Spin Crossover Properties Research Articles

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

Complexation of Fe[ClO4]2·6H2O by 1 equiv. 2,6-bis((4S)-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine ((S)-L1Ph) and 2,6-bis((4R)-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine ((R)-L2Ph) cleanly affords [Fe((S)-L1Ph)((R)-L2Ph)][ClO4]2; [Fe((R)-L1iPr)((S)-L2iPr)][ClO4]2 (L1iPr = 2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine; L2iPr = 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine) was prepared by a similar route. The compounds exhibit thermal spin-crossover in solution, at temperatures midway between the corresponding [Fe((R)-L1R)((S)-L1R)][ClO4]2 and [Fe((R)-L2R)((S)-L2R)][ClO4]2 (R = Ph or iPr) species. The spin states of [Fe(LR)(bimpy)][ClO4]2 and [Fe(LR)(bpp)][ClO4]2 (LR = L1R or L2R; bimpy = 2,6-bis(1H-benzimidazol-2-yl)pyridine; bpp = 2,6-di(pyrazol-1-yl)pyridine) are also reported, with most examples exhibiting gradual spin-crossover in solution and the solid state. Although some products undergo partial ligand exchange in solution by 1H NMR, their solution T½ values appear unaffected by this and correlate well with their spin state energies from gas phase DFT calculations. The high-spin state of [Fe(L2R)(bpp)]2+ is more stabilised than expected, compared to the other [Fe(LR)L]2+ complexes studied (L = bimpy, bpp or terpy). That is explained by an interplay between the relative σ-basicities and π-acidities of the two ligands in each molecule. The steric influence of their phenyl or isopropyl 'R' substituents stabilises the heteroleptic complexes by up to 5 kcal mol-1, compared to analogues lacking these groups.

SPIN-CROSSOVER IRON(II) COORDINATION COM­POUNDS: FABRICATION OF FUNCTIONAL MATERIALS AND THEIR INTEGRATION INTO MICRO- AND NANOCONSTRUCTIONS

Development of micro- and nanosized spin-crossover (SCO) materials has become an important research direction within the past decade. Such an interest is associated with high perceptive of practical application of these materials in nanoelectronic devices. Therefore, researches working in the field of SCO put considerable efforts to obtain SCO complexes in various functional forms, such as nanoparticles, thin films, etc. Fabrication of these materials is realized through different chemical and/or lithographical approaches, which allow to adjust size, shape and even organization of nanoobjects.
 In this review theoretical background of SCO phenomenon is described, additionally different classes of coordination compounds exhibiting spin crossover are covered. It is demonstrated that electric field, temperature and light irradiation can be effectively used for switching and control of spin state in nanosized SCO systems. Cooperative SCO with transition close to room temperature, wide hysteresis loop and distinct thermochromic effect is most often observed for Fe(II) coordination complexes. Therefore, Fe(II) SCO compounds form one of the most perspective classes of compounds for obtaining functional materials. It is shown that integration of Fe(II) compounds into micro- and nanohybrid devi­ces allows to combine unique functional pro­perties in one material due to synergy between SCO and physical properties (luminescent, electrical, etc.) of the other component. As a result, SCO compounds are interesting not only from the fundamental point of view, but also from practical, thanks to the possibility of integration of SCO Fe(II) complexes as active materials in devices of different configurations.
 It is expected that obtaining of new Fe(II) coordination polymers with unique SCO cha­racteristics will favor the development of new functional materials and devices on their basis in the nearest future.

Morphological Studies of Composite Spin Crossover@SiO2 Nanoparticles.

Spin crossover (SCO) iron (II) 1,2,4-triazole-based coordination compounds in the form of composite SCO@SiO2 nanoparticles were prepared using a reverse microemulsion technique. The thickness of the silica shell and the morphology of the as obtained core@shell nanoparticles were studied by modifying the polar phase/surfactant ratio (ω), as well as the quantity and the insertion phase (organic, aqueous and micellar phases) of the tetraethylorthosilicate (TEOS) precursor, the quantity of ammonia and the reaction temperature. The morphology of the nanoparticles was monitored by transmission electron microscopy (TEM/HRTEM) while their composition probed by combined elemental analyses, thermogravimetry and EDX analyses. We report that not only the particle size can be controlled but also the size of the silica shell, allowing for interesting perspectives in post-synthetic modification of the shell. The evolution of the spin crossover properties associated with the change in morphology was investigated by variable temperature optical and magnetic measurements.

