Stacks Of Cations Research Articles

Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations.

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation-anion H-bond-like motifs, cation-cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C4mim(+)) cation is coupled with the acetate (OAc(-)), chloride (Cl(-)), tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), or bis(trifluoromethanesulfonyl)amide (TFSA(-)) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc(-) and Cl(-), prefer to be located in the imidazolium ring plane next to the C-H(2/4-5) sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H(2) site are found to be particularly enhanced in comparison with the ones at H(4-5) in the case of asymmetric and/or more basic anions (C4mimOAc, C4mimCl, C4mimTfO, and C4mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation-cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion.

Stacking of bipyridinium radical cations incorporated in rigid conjugated polymers

Three rigid bipyridinium (BIPY2+)-incorporated coplanar polymers, with a degree of polymerisation of 27, 45 and 11, have been synthesised through the formation of hydrazone bonds from the condensation of a BIPY2+-derived dialdehyde and hydrazine, carbohydrazide or oxalohydrazide. Treatment of the polymers with zinc dust in acetonitrile led to the one-electron reduction of the BIPY2+ units to radical cation BIPY•+. UV–vis and electron paramagnetic resonance experiments revealed that the BIPY•+ units in these coplanar polymers stacked considerably into (BIPY•+)2 in acetonitrile through multivalent π interactions, leading to the formation of new homoduplexes. In contrast, a control compound containing only one BIPY•+ unit did not undergo dimerisation to a detectable extent. The new radical cation stacking-driven homoduplexes could be further stabilised by cyclobis(paraquat-4,4’-biphenylene)-derived bis(radical cation) square cyclophane through encapsulating the (BIPY•+)2 units in the polymer backbones.

A structural investigation of ionic liquid mixtures.

The structures of mixtures of ionic liquids (ILs) featuring a common 1-butyl-3-methylimidazolium ([C4C1im](+)) cation but different anions have been investigated both experimentally and computationally. (1)H and (13)C NMR of the ILs and their mixtures has been performed both on the undiluted liquids and those diluted by CD2Cl2. These experiments have been complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These techniques have identified the formation of preferential interactions between H(2) of the imidazolium cation and the most strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion to interact above the imidazolium ring through anion-π(+) interactions has been identified. The modelling of these data has identified that the magnitude of these preferences are small, of the order of only a few kJ mol(-1), for all IL mixtures. No clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the reorientation of the cation within a randomly assigned network of anions. π(+)-π(+) stacking of the imidazolium cations was also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily disrupted by the introduction of small proportions (<50 mol%) of a weakly coordinating anion due to their propensity to form anion-π(+) interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit ideal, or nearly ideal, behaviour.

Supramolecular radical polymers self-assembled from the stacking of radical cations of rod-like viologen di- and trimers

A series of π-conjugated oligomeric viologens have been synthesized, from which supramolecular radical polymers were constructed through the stacking of their radical cations.

Magnetic properties of 4,4′-bipiperidinediium tetrahalocuprates: crystal structures of 4,4′-bipiperidinediium tetrabromocuprate(II) monohydrate and bis(4,4′-bipiperidinediium) hexabromodicuprate(I)

Reaction of 4,4′-bipiperidine (bipip) with copper(II) bromide in aqueous HBr yielded the salts (H2bipip)CuBr4·H2O (2) and (H2bipip)2Cu2Br6 (3). Compound 2 crystallizes in the space group P21/m and forms alternating layers of the H2bipip cations and CuBr4 anions with the water molecules located in the anionic layer. Compound 3 contains Cu(I) and crystallizes in the space group P21/c and comprises stacks of H2bipip cations alternating with stacks of anions. Temperature-dependent magnetic measurements on 2 and its chloride analog, 1, show distinctly different behaviors supported by the two-halide superexchange pathway. The chloride compound (1) is best described as weakly interacting antiferromagnetic dimers (J ~ −13 K), while the bromide compound (2) exhibits very weak ferromagnetic interactions best modeled as a uniform chain with J = 2.45 (9) K (using the H = −JABΣSA·SB Hamiltonian).

