The cages, dynamics, and structuring of incipient methane clathrate hydrates

Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.     

Using the face-saturated incomplete cage analysis to quantify the cage compositions and cage linking structures of amorphous phase hydrates

Recent advances in the molecular dynamics simulations of spontaneous nucleation and growth of methane hydrate show that an amorphous phase of the hydrate is first reached. However, the amorphous hydrate has not been well described, due to the insufficient identification of cage structures. Here, we develop a method, called "face-saturated incomplete cage analysis", which can identify all face-saturated cages in a given system. As a result, it is found that thousands of cage types and abundant occupancy states are present in the amorphous hydrate. Moreover, the crystallinity of amorphous hydrate is evaluated according to the quantitative calculation of cage linking structures, and the critical nucleus of hydrate is also estimated on the basis of clustering analysis for all face-saturated cages.     

Unusual Photoreaction of Triquinacene within Self‐Assembled Hosts

Triquinacene is a concave tricyclic hydrocarbon with diverse photoreactivity. In the cavity of an electron‐accepting molecular host, triquinacene was specifically photooxidized at the peripheral allylic position into an alcohol, 1‐hydroxytriquinacene, via guest‐to‐host electron transfer. The unusual reactivity stems from the extremely electron‐deficient triazine panel ligand of the host cage, which allows the cage to function as a good electron acceptor. Thus, self‐assembled coordination cages can serve not only as molecular‐sized reaction vessels but also function electronically as redox media. Dissolved molecular oxygen is indispensable for the photoreaction and immediately traps a photogenerated radical.     

Cagearenes: synthesis,characterization, and application for programmed vapour release

Here, we announce the establishment of a new family of organic molecular cages, named cagearenes, by taking advantage of a versatile strategy. These cagearenes were prepared via the Friedel–Crafts reaction by condensing two equivalents of a precursor bearing three 1,4-dimethoxybenzene groups and three equivalents of formaldehyde. Two cages, namely cagearene-1 and cagearene-2, are obtained and well characterized. The cagearene-1 solid exhibits the ability to adsorb benzene vapour from an equimolar benzene/cyclohexane mixture with a purity of 91.1%. Then, the adsorbed benzene molecules can be released from the cage at a relatively lower temperature, namely 70 °C, as a consequence of which, cyclohexane with a high purity was left within the cage solid. Heating the cage solid further at 130 °C led to the production of cyclohexane with a purity up to 98.7%. As inferred from the single crystal structures and theoretical calculations, the ability of the cage in programmed release of benzene and cyclohexane results from the different binding modes of these two guests.

Two organic cages, cagearene-1 and cagearene-2, are prepared. The cagearene-1 solid selectively absorbs benzene vapor from a benzene/cyclohexane mixture and is used to achieve temperature-controlled programmed vapor release.     

Intermolecular hydrogen transfer between guest species in small and large cages of methane + propane mixed gas hydrates

To investigate the molecular interaction between guest species inside of the small and large cages of methane + propane mixed gas hydrates, thermal stabilities of the methyl radical (possibly induced in small cages) and the normal propyl and isopropyl radicals (induced in large cages) were investigated by means of electron spin resonance measurements. The increase of the total amount of the normal propyl and isopropyl radicals reveals that the methyl radical in the small cage withdraws one hydrogen atom from the propane molecule enclathrated in the adjacent large cage of the structure-II hydrate. A guest species in a hydrate cage has the ability to interact closely with the other one in the adjacent cages. The clathrate hydrate may be utilized as a possible nanoscale reaction field.     

