Ion‐Pair Complexation with a Cavitand Receptor

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K_ass) in the range of 10³–10⁴ M ^?1. Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H_down protons converging in the lower pocket, thereby maximizing the CH–anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen‐bonding‐acceptor P?O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by ¹H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.     

A Study of the Interaction Between Cucurbit[8]uril and Alkyl‐Substituted 4‐Pyrrolidinopyridinium Salts

The interaction between cucuribit[8]uril (Q[8]) and a series of 4‐pyrrolidinopyridinium salts bearing aliphatic substituents at the pyridinium nitrogen, namely 4‐(C₄H₈N)C₅H₅NRBr, where R=Et (g1), n‐butyl (g2), n‐pentyl (g3), n‐hexyl (g4), n‐octyl (g5), n‐dodecyl (g6), has been studied in aqueous solution by ¹H NMR spectroscopy, electronic absorption spectroscopy, isothermal titration calorimetry and mass spectrometry. Single crystal X‐ray diffraction revealed the structure of the host–guest complexes for g1, g2, g3, and g5. In each case, the Q[8] contains two guest molecules in a centrosymmetric dimer. The orientation of the guest molecule changes as the alkyl chain increases in length. Interestingly, in the solid state, the inclusion complexes identified are different from those observed in solution, and furthermore, in the case of g3, Q[8] exhibits two different interactions with the guest. In solution, the length of the alkyl chain plays a significant role in determining the type of host–guest interaction present.     

A Soft Copper(II) Porous Coordination Polymer with Unprecedented Aqua Bridge and Selective Adsorption Properties

Herein, the synthesis, crystal structure, and full characterization of a new soft porous coordination polymer (PCP) of ([Cu₂(dmcapz)₂(OH₂)]DMF_1.5)_n ( 1 ) formulation, which is easily obtained in the reaction of CuX₂ (X=Cl, NO₃) salts with 3,5‐dimethyl‐4‐carboxypyrazole (H₂dmcapz) is present. Compound 1 shows a copper(II) dinuclear secondary building unit (SBU), which is supported by two pyrazolate bridges and an unprecedented H₂O bridge. The dinuclear SBUs are further bridged by the carboxylate ligands to build a diamondoid porous network. The structural transformations taking place in 1 framework upon guest removal/uptake has been studied in detail. Indeed, the removal of the bridging water molecules gives rise to a metastable evacuated phase ( 1 b ) that transforms into an extremely stable porous material ( 1 c ) after freezing at liquid‐nitrogen temperature. The soaking of 1 c into water allows the complete and instantaneous recover of the water‐exchanged material ( 1 a′ ). Remarkably, 1 b and 1 c materials possess structural bistability, which results in the switchable adsorptive functions. Therefore, the gas‐adsorption properties of both materials have been studied by means of single‐component gas adsorption isotherms as well as by variable‐temperature pulse‐gas chromatography. Both materials present permanent porosity and selective gas‐adsorption properties towards a variety of gases and vapors of environmental and industrial interest. Moreover, the flexible nature of the coordination network and the presence of highly active convergent open metal sites confer on these materials intriguing gas‐adsorption properties with guest‐triggered framework‐breathing phenomena being observed. The plasticity of Cu^II metal center and its ability to form stable complexes with different coordination numbers is at the origin of the structural transformations and the selective‐adsorption properties of the studied materials.     

Influence of Solvent‐Like Sidechains on the Adsorption of Light Hydrocarbons in Metal–Organic Frameworks

A variety of strategies have been developed to adsorb and separate light hydrocarbons in metal–organic frameworks. Here, we present a new approach in which the pores of a framework are lined with four different C3 sidechains that feature various degrees of branching and saturation. These pendant groups, which essentially mimic a low‐density solvent with restricted degrees of freedom, offer tunable control of dispersive host–guest interactions. The performance of a series of frameworks of the type Zn₂(fu‐bdc)₂(dabco) (fu‐bdc^2?=functionalized 1,4‐benzenedicarboxylate; dabco=1,4‐diazabicyclo[2.2.2]octane), which feature a pillared layer structure, were investigated for the adsorption and separation of methane, ethane, ethylene, and acetylene. The four frameworks exhibit low methane uptake, whereas C2 hydrocarbon uptake is substantially higher as a result of the enhanced interaction of these molecules with the ligand sidechains. Most significantly, the adsorption quantities and selectivity were found to depend strongly upon the type of sidechains attached to the framework scaffold.     

