Cationic Guests Research Articles

Compositely modulated structures of phosphor materials SrxLi2+xAl2-xO4:Eu2.

Composite crystals SrxLi2+xAl2-xO4:Eu2+ were synthesized and their structures were determined using single-crystal X-ray diffraction. The commensurate structure with a modulation wavevector q = 5c*/6 was analyzed in a conventional manner in 3D space, while a structure model in (3+1)-dimensional superspace was used for the other two crystals with modulation wavevectors slightly differing from 5c*/6. The superstructure of the commensurate phase was described using the space group P4/n and a common superspace group I4/m(00γ)00 was used for the (3+1)D structures of all three crystals. The whole structure of each crystal consists of two substructures. Basis vectors a and b are common, but c is different for the two substructures. The first substructure is a host framework constructed by (Li/Al)O4 tetrahedra sharing edges. A linear connection of cavities is seen to be channel-like, in which Sr ions locate as guest cations forming the second substructure. The crystal of q= 5c*/6 contains five Sr ions per six cavities in a channel. Sr ions are distributed at seven sites, some of which are partially occupied. Statistical disorder of local structure models for the location of Sr ions in the channel was assumed to explain the results. Such a partially disordered character was also seen in the incommensurate phases and properly embodied by a (3+1)D model containing an atomic domain of the Sr ion with occupational modulation. Plots of the occupation factor, interatomic distances and the bond valence sum at each metal site as functions of t (= x4 - q·r) are roughly identical in the three crystals, which are considered as members of the same series of composite crystals.

Photoinduced interruption of interannular cooperativity for delivery of cationic guests in water.

The synergy among twelve carboxylates from two hexavalent baskets 16- assisted the encapsulation of one divalent diammonium guest 32+-62+ and the formation of ternary [3-6⊂12]10-. The reduction of basket's multivalency, by photoinduced α-decarboxylation of 16- to give 23-, intercepted the interannular cooperativity operating in the stabilization of capsulpex [3-6⊂12]10- to dramatically diminish the binding affinity towards diammonium guests. As a result, the cationic guests were released into bulk water with 23- assembling into nanoparticles. With numerous drugs carrying positive sites, the finding reported here could now be examined for their light-promoted spatiotemporal delivery.

Emission enhancement of cationic tetraphenylethylene derivatives by encapsulation in a cucurbit[10]uril host in water

Encapsulation of cationic tetraphenylethylene guests in a cucurbit[10]uril host in water results in strong emission enhancement of these guests since the host–guest interaction caused restriction of intramolecular rotation.

Cobalt(III) Metallacryptates and Their Guest Cation-Exchange in Solution Monitored by 59Co NMR.

One-pot reactions of the catechol-scaffolding aroylbis(N,N-diethylthiourea) H2Lcat with mixtures of CoCl2 and MCl (M+ = Cs+, Rb+, K+, Tl+, or NH4+) or with a Co(NO3)2/TlNO3 mixture lead to the self-assembly of a series of cationic Co(III) metallacryptates of the general formula [M ⊂ {Co2(Lcat)3}]+ (M+ = Cs+, Rb+, K+, Tl+, or NH4+). Crystalline PF6- salts were obtained after workup with (n-Bu4N)(PF6), and the single-crystal structures of all five metallacryptates have been determined. Depending on the nature of the guest cations, the directional interactions between guest cations and the metallacryptand {Co2(Lcat)3} are either weak coordination contacts or hydrogen bonds. The bonding mode and the size of the guest ions slightly influence the molecular skeleton of the host molecule. These small structural variations also exist in solution and could be detected by means of 59Co NMR spectroscopy, which is shown to be a unique tool for an easy characterization of such compounds. 59Co NMR chemical shifts are extraordinarily sensitive to the guest cation in the metallacryptates, and time-arrayed 59Co NMR experiments show that cation-exchange processes in biphasic organic/aqueous systems can be studied in detail. This leads to insights into the relative rates of cation exchange, as well as the relative conditional distribution coefficients of such Co(III) metallacryptates between the aqueous and organic phases. Thus, the extent and the relative rate of the NH4+ ion exchange in [NH4 ⊂ {Co2(Lcat)3}](PF6) by Cs+ and K+ ions across the organic/aqueous phase boundary at room temperature have been studied by in situ 59Co NMR experiments. Preliminary 59Co NMR experiments show that the K+ ion in [K ⊂ {Co2(Lcat)3}](PF6) can be removed by its competitive complexation with the highly potassium-selective [2.2.2]cryptand, to give a transient 59Co NMR signal of the relatively unstable "empty" {Co2(Lcat)3} complex, which slowly decomposes in solution.

