Disproportionation of Pyrazine in NH+···N Hydrogen-Bonded Complexes: New Materials of Exceptional Dielectric Response

Narrow-Pore Engineering of Vinylene-Linked Covalent Organic Frameworks with Weak Interaction-Triggered Multiple Responses

Vinylene-linked covalent organic frameworks (COFs) are emerging as promising crystalline materials, but their narrow pore engineering is severely impeded by the weak reversibility of the carbon-carbon double bond formation reaction, which has led to less exploration of their ultramicroporous structures and properties. Herein, we developed a single aromatic ring-based tetratopic monomer, tetramethylpyrazine, which undergoes a smooth Knoevenegal condensation at its four arylmethly carbon atoms with linear aromatic dialdehyde monomers upon the self-catalyzed activation of pyridine nitrogen-containing monomers in the presence of an organic anhydride. This has resulted in the formation of two vinylene-linked COFs, which both crystallized in orthorhombic lattices, and layered in AA stacking fashions along the vertical directions. They exhibit high surface areas and well-tailored ultramicropore sizes up to 0.5 nm. The unique cross-linking mode at two pairs of para-positions of each pyrazine unit through carbon-carbon double bonds afford them with π-extended conjugation over the in-plane backbones and substantial semiconducting characters. The resultant COFs can be well-dispersed in water to form stable sub-microparticles with negative charges (zeta potentials: ca. -30 mV), and exhibiting tunable aggregation behaviors through protonation/deprotonation. As a consequence, they exhibit pore-size-dependent colorimetric responses to various anions with different pKa values in high selectivity.

One-dimensional DABCO hydrogen-bonding chain in a hexagonal channel of magnetic [Ni(dmit)2]

Crystals of (HDABCO+)9(DABCO)[Ni(dmit)2]9·6CH3CN were shown to have a space group of R3[combining macron], a hexapetal flower-like channel of [Ni(dmit)2] anions, and a one-dimensional hydrogen bonding chain composed of protonated DABCO and CH3CN molecules. The crystals display antiferromagnetic and ferromagnetic interactions within and between hexamers, respectively, whereas the flexible DABCO-CH3CN array shows dielectric relaxation.

Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine

A proton-transfer reaction between squaric acid (H₂sq) and 2,3-dimethylpyrazine (2,3-Me₂pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me₂pyzH⁺)(Hsq^-)·H₂O (1), which possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me₂pyzH⁺ cations and strips featuring extensive hydrogen bonding between the Hsq^- anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me₂pyzH⁺)₂(Hsq₂^3-)(H₅O₂⁺), whereas the AFE phase can be described as (2,3-Me₂pyzH⁺)(Hsq^-)(H₂O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.

Synthesis and characterization of a new organic–inorganic hybrid ferroelectric: (C4H10N)6[InBr6][InBr4]3·H2O

Compound (C₄H₁₀N)₆[InBr₆][InBr₄]₃·H₂O undergoes a paraelectric–ferroelectric phase transition at 232 K, which triggered by the disorder–order transition of Br atoms in [InBr₄]⁻ anions.

Effect of carbon-skeleton isomerism on the dielectric properties and proton conduction of organic cocrystal compounds assembled from 1,2,4,5-benzenetetracarboxylic acid and piperazine derivatives

Two isostructural 2D supramolecular cocrystal compounds show different dielectric responses and proton conductivities due to the alteration of the carbon-skeleton of piperazine derivatives.

2-Aminoanilinium benzene-1,2-diaminium tris(4-methylbenzene-1-sulfonate)

In the title molecular salt, C6H10N22+·C6H9N2+·3C7H7O3S−, one of the cations is doubly protonated and one is singly protonated with charge balance achieved by three sulfonate anions. The crystal packing features N—H...O and C—H...O hydrogen bonds. The ions are arranged into a two-dimensional network along the (010) plane and the structure is further consolidated by weak C—H...π interactions.