Direct Detection of Supramolecular Reaction Centers in the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 by 13C–27Al Solid‐State NMR Spectroscopy

Hydrocarbon‐pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon‐pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H‐ZSM‐5 zeolite by advanced ¹³C–²⁷Al double‐resonance solid‐state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the ¹³C nuclei (associated with HP species) and the ²⁷Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.     

Tunable Spin‐Crossover Behavior of the Hofmann‐like Network {Fe(bpac)[Pt(CN)4]} through Host–Guest Chemistry

A study of the spin‐crossover (SCO) behavior of the tridimensional porous coordination polymer {Fe(bpac)[Pt(CN)₄]} (bpac=bis(4‐pyridyl)acetylene) on adsorption of different mono‐ and polyhalobenzene guest molecules is presented. The resolution of the crystal structure of {Fe(bpac)[Pt(CN)₄]} ? G (G=1,2,4‐trichlorobenzene) shows preferential guest sites establishing π???π stacking interactions with the host framework. These host–guest interactions may explain the relationship between the modification of the SCO behavior and both the chemical nature of the guest molecule (electronic factors) and the number of adsorbed molecules (steric factors).     

Stable Single‐Molecule Lines of Terrylene in Polycrystalline para‐Dichlorobenzene at 1.5 K

The spectroscopic properties of single terrylene (Tr) molecules are studied in a polycrystalline matrix of para‐dichlorobenzene (p‐DCB) at 1.5 K. Samples grown in a glass capillary show a very strong site at 597 nm, which is redshifted by more than 700 cm^?1 from the observed transition energy for Tr in p‐DCB prepared as a film on a coverslip (572 nm). Each of these two sites is characterized by measuring their single‐molecule spectroscopic parameters at 1.5 K. Lifetime‐limited linewidths of 45±5 MHz are found for both sites. Fluorescence detection rates reach 8×10⁴ count s^?1 at saturation. The spectral trails of the majority of single molecules show no spectral jumps, indicating an absence of interacting two‐level systems; however, the small distribution of linewidths may indicate weak interactions with low‐frequency modes. Frequency jumps are observed for 10 % of the molecules. The complete emission spectra from two different single molecules at the center of each of the two sites is presented. Debye–Waller factors of α_DW=0.33±0.05 for the normal site (572 nm) and α_DW=0.30±0.05 for the red site (597 nm) are reported. This new host–guest system provides a quick and easy way to obtain lifetime‐limited single‐molecule lines.     

Main‐Chain Supramolecular Polymers Based on Orthogonal Benzo‐21‐Crown‐7/Secondary Ammonium Salt and Terpyridine/Metal Ion Recognition Motifs

Orthogonal self‐assembly of multiple components represents an efficient strategy to afford hierarchical and multifunctional assemblies. Here, we demonstrate the orthogonal recognition behaviors between benzo‐21‐crown‐7/secondary ammonium salt and terpyridine/metal ions (Fe²⁺ or Zn²⁺) recognition motifs. Main‐chain supramolecular polymers are subsequently achieved via “one‐pot” mixing of the three monomers together (heteroditopic monomer 1 , homoditopic secondary ammonium salt monomer 2, and Fe(BF₄)₂•6H₂O or Zn(OTf)₂), which are confirmed by ¹H NMR, UV–Vis, DOSY, and viscosity measurements. Moreover, different metal ions (Fe²⁺ or Zn²⁺) exert considerable effects on the size of the resulting supramolecular polymers. Integration of two different types of non‐covalent interactions renders dynamic and responsive properties for the resulting supramolecular polymers, as triggered by a variety of external stimuli such as temperature, potassium cation, as well as stronger chelating ligands. Therefore, the current work is a prerequisite for the future application of such orthogonal assemblies as intelligent supramolecular materials.

