Two-step Spin Transition Research Articles

Two-Step Spin Crossover and Contact-Tunable Giant Magnetoresistance in Cyclopentadienyl Metalloporphyrin

In this article, we study the optimized structures, spin crossover, and coherent quantum transport properties of a series of cyclopentadienyl metalloporphyrin (PTMCp, TM = transition metal) complexes using the density functional theory combined with the non-equilibrium Green’s function method. The structure of the complexes can be classified into the sandwich type and the biplanar type. Energetic analyses of spin states reveal that the IS(intermediate-spin)-HS(high-spin) spin transition may appear in PMnCp and PFeCp, and the LS(low-spin)-IS(intermediate-spin)-HS(high-spin) two-step spin transition may appear in PCoCp under external stimulus. We predict that giant magnetoresistance may be observed in the ground-state sandwich PTMCp depending on the contact between the electrodes and the molecule in measurements. These results indicate that PTMCp complexes could be promising materials for spintronics.

Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer.

A novel two-dimensional (2D) Hofmann-type coordination polymer, {FeII(PyHbim)2[Pd(CN)4]}·2CH3OH [1·2CH3OH, PyHbim = 2-(4-pyridyl)benzimidazole], has been synthesized, which can undergo a spontaneous guest exchange, transforming to 1·2H2O in a single-crystal-to-single-crystal (SCSC) manner, shifting from orthorhombic Cmmm to monoclinic C2/m involving the displacement of 2D layers. The solvent-induced SCSC transformation process was reversible and verified through powder X-ray diffraction (PXRD) and single-crystal X-ray crystallography analyses. Both 1·2CH3OH and 1·2H2O exhibit complete and abrupt spin crossover (SCO) behaviors in two steps, while their SCO temperature ranges drastically shift by ca.100 K, spanning room temperature, owing to different intermolecular interactions resulting from diverse interlayer packing manners and host-guest interactions. Besides, a structural phase transition is observed in 1·2CH3OH, contributing to the two-step spin transition.

Two-step spin transition around room temperature in a FeIII complex.

Spin-crossover (SCO) at room temperature is a pivotal goal within the field of molecular magnetism. Herein, we attempt to assemble FeIII SCO complexes using a substituted Hqsal ligand, H2L (N-(8-quinolyl)-2,3-dihydroxybenzaldimine). Two complexes [Fe(HL)2]·X·2MeCN (X = BF4- for 1 and X = ClO4- for 2) were obtained and characterized. Single-crystal X-ray diffraction study reveals that the solvent and counteranion contact with the main structure through hydrogen bonding that significantly influences the SCO properties. Magnetic study reveals that both complexes show a one-step reversible spin transition below room temperature with a hysteresis loop width of 10 K for complex 1 and 4 K for complex 2. After removing the solvents, two-step SCO with a hysteresis loop width of 32 and 62 K is observed around room temperature for complex 1, while one-step SCO is found in complex 2. Magneto-structural study reveals that the differences in the SCO properties are related to the hydrogen bonding and solvent effects, which facilitates the spin transition.

Desolvation-induced magnetic switching from single-ion magnetism to spin-crossover in a halogen-functionalized cobalt(II) complex

The switching between spin-crossover (SCO) and single-ion magnet (SIM) are fascinating because the process provides potential in-depth understating of different bistability. Herein, we present a rare example of the desolvation-induced magnetic switching from single-ion magnetism to SCO between a halogen-functionalized cobalt(II) complex, [Co(Brphtpy)2](PF6)2·2MeOH (Brphtpy = 4′-(4-Bromophenyl)-2,2′:6′,2′'-terpyridine, 1·2MeOH) and its desolvated material of [Co(Brphtpy)2](PF6)2 (1). The temperature-dependent magnetic susceptibility and variable-temperature single crystal X-ray diffraction results show a high spin state of the Co(II) ions in 1·2MeOH. At lower temperature, frequency- and temperature-dependent slow magnetic relaxation behaviors were evidenced under an applied dc field of 1000Oe, originating from the easy-plane magnetic anisotropy (D=43.5 cm−1) of Co(II) ions. After the release of methanol molecules, interestingly, the desovated material 1 display a complete and cooperative two-step spin transitions with transition temperatures being 350 and 125K. Differential scanning calorimetry measurements support the stepwise SCO behavior and reveal the related ΔH and ΔS.

Observation of Two-Step Spin Transition in Graphene Oxide-Based Hybrids with Iron(II) 4-amino-1,2,4-triazole Spin Crossover Nanoparticles.

