Pillararenes, a new class of macrocycles for supramolecular chemistry

Because of the importance of novel macrocycles in supramolecular science, interest in the preparation of these substances has grown considerably. However, the discovery of a new class of macrocycles presents challenges because of the need for routes to further functionalization of these molecules and good host-guest complexation. Furthermore, useful macrocylic hosts must be easily synthesized in large quantities. With these issues in mind, the recently discovered pillararenes attracted our attention. These macrocycles contain hydroquinone units linked by methylene bridges at para positions. Although the composition of pillararenes is similar to that of calixarenes, they have different structural characteristics. One conformationally stable member of this family is pillar[5]arene, which consists of five hydroquinone units. The symmetrical pillar architecture and electron-donating cavities of these macrocycles are particularly intriguing and afford them with some special and interesting physical, chemical, and host-guest properties. Due to these features and their easy accessibility, pillararenes, especially pillar[5]arenes, have been actively studied and rapidly developed within the last 4 years. In this Account, we provide a comprehensive overview of pillararene chemistry, summarizing our results along with related studies from other researchers. We describe strategies for the synthesis, isomerization, and functionalization of pillararenes. We also discuss their macrocyclic cavity sizes, their host-guest properties, and their self-assembly into supramolecular polymers. The hydroxyl groups of the pillararenes can be modified at all positions or selectively on one or two positions. Through a variety of functionalizations, researchers have developed many pillararene derivatives that exhibit very interesting host-guest properties both in organic solvents and in aqueous media. Guest molecules include electron acceptors such as viologen derivatives and (bis)imidazolium cations and alkyl chain derivatives such as n-hexane, alkanediamines, n-octyltrimethyl ammonium, and neutral bis(imidazole) derivatives. These host-guest studies have led to the fabrication of (pseudo)rotaxanes or poly(pseudo)rotaxanes, supramolecular dimers or polymers, artificial transmembrane proton channels, fluorescent sensors, and other functional materials.     

Nitrogen and oxygen bridged calixaromatics: synthesis, structure, functionalization, and molecular recognition

Pedersen, Lehn, and Cram established supramolecular chemistry through their pioneering work with crown ethers, cryptands, and spherands. Since then, the hallmark of supramolecular science has been an increasing sophistication in the design and construction of macrocyclic molecules, as manifested in cyclodextrin derivatives, calixarenes, resorcinarenes, cyclotriveratrylenes, cucurbiturils, calixpyrroles, cyclophanes, and many other examples. Indeed, macrocyclic compounds provide unique models for the study of noncovalent molecular interactions. They also constitute building blocks for constructing high-level molecular and supramolecular architectures and fabricating molecular devices and advanced materials. As a postgraduate in the Huang laboratory in the late 1980s, I became interested in the calix[n]arenes because of their unique conformational structures and versatile complexation properties. The notion of introducing heteroatoms, and particularly nitrogen, into the bridging position of conventional calixarenes was particularly intriguing. Nitrogen, unlike methylene, can adopt either an sp(2) or sp(3) electronic configuration, providing different conjugation systems with adjacent aromatic rings. Consequently, depending on the configuration and conjugation, a range of C-N bond lengths and C-N-C bond angles is possible. The conformation and cavity size in heteroatom-bridged calixarenes might thus be tuned through the bridging heteroatoms and the number of aromatic rings. Furthermore, because heteroatom linkages significantly affect the electron density and distribution on aromatic rings, the electronic features of macrocyclic cavities might be regulated by heteroatoms. Given the essentially limitless combinations possible, only synthetic hurdles would prevent access to numerous diverse heteracalixaromatics. We began a systematic study on nitrogen- and oxygen-bridged calixarenes in 2000, years later than originally envisioned. Before this study, very few heteracalixaromatics had been reported, owing to the formidable synthetic challenges involved. Apart from thiacalixarene, the synthesis of nitrogen- and oxygen-bridged calixarenes appeared very difficult. But since our first publications in 2004, we have been delighted to see the rapid and tremendous development of the supramolecular chemistry of this new generation of macrocycles. In this Account, I summarize the synthesis of N- and O-bridged calixaromatics and their regiospecific functionalization on the rims and bridging positions, focusing on the fragment coupling approach and contributions from our laboratory. I describe the construction of molecular cages based on heteracalixaromatics and discuss the effect of both bridging heteroatoms and substituents on macrocyclic conformations and cavity sizes. Molecular recognition of neutral organic molecules and charged guest species is also demonstrated. The easy accessibility, rich molecular diversity, unique conformation, and cavity tunability of heteracalixaromatics make them invaluable macrocycles for research in supramolecular chemistry. New heteracalixaromatics, with well-defined conformations and cavity properties, will provide powerful tools for probing noncovalent interactions, leading to the development of new molecular sensing and imaging systems. Multicomponent molecular self-assembly of heteracalixaromatics as functional modules with metals, metal clusters, or charge-neutral species should result in multidimensional solid and soft materials with diverse applications. The profitable incorporation of heteracalixaromatics into molecular devices can also be anticipated in the future. Moreover, the construction of enantiopure, inherently chiral heteracalixaromatics should provide important applications in chiral recognition and asymmetric catalysis.     