Synthesis, crystal structures and magnetic properties of three complexes containing bis(maleonitriledithiolate) copper(II) anion and meta-substituted benzyl isoquinolinium cations

Three complexes, [3RBzIQl]2[Cu(mnt)2] (mnt2− = maleonitriledithiolate; [3RBzIQl]+ = 1-(3′-R-benzyl)isoquinolinium, R = Cl (1), I (2) or NO2 (3)), have been prepared and characterized by spectroscopic and physicochemical methods as well as by X-ray crystallography. The [Cu(mnt)2]2− anions formed a 1D chain in complex 1, a dimer in 2 and a sheet structure in 3 through C···N and/or N···N interactions. The neighboring dimers of the cations arranged in a boat-type pattern stack into columns through weak π···π, Cl···π or O···π, C–H···π interactions. The anions and cations stack together into a columnar structure with a ···A–CC–A–CC–A··· sequence. The magnetic susceptibilities measured in the temperature range 2.0–300 K reveal that complex 1 undergoes a change from weak ferromagnetic to antiferromagnetic exchange around 132 K, while 2 shows antiferromagnetic behavior and 3 shows a weak ferromagnetic interaction when the temperature is lowered.

Crystal and molecular structure of [Au(C14H22N4)]ReO4

Perrhenate of the tetraazamacrocyclic complex of gold(III) [Au(C14H22N4)]ReO4 is synthesized. The crystal structure of the compound is determined by single crystal X-ray diffraction. The crystallographic data are as follows: а = 7.733(2) A, b = 11.368(2) A, c = 11.685(2) A, α = 116.23°, β = 104.26°, γ = 94.96°; V = 870.3(3) A3, space group P(–1), Z = 2, dcalc = 2.647 g/cm3, R = 0.0245. The packing of the complex is comprised of double stacks of cations and chains of anions linked into two-dimensional layers due to weak intermolecular interactions (non-classical hydrogen bonds C–H…O, C–H…Au, C–H…π and the Au…Au contacts). The thermal decomposition of the obtained compound in the hydrogen atmosphere is studied. According to the powder X-ray diffraction data, the product of the reduction of the synthesized salt is a mixture of nanocrystalline phases of Re and Au with the coherent-scattering regions of 17 nm and 13 nm respectively.

Syntheses, crystal structures and properties of two new molecular solids by self-assembly of bis(maleonitriledithiolate)nickel(III)/copper(II) anion and substituted 4-fluorobenzyl isoquinolinium cation

Two new molecular solids, [4FBzIQl][Ni(mnt)2](1) and [4FBzIQl]2[Cu(mnt)2](2) ([4FBzIQl]+=[1-(4′-fluorobenzyl)isoquinolinium; mnt2−=maleonitriledithiolate) have been obtained by complexing of the [M(mnt)2]n− (M=Ni(III) or Cu(II), n=1 or 2) anion with [4FBzIQl]+ cation. Single-crystal structural analysis reveals that both molecular metals crystallize in the triclinic space group P-1. The [Ni(mnt)2]− anions of 1 form a column by short C⋯N and C⋯C interactions between the cyano groups of the anions, while two adjacent [Cu(mnt)2]2− anions of 2 form a dimer through S⋯S interaction. The neighboring cations form a dimer or 2D layered structure through CH⋯F hydrogen bonds, F⋯F interactions, and π⋯π stack interactions. The anions and cations stack alternately into a 1D column through CH⋯N, π⋯π or/and CH⋯S, CHCu interactions. Magnetic susceptibility measurements in the temperature range 2–300K reveal that 1 exhibits a characteristic of two spin-gaps around 103.8K and 7.5K, respectively, while 2 shows a change of magnetic exchange from weak ferromagnetic to antiferromagnetic interaction around 43.9K when the temperature is lowered.

Crystal structure of 1-butyl-2,3-di-methyl-imidazolium dicarba-7,8-nido-undeca-borate.