Perethylated pillar[n]arenes versus pillar[n]arenes: theoretical perspectives.

In this article we obtain the electronic structure, 1H NMR, and the vibrational spectra of perethylated pillar[n]arene (EtPn) (n = 5-8, 10) macrocycles using the ωB97x-based density functional theory. A single cavitand structure endowed with the robust pillar-like architecture was derived for the EtPn (n = 5-7) hosts while for the EtP8 receptor, two conformers one possessing the single cavitand and other double cavitand structure having four hydroquinone monomeric units in each cavity were obtained as the local minima. A double cavitand conformer of the EtP8 was found to be ~ 50kJmol-1 lower in energy than the single cavitand one which was attributed to the increased hydrogen bonding cooperativity and attractive H-H interactions. Further increase of hydroquinone monomers as in EtP10 host engenders only the double cavitand structure, composed of five hydroquinone monomeric units in each cavity. The molecular electrostatic potential investigations along the EtPn series of hosts reveal large electron-rich aromatic moieties compared to unsubstituted pillararenes which suggest the EtPn to be better artificial receptors toward cationic or neutral guests than their unsubstituted analogs. Calculated vibrational frequencies of EtPn (n = 5-8) show the cooperative stretching vibrations in a narrow span of 3241cm-1 to 3228cm-1. Besides, the most deshielded signals from aromatic protons along the EtPn (n = 5-8) series show up near 7.35ppm in their 1H NMR spectra. An increased cooperativity in the lowest energy of the EtP8 conformer further is evident from up-field signal corresponding to methylene protons of ethyl substituent compared with the single cavitand structure. Graphical abstractMolecular electrostatic potential for EtP5 and correlations obtained for δH VsρBCP and ν Vs G(r) plots.

Thermodynamic Characterization of the Interaction between the Antimicrobial Drug Sulfamethazine and Two Selected Cyclodextrins.

Sulfamethazine is a representative member of the sulfonamide antibiotic drugs; it is still used in human and veterinary therapy. The protonation state of this drug affects its aqueous solubility, which can be controlled by its inclusion complexes with native or chemically-modified cyclodextrins. In this work, the temperature-dependent (298–313 K) interaction of sulfamethazine with native and randomly methylated β-cyclodextrins have been investigated at acidic and neutral pH. Surprisingly, the interaction between the neutral and anionic forms of the guest molecule and cyclodextrins with electron rich cavity are thermodynamically more favorable compared to the cationic guest. This property probably due to the enhanced formation of zwitterionic form of sulfamethazine in the hydrophobic cavities of cyclodextrins. Spectroscopic measurements and molecular modeling studies indicated the possible driving forces (hydrophobic interaction, hydrogen bonding, and electrostatic interaction) of the complex formation, and highlighted the importance of the reorganization of the solvent molecules during the entering of the guest molecule into the host’s cavity.

Bacteriophage M13 Aggregation on a Microhole Poly(ethylene terephthalate) Substrate Produces an Anionic Current Rectifier: Sensitivity toward Anionic versus Cationic Guests.

Bacteriophage material (M13, wild-type) deposited as a film onto a poly(ethylene terephthalate) (PET) substrate (6 μm thick with a 20 μm diameter laser-drilled microhole) has been investigated for ion conductivity and ionic current rectification effects for potential applications in membranes. The M13 aggregate membrane forms under acidic conditions (in aqueous 10 mM acids) and behaves like a microporous anion conductor with micropores defined by the packing of cylindrical virus particles. Asymmetric deposition on the PET film substrate in conjunction with semipermeability leads to anionic diode behavior. Typical rectification ratio values are around 10 (determined at ±1 V) in aqueous 10 mM acids. Cationic guest species (aqueous Cu2+, Co2+, Ag+) consistently lead to a rectification minimum at 0.5 mM guest concentration. In contrast, anionic guest species (indigo carmine) lead to a similar rectification minimum already at 5 μM concentration. The behavior is proposed to be associated with cation exclusion effects on transport.

