Switching Between Giant Positive and Negative Thermal Expansions of a YFe(CN)6-based Prussian Blue Analogue Induced by Guest Species

The control of thermal expansion of solid compounds is intriguing but remains challenging. The effect of guests on the thermal expansion of open-framework structures was investigated. Notably, the presence of guest ions (K⁺) and molecules (H₂O) can substantially switch thermal expansion of YFe(CN)₆ from negative (α_v=−33.67×10⁻⁶ K⁻¹) to positive (α_v=+42.72×10⁻⁶ K⁻¹)—a range that covers the thermal expansion of most inorganic compounds. The mechanism of such substantial thermal expansion switching is revealed by joint studies with synchrotron X-ray diffraction, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations. The presence of guest ions or molecules plays a critical damping effect on transverse vibrations, thus inhibiting negative thermal expansion. An effective method is demonstrated to control the thermal expansion in open-framework materials by adjusting the presence of guests.     

Recognition of N‐Alkyl and N‐Aryl Acetamides by N‐Alkyl Ammonium Resorcinarene Chlorides

N‐Alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra‐ and intermolecular hydrogen bonds that leads to cavitand‐like structures. Depending on the upper‐rim substituents, self‐inclusion was observed in solution and in the solid state. The self‐inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self‐included dimers spontaneously reorganize to 1:1 host–guest complexes. These host compounds show an interesting ability to bind a series of N‐alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C?O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl^?) and ammonium (NH₂⁺) cations of the hosts, and also through CH ??? π interactions between the hosts and guests. The self‐included and host–guest complexes were studied by single‐crystal X‐ray diffraction, NMR titration, and mass spectrometry.     

Lateral thermal expansion of chitin crystals

Measurements of the thermal expansion coefficients (TECs) of chitin crystals in the lateral direction are reported. We investigated highly crystalline α chitin from the Paralithodes tendon and an anhydrous form of β chitin from a Lamellibrachia tube from room temperature to 250 °C, using X‐ray diffraction at selected temperatures in the heating process. For α chitin, the TECs of the a and b axes were α_a = 6.0 × 10⁻⁵ °C⁻¹ and α_b = 5.7 × 10⁻⁵ °C⁻¹, indicating an isotropic thermal expansion in the lateral direction. However, the anhydrous β chitin exhibited an anisotropic thermal expansion in the lateral direction. The TEC of the a axis was constant at α_a = 4.0 × 10⁻⁵ °C⁻¹, but the TEC of the b axis increased linearly from room temperature to 250 °C, with α_b = 3.0–14.6 × 10⁻⁵ °C⁻¹. These differences in the lateral thermal expansion behaviors of the α chitin and the anhydrous β chitin are due to their different intermolecular hydrogen bonding systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 168–174, 2001     

Glycoluril‐Derived Molecular Clips are Potent and Selective Receptors for Cationic Dyes in Water

Molecular clip 1 remains monomeric in water and engages in host–guest recognition processes with suitable guests. We report the K_a values for 32 1? guest complexes measured by ¹H NMR, UV/Vis, and fluorescence titrations. The cavity of 1 is shaped by aromatic surfaces of negative electrostatic potential and therefore displays high affinity and selectivity for planar and cationic aromatic guests that distinguishes it from CB[n] receptors that prefer aliphatic over aromatic guests. Electrostatic effects play a dominant role in the recognition process whereby ion–dipole interactions may occur between ammonium ions and the C=O groups of 1 , between the SO₃^? groups of 1 and pendant cationic groups on the guest, and within the cavity of 1 by cation–π interactions. Host 1 displays a high affinity toward dicationic guests with large planar aromatic surfaces (e.g. naphthalene diimide NDI+ and perylene diimide PDI+) and cationic dyes derived from acridine (e.g. methylene blue and azure A). The critical importance of cation–π interactions was ascertained by a comparison of analogous neutral and cationic guests (e.g. methylene violet vs. methylene blue; quinoline vs. N‐methylquinolinium; acridine vs. N‐methylacridinium; neutral red vs. neutral red H⁺) the affinities of which differ by up to 380‐fold. We demonstrate that the high affinity of 1 toward methylene blue (K_a=3.92×10⁷ m ^?1; K_d=25 nm ) allows for the selective sequestration and destaining of U87 cells stained with methylene blue.     

