Temperature controlled formation of polar copper phosphonates showing large dielectric anisotropy and a dehydration-induced switch from ferromagnetic to antiferromagnetic interactions

The reaction of copper(ii) chloride with 2-carboxyphenylphosphonic acid (2-cppH3) under hydrothermal conditions at 100 °C results in the formation of a polar compound, Cu3(2-cpp)2(H2O)5 (1), which shows large dielectric anisotropy. A similar reaction at 140 °C results in a centrosymmetric phase, Cu3(2-cpp)2(H2O)2 (2). Compound 1 can convert into 2 upon increasing the reaction temperature under hydrothermal conditions, and further to Cu3(2-cpp)2 (3) upon thermal treatment at 240 °C. This process is accompanied by a switch from ferromagnetic in 1 to antiferromagnetic interactions in 2 and 3.

Crystal-to-crystal structural transformation of hydrogen-bonding molecular crystals of (imidazolium)(3-hydroxy-2-quinoxalinecarboxylate) through H2O adsorption–desorption

Crystal-to-crystal structural transformation was observed following H₂O adsorption–desorption of hydrogen-bonding molecular crystals.

Quasi-one-dimensional molecular magnets based on derivatives of (fluorobenzyl)pyridinium with the [M(mnt)2] monoanion (M = Ni, Pd or Pt; mnt2- = maleonitriledithiolate): syntheses, crystal structures and magnetic properties

The syntheses, structural characterizations and magnetic behaviors of three new complexes, 1-(3',4',5'-trifluorobenzyl)pyridinium [M(mnt)2]- [M = Ni (1), Pd (2) or Pt (3)], are reported. These complexes are isomorphous and their prominent structural character is that the [M(mnt)2]- anions form columnar stacks, in which the dimerization was observed. Complexes 2 and 3 are diamagnetic, while 1 possesses an energy gap of 2474 K. For crystal 4, 1-(4'-fluorobenzyl)pyridinium [Ni(mnt)2] (its structure and magnetic susceptibility were briefly reported earlier), the magnetic behavior can be divided into two regimes, namely, weakly ferromagnetic coupling above 93 K and strongly antiferromagnetic coupling below 93 K. A transition occurs at 93 K which switches the magnetic exchange nature from ferromagnetic to antiferromagnetic. A sharp thermal abnormality with lambda-shape, associated with the transition, appears from its heat capacity measurement to indicate that the transition is first order. The temperature dependences of the superlattice diffractions revealed the existence of the pretransitional phenomena up to at least 140 K. The unusual magnetic behavior of 4, such as the origin of the ferromagnetic interaction in the high temperature phase and what causes the spin transition, are discussed further.

Structural and magnetic investigations for the doping effect of nonmagnetic impurity on the spin-Peierls-like transition in a quasi-one-dimensional magnet: 1-(4'-nitrobenzyl)pyridinium bis(maleonitriledithiolato)nickelate

A nonmagnetic compound, [NO(2)BzPy][Au(mnt)(2)] (NO(2)BzPy(+) = 1-(4'-nitrobenzyl)pyridinium; mnt(2-) = maleonitriledithiolate), was synthesized and characterized structurally, which is isostructural with [NO(2)BzPy][Ni(mnt)(2)] that is a quasi-one-dimensional magnet and possesses a spin-Peierls-like transition with J = 192 K in the gapless state and spin energy gap = 738 K in the dimerization state, respectively. Further, ten nonmagnetic impurity doped compounds with a formula [NO(2)BzPy][Au(x)Ni(1-x)(mnt)(2)] (x = 0.01-0.73) were prepared and investigated by crystal structural determinations and magnetic susceptibility measurements. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 221(12) K/percentage of dopant concentration. From the plots of chi(m)-T, the transition collapse (the characteristic of the transition is the sudden drop of chi(m) upon cooling, and the disappearance of this characteristic is considered as the criterion for the transition collapse) is estimated at around x > 0.27. In heavier doped system x = 0.49, the spin gap vanishes and a gapless phase is achieved again.

Design of a Magnetic Bistability Molecular System Constructed by H-Bonding and π···π-Stacking Interactions

Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H2DABCO][Ni(mnt)2] [(H2DABCO)2+ = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt2- = maleonitriledithiolate]. For each polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around 120 K with about 1 K hysteresis on heating for the alpha phase and at 112 K with about 10 K hysteresis for the beta phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a singlet-triplet model of an antiferromagnetically coupled S = 1/2 dimer, which is supported by the crystal structures and spin dimer analyses based on extended Hückel molecular orbital calculations.

