One-dimensional (1D) [Ni(mnt)2]−-based spin-Peierls-like complexes: Structural, magnetic and transition properties

Synergetic Responses of Multiple Functions Induced by Phase Transition in Molecular Materials

Materials that integrate magnetism, electricity and luminescence can not only improve the operational efficiency of devices, but also potentially generate new functions through their coupling. Therefore, multifunctional synergistic effects have broad application prospects in fields such as optoelectronic devices, information storage and processing, and quantum computing. However, in the research field of molecular materials, there are few reports on the synergistic multifunctional properties. The main reason is that there is insufficient awareness of how to obtain such material. In this brief review, we summarized the molecular materials with this characteristic. The structural phase transition of substances will cause changes in their physical properties, as the electronic configurations of the active unit in different structural phases are different. Therefore, we will classify and describe the multifunctional synergistic complexes based on the structural factors that cause the first-order phase transition of the complexes. This enables us to quickly screen complexes with synergistic responses to these properties through structural phase transitions, providing ideas for studying the synergistic response of physical properties in molecular materials.

Polymorph transformation in a mixed-stacking nickel-dithiolene complex with the derivative of 4,4'-bipyridinium

In this study, two polymorphs of the [1,1'-dibutyl-4,4'-bipyridinium][Ni(mnt)2] salt (1) were synthesized. The dark-green polymorph (designated as 1-g) was prepared under ambient conditions by the rapid precipitation method in aqueous solutions. Subsequently, the red polymorph (labeled as 1-r) was obtained by subjecting 1-g to ultrasonication in MeOH at room temperature. Microanalysis, infrared spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD) techniques were used to characterize the two polymorphs. Both 1-g and 1-r exhibit structural phase transitions: a reversible phase transition at ∼403 K (∼268 K) upon heating and 384 K (∼252 K) upon cooling for 1-g (1-r) within the temperature range below 473 K. Interestingly, on heating 1-r to 523 K, an irreversible phase transition occurred at about 494 K, resulting in the conversion of 1-r into 1-g. Relative to 1-r, 1-g represents a thermodynamically metastable phase wherein numerous high-energy conformations in butyl chains of cations are confined within the lattice owing to quick precipitation or rapid annealing from higher temperatures. Through variable-temperature single crystal and powder X-ray diffractions, UV-visible spectroscopy, dielectric spectroscopy, and DSC analyses, this study delves into the mechanism underlying phase transitions for each polymorph and the manual transformation between 1-g and 1-r as well.

Crystal structure of 1,1′-(1,2-ethanediyl)bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S:S)zinc(II), C20H14N6ZnS4

C20H14N6ZnS4, monoclinic, C2/c (no. 15), a = 8.5063(13) Å, b = 12.3254(19) Å, c = 21.843(3) Å, β = 96.550(2)°, V = 2275.1(6) Å3, Z = 4, R gt (F) = 0.0288, wR ref (F 2) = 0.0751, T = 293(2) K.

Crystal structure of 1,1′-(methylene)bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S:S)nickel(II), C42H30N14Ni2S8

C42H30N14Ni2S8, triclinic, P 1 ‾ $P\overline{1}$ (no. 2), a = 9.5083(17) Å, b = 10.744(2) Å, c = 12.188(2) Å, α = 80.109(4)°, β = 84.876(4)°, γ = 88.758(3)°, V = 1221.7(4) Å3, Z = 1, R gt (F) = 0.0268, wR ref(F 2) = 0.0701, T = 296.15 K.

Crystal structure of 1,1′-(1,2-ethanediyl)bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S:S)nickel(II), C20H14N6NiS4

C20H14N6NiS4, triclinic, P 1 ‾ $P\bar{1}$ (no. 2), a = 8.519(3) Å, b = 8.5956(16) Å, c = 8.6531(17) Å, α = 116.404(3)°, β = 94.574(4)°, γ = 103.031(4)°, V = 541.0(2) Å3, Z = 1, R gt (F) = 0.0445, wR ref(F 2) = 0.1147, T = 296.15 K.