Solvo‐thermal Preparation of One Novel Cadmium(II) Coordination Polymer with 1‐(4‐Aminobenzyl)‐1,2,4‐Triazole and Bi‐functional Photo‐Luminescent Sensing for Acetone and Dichromate

In this work a 1,2,4‐triazole derivative 1‐(4‐aminobenzyl)‐1,2,4‐triazole (abtz) was utilized, one new cadmium(II) coordination polymer, namely [Cd(abtz)I₂]_n ( 1 ) was prepared through the powerful solvo‐thermal synthetic strategy. In compound 1 , the abtz building blocks are interlinked through the central Cd^II ions forming the two‐dimensional (2D) layer coordination framework. Powder X‐ray diffraction (PXRD) characterization also reveals that we have prepared the pure phases of coordination polymer 1 . Optical properties have been determined, which can behave the excellent photo‐luminescent emission of coordination polymer 1 . Photo‐luminescent experiment also reveals that coordination polymer 1 can behave the highly sensitive detection for acetone molecules with high K_sv value (K_sv = 4.12 ×10⁴ L · mol^–1) in the recyclable detection fashion. Additionally, coordination polymer 1 also can behave the highly sensitive detection for pollutant dichromate with excellent quenching efficiency K_sv (K_sv = 2.12 × 10⁴ L · mol^–1) and low detection limit [38 × 10^–3 mM (S/N = 3)]. UV/Vis, photo‐luminescent lifetime, and PXRD patterns also have been determined to analyze the detection mechanism.     

Surface plasmon resonance study on binding interactions of multivalent cyclophane hosts with immobilized guests

Guest‐binding affinities of water‐soluble cyclophane heptadecamer (1) and pentamer (2) with immobilized guests such as 1‐pyrenylmethylamine (PMA) and 2‐(1‐ naphthyl)ethylamine (NEA) were investigated by surface plasmon resonance (SPR) measurements. As a typical example, the binding constants (K) for 1 and 2 with the immobilized PMA as a guest were evaluated to be 2.5 × 10⁷ and 2.7 × 10⁶ M^?1, respectively, and were much larger than that of a monocyclic reference cyclophane (K, 2.5 × 10⁴ M^?1). Interestingly, in the complexation of 1 and 2 with the immobilized guests, more favorable association and dissociation rate constant values (k_a and k_d, respectively) were observed in comparison with those for the monocyclic cyclophane, reflecting multivalent effects in macrocycles. The multivalent effects in macrocycles as well as molecular recognition abilities of the cyclophane oligomers were confirmed even when the guest molecules were immobilized on SPR sensor chip surfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrophilic Oligo(lactic acid)s Captured by a Hydrophobic Polyaromatic Cavity in Water

Biologically relevant hydrophilic molecules rarely interact with hydrophobic compounds and surfaces in water owing to effective hydration. Nevertheless, herein we report that the hydrophobic cavity of a polyaromatic capsule, formed through coordination‐driven self‐assembly, can encapsulate hydrophilic oligo(lactic acid)s in water with relatively high binding constants (up to K_a=3×10⁵ m ⁻¹). X‐ray crystallographic and ITC analyses revealed that the unusual host–guest behavior is caused by enthalpic stabilization through multiple CH–π and hydrogen‐bonding interactions. The polyaromatic cavity stabilizes hydrolyzable cyclic di(lactic acid) and captures tetra(lactic acid) preferentially from a mixture of oligo(lactic acid)s even in water.     

Halogen‐Bonded Supramolecular Capsules in the Solid State,in Solution,and in the Gas Phase

Supramolecular capsules were assembled by neutral halogen bonding (XB) and studied in the solid state, in solution, and in the gas phase. The geometry of the highly organized capsules is shown by an X‐ray crystal structure which features the assembly of two XB hemispheres, geometrically rigidified by H‐bonding to eight MeOH molecules and encapsulation of two benzene guests. To enhance capsular association strength, tuning the XB donor is more efficient than tuning the XB acceptor, due to desolvation penalties in protic solvents, as shown for a tetraquinuclidine XB acceptor hemisphere. With a tetra(iodoethynyl) XB donor and a tetralutidine XB acceptor, the association in deuterated benzene/acetone/methanol 70:30:1 at 283 K reaches K _a=(2.11±0.39)×10⁵ m ⁻¹ (ΔG =−6.9±0.1 kcal mol⁻¹). The stability of the XB capsules in the gas phase was confirmed by electrospray ionization mass spectrometry (ESI‐MS). A new guest binding site was uncovered within the elongated iodoethynyl capsule.     

