Spin-crossover Transition Research Articles

Water Molecule-Induced Reversible Magnetic Switching in a Bis-Terpyridine Cobalt(II) Complex Exhibiting Coexistence of Spin Crossover and Orbital Transition Behaviors.

The development of molecule-based switchable materials remains an important challenge in the field of molecular science. Achievement of a structural phase transition induced by adsorption/desorption of guest molecules in spin crossover (SCO) Co(II) compounds is of significant interest because of the possibility that the spin state of the magnetic anisotropic high-spin (HS, S = 3/2) and low-spin (LS, S = 1/2) states can be switched via the induced changes in associated intermolecular interactions. In this study, we demonstrated a reversible magnetic switching associated with spin state conversion, along with a single-crystal to single-crystal (SCSC) phase transition induced by dehydration/rehydration. [Co(terpy)2](BF4)2·H2O (1·H2O; terpy = 2,2':6',2''-terpyridine) assembles in the solid state via π-π and CH-π interactions involving adjacent terpyridine cores along the ab direction to form two-dimensional (2D) layered domains. 1·H2O exhibits gradual and incomplete SCO, from fully HS to ca. 0.5 HS, and the field-induced single-molecule magnet (SMM) behavior attributed to the presence of the anisotropic partial high-spin Co(II) species. 1·H2O undergoes a SCSC transformation accompanied by a change from the tetragonal space group I41/a to P42/n via a dehydration process. Dehydrated 1 exhibits a reverse thermal hysteresis behavior (T1/2↑ = 287 K; T1/2↓ = 270 K) in the gradual SCO region from fully HS to ca. 0.5 HS, followed by an ordinary thermal hysteresis (T'1/2↑ = 195 K; T'1/2↓ = 155 K) to fully LS Co(II). A temperature-dependent single-crystal X-ray structural analysis revealed that the reverse hysteresis can be attributed to an order/disorder structural phase transition of the BF4- anions involving a symmetry breaking to yield the monoclinic space group P21/n and orbital (angular momentum) transition (LT). Both the SCSC phase transition and magnetic behavior are switchable by dehydration/rehydration processes; thus 1 again adsorbs water at room temperature to give both the original structure and its magnetic behavior.

Guest Removal and External Pressure Variation Induce Spin Crossover in Halogen-Functionalized 2-D Hofmann Frameworks.

The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [FeIIPd(CN)4(thioX)2]·2H2O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloro- and bromo-functionalized ligands into the Hofmann-type frameworks (1Cl·2H2O and 2Br·2H2O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1Cl and 2Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1Cl·2H2O and 2Br·2H2O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements).

Pressure effects on magnetism in Ca2Mn2O5 -type ferrites and manganites

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (with a ${d}^{5}$ electronic configuration) and ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$ configuration), exhibiting the ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (${d}^{5}$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\ensuremath{\approx}24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ and shifts it to higher pressures.

The Impact of the Next-Nearest Neighbor Dispersion Interactions on Spin Crossover Transition Enthalpy Evidenced by Experimental and Computational Analyses of Neutral π-Extended Heteroleptic Fe(III) Complexes.

A neutral heteroleptic Fe(III) complex 1 derived from a π-extension of the parent complex 2 was prepared and characterized. Complex 1 exhibited an abrupt spin crossover (SCO) transition exactly at room temperature (TSCO = 298 K). A crystal structure analysis of 1 revealed that the Fe(III) complex molecules formed a three-dimensional π-stacking interaction network. To thermodynamically clarify the mechanism of the SCO transition, the thermodynamic parameters of the SCO transitions for 1 and 2 were deduced from the temperature dependence of the magnetic susceptibility in the solid and solution states using the regular solution model. A comparison of the SCO enthalpy difference between the solid and molecule for 1 and 2 revealed that the lattice enthalpy difference would largely contribute to the SCO transition enthalpy difference. A computational evaluation of intermolecular interactions and lattice energies before and after the SCO transitions in 1 and 2 disclosed the significant contribution of the next-nearest neighbor dispersion interactions to the lattice enthalpy differences. This finding indicates that not only conventional nearest neighbor intermolecular interactions but also next-nearest neighbor dispersion interactions should be taken into account to understand the fundamental mechanism of a phase transition in molecular solids.