Supramolecular engineering of intrinsic and extrinsic porosity in covalent organic cages

Control over pore size, shape, and connectivity in synthetic porous materials is important in applications such as separation, storage, and catalysis. Crystalline organic cage molecules can exhibit permanent porosity, but there are few synthetic methods to control the crystal packing and hence the pore connectivity. Typically, porosity is either 'intrinsic' (within the molecules) or 'extrinsic' (between the molecules)--but not both. We report a supramolecular approach to the assembly of porous organic cages which involves bulky directing groups that frustrate the crystal packing. This generates, in a synthetically designed fashion, additional 'extrinsic' porosity between the intrinsically porous cage units. One of the molecular crystals exhibits an apparent Brunauer-Emmett-Teller surface area of 854 m(2) g(-1), which is higher than that of unfunctionalized cages of the same dimensions. Moreover, connectivity between pores, and hence guest uptakes, can be modulated by the introduction of halogen bonding motifs in the cage modules. This suggests a broader approach to the supramolecular engineering of porosity in molecular organic crystals.     

Trackable Supramolecular Fusion: Cage to Cage Transformation of Tetraphenylethylene-Based Metalloassemblies

Herein, the trackable supramolecular transformation of a two-component molecular cage to a three-component cage through supramolecular fusion with another two-component molecular square is described. The use of tetraphenylethene (TPE), a chromophore with aggregation-induced emission (AIE) character, as a component for the molecular cages enables facile fluorescence monitoring of the transformation process: while both cages exhibit fluorescence emission via the restriction of intramolecular motion of the TPE motif, the interactions between TPE and 4,4′-bipyridine introduced in the supramolecular fusion process result in partial fluorescence quenching and shifts in the emission maximum. This study provides a simple and efficient approach towards complex supramolecular cages with emergent functions and demonstrates that AIE features could provide unique opportunities for the characterization of complex, dynamic supramolecular transformation processes.     

Density functional theory study of anionic and neutral per-substituted 12-vertex boron cage systems, B(12)X(12)(n-) (n = 2, 1, 0)

The 12(12) closomers form a rapidly expanding class of compounds where a 12-vertex cage is surrounded by 12 identical substituents. Density functional theory (B3LYP/6-31G(d)) is used to study a number of these closomers in different states of oxidation (dianion, radical anion, and neutral cages). The cage stability increases as the group electronegativity of the substituent increases. Also, the 12(12) closomer becomes easier to oxidize as the Hammett sigma(p) parameter becomes more negative (electron-donating). As the closomer is oxidized, the size of the cage increases and the B-B distances become more asymmetric. The Raman-active breathing mode in the 404-434 cm(-1) range moves to lower frequency as the cage is oxidized, which is caused by removing one or two electrons from a cage-bonding molecular orbital.     

Desymmetrized Vertex Design toward a Molecular Cage with Unusual Topology

A novel low-symmetry organic molecular cage with distinctive geometry was successfully synthesized from 5,5′-(propane-2,2-diyl)bis(2-hydroxyisophthalaldehyde) and 1,2-cyclohexanediamine building blocks, through the desymmetrized vertex design strategy. Single-crystal X-ray crystallographic analysis shows that the cage contains asymmetrical and nonplanar windows, exhibiting an unprecedented C₂ symmetry and an efficient packing. The molecular cage structure was also characterized by FTIR, NMR, and MALDI-TOF. Quantum chemistry studies show that the cage structure contains rare intramolecular hydrogen-hydrogen (C−H⋅⋅⋅H−C) bonding interactions. The cage crystals exhibit high iodine vapor uptake (3.78 g g⁻¹), which is among the highest for porous molecular materials. The knowledge gained in this study would open new possibilities for the design and synthesis of molecular cages with novel topologies targeting a broad range of applications.     

Trackable Supramolecular Fusion: Cage to Cage Transformation of Tetraphenylethylene‐Based Metalloassemblies

Herein, the trackable supramolecular transformation of a two‐component molecular cage to a three‐component cage through supramolecular fusion with another two‐component molecular square is described. The use of tetraphenylethene (TPE), a chromophore with aggregation‐induced emission (AIE) character, as a component for the molecular cages enables facile fluorescence monitoring of the transformation process: while both cages exhibit fluorescence emission via the restriction of intramolecular motion of the TPE motif, the interactions between TPE and 4,4′‐bipyridine introduced in the supramolecular fusion process result in partial fluorescence quenching and shifts in the emission maximum. This study provides a simple and efficient approach towards complex supramolecular cages with emergent functions and demonstrates that AIE features could provide unique opportunities for the characterization of complex, dynamic supramolecular transformation processes.     

