Cuboctahedral Cages Research Articles

A Porous Zirconium-Based Metal-Organic Framework with the Potential for the Separation of Butene Isomers.

By using a novel C3 -symmetrical tricarboxylate (4,4',4''-benzene-1,3,5-triyl-1,1',1''-trinaphthoic acid), a novel zirconium-based metal-organic framework ZJNU-30 was solvothermally synthesized and structurally characterized. Single-crystal X-ray structural analyses show that ZJNU-30 consists of Zr6 -based nodes connected by the organic linkers to form a (3,8)-connected network featuring the coexistence of two different polyhedral cages: octahedral and cuboctahedral cages with the dimensions of about 14 and 22 Å, respectively. Remarkably, ZJNU-30 is very stable when exposed to air for one month. More importantly, with a moderately high surface area, hierarchical pore structures, and an aromatic-rich pore surface in the framework, ZJNU-30, after activation, exhibits a promising potential for the selective adsorptive separation of industrially important butene isomers consisting of cis-2-butene, trans-2-butene, 1-butene, and iso-butene at ambient temperature. This separation was established exclusively by gas adsorption isotherms and simulated breakthrough experiments. To the best of our knowledge, this is the first study investigating porous metal-organic frameworks for butene-isomer separation.

Accurate H2 Sorption Modeling in the rht-MOF NOTT-112 Using Explicit Polarization

Hydrogen sorption was characterized theoretically in NOTT-112, an rht-metal–organic framework (MOF) that consists of Cu2+ ions coordinated to 1,3,5-tris(3′,5′-dicarboxy[1,1′-biphenyl]-4-yl)benzene ligands. Grand canonical Monte Carlo (GCMC) simulations of H2 sorption were performed using three different sorbate potentials of increasing complexity to elucidate the mechanism of interaction in NOTT-112. Reasonable agreement with the experimental H2 sorption isotherms, isosteric heats of adsorption (Qst), and H2–Cu2+ distances was obtained with the model that includes explicit many-body polarization. This highlights the physical importance of induction in H2 sorption in MOFs with open-metal sites. The two Cu2+ ions within the paddlewheels in NOTT-112 have distinct electrostatic profiles and sorption characteristics. The Cu2+ ions projecting into the cuboctahedral (cub–Oh) cage of the MOF are more electropositive than the Cu2+ ions projecting toward the organic linker, and therefore serve as the initial loadin...

A New Set of Isoreticular, Homochiral Metal–Organic Frameworks with ucp Topology

A new isoreticular series of metal–organic frameworks, called UHM-25 (UHM: University of Hamburg Materials), based on the copper paddle wheel motif and a novel set of homochiral linkers has been synthesized. Starting from amino acids available from the chiral pool a synthesis procedure was established that allows a straightforward, multigram scale synthesis of homochiral linkers in 4–5 steps. These linkers carry substituents that have been proven useful in stereoselective organic chemistry, such as the Evans auxiliary or chiral amino alcohols. The resulting MOFs only differ in the chiral moiety provided by the amino acid starting material. The structure of UHM-25 is composed of cuboctahedral cages of Cu2 paddle wheel motifs connected by the isophthalate moieties of the linker. These cages are linked via the bent backbone of the linker resulting in a primitive cubic arrangement, giving rise to the rare underlying (3,4)-c binodal net ucp. MOFs of the UHM-25 series show surface areas up to SBET = 1900 m2/g. ...

Metal-Ion Metathesis and Properties of Triarylboron-Functionalized Metal-Organic Frameworks.

