Metal-organic Polyhedra Research Articles

Metal-Organic Polyhedra as Building Blocks for Porous Extended Networks.

Metal–organic polyhedra (MOPs) are a subclass of coordination cages that can adsorb and host species in solution and are permanently porous in solid‐state. These characteristics, together with the recent development of their orthogonal surface chemistry and the assembly of more stable cages, have awakened the latent potential of MOPs to be used as building blocks for the synthesis of extended porous networks. This review article focuses on exploring the key developments that make the extension of MOPs possible, highlighting the most remarkable examples of MOP‐based soft materials and crystalline extended frameworks. Finally, the article ventures to offer future perspectives on the exploitation of MOPs in fields that still remain ripe toward the use of such unorthodox molecular porous platforms.

Photo/Electrochromic Dual Responsive Behavior of a Cage-like Zr(IV)-Viologen Metal-Organic Polyhedron (MOP).

Stable stimulus-responsive materials are highly desirable due to their widespread potential applications and growing demand in recent decades. Despite the fact that viologen derivatives have long been known as excellent photochromic and electrochromic materials, the development of stable viologen-based multifunctional smart materials with short coloration times remains an exciting topic. To obtain photochromic and electrochromic dual responsive materials, embedding the viologen ligand into a robust metal oxide cluster to increase its stability and sensitivity is an effective strategy. Herein, a viologen-based metal-organic polyhedron (MOP) {[Zr6L3(μ3-O)2(μ2-OH)6Cp6]·8Cl·CH3OH·DMF} [Zr-MOP-1; H2L·2Cl = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium dichloride, and Cp = η5-C5H5] was successfully prepared and characterized. It consists of trinuclear Zr-oxygen secondary building units and exhibits reversible photochromic and electrochromic dual responsive behaviors. As expected, the designed robust viologen-based nanocage with a V2E3 (V = vertex, and E = edge) topology can maintain its stability and rapid photo/electrochromic behaviors with an obvious reversible change in color from purple (brown) to green, mainly due to the enclosed cluster structure and the abundant free viologen radicals that originate from the effective Cl → N and O → N electron transfers. Spectroelectrochemistry and theoretical calculations of this Zr-MOP were also performed to verify the chromic mechanism.

Co-assembly of soluble metal–organic polyhedrons for high-flux thin-film nanocomposite membranes

Well-developed pore channels and large effective permeation area are sorely needed in developing highly permeable synthetic membranes for the reduction of energy consumption in separation processes. Herein, water stable, soluble and reactive amino-functionalized metal–organic polyhedron (MOP) ZrT-1-NH2 with intrinsic porosity is co-assembled with lysine (Lys) into a new kind of thin-film nanocomposite (TFN) membranes through in situ interfacial polymerization (IP) with trimesoyl chloride (TMC). Striped structure on the membrane surface is induced by the participation of the amino-MOP in polymerization, which greatly enhances the effective permeation area of the separation layer compared with the membranes feature smooth surface. Meanwhile, the suitable pore aperture of the introduced ZrT-1-NH2 can also benefit the diffusion of water molecules and block the transport of molecules with bulkier molecular volumes. The results showed that the formed membrane exhibits significantly improved water permeance (27.3 L m−2 h−1 bar−1, 480% increase) compared with that of the pristine Lys/TMC membrane (4.7 L m−2 h−1 bar−1). The compositing of ZrT-1-NH2 also leads to an increase in the rejection selectivity of dye and salt, rejection selectivity of dye/salt mixture, as well as in antifouling and antibacterial properties, making the ZrT-1-NH2-Lys/TMC composite membrane a potential choice for dye/salt separation. Moreover, this work demonstrates the bright prospects of applying soluble and reactive porous cages in the development of high-loading defect-less TFN membranes for efficient molecular separations.

Metal-Organic Polyhedron with Four Fe(III) Centers Producing Enhanced T1 Magnetic Resonance Imaging Contrast in Tumors.

A metal-organic polyhedron (MOP) with four paramagnetic Fe(III) centers was studied as a magnetic resonance imaging (MRI) probe. The MOP was characterized in solution by using electron paramagnetic resonance (EPR), UV-visible (UV-vis) spectroscopies, Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, and in the solid state with single-crystal X-ray diffraction. Water proton T1 relaxation properties were examined in solution and showed significant enhancement in the presence of human serum albumin (HSA). The r1 relaxivities in the absence and presence of HSA were 8.7 mM-1 s-1 and 21 mM-1 s-1, respectively, per molecule (2.2 mM-1 s-1 and 5.3 mM-1 s-1 per Fe) at 4.7 T, 37 °C. In vivo studies of the iron MOP show strong contrast enhancement of the blood pool even at a low dose of 0.025 mmol/kg with prolonged residence in vasculature and clearance through the intestinal tract of mice. The MOP binds strongly to serum albumin and shows comparable accumulation in a murine tumor model as compared to a covalently linked Gd-HSA contrast agent.

