Spherical Anions Research Articles

A New Bis-Urea Based Cage Receptor for Anions: Synthesis, Solid State Structures and Binding Studies.

The synthesis and characterization of a novel bis-urea-based cage receptor for anions (3S,15S)-3,15,20,25-tetramethyl-1,4,6,12,14,17,20,25-octaazatricyclo[15.5.5.17.11]octacosa-7(28),10-diene-2,5,13,16-tetraone (L) is reported. L is a macro-bicyclic ligand built on the 1,7-dimethyl-1,4,7,10-tetraazacyclododecane scaffold to obtain a cage topology in which two ureido moieties have been inserted as binding sites for anions. L can interact with anion guests (G) via H-bonding; in particular, it binds both spherical (Cl-) and V-shaped anions (AcO-) as well as more complex carboxylate anions, such as the norfloxacin (Nor-). NMR experiments highlight that the interaction between L and G mainly occurs at the ureido moieties. L forms L-G adducts of 1 : 1 ([LG]-) and 1 : 2 ([LG2]2-) stoichiometry with Cl- and AcO-. Otherwise, in the case of Nor- only the formation of the [LG]- complex is observed. L shows higher formation constants values for [LAcO]- (2.9) and [LNor]- (3.6) than [LCl]- suggesting a stronger interaction with the carboxylate anions. In the solid state, three crystal structures of the HL⋅G species were obtained (G=Cl-, AcO-, ClO4 -) highlighting the H-bonding interaction between the chloride, acetate or perchlorate anions and the -NH functions of the ureido fragment. The comparison between the two parent open chain receptors (Lb-c) and L has been reported and discussed.

Sidey V.I. ON THE IONIC RADIUS OF THE CYANIDE ION

Sidey V.I. ON THE IONIC RADIUS OF THE CYANIDE ION

Hydrogen-Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation.

Hybrid hydrogen-bonded (H-bonded) frameworks built from charged components or metallotectons offer diverse guest-framework interactions for target-specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F- , Cl- , Br- , and I- , with the aim to develop hybrid H-bond synthons that enable the controllable construction of microporous H-bonded frameworks exhibiting fine-tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl- , Br- , and I- , are found to readily participate in the charge-assisted H-bonding assembly with well-defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record-high separation factor (SF=7.0) among H-bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2 -containing gas streams exactly mimicking the off-gas of spent nuclear fuel (SNF) reprocessing.

Hydrogen‐Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

AbstractHybrid hydrogen‐bonded (H‐bonded) frameworks built from charged components or metallotectons offer diverse guest‐framework interactions for target‐specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F−, Cl−, Br−, and I−, with the aim to develop hybrid H‐bond synthons that enable the controllable construction of microporous H‐bonded frameworks exhibiting fine‐tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl−, Br−, and I−, are found to readily participate in the charge‐assisted H‐bonding assembly with well‐defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record‐high separation factor (SF=7.0) among H‐bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2‐containing gas streams exactly mimicking the off‐gas of spent nuclear fuel (SNF) reprocessing.

The Role of Chemical Pressure in the Formation of the Structure and Barocaloric Properties of Complex Fluorides and Oxyfluorides

The role of chemical pressure as an effective tool in the processes of formation of initial and distorted (as a result of structural transformations) phases, thermodynamic properties, and direct and inverse barocaloric effects in some complex oxyfluorides and fluorides with octahedral, tetrahedral, and spherical anion and cation groups in the structure has been studied. It is found that, due to the small temperature hysteresis and high baric sensitivity of materials, the maximum values of absolute and integral barocaloric characteristics can be implemented at low pressures. Correspondingly, the temperature range of reversibility of thermodynamic cycles based on fluorides/oxyfluorides as solid-state coolants can be expanded.

The influence of counterion structure identity on conductivity, dynamical correlations, and ion transport mechanisms in polymerized ionic liquids.

We used equilibrium and non-equilibrium atomistic simulations to probe the influence of anion chemistry on the true conductivity, dynamical correlations, and ion transport mechanisms in polymeric ionic liquids. An inverse correlation was found between anion self-diffusivities, ionic mobilities, and the anion size for spherical anions. While some larger asymmetric anions had higher diffusivities than smaller spherical anions, their diffusivities and mobilities did not exhibit a direct correlation to the anion volumes. The conductivity and anion dynamical correlations also followed the same trends as displayed by the diffusivity and mobility of anions. All the systems we examined displayed positively correlated motion among anions, suggesting a contribution that enhances the conductivity beyond the ideal Nernst-Einstein value. Analysis of ion transport mechanisms demonstrated very similar hopping characteristics among the spherical anions despite differences in their sizes.

