strategy that introduces a secondary linker, such as a small ditopic molecule, to mediate the bonds between the colloids. Here, we consider the case where the ditopic linker molecules are low-molecular-weight polymers and demonstrate using a model colloid-polymer mixture how macroscopic properties such as the phase behavior as well as the microstructure of the gel can be designed through the polymer molecular weight and concentration. The low-density window for equilibrium gel formation is favorably expanded using longer linkers while necessarily increasing the spacing between all colloids. However, we show that blends of linkers with different sizes enable wider variation in microstructure for a given target phase behavior. Our computational study suggests a robust and tunable strategy for the experimental realization of equilibrium colloidal gels.

Synthesis, characterization and catalytic activity of single site, Lewis acidic aluminosilicates

A general synthetic strategy for the preparation of atomically dispersed, well-defined Lewis acidic aluminum sites in silicate matrices is described and illustrated. These amorphous matrices are composed of the spherosilicate cubes (Si8O20) linked together by combinations of aluminum and siloxane groups. The connectivity of the catalytically active aluminum centers to the surrounding silicate matrix can be varied by choice of aluminum precursor. “Extraframework” 3-connected Al⟵Py and Al⟵THF sites are obtained from the corresponding AlCl3·L complexes and 4-connected “framework” sites from Y[AlCl4] (Y = Lutidinium, [Bu4N]+). The aluminum sites in these porous silicates are characterized by gravimetry, IR and multinuclear SSNMR spectroscopies as well as ligand exchange reactions. Their catalytic activities as Lewis acids are investigated for the aminolysis of styrene oxide by aniline. These single site aluminosilicate catalysts exhibit activities that are comparable to or better than analogous, well-known zeolite and amorphous aluminosilicate materials. Their activities are dependent on the connectivity of aluminum to the surrounding silicate matrix as well as the length and flexibility of secondary linkers in the matrix.

Topology-Guided Stepwise Insertion of Three Secondary Linkers in Zirconium Metal-Organic Frameworks.

We report a topology-guided, precise insertion of three distinct secondary linkers into a zirconium-based metal-organic framework, NPF-300. Constructed from a tetratopic linker L and Zr6 cluster, NPP-300 exhibits a unique scu topology and certain flexibility along the crystallographic a axis, and in conjunction with the conformation change of the primary ligand, is able to accommodate the stepwise insertion of three different secondary linkers along the a and c axes. Size-matching and mechanic strain of the resulting framework are two important factors that determine the chemical stability of the inserted linkers. Secondary linker insertion in NPF-300 significantly enables not only its porosity but also potentials to install up to three different functional groups for the construction of multivariate MOFs with homogeneity.

High Antimicrobial Activity of Metal-Organic Framework-Templated Porphyrin Polymer Thin Films.

Development of surface coatings with high antimicrobial activity is urgently required to fight bacteria and other microorganisms on technical and hygiene relevant surfaces. Control over structure and topology of the surface coatings, combined with the ability to include functional molecules within the structure, is crucial for optimizing their performance. Herein, we describe a novel strategy to synthesize structurally well-defined porphyrin polymer thin films via a template approach. In this approach, bisazido-functionalized porphyrin molecules are preorganized within a metal-organic framework (MOF) structure. Afterward, porphyrin units within the MOF are covalently connected via a secondary linker. Removal of the metal ions of the MOF results in water-stable porphyrin polymer thin films that demonstrate high antibacterial activity against pathogens via visible-light-promoted generation of reactive oxygen species. In addition, this approach offers the inherent possibility to incorporate guest molecules within the structures, to functionalize the surface with biomolecules, and to create hierarchically structured materials.

Metallomacrocycle-supported interpenetration networks assembled from binary N-containing ligands

A new semi-rigid ligand of 4,4′-bis(1H-tetrazole)oxydibenzene (H2btzb) was designed and employed to assemble coordination compounds with the help of a secondary linker of 1,4-bis(1-imidazolyl)benzene (bib). Three compounds, including [Zn(btzb)(bib)]n·(H2O)3n (1), [Cd(btzb)(bib)0.5]n (2) and [Cd(btzb)(bib)(H2O)]n·(DMAC)n·(H2O)2n (3) (DMAC = N,N′-dimethylacetamide) with diverse structures, were successfully prepared in the hydro/solvothermal reaction. Single crystal X-ray diffraction analysis demonstrated that compound 1 is composed of two-fold two-dimensional (2D) sheets interpenetrated. 2 displayed a three-dimensional (3D) interpenetrated network. As for the structure of 3, it adopts a 3D network made up of two interpenetrated α-Po nets. Systematical and comparative investigations on these crystal structures of 1–3 disclosed that all compounds share interesting 2-membered metallomacrocycles [M2(btzb2)] (M = Zn and Cd) as a secondary building block. Furthermore, a gas sorption study revealed the porous 3 exhibited a selective adsorption of CO2 over N2 at 273 K.