Multi-step phase transition crystal with dielectric constant bistability and temperature-dependent conductivity

[C4-bmim][Ni(mnt)2] (1) undergoes three-step phase transition with four phases before melting and has two-step dielectric constant bistability and temperature-dependent conducting properties.

Insight into understanding magnetic transition quite sensitive to nonmagnetic impurity in a one-dimensional S = ½ regular linear chain system

Herein, we demonstrate that the antiferromagnetic couplings in a [Ni(mnt)2]− stack are realized mainly through spatial dipole–dipole type interactions.

Crystal structure of 1,1′-(1,4-phenylenebis(methylene))bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S,S) platinum(II), C26H18N6PtS4

C26H18N6PtS4, triclinic, P 1 ‾ $P‾{1}$ (no. 2), a = 8.2661(5) Å, b = 9.1519(6) Å, c = 9.4673(6) Å, α = 70.473(1)°, β = 80.770(1)°, γ = 83.340(1)°, V = 664.77(7) Å3, Z = 1, R gt (F) = 0.0477, wR ref (F 2) = 0.1168, T = 296(2) K.

A Semiconductive Nature of Bis(dithiolato)nickelate Radical Salt Exhibiting Broadband Photoconduction

The fractional oxidation state [M(dmit)₂] (dmit^2- = 2-thioxo-1, 3-dithiole-4, 5-dithiolate) salts have long attracted attention in the molecular metal area owing to high conductivity and even superconductivity. In this study, we achieved a mixed-valence salt (1) of [Ni(dmit)₂]^0.5- with monovalent 1,3-N,N-dimethyl-imidazolium (DiMIm⁺) by a solvent evaporation approach under ambient conditions. The mixed valence of [Ni(dmit)₂]^0.5- has been characterized by an analysis of the IR spectrum and crystal structure. In the crystal structure of 1, two [Ni(dmit)₂]^0.5- anions overlap in an eclipsed mode to form a [Ni(dmit)₂]₂^1- dimer, featuring a radical bearing an S = 1/2 spin; the dimeric radicals stack into a column along the b axis, and the adjacent columns connect together via the lateral-to-lateral S···S contacts along the a axis, and through the head-to-head S···S contacts along the [101] direction. Salt 1 shows the magnetic behavior of an S = 1/2 Heisenberg antiferromagnetic uniform linear chain with J/k_B = -47.5(4) K and a semiconducting feature with σ = 2.52 × 10^-3 S cm^-1 at 293 K, 2.32 × 10^-2 S cm^-1 at 373 K, and E_a = 0.22 eV, as well as broadband photoconductivity under irradiation of green and white lights. This study suggests the possibility of designing new photoconductors based on the mixed-valence [Ni(dmit)₂]^0.5- salt.

Crystal structure of 1,1′-(1,3-phenylenebis(methylene))bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S:S) palladium(II), C26H18N6PdS4

C26H18N6PdS4, monoclinic, C2/c (no. 15), a = 16.658(2) Å, b = 10.398(3) Å, c = 17.284(2) Å, β = 116.928(3)°, V = 2669.2(9) Å3, Z = 4, R gt (F) = 0.0330, wR ref (F 2) = 0.0912, T = 293(2) K.

Crystal structure of 1,1′-(1,4-phenylenebis(methylene))bis(pyridin-1-ium) bis(1,2-dicyanoethene-1,2-dithiolato-k2 S:S)zinc(II), C26H18N6ZnS4

C26H18N6ZnS4, triclinic, P 1 ‾ $P‾{1}$ (no. 2), a = 9.4677(13) Å, b = 12.0147(17) Å, c = 13.571(3) Å, α = 102.907(3)°, β = 102.302(3)°, γ = 110.419(2)°, V = 1336.7(4) Å3, Z = 2, R gt (F) = 0.0441, wR ref (F 2) = 0.1202, T = 296(2) K.