An Electrically Conductive Single-Component Donor-Acceptor-Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D-A-D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex 1H₂). The crystal structure of complex 1H₂ has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex 1H₂ is semiconducting (E_a = 0.29 eV, ρ_rt = 9.1 × 10⁴ Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (E_a = 0.13 eV, ρ_rt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π-π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D-A-D aggregate.

Reversible phase transition of 2-carboxypyridinium perchlorate-pyridinium-2-carboxylate (1/1)

The title salt, C6H6NO2(+)·ClO4(-)·C6H5NO2, was crystallized from an aqueous solution of equimolar quantities of perchloric acid and pyridine-2-carboxylic acid. Differential scanning calorimetry (DSC) measurements show that the compound undergoes a reversible phase transition at about 261.7 K, with a wide heat hysteresis of 21.9 K. The lower-temperature polymorph (denoted LT; T = 223 K) crystallizes in the space group C2/c, while the higher-temperature polymorph (denoted RT; T = 296 K) crystallizes in the space group P2/c. The relationship between these two phases can be described as: 2a(RT) = a(LT); 2b(RT) = b(LT); c(RT) = c(LT). The crystal structure contains an infinite zigzag hydrogen-bonded chain network of 2-carboxypyridinium cations. The most distinct difference between the higher (RT) and lower (LT) temperature phases is the change in dihedral angle between the planes of the carboxylic acid group and the pyridinium ring, which leads to the formation of different ten-membered hydrogen-bonded rings. In the RT phase, both the perchlorate anions and the hydrogen-bonded H atom within the carboxylic acid group are disordered. The disordered H atom is located on a twofold rotation axis. In the LT phase, the asymmetric unit is composed of two 2-carboxypyridinium cations, half an ordered perchlorate anion with ideal tetrahedral geometry and a disordered perchlorate anion. The phase transition is attributable to the order-disorder transition of half of the perchlorate anions.

Temperature-induced reversible structural phase transition of 1,4-dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate)

A novel molecular-based phase change material was synthesized. The phase transition from a room temperature to a low temperature paraelectric phase might be driven by the order–disorder transition of ClO4⁻ anions and the ordering of twisting motions of the dabco ring.

2',6'-Bis(4-carb-oxy-phen-yl)-4,4'-bipyridin-1-ium nitrate 0.25-hydrate

In the title compound, C₂₄H₁₇N₂O₄⁺·NO₃⁻·0.25H₂O, the central pyridine ring of the 2′,6′-bis­(4-carb­oxy­phen­yl)-4,4′-bipyridin-1-ium cation is almost coplanar with one benzene ring [dihedral angle = 1.03 (5)°], while it makes dihedral angles of 9.59 (5)° with the other benzene ring and 13.66 (6)° with the pyridinium ring. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds link the cations and nitrate anions into a sheet in the (302) plane. The crystal structure also exhibits π–π inter­actions between the central pyridine ring and the benzene rings of neighboring mol­ecules [centroid–centroid distance = 3.6756 (13) Å].

4-Chloro-anilinium 4-methyl-benzene-sulfonate

In the crystal structure of the title salt, C(6)H(7)ClN(+)·C(7)H(7)O(3)S(-), the cations and anions are linked via N-H⋯O hydrogen bonds into double chains in [101]. Weak inter-molecular C-H⋯π-ring inter-actions link these chains into layers parallel to the ac plane.

3-Chloro-anilinium 4-methyl-benzene-sulfonate

In the crystal structure of the title salt, C₆H₇ClN⁺·C₇H₇O₃S⁻, the cations and anions are linked via N—H⋯O hydrogen bonds into doubled chains in [010]. Weak inter­molecular C—H⋯π inter­actions further link these chains into layers parallel to the bc plane.

4-Ethylanilinium 4-methylbenzenesulfonate

In the crystal structure of the title molecular salt, C₈H₁₂N⁺·C₇H₇O₃S⁻, the 4-ethyl­anilinium cations and 4-methyl­benzene­sulfonate anions are linked into chains parallel to the b axis by inter­molecular N—H⋯O hydrogen bonds.