     

N‐Alkyl Ammonium Resorcinarene Salts as High‐Affinity Tetravalent Chloride Receptors

N‐Alkyl ammonium resorcinarene salts (NARYs, Y=triflate, picrate, nitrate, trifluoroacetates and NARBr) as tetravalent receptors, are shown to have a strong affinity for chlorides. The high affinity for chlorides was confirmed from a multitude of exchange experiments in solution (NMR and UV/Vis), gas phase (mass spectrometry), and solid‐state (X‐ray crystallography). A new tetra‐iodide resorcinarene salt (NARI) was isolated and fully characterized from exchange experiments in the solid‐state. Competition experiments with a known monovalent bis‐urea receptor ( 5 ) with strong affinity for chloride, reveals these receptors to have a much higher affinity for the first two chlorides, a similar affinity as 5 for the third chloride, and lower affinity for the fourth chloride. The receptors affinity toward chloride follows the trend K₁?K₂?K₃≈ 5 >K₄, with K_a=5011 m ^?1 for 5 in 9:1 CDCl₃/[D₆]DMSO.     

A Boiling‐Water‐Stable,Tunable White‐Emitting Metal–Organic Framework from Soft‐Imprint Synthesis

A new avenue for making porous frameworks has been developed by borrowing an idea from molecularly imprinted polymers (MIPs). In lieu of the small molecules commonly used as templates in MIPs, soft metal components, such as CuI, are used to orient the molecular linker and to leverage the formation of the network. Specifically, a linear dicarboxylate linker with thioether side groups reacted simultaneously with Ln³⁺ ions and CuI, leading to a bimetallic net featuring strong, chemically hard Eu³⁺–carboxylate links, as well as soft, thioether‐bound Cu₂I₂ clusters. The CuI block imparts water stability to the host; with the tunable luminescence from the lanthanide ions, this creates the first white‐emitting MOF that is stable in boiling water. The Cu₂I₂ block also readily reacts with H₂S, and enables sensitive colorimetric detection while the host net remains intact.     

D3h‐Symmetrical Shape‐Persistent Macrocycles Consisting of Pyridine–Acetylene–Phenol Conjugates as an Efficient Host Architecture for Saccharide Recognition

Hexagonal shape‐persistent macrocycles (SPMs) consisting of three pyridine and three phenol rings linked with acetylene bonds were developed as a preorganized host for saccharide recognition by push–pull‐type hydrogen bonding. Three tert‐butyl or 2,4,6‐triisopropylphenyl substituents were introduced on the host to suppress self‐aggregation by steric hindrance. In spite of the simple architecture, association constants K_a of the host with alkyl glycoside guests reached the order of 10⁶ m ^?1 on the basis of UV/Vis titration experiments. This glycoside recognition was much stronger than that in the cases of acyclic equivalent hosts because of the entropic advantage brought by preorganization of the hydrogen‐bonding sites. Solid–liquid extraction and liquid–liquid transport through a liquid membrane were demonstrated by using native saccharides, and much preference to mannose was observed.     

Supramolecular Recognition Influences Magnetism in [X@HVIV8VV14O54]6− Self‐Assemblies with Symmetry‐Breaking Guest Anions

Mixed‐valence polyoxovanadates(IV/V) have emerged as one of the most intricate class of supramolecular all‐inorganic host species, able to encapsulate a wide variety of smaller guest templates during their self‐assembly formation process. As showcased herein, the incorporation of guests, though governed solely by ultra‐weak electrostatic and van der Waals interactions, can cause drastic effects on the electronic and magnetic characteristics of the shell complex of the polyoxovanadate. We address the question of methodology for the magnetochemical analysis of virtually isostructural {V^IV/V₂₂O₅₄}‐type polyoxoanions of D_2d symmetry enclosing diamagnetic VO₂F₂^? (C_2v), SCN^? (C_∞v), or ClO₄^? (T_d) template anions. These induce different polarization effects related to differences in their geometric structures, symmetry, ion radii, and valence shells, eventually resulting in a supramolecular modulation of magnetic exchange between the V(3d) electrons that are partly delocalized over the {V₂₂O₅₄} shells. We also include the synthesis and characterization of the novel [V^VO₂F₂@HV^IV₈V^V₁₄O₅₄]^6? system that comprises the rarely encountered discrete difluorovanadate anion as a quasi‐isolated guest species.     

