Guest Size Research Articles

Revealing Disorder, Sorption Locations and a Sorption-Induced Single Crystal-Single Crystal Transformation in a Rare-Earth fcu-Type Metal-Organic Framework.

Rare-earth metal-organic frameworks (RE-MOFs) formed in the presence of fluoride donors are a group of complex and applicable MOFs. Determining structural complexity is crucial in applying such MOFs and has been achieved to uncover framework disorders in the important fcu framework topology MOF, Y-ndc-fcu-MOF (1). 1 is found to contain F- groups disordered over the μ3-face-capping sites in its secondary building unit (SBU) and framework distortions upon sorption of different guest molecules. The favored location of the guests is within the octahedral cage of 1 where they interact with the Y3+ centers. The size, shape, and interactions of the different guests lead to subtle distortions within the SBU and adoption of specific orientations of the naphthalene group of the 1,4-naphthalenedicarboxylate framework linkers. The sorption of DMF(l)/H2O(l) lowers the symmetry from cubic Fm3̅m (for MeOH(l), N2(g), CO2(gorl)) to cubic Pa3̅ (for DMF(l)/H2O(l)) symmetry with retention of the fcu topology, and conversion between the Pa3̅ and Fm3̅m structures is induced by solvent exchange. Such disorder and sorption locations and transformation are important considerations during the optimization and application of MOFs for sorption-based technologies.

Adaptive maleimide-fused macrocyclic clamps: Synthesis, structures, and complexations with polycyclic aromatic hydrocarbons

The photoactive macrocyclic molecules attract attention because of their important roles in supramolecular chemistry as well as photoelectronic applications. Herein, two D-A conjugated macrocycles containing two maleimide nodes have been successfully obtained via an one-pot cascade reaction of Perkin and imidization condensations. Owing to the incorporation of two bulky 1,4-dimethoxybenzene moieties at the narrow maleimide corners, the obtained conjugated macrocycles have been equipped with two adaptive clamps, which can adjust to the guest sizes to capture the guest molecules. Interestingly, the co-crystallization with larger-sized polycyclic aromatic hydrocarbons (PAHs) leads to the 1:1 host-guest complexes, while with smaller-sized solvents results in the 1:2 complexes. Furthermore, the rotatable 1,4-dimethoxybenzene moieties endowed such macrocyclic fluorophores with pronounced solvatochromic and aggregation-induced emission enhancement (AIEE) phenomena.

Tuning the spin-crossover properties of [Fe2] metal-organic cages.

A computational study on the interplay between ligand functionalization and guest effects on the transition temperature (T1/2) in the [Fe2(L1R)3]@X (L1 = 1,3-bis-(3-(pyridin-2-yl)-1H-pyrazol-5-yl)benzene, X = H-, F-, Cl-, Br-, I- and [BF4]-, R = H, F, or CH3) family of metal-organic cages (MOCs) is presented. Our results indicate that ligand functionalization with electron-donating or electron-withdrawing groups can significantly impact the T1/2 as expected, while the guest effect in lowering the T1/2 has a linear correlation with the increasing guest size. More importantly, small guests can move away from the center of the cavity, thus enhancing the two-step characteristic of the transition. All the data can be understood by analyzing the underlying electronic structure of the studied systems in terms of the relevant d-based molecular orbitals. These results can help in the rational design of new MOCs that can operate as sensors at specific temperatures, thus accelerating the discovery of new SCO devices with tailored properties.

Guest size effects on a robust structure of semiclathrate hydrates and their thermophysical properties.