Cooperative Large-Hysteresis Spin-Crossover Transition in the Iron(II) Triazolate [Fe(ta)2] Metal-Organic Framework.

The metal-organic framework [Fe(ta)2] (Hta = 1H-1,2,3-triazole) containing Fe(II) ions and 1,2,3-triazolate ligands shows a reversible phase transition while retaining the cubic crystal symmetry and space group Fd3m (no. 227). The phase transition between room temperature (RT-[Fe(ta)2]; a = 16.6315(2) Å, V = 4600.39(8) Å3) and high temperature (HT-[Fe(ta)2]; a = 17.7566(4) Å, V = 5598.6(1) Å3) phases occurs at a temperature above 290 °C, whereas the phase transition between HT- and RT-[Fe(ta)2] starts at a temperature below 210 °C. Both [Fe(ta)2] polymorphs have identical bond topologies, but they differ by a large increase of the unit cell's volume of 22% for HT-[Fe(ta)2]. The compounds are characterized by powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Additionally, Mössbauer spectroscopy, magnetic studies, and the electronic structure of both phases are discussed in detail with respect to the spin-crossover transition from the low-spin (RT-[Fe(ta)2]) to the high-spin phase (HT-[Fe(ta)2]).

Three Jahn-Teller States of Matter in Spin-Crossover System Mn(taa).

Three high-spin phases recently discovered in the spin-crossover system Mn(taa) are identified through analysis by a combination of first-principles calculations and MonteCarlo simulation as a low-temperature Jahn-Teller ordered (solid) phase, an intermediate-temperature dynamically correlated (liquid) phase, and an uncorrelated (gas) phase. In particular, the Jahn-Teller liquid phase arises from competition between mixing with low-spin impurities, which drive the disorder, and intermolecular strain interactions. The latter are a key factor in both the spin-crossover phase transition and the magnetoelectric coupling. Jahn-Teller liquids may exist in other spin-crossover materials and materials that have multiple equivalent Jahn-Teller axes.

Experimental evidence for the elastic long-range character of the spin crossover transition in cooperative single crystals

We evidenced by means of optical microscopy the long-range character of the interactions acting in spin crossover (SCO) single crystals through a study of the optical density (OD) variation during the spin transition between high-spin (HS) and low-spin (LS) states. The time evolution of the local OD, measured in four single crystals, revealed the existence of precursor phenomena, which are manifest through the appearance of significant changes in the OD of the probed area well before the arrival of the front interface in the case of HS to LS transition and after the passage of the interface in the case of LS to HS transition. These two effects are attributed to the manifestation of long-range interactions that develop during the propagation of the HS/LS interface, which is accompanied by a volume change, generating elastic stresses in the material that deploy far from the sources. Interestingly, the experimental investigations indicate that the precursor and the after effect phenomena occur mostly in the HS phase, which is softer than the LS phase. These original experimental results are well reproduced using an electroelastic modeling in which we considered, in addition to the lattice misfit between HS and LS states, the existence of different elastic constants in the HS and LS phases.

Intercalation of 3X-pyridine with X = F, Cl, Br, I, in 2D ferrous nitroprusside. Thermal induced spin transition in Fe(3F-pyridine)2[Fe(CN)5NO