Cooperativity and Metal–Linker Dynamics in Spin Crossover Framework Fe(1,2,3-triazolate)2

Cooperative interactions are responsible for the useful properties of spin crossover (SCO) materials─large hysteresis windows, critical temperatures near room temperature, and abrupt transitions─with hybrid framework materials exhibiting the greatest cooperativity and hysteresis of all SCO systems. However, little is known about the chemical origin of cooperativity in frameworks. Here, we present a combined experimental–computational approach for identifying the origin of cooperativity in the metal–organic framework (MOF) Fe(1,2,3-triazolate)2 (Fe(TA)2), which exhibits the largest known hysteresis window of all SCO materials and unusually high transition temperatures, as a roadmap for understanding the manipulation of SCO behavior in general. Variable-temperature vibrational spectroscopy provides evidence that “soft modes” associated with dynamic metal–linker bonding trigger the cooperative SCO transition. Thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems, while electron density calculations of Fe(TA)2 support previous theoretical predictions that large cooperativity arises in materials where SCO produces considerable differences in metal–ligand bond polarities between different spin states. Taken together, this combined experimental–computational study provides a microscopic basis for understanding cooperative magnetism and highlights the important role of dynamic bonding in the functional behavior of framework materials.

Abundant Solvatomorphism-Tuned Spin Crossover in a Dinuclear Fe(II) Compound: Computational Insights on Molecular Distortion and Packing Effects

A series of solvates based on a dinuclear Fe(II) complex [(TPA)2Fe2(μ-DHBQ)][BF4]2·S [S = CH3OH (1·CH3OH), 2CH3OH (2·2CH3OH), 4DMF (3·4DMF), 2Et2O (4·2Et2O), Et2O·MeCN (5·Et2O·MeCN), and 2CH2Cl2 (6·2CH2Cl2)] were synthesized. Upon solvent removal, single-crystal-to-single-crystal (SC-to-SC) transitions could occur for the first four solvates to give 1, 2, 3, and 4·0.5Et2O. Within the temperature range of 400–10 K, these compounds exhibited abundant variations in their spin crossover (SCO) properties. 2,5·Et2O·MeCN and 6·2CH2Cl2 displayed half high-spin (HS) to low-spin (LS) transitions, and 4·2Et2O underwent incomplete LS-to-HS conversion, whereas other solvatomorphs showed complete SCO. As all these solvatomorphs possessed the identical [(TPA)2Fe2(μ-DHBQ)][BF4]2 core, these variations in SCO behavior emphasized the critical role of the crystal lattice contributions. With the aim of deciphering their origin, periodic DFT+U+D3 computations were performed on these solvatomorphs to quantify the importance of all possible intramolecular and intermolecular effects on their spin-state energetics. Computationally, the isolated [(TPA)2Fe2(μ-DHBQ)]2– molecule in the gas phase would undergo SCO around 350 K in one step intrinsically. However, distinctive roles of the crystal-lattice effects in the solid state resulted in varying SCO behaviors. Different reasons were discovered for the incomplete spin transitions of different solvatomorphs. For 2, although serious volume shrinkage of the LS state caused efficient packing of the overall crystal-lattice organization, the originally close-packing SCO cations got loose and thus strongly destabilized its LS state. For 5·Et2O·MeCN and 6·2CH2Cl2, severe molecular distortions kinetically trapped their LS state. These computationally magneto-structural studies on dinuclear solvatomorphs have great importance for designing SCO compounds with selected properties, which is critical for their actual application.

Spin crossover in metal–organic framework for improved separation of C2H2/CH4 at room temperature

C2H2 is a very promising fuel substitute in the future, but it is often accompanied by the presence of CH4 in its production process, and high-purity C2H2 cannot be obtained. In this work, the mixed metal-organic framework (M’MOF), [Fe(pyz)Ni(CN)4] (FeNi-M’MOF, pyz ​= ​pyrazine), was used to induce changes in Fe–N bonds at room temperature using spin-crossover (SCO) properties, and highly selective adsorption of C2H2/CH4 was carried out. The MOFs has a high C2H2 adsorption capacity (96.5 ​cm3 ​g−1 for HS state; 96.1 ​cm3 ​g−1 for LS state), and has a remarkable selectivity for C2H2/CH4 (198.9 for HS state; 273.5 for LS state) at room temperature and standard atmospheric pressure. Under the condition that the C2H2 adsorption capacity is less affected, the selectivity of the polymer to C2H2/CH4 in the LS state is much higher than that in the HS state. Moreover, the preparation method of this material is simple and the performance after synthesis is stable, which indicates that FeNi-M’MOF is a promising C2H2/CH4 separation material.