The ability to tune the pore size, shape, and functionality of semiclathrate hydrates, host-guest materials formed from aqueous solutions of ionic guest materials and water, makes them attractive materials for thermal storage and gas storage applications. The flexibility of semi-clathrate hydrates and their guest-molecule-dependent reactions produce these unexpected and desirable properties. As an ionic guest, tetra-n-butylammonium cation is known for best-fit in hydrogen-bonded water structures. Few investigations have been conducted for other cations, while there are numerous candidates. Relationships between the molecular structures of ionic guest substances and their hydrate structure and relevant thermodynamic properties are yet to be understood. In this study, the semiclathrate hydrates formed with two variations of tetra-n-butylammonium chloride (N4444Cl) that are n-propyl, tri-n-butylammonium chloride (N3444Cl) and tri-n-butyl, n-pentylammonium chloride (N4445Cl) were investigated. Structure analyses found that both salts formed Jeffrey's type III tetragonal hydrate structure which is the same as that of tetra-n-butylammonium chloride hydrate, although their lattice parameters are significantly different. The present data found that this hydrate structure can cover a wide range of melting temperature compared to the other two main semiclathrate structures. The present N4445Cl hydrate is an example in which its melting temperature was adjusted to be suitable for air conditioning, i.e., ∼282 K, compared to that of the N4444Cl hydrate, the melting temperature of which is slightly too high for this purpose. The results provide insight that the thermal properties of the tetragonal P42/m hydrate structure can be widely tuned by ionic guests for various practical requirements.

Cooperativity in Photofoldamer Chloride Double Helices Turned On with Sequences and Solvents, Around with Guests, and Off with Light.

Photofoldamers are sequence-defined receptors capable of switching guest binding on and off. When two foldamer strands wrap around the guest into 2:1 double helical complexes, cooperativity emerges, and with it comes the possibility to switch cooperativity with light and other stimuli. We use lessons from nonswitchable sequence isomers of aryl-triazole foldamers to guide how to vary the sequence location of azobenzenes from the end (FEND) to the interior (FIN) and report their impact on the cooperative formation of 2:1 complexes with Cl-. This sequence change produces a 125-fold increase from anti-cooperative (α = 0.008) for FEND to non-cooperative with FIN (α = 1.0). Density functional theory (DFT) studies show greater H-bonding and a more relaxed double helix for FIN. The solvent and guest complement the synthetic designs. Use of acetonitrile to enhance solvophobicity further enhances cooperativity in FIN (α = 126) but lowers the difference in cooperativity between sequences. Surprisingly, the impact of the sequence on cooperativity is inverted when the guest size is increased from Cl- (3.4 Å) to BF4- (4.1 Å). While photoconversion of interior azobenzenes was poor, the cis-cis isomer forms 1:1 complexes around chloride consistent with switching cooperativity. The effect of the guest, solvent, and light on the double-helix cooperativity depends on the sequence.

Host Spin‐Crossover Thermodynamics Indicate Guest Fit

AbstractSpin‐crossover (SCO) metal‐organic cages capable of switching between high‐spin and low‐spin states have the potential to be used as magnetic sensors and switches. Variation of the donor strength of heterocyclic aldehyde subcomponents in imine‐based ligands can tune the ligand field for a FeIIcenter, which results in both homoleptic and heteroleptic cages with diverse SCO behaviors. The tetrahedral SCO cage built from 1‐methyl‐1H‐imidazole‐2‐carbaldehyde is capable of encapsulating various guests, which stabilize different cage spin states depending on guest size. Conversely, the SCO tetrahedron exhibits different affinities for guests in different spin states, which is inferred to result from subtle structural differences of the cavity caused by the change in metal center spin state. Examination of SCO thermodynamics across a series of host–guest complexes enabled sensitive probing of guest fit to the host cavity, providing information complementary to binding‐constant determination.

Host Spin-Crossover Thermodynamics Indicate Guest Fit.