An anionic metal-organic framework, H3[(Mn4Cl)3L8]⋅30H2O⋅2.5 DMF⋅5 Diox (UPC-15), was successfully prepared by the reaction of MnCl2 with tris(p-carboxylic acid)tridurylborane (H3 L) under solvothermal conditions. UPC-15 with wide-open pores (∼18.8 Å) is constructed by packing of octahedral and cuboctahedral cages, and exhibits high gas-sorption capabilities. Notably, UPC-15 shows selective adsorption of cationic dyes due to the anion framework. Moreover, the catalytic and magnetic properties were investigated, and UPC-15 can highly catalyze the cyanosilylation of aromatic aldehydes. UPC-15 exhibits the exchange of metal ions from Mn to Cu in a single-crystal-to-single-crystal manner to generate UPC-16, which could not be obtained by the direct solvothermal reaction of CuCl2 and H3L. UPC-16 exhibits similar properties for gas sorption, dye separation, and catalytic activity. However, the magnetic behaviors for UPC-15 and UPC-16 are distinct due to the metal-specific properties. Below 47 K, UPC-15 exhibits a ferromagnetic coupling but UPC-16 shows a dominant antiferromagnetic behavior.

Modeling PCN-61 and PCN-66: Isostructural rht-Metal–Organic Frameworks with Distinct CO2 Sorption Mechanisms

Simulations of CO2 sorption were performed in two members of the highly tunable rht-metal–organic framework (MOF) platform: PCN-61 and PCN-66. These MOFs differ only in the triisophthalate ligand used to synthesize the respective MOFs. In PCN-61, the center of the ligand contains a benzene ring; this ring is substituted with a triphenylamine group in PCN-66. There are two chemically distinct Cu2+ ions that comprise the copper paddlewheels, [Cu2(O2CR)4], in all rht-MOFs. One type of Cu2+ ion, denoted Cu1, projects into the truncated tetrahedral (T–Td) and truncated octahedral (T–Oh) cages, while the other Cu2+ ion, denoted Cu2, projects into the cuboctahedral (cub–Oh) cages. Electronic structure calculations revealed that, in PCN-61, the Cu2 ions have a significantly higher partial positive charge than the Cu1 ions, whereas the opposite was observed in PCN-66. The simulations revealed that the CO2 molecules sorb initially onto the Cu2+ ions that have the higher partial positive charge, i.e., the Cu2 ions i...

Metal–organic polyhedra constructed from dinuclear ruthenium paddlewheels

Reactions of Ru2n+ [n=4 for Ru2(OAc)4 and n=5 for Ru2(OAc)4Cl] with H2CDC (9H-3,6-carbazoledicarboxylic acid), H2BBDC (5-tert-butyl-1,3-benzenedicarboxylic acid) and H2BDC (1,3-benzenedicarboxylic acid) were conducted in inert atmospheres under solvothermal conditions in a sealed tube. Two octahedral cages, [Ru2(CDC)2Cl]6, 1 and [Ru2(CDC)2]6, 2 were obtained when starting from Ru2n+ with CDC, and two cuboctahedral cages, [Ru2(BBDC)2Cl]12, 3 and [Ru2(BDC)2]12, 4 were yielded when starting from Ru2n+ with BDC based linkers. Structures of these MOPs were characterized by single crystal X-ray analyses. The resulted MOPs have shown redox activity and magnetic properties similar to their parent metal complexes.

Crystal Engineering of an nbo Topology Metal–Organic Framework for Chemical Fixation of CO2 under Ambient Conditions

AbstractCrystal engineering of the nbo metal–organic framework (MOF) platform MOF‐505 with a custom‐designed azamacrocycle ligand (1,4,7,10‐tetrazazcyclododecane‐N,N′,N′′,N′′′‐tetra‐p‐methylbenzoic acid) leads to a high density of well‐oriented Lewis active sites within the cuboctahedral cage in MMCF‐2, [Cu2(Cu‐tactmb)(H2O)3(NO3)2]. This MOF demonstrates high catalytic activity for the chemical fixation of CO2 into cyclic carbonates at room temperature under 1 atm pressure.

Crystal Engineering of an nbo Topology Metal–Organic Framework for Chemical Fixation of CO2 under Ambient Conditions

Crystal engineering of the nbo metal-organic framework (MOF) platform MOF-505 with a custom-designed azamacrocycle ligand (1,4,7,10-tetrazazcyclododecane-N,N',N'',N'''-tetra-p-methylbenzoic acid) leads to a high density of well-oriented Lewis active sites within the cuboctahedral cage in MMCF-2, [Cu2(Cu-tactmb)(H2O)3(NO3)2]. This MOF demonstrates high catalytic activity for the chemical fixation of CO2 into cyclic carbonates at room temperature under 1 atm pressure.

