Symmetric Cations Research Articles

Ionic liquids and plastic crystals with a symmetrical pyrrolidinium cation

New salts using the symmetrical diethylpyrrolidinium cation and a range of anions show varied thermal behaviour and good transport properties.

Halogen-free guanidinium-based perovskite solar cell with enhanced stability.

Herein, we report a new halogen-free and excellent stable perovskite, GAPb(SCN)3, which was prepared from the symmetric guanidinium cation (GA+) with nearly zero dipole moment, and pseudohalogen (SCN−), for the fabrication of stable perovskite solar cells. GAPb(SCN)3 exhibits an orthorhombic crystal phase, with optical band gap of 1.43 eV. The orthorhombic crystal phase shifted to a cubic phase, due to the formation of PbS beyond 150 °C. GAPb(SCN)3 itself showed excellent stability upon being exposed to ambient conditions for 30 days, without degradation of its optical or crystallographic properties. This superior stability could be attributable to the strong electrostatic interaction between SCN− and Pb2+, and hydrogen bonding between SCN− and GA+. Even though it has a suitable band gap, it exhibited a significantly lower efficiency of ca. 0.11%. The very low performance could be attributable to the significantly low light absorption coefficient, and large non-radiative recombination of photo-induced charges via an oxidized form of S at the TiO2/perovskite interface.

Supramolecular self-assembly for designing non-centrosymmetric crystals based on Keggin polyoxometallates and crown ether

We report the synthesis of five novel crystals [(4-BrAni+)(DB[18]crown-6)]2[SMo12O402-]·2CH3CN (1), [(4-BrAni+)(B[18]crown-6)]2[SMo12O402-]·CH3CN (2), [(4-BrAni+)(B[18]crown-6)]3[PMo12O402-]·2CH3CN (3), [(3-AP+)3(B[18]crown-6)2][PMo12O403-] (4) and [NBu4+][(3-AP2+)(DB[30]crown-10)][PMo12O403-]·CH3CN (5) (4-BrAni+ = 4-bromoanilinium; B[18]crown-6 = benzo[18]-crown-6; DB[18]crown-6 = dibenzo[18]-crown-6; DB[30]crown-10 = dibenzo[30]-crown-10; 3-AP+ = 3-aminopyridinium; 3-AP2+ = 3-ammoniumpyridinium; NBu4+ = tetrabutylammonium). In order to construct non-centrosymmetric crystals, the five crystals were designed using a method to gradually introduce asymmetry into the building units. Crystal 1 was constructed with a symmetric supramolecular cation (SPC) [(4-BrAni+)(DB[18]crown-6)], resulting in a P21/n space group. The asymmetric SPC [(4-BrAni+)(B[18]crown-6)] was introduced into [SMo12O402-] to obtain crystal 2, which belongs to the symmetric P1[combining macron] space group. Introducing trivalent [PMo12O403-], [(4-BrAni+)(B[18]crown-6)] produced crystal 3 with a non-centrosymmetric Pc space group. The asymmetric sandwich SPC [(3-AP+)3(B[18]crown-6)2] was designed with multiple hydrogen bonding sites on the 3-AP+ cation, and crystal 4 was obtained with trivalent [PMo12O403-]. Crystal 4 has the properties of the chiral P1 space group. The distorted SPC (3-AP2+)(DB[30]crown-10) was constructed using flexible DB[30]crown-10, resulting in crystal 5 which matched the chiral P21 space group with trivalent [PMo12O403-]. This work focuses on strategies for the rational design of novel non-centrosymmetric crystals without a chiral synthon.

