Expanding the Structural Topologies of Rare-Earth Porphyrinic Metal-Organic Frameworks through Ligand Modulation.

Expanding the structural diversity of porphyrinic metal-organic frameworks (PMOFs) is essential to develop functional materials with novel properties or enhanced performance in different applications. Herein, we establish a strategy to construct rare-earth (RE) PMOFs with unprecedented topology via rational functionalization of porphyrinic ligands. By introducing phenyl/pyridyl groups to the meso-positions of the porphyrin core, the symmetries and connectivities of the ligands are tuned, and three RE-PMOFs (BUT-224/-225/-226) with new topologies are successfully obtained. In addition, BUT-225(Co), with both the Lewis basic and acidic sites, exhibits enhanced CO2 uptake and higher catalytic activity for the cycloaddition of CO2 and epoxides under mild conditions. This work reveals that the RE-PMOFs with novel topologies can be rationally designed and constructed through ligand functionalization, which provides insights into the construction of tailored PMOFs for various applications.

A Practice of Reticular Chemistry: Construction of a Robust Mesoporous Palladium Metal-Organic Framework via Metal Metathesis.

Constructing stable palladium(II)-based metal-organic frameworks (MOFs) would unlock more opportunities for MOF chemistry, particularly toward applications in catalysis. However, their availability is limited by synthetic challenges due to the inertness of the Pd-ligand coordination bond, as well as the strong tendency of the Pd(II) source to be reduced under typical solvothermal conditions. Under the guidance of reticular chemistry, herein, we present the first example of an azolate Pd-MOF, BUT-33(Pd), obtained via a deuterated solvent-assisted metal metathesis. BUT-33(Pd) retains the underlying sodalite network and mesoporosity of the template BUT-33(Ni) and shows excellent chemical stability (resistance to an 8 M NaOH aqueous solution). With rich Pd(II) sites in the atomically precise distribution, it also demonstrates good performances as a heterogeneous Pd(II) catalyst in a wide application scope, including Suzuki/Heck coupling reactions and photocatalytic CO2 reduction to CH4. This work highlights a feasible approach to reticularly construct noble metal based MOFs via metal metathesis, in which various merits, including high chemical stability, large pores, and tunable functions, have been integrated for addressing challenging tasks.

Linker Desymmetrization: Access to a Series of Rare-Earth Tetracarboxylate Frameworks with Eight-Connected Hexanuclear Nodes.

The exploration of metal-organic frameworks (MOFs) through the rational design of building units with specific sizes, geometries, and symmetries is essential for enriching the structural diversity of porous solids for applications including storage, separation, and conversion. However, it is still a challenge to directly synthesize rare-earth (RE) MOFs with less connected clusters as a thermodynamically favored product. Herein, we report a systematic investigation on the influence of size, rigidity, and symmetry of linkers over the formation of RE-tetracarboxylate MOFs and uncover the critical role of linker desymmetrization in constructing RE-MOFs with eight-connected hexanuclear clusters. Our results on nine new RE-MOFs, PCN-50X (X = 1-9), indicate that utilization of trapezoidal or tetrahedral linkers provides accesses to traditionally unattainable RE-tetracarboxylate MOFs with 8-c hexanuclear nodes, while the introduction of square or rectangular linkers during the assembly of RE-MOFs based on polynuclear clusters typically leads to the MOFs constructed from 12-c nodes with underlying shp topology. By rational linker design, MOFs with two unprecedented (4, 8)-c nets, lxl and jun, can also be obtained. This work highlights linker desymmetrization as a powerful strategy to enhance MOFs' structural complexity and access MOF materials with nondefault topologies that can be potentially used for separation and catalysis.

A Series of Mesoporous Rare-Earth Metal-Organic Frameworks Constructed from Organic Secondary Building Units.