A novel experimental protocol based on a reverse micellar method is presented for the synthesis of graphene oxide (GO)-based hybrids with spin crossover nanoparticles (SCO NPs) of the 1D iron(II) coordination polymer with the formula [Fe(NH2trz)3](Br2). By introducing different quantities of 0.5% and 1.0% of GO (according to iron(II)) into the aqueous phase, two hybrids, NP4 and NP5, were synthesized, respectively. The morphological homogeneity of the NPs on the surface of the GO flakes is greatly improved in comparison to the pristine [Fe(NH2trz)3](Br2) NPs. From the magnetic point of view and at a low magnetic sweep rate of 1 K/min, a two-step hysteretic behavior is observed for NP4 and NP5, where the onset of the low-temperature second step appeared at 40% and 30% of the HS fraction, respectively. For faster sweep rates of 5-10 K/min, the two steps from the cooling branch are progressively smeared out, and the critical temperatures observed are T1/2↑ = 343 K and T1/2↓ = 288 K, with a thermal width of 55 K for both NP4 and NP5. A Raman laser power-assisted protocol was used to monitor the thermal tolerance of the hybrids, while XPS analysis revealed electronic interactions between the SCO NPs and the GO flakes.

Theoretical approach to the one-step versus two-step spin transitions in Hofmann-like FeII SCO metal-organic frameworks

Guest molecules in the 3D Hofmann-type FeII spin-crossover (SCO) metal-organic frameworks modulate the magnetic properties of the host, modifying the spin state, transition temperature, width of the hysteresis loop and are responsible for the occurrence of a single-step or multistep spin transition. In this work we explore the guest-dependent SCO properties of the Hofmann-like FeII SCO clathrates with formula {Fe(bpb)[Pt(CN)4]}⋅2G. We use a computational strategy based on DFT periodic calculations on the whole system, CASSCF/NEVPT2 calculations on single metal fragments and plots of the reduced density gradients to identify and quantify the dominant effects governing the occurrence of one-step or two-step spin transitions in these systems. Our results inform about the strength of the ligand field around Fe sites, the relative stability of the different spin states, the amplitude and nature of the host-guest and guest-guest interactions, and the role of the vibronic effects on the SCO properties of Hofmann-type FeII clathrates.

Diverse Magnetic Properties of Two New Binuclear Complexes Affected by [FeN6] Octahedral Distortion: Two-Step Spin Crossover versus Antiferromagnetic Interactions

Polymetallic complexes with covalently bridged metal centers that interact magnetically are important in the molecular magnetism field, with binuclear compounds receiving special attention because they represent the simplest type of multinuclear species with covalently bridged metal centers. Herein, we report the synthesis and properties of two new binuclear FeII complexes, namely, {[Fe(abpt-TPE)(NCS)2]2(bpym)}·2MeOH·2MeCN (1) and {[Fe(abpt-TPE)(NCS)2]2(bpym)}·2CH2Cl2 (2) (bpym = 2,2′-bipyrimidine). The crystal structure is analyzed at different temperatures, and its properties are analyzed by variable-temperature magnetic susceptibility and variable-temperature fluorescence emission spectroscopy tests. Variable-temperature magnetic susceptibility measurements of two binuclear compounds show different types of magnetic behavior. Complex 1 exhibits two-step spin transition behavior with an intermediate state near 150 K (Tc1 = 191 K, Tc2 = 111 K); 1 undergoes an [LS–LS] ↔ [LS–HS] ↔ [HS–HS] spin transition during thermal induction. On the other hand, complex 2 exhibits intramolecular antiferromagnetic coupling, with J = −0.47 cm−1. The analysis of correlations between the structural characteristics and different types of magnetic behaviors for two binuclear complexes, revealed that the different magnetic behaviors shown by the two complexes are attributable to different degrees of [FeN6] octahedral distortion caused by different lattice solvents, ligand strain and crystal stacking.

Observation of two-step spin transition in iron(II) 4-amino-1,2,4-triazole based spin crossover nanoparticles.

A synthetically controllable two-step spin transition was observed in iron(II) spin crossover nanoparticles of the dehydrated one-dimensional coordination polymer [Fe(NH2trz)3]Br2 (NH2trz = 4-amino-1,2,4-triazole) using the reverse micellar method. The change from two-step to one-step hysteretic characteristics succeeded by changing the reaction time.

Near room temperature stepwise spin state switching and photomagnetic effect in a mixed-valence molecular square.