Two novel pillar-layered Mn(II) coordination networks based on aromatic carboxylic acids with aminodiacetate functionality

By utilizing 2-aminodiacetic terephthalic acid (H4adtp) and N-(4-carboxyphenyl)iminodiacetic acid (H34-cpida), two coordination polymers formulated as [Mn(H2adtp)(H2O)]n (1) and [Mn3(4-cpida)2(H2O)2]n (2) have been synthesized under solvothermal conditions without any secondary linker. 1 is a three-dimensional (3D) supramolecular pillar-layered structure, where double hydrogen bonding two H2adtp ligands act as one pillar. 2 is a genuine 3D pillar-layered framework with phenyl moiety as strut. Aminodiacetate functionalities in 1 and 2 demonstrate unprecedented ability in connecting the metal ions into 2D layers. Magnetism studies indicate that two complexes both exhibit dominant antiferromagnetic behaviors. In addition, the fitting of magnetic susceptibility (χM) of 1 is performed based on the model of the alternating –J1J2– coupling chain and considering molecular-field approximation.

Utilizing mixed-linker zirconium based metal-organic frameworks to enhance the visible light photocatalytic oxidation of alcohol

A series of mixed-linker zirconium-based metal-organic frameworks (Zr-MOFs) have been synthesized in one-pot reactions. The Zr-MOFs, containing 2-amino-1,4-benzenedicarboxylate (NH2-BDC) as the primary linker and 2-X-1,4-bezenedicarboxylate (X-BDC, X=H, F, Cl, Br) as a secondary linker, have been used as visible light photocatalysts. The incorporation of multi-functional groups into the catalysts was characterized by PXRD, STEM, NMR, N2 physisorption, diffuse reflectance FTIR, and diffuse reflectance UV–vis. The effects of different linkers on the photocatalytic property of the Zr-MOFs were evaluated in the oxidation of benzyl alcohol. The photocatalytic oxidation reaction was performed using a 26W helical bulb as the visible light source, and the temperature of the reaction was kept at 80°C. The Zr-MOF containing mixed NH2-BDC and F-BDC linkers gives five times more conversion in the oxidation of benzyl alcohol compared to the Zr-MOF made of mixed NH2-BDC and H-BDC linkers. We only observed partial oxidation product, benzaldehyde, from the photocatalytic oxidation reaction.

Construction of acylhydrazidate-extended metal-organic frameworks.

Under hydrothermal conditions, the reactions of Ba(2+)/Zn(2+), aromatic polycarboxylic acids and N2H4 with or without oxalic acid were carried out, affording four new acylhydrazidate-extended metal-organic frameworks (MOFs) [Ba(pmdh)] (pmdh = pyromellitdihydrazidate) 1, [Ba(sdpth)(H2O)2]·0.5H2O (sdpth = 4,4'-sulfoyldiphthalhydrazidate) 2, [Ba2(cpth)2(H2O)2] (cpth = 4-carboxylphthalhydrazidate) 3 and [Zn2(pdh)2(ox)]·H2O (ox = oxalate, pdh = pyridine-2,3-dicarboxylhydrazidate) 4. The acylhydrazidate molecules pmdh, sdpth, cpth and pdh in compounds 1-4 derived from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. X-ray single-crystal diffraction analysis revealed that (i) in compound 1, the pmdh I molecules link the Ba(2+) ions into a two-dimensional (2D) layer with a (4,4) topology, and then the pmdh II molecules extend these layers into a three-dimensional (3D) network; (ii) in compound 2, the sdpth molecules link the Ba(2+) ions to form a one-dimensional (1D) square tube. Interestingly, the tubes are further linked into a 3D supramolecular network via the N-H···O interactions, creating synchronously big channels; (iii) in compound 3, the cpth I molecules link the Ba1 ions into a 3D network with a (10,3) topology. Ba2 and cpth II are distributed on the channels; (iv) in compound 4, Zn(2+) and pdh aggregate to form two types of Zn4(pdh)4 clusters. The ox molecules act as the secondary linkers, extending the Zn4(pdh)4 secondary building units (SBUs) into a 3D network with a 6(6) topology. The photoluminescence analysis indicates that compounds 3 and 4 emit green light with maxima at 495 nm for 3 (λ(ex) = 397 nm), and 522 nm for 4 (λ(ex) = 395 nm), respectively. At 77 K, the activated 2 and 4 can adsorb N2 in amounts of 58.31 cm(3) g(-1) for 2 and 38.38 cm(3) g(-1) for 4, respectively.

