Liqing Ma

Isoreticular Chiral Metal-Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes

A family of isoreticular chiral metal-organic frameworks (CMOFs) of the primitive cubic network topology was constructed from [Zn(4)(μ(4)-O)(O(2)CR)(6)] secondary building units and systematically elongated dicarboxylate struts that are derived from chiral Mn-Salen catalytic subunits. CMOFs 1-5 were synthesized by directly incorporating three different chiral Mn-Salen struts into the frameworks under solvothermal conditions, and they were characterized by a variety of methods, including single-crystal X-ray diffraction, PXRD, TGA, and (1)H NMR. Although the CMOFs 1 vs 2 and CMOFs 3 vs 4 pairs were constructed from the same building blocks, they exhibit two-fold interpenetrated or non-interpenetrated structures, respectively, depending on the steric sizes of the solvents that were used to grow the MOF crystals. For CMOF-5, only a three-fold interpenetrated structure was obtained due to the extreme length of the Mn-Salen-derived dicarboxylate strut. The open channel and pore sizes of the CMOF series vary systematically, owing to the tunable dicarboxylate struts and controllable interpenetration patterns. CMOFs 1-5 were shown to be highly effective catalysts for asymmetric epoxidation of a variety of unfunctionalized olefins with up to 92% ee. The rates of epoxidation reactions strongly depend on the CMOF open channel sizes, and the catalytic activities of CMOFs 2 and 4 approach that of a homogeneous control catalyst. These results suggest that, although the diffusion of bulky alkene and oxidant reagents can be a rate-limiting factor in MOF-catalyzed asymmetric reactions, the catalytic activity of the CMOFs with large open channels (such as CMOFs 2 and 4 in the present study) is limited by the intrinsic reactivity of the catalytic molecular building blocks. The CMOF catalysts are recyclable and reusable and retain their framework structures after epoxidation reactions. This work highlights the potential of generating highly effective heterogeneous asymmetric catalysts via direct incorporation of well-defined homogeneous catalysts into framework structures of MOFs.

Porous Phosphorescent Coordination Polymers for Oxygen Sensing

Phosphorescent cyclometalated iridium tris(2-phenylpyridine) derivatives were designed and incorporated into coordination polymers as tricarboxylate bridging ligands. Three different crystalline coordination polymers were synthesized using a solvothermal technique and were characterized using a variety of methods, including single-crystal X-ray diffraction, PXRD, TGA, IR spectroscopy, gas adsorption measurements, and luminescence measurements. The coordination polymer built from Ir[3-(2-pyridyl)benzoate](3), 1, was found to be highly porous with a nitrogen BET surface area of 764 m(2)/g, whereas the coordination polymers built from Ir[4-(2-pyridyl)benzoate](3), 2 and 3, were nonporous. The (3)MLCT phosphorescence of each of the three coordination polymers was quenched in the presence of O(2). However, only 1 showed quick and reversible luminescence quenching by oxygen, whereas 2 and 3 exhibited gradual and irreversible luminescence quenching by oxygen. The high permanent porosity of 1 allows for rapid diffusion of oxygen through the open channels, leading to efficient and reversible quenching of the (3)MLCT phosphorescence. This work highlights the opportunity of designing highly porous and luminescent coordination polymers for sensing other important analytes.

Chirality-Controlled and Solvent-Templated Catenation Isomerism in Metal−Organic Frameworks

A family of highly porous homochiral, racemic, and meso metal-organic frameworks (MOFs) were synthesized based on a new elongated tetra-carboxylate ligand and the copper paddle-wheel building units. These MOFs exhibited remarkable catenation isomerism that is controlled by both chirality of the bridging ligand and the size of solvent molecules. The ability to manipulate framework interpenetration is key to future synthesis of mesoporous homochiral MOFs which hold great promise in heterogeneous asymmetric catalysis and chiral separations.

Iodinated Nanoscale Coordination Polymers as Potential Contrast Agents for Computed Tomography