Low-temperature thermodynamic and magnetic properties of clathrate-like arsenide Eu7Cu44As23

Experimental investigation of heat capacity, CP, unit cell parameter, a, as well as the ESR absorption for a clathrate-like compound Eu7Cu44As23 has been performed in the temperature range of 2–300 K. The CP(T) function exhibits a sharp maximum at the temperature of ferromagnetic ordering, Tc ≈ 17 K, whereas the a(T) shows no anomaly at this temperature. This unusual behavior is governed by the clathrate-like structure of Eu7Cu44As23, where Eu2+ guest cations residing in oversized cavities of the host framework order ferromagnetically imposing no influence on the framework dynamics. The analysis of maximum of the Cp(T) function at 2–20 K allows determining the exchange integral, J, which is a characteristic of the magnetic interaction Eu2+ – Eu2+: J ≈ 0.27 K The analysis of the temperature dependencies of heat capacity, CP(T), and unit cell volume, V(T), in the frames of the Debye-Einstein model enabled determining characteristic temperatures of host and guest subsystems. Along with this, the contribution of two-level systems into thermodynamic properties of Eu7Cu44As23 was evaluated and related to the possibility of guest Eu2+ cations to occupy nonequivalent positions inside the cages of the host framework. The ESR of Eu2+ in Eu7Cu44As23 was studied at 5–300 K. The intensity of the ESR signal shows a sharp drop at low temperatures, which confirms the ferromagnetic character of the magnetic phase transition in Eu7Cu44As23. The experimental ESR spectrum was approximated by two Dyson lines, L1 and L2, which correspond to two g-factors: g1 = 2.081 ± 0,030 and g2 = 2,094 ± 0,016. The increase in L1 linewidth with temperature at 75–275 K is d(ΔH1)/dT = 0.027 G/K. These values of g-factors and d(ΔH1)/dT are derived the exchange integrals of the interaction of the Eu2+ local moment with conduction electrons J1* = 0.241 eV and J1** = 0.252 eV accordingly. For L2 J2* = 0.255 eV. Value J2** ≈ 0.26 eV is valid in 100–130 K interval only. The change of ΔH2(T) slope at higher temperatures may be due to crystal field effect.

New Barium Vanadate BaxV2O5 (x ≈ 0.16) for Fast Lithium Intercalation: Lower Symmetry for Higher Flexibility and Electrochemical Durability

AbstractNa0.33V2O5‐type metal vanadates generally show better cycling stability than α‐V2O5 as a cathode in Li‐ion batteries, because they contain enough crystallographic voids that allow for lithium intercalation without significant structural deformation, but metal leaching upon cycling deteriorates their capacity retention. Here a new barium vanadate Ba0.16(1)V2O5 is reported that does not undergo Ba leaching during cycling and shows a great promise for fast Li intercalation. Sr0.15(1)V2O5 is isostructural to Na0.33V2O5 (space group C2/m), while Ba0.16(1)V2O5 adopts a new structure (space group P21/c), a derivative of Na0.33V2O5. The framework of Ba0.16(1)V2O5 consists of edge‐shared VO6 octahedral layers and the VO5 pyramidal bridge, generating unidirectional tunnels. Unlike the in‐plane arrangement of atoms in Sr0.15(1)V2O5, the nonplanar arrangement in Ba0.16(1)V2O5 makes the host framework more flexible and creates larger voids for Ba. Compared with the combination of displacement and intercalation mechanisms in the Sr0.15(1)V2O5 cathode, Li intercalation looks dominant in the Ba0.16(1)V2O5 cathode due to its increased flexibility. Hence, Ba0.16(1)V2O5 exhibits improved cycling stability compared to Sr0.15(1)V2O5, suggesting that lower symmetry leads to higher cyclic stability in the V2O5‐related compounds. This study illustrates how guest cations can tune the structural symmetry to modulate the reaction mechanism of electrode materials toward superior performances.

Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules.

We present here the construction of a self-assembled two-dimensional network at the liquid/solid interface using a hexagonal pyridine macrocycle which binds an organic cation in its intrinsic porous space by electrostatic interactions. For this purpose, a hexagonal pyridinylene-butadiynylene macrocycle (PyBM) having six octyloxymethyl groups, PyBM-C8, was synthesized. As guests, tropylium (Tr) tetrafluoroborate and trioxatriangulenium (TOTA) hexafluorophosphate were used. In this study, we focused on (i) the network patterns of PyBM-C8 which change in response to its concentration and (ii) the position of the guest immobilized in the porous space of the macrocycle. Scanning tunneling microscopy (STM) observations at the interface of 1,2,4-trichlorobenzene (TCB) and highly oriented pyrolytic graphite (HOPG) revealed that PyBM-C8 formed four different polymorphs, oblique, loose hexagonal, linear, and rectangular, depending on the solute concentration and annealing treatment. Solvent TCB molecules are likely coadsorbed to not only the intrinsically porous space of PyBM-C8 (internal TCB) but also the space outside of the macrocycle between its alkyl chains (external TCB) in most of the cases. Upon adding the guest cation, whereas small Tr was not visualized in the pore due to size mismatching, larger TOTA was clearly observed in each pore. In addition, based on high-resolution STM images of the rhombus packing pattern of PyBM-C8, we revealed experimentally that TOTA was placed at an off-center position of the deformed hexagonal macrocyclic core in the rhombus pattern. On the basis of the molecular mechanics calculations, we hypothesize that the off-center location of TOTA is due to deformation of the hexagonal macrocycle through interaction with two external TCB molecules located at opposite edges of the macrocyclic core. Symmetry breaking of the macrocyclic host framework induced by coadsorbed surrounding solvent molecules thus plays a significant role in host-guest complexation at the liquid/solid interface.

