Spin Transition Temperature Research Articles

Lattice-solvent controlled spin transitions in iron(ii) complexes

A series of spin transition (ST) iron(II) compounds of the type [FeII2](X)2.{S}2 (where is 4'-(4'''-cyanophenyl)-1,2':6'1''-bispyrazolylpyridine, X=ClO4- or BF4-, and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A.{S}2) crystals of the composition [FeII()2](ClO4)2.{S}2, and two polymorphic compounds of the stoichiometry [FeII()2](BF4)2.{S}2 as red coffin (B.{S}2) and orange block (C.{S}2) crystals. The Fe-N bond distances of A.{S}2 (from 1.921(9) to 1.992(3) A; at 150 K), B.{S}2 (from 1.943(2) to 2.017(2) A; at 180 K) and C.{S}2 (from 1.883(3) to 1.962(3) A; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe-N distances at 180 K for the two polymorphs B.{S}2and C.{S}2 are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS)<-->orange (HS)) for A.{S}2 and B.{S}2 and ca. 3.6% decrease in the unit cell volume of A.{S}2 from 4403 A3 at 300 K to 4278 A3 at 150 K. The temperature dependent magnetic susceptibilities of A.{S}2 and B.{S}2 demonstrate systematic increase of the spin transition temperatures (T1/2) and continuous decreases of the hysteresis loop width (DeltaT1/2) upon slow lattice-solvent exclusion.

Spin‐Crossover Physical Gels: A Quick Thermoreversible Response Assisted by Dynamic Self‐Organization

Iron(II) triazolate coordination polymers with lipophilic sulfonate counterions with alkyl chains of different lengths have been synthesized. In hydrocarbon solvents, these polymers formed a physical gel and showed a thermoreversible spin transition upon the sol-gel phase transition. The formation of a hydrogen-bonding network between the triazolate moieties and sulfonate ions, bridged by water molecules, was found to play an important role in the spin-crossover event. The spin-transition temperature was tuned over a wide range by adding a small amount of 1-octanol, a scavenger for hydrogen-bonding interactions. This additive was essential for the iron(II) species to adopt a low-spin state. Compared with nongelling references in aromatic solvents, the spin-crossover physical gels are characterized by their quick thermal response, which is due to a rapid restoration of the hydrogen-bonding network, possibly because of a dynamic structural ordering through an enhanced lipophilic interaction of the self-assembling components in hydrocarbon solvents.

Mössbauer studies of mixed-valence spin-crossover iron complexes with a hexadentate tripod ligand

The spin-crossover behaviors of mixed-valence iron compounds [FeIIH3L][FeIIIL](NO3)2 (1) and [FeIIH3LMe][FeIIILMe](NO3)2 (2) have been investigated by 57Fe Mossbauer spectroscopy, where H3LMe is a hexadentate N6 tripod ligand containing three imidazole groups and H3LMe is its 2-methylimidazole derivative. Deconvolution analyses of the Mossbauer spectra revealed that a two-step SCO (LS FeII-LS FeIII→HS FeII-LS FeIII→HS FeII-HS FeIII) proceeds in each compound on elevating the temperature. Compound 2 exhibited lower spin-transition temperatures than 1. “Frozen-in effect” was observed below 120 and 50 K for 1 and 2, respectively.

Photomagnetism of Iron(II) Spin Crossover Complexes — The T(LIESST) Approach

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On the dielectric properties of the spin crossover complex [Fe(bpp)2] [BF4]2

AbstractThe thermal variation of the dielectric constant in the spin crossover complex [Fe(bpp)2] [BF4]2 displays a 16 K large hysteresis loop centered around 178 K corresponding to the spin transition temperatures. Unexpectedly, the dielectric constant has a higher value in the low‐spin state. DFT calculations were carried out to determine the microscopic electronic polarizabilities, but these latter could not be correlated with the macroscopic permittivity changes pointing the importance of the effect of the counter‐anions which must be necessarily taken into account in future DFT calculations. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On the dielectric properties of the spin crossover complex [Fe(bpp)2] [BF4]2

The thermal variation of the dielectric constant in the spin crossover complex [Fe(bpp) 2 ] [BF 4 ] 2 displays a 16 K large hysteresis loop centered around 178 K corresponding to the spin transition temperatures. Unexpectedly, the dielectric constant has a higher value in the low-spin state. DFT calculations were carried out to determine the microscopic electronic polarizabilities, but these latter could not be correlated with the macroscopic permittivity changes pointing the importance of the effect of the counter-anions which must be necessarily taken into account in future DFT calculations.