Computed tomography (CT) is a powerful diagnostic tool based on X-ray attenuation by a specimen, and is capable of providing three-dimensional (3D) images with excellent spatial resolution. A contrast agent with high X-ray attenuation, typically materials containing elements with a high Z number such as iodine, barium, and bismuth, is often used in CT imaging to provide better contrast between the tissue of interest and its surroundings. The only CT contrast agents currently approved for clinical use are iodinated aromatic molecules, and barium sulfate for gastrointestinal tract imaging. CT imaging with small-molecule contrast agents is limited by their nonspecific distribution, rapid renal clearance, and rapid extravasation from blood and lymphatic vessels. Large doses of small-molecule agents (tens of grams) are typically needed to provide adequate contrast, which sometimes cause adverse reactions for the patients. These limitations can be overcome by nanoparticulate contrast agents that can carry a high payload and be functionalized to increase blood circulation times and to endow target specificity. Nanoparticles do not readily diffuse into extravascular space or undergo rapid renal clearance, thus allowing adequate time for accumulation at a disease site. These advantages could allow for a larger time window for imaging and enhanced image contrast at a lower dose. Several nanoparticle systems including Bi2S3, [4] gold, and iodinated organic nanoparticles, have recently been evaluated as next-generation CT contrast agents. However, it is challenging to formulate nanoparticles with high loadings for elements having high Z numbers that are also nontoxic and able to be cleared from the body in a timely fashion. Coordination polymers have recently emerged as interesting functional materials that are readily synthesized by coordination-directed self-assembly of metal ions and organic bridging ligands. Our research group and others have recently demonstrated the synthesis of a new class of nanomaterials by scaling down coordination polymers to the nanoregime. These nanoscale coordination polymers (NCPs) have already shown great potential in biosensing, magnetic resonance imaging, and drug delivery. Given the clinical utility of iodinated aromatic molecules in CT imaging, we surmised that iodinated NCPs could have potential applications as CT contrast agents owing to their ability to carry a very high payload of iodine. Herein we report the synthesis of new iodinated coordination polymers and their scale down to the nano-regime. We also demonstrate the ability of iodinated NCPs to attenuate X-rays in phantom studies. As shown in Scheme 1, five new coordination polymers were synthesized using 2,3,5,6-tetraiodo-1,4-benzenedicarboxylic acid (I4-BDC-H2) bridging ligands and Cu II or Zn

Highly porous and robust 4,8-connected metal-organic frameworks for hydrogen storage.

Highly porous and robust metal-organic frameworks (MOFs) were constructed based on aromatics-rich octa-carboxylate ligands and copper paddle-wheel building units. Each octa-carboxylate ligand is linked to eight copper paddle wheels via the bridging carboxylate groups in a rectangular prismatic fashion to lead to very rare (4,8)-connected networks of the scu topology. The high-connectivity MOFs show remarkably high porosity and framework stability, as evidenced by a perfect agreement between experimental and theoretical surface areas and the maintenance of framework powder X-ray diffraction patterns after solvent removal. These aromatics-rich MOFs exhibit an exceptionally high hydrogen uptake of up to 2.5 wt% at 77 K and 1 atm. This work thus demonstrates the ability to construct highly porous and robust functional MOFs using multidentate bridging ligands of high connectivity. Such a rational synthetic strategy is complementary to the common reliance on high-nuclearity metal clusters for building stable and porous MOFs.

Single-Crystal to Single-Crystal Cross-Linking of an Interpenetrating Chiral Metal–Organic Framework and Implications in Asymmetric Catalysis†

Metal–organic frameworks (MOFs) have received extensive interest in the past decade because of their interesting properties. MOFs exhibit exceptional gas-uptake capacity owing to their extreme porosity. The ability to incorporate desired functional groups allows MOFs to perform in a variety of applications, such as catalysis, chemical sensing, and drug delivery. In particular, MOFs represent ideal candidates as heterogeneous catalysts by simultaneously imparting porosity and introducing catalytic sites into MOFs. Unlike other catalyst heterogenization strategies, the MOF-derived heterogeneous catalysts can remain singlecrystalline, thus providing a unique opportunity for detailed structural interrogation and therefore delineating the relationships between the MOF structures and their catalytic activities and selectivities. Moderate hydrolytic and thermal stabilities of most MOFs limit the scope of reactions that can be heterogeneously catalyzed by MOFs. We have recently focused our efforts on the design of chiral porous MOFs for heterogeneous asymmetric catalysis as most asymmetric catalytic reactions are carried out under mild conditions in aprotic solvents. In order for chiral MOFs to be useful asymmetric catalysts, they must possess large nanometer-scale open channels for the facile transport of sterically demanding substrates and products. We have recently synthesized isoreticular chiral MOFs based on tetracarboxylate bridging ligands derived from 1,1’-bi-2-naphthol (BINOL) and copper paddle-wheel secondary building units (SBUs), and observed the remarkable dependence of the enantioselectivities of the addition of Et2Zn to aromatic aldehydes on the MOF openchannel sizes. In order to expand the scope of applications of such chiral tetracarboxylate ligands, and to further understand the relationships between framework structures and catalytic activities, we have used these ligands in combination with other metal-connecting points or metal-cluster SBUs. Herein we report the synthesis and characterization of two interpenetrating chiral MOFs [Zn2(L)(dmf)(H2O)]·2EtOH·4.3DMF·H2O (1, where L is (R)-2,2’-diethoxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate) and [Zn2(L’)(dmf)(H2O)]·2EtOH·4.3DMF·4 H2O (2, L’= (R)-2,2’-dihydroxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate), which are constructed from dizinc SBUs and chiral tetracarboxylate ligands. More importantly, we observed unprecedented single-crystal to single-crystal crosslinking of the two interpenetrating networks in 2 by Ti(OiPr)4 to lead to intermolecular [Ti(BINOLate)2] complexes that exhibit modest enantioselectivity in catalyzing the addition of diethylzinc to aromatic aldehydes to afford chiral secondary alcohols. This result provides unambiguous structural identification of an immobilized homogeneous catalyst and has significant implications in rational design of MOF-based heterogeneous asymmetric catalysts. MOF 1 was synthesized by heating a mixture of ZnI2 and (R)-H4L in DMF/EtOH at 90 8C for five days, while MOF 2 was produced by heating ZnI2 and (R)-H4L’ in DMF/EtOH at 100 8C for one week (Scheme 1). The formulae of 1 and 2 were established by single-crystal X-ray diffraction studies, NMR analysis, and thermogravimetric analysis (TGA).