Molecular Recognition of Water-soluble Pillar[n]arenes Towards Biomolecules and Drugs

The exploration of molecular recognition of synthetic macrocycles in aqueous media is significantly important since most biological functions, activities and processes take place in water. Pillar[n]arenes (n=5–10) have been recognized as a new generation of macrocyclic arenes; much attention has been devoted to their host-guest chemistry. This review summarizes the host-guest properties of water-soluble pillararenes towards biologically significant molecules and drugs. These water-soluble pillararenes are classified into three major classes according to the electric charges of hydrophilic groups at their portals: anionic pillar[n]arenes ( AP[n]s ), cationic pillar[n]arenes ( CP[n]s ), and neutral pillar[n]arenes ( NP[n]s ). The host–guest affinities, binding modes, binding selectivity, stimuli-responsiveness, as well as biomedical applications are comprehensively discussed.     

新型环糊精衍生物的制备及应用

简要介绍了环糊精化学的产生、发展、性能、应用及结构特征。详细介绍了:新型环糊精衍生物的制备及在医药学中的应用;环糊精超分子作用力的自愈合杂化功能材料的制备及应用;环糊精自组装及主客体识别在分析分离科学中的应用。并对环糊精化学的发展进行了展望。     

金属有机框架材料的绿色合成

金属有机框架化合物（Metal-organic frameworks,简称MOFs）是由金属离子（或簇）与有机配体配位并经由自组装而形成的一类多孔材料^[1]。MOFs具有极其发达的孔道结构,比表面积和孔容远超其他多孔材料。有机/无机杂化这一特点也赋予了MOFs其他材料（例如沸石、活性炭等）所不具备的无限结构功能可调性^[2]。此外,MOFs具有移除客体分子而主体框架完好保持的持久孔道或孔穴,这使得MOFs具有超乎寻常的化学及物理稳定性。正是基于以上这些特点,MOFs在许多领域有着丰富的应用^[3-4],例如催化^[5]、H₂储存^[6]、CO₂捕集^[7]、药物运输^[8]、污染物吸附^[9]、生物医学成像^[10]等方面。MOFs的商业化探索成为了目前的热点。MOFs的很多应用都与可持续发展及“绿色材料”有关,但MOFs本身的合成过程也需要考虑可持续性和环境影响。金属有机化学所面临的环境挑战是独特的,因为它将金属离子、有机配体的危害联系在一起,且合成过程大多需要大量能耗。主要介绍了金属有机框架材料的绿色可持续合成,主要分为4个方面：1）使用更安全或生物相容性的配体;2）使用更绿色、低成本的金属源;3）绿色溶剂的开发;4）无溶剂合成法。     

Hemicucurbit[n]urils and Their Derivatives – Synthesis and Applications

The first hemicucurbit[n]uril macrocycles were described in 2004, and since then a series of functionalised hemicucurbit[n]uril derivatives have been prepared and studied. The original hemicucurbit[n]urils suffer from sparse solubility, which limits their applications in supramolecular chemistry. Recent progress in the synthesis of a range of derivatives such as the biotin[n]urils, the bambus[n]urils, and the cyclohexylhemicucurbit[n]urils have enabled studies of the supramolecular chemistry of the hemicucurbituril system, both in water and in a range of organic solvents. In contrast to the popular cucurbit[n]uril macrocycles that are state-of-the-art hosts for cations, the hemicucbit[n]urils host anions. Herein we provide a survey of the literature covering the syntheses of the hemicucurbituril derivatives and their supramolecular chemistry.     