Bis(bipyridinium) Salts as Multicolored Electrochromic Devices

Symmetrical and nonsymmetrical oxobis(bipyridinium) cations combined with iodide and bistriflimide anions were prepared. In the case of the bistriflimide-based bis(bipyridinium) salts, two room-temperature ionic liquids (RTILs) with a combination of linkers and terminal ethers, [(C3 O)bpy(C4 O)bpy(C3 O)][NTf2 ]4 and [(C10 )bpy(C4 O)bpy(C3 O)][NTf2 ]4 , were obtained, with glass transition temperatures (between -6.9 and 6.0 °C) higher than those reported previously for salts of dication bipyridinium scaffolds (-17.5 to -30.0 °C). The prepared salt containing [(C10 )bpy(C4 O)bpy(C10 )][NTf2 ] can form an ionic liquid crystal structure. The most promising salts were tested as efficient and reversible multicolored electrochromic devices through cyclic voltammetry studies and detailed characterizations of their electrochromic performances. In general, tetracations based on oxobis(bipyridinium) salts show higher chromatic contrast and lower coloration efficiency than similar dication salts. Exchange of the anion from iodide to [NTf2 ] increases the transition times and coloration efficiencies for all the prepared di- and tetracation salts. The electrochromic device stability of [(C3 O)bpy(C4 O)bpyC3 O]I4 was tested by on/off cycling; this device loses around 23 % of the initial color after 8640 redox cycles and shows very good cycling stability compared with previously reported diiodide salts.

Triazatruxene radical cation: a trigonal class III mixed valence system

The highly symmetric and stable triazatruxene radical cation was generated chemically for the first time.

Short- and long-range corrected hybrid density functionals with the D3 dispersion corrections.

We propose a short- and long-range corrected (SLC) hybrid scheme employing 100% Hartree-Fock exchange at both zero and infinite interelectronic distances, wherein three SLC hybrid density functionals with the D3 dispersion corrections (SLC-LDA-D3, SLC-PBE-D3, and SLC-B97-D3) are developed. SLC-PBE-D3 and SLC-B97-D3 are shown to be accurate for a very diverse range of applications, such as core ionization and excitation energies, thermochemistry, kinetics, noncovalent interactions, dissociation of symmetric radical cations, vertical ionization potentials, vertical electron affinities, fundamental gaps, and valence, Rydberg, and long-range charge-transfer excitation energies. Relative to ωB97X-D, SLC-B97-D3 provides significant improvement for core ionization and excitation energies and noticeable improvement for the self-interaction, asymptote, energy-gap, and charge-transfer problems, while performing similarly for thermochemistry, kinetics, and noncovalent interactions.

Twinned CsLn2F7 compounds (Ln=Nd, Gd, Tb, Er, Yb): the role of a highly symmetrical cation lattice with an arrangement analogous to the Laves phase MgZn2

Abstract The occurrence of twinning can often be related to higher symmetrical structures. Fluorides are frequently twinned due to their close relation to high symmetry structures like fluoride, tysonite or pyrochlores. The series of compounds CsLn2F7 is no exception. We refined the structures of the twinned compounds with Ln=Nd, Gd, Tb, Er, Yb in space group P1121/b. An analysis of the pseudosymmetry of the resulting structures shows a highly symmetrical cation partial structure with a cation distribution similar to the one in the hexagonal Laves phase MgZn2. Several other compounds ALn2F7, which have been described in the literature, show a similar cation array. The diversity of different space groups which have been reported for ALn2F7 compounds can be better understood using group-subgroup relationships assuming the hypothetical structure of the cation array with space group P63/mmc as aristotype. Furthermore, the twinning is easily understood on the basis of the lost symmetry operations in the symmetry reduction from point group 6/mmm, e.g. to 2/m in the case of the CsLn2F7 compounds.

15N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

Ionization dynamics of the branched water cluster: A long-lived non-proton-transferred intermediate

The proton transfer (PT) reaction after water cluster ionization is known to be a very fast process occurring on the 10–30fs time scale. In the present study, the ionization dynamics of the branched water tetramer (H2O)4 were investigated by means of a direct ab initio molecular dynamics (AIMD) method to elucidate the time scale of PT in the water cluster cation. A long-lived non-proton-transferred intermediate was found to exist after the ionization of the branched-type water cluster. The lifetimes of the intermediate were calculated to be ca. 100–150fs. PT occurred after the formation of the intermediate. The structure of the intermediate was composed of a symmetric cation core: H2O–H2O+–H2O. The broken symmetry of the structure led to PT from the intermediate. The reaction mechanism is discussed based on the theoretical results.