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe III 4 Fe II 4 } Cubes

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe ^III ₄ Fe ^II ₄ } Cubes

Nitrile orientation versus crystal lattice as a tool for tuning the spin crossover properties in the one-dimensional coordination polymers [Fe(ebtz)2(RCN)2](BF4)2 (RCN = nitrile)

Nitrile orientation <i>versus</i> crystal lattice as a tool for tuning the spin crossover properties in the one-dimensional coordination polymers [Fe(ebtz)₂(<i>R</i>CN)₂](BF₄)₂ (<i>R</i>CN = nitrile)

Enhanced Interplay between Host-Guest and Spin-Crossover Properties through the Introduction of an N Heteroatom in 2D Hofmann Clathrates.

Controlled modulation of the spin-crossover (SCO) behavior through the sorption–desorption of invited molecules is an extensively exploited topic because of its potential applications in molecular sensing. For this purpose, understanding the mechanisms by which the spin-switching properties are altered by guest molecules is of paramount importance. Here, we show an experimental approach revealing a direct probe of how the interplay between SCO and host–guest chemistry is noticeably activated by chemically tuning the host structure. Thus, the axial ligand 4-phenylpyridine (4-PhPy) in the 2D Hofmann clathrates {Fe(4-PhPy)2[M(CN)4]} (PhPyM; M = Pt, Pd) is replaced by 2,4-bipyridine (2,4-Bipy), resulting in the isomorphous compounds {Fe(2,4-Bipy)2[M(CN)4]} (BipyM; M = Pt, Pd), which basically differ from the former in that they have a noncoordinated N heteroatom in the ancillary aromatic substituent, i.e., 2-pyridyl instead of phenyl. Our chemical, magnetic, calorimetric, and structural characterizations demonstrate that this subtle chemical composition change provokes outstanding modifications not only in the capability to adsorb small guests as water or methanol but also in the extent to which these guests affect the SCO characteristics.

Light and Thermal Induced Spin Crossover Properties of Fe(II) Compound

We synthesized a novel phenanthroline-based mononuclear Fe(II) compound [Fe(TTD)2(NCS)2]⋅CH3CN (1, TTD = 1,4,8,9-tetraaza-triphenylene-2,3-dicarbonitrile). And its crystal structure, optical and magnetic properties were analyzed. The thermally induced SCO of the compound is a one-step abrupt transition. In addition, light can also control the spin state of the compound. By using 660 nm light-excited compound, LIESST (light-induced excited spin-state trapping) effect can be achieved.

Spin Crossover in a Series of Non-Hofmann-Type Fe(II) Coordination Polymers Based on [Hg(SeCN)3]- or [Hg(SeCN)4]2- Building Blocks.

Self-assembly of [Hg(SeCN)4]2- tetrahedral building blocks, iron(II) ions, and a series of bis-monodentate pyridyl-type bridging ligands has afforded the new heterobimetallic HgII-FeII coordination polymers {Fe[Hg(SeCN)3]2(4,4'-bipy)2}n (1), {Fe[Hg(SeCN)4](tvp)}n (2), {Fe[Hg(SeCN)3]2(4,4'-azpy)2}n (3), {Fe[Hg(SeCN)4](4,4'-azpy)(MeOH)}n (4), {Fe[Hg(SeCN)4](3,3'-bipy)}n (5) and {Fe[Hg(SeCN)4](3,3'-azpy)}n (6) (4,4-bipy = 4,4'-bipyridine, tvp = trans-1,2-bis(4-pyridyl)ethylene, 4,4'-azpy = 4,4'-azobispyridine, 3,3-bipy = 3,3'-bipyridine, 3,3'-azpy = 3,3'-azobispyridine). Single-crystal X-ray analyses show that compounds 1 and 3 display a two-dimensional robust sheet structure made up of infinite linear [(FeL)n]2n+ (L = 4,4'-bipy or 4,4'-azpy) chains linked by in situ formed {[Hg(L)(SeCN)3]2}2- anionic dimeric bridges. Complexes 2 and 4-6 define three-dimensional networks with different topological structures, indicating, in combination with complexes 1 and 3, that the polarity, length, rigidity, and conformation of the bridging organic ligand play important roles in the structural nature of the products reported here. The magnetic properties of complexes 1 and 2 show the occurrence of temperature- and light-induced spin crossover (SCO) properties, while complexes 4-6 are in the high-spin state at all temperatures. The current results provide a new route for the design and synthesis of new SCO functional materials with non-Hofmann-type traditional structures.

Downsizing of Nanocrystals While Retaining Bistable Spin Crossover Properties in Three-Dimensional Hofmann-Type {Fe(pz)[Pt(CN)4]}-Iodine Adducts.