The series of 2D solids, Fe(3XPy)2[Fe(CN)5NO] with 3X ​= ​3-Fluoro, 3-Chloro, 3-Bromo, 3-Iodo and Py ​= ​Pyridine, was obtained by intercalation of the organic molecule (3XPy) between neighboring layers of 2D ferrous nitroprusside. The organic ligand was found coordinated to the axial positions for the iron atom linked at the equatorial N ends of the CN groups in the nitroprusside building block, [Fe(CN)5NO]. The axial CN and NO ligands remain unlinked and contribute to the solid stability through attractive dipole-dipole interactions. The formed hybrid solids preserve the 2D nature because no chemical bond is established between neighboring organic molecules in the interlayer region. For X ​= ​F, a thermal induced spin transition was observed, but not for the remaining derivatives. The nature of such behavior is discussed according to the recorded XRD, Raman and Mössbauer data and computational calculations. The spin crossover transition in Fe(3FPy)2[Fe(CN)5NO] is an incomplete process, a sample fraction of about 50% remains in paramagnetic state, even at 5 ​K. The spin transition is accompanied of an atypical small structural change, corresponding to a variation of 0.7% for the unit cell volume. Such behavior finds explanation in both, the bonding properties of the N end of the CN ligands in the nitroprusside pseudo-octahedral block, and in the intermolecular interactions in the interlayer region.

Enhanced dielectricity coupled to spin-crossover in a one-dimensional polymer iron(ii) incorporating tetrathiafulvalene.

In designing multifunctional materials for potential switches that can be used as memory devices, the high-spin (HS) to low-spin (LS) crossover (SCO) one-dimensional polymer, [FeII(L)(4,4'-bpy)] n , was constructed from a designed redox-active tetrathiafulvalene (TTF) functionalized Schiff-base and the ditopic linker 4,4'-bipyridine (bpy). It exhibits an 8 K hysteretic SCO centred at T 1/2 = 325 K which is coupled to changes in its dielectric constant. The crystal structures above and below the transition temperature reveal similar parallel linear ···Fe-bpy-Fe-bpy··· chains displaying expansion of the FeII octahedron in the HS state. Density functional theory (DFT) calculations reveal a concerted electronic charge and spin change represented by the Mülliken charge of the Fe and the magnitude and direction of the dipole moment which substantiate the experimental observations.

Three Cyanide-Bridged Fe <sup>II</sup>-M <sup>II</sup> (M = Fe, Co and Mn) Complexes: Synthesis, Crystal Structures and Magnetic Properties

Three dinuclear cyanide-bridged complexes Fe II (PY 5 OMe 2 )CNM II (PY 5 OMe 2 )[OTf] 3 (M = Fe 1 , Co 2 , Mn 3 ; PY 5 OMe 2 = 2,6-bis-((2-pyridyl)methoxymethane)pyridine OTf = CF 3 SO 3- ) have been synthesized and characterized. Single-crystal diffraction analyses show these three dinuclear compounds are very similar in structure. And the M-N (M = Fe, Co and Mn) bond lengths for complexes 1 - 3 indicate that 2 and 3 are typical of a high spin (HS) complexes and 1 is evident a low spin (LS) Fe (II) complex at room temperature, confirmed by the temperature dependence of magnetic susceptibilities of complexes 1-3 . The different magnetic behaviors of complexes 1 - 3 reveal that in the three complexes all the cyanide-carbon coordinated Fe (II) are low-spin, and Co (II) for 2 and Mn (II) for 3 are both in a high-spin state through 2-400 K but the cyanide-nitrogen coordinated Fe (II) for complex 1 begins to occur a spin crossover (SCO) transition from 300 K.

Spin crossover in the Prussian blue analogue FePt(CN)6 induced by pressure or X-ray irradiation.

The spin state of the Prussian blue analogue FeIIPtIV(CN)6 is investigated in response to temperature, pressure, and X-ray irradiation. While cooling to 10 K maintains the high-spin state of FeII, compression at ambient temperature induces a first-order spin-crossover (SCO) transition with a small hysteresis loop (p↑ = 0.8 GPa, p↓ = 0.6 GPa). In addition, the high-spin to low-spin transition can be initiated at lower pressure through increased X-ray irradiation. Our study highlights a cooperative SCO with moderate pressure in a porous Prussian blue analogue.

Simultaneous Spin-Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7',8,8'-Tetracyano-p-quinodimethane.