Structural Phase Transition Driven by Spin−Lattice Interaction in a Quasi-One-Dimensional Spin System of [1-(4‘-Iodobenzyl)pyridinium][Ni(mnt)2]

Crystal structures and magnetic properties were determined for two novel compounds, [1-(4'-iodobenzyl)pyridinium][M(mnt)2] (mnt2- = maleonitriledithiolate; M = Ni (1) or Cu (2)). At room temperature, single crystals of 1 and 2 were isostructural, featuring the formation of segregated columnar structures with regular stacks of cations and anions. For crystal 1, a magnetic transition was observed at approximately 120 K; furthermore, its magnetic behavior was consistent with that of a regular Heisenberg antiferromagnetic (AFM) chain of S = 1/2 in the high-temperature phase (HT phase) and that of a spin-gap system in the low-temperature phase (LT phase). Such a phenomenon is similar to the spin-Peierls transition. However, the crystal structure of 1 in the LT phase at 100 K revealed that its structural transition is associated with the magnetic transition. Because crystal 2 (S = 0) did not exhibit a structural transition, the structural transition of 1 is driven by spin-lattice interaction.

Ionic channel structures in [(M+)x([18]crown-6)][Ni(dmit)2]2 molecular conductors

The [(M+)x[18]crown-6)] supramolecular cations (SC+), in which M+ and [18]crown-6 are alkali metal ions (M+ = Li+, Na+, and Cs+) and 1,4,7,10,13,16-hexaoxacyclooctadecane, respectively, form ionic channel structures through the regular stacks of [18]crown-6 in [Ni(dmit)2]-based molecular conductors (dmit2+ = 2-thioxo-1,3-dithiole-4,5-dithiolate). In addition to the [Ni(dmit)2] salts that have the ionic channel structures (these salts are abbreviated as type I salts), Li+ and Na+ form dimerized [(M+)2([18]crown-6)2] units in the crystals (type II salts). The K+ and Rb+ are coordinated tightly into the [18]crown-6 cavity to form typical disk-shape SC+ units in the corresponding [Ni(dmit)2] salts (type III salts). The type I, II, and III salts have typical stoichiometries of [(M+)x([18]crown-6)][Ni(dmit)2]2, [(M+)([18]crown-6)(H2O)x(CH3CN)(1.5 - x)][Ni(dmit)2]3 (x = 1 for Li+ or 0.5 for Na+), and [M+([18]crown-6)][Ni(dmit)2]3, respectively: the salts of the same type are isostructural. In agreement with the trimer structures of [Ni(dmit)2] in the type II and III salts, they exhibit semiconducting behavior with electrical conductivities at 300 K (sigma(300 K)) of 0.01-0.1 S cm(-1). Type I salts contain a regular stack of partially oxidized [Ni(dmit)2] units, which form a quasi one-dimensional metallic band within the tight-binding approximation regime. The electrical conductivities at 300 K are 10-30 S cm(-1), and an almost temperature-independent conductivity was observed at higher temperatures. However, the one-dimensional electronic structures in these salts are strongly influenced by the static and dynamic structures of the coexisting ionic channel. The Na+ salt is a semiconductor, whose magnetic behavior is described by the disordered one-dimensional antiferromagnetic chain. On the other hand, the Cs+ salt is a exhibits metallic properties with 2 kF instability at room temperature. The Li+ salt shows a gradual transition from the high-temperature metallic phase to the low-temperature one-dimensional antiferromagnetic semiconductor phase, which was associated with the freezing of Li+ motion at lower temperatures. The preferential crystallization of type I salts was possible by controlling the equilibrium constant (Kc) of the complex formation between M+ ions and the [18]crown-6 molecule. The ionic channel structures were obtained when the KC was low in the electrocrystallization solution, while type II or III salts were formed in the high Kc region.

Potassium bis(4,5-dimercapto-1,3-dithiole-2-thionato)nickelate 1,4,7,10,13,16-hexaoxa-2,3:11,12-dibenzocyclooctadeca-2,11-diene propanone solvate

In the title compound, K[Ni(C(3)S(5))(2)] x C(20)H(24)O(6).C(3)H(6)O, K(+) is incorporated in the cavity of the 1,4,7,10,13,16-hexaoxa-2,3:11,12-dibenzocyclooctadeca-2,11-diene (DB18c6) molecule and is coordinated by the six DB18c6 O atoms and the propanone O atom. Two [K(+)(DB18c6)[(CH(3))(2)CO]] units form a dimer which is aligned in a one-dimensional manner along the a axis through a face-to-face interaction between the benzene rings of neighboring DB18c6 molecules. [Ni(dmit)(2)](-) anions are also aligned along the a axis through side-by-side S.S interactions.