Further development of metal-organic frameworks (MOFs) requires an establishment of hierarchical interaction within the framework. Herein, we report a series of mesoporous rare-earth (RE) MOFs that are constructed from an unusual 12-connected π-stacked pyrene secondary building unit (SBU) and a typical 12-connected RE6 cluster (RE=Eu, Y, Yb, Tb, Ce). The judicious design of a butterfly-shape pyrene ligand with a tert-butyl substituent enables the formation of the disordered 12-connected organic SBUs on its strong intermolecular π-π interactions. The assembly of 12-connected inorganic cuboctahedron SBUs and 12-connected organic distorted hexagonal prism SBUs generates an unprecedented network that can be further simplified into a 4,4-connected pts net linked from planar square and tetrahedra. This work provides fresh insights into the design and synthesis of frameworks constructed from coordinatively, covalently, and noncovalently linked building units.

Kinetically Controlled Reticular Assembly of a Chemically Stable Mesoporous Ni(II)-Pyrazolate Metal-Organic Framework.

The application scope of metal-organic frameworks (MOFs) is severely restricted by their weak chemical stability and limited pore size. A robust MOF with large mesopores is highly desired, yet poses a great synthetic challenge. Herein, two chemically stable Ni(II)-pyrazolate MOFs, BUT-32 and -33, were constructed from a conformation-matched elongated pyrazolate ligand through the isoreticular expansion. The two MOFs share the same sodalite-type net, but have different pore sizes due to the network interpenetration in BUT-32. Controlled syntheses of the two MOFs have been achieved through precisely tuning reaction conditions, where the microporous BUT-32 was demonstrated to be a thermodynamically stable product while the mesoporous BUT-33 is kinetically favored. To date, BUT-32 represents the first example of Ni4-pyrazolate MOF whose structure was unambiguously determined by single-crystal X-ray diffraction. Interestingly, the kinetic product BUT-33 integrates 2.6 nm large mesopores with accessible Ni(II) active sites and remarkable chemical stability even in 4 M NaOH aqueous solution and 1 M Grignard reagent. This MOF thus demonstrated an excellent catalytic performance in carbon-carbon coupling reactions, superior to other Ni(II)-MOFs including BUT-32. These findings highlight the importance of kinetic control in the reticular synthesis of mesoporous MOFs, as well as their superiority in heterogeneous catalysis.

Modular Total Synthesis in Reticular Chemistry.

The idea of modularity in organic total synthesis has promoted the construction of diverse targeted natural products by varying the building blocks and assembly sequences. Yet its utilization has been mainly limited to the synthesis of molecular compounds based on covalent bonds. In this work, we expand the conceptual scope of modular synthesis into framework materials, which bridges metal- and covalent organic frameworks (MOFs and COFs) hierarchically in reticular chemistry. While the assembly sequences are determined by the coordination or the covalent bond strengths, a modular synthesis strategy which progressively links simple building blocks into increasingly sophisticated superstructures was reported. As a result, a series of hierarchical COF-on-MOF structures with architectural intricacy were obtained through sequence-defined reactions of diverse building blocks. The tunability of spatial apportionment, compositions, and functionality was successfully managed in these framework materials. To the best of our knowledge, this is the first report on the synthesis of COF@MOF composites and also the first discovery of controlled COF alignment. This generalizable modularity strategy will not only accelerate the discovery of multicomponent framework materials by the hierarchical assembly of MOFs and COFs but also offer a predictable retrosynthetic route to smart materials with unusual tunability owing to the diverse inorganic or organic building units.

Hierarchically porous metal-organic frameworks: synthetic strategies and applications.

Despite numerous advantages, applications of conventional microporous metal-organic frameworks (MOFs) are hampered by their limited pore sizes, such as in heterogeneous catalysis and guest delivery, which usually involve large molecules. Construction of hierarchically porous MOFs (HP-MOFs) is vital to achieve the controllable augmentation of MOF pore size to mesopores or even macropores, which can enhance the diffusion kinetics of guests and improve the storage capacity. This review article focuses on recent advances in the methodology of HP-MOF synthesis, covering preparation of HP-MOFs with intrinsic hierarchical pores, and modulated, templated and template-free synthetic strategies for HP-MOFs. The key factors which affect the formation of HP-MOF architectures are summarized and discussed, followed by a brief review of their applications in heterogeneous catalysis and guest encapsulation. Overall, this review presents a roadmap that will guide the future design and development of HP-MOF materials with molecular precision and mesoscopic complexity.

A novel mesoporous hydrogen-bonded organic framework with high porosity and stability.