Invited for the cover of this issue are Ryuta Ishikawa and Satoshi Kawata at Fukuoka University and co-workers at Osaka University, Tohoku University, and Kumamoto University, Japan, collaborating within the research project "SOFT CRYSTALS". The image depicts the thermally induced simultaneous switching of magnetism and electrical conductivity in a two-dimensional supramolecular architecture composed of dinuclear FeII spin-crossover complexes and partially charged 7,7',8,8'-tetracyano-p-quinodimethanide radicals. 10.1002/chem.201903934.

Heterospin magnetically active bimetallic Fe and Co complexes of aldiminato-functionalized catechol: a DFT study

Density functional theory calculations (UTPSSh/6-311++G(d,p)) of electronic structure, energy characteristics, and magnetic properties of the electromeric forms of homo- and heterometallic binuclear complexes based on a redox-active aldiminato-functionalized catechol were performed. By means of completion of the coordination sphere of the metal ions (Fe and Co) by pyridine molecules in the aldiminate moiety and N,N'-di-tert-butyl-2,11-diaza[3.3]-(2,6)pyridinophane in the dioxolene fragment, the new heterospin magnetically active systems have been revealed. The studied compounds are shown to be capable of undergoing thermally initiated stepwise or synchronized spin-crossover transitions, in particular involving all the electromeric forms. The expected spin-state switching rearrangements allow one to consider the binuclear complexes with aldiminato-functionalized catechol as building blocks for molecular and quantum electronics devices.

Host Polytypism and Structural Modulation in Two-Dimensional Fe(NCS)2-Based Metal–Organic Frameworks: Can Spin-Crossover Transitions Be Predicted?

The properties and crystal structure of two new polymorphs of [Fe(tvp)2(NCS)2]·tvp [tvp = trans-(4,4′-vinylenedipyridine)] are investigated. Despite being unusual in metal–organic frameworks (MOFs), one of them shows a commensurate modulation with q = (1/4, 0, −1/4) at room temperature and an unmodulated triclinic structure at lower temperatures that requires a noticeable reconstruction of the H-bond schema. Magnetically, it consists of weakly interacting FeII zigzag chains. The second compound does not show any structural instability, and the distribution of the almost isolated magnetic ions is square planar. The refined structure models are compared applying fully the superspace approach and group theoretical tools. The structural differences are deeply discussed in terms of host–host and host–guest interactions. Unexpectedly, none of the polymorphs exhibit spin crossover, although they seem to fulfill the structural requirements derived from a thorough comparison with a large family of Fe(NCS)2-based M...

Manipulation of the molecular spin crossover transition of Fe(H2B(pz)2)2(bipy) by addition of polar molecules

The addition of various dipolar molecules is shown to affect the temperature dependence of the spin state occupancy of the much studied spin crossover Fe(II) complex, [Fe{H2B(pz)2}2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine). Specifically, the addition of benzimidazole results in a re-entrant spin crossover transition, i.e. the spin state starts in the mostly low spin state, then high spin state occupancy increases, and finally the high spin state occupancy decreases with increasing temperature. This behavior contrasts with that observed when the highly polar p -benzoquinonemonoimine zwitterion C6H2(…NH2)2(…O)2 was mixed with [Fe{H2B(pz)2}2(bipy)], which resulted in locking [Fe{H2B(pz)2}2(bipy)] largely into a low spin state while addition of the ethyl derivative C6H2(…NHC2H5)2(…O)2 did not appear to perturb the spin crossover transition of [Fe{H2B(pz)2}2(bipy)].