Induced‐Fit Binding of π‐Electron‐Donor Substrates to Macrocyclic Aromatic Ether Imide Sulfones: A Versatile Approach to Molecular Assembly

Novel macrocyclic receptors that bind electron‐donor aromatic substrates through π‐stacking donor–acceptor interactions are obtained by cycloimidisation of an amine‐functionalised aryl ether sulfone with pyromellitic and 1,4,5,8‐naphthalenetetracarboxylic dianhydrides. These macrocycles can form complexes with a wide variety of π‐donor substrates, including tetrathiafulvalene, naphthalene, anthracene, pyrene, perylene and functional derivatives of these polycyclic hydrocarbons. The resulting supramolecular assemblies range from simple 1:1 complexes to [2]‐ and [3]pseudorotaxanes and even (as a result of crystallographic disorder) an apparent polyrotaxane. Direct five‐component self‐assembly of a metal‐centred [3]pseudorotaxane is also observed on complexation of a macrocyclic ether imide with 8‐hydroxyquinoline in the presence of palladium(II) ions. Binding studies in solution were carried out by using ¹H NMR and UV/Vis spectroscopy, and the stoichiometries of binding were confirmed by Job plots based on the charge‐transfer absorption bands. The highest association constants were found for strong π‐donor guests with large surface areas, notably perylene and 1‐hydroxypyrene, for which K_a values of 1.4×10³ and 2.3×10³ M ^?1, respectively, were found. Single‐crystal X‐ray analyses of the receptors and their derived complexes reveal large induced‐fit distortions of the macrocyclic frameworks as a result of complexation. These structures provide compelling evidence for the existence of strong attractive forces between the electronically complementary aromatic π systems of host and guest.     

Size‐Selective Encapsulation of Hydrophobic Guests by Self‐Assembled M4L6 Cobalt and Nickel Cages

Subtle differences in metal–ligand bond lengths between a series of [M₄L₆]^4? tetrahedral cages, where M=Fe^II, Co^II, or Ni^II, were observed to result in substantial differences in affinity for hydrophobic guests in water. Changing the metal ion from iron(II) to cobalt(II) or nickel(II) increases the size of the interior cavity of the cage and allows encapsulation of larger guest molecules. NMR spectroscopy was used to study the recognition properties of the iron(II) and cobalt(II) cages towards small hydrophobic guests in water, and single‐crystal X‐ray diffraction was used to study the solid‐state complexes of the iron(II) and nickel(II) cages.     

Synthesis of O6‐Corona[3]arene[3]pyridazines and Their Molecular Recognition Property in Organic and Aqueous Media

O₆‐Corona[3]arene[3]pyridazines were synthesized from the one‐pot macrocyclic condensation reaction of 3,6‐dichlorotetrazine with 1,4‐dihydroquinone derivatives followed by the inverse electron demand Diels‐Alder reaction of the tetrazine rings with a cyclopentanone‐derived enamine. Conversion of six ester groups within macrocycle into all sodium acetate moieties afforded a water soluble O₆‐corona[3]arene[3]pyridazine. The coronary macrocycle host formed complexes selectively with organic ammoniums and dinitrile guests in a 1: 1 stoichiometric ratio in organic solvents with association constants ranging from (2.96 ± 0.10) × 10¹ to (2.53 ± 0.33) × 10⁵ L·mol⁻¹. Water soluble O₆‐corona[3]arene[3]pyridazine was also able to complex strongly with organic ammoniums in water to give an association constant up to (2.67 ± 0.21) × 10⁴ L·mol⁻¹. The pseudo‐rotaxane and inclusion structures of the host‐guest complexes were revealed by the X‐ray crystallography.     

Cucurbit[7]uril⋅Guest Pair with an Attomolar Dissociation Constant

Host?guest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by ¹H NMR spectroscopy and X‐ray crystallography. ¹H NMR competition experiments revealed that CB[7]? 6 is among the tightest monovalent non‐covalent complexes ever reported in water with K_a=7.2×10¹⁷ M ^?1 in pure D₂O and 1.9×10¹⁵ M ^?1 in D₂O buffered with NaO₂CCD₃ (50 mM ). The crystal structure of CB[7]? 6 allowed us to identify some of the structural features responsible for the ultratight binding, including the distance between the NMe₃⁺ groups of 6 (7.78 Å), which allows it to establish 14 optimal ion‐dipole interactions with CB[7], the complementarity of the convex van der Waals surface contours of 6 with the corresponding concave surfaces of CB[7], desolvation of the C?O portals within the CB[7]? 6 complex, and the co‐linearity of the C₇ axis of CB[7] with the N⁺???N⁺ line in 6 . This work further blurs the lines of distinction between natural and synthetic receptors.     

