Chemistry and Structure of a Host–Guest Relationship: The Power of NMR and X-ray Diffraction in Tandem

Diphenylpyrrole-Strapped Calix[4]pyrrole Extractant for the Fluoride and Chloride Anions

The anion binding features of diphenylpyrrole-strapped calix[4]pyrrole 1 have been investigated by means of 1 H NMR spectroscopy and ITC (isothermal titration calorimetry), as well as single crystal X-ray diffraction analyses. Receptor 1 bearing an auxiliary pyrrolic NH donor and solubilizing phenyl groups on the strap was found to bind F- , Cl- , and Br- as their tetrabutylammonium salts with high affinity in DMSO-d6 . In addition, receptor 1 was found to extract the fluoride anion (as both its tetraethylammonium (TEA+ ) and tetrabutylammonium (TBA+ ) salts), as well as the chloride anion into chloroform-d from an aqueous source phase. Cation metathesis using TBAI or the use of a dual host approach involving crown ethers enabled receptor 1 to extract simple alkali metal fluoride or chloride salts from water. Quantitative binding of NaF by receptor 1 was observed in 20 % D2 O-DMSO-d6 allowing for the direct determination of the NaF concentration in an unknown sample.

Accurate Matching of a Secondary Amino-Functionality Metal-Organic Cage for Selective Recognition and Supramolecular Binding during Photoinduced Hydrogen Evolution

Accurate matching of the active sites between the host and guest molecules has a great effect on the selective recognition of different but similar guest molecules or different binding abilities toward the same molecule. Herein, a pseudotetrahedral metal-organic cage (MOC, Co-TAP) that contains secondary amino groups designed as guest-interacting sites was achieved. Co-TAP exhibits the selective recognition of uridine over other similar natural molecules via a fluorescent response. However, a reference structure (Co-TOP) with the same configuration was also synthesized by replacing the secondary amine group with an oxygen atom of the ligand, and it reveals the selective recognition of guanosine. In addition, the accurate matching also enables Co-TAP to strongly bind the organic dye as a guest molecule via host-guest interactions, thus facilitating photoinduced electron transfer between the redox catalytic sites in MOC and the excited guest via a pseudointramolecular pathway.

Metal-Organic Cubane Cage with Trimethylplatinum(IV) Vertices

Herein we describe the synthesis and characterization of the first platinum(IV) metal-organic cage [(Me₃Pt^IV)₈(byp)₁₂](OTf)₈ (2), in which the organometallic moieties trimethylplatinum(IV) (PtMe₃) occupied the corners of a cubane structure and 4,4'-bipyridine ligands used as linkers. The first-principles density functional theory calculations showed that the highest occupied molecular orbitals were localized on the PtMe₃ moieties, while the lowest unoccupied molecular orbitals were distributed on the organic linkers.

Glucose Binding Drives Reconfiguration of a Dynamic Library of Urea-Containing Metal-Organic Assemblies

A bis-urea-functionalized ditopic subcomponent assembled with 2-formylpyridine and Fe^II , resulting in a dynamic library of metal-organic assemblies: an irregular Fe^II ₄ L₆ structure and three Fe^II ₂ L₃ stereoisomers: left- and right-handed helicates and a meso-structure. This library reconfigured in response to the addition of monosaccharide derivatives, which served as guests for specific library members, and the rate of saccharide mutarotation was also enhanced by the library. The (P) enantiomer of the Fe^II ₂ L₃ helical structure bound β-D-glucose selectively over α-D-glucose. As a consequence, the library collapsed into the (P)-Fe^II ₂ L₃ helicate following glucose addition. The α-D-glucose was likewise transformed into the β-D-anomer during equilibration and binding. Thus, β-D-glucose and (P)-3 amplified each other in the product mixture, as metal-organic and saccharide libraries geared together into a single equilibrating system.