A two-step thermo-induced spin-state switching was observed in a cyanide-bridged [Fe2Fe2] molecular square complex, {[Fe(pzTp)(CN)3]2[Fe(L)2]2}[Fe(pzTp)(CN)3]2·4CH3OH·2H2O [1·4MeOH·2H2O; pzTp = tetrakis(pyrazol-1-yl)borate and L = bis(1-ethylimidazol-2-yl)ketone (bik*)], which was characterized fully by single-crystal X-ray diffraction, (photo)magnetic measurements, and spectroscopic techniques. 1·4MeOH·2H2O exhibited a two-step thermo-induced spin transition with T1/2 (1) ↑ = 306 K and T1/2 (2) ↑ = 370 K converting the low-temperature ground state, {[(FeIIILS)2(FeIILS)2](FeIIILS)2} into the high-temperature state, {[(FeIIILS)2(FeIIHS)2](FeIIILS)2} via a stable intermediate phase. The desolvated phase, 1 also exhibited a gradual but reversible thermo-induced spin state change with a T1/2 value of 190 K, significantly shifted to a lower temperature. It also exhibited photo-induced spin-state switching at 20 K with the TLIESST value of 60 K.

High-efficiency X-ray emission spectroscopy of cold-compressed Fe2O3and laser-heated pressurized FeCO3using a von Hámos spectrometer

Fast and efficient (resonant) X-ray emission spectroscopy setup for the electronic structure at lower mantle conditions: high spin in laser-heated FeCO3and a two-step spin transition in pressurized Fe2O3with increasing valence to core intensity.

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Two-step spin crossover by guest-disorder induced local symmetry breaking within a 3D Hofmann-like framework.

A 3D Hofmann-like metal-organic framework has been prepared which contains a 2,1,3-benzothiadiazole-based pillaring ligand. Encapsulation of a polycyclic aromatic hydrocarbon, chrysene, within the pore structure leads to a new pathway to multi-step spin crossover behaviour in which the observed two-step spin transition arises due to the presence of multiple site environments associated with local guest positional effects within the host lattice.

Effects of water removal on the structure and spin-crossover in an anilato-based compound

The crucial role played by a crystallization water molecule in the spin crossover (SCO) temperature and its hysteresis is described and discussed in compound [NBu4][Fe(bpp)2][Cr(C6O4Br2)3]⋅2.5H2O (1), where bpp = 2,6-bis(pyrazol-3-yl)pyridine and (C6O4Br2)2− = dianion of the 3,6-dibromo-2,5-dihydroxy-1,4-benzoquinone. The compound has isolated [Fe(bpp)2]2+ cations surrounded by chiral [Cr(C6O4Br2)3]3− anions, NBu4+ cations, and a water molecule H-bonded to one of the non-coordinated N–H groups of one bpp ligand. This complex shows a gradual almost complete two-step spin transition centered at ca. 180 and 100 K with no hysteresis. The loss of the water molecules results in a phase transition from a P21/n phase with only one independent [Fe(bpp)2]2+ cation to a chiral Pn phase with two independent [Fe(bpp)2]2+ cations. Besides, there is an increase in the SCO temperature to 195/202 K with a hysteresis of ca. 7 K. In the dehydrated phase, only one of the two independent [Fe(bpp)2]2+ cations shows the SCO, whereas the second one remains in a high spin configuration at any temperature. In addition, compound 1 exhibits the LIESST (light-induced excited spin-state trapping) effect with a TLIESST of ca. 70 K.

Variable Cooperative Interactions in the Pressure and Thermally Induced Multistep Spin Transition in a Two-Dimensional Iron(II) Coordination Polymer.

Two types of experiments conducted to investigate the effect of pressure on the spin crossover (SCO) properties of the 2D Fe(II) coordination polymer formulated {Fe[bipy(ttr)2]}n are reported, namely, (1) magnetic measurements performed at variable temperature and at fixed pressure and (2) visible spectroscopy at variable pressure and fixed temperature. The magnetic experiments carried out under a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal a two-step spin transition behavior. The characteristic critical temperatures of the spin transition are shifted upward in temperature as pressure increases. The slope of the straight-line of the Tc vs P plot, dTc/dP, is 775 K/GPa and 300 K/GPa, for the high temperature and the low temperature steps, respectively. These values are remarkably large and denote the extreme sensitivity of the material to the application of pressure. Indeed, the visible spectroscopic measurements performed at 293 K show that a complete spin transition is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced spin transition is reversible and shows an asymmetric hysteresis. An analysis of the cooperative interactions of the thermal- and pressure-induced spin transition in the framework of the model of elastic interactions reveals that the elastic energy of the lattice as well as the interaction parameter between the SCO centers change during the course of the spin transition. Consequently, the character of the spin transition varies from abrupt for the high temperature step to continuous for the low temperature step.