Studies on Metal–Organic Frameworks of Cu(II) with Isophthalate Linkers for Hydrogen Storage

Hydrogen (H2) is a promising alternative energy carrier because of its environmental benefits, high energy density, and abundance. However, development of a practical storage system to enable the "Hydrogen Economy" remains a huge challenge. Metal-organic frameworks (MOFs) are an important class of crystalline coordination polymers constructed by bridging metal centers with organic linkers. MOFs show promise for H2 storage owing to their high surface area and tuneable properties. In this Account, we summarize our research on novel porous materials with enhanced H2 storage properties and describe frameworks derived from 3,5-substituted dicarboxylates (isophthalates) that serve as versatile molecular building blocks for the construction of a range of interesting coordination polymers with Cu(II) ions. We synthesized a series of materials by connecting linear tetracarboxylate linkers to {Cu(II)2} paddlewheel moieties. These materials exhibit high structural stability and permanent porosity. Varying the organic linker modulates the pore size, geometry, and functionality to control the overall H2 adsorption. Our top-performing material in this series has a H2 storage capacity of 77.8 mg g(-1) at 77 K, 60 bar. H2 adsorption at low, medium, and high pressures correlates with the isosteric heat of adsorption, surface area, and pore volume, respectively. Another series, using tribranched C3-symmetric hexacarboxylate ligands with Cu(II), gives highly porous (3,24)-connected frameworks incorporating {Cu(II)2} paddlewheels. Increasing the length of the hexacarboxylate struts directly tunes the porosity of the resultant material from micro- to mesoporosity. These materials show exceptionally high H2 uptakes owing to their high surface area and pore volume. The first member of this family reported adsorbs 111 mg g(-1) of H2, or 55.9 g L(-1), at 77 K, 77 bar, while at 77 K, 1 bar, the material adsorbs 2.3 wt % H2. We and others have since achieved enhanced H2 adsorption in these frameworks using combinations of polyphenyl groups linked by alkynes. The maximum storage achieved for one of the enhanced materials is 164 mg g(-1) at 77 K, 70 bar, but because of its low density, its volumetric capacity is only 45.7 g L(-1). We attribute the significant adsorption of H2 at low pressures to the arrangement of the {Cu24(isophthalate)24} cuboctahedral cages within the polyhedral structure. Free metal coordination positions are the first binding sites for D2, and these frameworks have two types of Cu(II) centers, one with its vacant site pointing into the cuboctahedral cage and another pointing externally. D2 molecules bind first at the former position and then at the external open metal sites. Design of ligands and complexes is key for enhancing and maximizing H2 storage, and although current materials operate at 77 K, research continues to explore routes to high capacity H2 storage materials that can function at higher temperatures.

A 3D Iron(II)-Based MOF with Squashed Cuboctahedral Nanoscopic Cages Showing Spin-Canted Long-Range Antiferromagnetic Ordering

The reaction of dilithium squarate with Fe(II) perchlorate led to the formation of a new Fe(II)-based 3D MOF, [Fe3(OH)3(C4O4)(C4O4)0.5]n (1), with homoleptic squashed cuboctahedral cages. Complex 1 crystallizes in the monoclinic C2/c space group. Fe(II) centers in the complex are octahedrally coordinated by four squarate dianions in axial and equatorial positions and two hydroxyl groups in the remaining equatorial positions. The interesting structural feature of 1 is that the three-dimensional framework is an infinite extension of nanoscopic cuboctahedral cages. The framework also contains two types of voids; the larger hydrophobic ones are surrounded by aromatic squarate ligands, while the smaller ones are hydrophilic with hydroxyl groups on the surface connected by bifurcated hydrogen bonding interaction. A variable temperature magnetic study shows spin-canted long-range antiferromagnetic ordering in the low temperature regime.