Gold-Nanoparticle-Decorated Metal-Organic Frameworks for Anticancer Therapy

Confinement of Au nanoparticles (NPs) within the porous materials with few nanometers (2-3 nm) has been a well established research area in the past decades in heterogeneous catalysis mainly due to the unique behaviour of Au NPs than its bulk counterpart. In this aspect, Au NPs encapsulated within the pore volumes of metal−organic frameworks (MOFs) have been intensively explored as heterogeneous solid catalysts for wide range of reactions. In recent years, Au NPs confined within the porous MOFs along with the photosensitizer or drug have been effectively used for the treatment of tumor cells through the generation of reactive oxygen species via cascade reactions. This work highlights the benefits of MOFs pores in the preparation of nanomedicine with high efficiency by assembling Au NPs, photosensitizer/drug with the combination of laser either for imaging or treatment of tumor cells. Further, the existing literature is grouped based on the nature of porous materials employed in the preparation of nanomedicine. The final section comments on our view on future developments in the field.     

Modification of porous suspension‐PVC particles by stabilizer‐free aqueous dispersion polymerization of absorbed monomers

The modification of porous PVC particles by an in‐situ stabilizer‐free polymerization/crosslinking of a monomer/crosslinker/peroxide solution absorbed within the PVC particles is presented. The modifying crosslinked polymers are polystyrene (PS) crosslinked with DVB (divinyl benzene), polymethyl methacrylate (PMMA) crosslinked with ethylene glycol dimethacrylate (EGDMA), and styrene‐MMA copolymer crosslinked with DVB. The modified PVC particles characterization includes polymerization yield, non‐extractables, ¹³C solid‐state CPMAS NMR, porosity measurements and also morphology and dynamic mechanical behavior (DMTA). The levels of nonextractable fractions found and ¹³C solid‐state CPMAS NMR results are indicative of low chemical interaction in the semi‐IPN PVC particles. Particle porosity levels and SEM observations indicate that styrene and MMA mainly polymerize within the PVC particles' bulk and just small amounts in the pores. MMA polymerization in the PVC pores is as crusts covering the PVC pore surfaces, whereas styrene polymerization in the PVC pores is by filling the pores. Dynamic mechanical studies show that tanδ and the storage modulus curves are influenced by the incorporation of PS and XPS but not by the incorporation of PMMA and XPMMA.     

Fullerene-porphyrin constructs

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element can be used to construct discrete host-guest complexes, as well as ordered arrays of interleaved porphyrins and fullerenes. The fullerene-porphyrin interaction underlies successful chromatographic separations of fullerenes, and there are promising applications in the areas of porous framework solids and photovoltaic devices.     

Electroactive Metalorganic Frameworks

Crystalline and porous framework materials with appreciable charge carrier mobility hold promise as a new semiconducting platform. Through the concepts of reticular chemistry, numerous two- and three-dimensional crystalline and porous frameworks with encoded properties enabling significant electrical conductivity are in reach. Prominent members of this group are the metal-organic frameworks (MOFs), which recently created scientific excitement as highly conductive materials. Herein, we discuss the recent prominent examples reported for electrically conducting MOFs with regards to their synthesis, structural and electronic properties and possible applications. We frame our discussion according to structural features of the MOFs, covering both layered 2D and 3D frameworks.     

新型超分子化合物的合成分子识别自组装及应用新进展

简要介绍了超分子化学的产生发展及应用。详细介绍了:超分子配体的合成及其应用;新型超分子配体的合成分子识别及应用;超分子金属配合物的合成及催化作用。并对超分子化学的发展进行了展望。     

Pillar[n]arenes for Construction of Artificial Transmembrane Channels

Pillar[n]arenes have been intensively used during the last decade in supramolecular chemistry as host systems for guest recognition and self-assembly, toward functional materials and devices. They present electron-rich and tuneable size cavities, as well as reactive rims for further functionalization. Their unique pillar shape promotes pillar[n]arenes as molecular analogues of carbon nanotubes. For this reasons pillar[n]arenes have been embedded into membranes and used to construct artificial water, proton, ions and molecular channels. This review discusses the incipient developments of the first artificial channels based on pillar[n]arenes. We include also systems that integrate pillar[n]arenes as synthetic elements for the construction of selective pores of nanodevices for ion rectification.     