A large dinuclear Fe(ii) triple helicate demonstrating a two-step spin crossover

Reported herein, the synthesis as well as the structural and magnetic characterisation of the largest reported dinuclear Fe(ii) triple helicate system to exhibit spin crossover-and also a rare example of a 273° helical twist using aromatic spacers-is presented, with exploration of the two-step spin-transition observed.

Structural Insights into the Two-Step Spin-Crossover Compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2

The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred at ca. 170 K and 145 K and a plateau at around 155 K. The origin of the two-step transition is discussed in light of these structural studies. The observations are compatible with a mostly disordered state between the two steps, consisting of mixing of high-spin and low-spin species, while weak substructure reflections in the mixed phase could indicate some degree of long-range order of the high-spin and low-spin sites.

The structurally variable network of spin couplings and migrating paramagnetic centers in binuclear o-quinone CoII complexes with biradical acene linkers: a computational DFT study.

A series of new magnetically-active coordination compounds comprising binuclear mixed-ligand complexes of cobalt bis-diketonates with acene linkers functionalized by two redox-active o-quinone moieties has been designed by means of density functional theory (DFT UB3LYP*/6-311++G(d,p)) calculations of their electronic structure, energy characteristics and magnetic properties. Two types of redox-active ligands include those with an acene linker bridging two o-benzoquinone fragments and the ligands containing an integrated π-conjugated system formed by annulation of o-quinone rings to the polycyclic core. The calculations reveal the dependence of spin density distribution in the compounds under study on the type of ligand. The considered binuclear CoII diketonate adducts manifest the capability of undergoing one- and two-step spin transitions induced by intramolecular electron transfers between the metal ions and the ligand system. An increase in the number of condensed rings of the acene linker promotes the stabilization of the biradicaloid state of the linker and enhances exchange interactions between all paramagnetic centers of the complexes giving rise to the formation of a flexible spin coupling network. The predicted unusual magnetic properties make the binuclear cobalt complexes with di-o-quinone ligand containing acene linker groups the promising building blocks for molecular electronic and spintronic devices.

Multiple spin phases in a switchable Fe(ii) complex: polymorphism and symmetry breaking effects

Three polymorphs of a new Fe(ii) complex have been selectively prepared in their pure forms. They display different SCO properties: the α-form displays a hysteretic one-step spin transition centered at 134 K, the β-form undergoes a hysteretic two-step spin transition (T1/2 = 153 and 144 K) and the γ-form is paramagnetic over the whole temperature region.

Pressure Effect Studies on the High Spin ⇆ Low Spin Transition Behavior Observed in [Fe(II)(bpp)2](NCS)2.2H2O

Background: [Fe(bpp)2](NCS)2.2H2O exhibits a two-step spin transition between the 5T2 state (HS, S = 2) and the 1A1 state (LS, S = 0) below room temperature, both steps being associated with thermal hysteresis. The present study explored the nature of spin transition (ST) in [Fe(bpp)2](NCS)2.2H2O under pressure to determine if ST could be induced to go to completion and whether a two-step process was then involved. Keywords: Spin transition, Fe(II), high pressure, magnetization, pressure effect studies, spin crossover.

Modification of Cooperativity and Critical Temperatures on a Hofmann-Like Template Structure by Modular Substituent

In a series of Hofmann-like spin crossover complexes, two new compounds, {Fe(3-F-4-Methyl-py)2[Au(CN)2]2} (1) and {Fe(3-Methyl-py)2[Au(CN)2]2} (2) (py = pyridine) are described. The series maintains a uniform 2-dimentional (2-D) layer structure of {Fe[Au(CN)2]2}. The layers are combined with another layer by strong aurophilic interactions, which results in a bilayer structure. Both coordination compounds 1 and 2 at 293 K crystallize in the centrosymmetric space groups P21/c. The asymmetric unit contains two pyridine derivative ligands, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2]− units. Compound 1 undergoes a complete two-step spin transition. On the other hand, 2 maintains the characteristic of the high-spin state. The present compounds and other closely related bilayer compounds are compared and discussed in terms of the cooperativity and critical temperature. The bilayer structure is able to be further linked by substituent-substituent contact resulting in 3-dimentional (3-D) network cooperativity.