Thermally Induced Spin Transition in a 2D Ferrous Nitroprusside

This study reports the intercalation of pyridine molecules between neighboring layers of two‐dimensional (2D) ferrous nitroprusside. In the material under study, the stacking of neighboring layers results in the formation of a long range ordered solid, where the 3D structure is supported by dipole‐dipole attractive interactions between neighboring pyridine molecules in the interlayer region. No chemical interactions were observed between layers, which preserve their identity as a 2D material. In this hybrid inorganic–organic solid, a thermal induced spin transitions from high to low spin on cooling and then from low to high spin on heating were observed. Such thermal induced spin crossover transition takes place with a pronounced hysteresis of 18 K, according to the magnetic and DSC measurements. That spin crossover transition is characterized by an extremely small structural change, involved a unit cell volume reduction from the high to low spin states of only 0.7 % and a related Fe–NPyridine distance shortening of 0.10 Å. The two spin states and the transition between them were additionally characterized from magnetic and DSC data and, Raman and Mössbauer spectra.

Multistep spin-crossover transitions induced by the interplay between short- and long-range interactions with frustration on a triangular lattice

We investigate the phase diagram of an elastic interaction model for spin crossover (SC) materials on a triangular lattice, in which the long-range (LR) interaction of elastic origin, antiferromagnetic-like nearest-neighbor (NN) interaction ${J}_{1}$, and ferromagnetic (F)-like next-NN interaction ${J}_{2}$ interplay nontrivially. We find that for relatively weak elastic interactions in the model, two critical lines with the three-state Potts universality and the Berezinskii-Kosterlitz-Thouless (BKT) phase line merge at the BKT high temperature end point at zero effective field $H=0$. In contrast, for relatively large elastic interactions, the two critical lines and the coexistence line between the ferrimagnetic (FR)-like phases merge at a critical end point at $H=0$. The phase diagrams are also characterized by tricritical points, at which F-like spinodal, FR-like spinodal, and critical lines merge, and the metastable regions of the F-like and FR-like phases expand for stronger elasticity. We also find that, in the model with only the NN interaction ${J}_{1}$, for relatively weak elastic interactions, the critical end point is located at a very low temperature, the disordered phase appears at low temperatures, and the metastable regions of the F-like and FR-like phases are very small. The SR interactions are relevant for the critical lines, while the spinodal lines are caused by the LR interaction of elastic origin, which is independent of the strength of elasticity. Making use of these phase diagrams with the different characteristics, we show a wide variety of SC transitions with one to four steps, in which a second-order (continuous), first-order (discontinuous), or BKT (continuous) transition can occur at each step.

Electronic Structure and Magnetic Properties of o-Benzoquinone Iron Complexes with Tetraazamacrocyclic Ligands

Structure, energy characteristics, and magnetic properties of o-benzoquinone iron complexes with tetradentate macrocyclic nitrogen containing bases (pyridinophane and tris(2-pyridylmethyl)amine) are studied with a DFT (UTPSSh/6-311++G(d,p)) quantum chemical method. Electron withdrawing substituents in the redox-active ligand stabilize its dianionic form but do not significantly affect the energy difference between the electromers. Strong ferromagnetic exchange interactions are predicted in the isomers of complexes containing a high-spin divalent iron ion and the semiquinone form of o-benzoquinone. According to the calculations, all studied compounds can undergo thermally induced spin crossover transitions.

Magnetocapacitance effect and magnetostriction by the field-induced spin-crossover in [MnIII(taa)

We report the magnetocapacitance effect and the magnetostriction in the spin-crossover (SCO) complex [MnIII(taa)] (H3taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine). It is shown that huge enhancements of dielectric constant and lattice volume are caused by the field-induced SCO transition from the low-spin to the high-spin states in [MnIII(taa)]. In addition, the temperature-field phase diagram of the field-induced SCO in thermal equilibrium state is clarified up to 28 T. Our analysis suggests that a large magnetocaloric effect can also be caused by the field-induced SCO. These results indicate the possibility of multiple control of various material properties by the field-induced SCO.

Tunable spin-state bistability in a spin crossover molecular complex

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H2B(pz)2}2(bipy)] polycrystalline films on Al2O3 substrates have been studied, where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. For [Fe{H2B(pz)2}2(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H2B(pz)2}2(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H2B(pz)2}2(bipy)]/Al2O3 interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H2B(pz)2}2(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.