Counterion Influenced Metal‐Organic Frameworks of Cyclam with CuII

AbstractCyclam is one of the simplest macrocycle that forms stable metal chelates, much explored for their material and biological applications. Varying anion shape is an effective strategy to modulate the supramolecular structure, inspite of same molecular structure. Here, we report six new complexes of cyclam with CuII bound to counterions of different shapes viz., linear (NCS−/N3−), bent (N(CN)2−) or spherical (Cl−/Br−/I−) in which the shape of the anion leads to distinctly different supramolecular structures (ladder/linear chains with linear anions), braid like (with bent anion) and 3‐D hydrogen bonded networks (with spherical anions) due to changes in the non‐covalent interactions, inspite of the same molecular structure. The variations in packing are responsible for their different thermal properties. With metallo‐cyclams being explored for their efficacy in medical and material fields, studies of this type may offer potential candidates to explore.

Switchable Dielectric Materials Based on a Diprotonated Organic Cation Containing Two Different Anions: (C5H16N2)BF4X (X = Cl, Br, NO3)

Three molecular-based phase transition compounds, namely, (C5H16N2)BF4Cl (1, C5H16N2 = diprotonated 2,2-dimethyl-1,3-propanediamine), (C5H16N2)BF4Br (2), and (C5H16N2)BF4NO3 (3), were synthesized and characterized. Differential scanning calorimetry and the step-like anomaly of the real part (ε′) of the temperature-dependent dielectric constant revealed that the compounds underwent reversible first-order phase transition with high phase transition temperature under the stimulation of temperature. Variable-temperature X-ray diffraction analysis revealed that the microcosmic driving force of phase transition was the order–disordered change of the organic cation and spherical anion. Notably, the diprotonated organic cations were charge-balanced by two different anions of the combination BF4– and Cl–, Br– and NO3–. Our research paves a new avenue to construct molecular-based phase transition materials with temperature stimulus response.

Anion‐Directed Conformation Switching and Trigonal Distortion in Hexakis(methylamine)nickel(II) Cations

AbstractThe homoleptic [Ni(NH2CH3)6]2+ cations have dozens of energetically feasible conformational isomers (conformers), and we herein show their conformation is tunable using counter anions. The reaction of Ni(NO3)2 or NiCl2 with aqueous methylamine in dimethylformamide gave [Ni(NH2CH3)6](NO3)2 (1) or [Ni(NH2CH3)6]Cl2 (2) in which methylamine ligands have axial or equatorial orientation with respect to the three‐fold principal axis, respectively. Interestingly, the conformation of methylamine ligands affects geometry around nickel(II) so that the nickel octahedron is trigonally compressed in 1, whereas it is trigonally elongated in 2, causing different visible light absorption properties between 1 and 2. Density functional theory calculations showed that nitrate anions stabilize the axial conformer through two‐point hydrogen bonding interactions with methylamines, whereas spherical chloride anions promote deformation of the axial conformer into the equatorial conformer through formation of three‐point hydrogen bonding interactions.

Preparation and characterization of spherical lignocellulose-based anion exchanger from sugarcane bagasse

Biosorption is considered a promising technique for removing heavy metals from water. However, a biosorbent is usually prepared in the form of biomass powder that has drawbacks in stability and uniformity. Herein, a spherical lignocellulose-based anion exchanger (LCB-AE) was prepared from sugarcane bagasse through the method of dissolution-regeneration of biomass followed by quaternary ammonium modification. Dissolution-regeneration conditions of raw biomass were optimized, and the prepared materials were characterized by element composition analysis, pore-structure analysis (mercury intrusion porosimetry), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and carbon-13 nuclear magnetic resonance (13C-NMR) analyses. The LCB-AE has a macro-porous structure and a rough surface occupied mainly by quaternary ammonium and hydroxyl groups. Adsorption selectivity of LCB-AE follows the order of CrO42- > PO43- > SO42- > NO3-, and adsorption isotherms agree well with the Langmuir model, which suggests the experimental exchange abilities are approximately 0.8 to 0.9 mEq/g. These results show that LCB-AE as a new spherical biosorbent has the potential to be used for anions removal from water.