Synthesis of the two isomers of heteroleptic Rh12L6L′6metal–organic polyhedra by screening of complementary linkers

We have synthesised and characterised the two possible isomers of heteroleptic trigonal antiprismatic M12L6L'6 MOPs by screening reactions of rhodium acetate with different pairs of complementary dicarboxylate linkers. The resulting 12 new MOPs (eight of isomer A + four of isomer B) are microporous in the solid state, exhibiting Brunauer-Emmett-Teller (BET) surface areas as high as 770 m2 g-1.

Permanent cavities in ionic liquids created by metal–organic polyhedra

A new porous liquid system based on the coalition of MOPs and bulky IL was designed. The liquid materials exhibit permanent cavities, long-term durability, and good ability to separate CO2 from N2 and CH4 not only as an entity but also as a membrane.

Surface chemistry of metal-organic polyhedra.

Metal-organic polyhedra (MOPs) are discrete, intrinsically-porous architectures that operate at the molecular regime and, owing to peripheral reactive sites, exhibit rich surface chemistry. Researchers have recently exploited this reactivity through post-synthetic modification (PSM) to generate specialised molecular platforms that may overcome certain limitations of extended porous materials. Indeed, the combination of modular solubility, orthogonal reactive sites, and accessible cavities yields a highly versatile molecular platform for solution to solid-state applications. In this feature article, we discuss representative examples of the PSM chemistry of MOPs, from proof-of-concept studies to practical applications, and highlight future directions for the MOP field.

A self-arranged metal–organic polyhedron/fullerene asymmetric structure improves the performance of inverted perovskite solar cells

A hybrid dirhodium tetracarboxylate-based metal–organic polyhedron (MOP):PCBM electron transport layer is demonstrated to give rise to a 9% enhancement in power conversion efficiency for the derived inverted perovskite solar cell.

SO2 capture and detection using a Cu(II)-metal-organic polyhedron.

The SO2 adsorption-desorption capacity at room temperature and 1 bar of the metal-organic polyhedron MOP-CDC was investigated. In addition, the qualitative solid-state absorption-emission properties of this material (before and after SO2 exposure) were measured and tested, and it demonstrated remarkable capability for SO2 detection. Our results represent the first example of fluorimetric SO2 detection in a MOP.

An Adamantane Capsule and its Efficient Uptake of Spherical Guests up to 3 nm in Water.

Efficient uptake of small to large guests, with a large difference in relative size, is quite rare for synthetic host compounds. Herein we designed and prepared a micellar capsule, composed of bent amphiphiles bearing two adamantyl groups, as a new host with a well-defined nanostructure. Unlike previous covalent, coordination, and hydrogen-bonding hosts, the adamantane-based capsule displays unusual uptake abilities toward spherical molecules with small (∼0.6 nm in diameter; e.g., adamantane) to medium size (∼1 nm; e.g., fullerene) as well as large size (∼3 nm; i.e., metal-organic polyhedra (MOP)), where the size differences are up to 5-fold, in water. Moreover, the resultant MOP-uptaking capsule incorporates medium-sized molecules (e.g., perylene and eosin Y) into the polyhedral cavity to generate ternary core-shell structures.

Amino-functionalized Zr (IV) metal-organic polyhedron as water-stable catalyst for the photocatalytic degradation of tetracycline

In this work, an amino-functionalized Zr-based metal-organic polyhedron (denoted ZrT-1-NH2), synthesised under the solvothermal method, has been selected as the photocatalyst for the removal of tetracycline (TC). The effects of adding dosages, pH values of TC degradation efficiency were investigated, and the results showed that this material was better effective in photocatalysis degradation of tetracycline antibiotic. In addition, the trapping experiments reveal that hydroxyl radical (•OH) and superoxide radical (•O2−), serving as active species, play major roles in the photocatalytic process. Furthermore, a possible mechanism for the photocatalytic process is proposed. This work presents that zirconium-based metal-organic polyhedra (Zr-MOPs) may be promising photocatalysts for the removal of antibiotics in wastewater.

A Copper Iodide Cluster‐Based Metal–Organic Polyhedra for Photocatalytic Click Chemistry

Self‐assembly from versatile inorganic secondary building units (SBUs) is an important strategy for constructing metal–organic compounds with sophisticated structures and desired functionality. Herein, a copper(I) iodide cluster‐based metal–organic polyhedra (MOP) (Cu3I3)4L1 4 (1) (L 1 = 1,3,5‐trispyridylsulfanylethyl)‐2,4,6‐triethylbenzene, 2‐mercaptopyridine) is synthesized. Due to the abnormal geometry of [Cu3I3] SBUs, MOP 1 exhibits a half‐truncated cube topology, which is rarely observed up to date. Compared with commonly used metal ion or clusters, the coordination bond between the [Cu3I3] SBU and the ligand are rather labile in solution evidenced by the theoretical calculations and experimental results. Interestingly, the liable [Cu3I3] SBU endows MOP 1 to photocatalyze the azide–alkyne cycloaddition reaction in aqueous solution with high efficiency, excellent tolerance (>80% isolated yield for 18 examples), and good recyclability. The mechanistic studies reveal a photoinduced electron transfer from the MOP to azide, which is unusual in copper‐catalyzed click reactions.