Functional Material Systems Based on Soft Cages

Discrete molecular soft cages integrate multiple functionalities in one molecule. They express their functions from the confined space in their cavity, functional groups in the cavity interior wall and exterior wall, and the chelating nodes in many chelating cages. Such functional integrity render cage molecules special applications in material engineering. Increasing applications of cage molecules in material design have been reported in recent years. Compared with other cavity-rich molecular structures such as metal-organic framework (MOF) or covalent organic frameworks (COF), discrete soft cages present the unique advantage of material design flexibility, that they can easily composite with nanoparticles or polymers and exist in materials of various forms. We document the development of cage-based materials in recent years and expect to further inspire materials engineering to integrate contribution from the functionality specificity of cage molecules and ultimately promote the development of functional materials and thus human life qualities.     

π-Face Promoted Catalysis in Water: From Electron-deficient Molecular Cages to Single Aromatic Slides

Exploiting noncovalent π-interactions particularly emerging anion-π interactions to drive efficient catalysis is fascinating. Even with exciting progresses, can anion-π activation operate in water remains elusive. Here we report the design, synthesis and catalytic studies of a class of water-soluble electron-deficient molecular cages and relevant aromatic slide compounds. The prism-like cages contain three divided, long, cationic aromatic walls which constitute three highly electron-deficient V-shape cavities. They were efficiently synthesized in two steps from a parent triformyl cage in gram-scale. Crystal structure showed the π-walls bind to the counter bromide through strong anion-π interactions. Just 5 mol% of cages were effective in catalyzing decarboxylative Aldol reactions of aldehydes and malonic acid half thioesters in water but not in organic solvents, showing a pronounced hydrophobic amplification effect. Meantime, a series of single π-slides resembling the π-wall of the cage performed equally well, while those lacking an extended π-surface were ineffective, highlighting the essential role of electron-deficient π-face on promoting the conversion.     

Recent advances of application of porous molecular cages for enantioselective recognition and separation

Porous materials with well‐defined pore structures have received considerable attention in the past decades due to their unique structures and wide applications. Most porous materials such as zeolites, metal‐organic frameworks, covalent organic frameworks, and porous organic polymers are extended to infinite frameworks or networks by robust covalent or coordination bonds. Porous molecular cages composed of discrete molecules with permanent cavities are an emerging class of porous material and the discrete molecules assemble into solids by weak intermolecular interaction. In comparison to porous extended solids such as metal‐organic frameworks and covalent organic frameworks, porous molecular cage solids are generally soluble in organic solvents thus allowing solution processing, making them more convenient to apply in many fields. This review mainly focuses on the recent advances of application of porous molecular cages (porous organic cages and metal‐organic cages) for enantioselective recognition and separation from 2010 to present, including gas chromatography, capillary electrochromatography, chiral fluorescent recognition, chiral potentiometric sensing, and enantioselective adsorption. Furthermore, the two important family members of porous molecular cages, porous organic cages and metal‐organic cages, are also discussed.     

Exohedral silicon fullerenes: SiNPtN/2 (20<or=N<or=60)

Using density functional theory method we show that hollow silicon fullerene cages, SiN (20相似文献   

A Multifunctional,Charge-Neutral,Chiral Octahedral M12L12 Cage

A chiral, octahedral M₁₂L₁₂ cage, which is charge neutral and contains an internal void of about 2000 ų, is reported. The cage was synthesised as an enantiopure complex by virtue of amino-acid-based dicarboxylate ligands, which assemble around copper paddlewheels at the vertices of the octahedron. The cage persists in solution with retention of the fluorescence properties of the parent acid. The solid-state structure contains large pores both within and between the cages, and displays permanent porosity for the sorption of gases with retention of crystallinity. Initial tests show some enantioselectivity of the cage towards guests in solution.     