Piperazine-1,4-diium bis-(perchlorate) dihydrate

The asymmetric unit of the title compound, C(4)H(12)N(2) (2+)·2ClO(4) (-)·2H(2)O, contains half of a piperazinediium cation, one perchlorate anion and one water mol-ecule. The diprotonated piperazine ring, which is completed by crystallographic inversion symmetry, adopts a chair conformation. In the crystal structure, the cations and anions are linked by inter-molecular N-H⋯O and O-H⋯O hydrogen bonds into a three-dimensional network.

4-Ethyl-anilinium 2-carb-oxy-acetate

In the crystal structure of the title compound, C₈H₁₂N⁺·C₃H₃O₄⁻, the hydrogen malonate anions are linked into infinite chains parallel to the b axis by inter­molecular O—H⋯O hydrogen bonds of the type COO⁻⋯HO₂C in a head-to-tail fashion. The 4-ethyl­anilinium cations link adjacent anion chains by inter­molecular N—H⋯O hydrogen bonds into a two-dimensional network parallel to the b and c axes.

Reversible structural phase transition of pyridinium-4-carboxylic acid perchlorate

Pyridinium-4-carboxylic acid perchlorate (C6H6NO2·ClO4) was synthesized and separated as crystals. Differential scanning calorimetry measurement shows that this compound undergoes a reversible phase transition at about 122 K with a heat hysteresis of 1.8 K. A dielectric anomaly observed at 127 K further confirms the phase transition. The low-temperature (LT;T= 103 K) structure has space groupP21/cand cell parametersa= 17.356 (6),b= 13.241 (3),c= 16.161 (7) Å, β = 138.055 (17)°. The high-temperature (HT;T= 298 K) structure has space groupP21/cand cell parametersa= 5.5046 (11),b= 13.574 (3),c= 11.834 (2) Å, β = 99.35 (3)°, but can be re-described using new axesa′ =a,b′ =b,c′ = −2a+c,V′ =Vto give the cella′ = 5.5046 (11),b′ = 13.574 (3),c′ = 17.424 (3) Å, β′ = 137.92 (3)° and space groupP21/c. The associated coordinate transformation isx′ =x+ 2z,y′ =y,z′ =zand the associated reflection index transformation ish′ =h,k′ =k,l′ =l− 2h. The relationship between the two cells is 3a,b,c(HT) approximatesa,b,c(LT). The crystal comprises one-dimensional hydrogen-bonded chains of the pyridinium-4-carboxylic acid cations and perchlorate anions. A precise analysis of the main packing and structural differences as well as the changes in the intermolecular interactions between the HT phase and the LT phase reveals that the disorder–order transition of the perchlorate anions may be the driving force of the transition, and the hydrogen-bonding effect may contribute to the transition as a secondary parameter.

(±)-2-Methylpiperazin-1-ium perchlorate

In the title compound, C₅H₁₃N₂⁺·ClO₄⁻, the monoprotonated piperazine ring adopts a chair conformation. In the crystal structure, cations and anions are linked by inter­molecular N—H⋯O and N—H⋯N hydrogen bonds into layers parallel to (01).

Structural polymorphism of pyrazinium hydrogen sulfate: extending chemistry of the pyrazinium salts with small anions

Two polymorphs (α, β) of pyrazinium hydrogen sulfate (pyzH+HSO_4^-, abbreviated as PHS) with distinctly different hydrogen-bond types and topologies but close electronic energies have been synthesized and characterized for the first time. The α-polymorph (P212121) forms distinct blocks in which the pyzH+and HSO_4^- ions are interconnected through a network of NH...O and OH...O hydrogen bonds. The β-form (P\bar 1) consists of infinite chains of alternating pyzH+and HSO_4^- ions connected by NH...O and OH...N hydrogen bonds. Density functional theory (DFT) calculations indicate the possible existence of a hypothetical polarP1 form of the β-polymorph with an unusually high dipole moment.

4-(N-Propan-2-ylcarbamo-yl)pyridinium perchlorate

In the title compound, C₉H₁₃N₂O⁺·ClO₄⁻, the dihedral angle between the planes of the amide group and the pyridinium fragment is 34.11 (14)°. In the crystal, the cations are connected by N—H⋯O hydrogen bonds between the amide groups into chains extended along the a axis. Hydrogen bonds between the pyridinium N—H group and the perchlorate anions organize the chains into a two-dimensional network.