Towards the Application of Structure-Property Relationship Modeling in Materials Science: Predicting the Seebeck Coefficient for Ionic Liquid/Redox Couple Systems.

This work focuses on determining the influence of both ionic-liquid (IL) type and redox couple concentration on Seebeck coefficient values of such a system. The quantitative structure-property relationship (QSPR) and read-across techniques are proposed as methods to identify structural features of ILs (mixed with LiI/I2 redox couple), which have the most influence on the Seebeck coefficient (Se ) values of the system. ILs consisting of small, symmetric cations and anions with high values of vertical electron binding energy are recognized as those with the highest values of Se . In addition, the QSPR model enables the values of Se to be predicted for each IL that belongs to the applicability domain of the model. The influence of the redox-couple concentration on values of Se is also quantitatively described. Thus, it is possible to calculate how the value of Se will change with changing redox-couple concentration. The presence of the LiI/I2 redox couple in lower concentrations increases the values of Se , as expected.

The salts of chloronium ions R-Cl(+)-R (R = CH3 or CH2Cl): formation, thermal stability, and interaction with chloromethanes.

The interaction of CH3Cl/CD3Cl or CH2Cl2/CD2Cl2 with the carborane acid H(CHB11Cl11) (abbreviated as H{Cl11}) generates the salts of CH3-{Cl11} and CH2Cl-{Cl11} and their deuterio analogs, respectively, which are analogs of the salts of asymmetric chloronium cations. Next, salts of chloronium cations CH3-Cl(+)-CH3, ClCH2-Cl(+)-CH2Cl, and ClCH2-Cl(+)-CH3 and their deuterio analogs were obtained from the above compounds. The asymmetric ClCH2-Cl(+)-CH3 cation was found to be unstable, and at ambient temperature, slowly disproportionated into symmetric cations (CH3)2Cl(+) and (CH2Cl)2Cl(+). At a high temperature (150 °C), disproportionation was completed within 5 minutes, and the resulting cations further decomposed into CH3-{Cl11} and CH2Cl-{Cl11}. The molecular fragment ClCH2-(X) of the compounds (X = {Cl11}, -Cl(+)-CH2Cl, or -Cl(+)-CH3) is involved in exchange reactions with CH2Cl2 and CHCl3, converting into CH3-(X) with the formation of chloroform and CCl4, respectively.

Protic organic ionic plastic crystals based on a difunctional cation and the triflate anion: a new solid-state proton conductor.

Two protic organic ionic plastic crystalline materials based on the mono- and di-protonated cations of N,N-dimethylethylenediamine, (DMEDA) and the triflate anion have been studied. The symmetrical di-protonated cation salt shows 54 °C higher melting point and 24-fold higher conductivity at room temperature as compared to the mono-protonated cation salt, making it a promising candidate for a solid-state proton conductor. The conductivity of this salt was further enhanced by acid doping.

Guanidinium nonaflate as a solid-state proton conductor

Guanidinium nonaflate, a novel pure protic organic ionic plastic crystal (POIPC) with an elegant symmetrical cation, is discovered to be a new solid-state proton conductor.

Self-electrophoresis of spheroidal electrocatalytic swimmers

Using the method of matched asymptotic expansions, we derive a general expression for the speed of a prolate spheroidal electrocatalytic nanomotor in terms of interfacial potential and physical properties of the motor environment in the limit of small Debye length and Péclet number. This greatly increases the range of geometries that can be handled without resorting to numerical simulations, since a wide range of shapes from spherical to needle-like, and in particular the common cylindrical shape, can be well-approximated by prolate spheroids. For piecewise-uniform distribution of surface cation flux with fixed average absolute value, the mobility of a prolate spheroidal motor with a symmetric cation source/sink configuration is a monotonically decreasing function of eccentricity. A prolate spheroidal motor with an asymmetric sink/source configuration moves faster than its symmetric counterpart and can exhibit a non-monotonic dependence of motor speed on eccentricity for a highly asymmetric design.