Spin-crossover (SCO) metal-organic cages capable of switching between high-spin and low-spin states have the potential to be used as magnetic sensors and switches. Variation of the donor strength of heterocyclic aldehyde subcomponents in imine-based ligands can tune the ligand field for a FeII center, which results in both homoleptic and heteroleptic cages with diverse SCO behaviors. The tetrahedral SCO cage built from 1-methyl-1H-imidazole-2-carbaldehyde is capable of encapsulating various guests, which stabilize different cage spin states depending on guest size. Conversely, the SCO tetrahedron exhibits different affinities for guests in different spin states, which is inferred to result from subtle structural differences of the cavity caused by the change in metal center spin state. Examination of SCO thermodynamics across a series of host-guest complexes enabled sensitive probing of guest fit to the host cavity, providing information complementary to binding-constant determination.

Modulating Supramolecular Charge‐Transfer Interactions in the Solid State using Compressible Macrocyclic Hosts

AbstractModulating intermolecular charge‐transfer (ICT) interactions between specific donor and acceptor species in host–guest systems is a big challenge and full of research value in supramolecular chemistry and materials science. In this work, a strategy to modulate the supramolecular ICT interactions in the solid state is developed by compressing the binding cavity of a macrocyclic host named perethylated leaning pillar[6]arene (p‐EtLP6). The solid‐state ICT affinities ofp‐EtLP6toward multi‐types of electron‐deficient planar guests could be significantly enhanced by transforming the macrocyclic backbone from the originalpara‐bridged mode into a hybridpara‐ andmeta‐bridged isomeric form (m‐EtLP6). X‐ray single‐crystal structural analyses incorporating theoretical calculation demonstrate that the improved ICT affinities are mainly attributed to the superior host–guest size fit arising from the compressed binding cavity inm‐EtLP6as compared withp‐EtLP6.

Helicate versus Mesocate in Quadruple-Stranded Lanthanide Cages: A Computational Insight

To drive the synthesis of metallo-supramolecular assemblies (MSAs) and to fully exploit their functional properties, robust computational tools are crucial. The capability to model and to rationalize different parameters that can influence the outcome is mandatory. Here, we report a computational insight on the factors that can determine the relative stability of the supramolecular isomers helicate and mesocate in lanthanide-based quadruple-stranded assemblies. The considered MSAs have the general formula [Ln2L4]2− and possess a cavity suitable to allocate guests. The analysis was focused on three different factors: the ligand rigidity and the steric hindrance, the presence of a guest inside the cavity, and the guest dimension. Three different quantum mechanical calculation set-ups (in vacuum, with the solvent, and with the solvent and the dispersion correction) were considered. Comparison between theoretical and experimental outcomes suggests that all calculations correctly estimated the most stable isomer, while the inclusion of the dispersion correction is mandatory to reproduce the geometrical parameters. General guidelines can be drawn: less rigid and less bulky is the ligand and less stable is the helicate, and the presence of a guest can strongly affect the isomerism leading to an inversion of the stability by increasing the guest size when the ligand is flexible.

On the Role of Flexibility for Adsorptive Separation.

Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world's energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal-organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view.As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host-guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism.The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host-guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-generation separation materials beyond MOFs and adsorbents.

Mechanical stability of fluorinated-methane clathrate hydrates

• Lattice constant of clathrate hydrates formed by encapsulating fluorinated methane derivatives is greatly dictated by the number of fluorine atoms. • Polar fluorinated methane derivatives@cage show distinct rotational dynamics that vary with global strain. • Mechanical properties of clathrate hydrates are greatly influenced by the size and dipole moment of guests. • Unconventional water cages form as clathrate hydrates are mechanically failed via fractures of water cages. Clathrate hydrates recently find important practical applications in the capture and recovery of greenhouse gases, cold storage and refrigeration systems. Nevertheless, their properties at microscopic scale remains largely insufficient yet. Herein, the structure and stability of clathrate hydrates encapsulating fluorinated methane derivatives under mechanical load are investigated by molecular dynamics simulations. All investigated clathrate hydrates are structurally stable host-guest molecular crystals yet show distinct structural and mechanical behaviors. Lattice constant of those clathrate hydrates is dictated by the size and dipole moment of fluorinated methane, for example, it is initially enlarged with increasing fluorine atom in the methane guest molecule but followed by reduction as the guest molecule becomes tetrafluoromethane. However, clathrate hydrates encapsulating non-polar fluorinated methane show superior mechanical properties over those encapsulating polar ones. Polar fluorinated methane derivatives@clathrate cages exhibit distinct rotational dynamics that are influenced by strain. Moreover, all studied clathrate hydrates are mechanically failed by fractures of water cage accompanied by formation of unconventional clathrate cages. Those findings give insights into understanding the structural, thermodynamic stability and mechanical properties of clathrate hydrates encapsulating fluorinated guests.