Capacitance and Structure of Electric Double Layers: Comparing Brownian Dynamics and Classical Density Functional Theory

We present a study of the structure and differential capacitance of electric double layers of aqueous electrolytes. We consider electric double layer capacitors (EDLC) composed of spherical cations and anions in a dielectric continuum confined between a planar cathode and anode. The model system includes steric as well as Coulombic ion-ion and ion-electrode interactions. We compare results of computationally expensive, but “exact” , Brownian Dynamics (BD) simulations with approximate, but cheap, calculations based on classical Density Functional Theory (DFT). Excellent overall agreement is found for a large set of system parameters, including variations in concentration, ionic size- and valency-asymmetries, applied voltages and electrode separation, provided the differences between the canonical ensemble of the BD simulations and the grand-canonical ensemble of DFT are properly taken into account. In particular, a careful distinction is made between the differential capacitance C_N at fixed number of ions and C_mu at fixed ionic chemical potential. Furthermore, we derive and exploit their thermodynamic relations. In the future these relations will also be useful for comparing and contrasting experimental data with theories for supercapactitors and other systems. The quantitative agreement between simulation and theory indicates that the presented DFT is capable of accounting accurately for coupled Coulombic and packing effects. Hence it is a promising candidate to cheaply study room temperature ionic liquids at much lower dielectric constants than that of water.

Tripodal Squaramide Derivative as a Neutral Chloride Ionophore for Whole Blood and Sweat Chloride Measurement

AbstractA tripodal squaramide derivative was synthesized and characterized as a new neural ionophore for chloride ion. Squaramide based compounds are known to be better hydrogen bonding receptors than urea derivatives because they donate four hydrogen bonds to anions, while the tripodal structure provides a 3D geometry optimum for spherical anion complexation. The ionophore demonstrated high selectivity over salicylate and heparin that are major interferences for chloride measurement in biological samples with direct potentiometric method. The new ionophore based solid‐contact electrodes were fabricated and showed the measuring range of 10−5 to 1 M chloride ion in the presence of 1 mM of salicylate and heparin with near Nernstian slope of 55 mV/decade. The sensor was successfully applied for the chloride measurement in undiluted heparinized whole blood and artificial sweat samples with our clinical analyzer.

Hybrid Organic-Inorganic Antiperovskites.

Substitution of A-site and/or X-site ions of ABX3 -type perovskites with organic groups can give rise to hybrid perovskites, many of which display intriguing properties beyond their parent compounds. However, this method cannot be extended effectively to hybrid antiperovskites. Now, the design of hybrid antiperovskites under the guidance of the concept of Goldschmidt's tolerance factor is presented. Spherical anions were chosen for the A and B sites and spherical organic cations for the X site, and seven hybrid antiperovskites were obtained, including (F3 (H2 O)x )(AlF6 )(H2 dabco)3 , ((Co(CN)6 )(H2 O)5 )(MF6 )(H2 dabco)3 (M=Al3+ , Cr3+ , or In3+ ), (Co(CN)6 )(MF6 )(H2 pip)3 (M=Al3+ or Cr3+ ), and (SbI6 )(AlF6 )(H2 dabco)3 . These new structures reveal that all ions at A, B, and X sites of inorganic antiperovskites can be replaced by molecular ions to form hybrid antiperovskites. This work will lead to the synthesis of a large family of hybrid antiperovskites.

Hybrid Organic–Inorganic Antiperovskites

AbstractSubstitution of A‐site and/or X‐site ions of ABX3‐type perovskites with organic groups can give rise to hybrid perovskites, many of which display intriguing properties beyond their parent compounds. However, this method cannot be extended effectively to hybrid antiperovskites. Now, the design of hybrid antiperovskites under the guidance of the concept of Goldschmidt's tolerance factor is presented. Spherical anions were chosen for the A and B sites and spherical organic cations for the X site, and seven hybrid antiperovskites were obtained, including (F3(H2O)x)(AlF6)(H2dabco)3, ((Co(CN)6)(H2O)5)(MF6)(H2dabco)3 (M=Al3+, Cr3+, or In3+), (Co(CN)6)(MF6)(H2pip)3 (M=Al3+ or Cr3+), and (SbI6)(AlF6)(H2dabco)3. These new structures reveal that all ions at A, B, and X sites of inorganic antiperovskites can be replaced by molecular ions to form hybrid antiperovskites. This work will lead to the synthesis of a large family of hybrid antiperovskites.