Unusual interlocking and interpenetration lead to highly porous and robust metal-organic frameworks.

Stop the breathing: As the organic bridging ligands become more elaborate and large, the resulting MOFs tend to experience significant framework distortion (i.e., breathing) upon solvent removal. Rigidification of MOFs that are built from elongated tetracarboxylate bridging ligands by unusual interlocking and interpenetration leads to highly porous and robust hybrid materials.

Designing Metal-Organic Frameworks for Catalytic Applications

Metal-organic frameworks (MOFs) are constructed by linking organic bridging ligands with metal-connecting points to form infinite network structures. Fine tuning the porosities of and functionalities within MOFs through judicious choices of bridging ligands and metal centers has allowed their use as efficient heterogeneous catalysts. This chapter reviews recent developments in designing porous MOFs for a variety of catalytic reactions. Following a brief introduction to MOFs and a comparison between porous MOFs and zeolites, we categorize catalytically active achiral MOFs based on the types of catalytic sites and organic transformations. The unsaturated metal-connecting points in MOFs can act as catalytic sites, so can the functional groups that are built into the framework of a porous MOF. Noble metal nanoparticles can also be entrapped inside porous MOFs for catalytic reactions. Furthermore, the channels of porous MOFs have been used as reaction hosts for photochemical and polymerization reactions. We also summarize the latest results of heterogeneous asymmetric catalysis using homochiral MOFs. Three distinct strategies have been utilized to develop homochiral MOFs for catalyzing enantioselective reactions, namely the synthesis of homochiral MOFs from achiral building blocks by seeding or by statistically manipulating the crystal growth, directing achiral ligands to form homochiral MOFs in chiral environments, and incorporating chiral linker ligands with functionalized groups. The applications of homochiral MOFs in several heterogeneous asymmetric catalytic reactions are also discussed. The ability to synthesize value-added chiral molecules using homochiral MOF catalysts differentiates them from traditional zeolite catalysis, and we believe that in the future many more homochiral MOFs will be designed for catalyzing numerous asymmetric organic transformations.

3D Metal-Organic Frameworks Based on Elongated Tetracarboxylate Building Blocks for Hydrogen Storage

Two 3D metal-organic frameworks (MOFs) with a new biphenol-derived tetracarboxylate linker and Cu(II) and Zn(II) metal-connecting points were synthesized and characterized by single-crystal X-ray crystallographic studies. The two isostructural MOFs exhibit distorted PtS network topology and show markedly different framework stability. The porosity and hydrogen uptake of the frameworks were determined by gas adsorption experiments.

Three-Dimensional Metal−Organic Frameworks Based on Tetrahedral and Square-Planar Building Blocks: Hydrogen Sorption and Dye Uptake Studies

Two three-dimensional metal-organic frameworks (MOFs) with tetraphenylmethane-derived tetracarboxylate linkers and copper paddle-wheel secondary building units were synthesized and characterized by single-crystal X-ray crystallographic studies. The two MOFs show a similar 4,4-connectivity but very different homocrossing and interpenetrating PtS network topologies and exhibit permanent porosity as probed by gas adsorption and dye inclusion experiments.