A Tunable Cyclic Container: Guest‐Induced Conformational Switching,Efficient Guest Exchange,and Selective Isolation of C70 from a Fullerene Mixture

An adaptable cyclic porphyrin dimer with highly flexible linkers has been used as an artificial molecular container that can efficiently encapsulate various aromatic guests (TCNQ/C₆₀/C₇₀) through strong π–π interactions by adjusting its cavity size and conformation. The planar aromatic guest (TCNQ) can be easily and selectively exchanged with larger aromatic guests (C₆₀/C₇₀). During the guest‐exchange process, the two porphyrin rings switch their relative orientation according to the size and shape of the guests. This behavior of the cyclic container has been thoroughly investigated by using UV/Vis spectroscopy, NMR spectroscopy, and X‐ray crystal structure determination of the host–guest assemblies. The electrochemical and photophysical studies demonstrated the occurrence of photoinduced electron transfer from bisporphyrin to TCNQ/C₆₀/C₇₀ in the respective host–guest assemblies. The cyclic host can form complexes with C₆₀ and C₇₀ with association constants of (2.8±0.2)×10⁵ and (1.9±0.3)×10⁸ m ⁻¹, respectively; the latter value represents the highest binding affinity for C₇₀ reported so far for zinc(II) bisporphyrinic receptors. This high selectivity for the binding of C₇₀ versus C₆₀ allows the easy extraction and efficient isolation of C₇₀ from a C₆₀/C₇₀ fullerene mixture. Experimental evidence was substantiated by DFT calculations.     

K3Eu5(PO4)6: hydrothermal synthesis,crystal structure and photoluminescence properties

An anhydrous orthophosphate, K₃Eu₅(PO₄)₆ (tripotassium pentaeuropium hexaphosphate), has been prepared by a high‐temperature solid‐state reaction combined with hydrothermal synthesis, and its crystal structure was determined by single‐crystal X‐ray diffraction analysis (SC‐XRD). The results show that the compound crystallizes in the monoclinic space group C2/c and the structure features a three‐dimensional framework of [Eu₅(PO₄)₆]_∞, with the tunnel filled by K⁺ ions. The IR spectrum, UV–Vis spectrum and luminescence properties of polycrystalline samples of K₃Eu₅(PO₄)₆, annealed at temperatures of 650, 700, 750, 800 and 850 °C, were investigated. Although with a full Eu³⁺ concentration (9.96 × 10²¹ ions cm^?3), the self‐activated phosphor K₃Eu₅(PO₄)₆ shows s strong luminescence emission intensity with a quantum yield of 37%. Under near‐UV light excitation (393 nm), the series of samples shows the characteristic emissions of Eu³⁺ ions in the visible region from 575 to 715 nm. The sample sintered at 800 °C gives the strongest emission and its lifetime sintered at 800 °C (1.88 ms) is also the longest of all.     

A Dynamic Crystal‐to‐Amorphous Transformation Microporous ZnII Mixed Neutral‐ligand Metal‐organic Polymer {[Zn(bpp)2(μ‐4,4′‐bipy)(H2O)2](ClO4)2·H2O}n

A new rarely reported Zn^II mixed‐polypyridine coordination polymer with both rigid and flexible spacers, {[Zn(bpp)₂(μ‐4,4′‐bipy)(H₂O)₂](ClO₄)₂ · H₂O}_n ( 1 ), has been synthesized and characterized by elemental analysis, IR‐, ¹H NMR‐, ¹³C NMR spectroscopy and single‐crystal X‐ray diffraction. The thermal stability of compound 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The single‐crystal X‐ray structure of 1 shows that the complex has been formed from a 1D polymer as a result of bridging by the 4,4′‐bipy ligands. Solution and solid‐state luminescent spectra of the compound 1 indicate intense fluorescent emissions at ca. 353.6 and 468.8 nm, respectively. Removal of the interstitial water guest molecules results in a loss of crystallinity, but exposure to water vapor reestablishes the original structure, thus constituting 1 as a third‐generation porous framework.     

Self‐Assembly and Host–Guest Chemistry of a 3.5‐nm Coordination Nanotube

Upon complexation with Pd^II ions, precisely designed strandlike ligands with two tris(3,5‐pyridine) units at both terminals were assembled, with the aid of a linear template molecule, into a discrete tubular complex with a length of 3.5 nm. The high stability and the well‐defined structure of the coordination nanotube were revealed by NMR spectroscopy, cold‐spray ionization MS, and single‐crystal X‐ray analysis. Guest lengths were discriminated by the tube: When the association of strandlike guest molecules, in which two biphenylene units are linked with an (OCH₂CH₂)_n linker, were compared, the tube selectively recognized an appropriate guest whose length was comparable to that of the tube. Tetrathiafulvalene (TTF)‐terminated linear guests were directly oxidized to TTF²⁺ in the tube, but reduced stepwise via TTF^+? outside the tube.     