Clip‐off Chemistry: Synthesis by Programmed Disassembly of Reticular Materials**

AbstractBond breaking is an essential process in chemical transformations and the ability of researchers to strategically dictate which bonds in a given system will be broken translates to greater synthetic control. Here, we report extending the concept of selective bond breaking to reticular materials in a new synthetic approach that we call Clip‐off Chemistry. We show that bond‐breaking in these structures can be controlled at the molecular level; is periodic, quantitative, and selective; is effective in reactions performed in either solid or liquid phases; and can occur in a single‐crystal‐to‐single‐crystal fashion involving the entire bulk precursor sample. We validate Clip‐off Chemistry by synthesizing two topologically distinct 3D metal‐organic frameworks (MOFs) from two reported 3D MOFs, and a metal‐organic macrocycle from metal‐organic polyhedra (MOP). Clip‐off Chemistry opens the door to the programmed disassembly of reticular materials and thus to the design and synthesis of new molecules and materials.

Clip-off Chemistry: Synthesis by Programmed Disassembly of Reticular Materials.

Bond breaking is an essential process in chemical transformations and the ability of researchers to strategically dictate which bonds in a given system will be broken translates to greater synthetic control. Here, we report extending the concept of selective bond breaking to reticular materials in a new synthetic approach that we call Clip‐off Chemistry. We show that bond‐breaking in these structures can be controlled at the molecular level; is periodic, quantitative, and selective; is effective in reactions performed in either solid or liquid phases; and can occur in a single‐crystal‐to‐single‐crystal fashion involving the entire bulk precursor sample. We validate Clip‐off Chemistry by synthesizing two topologically distinct 3D metal‐organic frameworks (MOFs) from two reported 3D MOFs, and a metal‐organic macrocycle from metal‐organic polyhedra (MOP). Clip‐off Chemistry opens the door to the programmed disassembly of reticular materials and thus to the design and synthesis of new molecules and materials.

Exploring the Geometric Space of Metal-Organic Polyhedrons (MOPs) of Metal-Oxo Clusters.

Metal organic polyhedra (MOPs) such as coordination cages and clusters are increasingly utilized across many fields, but their geometrically selective assembly during synthesis is nontrivial. When ligand coordination along these polyhedral edges is arranged in an unsymmetrical mode or the bridging ligand itself is nonsymmetric, a vast combinatorial space of potential isomers exists complicating formation and isolation. Here we describe two generalizable combinatorial methodologies to explore the geometrical space and enumerate the configurational isomers of MOPs with discrimination of the chiral and achiral structures. The methodology has been applied to the case of the octahedron {Bi6Fe13L12} which has unsymmetrical coordination of a carboxylate ligand (L) along its edges. For these polyhedra, the enumeration methodology revealed 186 distinct isomers, including 74 chiral pairs and 38 achiral. To explore the programming of these, we then used a range of ligands to synthesize several configurational isomers. Our analysis demonstrates that ligand halo-substituents influence isomer symmetry and suggests that more symmetric halo-substituted ligands counterintuitively yield lower symmetry isomers. We performed mass spectrometry studies of these {Bi6Fe13L12} clusters to evaluate their stability and aggregation behavior in solution and the gas phase showing that various isomers have different levels of aggregation in solution.

Catalytic Formation of Coordination-Based Self-Assemblies by Halide Impurities.

The dynamics of metal organic polyhedra (MOP) play a crucial role for their application in catalysis and host–guest chemistry and as functional materials. In this contribution, we study the influence of possible contaminations of different metal precursors on the kinetic properties of MOP. Exemplary five different MOP are studied with metal precursors of varying quality. The metal precursors are either obtained from commercial sources or prepared by various literature procedures. Studies into the self-assembly process using 1H NMR and MS analyses were performed on Pt2L4, Pd2L4, Pd6L12, Pd12L24, and Ni4L6 assemblies. Commonly found impurities are shown to play a prominent role guiding selective formation of MOP, as they allow for an escape from otherwise kinetically trapped intermediates. The energy requirement for selective sphere formation is significantly lowered in many examples providing evidence for a catalytic role of halide impurities/additives in the self-assembly process. Furthermore, even though most analytical features such as 1H NMR and MS analyses show identical results for assemblies with different types of metal precursors, the dynamics of formed assemblies differs significantly if slightly less pure starting materials are used. Tiny amounts of halide contaminations make the MOP more dynamic, which can play an important role for substrate diffusion especially if bulky substrates are used. We believe that this study on the influence of impurities (which were shown to be present in some commercial sources) on the kinetic properties of MOP together with procedures of obtaining high purity metal precursors provides important information for future material preparation and provides a better understanding of already known examples.