A highly stable hydrogen-bonded organic framework, HOF-14, has been successfully constructed and structurally characterized. It possesses a permanent three dimensional (3D) porous structure. The activated HOF-14 has a high BET surface area of 2573 m2 g-1 and a record large pore volume of 1.36 cm3 g-1 among HOF materials. In addition, HOF-14 also exhibits high chemical and thermal stability and is capable of highly selective separation of light hydrocarbons under ambient conditions.

Fixing Flexible Arms of Core-Shared Ligands to Enhance the Stability of Metal-Organic Frameworks.

In recent years, more and more research on metal-organic frameworks (MOFs) has focused on exploring their practical applications, where the stability is crucial. Besides the metal-ligand coordination bond, the configuration of the ligand also plays an important role in determining the stability of resulting MOFs. In this work, we demonstrate that fixing flexible arms of core-shared ligands can enhance the stability of their Zr(IV)-MOFs. Two groups, four core-shared tetracarboxylate ligands, 3,3',3″,3‴-(pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1-diyl))tetraacrylate (PTSA4-) and 6,6',6″,6‴-(pyrene-1,3,6,8-tetrayl)tetrakis(2-naphthoate) (PTNA4-) with the pyrene core and 3,3',3″,3‴-((9H-carbazole-1,3,6,8-tetrayl)tetrakis(benzene-4,1-diyl))-tetraacrylate (CTSA4-) and 6,6',6″,6‴-(9H-carbazole-1,3,6,8-tetrayl)tetrakis-(2-naphthoate) (CTNA4-) with the carbazole core are rationally designed. Two ligands in each group have different flexibilities due to the distinct side arms: the styrene arm is flexible, whereas the naphthalene is rigid. Constructed with Zr6 clusters, four 4,8-connected Zr(IV)-MOFs, Zr6O4(OH)8(H2O)4(PTSA)2 (BUT-72) and Zr6O4(OH)8(H2O)4(PTNA)2 (BUT-73) with a sqc-a topologic framework structure and Zr6O4(OH)8(H2O)4(CTSA)2 (BUT-74) and Zr6O4(OH)8(H2O)4(CTNA)2 (BUT-63) with a scu-a structure are obtained, respectively. It is found that the stability of BUT-73 and -63 with the rigid naphthoate-based ligands is significantly enhanced compared with that of BUT-72 and -74 with the flexible phenyl acrylate-based ones. Moreover, stable BUT-63 represents outstanding performance in the molecular recognition of most solvents commonly used in organic synthesis and industrial manufacture.

Imprinted Apportionment of Functional Groups in Multivariate Metal-Organic Frameworks.

Sophisticated chemical processes widely observed in biological cells require precise apportionment regulation of building units, which inspires researchers to develop tailorable architectures with controllable heterogeneity for replication, recognition and information storage. However, it remains a substantial challenge to endow multivariate materials with internal sequences and controllable apportionments. Herein, we introduce a novel strategy to manipulate the apportionment of functional groups in multivariate metal-organic frameworks (MTV-MOFs) by preincorporating interlocked linkers into framework materials. As a proof of concept, the imprinted apportionment of functional groups within ZIF-8 was achieved by exchanging imine-based linker templates with original linkers initially. The removal of linker fragments by hydrolysis can be achieved via postsynthetic labilization, leading to the formation of architectures with controlled heterogeneity. The distributions of functional groups in the resulting imprinted MOFs can be tuned by judicious control of the interlocked chain length, which was further analyzed by computational methods. This work provides synthetic tools for precise control of pore environment and functionality sequences inside multicomponent materials.

Modular Programming of Hierarchy and Diversity in Multivariate Polymer/Metal-Organic Framework Hybrid Composites.