Anion-driven encapsulation of cationic guests inside pyridine[4]arene dimers.

Pyridine[4]arenes have previously been considered as anion binding hosts due to the electron-poor nature of the pyridine ring. Herein, we demonstrate the encapsulation of Me4N+ cations inside a dimeric hydrogen-bonded pyridine[4]arene capsule, which contradicts with earlier assumptions. The complexation of a cationic guest inside the pyridine[4]arene dimer has been detected and studied by multiple gas-phase techniques, ESI-QTOF-MS, IRMPD, and DT-IMMS experiments, as well as DFT calculations. The comparison of classical resorcinarenes with pyridinearenes by MS and NMR experiments reveals clear differences in their host–guest chemistry and implies that cation encapsulation in pyridine[4]arene is an anion-driven process.

Tailoring Cu+ for Ga3+ Cation Exchange in Cu2-xS and CuInS2 Nanocrystals by Controlling the Ga Precursor Chemistry.

Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl3–L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu+ for Ga3+ in Cu2–xS nanocrystals. We find that noncomplexed GaCl3 etches the nanocrystals by S2– extraction, whereas GaCl3–TOP is unreactive. Successful exchange of Cu+ for Ga3+ is only possible when GaCl3 is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu2–xS–GaCl3–L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga3+ insertion and Cu+ extraction can occur. This requisite is only met if GaCl3 is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl3–DPP to convert CuInS2 into (Cu,Ga,In)S2 nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.

Unique Freezing Dynamics of Flexible Guest Cations in the First Molecular Postperovskite Ferroelectric: (C 5 H 13 NBr)[Mn(N(CN) 2 ) 3

Ferroelectricity is usually found in cubic and hexagonal perovskites consisting of corner- and faced-shared BX6 octahedra, respectively. In contrast, another important branch of perovskites, postpe...

Sulfonatocalixarene Counterion Exchange Binding Model in Action: Metal‐Ion Catalysis Through Host‐Guest Complexation

Abstractp‐Sulfonatocalixarene water soluble macrocyclic host receptors are known to form cooperative ternary complexes with complementary organic guest and metal cations. This property may be explored to enhance the interaction of weak nitrogen ligands with metal cations in a confined space showing some resemblance to metal‐containing enzymes. However, the best of our knowledge, catalytic potential of this property remains unexplored. In this work the Ni2+ catalyzed hydrolysis of a picolinate ester (2,4‐dinitrophenyl picolinate, 1) was used as a model reaction to evaluate the effect of sulfonatocalixarene macrocycles in the kinetics of this reaction. The results show that the host molecules promote the reaction through simultaneous complexation of the metal cation and the substrate and, in the case of the larger calixarenes containing more basic phenol groups, substantially higher rate enhancements are observed owing to additional assistance provided by base/nucleophilic catalysis. However, due the ionic nature of these receptors auto‐inhibition of the reaction is observed at higher concentrations due counterion (Na+) binding that competes with the catalytically active Ni2+ ‐complexes.

Pillar[5]arene-Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host-Guest Interactions.

Herein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)-based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene-stabilized NCs (Ag29 (LA-P5)12 (TPP)2 ) endorse reversible host-guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000-fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.

Pillar[5]arene‐Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host–Guest Interactions

AbstractHerein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)‐based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene‐stabilized NCs (Ag29(LA‐P5)12(TPP)2) endorse reversible host–guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000‐fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.