Synthesis and Anion Recognition Features of a Molecular Cage Containing Both Hydrogen Bond Donors and Acceptors.

A molecular cage, macrobicycle 2, containing amide and pyrrole groups as hydrogen-bonding donors and imine groups as hydrogen-bonding acceptors has been synthesized. Compound 2 was found to recognize tetrahedral oxyanions with high affinities, such as H2PO4-, HSO4-, SO42-, and HP2O73-, as well as the spherical halide anions, in chloroform. A single-crystal X-ray diffraction analysis revealed that compound 2 formed a 1:1 complex with H2PO4- in the solid state.

Halide-Controlled Extending-Shrinking Motion of a Covalent Cage.

Herein, we present an example of covalent cages, whose flexible framework undergoes extending-shrinking motion under halide control. In the absence of halide anions, the free cage assumes a flattened conformation: the cavity is compressed along the C3 axis passing through the tertiary amines, and the two tribenzylamine platforms are eclipsed. Halide encapsulation promotes a large conformational rearrangement of the cage, involving an extension of the cavity along the C3 axis and shrinkage along the equatorial plane. Interestingly, the rearrangement is accompanied by the pyramidal inversion of the tertiary amines and by the rotation of the tribenzylamine-based platforms, which become staggered. The imidazolium-containing arms wrap around the spherical anion, leading to a racemic mixture of the M and P helical complexes. As expected from the flexible structure of the cage, the switch between the two limit conformations can be repeated for several cycles under alternating chemical stimuli (AgNO3/TBACl). This result is consistent with the low activation barriers determined by computational investigations. These also allowed us to quantify the energy difference between the shrunk and expanded cage conformations and to hypothesize an energetic pathway along which the conformational rearrangement can occur.

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Pyrrolidinium-Based Ionic Liquids: Effects of Anion Species.

We investigated the temperature dependence of the intermolecular vibrational dynamics of pyrrolidinium-based ionic liquids (ILs) with 10 different anion species using femtosecond Raman-induced Kerr effect spectroscopy. The features of the temperature-dependent vibrational spectra vary with the different anions. In the case of the ILs with spherical top anions, such as tetrafluoroborate and hexafluorophosphate, and trifluoromethanesulfonate, the spectral intensity in the low-frequency region below 50 cm-1 increases with rising temperature, while that in the high-frequency region above 50 cm-1 remains almost unchanged. Similar temperature-dependent features were also found in the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts. However, the difference spectra at respective temperature relative to 293 K indicate that the spectra of the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts are more temperature-sensitive in the low-frequency region below 50 cm-1 compared to those of the tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate salts. The spectra of 1-butyl-1-methylpyrrolidinium-based ILs with dicyanamide and tricyanomethide anions show a characteristic temperature dependence; in addition to an increase of the spectral intensity in the low-frequency region below 50 cm-1, a red shift of the spectra in the high-frequency side above 50 cm-1 was observed with increasing temperature. This implies that the librational motions of planar dicyanamide and tricyanomethide anions contribute substantially to the low-frequency spectra. We also compared the temperature-dependent low-frequency spectra of 1-butyl-1-methylpyrrolidinium- and 1-(2-methoxyethyl)-1-methylpyrrolidinium-based ILs with some anions. Although the spectral shapes are slightly different in the range of 70-150 cm-1, which can be attributed to the intramolecular vibrational modes of the cations, the temperature dependence of the spectral shapes is quite similar, indicating that the ether substitution in the cation side groups has little effects on the temperature dependence of the low-frequency spectra. The fragilities of the ILs were also estimated from the temperature-dependent viscosities and the glass-transition temperatures. The fragility parameter seems to be correlated with the temperature dependence of the first moment of the low-frequency spectral bands mainly arising from the intermolecular vibrations of the ILs.

Molecular dynamics investigation of the influence of the shape of the cation on the structure and lubrication properties of ionic liquids.