Solvent-Dependent Linear Free-Energy Relationship in a Flexible Host-Guest System

Supramolecular chemistry provides an effective strategy for the molecular recognition of diverse molecules. Significant efforts to design synthetic hosts have enabled the successful binding of many types of guests; however, less is known about how host-guest environments influence binding. Herein, we present a comprehensive study in which we measure the host-guest binding of a bis(arylethynyl phenylurea) host with a chloride guest in eight solvents spanning ET(30) values ranging from nonpolar (40.7 kcal mol-1) to polar (47.4 kcal mol-1). Polar solvents show significantly weaker binding in comparison to nonpolar solvents, and the bulk solvent polarity parameter, ET(30), shows a linear free-energy relationship with respect to the free energy of binding in the host-guest complex. These studies provide a better understanding of how host-guest binding in flexible receptors is governed by their environments and highlight the importance of host reorganization contributions in the free energy of binding. In addition, these studies highlight that preorganization may not be as important as previously thought for weak binding in which enthalpic contributions are smaller versus in polar solvents where solvent effects are magnified.

Reversible reduction drives anion ejection and C60 binding within an FeII 4L6 cage

Fe^II₄L₆ tetrahedral cage 1 was prepared from a redox-active dicationic naphthalenediimide (NDI) ligand. The +20 charge of the cage makes it a good host for anionic guests, with no binding observed for neutral aromatic molecules. Following reduction by Cp₂Co, the cage released anionic guests; subsequent oxidation by AgNTf₂ led to re-uptake of anions. In its reduced form, however, 1 was observed to bind neutral C₆₀. The fullerene guest was subsequently ejected following cage re-oxidation. The guest release process was found to be facilitated by anion-mediated transport from organic to aqueous solution. Cage 1 thus employs electron transfer as a stimulus to control the uptake and release of both neutral and charged guests, through distinct pathways.

Fe^II₄L₆ cage 1 binds anionic guests but not neutral guests. In its reduced form, the cage can bind neutral C₆₀. Reduction and oxidation of the cage could thus be used as a stimulus to control the uptake and release of both neutral and charged guests.

Selective Separation of Polyaromatic Hydrocarbons by Phase Transfer of Coordination Cages

Here we report a new supramolecular strategy for the selective separation of specific polycyclic aromatic hydrocarbons (PAHs) from mixtures. The use of a triethylene glycol-functionalized formylpyridine subcomponent allowed the construction of an Fe^II₄L₄ tetrahedron 1 that was capable of transferring between water and nitromethane layers, driven by anion metathesis. Cage 1 selectively encapsulated coronene from among a mixture of eight different types of PAHs in nitromethane, bringing it into a new nitromethane phase by transiting through an intermediate water phase. The bound coronene was released from 1 upon addition of benzene, and both the cage and the purified coronene could be separated via further phase separation.

Macrocycle-Directed Construction of Tetrahedral Anion-π Receptors for Nesting Anions with Complementary Geometry

Manipulation of the emerging anion-π interactions in a highly cooperative manner through sophisticated host design represents a very challenging task. In this work, unprecedented tetrahedral anion-π receptors have been successfully constructed for complementary accommodation of tetrahedral and relevant anions. The synthesis was achieved by a macrocycle-directed approach by using large macrocycle precursors bearing four reactive sites, which enabled a kinetic-favored pathway and afforded the otherwise inaccessible tetrahedral cages in considerable yields. Crystal structure suggested that the tetrahedral cages have an enclosed three-dimensional cavity surrounded by four electron-deficient triazine faces in a tetrahedral array. The complementary accommodation of a series of tetrahedral and relevant anions including BF₄ ^- , ClO₄ ^- , H₂ PO₄ ^- , HSO₄ ^- , SO₄ ^2- and PF₆ ^- was revealed by ESI-MS and DFT calculations. Crystal structures of ClO₄ ^- and PF₆ ^- complexes showed that the anion was nicely encapsulated within the tetrahedral cavity with up to quadruple cooperative anion-π interactions by an excellent shape and size match. The strong anion-π binding was further confirmed by negative ion photoelectron spectroscopy measurements.

Molecular recognition and activation by polyaza macrocyclic compounds based on host-guest interactions

The design and syntheses of supramolecular hosts for the recognition and activation of molecules and anions are one of the most active research fields in supramolecular chemistry, in which polyaza macrocyclic ligands and their complexes have drawn particular attention due to their strong host-guest interactions. This review mainly focuses on the recent progress in the recognition of molecules and anions by polyaza macrocyclic compounds including polyaza macrocycles, polyaza macrobicycles and polyaza macrotricycles, as well as the activation of molecules by polyaza macrocyclic ligands and their metal complexes.