金属有机骨架基复合相变储热材料研究进展

固-液相变材料的品种多且潜热大,是潜热储热技术的重要工作介质。因其存在的液相泄漏问题,现阶段常将此类相变材料与多孔载体复合以提升相变材料的应用性能及使用寿命。金属有机骨架（MOFs）是一种新型多孔材料,具有高比表面积、高孔隙率以及孔径和表面性质可调控等优势,将其用作相变材料的载体具有潜在的发展前景。本文对MOF基复合相变材料的研究进行了全面综述,详细介绍了以MOFs为载体、以MOFs衍生多孔碳为载体和以MOFs原位生长于高导热基体所得复合材料为载体而制得的多种复合相变材料。MOFs的微孔结构所产生的强毛细管力对固-液相变材料有很强的固定作用;制备较大孔径的MOFs或者对MOFs进行修饰以调节MOF与相变材料间的相互作用,都有利于提高相变材料的负载率,从而提升复合相变材料的潜热;对MOFs进行高温碳化处理得到MOFs衍生多孔碳能有效解决MOFs孔径过小的问题,并能通过对其进行氮掺杂或磷掺杂来增强载体与相变材料间的氢键作用,从而获得具有高负载率和相变潜热的复合相变材料;为了增强MOF基复合相变材料的导热性能,先将MOFs原位合成在高导热基体上以利用高导热基体提供连续的传热网络,可以有效提升复合相变材料的导热系数。将原位生长在高导热基体上的MOFs进行高温碳化处理可以得到MOFs衍生多孔碳与高导热基体的复合材料,将其作为载体可以进一步增强复合相变材料的导热性能。文中最后指出,今后对于MOF基复合相变储热材料所用MOFs和相变材料的种类、MOFs与相变材料间相互作用对储热性能的影响、MOFs与相变材料复合后的稳定性等方面还需进一步探索,将MOFs的催化、检测等功能与相变材料的储热控温功能相结合制备多功能材料也是未来的发展方向之一。     

基于超分子化学方法制备自愈合聚合物材料的研究进展

本文综述了超分子化学方法制备自愈合聚合物材料的研究进展,着重介绍了利用氢键、π-π键、离子键等非共价键主-客体相互作用来制备自愈合聚合物材料的研究现状。     

用于燃烧后CO2捕集系统的胺基固态吸附材料研究进展

随着工业化的发展和大量化石燃料的消耗,大量的CO₂气体排放到大气中并引发了一系列严重的环境问题,而采用燃烧后CO₂捕集技术可以有效地应对这一问题。寻找一种高效吸附、稳定、价格低廉的固态吸附材料对于开展燃烧后CO₂捕集系统的研究具有重要的实际意义。近年来,胺基固态吸附材料因其高CO₂吸附能力和高吸附选择性成为研究的热点。本文综述了近年来国内外学者对不同胺基固态吸附材料在合成方法、载体材料选择以及性能测试等方面进行的研究,重点讨论了以沸石分子筛、介孔硅分子筛、多孔碳和金属有机骨架为载体的胺基固态吸附材料对CO₂的吸附行为,并指出多孔载体材料的结构改进及有机胺和促进剂的合理选择将会成为未来胺基固态吸附材料的重点研究方向。     

Supramolecular control of reactivity in the solid state: from templates to ladderanes to metal-organic frameworks