Formation of a Metal–Organic Framework with High Surface Area and Gas Uptake by Breaking Edges Off Truncated Cuboctahedral Cages

Breaking edges off regular truncated cuboctahedra leads to a metal-organic framework (MOF) with polybenzene topology. This fully characterized MOF with lowest connectivity has the largest BET surface area among interpenetrated MOFs.

Formation of a Metal–Organic Framework with High Surface Area and Gas Uptake by Breaking Edges Off Truncated Cuboctahedral Cages

Breaking edges off regular truncated cuboctahedra leads to a metal-organic framework (MOF) with polybenzene topology. This fully characterized MOF with lowest connectivity has the largest BET surface area among interpenetrated MOFs.

Construction of a Polyhedral Metal–Organic Framework via a Flexible Octacarboxylate Ligand for Gas Adsorption and Separation

A flexible octacarboxylate ligand, tetrakis[(3,5-dicarboxyphenyl)oxamethyl]methane (H8X), has been used to construct a highly porous metal-organic framework (In2X)(Me2NH2)2(DMF)9(H2O)5 (1), which is comprised of octahedral and cuboctahedral cages and shows a rare (4,8)-connected scu topology. Gas adsorption studies of N2, H2 on the actived 1 at 77 K reveal a Langmuir surface area of 1707 m(2) g(-1), a BET surface area of 1555 m(2) g(-1), a total pore volume of 0.62 cm(3) g(-1), and a H2 uptake of 1.49 wt % at 1 bar and 3.05 wt % at 16 bar. CO2, CH4, and N2 adsorption studies at 195, 273, 285, and 298 K and also ideal adsorbed solution theory (IAST) calculations demonstrate that 1 has high selectivites of CO2 over CH4 and N2. The resulting framework represents a MOF with the highest gas uptakes and gas selectivities (CO2 over CH4 and N2) constructed by flexible ligands.

An Interpenetrated Metal–Organic Framework and Its Gas Storage Behavior: Simulation and Experiment

A new metal-organic framework, called UHM-6 (UHM: University of Hamburg Materials), based on the copper paddle wheel motif and a novel organosilicon linker, 4',4″-(dimethylsilanediyl)bis(biphenyl-3,5-dicarboxylic acid) (sbbip), has been synthesized and characterized with regard to its gas storage behavior up to 1 bar for hydrogen, methane, and carbon dioxide. The 2-fold interpenetrated microporous framework of UHM-6 is isoreticular to PMOF-3 (Inorg. Chem.2009, 48, 11507) and is composed of cuboctahedral cages of Cu(2) paddle wheels connected via nonlinear organosilicon units. The structure (SG I422, No. 97) is characterized by straight channels running along the [001] and [110] direction. UHM-6 reveals a specific surface area of S(BET) ~ 1200 m(2) g(-1) and a specific micropore volume of V(micropore) ~ 0.48 cm(3) g(-1). At 1 bar the activated form of UHM-6 shows a hydrogen uptake of 1.8 wt % (77 K), a methane uptake of 0.8 mmol g(-1) (293 K), and a carbon dioxide uptake of 3.3 mmol g(-1) (273 K). Accompanying theoretical grand-canonical Monte Carlo (GCMC) simulations show an overall good agreement with the experimental results. Furthermore, GCMC adsorption simulations for three binary equimolar mixtures (CH(4)/H(2), CO(2)/H(2), and CO(2)/CH(4)) were carried out (T = 298 K) to assess the potential for gas separation/purification applications.

Designed 4,8-Connected Metal–Organic Frameworks Based on Tetrapodal Octacarboxylate Ligands

A flexible tetrapodal octacarboxylate ligand with four 1,3-benzenedicarboxylate units has been used to construct three polyhedral metal–organic frameworks, which show a (4,8)-connected structure of a rare scu topology comprising octahedral and cuboctahedral cages.