Mechanistic Aspects for the Formation of Copper Dimer Bridged by Phosphonic Acid and Extending Its Dimensionality by Organic and Inorganic Linkers: Synthesis, Structural Characterization, Magnetic Properties, and Theoretical Studies

Six new copper metal complexes with formulas [Cu(H2O)(2,2′-bpy)(H2L)]2·H4L·4H2O (1), [{Cu(H2O)(2,2′-bpy)(H3L)}2(H2L)]·2H2O (2), [Cu(H2O)(1,10-phen)(H2L)]2·6H2O (3), [Cu(2,2′-bpy)(H2L)]n·nH2O (4), [Cu(1,10-phen)(H2L)]n·3nH2O (5), and [{Cu(2,2′-bpy)(MoO3)}2(L)]n·2nH2O (6) have been synthesized starting from p-xylylenediphosphonic acid (H4L) and 2,2′-bipyridine (2,2′-bpy) or 1,10-phenanthroline (1,10-phen) as secondary linkers and characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) analysis. All the complexes were synthesized by hydrothermal methods. A dinuclear motif (Cu-dimer) bridged by phosphonic acid represents a new class of simple building unit (SBU) in the construction of coordination architectures in metal phosphonate chemistry. The initial pH of the reaction mixture induced by the secondary linker plays an important role in the formation of the molecular phosphonates 1, 2, and 3. Temperature dependent hydrothermal synthesis of the compounds 1, 2, and 3 reveals the mechanism of the self-assembly of the compounds based on the solubility of the phosphonic acid H4L. Two-dimensional coordination polymers 4, 5, and 6, which are formed by increasing the pH of the reaction mixture, comprise Cu-dimers as nodes, organic (H2L) and inorganic (Mo4O12) ligands as linkers. The void space-areas, created by the (4,4) connected nets in compounds 4 and 5, are occupied by lattice water molecules. Thus compounds 4 and 5 have the potential to accommodate guest species/molecules. Variable temperature magnetic studies of the compounds 3, 4, 5, and 6 reveal the antiferromagnetic interactions between the two Cu(II) ions in the eight-membered ring, observed in their crystal structures. A density functional theory (DFT) calculation correlates the conformation of the Cu-dimer ring with the magnitude of the exchange parameter based on the torsion angle of the conformation.

Directed Immobilisation of Modificated Galactitol-Dehydrogenase on Gold Electrodes for Electrochemical Cofactor Regeneration

A supplemental cysteine-tag modification of the enzyme galactitol dehydrogenase (GatDH), expressed in Escherichia coli has enabled a directed and covalent immobilisation of the protein on gold surfaces without secondary linkers. The successful enzyme immobilisation was verified by SPR measurements, and the activity of the immobilized GatDH was demonstrated by cyclic voltammetry (CV) while NAD+ and the redox mediator 4-carboxy-2,5,7-trinitrofluorenyliden-malonnitrile (CTFM) were both kept in solution. The results represent a proof of concept for the development of electrochemical reactors for the generation of enantiopure fine chemicals as building blocks for pharmaceutical products.

CGI-58 Is an α/β-Hydrolase within Lipid Transporting Lamellar Granules of Differentiated Keratinocytes

CGI-58 is the causative molecule underlying Dorfman-Chanarin syndrome, a neutral lipid storage disease exhibiting apparent clinical features of ichthyosis. CGI-58, associated with triacylglycerol hydrolysis, has an alpha/beta-hydrolase fold and is also known as the alpha/beta-hydrolase domain-containing protein 5. The purpose of this study was to elucidate the function of CGI-58 and the pathogenic mechanisms of ichthyosis in Dorfman-Chanarin syndrome. Using an anti-CGI-58 antibody, we found CGI-58 to be expressed in the upper epidermis, predominantly in the granular layer cells, as well as in neurons and hepatocytes. Immunoelectron microscopy revealed that CGI-58 was also localized to the lamellar granules (LGs), which are lipid transport and secretion granules found in keratinocytes. CGI-58 expression was markedly reduced in the epidermis of patients with harlequin ichthyosis, demonstrating defective LG formation. In cultured keratinocytes, CGI-58 expression was mildly up-regulated under high Ca(2+) conditions and markedly up-regulated in three-dimensional, organotypic cultures. In the developing human epidermis, CGI-58 immunostaining was observed at an estimated gestational age of 49 days, and CGI-58 mRNA expression was up-regulated concomitantly with both epidermal stratification and keratinocyte differentiation. CGI-58 knockdown reduced expression of keratinocyte differentiation/keratinization markers in cultured human keratinocytes. Our results indicate that CGI-58 is expressed and packaged into LGs during keratinization and likely plays crucial role(s) in keratinocyte differentiation and LG lipid metabolism, contributing to skin lipid barrier formation.