In the title mol­ecular salt, C9H17N2 +·C2H12B9 −, the carborane cage has a bridging B—H—B bond on the open B3C2 face. The butyl side chain of the cation adopts an extended conformation [C—C—C—C = 179.6 (1)°]. In the crystal, the imidazolium ring is almost coplanar with the open face of the carborane anion. The cations stack in the [010] direction and the dihedral angle between the imidazolium rings of adjacent cations is 68.45 (6)°. The butyl chains extend into the space between carborane anions.

Stacking of hydrazone-bridged linear tetrathiafulvalene radical cations

Four compounds M1, M2, M3, M4, and one polymer P1 containing tetrathiafulvalene (TTF) units have been synthesized by the condensation of TTF-derived aldehyde and acylhydrazine precursors. UV–vis absorption and electron paramagnetic resonance spectroscopy as well as scanning electron microscopy reveal that the TTF+ stacking of the polymer is considerably enhanced compared to that of the shorter compounds in polar solvents.

Syntheses, crystal structures and magnetic properties of three new molecular solids based on bis(maleonitriledithiolate)copper(II) anion and ortho-position substituted benzyl isoquinolinium cations

Three molecular solids, [2RBzIQl]2[Cu(mnt)2] (mnt2−=maleonitriledithiolate; [2RBzIQl]+=1-(2′-R-benzyl) isoquinolinium, R=F(1), Cl(2) and Br(3)), have been prepared and systemically investigated. The crystal structural analysis reveals that three molecular metals crystallize in the monoclinic with space group P21/c, and the [2RBzIQl]+ cations stack into an one-dimensional column through CH⋯π, π⋯π and CH⋯X (X=F or Cl) interactions while the [Cu(mnt)2]2− anions in 1 and 2 form a dimer by the side-to-side S⋯S, N⋯N and S⋯N interactions. The anions (A) and cations (C) are arranged alternatively into one 1D column for 1 and 2 and two 1D columns for 3 with a ⋯ACCACCA⋯ sequence though CH⋯S, CH⋯π, CH⋯N or/and CH⋯Cu hydrogen bonds, and weak X⋯N (X=F, Cl), Br⋯S, Cu⋯N or/and π⋯π interactions. The magnetic susceptibilities measured in the temperature range 2.0–300K reveal that 1 shows an antiferromagnetic behavior, while 2 and 3 exhibit a change of magnetic exchange from weak ferromagnetic to antiferromagnetic around 48K upon the temperature is lowered.

The formation of organogels and helical nanofibers from simple organic salts.

Simple organic salts based on aniline-derived cations and D-tartrate anions formed organogels and helical nanofibers. The organic salt (p-fluoroanilinium)(D-tartrate) was found to generate an organogel despite the absence of a hydrophobic alkyl chain, whereas (p-iodoanilinium)(D-tartrate) formed helical nanofibers in braided ropelike structures through a rolling-up process. The helicity of these nanofibers could be reversed by changing the growth solvent. The driving forces responsible for the formation of the nanofibers were determined to be 1D OH⋅⋅⋅O(-) hydrogen-bonding interactions between D-tartrate anions and π stacking of anilinium cations, as well as steric hindrance between the hydrogen-bonded chains.

Crystal structure of 4,4'-bipyridine-1,1'-diium naphthalene-2,6-di-sulfonate dihydrate.

The title hydrated mol­ecular organic salt, C10H10N2 2+·C10H6O6S2 2−·2H2O, crystallized with half a bipyridinium cation, half a naphthalene-2,6-di­sulfonate anion and a water mol­ecule in the asymmetric unit. The whole cation and anion are generated by inversion symmetry, the inversion centers being at the center of the bridging C—C bond of the cation, and at the center of the fused C—C bond of the naphthalene group of the anion. In the crystal, the anions and cations stack alternately along the a axis with π–π inter­actions [inter-centroid distance = 3.491 (1) Å]. The anions are linked via O—H⋯O(sulfonate) hydrogen bonds involving two inversion-related water mol­ecules, forming chains along [10-1]. These chains are bridged by bifurcated N—H⋯(O,O) hydrogen bonds, forming a three-dimensional framework structure. There are also C—H⋯O hydrogen bonds present, reinforcing the framework structure.