Promotion of Proton Conductivity by Encapsulation of Metal-Organic Polyhedra in Metal-Organic Frameworks.

A Zr-based metal-organic polyhedron (MOP) was self-assembled in a porous MOF host, DUT-68, successfully to synthesize MOP-1@DUT-68. The MOP guest (MOP-1) has a diameter of about 20 Å, larger than that of the square windows (pore sizes of ∼14 Å) of DUT-68 but smaller than that of the rhombicuboctahedral cage (27.7 Å), which means that the migration and leaching of MOP-1 could be effectively prohibited if MOP-1 is encapsulated in the MOF's cavities. The proton conductivity of MOP-1@DUT-68 is 1.14×10-3 S cm-1 (at 80 °C under 98 % relative humidity), which is three orders of magnitude higher than that of DUT-68. Compared with MOP-1⊂DUT-68, which was synthesized by impregnation, MOP-1@DUT-68 is more prone to form faster proton-conduction pathways and thus provides higher proton conductivity.

Multivariate porous platform based on metal-organic polyhedra with controllable functionality assembly

<h2>Summary</h2> The chemical environment of pores is important for various applications of porous solids. Thanks to reticular chemistry, metal-organic frameworks (MOFs) have become a versatile platform targeting numerous applications through multiple functionalizations. Although multivariate MOFs often display novel properties, identifying and manipulating pore types remain a daunting challenge. Here, we present an isoreticular series of Zr-based metal-organic polyhedra (MOPs) as a porous platform to achieve controllable intrinsic pores. Two multivariate synthetic approaches were demonstrated: a mixed-cage strategy, whereby functionalized cages are mixed, compared with the conventional mixed-linker strategy, which yields a random distribution of functionalities. A remarkable difference in functionality assembly was achieved between the strategies, with complexity increasing from binary to senary systems. More interestingly, distinct photophysical properties were observed between mixed-linker and mixed-cage samples and attributed to radiative decay kinetics. This study highlights the potential of MOPs as a unique multivariate platform with tunable component assembly to study the emerging properties of multivariate porous solids.

Metal-organic polyhedron membranes for molecular separation

Pore constructing in polyamide thin film composite (TFC) membranes through the mixed matrix strategy is of great interest to the production of nanofiltration membranes with high liquid permeance, however still facing challenges in fine control of filler distribution and filler-polymer interfacial microstructure. Herein, ZrT-1-NH2, one kind of water stable metal-organic polyhedron (MOP) molecular cages with intrinsic porosity, was exploited to construct porous MOP membranes through interfacial polymerization assembly for the first time. Continuous and defect-free separating layer can be facilely assembled on polymer ultrafiltration support by crosslinking ZrT-1-NH2 with trimesoyl chloride (TMC). The permanent pores inherited from ZrT-1-NH2 and the network pores originated from the mismatch in molecular volume of ZrT-1-NH2 and TMC offer the MOP membrane great water permeance (82.0 L·m−2·h−1·bar−1), as well as attractive dye rejection (99.7% for congo red) and dye/salt separation performance. Moreover, the ZrT-1-NH2/TMC composite membrane also exhibited good antifouling and antibacterial properties. The MOP membrane can well combine the developed porous structure of thin film nanocomposite membranes with the advantages of polyamide membranes in simplicity and scalability of membrane fabrication, showing good potential for application in practical molecular separations.

Controllable Microporous Framework Isomerism within Continuous Mesoporous Channels: Hierarchically Porous Structure for Capture of Bulky Molecules.

To date, some attempts have been made to synthesize hierarchically porous metal-organic frameworks (HPMOFs), and in most cases, mesopores are formed in microporous frameworks. However, mass transfer and diffusion are still limited in such HPMOFs since micropores connect mesopores and mesopores are noncontinuous. Here, we fabricate a new hierarchical structure through the formation of microporous MOFs within continuous mesoporous channels. Confined space in the as-prepared mesoporous silica-containing template was used to prepare well-dispersed metal precursor of ZnO. The strategy of ligand vapor-induced crystallization was then designed to construct MOFs inside mesoporous channels, in which vapored ligand at elevated temperature diffuses and reacts with metal precursor. Our results indicate that framework isomerism is controllable by adjusting the crystallization conditions. In comparison to their microporous and mesoporous counterparts, the hierarchically porous materials show obviously enhanced adsorption performance on a series of bulky molecules including dye, enzyme, and metal-organic polyhedron.