Mastering nanostructuration of functional materials into electronic devices is presently an essential task in materials science. This is particularly relevant for spin crossover (SCO) compounds, whose properties are extremely sensitive to size reduction. Indeed, the search for materials displaying strong cooperative hysteretic SCO properties operative at the nanoscale close near room temperature is extremely challenging. In this context, we describe here the synthesis and characterization of 20-30 nm surfactant-free nanocrystals of the FeII Hofmann-type polymer {FeII(pz)[PtII,IVIx(CN)4]} (pz = pyrazine), which affords the first example of a robust three-dimensional coordination polymer, substantially keeping operational thermally induced SCO bistability at such a scale.

Kompleks Besi(II) dengan ligan 3-bpp: review

The change of two different spin electronic states under the influence of temperature, pressure and light irradiation is known as the Spin Transition or Spin Crossover (SCO) phenomenon. &nbsp;Spin transition exists on octahedral complexes with an intermediate ligand field, one of the ligands is 2,6-bis (pyrazol-3-yl)pyridine or 3-bpp. Spin transition complexes were intensively investigated because they are highly potential candidates for electronic materials, such as sensors, molecular switches and information storages. &nbsp;The iron(II) complex with 3-bpp ligand has shown SCO characteristics including different transition pattern and transition temperature which were influenced by type of the counter anions. &nbsp;The iron(II) complexes with different types of counter anions have exhibited SCO phenomenon, due to the effect of temperature with abrupt transition. &nbsp;However, the presence of hydrate water in the iron(II) complex results in the higher temperature transition. &nbsp;In addition of that, the transition pattern has changed from the abrupt transition into the gradual transition types. &nbsp;Several studies of SCO properties of the complex Fe(II)-3-bpp reported that the transition type and the temperature transition were influenced by the type of counter anion and the amount of hydrate molecules. &nbsp;With various transition patterns and transition temperatures, the potential of Fe(II)-bpp complexes as SCO materials is more interesting to explored. &nbsp; Keywords: Iron(II) Complex, 3-bpp Ligand, Spin Crossover

Hexagonal-Shaped Spin Crossover Nanoparticles Studied by Ising-Like Model Solved by Local Mean Field Approximation

The properties of spin crossover (SCO) nanoparticles were studied for five 2D hexagonal lattice structures of increasing sizes embedded in a matrix, thus affecting the thermal properties of the SCO region. These effects were modeled using the Ising-like model in the framework of local mean field approximation (LMFA). The systematic combined effect of the different types of couplings, consisting of (i) bulk short- and long-range interactions and (ii) edge and corner interactions at the surface mediated by the matrix environment, were investigated by using parameter values typical of SCO complexes. Gradual two and three hysteretic transition curves from the LS to HS states were obtained. The results were interpreted in terms of the competition between the structure-dependent order and disorder temperatures (TO.D.) of internal coupling origin and the ligand field-dependent equilibrium temperatures (Teq) of external origin.

Complete and Versatile Post‐Synthetic Modification on Iron‐Triazole Spin Crossover Complexes: A Relevant Material Elaboration Method

AbstractIn this paper we study the post‐synthetic modification (PSM) reaction on solid spin crossover (SCO) [Fe(NH2trz)3]X2 (X=NO3, OTs, Cl, SO4, BF4) complexes with different substrates. The wide access to a diversity of functionalized complexes with imine, amide and carbamide groups from the same amino parent compound demonstrates the synthetic approach value of this method. The as‐obtained post‐synthetic complexes were studied by IR, solid NMR, elemental analyses, and powder X‐ray diffraction, and compared to the corresponding compounds obtained by direct synthesis (DS) routes. Moreover, after digestion of the complexes obtained by PSM reactions, the free ligands were characterized by NMR in solution, which allowed us to indirectly confirm the formation of complexes we wished to synthesize. The study reveals in numerous cases that a complete post‐synthetic modification is possible despite the structural cohesion that is established between the 1D coordination chains within these materials. Spin crossover properties of some complexes obtained by both methods are also reported and compared.

Front Cover: An Iron(II) Spin Crossover Complex with a Maleonitrile Schiff base‐like Ligand and Scan Rate‐dependent Hysteresis above Room Temperature (Z. Anorg. Allg. Chem. 8/2021)

The cover picture shows a scan-rate dependent thermal hysteresis loop for an iron(II) spin crossover complex. Intermolecular interactions between the iron centres, illustrated as dashed lines in the packing diagram, are responsible for the observation of bistability of the high spin and the low spin state, but also for kinetic effects. A faster scan rate leads to an apparent wider hysteresis loop, thus very slow measurements are necessary to correctly determine the spin crossover properties of the material. More details can be found in the Article by Birgit Weber et al.