Dielectric Phase Transition Behavior in an N-salicyliden Based Ion Crystal

A new Schiff base iodides with a formula of [o-OH-Bz-1-APy]I have been synthesized and characterized structurally (o-OH-Bz-1-APy+ = ortho-hydroxy-benzylidene-1-aminopyridinium Schiff base derivative (1)). Two adjacent cations adopt an antiparallel arrangement forming π-type cation dimmer, along the a-axis direction, the cation dimmer formed irregular cation stacks and counterion I− anions are embedded in the cation stacks in the crystals of 1. A strong frequency dispersion of e’ is observed in the low frequency region and can be attributed to local dipole motion in 1. Furthermore, this ion-pair compound shows novel dielectric phase transition around 125 °C. DSC measurement of 1 exhibited gradual endothermic peak around the transition temperature and small enthalpy change means that the difference in lattice energy between the two phases is very small. This study provided a new strategy for the design of dielectric phase transition system. Compound 1 shows a strong frequency dispersion of dielectric loss in the low frequency region and novel dielectric phase transition.

Unusual layer structure in an ion-paired compound containing tetra(isothiocyanate) cobalt(II) dianion and 4-nitrobenzylpyridinium: Crystal structure and magnetic properties

A new ion-paired compound [4NO2BzPy]2[Co(NCS)4] (1) ([4NO2BzPy]+ = 1-(4′-nitrobenzyl)pyridinium, NCS− = isothiocyanate) is synthesized and characterized by elemental analysis, IR and UV-Vis spectra, ESI-MS and single crystal X-ray diffraction. Compound 1 is orthorhombic, space group Pbcn, with a = 13.188(2) A, b = 8.458(1) A, c = 29.281(4) A, V = 3266.2(7) A3, D calc = 1.468 g/cm3, Z = 4, F(000) = 1476, R 1 = 0.0332. The [Co(NCS)4]2− anions form an unusual layer structure by S…N and S…Co interactions, while the [4NO2BzPy]+ cations stack into a 1D column by the p…π stacking interaction in the solid state of 1. A magnetic measurement in the range 2–300 K shows a weak antiferromagnetic exchange with θ = −2.42 K in 1.

Syntheses, crystal structures and magnetic properties of three new molecular metals based on bis(maleonitriledithiolate)copper(II) anion and substituted benzyl isoquinolinium cations

Syntheses, crystal structures and magnetic properties of three novel molecular metals, [4RBzIQl]2[Cu(mnt)2] (mnt2−=maleonitriledithiolate; [4RBzIQl]+=1-(4′-R-benzyl)isoquinolinium, R=H(1), Cl(2) and NO2(3)) are systemically investigated. The crystal structural analysis reveals that three molecular metals crystallize in the triclinic space group P-1, and the [4RBzIQl]+ cations stack into a one-dimensional column through C–H⋯π and π⋯π interactions. The anions (A) and cations (C) are arranged alternatively into a column in the sequence of ···A-CC-A-CC··· for 1 and ···AA-CC-AA-CC··· for 2 and 3 though Cu⋯N, C⋯N, C⋯O, C–H⋯π or/and π⋯π interactions. The magnetic susceptibilities measured in the temperature range 2.0–300K reveals that these molecular show a change of magnetic exchange from weak ferromagnetic to antiferromagnetic around 20K, 19K and 47.6K for 1, 2 and 3.