Crystal packing modes in the salts of ethane-1,2-diamine with acetic acid and its halogeno derivatives

In a series of salts of ethane-1,2-diamine with acetic (1), trifluoroacetic (2), trichloroacetic (3), and dichloroacetic (4) acids, the amine is doubly protonated so the cation:anion stoichiometry is 2:1. In 1 and 2, dications are lying across the center of symmetry (Z′ = 1/2), in 4 dication is nonsymmetric (Z′ = 1), while the structure of 3 contains both symmetric and nonsymmetric cations (Z′ = 3/2). All dications, either symmetric or not, are in extended (trans) form, by far the most popular among the salts of ethane-1,2-diamine. Strong hydrogen N–H···O bonds constitute the main specific, directional force determining the crystal packing. In 1, one of the N–H hydrogen atoms is involved in bifurcated hydrogen bonds, and in this case, four alresaacceptor oxygen atoms are almost equally involved in accepting the hydrogen bonds; in consequence, the C–O bond lengths are almost equal. In all other cases, the C–O bonds with oxygen atoms involved in more hydrogen bonds are systematically longer than those which accept less such interactions. Halogen C–Cl···O interactions are observed in two (out of five) symmetry-independent anions of 3 and 4, suggesting that at least in this series, the roles of these interactions are only secondary in nature. No significant involvement of fluorine atoms in directional interactions is detected, however. The hydrogen-bond networks are described by means of graph-set method. In addition, Hirshfeld surfaces are used to detect and visualize the differences between similar moieties, and fingerprint plots are presented in order to analyze the mutual importance of different kinds of interactions.

Quantitative reflectance spectra of solid powders as a function of particle size.

We have recently developed vetted methods for obtaining quantitative infrared directional-hemispherical reflectance spectra using a commercial integrating sphere. In this paper, the effects of particle size on the spectral properties are analyzed for several samples such as ammonium sulfate, calcium carbonate, and sodium sulfate as well as one organic compound, lactose. We prepared multiple size fractions for each sample and confirmed the mean sizes using optical microscopy. Most species displayed a wide range of spectral behavior depending on the mean particle size. General trends of reflectance versus particle size are observed such as increased albedo for smaller particles: for most wavelengths, the reflectivity drops with increased size, sometimes displaying a factor of 4 or more drop in reflectivity along with a loss of spectral contrast. In the longwave infrared, several species with symmetric anions or cations exhibited reststrahlen features whose amplitude was nearly invariant with particle size, at least for intermediate and large size sample fractions: that is, ≳150 μm. Trends of other types of bands (Christiansen minima, transparency features) are also investigated as well as quantitative analysis of the observed relationship between reflectance versus particle diameter.

Comprehensive study on the impact of the cation alkyl side chain length on the solubility of water in ionic liquids

A comprehensive study on the phase behaviour of two sets of ionic liquids (ILs) and their interactions with water is here presented through combining experimental and theoretical approaches. The impact of the alkyl side chain length and the cation symmetry on the water solubility in the asymmetric [CN−1C1im][NTf2] and symmetric [CN−1CN−1im][NTf2] series of ILs (N up to 22), from 288.15K to 318.15K and at atmospheric pressure, was studied. The experimental data reveal that the solubility of water in ILs with an asymmetric cation is higher than in those with the symmetric isomer. Several trend shifts on the water solubility as a function of the alkyl side chain length were identified, namely at [C6C1im][NTf2] for asymmetric ILs and at [C4C4im][NTf2] and [C7C7im][NTf2] for the symmetric ILs. To complement the experimental data and to further investigate the molecular-level mechanisms behind the dissolution process, density functional theory calculations, using the Conductor-like Screening Model for Real Solvents (COSMO-RS) and the electrostatic potential-derived CHelpG, were performed. The COSMO-RS model is able to qualitatively predict water solubility as a function of temperature and alkyl chain lengths of both symmetric and asymmetric cations. Furthermore, the model is also capable to predict the somewhat higher water solubility in the asymmetric cation, as well as the trend shift as a function of alkyl chain lengths experimentally observed. Both COSMO-RS and the electrostatic potential-derived CHelpG show that the interactions of water and the IL cation take place on the IL polar region, namely on the aromatic head and adjacent methylene groups that explains the differences in water solubility observed for cations with different chain lengths. Furthermore, the CHelpG calculations for the isolated cations in the gas phase indicates that the trend shift of water solubility as a function of alkyl chain lengths and the difference of water solubility in symmetric may also result from the partial positive charge distribution/contribution of the cation.