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate

Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at ∼ 135 K with a wide hysteresis of 15 K. Dielectric measurements confirm the transition at ∼ 127 K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and β angle change abruptly and remarkably at 129 (2) K, indicating that the system undergoes a first-order transition at T c = 129 K. The crystal structures determined at 103 and 298 K are all monoclinic in P21/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are further linked to perchlorate anions by hydrogen bonds to form well separated infinite planar layers. The most distinct differences between the structures of the higher-temperature phase and the lower-temperature phase are the change of the distance between the adjacent pyridinium ring planes within the hydrogen-bonded chains and the relative displacement between the hydrogen-bonded layers. Structural analysis shows that the driving force of the transition is the reorientation of the pyridinium-3-carboxylic acid cations. The degree of order of the perchlorate anions may be a secondary order parameter.

1,4-Diazoniabicyclo[2.2.2]octane tetrachloroiodate(III) chloride

In the title compound, C₆H₁₄N₂²⁺·Cl₄I⁻·Cl⁻, the dication and the anions lie on special positions. The dication has mm2 symmetry with two bonded C atoms and the two N atoms located on a crystallographic mirror plane parallel to bc, and with a mirror plane parallel to ab passing through the mid points of the three C—C bonds. In the square-planar Cl₄I⁻ anion, two Cl atoms and the I atom are located on the mm2 axis; the other two Cl atoms are disordered over two postions of equal occupancy (0.25) across the mirror parallel to the ab plane. The Cl⁻ anion is located on the mm2 axis. The crystal structure is stabilized by inter­molecular N—H⋯Cl hydrogen bonds.

4-(4-Pyrid-yl)pyridinium perchlorate methanol solvate

In the cation of the title hydrated molecular salt, C₁₀H₉N₂⁺·ClO₄⁻·CH₃OH, the dihedral angle formed by the pyridine rings is 28.82 (15)°. The crystal structure is stabilized by inter­molecular N—H⋯O and O—H⋯N hydrogen bonds and π–π stacking inter­actions, with centroid-to-centroid distances of 3.5913 (7) and 3.6526 (7) Å. Three O atoms of the perchlorate anion are disordered over two positions with refined occupancy factors of 0.649 (7):0.351 (7).

4-Carbamoylpyridinium perchlorate

In the cation of the title compound, C₆H₇N₂O⁺·ClO₄⁻, the amide group is oriented at a dihedral angle of 10.41 (17)° to the benzene ring. The crystal structure is stabilized by inter­molecular N—H⋯O hydrogen bonding.

Nicotinium hydrogen sulfate

The structure of title compound, C₆H₆NO₂⁺·HSO₄⁻, comprises discrete ions which are inter­conected by N—H⋯O and O—H⋯O hydrogen bonds, leading to a neutral one-dimensional network along [001]. These hydrogen bonds appear to complement the Coulombic inter­action and help to stabilize the structure further.

Isonicotinium hydrogen sulfate

The crystal structure of the title compound, C₆H₆NO₂⁺·HSO₄⁻, is stabilized by inter­molecular N—H⋯O and O—H⋯O hydrogen bonds.

Organic ferroelectrics

Ferroelectricity results from one of the most representative phase transitions in solids, and is widely used for technical applications. However, observations of ferroelectricity in organic solids have until recently been limited to well-known polymer ferroelectrics and only a few low-molecular-mass compounds. Whereas the traditional use of dipolar molecules has hardly succeeded in producing ferroelectricity in general, here we review advances in the synthesis of new organic materials with promising ferroelectric properties near room temperature, using design principles in analogy to inorganic compounds. These materials are based on non-covalent molecules formed by two or more components, in which ferroelectricity arises either from molecular displacements or from the collective transfer of electrons or protons. The principle of using multi-component molecular compounds leads to a much broader design flexibility and may therefore facilitate the development of future functional organics.