Magnetism, dielectrics and ion conduction in two polymorphs of a heteroleptic bis(dithiolato)nickelate molecular solid

A new heteroleptic compound [triethylpropylammonium][Ni(dmit)(mnt)] with two crystal polymorphs, which show different magnetic and dielectric properties, has been reported.

Single Crystal X-ray Diffraction Studies of Two Polymorphic Modifications of the Dicarbonyl-o-Semiquinonato Rhodium Complex at Different Temperatures. Destruction Stimulated by Cooling Versus Stability

It was found that the dicarbonyl-rhodium-o-semiquinonate complex (which thread-like crystals can bend reversibly under light/warm activation) can form two polymorphic modifications: isometric prisms (1) and sticks (2). Some thin sticks can bend as mentioned above. X-ray diffraction studies of polymorphic modifications at different temperatures were carried out. It was found that crystals 1 are destructed after cooling to 110 K as opposed to crystals 2. In turn, the reversible phase transition is detected in 2. In both polymorphic modifications, stack packaging motifs through the direct Rh-Rh bond are observed. The principal difference between packages of polymorphic modifications is that molecules 1 in the adjacent stacks are shifted relative to each other along the stack, in contrast to crystal 2. It was found that different packing of stacks leads to different anisotropic compression of crystals 1 and 2 during cooling, which is a key factor of their stability. Using the molecular invariom approach, the nature of the chemical bonds and charge distribution was investigated; the energy of the Rh-Rh bonds was estimated.

Crystal structure of 1,4-bis(methylpyridinium benzene) bis(1,2-dicyanoethene-1,2-dithiolato-κ2 S:S)nickel(II), C26H18N6NiS4

C26H18N6NiS4, triclinic, P 1 ¯ $\overline{1}$ (no. 2), a = 8.2947(4) Å, b = 8.9964(5) Å, c = 9.4210(5) Å, α = 70.988(1)°, β = 79.971(1)°, γ = 82.567(1)°, V = 652.52(6) Å3, Z = 1, R gt (F) = 0.0340, wR ref (F 2) = 0.0643, T = 296(2) K.

A Promising Phase Change Material with Record High Ionic Conductivity over a Wide Temperature Range of a Plastic Crystal Phase and Magnetic Thermal Memory Effect

The emerging organic ion plastic crystals (OIPCs) are the most promising candidates used as solid-state electrolytes in a range of ionic devices. To endow an OIPC with additional functionality may create a new type of material for multifunctional devices. Herein, we present an ion plastic crystal, [EMIm][Ni(mnt)₂] (1; [EMIm]⁺ = 1-ethyl-3-methylimidazolium and mnt^2- = maleonitriledithiolate), and its crystal consists of twin dimeric chains of [Ni(mnt)₂]^- anions, embraced by [EMIm]⁺ cations. A crystal-to-plastic crystal transformation with a large latent heat that occurred at ∼367/337 K on heating/cooling is confirmed by the differential scanning calorimetry (DSC) technique. The plastic crystal phase in 1, characterized by variable temperature powder X-ray diffraction (PXRD) and optical microscopy images, spans a broad temperature range with ΔT ∼123/153 K on heating/cooling (DSC measurement), and the wide ΔT is relevant to an extra stable anion chain owing to the strong antiferromagnetic (AFM) interactions protecting the chain from collapse in the plastic crystal state. 1 is a single-component ion plastic crystal with a record high ion conductivity, 0.21 S·cm^-1, at 453 K. The crystal-to-plastic crystal transformation in 1 is coupled to a bistable magnetic transition to give a multi-in-one multifunctional material. This study provides a creative thought for the design of OIPCs with striking thermal, electrical, and magnetic multifunctionality.

Polymorphism in a π stacked Blatter radical: structures and magnetic properties of 3-(phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

The first polymorphism example in Blatter radicals, is reported.