PPh3衍生准多孔有机笼在Rh/PPh3体系催化氢甲酰化反应中的应用:增强活性和选择性及可回收利用

多孔有机笼(POCs)由英国利物浦大学的Cooper教授在2009年首次合成,这种多孔小分子材料的出现具有两方面重要意义:(1)开拓了多孔材料领域的一个全新分支,改变了人们对多孔材料的传统认知;(2)由于POCs材料由离散的小分子堆积而成,可溶解于一些常用的有机溶剂中,因此其在材料制备方面具有很好的"溶液成型"性能,该优势是三维延伸网状多孔材料所不具备的.POCs本质上是一种"中心带孔"的有机小分子,由刚性有机分子砌块收敛堆叠而成,其特殊结构在气体吸附与分离等方面表现出很好的应用前景.不同于传统空间延伸网状框架材料(如金属-有机框架材料和共价有机框架材料)及多孔有机聚合物(POPs)材料,POCs是一种在大多数有机溶剂中可溶解的小分子材料,因此在均相催化领域也有很好的应用前景.作为最为经典的有机配体,三苯基膦(PPh3)在金属有机化学和均相催化领域应用十分广泛,如目前均相催化工业应用最成功的典范之一氢甲酰化反应,大多数情况下使用的是PPh3与Rh形成的络合物催化剂.本文首先将PPh3进行醛基官能团化,通过醛基和氨基的收敛缩合形成POCs材料,合成了基于PPh3配体的准多孔有机笼(POC-DICP),利用得到的多孔有机笼制备出类Rh/PPh3均相催化体系的Rh/POC-DICP络合催化体系,并将其应用于氢甲酰化反应.相比于经典的Rh/PPh3均相催化体系,该Rh/POC-DICP催化体系在氢甲酰化反应中不仅展示出了更高的活性和目标产物醛的选择性(醛的化学选择性为97％,醛的正异构比为1.89),而且可以很方便地从均相反应体系中沉淀回收(通过调整溶剂体系极性).在氢甲酰化反应中,Rh/POC-DICP体系显示出了良好的底物适用性,在己烯、庚烯、辛烯和苯乙烯的氢甲酰化反应中均表现出良好的催化活性和醛选择性,同时催化剂回收使用4次,未见催化性能明显下降.X射线单晶衍射、同步辐射及DFT计算等结果表明,Rh/POC-DICP催化体系在氢甲酰化反应中具有较高活性和选择性的原因是POC-DICP多孔有机笼分子的有利的空间咬合角(123.88o)和P原子上相对的缺电子效应.本文设计合成的PPh3衍生的多孔有机笼不仅拓宽了多孔有机笼材料在催化领域的应用,而且为新型配体及络合催化剂的设计、合成及修饰提供了新的思路.     