Synthesis, Three-Dimensional Network Structure, Weak Interactions, and Magnetic Properties of Bis(quinolinium) Tetrachlorocobaltate(II) Dihydrate

A new tetrachlorocobalt(II) salt, [QlH]2[CoCl4]·2H2O(1) ([QlH]+ = quinolinium), has been prepared and characterized by elemental analysis, IR, UV-Vis, ESI-MS, single-crystal X-ray diffraction, and magnetic susceptibility. Compound 1 crystallizes in the monoclinic space group C2/c with cell dimensions a = 11.848(2) Å, b = 10.069(1) Å, c = 18.494(2) Å, β = 101.35(1) Å, V = 2163.3(4) Å3, Z = 4. The unit cell contains two [QlH]+ cations, one [CoCl4]2– anion and two water molecules. The [QlH]+ cations stack and form a 1D column through p···π and π···π interactions. The water molecules are linked to the anions through O-H···Cl hydrogen bonds to form two-dimensional molecular networks. The cations and [CoCl4]2– anions are linked through N-H···O and C-H···O hydrogen bonds and give further rise to a 3D hydrogen-bonding network structure. Magnetic susceptibility measurement in the temperature range 2–300 K shows that 1 exhibits a weak antiferromagnetic coupling behavior with θ = –2.90 K.

Propane-1,2-diammonium chromate(VI)

In the title mol­ecular salt, (C3H12N2)[CrO4], each chromate anion accepts six N—H⋯O and C—H⋯O hydrogen bonds from nearby propane-1,2-di­ammonium cations. Three of the four O atoms of the chromate anion accept these bonds; the remaining Cr—O bond length is notably shorter than the others. In the crystal, the anions and cations stack in layers lying parallel to (100): the hydrogen-bonding pattern leads to a three-dimensional network.

Insight into the chemical bonding and electrostatic potential: A charge density study on a quinazoline derivative

2,3-Trimethylene-3,4-dihydroquinazoline shares the heterocyclic core with natural compounds and synthetic drugs. The hydrochloride of the compound forms excellent dihydrate crystals which have allowed us to collect high-resolution X-ray diffraction data and obtain the experimental charge density. The solid may be understood as built up from pairs of heterocyclic cations and chloride anions; a direct hydrogen bond links the halide to the formally cationic pyrimidine NH group. The hydrate water molecules interact with the anions, forming an infinite chain along the crystallographic a axis between the stacks of the heterocyclic cations. Based on the experimental charge density, a dipole moment of 16.1 Debye is calculated for a pair of the hydrogen-bonded quinazolinium cation and the chloride anion in the extended crystal structure.

Simulation of a cellulose fiber in ionic liquid suggests a synergistic approach to dissolution

Ionic liquids dissolve cellulose in a more efficient and environmentally acceptable way than conventional methods in aqueous solution. An understanding of how ionic liquids act on cellulose is essential for improving pretreatment conditions and thus detailed knowledge of the interactions between the cations, anions and cellulose is necessary. Here, to explore ionic liquid effects, we perform all-atom molecular dynamics simulations of a cellulose microfibril in 1-butyl-3-methylimidazolium chloride and analyze site–site interactions and cation orientations at the solute–solvent interface. The results indicate that Cl− anions predominantly interact with cellulose surface hydroxyl groups but with differences between chains of neighboring cellulose layers, referred to as center and origin chains; Cl− binds to C3-hydroxyls on the origin chains but to C2- and C6-hydroxyls on the center chains, thus resulting in a distinct pattern along glucan chains of the hydrophilic fiber surfaces. In particular, Cl− binding disrupts intrachain O3H–O5 hydrogen bonds on the origin chains but not those on the center chains. In contrast, Bmim+ cations stack preferentially on the hydrophobic cellulose surface, governed by non-polar interactions with cellulose. Complementary to the polar interactions between Cl− and cellulose, the stacking interaction between solvent cation rings and cellulose pyranose rings can compensate the interaction between stacked cellulose layers, thus stabilizing detached cellulose chains. Moreover, a frequently occurring intercalation of Bmim+ on the hydrophilic surface is observed, which by separating cellulose layers can also potentially facilitate the initiation of fiber disintegration. The results provide a molecular description why ionic liquids are ideal cellulose solvents, the concerted action of anions and cations on the hydrophobic and hydrophilic surfaces being key to the efficient dissolution of the amphiphilic carbohydrate.