Design of a One-Dimensional Stacked Spin Peierls System with Room-Temperature Switching from Quantum Mechanical Predictions

Planar bis-1,2-dithiolene complex anions of a transition metal (denoted as [M(dithiolato)₂]^- and M = Ni, Pd, or Pt ion) favor forming columnar stacks of anions in the crystal that feature S = 1/2 spin-chains, and such a spin-chain compound often undergoes a spin-Peierls-type transition, making this a promising material for conducting and magnetic switching. However, current examples show the transition temperatures are far too low for most applications. We use quantum mechanics to predict that changing the cation arrangement from the boat-type to the chair-type packing configuration in a spin-Peierls-type [Ni(dithiolato)₂]^- complex will substantially stabilize the antiferromagnetic coupling, dramatically increasing the transition temperature. We estimate that the [Ni(mnt)₂]-based complexes (mnt = maleonitriledithiolate) with chair-type packing of cations will lead to critical temperatures of ∼170, ∼252, and ∼310 K for S-, Se-, and Te-based mnt, respectively. We also suggest how to stabilize the chair-type configurations of these systems.

Wide Magnetic Thermal Memory Effect (∼55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = 1/2 Spin Chain Molecule Crystal

One-dimensional (1D) S = 1/2 Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From high-temperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction in lattice symmetry in LTP, called the symmetry breaking (SB) phase transition. Herein, we report the first example of 1D S = 1/2 AFM molecular crystal, [Et₃( n-Pr)N][Ni(dmit)₂] (Et₃( n-Pr)N⁺ = triethylpropylammonium, dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate), which shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with T_C↑ ≈ 375 K, T_C↓ ≈ 320 K, and surprisingly large thermal hysteresis (∼55 K). Additionally, LTP and HTP coexist in a temperature region near T_C but not at T_C in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into the relationship of spin-Peierls-type transition and structure phase transition but also offer a roadmap for searching molecular-scale magnetic bistable materials, which are in huge demand in future electronic, magnetic, and photonic technologies.

Cobalt complexes with redox-active anthraquinone-type ligands

Three anthraquinone-type multidentate ligands, HL1-3 (HL = 2-R-1H-anthra[1,2-d]imidazole-6,11-dione; HL1: R = (2-pyridyl), HL2; R = (4,6-dimethyl-2-pyridyl), HL3; R = (6-methoxy-2-pyridyl)), were prepared, and their complexation behaviour was investigated. Three bis-chelate cobalt complexes with the formula [CoII(L1-3)2]·n(solv.) (1, 2, and 3 for HL1, HL2, and HL3, respectively), in which the ligands adopted tridentate binding modes, were synthesized and structurally characterized by single-crystal X-ray analyses. Electrochemical studies of 1-3 in CH2Cl2 reveal three reversible redox waves, assigned to ligand and cobalt-centred processes. Additional complexes were obtained in which HL1 adopted a bidentate binding mode, stabilising the mono-chelate [CoII(HL1)(NO3)2(DMF)2] (4) species and tris-chelate [CoIII(L1)3] (5) complex in which the cobalt ion was in its 3+ state. The electrochemical properties of complex 5 were investigated in DMF, and the Co(ii)/Co(iii) redox couple was found to be negatively shifted compared to that of complex 1, while the ligand-based processes became irreversible. Tridentate chelation is found to stabilise the anthraquinone ligands and unlocks their redox multi-stability.

The origin of the antiferromagnetic behaviour of the charge-transfer compound (HMTTF)[Ni(mnt)2]

Herein we disclose the origin of the magnetic behaviour of the remarkable (HMTTF)[Ni(mnt)₂] charge-transfer compound.

Grain size effect on magnetic and phase transition features in one-dimensional S = 1/2 Heisenberg spin chain molecular crystals

Magnetic and thermal behaviors and the phase transition nature are strongly influenced by grain size in one-dimensional S = 1/2 molecular spin systems.