Encapsulation of AGE‐Breaker Alagebrium by Cucurbit[7]uril Improved the Stability of Both Its Carbonyl α‐Hydrogen and Thiazolium C2‐Hydrogen

As determined by both ¹H NMR and UV/Vis spectroscopic titration, ESI‐MS, isothermal titration calorimetry, and DFT molecular modeling, advanced glycation end products (AGE) breaker alagebrium (ALA) formed 1:1 guest–host inclusion complexes with cucurbit[7]uril (CB[7]), with a binding affinity, K_a, in the order of magnitude of 10⁵ m ^?1, thermodynamically driven by both enthalpy (ΔH=?6.79 kcal mol^?1) and entropy (TΔS=1.21 kcal mol^?1). For the first time, a dramatic inhibition of keto–enol tautomerism of the carbonyl α‐hydrogen of ALA has been observed, as evidenced by over an order of magnitude decrease of both the first step rate constant, k₁, and the second step rate constant, k₂, during hydrogen/deuterium exchange in D₂O. Meanwhile, as expected, the reactivity of C2‐hydrogen was also inhibited significantly, with an upshift of 2.09 pK_a units. This discovery will not only provide an emerging host molecule to modulate keto–enol tautomerism, but also potentially lead to a novel supramolecular formulation of AGE‐breaker ALA for improved stability and therapeutic efficacy.     

The Quest for PdII–PdII Interactions: Structural and Spectroscopic Studies and Ab Initio Calculations on Dinuclear [Pd2(CN)4(μ‐diphosphane)2] Complexes

Structural and spectroscopic properties of and theoretical investigations on dinuclear [Pd₂(CN)₄(P–P)₂] (P–P=bis(dicyclohexylphosphanyl)methane ( 1 ), bis(dimethylphosphanyl)methane ( 2 )) and mononuclear trans‐[Pd(CN)₂(PCy₃)₂] ( 3 ) complexes are described. Xray structural analyses reveal Pd???Pd distances of 3.0432(7) and 3.307(4) Å in 1 and 2 , respectively. The absorption bands at λ>270 nm in 1 and 2 have 4d →5p_σ electronic‐transition character. Calculations at the CIS level indicate that the two low‐lying dipole‐allowed electronic transition bands in model complex [Pd₂(CN)₄(μ‐H₂PCH₂PH₂)₂] at 303 and 289 nm are due to combinations of many orbital transitions. The calculated interaction‐energy curve for the skewed dimer [{trans‐[Pd(CN)₂(PH₃)₂]}₂] is attractive at the MP2 level and implies the existence of a weak Pd^II–Pd^II interaction.     

The Controlled Formation and Cleavage of an Intramolecular d8–d8 Pt–Pt Interaction in a Dinuclear Cycloplatinated Molecular “Pivot‐Hinge”

The bis(diphenylphosphino)methane (dppm)‐bridged dinuclear cycloplatinated complex {[Pt(L)]₂(μ‐dppm)}²⁺ (Pt₂ ? dppm; HL: 2‐phenyl‐6‐(1H‐pyrazol‐3‐yl)‐pyridine) demonstrates interesting reversible “pivot‐hinge”‐like intramolecular motions in response to the protonation/deprotonation of L. In its protonated “closed” configuration, the two platinum(II) centers are held in position by intramolecular d⁸–d⁸ Pt–Pt interaction. In its deprotonated “open” configuration, such Pt–Pt interaction is cleaved. To further understand the mechanism behind this hingelike motion, an analogous dinuclear cycloplatinated complex, {[Pt(L)]₂(μ‐dchpm)}²⁺ (Pt₂ ? dchpm) with bis(dicyclohexylphosphino)methane (dchpm) as the bridging ligand, was synthesized. From its protonation/deprotonation responses, it was revealed that aromatic π–π interactions between the phenyl moieties of the μ‐dppm and the deprotonated pyrazolyl rings of L was essential to the reversible cleavage of the intramolecular Pt–Pt interaction in Pt₂ ? dppm. In the case of Pt₂ ? dchpm, spectroscopic and spectrofluorometric titrations as well as X‐ray crystallography indicated that the distance between the two platinum(II) centers shrank upon deprotonation, thus causing a redshift in its room‐temperature triplet metal–metal‐to‐ligand charge‐transfer emission from 614 to 625 nm. Ab initio calculations revealed the presence of intramolecular hydrogen bonding between the deprotonated and negatively charged 1‐pyrazolyl‐N moiety and the methylene CH and phenyl C–H of the μ‐dppm. The “open” configuration of the deprotonated Pt₂ ? dppm was estimated to be 19 kcal mol^?1 more stable than its alternative “closed” configuration. On the other hand, the open configuration of the deprotonated Pt₂ ? dchpm was 6 kcal mol^?1 less stable than its alternative closed configuration.     