The idea that complex systems have a hierarchical arrangement has been widely observed on various scales. In this work, we introduce the concept of modular programming, which emphasizes isolating the functionality of a system into independent, interchangeable modules, to tailor the hierarchy and diversity in these complex systems. Guided by modular programming, a system with multiple compatible components, including modules A, B, C, and so forth, can be constructed and subsequently modified into modules A', B', C', and so forth independently. As a proof of concept, a series of multivariate hierarchical metal-organic frameworks (MOFs) with various compositions, ratios, and distributions were prepared as a compatible system. Sequential click reactions and acid treatments can be utilized to selectively modify a certain modular MOF into a polymer, while other modular MOFs either remain in their original state or dissolve upon treatment. As a result, a series of polymer/MOF composites that traditionally have been viewed as incompatible can be prepared with tailored properties and behaviors. The resulting polymer/MOF hierarchical composites represent a unique porous composite material which contains functional groups and metal clusters with controllable compositions and distribution, tunable hierarchically porous structures, and tailored diversity within one framework. This general synthesis approach guided by modular programming not only provides a facile method to tailor hierarchy and diversity in multivariate systems but also enables the investigation into hierarchy and its structured control flow, which is a critical design feature of future materials for their fast adaptivity and responses to variable environmental conditions.

Ligand Rigidification for Enhancing the Stability of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have been developing at an unexpected rate over the last two decades. However, the unsatisfactory chemical stability of most MOFs hinders some of the fundamental studies in this field and the implementation of these materials for practical applications. The stability in a MOF framework is mostly believed to rely upon the robustness of the M-L (M = metal ion, L = ligand) coordination bonds. However, the role of organic linkers as agents of stability to the framework, particularly the linker rigidity/flexibility, has been mostly overlooked. In this work, we demonstrate that a ligand-rigidification strategy can enhance the stability of MOFs. Three series of ligand rotamers with the same connectivity but different flexibility were prepared. Thirteen Zr-based MOFs were constructed with the Zr6O4(OH4)(-CO2) n units ( n = 8 or 12) and corresponding ligands. These MOFs allow us to evaluate the influence of ligand rigidity, connectivities, and structure on the stability of the resulting materials. It was found that the rigidity of the ligands in the framework strongly contributes to the stability of corresponding MOFs. Furthermore, water adsorption was performed on some chemically stable MOFs, showing excellent performance. It is expected that more MOFs with excellent stability could be designed and constructed by utilizing this strategy, ultimately promoting the development of MOFs with higher stability for synthetic chemistry and practical applications.

Tuning Water Sorption in Highly Stable Zr(IV)-Metal-Organic Frameworks through Local Functionalization of Metal Clusters.

Water adsorption of metal-organic frameworks (MOFs) is attracting intense interest because of their potential applications in atmospheric water harvesting, dehumidification, and adsorption-based heating and cooling. In this work, through using a hexacarboxylate ligand, four new isostructural Zr(IV)-MOFs (BUT-46F, -46A, -46W, and -46B) with rare low-symmetric 9-connected Zr6 clusters were synthesized and structurally characterized. These MOFs are highly stable in water, HCl aqueous solution (pH = 1), and NaOH aqueous solution (pH = 10) at room temperature, as well as in boiling water. Interestingly, the rational modification of the metal clusters in these MOFs with different functional groups (HCOO-, CH3COO-, H2O/OH, and PhCOO-) enables the precise tuning of their water adsorption properties, which is quite important for given application. Furthermore, all four MOFs show excellent regenerability under mild conditions and good cyclic performance in water adsorption.

Zr(IV)-Based Metal-Organic Framework with T-Shaped Ligand: Unique Structure, High Stability, Selective Detection, and Rapid Adsorption of Cr2O72- in Water.

Dichromate is known for severe health impairments to organisms. New and valid strategies have been developed to rapidly detect and efficiently remove this pollutant. Constructing stable luminescent metal-organic frameworks (MOFs) for dichromate recognition and removal from aqueous solution could provide a feasible resolution to this problem. Herein, a new luminescent Zr(IV)-MOF, Zr6O4(OH)7(H2O)3(BTBA)3 (BUT-39, BUT = Beijing University of Technology) was constructed through the reaction of a newly designed functionalized T-shaped ligand 4,4',4″-(1 H-benzo[ d]imidazole-2,4,7-triyl)tribenzoic acid (H3BTBA) with zirconium salt. BUT-39 has a unique porous framework structure, in which Zr6 cluster acts as a rare low-symmetric 9-connected node and BTBA3- as a T-shaped 3-connected linker. As far as we know, this represents the first case of a (3,9)-connected Zr(IV)-MOF. BUT-39 could retain its framework integrity in boiling water, 2 M HCl aqueous solution, and pH 12 NaOH aqueous solution. Due to its good water stability and strong fluorescent emission, BUT-39 is then employed in fluorescence sensing for various ions in aqueous solution and shows good performance toward Cr2O72- selectively, at a low concentration and a short response time (<1 min). Simultaneously, it also exhibits excellent capacity to rapidly capture Cr2O72- (within 1 min) with a high uptake up to 1 mmol g-1. Taking advantage of its excellent stability, sensitive and selective sensing, as well as rapid and high adsorption, BUT-39 is expected to be useful in Cr2O72- detection in and removal from water.