Zinc(II) {2}-metallacoronates and {2}-metallacryptates based on dipicolinoylbis(N,N-diethylthiourea): Structures and biological activities

Strict control of the stoichiometry of one-pot reactions between zinc(II) acetate and alkali earth chlorides MCl2 (M = Ca, Ba) with the bifunctional ligand dipicolinoylbis(N,N-diethylthiourea), H2Lpy, leads to the selective formation of {2}-metallacoronates [MZn2(μ-AcO)2(Lpy)2] (M = Ca (1a), Ba (1b)) and {2}-metallacryptates [MZn2(Lpy)3] (M = Ca (2a), Ba (2b)) in good yields. In 1a, the neutral metallacorand [Zn2(Lpy)2], resulting from the coordination of two Zn2+ ions with two deprotonated (Lpy)2− ligands, encapsulates a Ca2+ cation in the central void. The system is further stabilized by two bidentate acetato ligands bridging the Ca2+ guest and the two Zn2+ ions. In 2a and 2b, the anionic metallacryptands [Zn2(Lpy)3]2−, formed by coordination of three deprotonated (Lpy)2− ligands and two Zn2+ ions, capture a divalent guest cation, Ca2+ (2a) or Ba2+ (2b), in the interior cavity. All the complexes show weak antiproliferative effects against HepG2 and MCF7 cancer cell lines, but display relatively good antimicrobial activity against Gram-positive bacteria (E. faecali, S. aureus, B. cereus) and Gram-negative bacteria (E. coli, P. aeruginosa). Clear trends in the antimicrobial activities of the compounds were observed in which, for the same guest cations, complexes 1 show better activities than complexes 2 and for the structurally analogous complexes, the Ba2+ containing compounds are more active than the Ca2+ containing counterparts.

Switching of Recognition First and Reaction First Mechanisms in Host-Guest Binding Associated with Chemical Reactions.

Host-guest binding sometimes triggers the subsequent chemical reactions of the host framework as well aschanges in the physical properties. Since the host-guest binding generally occurs very quickly, it is sometimes difficult to differentiate the mechanism from the alternative one in which the guest binding occurs after the reaction. However, it is important to differentiate the two mechanisms when we develop new molecules based on time-dependent functions. Thus, we propose two distinct mechanisms, recognition first and reaction first, in a slowly reacting host system. We designed and synthesized a new cobalt(III) metallohost, [LCo2(pip)4](OTf)2 (pip = piperidine), which can take up a guest cation into its 18-crown-6-like cavity causing concomitant exchange of the axial piperidine ligands under solvolytic conditions. We investigated the mechanism to elucidate whether the guest recognition or ligand exchange occurs first. When Na+ (5-10 equiv) was present, the guest recognition occurred by the recognition first mechanism; i.e., Na+ was initially taken up into the cavity, then the axial piperidine ligands were replaced with methoxo ligands. On the other hand, when 1 equiv of M+ (= K+, Rb+) was present, the guest recognition occurred by the reaction first mechanism; i.e., M+ was taken up after one of the piperidine ligands was replaced with a methoxo ligand. Therefore, the recognition pathway can be switched by changing the guest cations.

Controlling the Electrochemical Properties of Spinel Intercalation Compounds

Many spinel intercalation compounds are well known for their high ionic conductivity and high voltage in Li-ion batteries. In this study, we investigate the effects of changing the (i) guest cation valence, (ii) guest cation size, and (iii) host ionicity on the site preference and, in turn, intercalation behavior of spinel compounds. We examine the model spinels of CoO2 and TiS2 from first-principles and choose intercalants of Li, Na, and Mg to investigate the parameters of interest. Our results indicate that larger guest cations prefer octahedral coordination of anions within the spinel framework. Electrostatic interactions cause the guest cations to reside on tetrahedral sites within the spinel framework for spinel hosts that are more ionic in nature. These competing factors dictate the site preference and electrochemical properties of spinel intercalation compounds. We propose simple design principles that suggest that Na intercalation into Co, Ni, or Mn oxide spinels would result in fast sodium ion diffusion and therefore make them promising candidates for Na-ion batteries. Figure 1

Tunable fluorescent pseudorotaxane from axle-length-dependent cucurbit[7]uril complexation

A series of aliphatic-chain-appended linear π-conjugated water-soluble cationic fluorophore axle guests with electronic acceptor–donor–acceptor properties have been synthesized. Analysis of cucurbit[7]uril (Q[7])-based host–guest interactions suggests that a dynamic sequence tunable [2]pseudorotaxane to [4]pseudorotaxane transition accompanied by a distinct smart enhancement of the fluorescence signal is elicited in aqueous solution, in which the length of the appended aliphatic-chain play crucial roles in the sequence controlling of the supramolecular assemblies. Meanwhile, the hydrophobic cavity and carbonyl-fringed portals of the Q[7] hosts are seemingly beneficial for switching of the fluorescence emissions of the guests by different binding fashions. This study thus provides new insight for the construction of pseudorotaxanes and fluorescent materials.