PPh3衍生准多孔有机笼在Rh/PPh3体系催化氢甲酰化反应中的应用:增强活性和选择性及可回收利用

多孔有机笼(POCs)由英国利物浦大学的Cooper教授在2009年首次合成,这种多孔小分子材料的出现具有两方面重要意义:(1)开拓了多孔材料领域的一个全新分支,改变了人们对多孔材料的传统认知;(2)由于POCs材料由离散的小分子堆积而成,可溶解于一些常用的有机溶剂中,因此其在材料制备方面具有很好的"溶液成型"性能,该优势是三维延伸网状多孔材料所不具备的.POCs本质上是一种"中心带孔"的有机小分子,由刚性有机分子砌块收敛堆叠而成,其特殊结构在气体吸附与分离等方面表现出很好的应用前景.不同于传统空间延伸网状框架材料(如金属-有机框架材料和共价有机框架材料)及多孔有机聚合物(POPs)材料,POCs是一种在大多数有机溶剂中可溶解的小分子材料,因此在均相催化领域也有很好的应用前景.作为最为经典的有机配体,三苯基膦(PPh3)在金属有机化学和均相催化领域应用十分广泛,如目前均相催化工业应用最成功的典范之一氢甲酰化反应,大多数情况下使用的是PPh3与Rh形成的络合物催化剂.本文首先将PPh3进行醛基官能团化,通过醛基和氨基的收敛缩合形成POCs材料,合成了基于PPh3配体的准多孔有机笼(POC-DICP),利用得到的多孔有机笼制备出类Rh/PPh3均相催化体系的Rh/POC-DICP络合催化体系,并将其应用于氢甲酰化反应.相比于经典的Rh/PPh3均相催化体系,该Rh/POC-DICP催化体系在氢甲酰化反应中不仅展示出了更高的活性和目标产物醛的选择性(醛的化学选择性为97％,醛的正异构比为1.89),而且可以很方便地从均相反应体系中沉淀回收(通过调整溶剂体系极性).在氢甲酰化反应中,Rh/POC-DICP体系显示出了良好的底物适用性,在己烯、庚烯、辛烯和苯乙烯的氢甲酰化反应中均表现出良好的催化活性和醛选择性,同时催化剂回收使用4次,未见催化性能明显下降.X射线单晶衍射、同步辐射及DFT计算等结果表明,Rh/POC-DICP催化体系在氢甲酰化反应中具有较高活性和选择性的原因是POC-DICP多孔有机笼分子的有利的空间咬合角(123.88o)和P原子上相对的缺电子效应.本文设计合成的PPh3衍生的多孔有机笼不仅拓宽了多孔有机笼材料在催化领域的应用,而且为新型配体及络合催化剂的设计、合成及修饰提供了新的思路.     

Connection of metallamacrocycles via dynamic covalent chemistry: a versatile method for the synthesis of molecular cages

A modular approach for the synthesis of cage structures is described. Reactions of [(arene)RuCl(2)](2) [arene = p-cymene, 1,3,5-C(6)H(3)Me(3), 1,3,5-C(6)H(3)(i-Pr)(3)] with formyl-substituted 3-hydroxy-2-pyridone ligands provide trinuclear metallamacrocycles with pendant aldehyde groups. Subsequent condensation reactions with di- and triamines give molecular cages with 3, 6, or 12 Ru centers in a diastereoselective and chemoselective (self-sorting) fashion. Some of the cages can also be prepared in one-pot reactions by mixing [(arene)RuCl(2)](2) with the pyridone ligand and the amine in the presence of base. The cages were comprehensively analyzed by X-ray crystallography. The diameter of the largest dodecanuclear complex is ～3 nm; the cavity sizes range from 290 to 740 ?(3). An amine exchange process with ethylenediamine allows the clean conversion of a dodecanuclear cage into a hexanuclear cage without disruption of the metallamacrocyclic structures.     

Structural and electronic properties of S-doped fullerene C58: where is the S atom situated?

Structural and electronic properties of S-doped fullerene C58 were calculated systematically via Hartree-Fock self-consistent field (SCF) and density functional B3LYP levels of theory with 6-31G(d) basis set. The most stable C58S represents an open cage structure with a nine-member ring orifice, which provides a large hole for large atoms or small molecules to pass through into the cage. The most stable endohedral S@C58 has the S atom seated near the center of the C58 cage. The calculated highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps of the isomers lie in the range of 1.42-2.50 eV. The electron affinity and the ionization potential were also presented as an indicator of the kinetic stability. Our results may aid in the design of experimental methods for controlling the nature of fullerene cages (for example, doping, opening, and reclosing them).     

Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Classical equilibrium molecular dynamics simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar and up to 250 K. For the mixed H(2)-THF system in which there is single H(2) occupation of the small cage (labelled "1SC 1LC"), we find that no H(2) migration occurs, and this is also the case for pure H(2) hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed "1SC 4LC"). However, for the more densely filled H(2)-THF and pure-H(2) systems, in which there is double H(2) occupation in the small cage (dubbed "2SC 1LC" and "2SC 4LC," respectively), there is an onset of inter-cage H(2) migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these "cage hopping" events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200-250 K range. The "breathing modes," or underlying frequencies governing the variation in the cavities' radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while there is some overlap of small cages' radial breathing modes with lattice acoustic modes.