Molecular Recognition in an Aqueous Medium Using Water-Soluble Prismarene Hosts.

The synthesis of water-soluble prism[n]arenes (n = 5 and 6) bearing anionic carboxylato groups on the rims is described. The binding properties of this novel class of water-soluble hosts are studied by nuclear magnetic resonance and calorimetry. The complexation of singly and doubly charged ammonium guests with the more rigid pentamer is enthalpically driven by secondary interactions, while the thermodynamic fingerprint for the larger hexamer reveals driving forces that strongly depend on the guest charge and/or size.

Conformation Selectivity and Disharmony of the Lattice Constant Depending on the Guest Size in Clathrate Hydrates

This study reports the guest molecular conformation selectivity of a clathrate hydrate and lattice shrunk by hydrogen bonds between the guest and host water frameworks. Cyclohexanol (CHnol) exhibited two conformations (equatorial and axial structures) with small energy differences. The Rietveld refinement results of the powder X-ray diffraction profiles of two structure II (sII) hydrates enclosing CHnol showed that encaged CHnol prefers equatorial conformation over axial conformation in the 51264 cage. In contrast, the hydrate samples were synthesized in both the equatorial and axial conformations in equal proportions. Generally, longer or larger guest molecules exhibit longer lattice constants of the hydrate structure. The equatorial CHnol was longer than cyclohexanone (CHnone), known as an sII former. Nevertheless, the sII (CH4 + CHnol) hydrate showed a smaller lattice constant than that of the sII (CH4 + CHnone) hydrate. The average radius of the 51264 cage in the sII (CH4 + CHnol) hydrate was smaller than that in the sII (CH4 + CHnone) hydrate. This disharmony was caused by the hydrogen bond between CHnol and the water framework of the 51264 cage.

Water content and guest size dictate the mechanical properties of cyclodextrin mediated hydrogels

Steric bulkiness and water content plays an important role in mechanical properties of supramolecular hydrogels consisting of host-guest complexation as cross-links. With low and high water contents, the network mobility and the kinetics of the cross-links become dominant to the mechanical properties, respectively.

Structural and mechanical stability of clathrate hydrates encapsulating monoatomic guest species

Clathrate hydrate (CH) structures find diversely promising applications such as gas and energy storage and transport, carbon capture and storage, gas separation, water desalination, and food and pharmaceutical engineering. However, the influence of guest molecules on their properties remains largely uncharacterized. Here, the critical roles of monoatomic guest size and interaction strength with water molecule in the structural and mechanical stability of clathrate hydrates are explicitly examined by classical molecular dynamics simulations. With increasing guest size or its interaction strength, the lattice constant and mechanical properties such as Young’s modulus, tensile strength, Poisson’s ratio of CHs vary nonlinearly. Based on the characteristics of the contour map of lattice constant, four regions of lattice constant of CHs can be divided. Interestingly, there are crossovers in the iso-lattice constant lines and tensile strength. Upon uniaxial tension, CH destabilizes via brittle or ductile failure, relying on the size and interaction strength of monoatomic guest. The guest size and interaction strength dependent properties can be critically understood by radial distribution function, cage volume and distribution of guest@cages.