Spin Glass transition on Fe0.6Cr0.1Al0.3 and Fe0.6Ti0.1Al0.3

DC-magnetization measurements on Fe0.6Cr0.1Al0.3 and Fe0.6Ti0.1Al0.3 reveals the disappearance of the wide ferromagnetic state exists in the free doped Fe0.7Al0.3 alloy. Our data shows that the spin glass transition temperature for Fe0.6Cr0.1Al0.3 alloy is 38 K and for Fe0.6Ti0.1Al0.3 alloy is 41 K. These values are below 70 K which is the spin transition temperature of the Fe0.7Al0.3 alloy. This is attributed to the paramagnetic nature of the metals Cr and Ti.

Photomagnetism of iron(ii) spin crossover complexes—the T(LIESST) approach

The Light-Induced Excited Spin State Trapping (LIESST) effect, encountered in some Spin-Crossover (SCO) complexes, is of major interest for the design of optical switches. Nevertheless, until now any applications have been prohibited, because the lifetimes of the photomagnetic states are long enough only at low temperatures. Hereby we review the recent progress made by using the T(LIESST) procedure, which consists of systematically measuring the limit temperature above which a photomagnetic effect in a material is erased by warming the material from 10 K at a rate of 0.3 K min−1. This method has been today applied to more than sixty SCO compounds and by comparing the various materials a relation between T(LIESST) and thermal spin transition (T1/2) temperatures has been obtained, i.e.T(LIESST) = T0 − 0.3T1/2. The second section reports part of works done to identify the parameters affecting the T0 factor; that is to find a guideline for the rational design of materials with long-lived photomagnetic lifetimes at working room temperature. Finally, we present the procedure used to simulate a T(LIESST) curve and illustrate it using the examples of a mononuclear SCO complex and of a binuclear SCO system displaying antiferromagnetic interactions.

Photomagnetic properties of iron(ii) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

Anab initioeffective Hamiltonian for magnetism including longitudinal spin fluctuations

We discuss the use of the magnetic force theorem (MFT) using different reference states upon which the perturbative approach is based. Using a disordered local moment (DLM) state one finds goo d Curie (or Ne\'el) temperatures, and good energetics for planar spin spirals in the 3d magnets Fe, Co (fcc), Mn, Cr. On the other hand the ferromagnetic reference state provides excellent energetics for small $\theta$ spin spirals in Fe, Co and Ni, and by extension magnon energies under the assumption of adiabacity. However, planar spin spiral energetics and transition temperatures for Ni, Fe, Mn, and Cr show worse agreement. The reasons for this, and for the case of fcc Co whe re both approaches work very well are discussed. We further provide an extension of the mapping of the quantum problem to include longitudinal fluctuations, and discuss the role they will play in magnetic phase transitions. This constru ction is tested using planar spin spirals where ${\bf q}$ is fixed but the moment is allowed to rel ax. It is demonstrated that results from the magnetic force theorem approach and directly calculated {\it ab initio} values agree very well.

Solid solutions: An efficient way to control the temperature of spin transition in heterospin crystals MxCu1−x(hfac)2L (M=Mn, Ni, Co; L=nitronyl nitroxide)

Abstract A series of mixed-metal solid solutions MxCu1 − x(hfac)2L (M = Mn, Ni, and Co) liable to undergo thermally induced spin transitions have been synthesized. The structure and magnetic properties of the compounds have been investigated. By varying the metal to be incorporated in the complex (Mn, Ni, or Co) and the value of x, one can control the character of the magnetic anomaly.

