Kecheng Wang

An Exceptionally Stable, Porphyrinic Zr Metal–Organic Framework Exhibiting pH-Dependent Fluorescence

A reaction between a Zr(IV) salt and a porphyrinic tetracarboxylic acid leads to a metal-organic framework (MOF) with two types of open channels, representing a MOF featuring a (4,8)-connected sqc net. The MOF remains intact in both boiling water and aqueous solutions with pH ranging from 1 to 11, a remarkably extensive pH range that a MOF can sustain. Given its exceptional stability and pH-dependent fluorescent intensity, the MOF can potentially be applied in fluorescent pH sensing.

Stable metal-organic frameworks containing single-molecule traps for enzyme encapsulation

Enzymatic catalytic processes possess great potential in chemical manufacturing, including pharmaceuticals, fuel production and food processing. However, the engineering of enzymes is severely hampered due to their low operational stability and difficulty of reuse. Here, we develop a series of stable metal-organic frameworks with rationally designed ultra-large mesoporous cages as single-molecule traps (SMTs) for enzyme encapsulation. With a high concentration of mesoporous cages as SMTs, PCN-333(Al) encapsulates three enzymes with record-high loadings and recyclability. Immobilized enzymes that most likely undergo single-enzyme encapsulation (SEE) show smaller Km than free enzymes while maintaining comparable catalytic efficiency. Under harsh conditions, the enzyme in SEE exhibits better performance than free enzyme, showing the effectiveness of SEE in preventing enzyme aggregation or denaturation. With extraordinarily large pore size and excellent chemical stability, PCN-333 may be of interest not only for enzyme encapsulation, but also for entrapment of other nanoscaled functional moieties.

Topology-Guided Design and Syntheses of Highly Stable Mesoporous Porphyrinic Zirconium Metal–Organic Frameworks with High Surface Area

Through a topology-guided strategy, a series of Zr6-containing isoreticular porphyrinic metal-organic frameworks (MOFs), PCN-228, PCN-229, and PCN-230, with ftw-a topology were synthesized using the extended porphyrinic linkers. The bulky porphyrin ring ligand effectively prevents the network interpenetration which often appears in MOFs with increased linker length. The pore apertures of the structures range from 2.5 to 3.8 nm, and PCN-229 demonstrates the highest porosity and BET surface area among the previously reported Zr-MOFs. Additionally, by changing the relative direction of the terminal phenyl rings, this series replaces a Zr8 cluster with a smaller Zr6 cluster in a topologically identical framework. The high connectivity of the Zr6 cluster yields frameworks with enhanced stability despite high porosity and ultralarge linker. As a representative example, PCN-230, constructed with the most extended porphyrinic linker, shows excellent stability in aqueous solutions with pH values ranging from 0 to 12 and demonstrates one of the highest pH tolerances among all porphyrinic MOFs. This work not only presents a successful example of rational design of MOFs with desired topology, but also provides a strategy for construction of stable mesoporous MOFs.

A Series of Highly Stable Mesoporous Metalloporphyrin Fe-MOFs

A series of mesoporous metalloporphyrin Fe-MOFs, namely PCN-600(M) (M = Mn, Fe, Co, Ni, Cu), have been synthesized using the preassembled [Fe3O(OOCCH3)6] building block. PCN-600 exhibits a one-dimensional channel as large as 3.1 nm and the highest experimental pore volume of 1.80 cm(3)g(-1) among all the reported porphyrinic MOFs. It also shows very high stability in aqueous solutions with pH values ranging from 2-11 and is to our knowledge the only mesoporous porphyrinic MOF stable under basic aqueous conditions. PCN-600(Fe) has been demonstrated as an effective peroxidase mimic to catalyze the co-oxidation reaction.

A Highly Stable Zeotype Mesoporous Zirconium Metal-Organic Framework with Ultralarge Pores

Through topological rationalization, a zeotype mesoporous Zr-containing metal-organic framework (MOF), namely PCN-777, has been designed and synthesized. PCN-777 exhibits the largest cage size of 3.8 nm and the highest pore volume of 2.8 cm(3)  g(-1) among reported Zr-MOFs. Moreover, PCN-777 shows excellent stability in aqueous environments, which makes it an ideal candidate as a support to incorporate different functional moieties. Through facile internal surface modification, the interaction between PCN-777 and different guests can be varied to realize efficient immobilization.

Symmetry-Guided Synthesis of Highly Porous Metal–Organic Frameworks with Fluorite Topology

Two stable, non-interpenetrated MOFs, PCN-521 and PCN-523, were synthesized by a symmetry-guided strategy. Augmentation of the 4-connected nodes in the fluorite structure with a rigid tetrahedral ligand and substitution of the 8-connected nodes by the Zr/Hf clusters yielded MOFs with large octahedral interstitial cavities. They are the first examples of Zr/Hf MOFs with tetrahedral linkers. PCN-521 has the largest BET surface area (3411 m(2) g(-1)), pore size (20.5×20.5×37.4 Å) and void volume (78.5%) of MOFs formed from tetrahedral ligands. This work not only demonstrates a successful implementation of rational design of MOFs with desired topology, but also provides a systematic way of constructing non-interpenetrated MOFs with high porosity.

Kinetically tuned dimensional augmentation as a versatile synthetic route towards robust metal-organic frameworks.

Metal-organic frameworks with high stability have been pursued for many years due to the sustainability requirement for practical applications. However, researchers have had great difficulty synthesizing chemically ultra-stable, highly porous metal-organic frameworks in the form of crystalline solids, especially as single crystals. Here we present a kinetically tuned dimensional augmentation synthetic route for the preparation of highly crystalline and extremely robust metal-organic frameworks with a preserved metal cluster core. Through this versatile synthetic route, we obtain large single crystals of 34 different iron-containing metal-organic frameworks. Among them, PCN-250(Fe2Co) exhibits high volumetric uptake of hydrogen and methane, and is also stable in water and aqueous solutions with a wide range of pH values.

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 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.

A Reversible Crystallinity-Preserving Phase Transition in Metal–Organic Frameworks: Discovery, Mechanistic Studies, and Potential Applications

A quenching-triggered reversible single-crystal-to-single-crystal (SC-SC) phase transition was discovered in a metal-organic framework (MOF) PCN-526. During the phase transition, the one-dimensional channel of PCN-526 distorts from square to rectangular in shape while maintaining single crystallinity. Although SC-SC transformations have been frequently observed in MOFs, most reports have focused on describing the resulting structural alterations without shedding light on the mechanism for the transformation. Interestingly, modifying the occupancy or species of metal ions in the extra-framework sites, which provides mechanistic insight into the causes for the transformation, can forbid this phase transition. Moreover, as a host scaffold, PCN-526 presents a platform for modulation of the photoluminescence properties by encapsulation of luminescent guest molecules. Through judicious choice of these guest molecules, responsive luminescence caused by SC-SC transformations can be detected, introducing a new strategy for the design of novel luminescent MOF materials.