Design and control of axial binding with flexible Zn(II) porphyrin dimer: Building-up novel polymer with exo–endo binding

Pyrrole-bridged Zn(II) porphyrin dimer, Zn2DEP, has been utilized as an efficient tweezer host for the binding of guest (substrate) inside the cavity due to its large vertical flexibility. Aromatic monodentate substrate binds in an exo−endofashion to form a discrete monomeric complex while it converts to anexo−exo upon increasing the bulk of the guest. Shorter bidentate guest, on the other hand, forms 1:1 sandwich complex. In contrast, a longer bidentate guest binds in an exo−endo fashion but to form a polymeric structure. The complexes have been isolated in the solid state and structurally characterized while their complexation process in solution has been monitored spectroscopically. We also provide here a comprehensive account of the substrate binding ability of flexible Zn2DEP towards preferential formation of discrete monomeric exo–endo, exo–exo and sandwich complexes as well as 1D polymer just upon varying the size and type of axial guests. This work can contribute towards the rational design of receptors as highly efficient probes for molecular recognition and chirality sensing devices.

Multiple phenotypic traits as triggers of host attacks towards ant symbionts: body size, morphological gestalt, and chemical mimicry accuracy

BackgroundAnt colonies are plagued by a diversity of arthropod guests, which adopt various strategies to avoid or to withstand host attacks. Chemical mimicry of host recognition cues is, for example, a common integration strategy of ant guests. The morphological gestalt and body size of ant guests have long been argued to also affect host hostility, but quantitative studies testing these predictions are largely missing. We here evaluated three guest traits as triggers of host aggression—body size, morphological gestalt, and accuracy in chemical mimicry—in a community of six Eciton army ant species and 29 guest species. We quantified ant aggression towards 314 guests in behavioral assays and, for the same individuals, determined their body size and their accuracy in mimicking ant cuticular hydrocarbon (CHC) profiles. We classified guests into the following gestalts: protective, myrmecoid, staphylinid-like, phorid-like, and larval-shaped. We expected that (1) guests with lower CHC mimicry accuracy are more frequently attacked; (2) larger guests are more frequently attacked; (3) guests of different morphological gestalt receive differing host aggression levels.ResultsArmy ant species had distinct CHC profiles and accuracy of mimicking these profiles was variable among guests, with many species showing high mimicry accuracy. Unexpectedly, we did not find a clear relationship between chemical host similarity and host aggression, suggesting that other symbiont traits need to be considered. We detected a relationship between the guests’ body size and the received host aggression, in that diminutive forms were rarely attacked. Our data also indicated that morphological gestalt might be a valuable predictor of host aggression. While most ant-guest encounters remained peaceful, host behavior still differed towards guests in that ant aggression was primarily directed towards those guests possessing a protective or a staphylinid-like gestalt.ConclusionWe demonstrate that CHC mimicry accuracy does not necessarily predict host aggression towards ant symbionts. Exploitation mechanisms are diverse, and we conclude that, besides chemical mimicry, other factors such as the guests’ morphological gestalt and especially their body size might be important, yet underrated traits shaping the level of host hostility against social insect symbionts.

Fluorescence-based investigations of the host–guest inclusion of anilinonaphthalene sulfonic acids (1,8- and 2,6-ANS) by dimethoxypillar[5]arene in nonaqueous solvents