A Base-Resistant Metalloporphyrin Metal-Organic Framework for C-H Bond Halogenation.

A base-resistant porphyrin metal-organic framework (MOF), namely PCN-602 has been constructed with 12-connected [Ni8(OH)4(H2O)2Pz12] (Pz = pyrazolate) cluster and a newly designed pyrazolate-based porphyrin ligand, 5,10,15,20-tetrakis(4-(pyrazolate-4-yl)phenyl)porphyrin under the guidance of the reticular synthesis strategy. Besides its robustness in hydroxide solution, PCN-602 also shows excellent stability in aqueous solutions of F-, CO32-, and PO43- ions. Interestingly, the Mn3+-porphyrinic PCN-602, as a recyclable MOF catalyst, presents high catalytic activity for the C-H bond halogenation reaction in a basic system, significantly outperforming its homogeneous counterpart. For the first time, a porphyrinic MOF was thus used as an efficient catalyst in a basic solution with coordinating anions, to the best of our knowledge.

Highly Stable Zr(IV)-Based Metal-Organic Frameworks for the Detection and Removal of Antibiotics and Organic Explosives in Water.

Antibiotics and organic explosives are among the main organic pollutants in wastewater; their detection and removal are quite important but challenging. As a new class of porous materials, metal-organic frameworks (MOFs) are considered as a promising platform for the sensing and adsorption applications. In this work, guided by a topological design approach, two stable isostructural Zr(IV)-based MOFs, Zr6O4(OH)8(H2O)4(CTTA)8/3 (BUT-12, H3CTTA = 5'-(4-carboxyphenyl)-2',4',6'-trimethyl-[1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid) and Zr6O4(OH)8(H2O)4(TTNA)8/3 (BUT-13, H3TTNA = 6,6',6″-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(2-naphthoic acid)) with the the-a topological structure constructed by D4h 8-connected Zr6 clusters and D3h 3-connected linkers were designed and synthesized. The two MOFs are highly porous with the Brunauer-Emmett-Teller surface area of 3387 and 3948 m(2) g(-1), respectively. Particularly, BUT-13 features one of the most porous water-stable MOFs reported so far. Interestingly, these MOFs represent excellent fluorescent properties, which can be efficiently quenched by trace amounts of nitrofurazone (NZF) and nitrofurantoin (NFT) antibiotics as well as 2,4,6-trinitrophenol (TNP) and 4-nitrophenol (4-NP) organic explosives in water solution. They are responsive to NZF and TNP at parts per billion (ppb) levels, which are among the best performing luminescent MOF-based sensing materials. Simultaneously, both MOFs also display high adsorption abilities toward these organic molecules. It was demonstrated that the adsorption plays an important role in the preconcentration of analytes, which can further increase the fluorescent quenching efficiency. These results indicate that BUT-12 and -13 are favorable materials for the simultaneous selective detection and removal of specific antibiotics and organic explosives from water, being potentially useful in monitoring water quality and treating wastewater.

Pyrazolate-Based Porphyrinic Metal-Organic Framework with Extraordinary Base-Resistance.

Guided by a top-down topological analysis, a metal-organic framework (MOF) constructed by pyrazolate-based porphyrinic ligand, namely, PCN-601, has been rationally designed and synthesized, and it exhibits excellent stability in alkali solutions. It is, to the best of our knowledge, the first identified MOF that can retain its crystallinity and porosity in saturated sodium hydroxide solution (∼ 20 mol/L) at room temperature and 100 °C. This almost pushes base-resistance of porphyrinic MOFs (even if MOFs) to the limit in aqueous media and greatly extends the range of their potential applications. In this work, we also tried to interpret the stability of PCN-601 from both thermodynamic and kinetic perspectives.