1,3‐Alternate Tetraamido‐Azacalix[4]arenes as Selective Anion Receptors

Six tetraaza[1.1.1.1]cyclophane derivatives bearing peripheral amide groups were prepared according to two distinct synthetic strategies that depend on the connection pattern between the aryl units. NMR experiments combined with the X‐ray structures of two tetraamide derivatives 4 b and 10 show that these cavitands adopt a 1,3‐alternate conformation both in solution and in the solid state. Consequently, the four amide groups of the aza[1.1.1.1]‐m,m,m,m‐cyclophane isomer 10 can contribute to the same recognition process towards neutral water molecules or anion guests. NMR experiments, mass spectrometry analyses and single‐crystal X‐ray structures confirm the anion‐binding ability of this receptor. Absorption spectrophotometric titrations in nonpolar solvents provided evidence for the selectivity of 10 to chloride anions in the halide series, with a corresponding association constant K_a reaching 2.5×10⁶ m ^?1.     

Hydrogen ADPs with Cu Kα data? Invariom and Hirshfeld atom modelling of fluconazole

For the structure of fluconazole [systematic name: 2‐(2,4‐difluorophenyl)‐1,3‐bis(1H‐1,2,4‐triazol‐1‐yl)propan‐2‐ol] monohydrate, C₁₃H₁₂F₂N₆O·H₂O, a case study on different model refinements is reported, based on single‐crystal X‐ray diffraction data measured at 100 K with Cu Kα radiation to a resolution of sin θ/λ of 0.6 Å⁻¹. The structure, anisotropic displacement parameters (ADPs) and figures of merit from the independent atom model are compared to `invariom' and `Hirshfeld atom' refinements. Changing from a spherical to an aspherical atom model lowers the figures of merit and improves both the accuracy and the precision of the geometrical parameters. Differences between results from the two aspherical‐atom refinements are small. However, a refinement of ADPs for H atoms is only possible with the Hirshfeld atom density model. It gives meaningful results even at a resolution of 0.6 Å⁻¹, but requires good low‐order data.     

Complexes of Diquat with Dibenzo-24-Crown-8

The complexation between dibenzo‐24‐crown‐8 ( 1 ) and diquat ( 2 ) was investigated in detail by NMR, MS and X‐ray analysis. It was found that dibenzo‐24‐crown‐8 and diquat formed a 1:1 complex 1 · 2 in acetone with K_a=2.0×10² L·mol^?1, but, as shown by X‐ray analysis, a crystalline 2:1 host:guest inclusion complex 1 ₂· 2 was isolated, in which a single molecule of diquat is enclosed in the concave cavity provided by two dibenzo‐24‐crown‐8 host molecules. Both results are different from the previously assumed stoichiometry of the complexation between dibenzo‐24‐crown‐8 and diquat. This result enriches the range of host‐guest complexes based on dibenzo‐24‐ crown‐8 and provides new opportunities for developing more complicated structures and chemosensors for diquat.     

Chiral Encapsulation by Directional Interactions

The complexation of chiral guests in the cavity of dimeric self‐assembled chiral capsule 1 ₂ was studied by using NMR spectroscopy and X‐ray crystallography. Capsule 1 ₂ has walls composed of amino acid backbones forming numerous directional binding sites that are arranged in a chiral manner. The polar character of the interior dictates the encapsulation preferences towards hydrophilic guests and the ability of the capsule to extract guests from water into an organic phase. Chiral discrimination towards hydroxy acids was evaluated by using association constants and competition experiments, and moderate de values were observed (up to 59 %). Complexes with one or two guest molecules in the cavity were formed. For 1:1 complexes, solvent molecules are coencapsulated; this influences guest dynamics and makes the chiral recognition solvent dependent. Reversal of the preferences can be induced by coencapsulation of a nonchiral solvent in the chiral internal environment. For complexes with two guests, filling of the capsule’s internal space can be very effective and packing coefficients of up to 70 % can be reached. The X‐ray crystal structure of complex 1 ₂?((S) ‐6 )₂ with well‐resolved guest molecules reveals a recognition motif that is based on an extensive system of hydrogen bonds. The optimal arrangement of interactions with the alternating positively and negatively charged groups of the capsule’s walls is fulfilled by the guest carboxylic groups acting simultaneously as hydrogen‐bond donors and acceptors. An additional guest molecule interacting externally with the capsule reveals a possible entrance mechanism involving a polar gate. In solution, the structural features and dynamic behavior of the D₄‐symmetric homochiral capsule were analyzed by variable‐temperature NMR spectroscopy and the results were compared with those for the S₈‐symmetric heterochiral capsule.