Pillar[n]arenes are a relatively new class of macrocyclic host molecules which have not been extensively studied for their ability to form inclusion complexes. The properties and host–guest inclusion complexes of dimethoxypillar[5]arene (DMPill[5]) hosts with the polarity-sensitive fluorescent probes 8-anilinonaphthalene-1-sulfonic acid (1,8-ANS) and 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) as guests were investigated in nonaqueous solvents via fluorescence spectroscopy. The binding properties of DMPill[5] were found to depend significantly on the shape of the guest molecule, as well as on the properties of the solvent. Formation of host–guest inclusion complexes of DMPill[5] with these two fluorescent ANS guests occurred in the polar aprotic solvents acetonitrile, acetone, and tetrahydrofuran. Fluorescence titration experiments were performed to determine the nature and strength of the complexation. In the case of 1,8-ANS, 1:1 host–guest complexation was observed in all three solvents, with average binding constant K values of 408, 1000 and 5500 M−1 in acetone, acetonitrile, and tetrahydrofuran, respectively. This large dependence of the binding constant on solvent provides insight into the nature of the binding in these complexes. In the case of 2,6-ANS, 2:1 host–guest complexation was observed; this difference with 1,8-ANS was explained in terms of the shape and size of these two isomeric guests. These results show that DMPill[5] is an excellent host for these neutral aromatic guests, and shows strong binding abilities even in these nonaqueous solvents. The strength (or lack) of binding in different solvents was found to be dependent on a range of factors beyond solvent polarity, including guest and solvent shape and size, and most importantly, specific solvent–solute interactions.

Applying the Crystalline Sponge Method to Agrochemicals: Obtaining X-ray Structures of the Fungicide Metalaxyl-M and Herbicide S-Metolachlor.

The crystalline sponge method is a technique that provides the ability to elucidate the absolute structure of noncrystalline or hard to crystallize compounds through single-crystal X-ray diffraction by removing the need to obtain crystals of the target compound. In this study the crystalline sponges {[(ZnX2)3(2,4,6-tris(4-pyridyl)-1,3,5-trazine)2].x(solvent)}n (X = I, Br) were used to obtain X-ray structures of the agrochemical active ingredients metalaxyl-M and S-metolachlor. The effect of the temperature used during guest uptake and the influence of changing the host framework ZnX2 nodes on guest encapsulation were investigated. Additionally, three compounds containing chemical fragments similar to those of metalaxyl-M and S-metolachlor (phenylacetaldehyde, N-ethyl-o-toluidine, and methyl phenylacetate) were also encapsulated. This allowed for the effect of guest size on the position that guests occupy within the host frameworks to be examined. The disorder experienced by the guest compounds was documented, and an analysis of the intermolecular host–guest interactions (CH···π and π ···π) used for guest ordering within the host frameworks was also undertaken in this study.

Pillar[5]-bis-trithiacrown: Influence of Host-Guest Interactions on the Formation of Coordination Networks.

A pillar[5]-bis-trithiacrown (L) capable of metal binding and organic guest threading simultaneously has been employed, and the influence of dinitrile guests [CN(CH2)nCN (n = 2-6: abbreviated C2-C6)] on the coordination behaviors has been investigated. When the ditopic ligand L was reacted with HgCl2 in the presence of the C2-C6 guests, the shorter guests C2 and C3 afforded a two-dimensional coordination polymer [Hg7Cl14(C2@L)2]n (1) and a one-dimensional coordination polymer [(Hg3Cl6)2(C3@L)2]n (2), respectively. In 1 and 2, each dinitrile guest threads into the pillararene cavity to form a C2@L or C3@L unit via the host-guest interaction. Further linking of these units by exocyclic Hg-S bonds and anion coordination lead to the formation of coordination products with different dimensionalities. While the use of the longer guests C4-C6 under the same reactions yielded a discrete dimercury(II) complex 3, [Hg2Cl4(CH3CN@L)] which contains one acetonitrile solvent molecule because the longer dinitriles do not serve as effective guests. In the NMR and UV-vis studies, the association constants (log K1:1) for the host-guest interactions of L with the dinitrile guests are C2 (4.75) > C3 (4.17) ≫ C4 (2.85) > C5 (2.45) > C6 (too small), indicating that the shorter guests C2 or C3 interact more strongly than longer ones due to the confined interior space of L. Taken collectively, the C2 and C3 guests with proper size-matching promote the formation of coordination polymers and vice versa, suggesting that the guest size could be a controlling factor.