Daqiang Yuan

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 laboratorys 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 Stable Porous Polymer Networks with Exceptionally High Gas‐Uptake Capacities

Figure 1 . a) Synthetic route for PPN-3 (X: Adamantane), PPN-4 (X: Si), PPN-5 (X: Ge), and PAF-1 (X: C). b) The default noninterpenetrated diamondoid network of PPN-4 (black, C; pale grey, H; grey, Si). For several decades, porous materials have been exploited for their applications in gas storage. [ 1 ] Recently, several metal–organic frameworks (MOFs) with high surface area and gas-uptake capacity have been reported. [ 2 ]

Polyamine‐Tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas

One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO2, which is largely correlated to the combustion of fossil fuels. For the foreseeable future, however, it seems that the ever-growing energy demand will most likely necessitate the consumption of these indispensable sources of energy. Carbon capture and sequestration (CCS), a process to separate CO2 from the flue gas of coal-fired power plants and then store it underground, has been proposed to reduce the anthropogenic CO2 emissions. Current CO2 capture processes employed in power plants worldwide are post-combustion “wet scrubbing” methods involving the chemical adsorption of CO2 by amine solutions such as monoethanolamine (MEA). The formation of carbamate from two MEA molecules and one CO2 molecule endows the scrubber with a high capacity and selectivity for CO2. However, this process suffers from a series of inherent problems, such as high regeneration costs that arise from heating the solution (ca. 30 % of the power produced by the plant), fouling of the equipment, and solvent boil-off. [1]

Sulfonate-Grafted Porous Polymer Networks for Preferential CO2 Adsorption at Low Pressure

A porous polymer network (PPN) grafted with sulfonic acid (PPN-6-SO(3)H) and its lithium salt (PPN-6-SO(3)Li) exhibit significant increases in isosteric heats of CO(2) adsorption and CO(2)-uptake capacities. IAST calculations using single-component-isotherm data and a 15/85 CO(2)/N(2) ratio at 295 K and 1 bar revealed that the sulfonate-grafted PPN-6 networks show exceptionally high adsorption selectivity for CO(2) over N(2) (155 and 414 for PPN-6-SO(3)H and PPN-6-SO(3)Li, respectively). Since these PPNs also possess ultrahigh physicochemical stability, practical applications in postcombustion capture of CO(2) lie well within the realm of possibility.

The current status of hydrogen storage in metal–organic frameworks

The theoretical and experimental hydrogen storage studies on metal–organic frameworks (MOFs) have been reviewed. Seven distinct factors influencing hydrogen uptake capacity in MOFs have been classified and discussed. Based on existing studies, some possible future developments have been proposed.

The current status of hydrogen storage in metal–organic frameworks—updated

Hydrogen storage in metal–organic frameworks (MOFs), or porous coordination polymers, has been extensively investigated in the last two years and this review is to serve as an up to date account of the recent progress. The effects of MOF sample preparation and activation, functionalization (including post-synthetic), catenation, unsaturated metal sites, metal doping and spillover have been discussed using recent examples. In addition to a condensed reference table of all recently reported MOFs for hydrogen storage, future directions are discussed based on promising new materials and reported computational analyses.

Stabilization of metal-organic frameworks with high surface areas by the incorporation of mesocavities with microwindows.

Reactions between two novel hexatopic carboxylate ligands and copper salts give rise to two isostructural metal-organic frameworks (MOFs) with a (3,24)-connected network topology containing both micro- and mesocavities with sizes of up to 3 nm. Both frameworks retain their porosity after guest-molecule removal, leading to high surface areas. Constructing MOFs containing mesocavities with microwindows may serve as a general approach toward stable MOFs with high surface areas.

Enhancing H2 Uptake by “Close-Packing” Alignment of Open Copper Sites in Metal–Organic Frameworks†

Inspired by close-packing of spheres, to strengthen the framework-H{sub 2} interaction in MOFs (metal-organic frameworks), a strategy is devised to increase the number of nearest neighboring open metal sites ofe ach H{sub 2}-hosting cage, and to align the open metal sites toward the H{sub 2} molecules. Two MOF polymorphs were made, one exhibiting a record high hydrogen uptake of 3.0 wt% at 1 bar and 77 k.

Surface Functionalization of Porous Coordination Nanocages Via Click Chemistry and Their Application in Drug Delivery