We describe how reactivity can be controlled in the solid state using molecules and self-assembled metal-organic complexes as templates. Being able to control reactivity in the solid state bears relevance to synthetic chemistry and materials science. The former offers a promise to synthesize molecules that may be impossible to realize from the liquid phase while also taking advantage of the benefits of conducting highly stereocontrolled reactions in a solvent-free environment (i.e., green chemistry). The latter provides an opportunity to modify bulk physical properties of solids (e.g., optical properties) through changes to molecular structure that result from a solid-state reaction. Reactions in the solid state have been difficult to control owing to frustrating effects of molecular close packing. The high degree of order provided by the solid state also means that the templates can be developed to determine how principles of supramolecular chemistry can be generally employed to form covalent bonds. The paradigm of synthetic chemistry employed by Nature is based on integrating noncovalent and covalent bonds. The templates assemble olefins via either hydrogen bond or coordination-driven self-assembly for intermolecular [2 + 2] photodimerizations. The olefins are assembled within discrete, or finite, self-assembled complexes, which effectively decouples chemical reactivity from effects of crystal packing. The control of the solid-state assembly process affords the supramolecular construction of targets in the form of cyclophanes and ladderanes. The targets form stereospecifically, in quantitative yield, and in gram amounts. Both [3]- and [5]-ladderanes have been synthesized. The ladderanes are comparable to natural ladderane lipids, which are a new and exciting class of natural products recently discovered in anaerobic marine bacteria. The organic templates function as either hydrogen bond donors or hydrogen bond acceptors. The donors and acceptors generate cyclobutanes lined with pyridyl and carboxylic acid groups, respectively. The metal-organic templates are based on Zn(II) and Ag(I) ions. The reactivity involving Zn(II) ions is shown to affect optical properties in the form of solid-state fluorescence. The solids based on both the organic and metal-organic templates undergo rare single-crystal-to-single-crystal reactions. We also demonstrate how the cyclobutanes obtained from this method can be applied as novel polytopic ligands of metallosupramolecular assemblies (e.g., self-assembled capsules) and materials (e.g., metal-organic frameworks). Sonochemistry is also used to generate nanostructured single crystals of the multicomponent solids or cocrystals based on the organic templates. Collectively, our observations suggest that the organic solid state can be integrated into more mainstream settings of synthetic organic chemistry and be developed to construct functional crystalline solids.     

Progress of superhydrophobic porous materials

Porous materials such as metal organic framework materials (MOFs), covalent organic framework materials (COFs), organic porous polymers (POPs), etc., have been used widely used in the fields of separation, catalysis, gas storage and drug release due to their diversity, designability, controllability and functionalization of pores. Despite these promising applications, some of the porous materials suffer from moisture-sensitivity and instability in aqueous media due to their inherent structural features. To overcome this problem, endowing them with hydrophobicity is an effective strategy. However, designing superhydrophobic porous materials has certain challenges. In this work, the progress of MOFs, COFs and POPs with (super-)hydrophobic property is introduced. Issues related to their design strategy, structures, and practical applications such as catalysis, oil/water separation and gas storage and separation were analyzed. Additionally, the current problems and the future research directions of the hydrophobic porous materials were discussed.     

超疏水多孔材料的研究进展

多孔材料如金属有机框架材料(MOFs)、共价有机框架材料(COFs)、有机多孔聚合物(POPs)等由于构筑单元的多样性、可设计性,孔道的可调控性和功能化,已经被广泛用于分离、催化、气体储存以及药物释放等领域。尽管如此,这些多孔材料固有的结构特征让它们普遍对水气非常敏感,最严重时多孔结构在水溶液环境下会坍塌。为解决此类问题,制备疏水的多孔材料是一个非常好的策略。然而,设计超疏水多孔材料具有一定的挑战。介绍了具有(超)疏水性能的MOFs、COFs和POPs的发展现状,对超疏水多孔材料合成思路和结构特点进行了分析,对这类材料在催化、油水分离、气体吸附和分离等方面的应用进行了总结,并进一步探讨了此类材料存在的问题和发展方向。     

连锁结构金属-有机骨架材料强化甲烷吸附的分子模拟研究

A systematic molecular simulation study was performed to investigate the effect of catenation on methane adsorption in metal-organic frameworks (MOFs). Four pairs of isoreticular MOFs (IRMOFs) with and without catenation were adopted and their capacities for methane adsorption were compared at room temperature. The pre-sent work showed that catenation could greatly enhance the storage capacity of methane in MOFs, due to the for-mation of additional small pores and adsorption sites formed by the catenation of frameworks. In addition, the simulation results obtained at 298 K and 3.5 MPa showed that catenated MOFs could easily meet the requirement for methane storage in porous materials.     

SynBio-SynChem Approaches to Diversifying the Pacidamycins through the Exploitation of an Observed Pictet-Spengler Reaction

A nonenzymatic Pictet-Spengler reaction has been postulated to give rise to a subset of naturally occurring uridyl peptide antibiotics (UPAs). Here, using a combination of strain engineering and synthetic chemistry, we demonstrate that Pictet-Spengler chemistry may be employed to generate even greater diversity in the UPAs. We use an engineered strain to afford access to meta-tyrosine containing pacidamycin 4. Pictet-Spengler diversification of this compound using a small series of aryl-aldehydes was achieved with some derivatives affording remarkable diastereomeric control.