Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide

A new series of iron­(II) 1D coordination polymers with the general formula [FeL1­(pina)]·<i>x</i>solvent with L1 being a tetradentate N₂O₂^2– coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis­(iminomethylidyne)]­bis­(2,4-pentanedionato)­(2-)-<i>N</i>,<i>N</i>′,<i>O</i>²,<i>O</i>²′], and pina being a bridging axial ligand <i>N</i>-(pyrid-4-yl)­isonicotinamide, are discussed. The X-ray crystal structure of [FeL1­(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i>, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1­(pina)]·<i>x</i>H₂O/MeOH. The 88 K wide thermal hysteresis loop (<i>T</i>_1/2↑ = 328 K and <i>T</i>_1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1­(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (<i>T</i>_1/2↑ = 296 K and <i>T</i>_1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1­(pina)]·0.5H₂O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (<i>T</i>_1/2↑ = 321 K and <i>T</i>_1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties

Fe^II Spin Transition Materials Including an Amino–Ester 1,2,4-Triazole Derivative, Operating at, below, and above Room Temperature

A new family of one-dimensional Fe^II 1,2,4-triazole spin transition coordination polymers for which a modification of anion and crystallization solvent can tune the switching temperature over a wide range, including the room temperature region, is reported. This series of materials was prepared as powders after reaction of ethyl-4<i>H</i>-1,2,4-triazol-4-yl-acetate (αEtGlytrz) with an iron salt from a MeOH/H₂O medium affording: [Fe­(αEtGlytrz)₃]­(ClO₄)₂ (<b>1</b>); [Fe­(αEtGlytrz)₃]­(ClO₄)₂·CH₃OH (<b>2</b>); [Fe­(αEtGlytrz)₃]­(NO₃)₂·H₂O (<b>3</b>); [Fe­(αEtGlytrz)₃]­(NO₃)₂ (<b>4</b>); [Fe­(αEtGlytrz)₃]­(BF₄)₂·0.5H₂O (<b>5</b>); [Fe­(αEtGlytrz)₃]­(BF₄)₂ (<b>6</b>); and [Fe­(αEtGlytrz)₃]­(CF₃SO₃)₂·2H₂O (<b>7</b>). Their spin transition properties were investigated by ⁵⁷Fe Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and differential scanning calorimetry (DSC). The temperature dependence of the high-spin molar fraction derived from ⁵⁷Fe Mössbauer spectroscopy in <b>1</b> reveals an abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (<i>T</i>_c^↑ = 296 K and <i>T</i>_c^↓ = 291 K). The properties drastically change with modification of anion and/or lattice solvent. The transition temperatures, deduced by SQUID magnetometry, shift to <i>T</i>_c^↑ = 273 K and <i>T</i>_c^↓ = 263 K for (<b>2</b>), <i>T</i>_c^↑ = 353 K and <i>T</i>_c^↓ = 333 K for (<b>3</b>), <i>T</i>_c^↑ = 338 K and <i>T</i>_c^↓ = 278 K for (<b>4</b>), <i>T</i>^↑ = 320 K and <i>T</i>^↓ = 305 K for (<b>5</b>), <i>T</i>_c^↑ = 106 K and <i>T</i>_c^↓ = 92 K for (<b>6</b>), and <i>T</i>^↑ = 325 K and <i>T</i>^↓ = 322 K for (<b>7</b>). Annealing experiments of <b>3</b> lead to a change of the morphology, texture, and magnetic properties of the sample. A dehydration/rehydration process associated with a spin state change was analyzed by a mean-field macroscopic master equation using a two-level Hamiltonian Ising-like model for <b>3</b>. A new structural-property relationship was also identified for this series of materials [Fe­(αEtGlytrz)₃]­(anion)₂·<i>n</i>Solvent based on Mössbauer and DSC measurements. The entropy gap associated with the spin transition and the volume of the inserted counteranion shows a linear trend, with decrease in entropy with increasing the size of the counteranion. The first materials of this substance class to display a complete spin transition in both spin states are also presented

Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide

A new series of iron­(II) 1D coordination polymers with the general formula [FeL1­(pina)]·<i>x</i>solvent with L1 being a tetradentate N₂O₂^2– coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis­(iminomethylidyne)]­bis­(2,4-pentanedionato)­(2-)-<i>N</i>,<i>N</i>′,<i>O</i>²,<i>O</i>²′], and pina being a bridging axial ligand <i>N</i>-(pyrid-4-yl)­isonicotinamide, are discussed. The X-ray crystal structure of [FeL1­(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i>, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1­(pina)]·<i>x</i>H₂O/MeOH. The 88 K wide thermal hysteresis loop (<i>T</i>_1/2↑ = 328 K and <i>T</i>_1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1­(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (<i>T</i>_1/2↑ = 296 K and <i>T</i>_1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1­(pina)]·0.5H₂O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (<i>T</i>_1/2↑ = 321 K and <i>T</i>_1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Quantitative Contact Pressure Sensor Based on Spin Crossover Mechanism for Civil Security Applications

Detailed studies of impacts on the new 1D spin transition polymer [Fe­(hyetrz)₃]­I₂·0.5EtOH have been performed under several controlled contact pressures, showing for high energy values a color change of the compound and allowing a visual detection of the spin transition from high-spin to low-spin states. By performing detailed investigations on freshly impacted samples, using spectroscopic diffuse optical reflectivity, we could follow the variation of the optical spectra as a function of the energy of the impact. The meticulous analysis of the obtained spectra allowed us to establish an absorption peak at 550 nm whose intensity and position well correlate to the energy of the impact. This concept provides a reliable method of measuring the energy of a chock even if the sample does not change its color so much in the visible range. This might be of high importance in several civil security applications, like transportation of artwork or other fragile valuable objects or even in the evaluation of the degree of alteration of a material after a collision

Selective and Reusable Iron(II)-Based Molecular Sensor for the Vapor-Phase Detection of Alcohols

A mononuclear iron­(II) neutral complex (<b>1</b>) is screened for sensing abilities for a wide spectrum of chemicals and to evaluate the response function toward physical perturbation like temperature and mechanical stress. Interestingly, <b>1</b> precisely detects methanol among an alcohol series. The sensing process is visually detectable, fatigue-resistant, highly selective, and reusable. The sensing ability is attributed to molecular sieving and subsequent spin-state change of iron centers, after a crystal-to-crystal transformation

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