Modifying Cage Structures in Metal–Organic Polyhedral Frameworks for H2 Storage

Three isostructural metal-organic polyhedral cage based frameworks (denoted NOTT-113, NOTT-114 and NOTT-115) with (3,24)-connected topology have been synthesised by combining hexacarboxylate isophthalate linkers with {Cu(2)(RCOO)(4)} paddlewheels. All three frameworks have the same cuboctahedral cage structure constructed from 24 isophthalates from the ligands and 12 {Cu(2)(RCOO)(4)} paddlewheel moieties. The frameworks differ only in the functionality of the central core of the hexacarboxylate ligands with trimethylphenyl, phenylamine and triphenylamine moieties in NOTT-113, NOTT-114 and NOTT-115, respectively. Exchange of pore solvent with acetone followed by heating affords the corresponding desolvated framework materials, which show high BET surface areas of 2970, 3424 and 3394 m(2) g(-1) for NOTT-113, NOTT-114 and NOTT-115, respectively. Desolvated NOTT-113 and NOTT-114 show high total H(2) adsorption capacities of 6.7 and 6.8 wt%, respectively, at 77 K and 60 bar. Desolvated NOTT-115 has a significantly higher total H(2) uptake of 7.5 wt% under the same conditions. Analysis of the heats of adsorption (Q(st)) for H(2) reveals that with a triphenylamine moiety in the cage wall, desolvated NOTT-115 shows the highest value of Q(st) for these three materials, indicating that functionalisation of the cage walls with more aromatic rings can enhance the H(2)/framework interactions. In contrast, measurement of Q(st) reveals that the amine-substituted trisalkynylbenzene core used in NOTT-114 gives a notably lower H(2)/framework binding energy.

Non-framework Coordination Polymers With Tunable Bimodal Porosities Based on Inter-connected Metal-organic Polyhedra

One discerns coordination polymers (CPs) and metalorganic frameworks (MOFs) because the former provides a more comprehensive concept for network solids based on metal ions and multitopic organic ligands, while the latter implies the presence of a crystal lattice in the structure. MOF materials with three-dimensional backbones and welldefined network structures possess many advantages compared to their non-framework analogues, such as homogeneous and reproducible material properties, and atomlevel information on the structures. However, the labile nature of some coordination bonds, such as those between first-row transition elements and carboxylate ligands may lead to a gradual or even complete degradation of the scaffold under ambient conditions. For most MOFs, therefore, the sustainability of the crystal lattice is a prerequisite to realize the fascinating potentials of these materials, including hydrogen storage or carbon capture and sequestration. Consequently, it is desirable to have a new type of materials that possess the advantages of porous MOFs but without the constraint of an ordered crystal lattice. Approaches taken in this direction include polymerizations using rigid building blocks such as phthalocyanins, liquid-crystal templation and the derivation of disordered structures using rapid precipitation methods. We provide a potential alternative in another perspective, and hereby report some examples of non-framework CPs that have intrinsic porosities tunable between microporous and mesoporous regimes. The idea is based on the realization that some of the well-known metal-organic polyhedral cages are equipped with solvent-coordinated sites. Cu2 paddlewheels and bent dicarboxylates are prototypical building blocks for such cases. Thus we decided to take advantage of the solvent sites of paddlewheel units to crosslink polyhedral cage molecules into a network. The polymeric materials would possess a mesoporosity depending on the size of the cage and linker molecules, as well as an intrinsic microporosity originating from the inner cage voids (Scheme 1). We used 2,7-naphthalenedicarboxylate and Cu to build a macromolecule with 12 Cu2 paddlewheel units, [Cu24(2,7ndc)24(DMF)10(H2O)14] (1). The structure of the cuboctahedral cage molecule was determined by X-ray crystallography on weakly diffracting single-crystals using synchrotron X-rays from Pohang Accelerator Laboratory, Korea (Figure 1). The approximately spherical molecule measures about 3.3 nm, and the inner-cage void can host an imaginary sphere with the diameter of 1.8 nm. Such metal-organic cuboctahedra were first reported by Yaghi and Zaworotko, and continue to be a subject of active research. In the packing structure, a given cuboctahedral cage

High capacity gas storage by a 4,8-connected metal–organic polyhedral framework

The polyhedral complex [Cu(4)L(H(2)O)(4)]solv (NOTT-140) shows a 4,8-connected structure of rare scu topology comprising octahedral and cuboctahedral cages; desolvated NOTT-140a shows a total CO(2) uptake of 314.6 cm(3) (STP) cm(-3) at 20 bar, 293 K, and a total H(2) uptake of 6.0 wt% at 20 bar, 77 K.