The ON–OFF switching of thermal spin crossover by interstitial solvent exchange in a layered ReV–CN–FeII coordination framework

We present an advanced material exhibiting a spin crossover (SCO) effect generated by the combined application of two external stimuli, temperature and chemical treatment related to the crystallization solvent exchange. The reported material is based on a bimetallic {[FeII(4-Brphpy)4]3[ReV(CN)8]2}⋅7MeOH [1, 4-Brphpy = 4-(4-bromophenyl)pyridine] coordination network built of cyanido-bridged layers and interstitial methanol molecules. A weakly bonded solvent can be exchanged into the water, which results in a hydrated phase, {[FeII(4-Brphpy)4]3[ReV(CN)8]2}⋅5H2O (1-hyd). The difference in solvent content between 1 and 1-hyd is sufficient to induce a remarkable change in spin transition properties. Despite the {N6} coordination environment around Fe(II), 1 reveals a stable high-spin state in the whole 2–300 K temperature range, which was assigned to the stiffening of the framework due to a series of non-covalent interactions involving solvent and 4-Brphpy ligands. Upon the exchange of solvent to water, the framework of 1-hyd becomes more flexible producing a thermally induced SCO effect, which occurs in two distinguishable steps in the broad 70–250 K range. The 1 to 1-hyd transformation can be reversed by immersion in the original solvent, thus, the chemical ON–OFF switching of a thermal SCO effect was achieved. This work shows that solvent exchange processes within bimetallic layered FeII–[ReV(CN)8]3– networks give an efficient route for reversible chemical modulation of thermally induced SCO properties.

Influence of the ultra-slow nucleation and growth dynamics on the room-temperature hysteresis of spin-crossover single crystals

Nucleation and growth phenomena associated with the thermally-induced spin transition was studied using optical microscopy in single crystals of a cyanoheterometallic coordination polymer [Fe(1,6-naphthyridine)2(Ag(CN)2)2]. Despite the fact that the spin-crossover phenomenon occurs near room temperature (ca. 290–300 K), the experimental results show the existence of significant kinetic effects on the hysteretic behaviour of the spin transition in this compound. Ultra-slow switching rates and phase-boundary propagation velocities were observed (down to 3 nm s−1), several orders of magnitude lower than customary. Our observations indicate that these peculiar spin-crossover properties (slow switching kinetics, scan-rate-dependent transition temperatures) are presumably related to the presence of numerous microstructural defects in the crystals, which is an intrinsic characteristic of this compound.

Controlling the spin-crossover behavior of the [Cr(indenyl)2] family via ligand functionalization.

In this work, a computational method to study the spin-crossover (SCO) behavior of the [Cr(n-Meindenyl)2] family was carried out. Using the TPSSh/Def2TZVP method with the GD3BJ dispersion correction scheme, we computed the thermochemistry and transition temperatures (T1/2) for all members of this family, which are in excellent agreement with the available experimental data. Moreover, the computed data allow us to build a model that describes the effect of functionalizing the indenyl ligand in different positions on the spin-state energy gap and transition temperature. Our results show that the C4 and C7 positions of the indenyl ligand have a greater effect on tuning the SCO properties of such complexes. The model quantitatively reproduces the DFT calculations, thus providing a powerful tool to analyze and predict the SCO properties in any member of the [Cr(n-Meindenyl)2] family.

Temperature dependence of desolvation effects in hydrogen-bonded spin crossover complexes.

The synthesis, crystal structure and (photo)magnetic properties of the anhydrous spin crossover salt of formula [Fe(bpp)2](C6H8O4) (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; C6H8O4 = adipate dianion), obtained by desolvation at 400 K of the original tetrahydrate in a single-crystal-to-single-crystal (SC-SC) transformation, are reported. This work offers a comparison between this compound and the previously reported hydrated material ([Fe(bpp)2](C6H8O4)·4H2O, 1·4H2O), highlighting the significance of the thermal conditions used in the dehydration-rehydration processes. In both compounds, a hydrogen-bonded network between iron(ii) complexes and adipate anions is observed. The original tetrahydrate (1·4H2O) is low-spin and desolvation at 450 K triggers a low-spin (LS) to high-spin (HS) transition to an amorphous phase that remains stable over the whole temperature range of study. Surprisingly, the dehydrated compound at 400 K (1) keeps the crystallinity, undergoes a partial spin crossover (T1/2 = 180 K) and a quantitative LS to HS photomagnetic conversion at low temperatures, with a T(LIESST) value of 61 K.