Synthesis and Crystal Structure of New Lacunary Wells–Dawson with an Unprecedented Eu-Substituted Sandwiched Cluster

A new red emitting sandwich-type phosphotungstate built on the lacunary Wells–Dawson polyanion [W17(PO4)1.75O52]10− and trivalent rare earth ion, $${\text{K}}_{ 8} \left[ {{\text{W}}_{ 3 5} {\text{Eu}}_{0. 5} \left( {{\text{PO}}_{ 4} } \right)_{ 1. 7 5} {\text{O}}_{ 10 5} } \right]_{2}$$ ·15·5H2O (1) has been synthesized and characterized by SEM-EDX, single crystal X-ray diffraction, and PL spectroscopy. It crystallizes in the monoclinic space group C2/c with unit cell dimensions a = 43.523(4) A, b = 12.3522(6) A, c = 29.4592(12) A, β = 100.933(6)°, Z = 2 and 15,549.8(16) A3. The crystal structure has been determined and refined to R1 and wR2 were 0.0682 and 0.1652 respectively. In this polyanion, two symmetrical EuIII cations sandwiched between C 2 symmetry related lacunary Wells–Dawson-like anions.

Phosphonium-Organophosphate Ionic Liquids as Lubricant Additives: Effects of Cation Structure on Physicochemical and Tribological Characteristics

Our previous work suggested great potential for a phosphonium-organophosphate ionic liquid (IL) as an antiwear lubricant additive. In this study, a set of five ILs were carefully designed and synthesized, with identical organophosphate anions but dissimilar phosphonium cations, to allow systematic investigation of the effects of cation alkyl chain length and symmetry on physicochemical and tribological properties. Symmetric cations with shorter alkyl chains seem to increase the density and thermal stability due to closer packing. On the other hand, either higher cation symmetry or longer alkyl moieties induce a higher viscosity, though the viscosity index is dependent more on molecular mass than on symmetry. While a larger cation size generally increases an IL's solubility in nonpolar hydrocarbon oils, six-carbon seems to be the critical minimum alkyl chain length for high oil miscibility. Both the two ILs, that are mutually oil miscible, have demonstrated promising lubricating performance at 1.04% treat rate, though the symmetric-cation IL moderately outperformed the asymmetric-cation IL. Characterizations on the tribofilm formed by the best-performing symmetric-cation IL revealed the film thickness, nanostructure, and chemical composition. Results here provide fundamental insights for future molecular design in developing oil-soluble ILs as lubricant additives.

Thermophysical properties of sulfonium- and ammonium-based ionic liquids

Experimental data for the density, viscosity, refractive index, and surface tension of four sulfonium- and ammonium-based ionic liquids (ILs) with the common bis(trifluoromethylsulfonyl)imide anion were measured in the temperature range between 288.15 and 353.15K and at atmospheric pressure. The ILs considered include butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, [N4111][NTf2], tributylmethylammonium bis(trifluoromethylsulfonyl)imide, [N4441][NTf2], diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, [S221][NTf2], and triethylsulfonium bis(trifluoromethylsulfonyl)imide, [S222][NTf2]. Based on the gathered results and on data taken from literature, the impact of the cation isomerism and of the size of the aliphatic tails, as well as the effect resulting from the substitution of a nitrogen by a sulfur atom as the cation central atom, on the thermophysical properties of sulfonium- and ammonium-based ILs is discussed here. Remarkably, more symmetric cations present a lower viscosity for the same, and sometimes even for higher, alkyl chain lengths at the cation. Additional derivative properties, such as the isobaric thermal expansion coefficient, the surface thermodynamic properties and the critical temperature for the investigated ILs were also estimated and are presented and discussed.