A Base-Resistant ZnII -Based Metal-Organic Framework: Synthesis, Structure, Postsynthetic Modification, and Gas Adsorption.

A ZnII -based metal-organic framework (MOF), [Zn2 (bdp-CHO)2 ]⋅(DMF)(CH3 CN)(H2 O)2 (BUT-31) is reported that was synthesized by the reaction between a newly designed aldehyde-tagged polypyrazole ligand 2,5-di(1H-pyrazol-4-yl)benzaldehyde (H2 bdp-CHO) and a zinc salt. BUT-31 has a unique pillared layered framework structure with 3D intersecting channels approximately 3.4-5.4 Šin size. Powder X-ray diffraction and N2 adsorption experiments revealed that BUT-31 is rigid and permanently porous with the Brunauer-Emmett-Teller surface area of 926 m2 g-1 . Notably, this MOF tolerates boiling water and even highly basic aqueous solution (4 m sodium hydroxide), although dilute acid gradually decomposes its framework. Owing the permanent porosity and chemical stability of BUT-31, covalent post-modification of the free aldehyde group exposed on the pore surface was accomplished by treating the MOF in a concentrated ammonia solution (25 %) at near room temperature, giving rise to an imine-functionalized analogue of BUT-31. Gas adsorption results show that the aldehyde- and imine-functionalized MOFs have high CO2 adsorption capacities, as well as CO2 /N2 and CO2 /CH4 adsorption selectivities.

Tuning CO2 selective adsorption over N2 and CH4 in UiO-67 analogues through ligand functionalization.

Introducing functional groups into pores of metal-organic frameworks (MOFs) through ligand modification provides an efficacious approach for tuning gas adsorption and separation performances of this type of novel porous material. In this work, two UiO-67 analogues, [Zr6O4(OH)4(FDCA)6] (BUT-10) and [Zr6O4(OH)4(DTDAO)6] (BUT-11), with functionalized pore surfaces and high stability were synthesized from two functional ligands, 9-fluorenone-2,7-dicarboxylic acid (H2FDCA) and dibenzo[b,d]thiophene-3,7-dicarboxylic acid 5,5-dioxide (H2DTDAO), respectively, and structurally determined by single-crystal X-ray diffraction. Notwithstanding skeleton bend of the two ligands relative to the linear 4,4'-biphenyldicarboxylic acid in UiO-67, the two MOFs have structures similar to that of UiO-67, with only lowered symmetry in their frameworks. Attributed to these additional functional groups (carbonyl and sulfone, respectively) in the ligands, BUT-10 and -11 show enhanced CO2 adsorption and separation selectivities over N2 and CH4, in spite of decreased pore sizes and surface areas compared with UiO-67. At 298 K and 1 atm, the CO2 uptake is 22.9, 50.6, and 53.5 cm(3)/g, and the infinite dilution selectivities of CO2/CH4 are 2.7, 5.1, and 9.0 and those of CO2/N2 are 9.4, 18.6, and 31.5 for UiO-67, BUT-10, and BUT-11, respectively. The selectivities of CO2/CH4 and CO2/N2 are thus enhanced 1.9 and 2.0 times in BUT-10 and 3.3 and 3.4 times in BUT-11, respectively, on the basis of UiO-67. The adsorption mechanism of CO2 in BUT-11 has also been explored through computational simulations. The results show that CO2 molecules locate around the sulfone groups in pore surfaces of BUT-11, verifying at the molecular level that sulfone groups significantly increase the affinity toward CO2 molecules of the framework. This provides thus an efficient strategy for the design of CO2 capture materials.

Direct solvent-free regioselective construction of pyrrolo[1,2-a][1,10]phenanthrolines based on isocyanide-based multicomponent reactions.

A novel efficient one-pot four-component regioselective synthesis of pyrrolo[1,2-a][1,10]phenanthrolines in excellent yields has been developed by 1,3-dipolar cycloaddition of aldehydes, malononitrile, and isocyanides with 1,10-phenanthroline under solvent-free conditions within 3 min without using any catalyst or activation. The products were preliminarily investigated as chromogenic and fluorescent sensors for Cu(2+) ions.