Crystal structure of bis-[1-(4-bromo-benz-yl)pyridinium] bis-(1,2-di-cyano-ethene-1,2-di-thiol-ato-κ(2) S,S')nickelate(II)

The asymmetric unit of the title salt, (C₁₂H₁₁BrN)₂[Ni(C₄N₂S₂)₂], consists of one 1-(4-bromo­benz­yl)pyridinium cation and one half of a complex [Ni(mnt)₂]²⁻ (mnt²⁻ is the maleo­nitrile­dithiol­ate dianion). The Ni²⁺ ion is located on an inversion centre and is coordinated by four S atoms from two mnt²⁻ ligands, exhibiting a square-planar coordination environment. In the cation, the planes of the pyridinium and benzene rings make a dihedral angle of 69.86 (19)°. The cations and anions are alternately arranged in layers parallel to (001) and are held together by non-classical C—H⋯N hydrogen bonds.

Crystal structure of bis-[1-(naphthalen-1-ylmeth-yl)pyridinium] bis-(2,2-di-cyano-ethene-1,1-di-thiol-ato-κ(2) S,S')nickelate(II)

In the ion-pair complex, bis­[1-(naphthalen-1-ylmeth­yl)pyridinium] bis­(2,2-di­cyano­ethene-1,1-di­thiol­ate-κ²S,S′)nickelate(II), C—H⋯N and C—H⋯Ni hydrogen bonds as well as π–π inter­actions between the ions result in the formation of a three-dimensional network.

A new ion-pair complex, (C₁₆H₁₄N)₂[Ni(C₄N₂S₂)₂] or (1-NaMePy)₂[Ni(imnt)₂], where 1-NaMePy is 1-(4-naphthyl­methyl­ene)pyridinium and imnt is 2,2-di­cyano­ethene-1,1-di­thiol­ate, was obtained by the direct reaction of NiCl₂, K₂imnt and (1-NaMePy)⁺Br⁻ in H₂O. The asymmetric unit contains a [1-NaMePy]⁺ cation and one half of an Ni(imnt)₂²⁻ anion. The Ni^II ion lies on an inversion centre and adopts a square-planar configuration with Ni—S bond lengths of 2.200 (1) and 2.216 (1) Å. In the [1-NaMePy]⁺ cation, the naphthyl ringsystem and the pyridinium ring make a dihedral angle of 90.0 (2)°. In the crystal, C—H⋯N and C—H⋯Ni hydrogen bonds, as well as π–π inter­actions between the chelate ring and the pyridinium ring [centroid–centroid distance = 3.675 (2) Å] link the ions into a three-dimensional network.

Spin-crossover phenomena of the mononuclear Mn(III) complex tuned by metal dithiolene counteranions

Three ion-pair complexes based on spin-crossover [Mn(5-Br-sal-N-1,5,8,12)]ClO4 with TBA2[Ni(mnt)2], TBA2[Pt(mnt)2] (mnt = maleonitriledithiolate) and TBA[Ni(dmit)2] respectively (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato) have been synthesized and structurally characterized. Complexes [Mn(5-Br-sal-N-1,5,8,12)]2[Ni(mnt)2] and [Mn(5-Br-sal-N-1,5,8,12)]2[Pt(mnt)2] are isomorphic and show the axial compression of the octahedral coordination environment of Mn(III) ions. With the temperature increasing the equatorial metal-ligand bond lengths show significant elongation, but the axial bond lengths remain unchanged. Complex [Mn(5-Br-sal-N-1,5,8,12)][Ni(dmit)2]·CH3CN contains π-π, p-π and H-bonds weak interactions. Magnetic investigation shows the spin-crossover phenomena for and , and T1/2 has been increased by 230 K comparing with the reactant complex. However, no spin-crossover was observed in complex , and theoretical calculations show that there are weak antiferromagnetic couplings mediated through π-π interactions.

Influence of the molecular structures of dithiolate ligands on crystal packing modes and magnetic properties in salts (Bz-Et3N)[Ni(dmit)x(mnt)2−x] (x = 0–2)

Nickel-bis-dithiolene salts, (Bz-Et₃N)[Ni(dmit)_x(mnt)_2−x] (x = 0–2 for 1–3 and mnt²⁻ = maleonitriledithiolate, dmit²⁻ = 2-thioxo-1,3-dithiole-4,5-dithiolate), have been prepared and characterized.