Water‐Stable Zirconium‐Based Metal–Organic Framework Material with High‐Surface Area and Gas‐Storage Capacities

We designed, synthesized, and characterized a new Zr‐based metal–organic framework material, NU‐1100 , with a pore volume of 1.53 ccg^?1 and Brunauer–Emmett–Teller (BET) surface area of 4020 m²g^?1; to our knowledge, currently the highest published for Zr‐based MOFs. CH₄/CO₂/H₂ adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g^?1, which corresponds to 43 g L^?1. The volumetric and gravimetric methane‐storage capacities at 65 bar and 298 K are approximately 180 v_STP/v and 0.27 g g^?1, respectively.     

Controlling the Reactivity of the SeSe Bond by the Supramolecular Chemistry of Cucurbituril

We describe a new strategy to control the reactivity of Se?Se bond by using supramolecular chemistry of cucurbituril. We have demonstrated that selenocystamine (SeCy) and cucurbit[6]uril (CB[6]) can form a stable supramolecular complex (K_a=5.5×10⁶ M ^?1). Before complexation, the free Se?Se bond in SeCy is rather sensitive to redox stimuli and gets disrupted quickly with addition of reductant or oxidant. However, after binding with CB[6], the Se?Se bond becomes quite inert and hardly reacts with reductant or oxidant. One advantage of this supramolecular protection is that it can be applied in a wide pH range from weakly acidic to basic. Additionally, the supramolecular complex formed by SeCy and CB[6] can be reversibly dissociated simply with addition of Ba²⁺.     

Exploiting Host–Guest Interactions for the Synthesis of a Rod–Rod Block Copolymer with Crystalline and Liquid‐Crystalline Blocks

By making use of the host–guest interactions between the host molecule tris‐o‐phenylenedioxycyclotriphosphazene (TPP) and the rod–coil block copolymer (BCP) poly(ethylene oxide)‐block‐poly(octyl 4′‐octyloxy‐2‐vinylbiphenyl‐4‐carboxylate) (PEO‐b‐PVBP), the supramolecular rod–rod block copolymer P(EO@TPP)‐b‐PVBP was constructed. It consists of a crystalline segment P(EO@TPP) with a hexagonal crystalline structure and a columnar nematic liquid‐crystalline segment (PVBP). As the PVBP segments arrange themselves as columnar nematic phases, the crystalline structure of the inclusion complex P(EO@TPP), which has a smaller diameter, is destroyed. The self‐assembled nanostructure is thus clearly affected by the interplay between the two blocks. On the basis of wide‐ and small‐angle X‐ray scattering analysis, we conclude that the supramolecular rod–rod BCP can self‐assemble into a cylinder‐in‐cylinder double hexagonal structure.     

Podand‐Based Dimeric Chromium(III)–Salen Complex for Asymmetric Henry Reaction: Cooperative Catalysis Promoted by Complexation of Alkali Metal Ions

A new kind of podand‐based dimeric salen ligand was synthesized, and its association with potassium cations was investigated by ¹H NMR spectroscopy. The corresponding Cr^III–salen dimer was assembled by a supramolecular host–guest self‐assembly process and was then used as a catalyst in highly efficient and enantioselective asymmetric Henry reactions. Regulation by KBAr_F (BAr_F=[3,5‐(CF₃)₂C₆H₃]₄B) led to remarkable improvements in yield (by up to 58 %) and enantioselectivity (for example, from 80 % ee to 96 % ee).