Tuning the Topology and Functionality of Metal−Organic Frameworks by Ligand Design

Metal-organic frameworks (MOFs)-highly crystalline hybrid materials that combine metal ions with rigid organic ligands-have emerged as an important class of porous materials. The organic ligands add flexibility and diversity to the chemical structures and functions of these materials. In this Account, we summarize our laboratory's experience in tuning the topology and functionality of MOFs by ligand design. These investigations have led to new materials with interesting properties. By using a ligand that can adopt different symmetry conformations through free internal bond rotation, we have obtained two MOFs that are supramolecular stereoisomers of each other at different reaction temperatures. In another case, where the dimerized ligands function as a D(3)-Piedfort unit spacer, we achieve chiral (10,3)-a networks. In the design of MOF-based materials for hydrogen and methane storage, we focused on increasing the gas affinity of frameworks by using ligands with different geometries to control the pore size and effectively introduce unsaturated metal centers (UMCs) into the framework. Framework interpenetration in PCN-6 (PCN stands for porous coordination network) can lead to higher hydrogen uptake. Because of the proper alignment of the UMCs, PCN-12 holds the record for uptake of hydrogen at 77 K/760 Torr. In the case of methane storage, PCN-14 with anthracene-derived ligand achieves breakthrough storage capacity, at a level 28% higher than the U.S. Department of Energy target. Selective gas adsorption requires a pore size comparable to that of the target gas molecules; therefore, we use bulky ligands and network interpenetration to reduce the pore size. In addition, with the help of an amphiphilic ligand, we were able to use temperature to continuously change pore size in a 2D layer MOF. Adding charge to an organic ligand can also stabilize frameworks. By ionizing the amine group within mesoMOF-1, the resulting electronic repulsion keeps the network from collapsing, giving rise to the first case of mesoporous MOF that demonstrates the type IV isotherm. We use dendritic hexacarboxylate ligands to synthesize an isoreticular series of MOFs with (3,24)-connected network topology. The cuboctahedral cages serve as building blocks that narrow the opening of the mesocavities into microwindows and stabilize these MOFs. The resulting materials have exceptionally high surface areas and hydrogen uptake capacities. Despite the many achievements in MOF development, there is still ample opportunity for further exploration. We will be continuing our efforts and look forward to contributing to this blossoming field in the next decade.

Highly Porous Ionic rht Metal−Organic Framework for H2 and CO2 Storage and Separation: A Molecular Simulation Study

The storage and separation of H2 and CO2 are investigated in a highly porous ionic rht metal-organic framework (rht-MOF) using molecular simulation. The rht-MOF possesses a cationic framework and charge-balancing extraframework NO3(-) ions. Three types of unique open cages exist in the framework: rhombicuboctahedral, tetrahedral, and cuboctahedral cages. The NO3(-) ions exhibit small mobility and are located at the windows connecting the tetrahedral and cuboctahedral cages. At low pressures, H2 adsorption occurs near the NO3(-) ions that act as preferential sites. With increasing pressure, H2 molecules occupy the tetrahedral and cuboctahedral cages and the intersection regions. The predicted isotherm of H2 at 77 K agrees well with the experimental data. The H2 capacity is estimated to be 2.4 wt % at 1 bar and 6.2 wt % at 50 bar, among the highest in reported MOFs. In a four-component mixture (15:75:5:5 CO2/H2/CO/CH4) representing a typical effluent gas of H2 production, the selectivity of CO2/H2 in rht-MOF decreases slightly with increasing pressure, then increases because of cooperative interactions, and finally decreases as a consequence of entropy effect. By comparing three ionic MOFs (rht-MOF, soc-MOF, and rho-ZMOF), we find that the selectivity increases with increasing charge density or decreasing free volume. In the presence of a trace amount of H2O, the interactions between CO2 and NO3(-) ions are significantly shielded by H2O; consequently, the selectivity of CO2/H2 decreases substantially.