M+(12-crown-4) supramolecular cations (M+ = Na+, K+, Rb+, and NH4+) within Ni(2-thioxo-1,3-dithiole-4,5-dithiolate)2 molecular conductor

Monovalent cations (M+ = Na+, K+, Rb+, and NH4+) and 12-crown-4 were assembled to new supramolecular cation (SC+) structures of the M+(12-crown-4)n (n = 1 and 2), which were incorporated into the electrically conducting Ni(dmit)2 salts (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate). The Na+, K+, and Rb+ salts are isostructural with a stoichiometry of the M+(12-crown-4)2[Ni(dmit)2]4, while the NH4+ salt has a stoichiometry of NH4+(12-crown-4)[Ni(dmit)2]3(CH3CN)2. The electrical conductivities of the Na+, K+, Rb+, and NH4+ salts at room temperature are 7.87, 4.46, 0.78, and 0.14 S cm-1, respectively, with a semiconducting temperature dependence. The SC+ structures of the Na+, K+, and Rb+ salts have an ion-capturing sandwich-type cavity of M+(12-crown-4)2, in which the M+ ion is coordinated by eight oxygen atoms of the two 12-crown-4 molecules. On the other hand, the NH4+ ion is coordinated by four oxygen atoms of the 12-crown-4 molecule. Judging from the M(+)-O distances, thermal parameters of oxygen atoms, and vibration spectra, the thermal fluctuation of the Na+(12-crown-4)2 structure is larger than those of K+(12-crown-4)2 and Rb+(12-crown-4)2. The SC+ unit with the larger alkali metal cation gave a stress to the Ni(dmit)2 column, and the SC+ structure changed the pi-pi overlap mode and electrically conducting behavior.

Interactions of lidocaine and calcium in blocking the compound action potential of frog sciatic nerve

The exact role of calcium in nerve conduction in neurons that have been blocked by local anesthetics remains controversial. Recently, attention has been drawn to the importance of examining both frequency-dependent and nonfrequency-dependent conduction block, since it is felt that frequency-dependent block provides a model that more closely approximates the normal physiologic state. The present study was designed to examine the effects of calcium on both the nonfrequency-dependent and frequency-dependent components of lidocaine nerve block. Desheathed, whole sciatic nerves from frogs were placed in a sucrose gap chamber and stimulated by trains of 20 impulses at frequencies from 3 to 90 Hz at supramaximal intensity for activation of the compound action potential. After control studies, the nerve was bathed by a frog Ringer's solution containing calcium concentrations, which increased from 0.0 mM to the physiologic value of 2.0 mM with or without 0.5 mM lidocaine. Compound action potentials were measured, and both frequency-dependent block and nonfrequency-dependent block were compared in each solution. Low calcium concentrations significantly enhanced both nonfrequency- and frequency-dependent lidocaine block. The effect of low concentrations of calcium was greater at higher frequencies of stimulation.

Magnesium enhances local anesthetic nerve block of frog sciatic nerve

The present study was designed to examine the effects of increased magnesium concentration on the nerve block produced by lidocaine, benzocaine, and QX 572 (a quaternary derivative of lidocaine). Desheathed whole sciatic nerves from northern Rana pipiens frogs were placed in a sucrose gap chamber and stimulated at various frequencies at supramaximal intensity for the activation of the compound action potential. After control studies, the nerve was bathed by a calcium-free frog Ringer's solution containing magnesium concentrations of 3.0, 10.0, or 20.0 mM with or without 0.5 mM lidocaine, 0.5 mM benzocaine, or 0.75 mM QX 572. Compound action potentials were measured, and nonfrequency and frequency dependent blocks were compared in each solution. Increased magnesium ion concentration, in the absence of local anesthetics, enhanced the nonfrequency dependent block but did not change the frequency dependent block. All three local anesthetics enhanced both types of block. Increased magnesium concentrations enhanced only the nonfrequency dependent benzocaine block. In contrast, increased magnesium enhanced both types of block produced by QX 572 and lidocaine. These results suggest a potentially important interaction between high magnesium concentrations and local anesthetic nerve blocks.