Covalent post-assembly modification in metallosupramolecular chemistry

This review examines the growing variety of covalent reactions used to achieve the post-assembly modification of self-assembled metallosupramolecular complexes.

Benzoins and cyclobenzoins in supramolecular and polymer chemistry

Benzoin condensation is one of the oldest rigorously described organic reactions, having been discovered in 1832 by Liebig and Wöhler. This Feature Article summarizes our work on cyclobenzoins: a class of macrocyclic compounds prepared by a benzoin cyclooligomerization of simple aromatic dialdehydes.

Tetrahedral cage complex with planar vertices: selective synthesis of Pt4L6 cage complexes involving hydrogen bonds driven by halide binding

The first synthesis of tetrahedral cage complexes with four square-planar Pt complexes at the vertex positions was achieved.

The strengths and weaknesses of NMR spectroscopy and mass spectrometry with particular focus on metabolomics research

Mass spectrometry (MS) and nuclear magnetic resonance (NMR) have evolved as the most common techniques in metabolomics studies, and each brings its own advantages and limitations. Unlike MS spectrometry, NMR spectroscopy is quantitative and does not require extra steps for sample preparation, such as separation or derivatization. Although the sensitivity of NMR spectroscopy has increased enormously and improvements continue to emerge steadily, this remains a weak point for NMR compared with MS. MS-based metabolomics provides an excellent approach that can offer a combined sensitivity and selectivity platform for metabolomics research. Moreover, different MS approaches such as different ionization techniques and mass analyzer technology can be used in order to increase the number of metabolites that can be detected. In this chapter, the advantages, limitations, strengths, and weaknesses of NMR and MS as tools applicable to metabolomics research are highlighted.

A unique fluoride nanocontainer: porous molecular capsules can accommodate an unusually high number of "rather labile" fluoride anions

The present work refers to the challenging issue of fluoride anion recognition/binding in water by taking advantage of the unique possibilities offered by the porous molecular nanocontainers of the {Mo132} Keplerate type allowing the study of a variety of new phenomena. Reaction of the highly reactive carbonate-type capsule with aqueous HF results in the release of carbon dioxide and integration of an unprecedentedly large number of fluoride anions--partly as coordinated ligands at both the pentagonal units and the linkers, partly as a disordered water/fluoride assembly inside the cavity. The internal assembly and some of the fluoride ligands are easily released, which provides interesting options for future studies regarding coordination chemistry and catalysis under confined conditions.

Open-cage fullerene with a stopper acts as a molecular vial for a single water molecule

An open-cage fullerene derivative with three carbonyl groups on the rim of the orifice reacts with o-diaminobenzene reversibly to form a tetrahydrofuran moiety above the orifice.

Arene platform based hexa-amide receptors for anion recognition: single crystal X-ray structural and thermodynamic studies

Compartmental recognition of [Cl₂(inf>O/inf>O)₂]²⁻ in the cavity of p-fluoro-phenyl substituted hexa-amide receptor.

Solvent effects upon guest binding and dynamics of a Fe(II)4L4 cage

Solvent-dependent host-guest chemistry and favoring of otherwise disfavored conformations of large guests has been achieved with an adaptive, self-assembled Fe(II)4L4 coordination cage. Depending on the counterion, this face-capped tetrahedral capsule is soluble either in water or in acetonitrile and shows a solvent-dependent preference for encapsulation of certain classes of guest molecules. Quantitative binding studies were undertaken, revealing that both aromatic and aliphatic guests bind in water, whereas only aliphatic guests bind in acetonitrile. The flexibility of its subcomponent building blocks allows this cage to expand or contract upon guest binding, as studied by VT-NMR, thereby ensuring strong binding of both small and large guests. Upon encapsulation, large guest molecules can adopt conformations which are not thermodynamically favored in the free state. In addition, the chirotopic inner phase of the cage renders enantiotopic guest proton signals diastereotopic in specific cases.