The discrete coordination-driven self assemblies have received continuous attention due to their molecular architecture esthetics and applications in recognition, catalysis, storage, etc. [ 1 ] Among these self assemblies, one species that has emerged recently is the porous coordination nanocages formed between carboxylate ligands and metal clusters, which are also known as metal-organic polyhedra (MOP). [ 2 ] Due to the robust porous structure and versatile functionality, they have found applications as plasticizer, gas sponge, ion channel, coatings, and building units. [ 3 ] Presumably, the porous shell and uniform yet tunable cavity make them good candidates for drug delivery purpose. However, almost all the coordination nanocages reported so far are hydrophobic, which greatly limits their applications in aqueous condition. We hypothesize this problem can be circumvented by turning these nanocages into colloids through surface functionalization with hydrophilic polymers. In this Communication, we report a porous coordination nanocage covered with alkyne groups and its surface functionalization by grafting with azide-terminated polyethylene glycol (PEG) through “click chemistry”. In addition, its drug load and release capacity has been evaluated using an anticancer drug 5-fl uorouracil as a model. The metal-organic cuboctahedron was chosen as the prototype of nanocage in this study. [ 2a , 2c ] It is composed of 12 dicopper paddlewheel clusters and 24 isophthalate moieties, with 8 triangular and 6 square windows that are roughly 8 and 12 Å across, respectively. The internal cavity has a diameter of around 15 Å. The 5-position of isophthalate moieties would be the reaction site for surface functionalization. The Cu(I)catalyzed Huisgen cycloaddition between azide and alkyne, a so-called “click reaction”, was chosen as the synthetic tool in

A Coordinatively Linked Yb Metal–Organic Framework Demonstrates High Thermal Stability and Uncommon Gas‐Adsorption Selectivity

Porous metal–organic frameworks (MOFs), which have emerged as new zeolite analogues, have attracted considerable research interest in the past decade as, compared to traditional zeolites, they possess a high surface area, modifiable surface, and tunable pore size. These characteristics have led to an enormous application potential for MOFs in catalysis, gas storage, and adsorptive separation. One of the main concerns regarding porous MOFs is their limited thermal stability, which prevents them from competing with inorganic zeolites in practical applications. Most porous MOFs can only be heated up to 150–350 8C without losing their framework integrity. Interpenetration, which often arises from weak interactions, has been widely used to improve the thermal stability of porous MOFs, and interpenetrated porous MOFs that are stable up to 400 8C have been reported. Interpenetration increases the wall thickness and reduces the pore size of an MOF, both of which lead to enhanced thermal stability. If two interpenetrated frameworks can be linked through coordinative bonds, the thermal stability should be boosted still further (Scheme 1). Herein we report such a coordinatively linked, doubly interpenetrated Yb MOF with improved thermal stability (up to 500 8C) and uncommon gas-adsorption selectivity. We have previously reported a cobalt-based porous MOF with doubly interpenetrated, (8,3)-connected nets (PCN-9; PCN: porous coordination network). PCN-9 contains a square-planar Co4(m4-O) secondary building unit (SBU) where each Co center is five-coordinate with a coordination site open toward the channel. As a consequence of this interpenetration, PCN-9 is thermally stable up to 400 8C (by thermogravimetric analysis (TGA)). If the interpenetrated, (8,3)-connected nets can be linked at the open metal sites by a bridging ligand, the thermal stability of the resulting MOF should be still higher. A short bridge is the best candidate due to the proximity of the two nets, and we chose SO4 2 as the bridging ligand because it can chelate the two metal centers and stabilize the MOF further. In addition, it can be generated slowly under solvothermal conditions through decomposition of DMSO (dimethyl sulfoxide), thereby facilitating the formation of the coordinatively linked interpenetrated MOF. Initial attempts to use sulfates to bridge the doubly interpenetrated, (8,3)-connected nets in PCN-9 failed. There are two possible reasons for this failure: the limited coordination number (maximum of six) of the cobalt center and the need for additional counterions to balance the overall charge. The coordination number of the metal center can be increased by using Ln cations instead of Co and no additional counterions will be needed in this case to balance the overall charge. With these considerations in mind, a ytterbium MOF with coordinatively linked, doubly interpenetrated, (8,3)-connected nets (PCN-17) was synthesized. Studies of similar MOFs containing other lanthanides are currently underway and will be reported in due course elsewhere. PCN-17 is stable up to 480 8C and exhibits selective adsorption of H2 and O2 over N2 and CO. Crystals of PCN-17 were obtained upon heating a mixture of H3TATB (TATB= 4,4’,4’’-S-triazine-2,4,6-triyl tribenzoate) and ytterbium nitrate in DMSO at 145 8C for 72 hours. The formula of PCN-17 (Yb4(m4-H2O)(C24H12N3O6)8/3(SO4)2·3H2O·10DMSO) was determined by X-ray crystallography, elemental analysis, and thermogravimetric analysis (TGA). X-ray structural analysis revealed that PCN-17 crystallizes in the space group Im3m. As expected, it adopts a square-planar Yb4(m4H2O) SBU, with the m4-H2O molecule, which is probably disordered over two or more orientations (see below), residing at the center of a square of four Yb atoms (Figure 1a). The four Yb atoms in the SBU lie in the same plane and each coordinates to seven O atoms (four from four carboxylate groups of four different TATBs, two from the bridging sulfate generated in situ, and one from the m4H2O). The Yb···m4-H2O distance (2.70 D) indicates very weak Scheme 1. a) A single net. b) Two doubly interpenetrated nets. c) Interpenetrated nets linked by a coordinative bond. The vertical gold dotted line represents a p–p interaction; the blue solid line represents coordinative bonding.