We present a theoretical study of the influence of the molecular geometry of the cation on the response of ionic liquids (ILs) to confinement and mechanical strain. The so-called tailed model includes a large spherical anion and asymmetric cation consisting of a charged head and a neutral tail. Despite its simplicity, this model recovers a wide range of structures seen in ILs: a simple cubic lattice for small tails, a liquid-like state for symmetric cation-tail dimers, and a molecular layer structure for dimers with large tails. A common feature of all investigated model ILs is the formation of a fixed (stable) layer of cations along solid plates. We observe a single anionic layer for small gap widths, a double anionic layer for intermediate ones, and tail-to-tail layer formation for wide gaps. The normal force evolution with gap size can be related to the layer formed inside the gap. The low hysteretic losses during the linear cyclic motion suggest the presence of strong slip inside the gap. In our model the specific friction is low and the friction force decreases with tail size.

Off-center charge model revisited: Electrical double layer with multivalent cations.

The off-center charge model of ions is a relatively simple model for introducing asymmetry in Coulomb interaction while retaining the simplicity and convenience of the spherical hard core geometry. A Monte Carlo simulation analysis of the planar electric double layer formed by this ionic model for 1+:1- valence systems [S. Lamperski et al., Langmuir 33, 11554-11560 (2017)] is extended to include solutions of multivalent (2+, 3+) hard spherical cations and single valence (1-) hard spherical anions near a uniformly charged, planar electrode. The solvent is modelled as a uniform dielectric continuum with a dielectric constant equal to that of the pure solvent, viz., the primitive model. Results are reported for the ion density, the cation charge profile, and the electrostatic potential profile at 1 mol/dm3 salt concentration. Additionally, the double layer potential drop, that is, the electrode potential, and the integral and the differential capacitances are computed as functions of the electrode surface charge density. The latter two quantities show an expected asymmetry as long as the cation valence is not too great and the charge of the off-center ion cannot approach too close to the electrode surface. It is unusual that the integral and differential capacitances are negative for high valence cations and a negatively charged electrode when the off-center charge is large and can be very near the surface of the electrode. The corresponding electrode potential versus surface charge density curve becomes non-monotonic and shows a change of slope, and thus the resultant integral and differential capacitances can become negative. This nonphysical result is the result of an incipient singularity when a large positive charge is too near a negatively charged electrode. Overall, the off-center charge model suggests a useful recipe to model electrical asymmetry within the broader context of the primitive model provided that the off-center charge is not too near the surface of the electrode.

Heteropoly Acid Functionalized Membranes for High Performance and Chemically Stable Fuel Cell Operation

There is still a need for membranes that operate in proton exchange membrane (PEM) fuel cells at hotter and drier conditions than can be achieved with current materials, >100°C and <50%RH. One approach pursued by us and independently by Kunz and co-workers was the use of inorganic super acids, such as the heteropoly acids (HPAs). HPAs are a sub group of the large class of metal oxygen clusters known as polyoxometalates in which a central heteroatom is surrounded by a number of W or Mo oxygen octahedra. For proton conductivity it is desirable that a strong negative charge be delocalized across the whole anion so that the proton will be as dissociated as possible. This limits the choice of HPA to the spherical tungsten based Keggin anion with as light as possible a heteroatom. This limit is reached with Si as the P based HPA is known to decompose in the presence of peroxide and the electron deficient nature of B renders the spherical Keggin anion unstable. Many fundamental studies have been undertaken on solid state HPA systems. In addition the Si based HPA are known to decompose oxygenated radicals and so offer a path to the development of a PEM that is oxidatively stable under fuel cell operating conditions. These studies indicated that despite the original report in the early 80’s that HPA had the highest proton conductivity reported at that time, that when dry there proton conductivity was disappointingly low at moderate temperatures. The key is to distribute the HPA on a perfluorinated polymer backbone in such a way that the natural tendency of the HPA to cluster and crystalize is suppressed. Using dehydroflourination chemistry we have learnt to functionalism PVDF type polymers with molecules that can act as anchor points for lacunary HPA, where a lacunary HPA has 1,2 or 3 tungsten oxygen clusters removed to give bonding points. After much research we now can produce these materials in large area thin film membranes. The base material outperforms the state of the art PEM under standard conditions and has the highest chemical stability of any PEM proposed to date. We are now varying the HPA structure to try and avoid the HPA clustering issue, these membranes are being sprayed on gas diffusion electrodes which act as the support to the tin films. This is allowing us to run these as fuel cells at higher temperatures to asses the potential of HPA functionalized membranes in these applications.