Encapsulation of [X2(H2O)4]2- (X = F/Cl) clusters by pyridyl terminated tripodal amide receptor in aqueous medium: single crystal X-ray structural evidence

A new tris-amide receptor L based on 1,3,5-methyl substituted benzene platform and pyridyl as an attached unit is synthesized and explored towards anion recognition in aqueous environment. The presence of pyridyl terminal in L facilitates its aqueous solubility. The binding of halides and oxyanions towards L are examined by (1)H-NMR technique in solution and by single crystal X-ray crystallography in solid state studies. Crystallization of fluoride and chloride with L is carried out in acetone-water (1 : 1, v/v) binary solvent mixture that yields crystals for respective host-guest complexes, [L]2·[F2(H2O)4]·[TBA]2 (1) and [L]2·[Cl2(H2O)4]·[TBA]2 (2) suitable for single crystal X-ray diffraction studies. On the other hand, complexation of L with fluoride in dioxane-acetone (1 : 1, v/v) solvent mixture, results the formation of SiF6(2-) encapsulated complex, [L]2·[SiF6(H2O)2]·[TBA]2 (3). Crystallographic result shows the formation of [F2(H2O)4](2-) and [Cl2(H2O)4](2-) zipped 1D-polymeric tweezer-like assemblies of L in acetone-water (1 : 1, v/v) binary solvent mixture in complexes 1 and 2 respectively. Solution state (1)H-NMR studies in D2O-acetone-d6 (1 : 19, v/v) support 1 : 4 (host-guest) binding stoichiometry of F(-), Cl(-), Br(-), NO3(-), HSO4(-) and H2PO4(-) with L. Binding constants of these investigated anions with L by 1 : 1 binding model are calculated which show the following binding order: NO3(-) ≈ HSO4(-) > F(-) ≈ Cl(-) ≈ Br(-) > H2PO4(-). Further, solution state (19)F-NMR studies are also carried out to establish the F(-) binding with L in DMSO-d6.

Current advances in precious metal core-shell catalyst design

Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core-shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core-shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core-shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core-shell nanoparticles in this paper. The current research trends of these core-shell catalysts are also highlighted.

Dual modes of binding on the hexafluorosilicate anion by a C3v symmetric flexible tripodal amide ligand in solid state

A para-nitrophenyl functionalized C_3v symmetric flexible tripodal amide ligand, L, shows remarkable solvent dependent dual binding behaviour towards the octahedral hexafluorosilicate anion in solid state.

Anion directed conformational diversities of an arene based hexa-amide receptor and recognition of the [F4(H2O)6]4− cluster

The TOC shows difference in binding energies between different conformers after binding with anions of different dimensionalities and conformers A, B & C show structural diversities with anions in case of L.

The syn and anti isomers of the porphyrinogen-like precursor of calix[4]phyrin: isolation, X-ray structure, anion binding and fluoride-ion-mediated proton-deuterium exchange studies

Porphyrinogen-like precursors of calix[4]phyrins are presumed to be unstable owing to their auto-oxidation. In contrast to this, the syn and the anti isomers of a calix[4]pyrrole molecule containing pyridine moieties at the meso positions were isolated and their structures were determined by single crystal X-ray diffraction studies. Both the isomers gave the same calix[4]phyrin molecule upon oxidation. The anion binding properties of both the isomers were studied in DMSO-d6 by the EQNMR method, which showed that they have a preference of binding with the F(-) ion over the other large sized halide and oxo anions. In addition, the F(-) ion mediated H/D exchange process was monitored by the (19)F NMR method. The solution state structures of the 1 : 1 F(-) ion complexes containing deuterium atoms formed by a random but sequential substitution of NH protons by deuterium atoms were identified from their multiplicity patterns observed in the proton coupled (19)F NMR spectrum, which are supported by the proton decoupled (19)F NMR spectrum showing one singlet for each type of F(-) ion complex in solution for both the syn and anti isomers, correlating with their solid state structures.

Encapsulation of a discrete cyclic halide water tetramer [X2(H2O)2]2-, X = Cl-/Br- within a dimeric capsular assembly of a tripodal amide receptor

A conformationally flexible C3v symmetric N-bridged tripodal amide receptor encapsulates a tetrameric halide water cluster [X2(H2O)2](2-) (X = Cl(-)/Br(-)) within its dimeric capsular assembly and forms a non-capsular 1D polymeric assembly with higher homologous iodide anions upon protonation.