Spin-Crossover Dendrimers: Generation Number-Dependent Cooperativity for Thermal Spin Transition

Poly(benzyl ether) dendrons having a focal triazole unit (Gntrz: trz = triazole; n = generation number = 0-2) were found to react with (MeSO(3))(2)Fe to form dendritic coordination polymers ([Fe(Gntrz)(3)](MeSO(3))(2).2H(2)O) that undergo the thermal spin transition. When the generation number of the dendritic unit was larger (n = 0 --> 1 --> 2), the average degree of polymerization (D(p) = 20 --> 10 --> 3) and spin-crossover temperature (T(c) = 335 --> 315 --> 300 K) of the resulting polymer were lower. However, the abruptness of the spin transition was not monotonically dependent on the generation number; (G1trz)Fe exhibited an abrupt spin transition with a temperature width of only 10 K, while the smallest and largest members of the (Gntrz)Fe family both displayed a rather broad spin-transition temperature width (30 (n = 0) and 50 K (n = 2)). X-ray diffraction and calorimetric analyses indicated the presence of a discotic columnar core-shell assembly with a crystal lattice best occupied by a C(3)(v)() symmetric array of medium-sized (G1trz)Fe.

Observation of the spin-crossover in [Fe(btr) 2(NCS) 2] · H 2O (btr = 4,4′-bis-1,2,4-triazole) with μSR

The electronic spin transition in [Fe(btr) 2(NCS) 2] · H 2O (btr = 4,4′-bis-1,2,4-triazole) is probed by longitudinal field muon spin relaxation measurements. The μSR spectra could be analyzed with two different components, corresponding to two muon sites. For both components, an increase in magnetic fluctuations is observed at the LS → HS transition, i.e. crossing the spin transition temperature coming from low temperature, if a sufficiently high field is applied. Even below the transition temperature, magnetic fluctuations are still found. For one of the two muon sites, muonium formation is clearly found below the spin-crossover temperature.

Synthesis and anomalous magnetic properties of heterometal dinuclear Fe(II)–Cr(III) complex bridged by 3,5-bis(pyridin-2-yl)-pyrazolate

A Cr(III)–Fe(II) dinuclear complex [(nta)Cr(μ-bpypz)Fe(dpea)]BF 4, where bpypz=3,5-bis(pyridin-2-yl)-pyazolate, nta=nitrilotriacetate and dpea=(2-aminoethyl)-bis(2-pyridylmethyl)amine, has been synthesized and structurally characterized by ESI-MS and 2H NMR spectra. The proposed structure is mostly more planar in the bridging moiety as compared to that of the previously reported [(nta)Cr(μ-bpypz)Fe(picen)]BF 4( N, N ′-bis-(2-pyridylmethyl)ethylenediamine(picen)). The dpea complex is found to be a high spin state, but to exhibit no spin-crossover by magnetic susceptibility and Mössbauer spectra measurements; showing a large antiferromagnetic interaction, which could make the spin transition temperature lower and/or destroy spin-crossover, suggesting complementarity between spin-crossover and magnetic interaction.

Variable temperature far and mid FT-IR as a valuable tool to determine the spin transition temperature of iron(II) spin-crossover coordination compounds

The thermally induced spin transition behaviour of selected iron(II) coordination compounds with substituted tetrazole ligands has been monitored using variable temperature FT-IR spectroscopy. The reliability of these results is discussed and compared with independent analytical techniques such as SQUID measurements and 57Fe Mössbauer spectroscopy proofing variable temperature IR spectroscopy to be a valuable tool in the determination of the spin transition temperature T 1/2.

Unexpected isotope effect on the spin transition of the coordination polymer Fe(C5H5N)2[Ni(CN)4]Dedicated to Patrick Cassoux on the occasion of his retirement.

The two-dimensional coordination polymer spin-crossover compound Fe(pyridine)2[Ni(CN)4] (1) and its isotope substituted analogues Fe(pyridine-D5)2[Ni(CN)4] (2) and Fe(pyridine-15N)2[Ni(CN)4] (3) have been synthesised and the isotope effect on the thermal spin-crossover behaviour has been studied using magnetic susceptibility, 57Fe Mossbauer, Raman and calorimetric techniques. All these methods confirmed that upon isotope substitution—contrary to previous observations—the spin transition temperature shifted downwards from 202 to 194 K. The theoretical analysis of spectroscopic data has revealed that this shift was a result of a subtle balance between different vibrational and electronic factors.

Triggering the spin-crossover of Fe(phen) 2(NCS) 2 by a pressure pulse. Pressure and magnetic field induce ‘mirror effects’

We have studied the static and dynamic effects of pressure on the spin-transition in the temperature range of the thermal hysteresis loop for the compound Fe(phen) 2(NCS) 2. The high-spin fraction ( n HS) as a function of pressure and temperature has been determined by optical reflectivity. In this compound, the pressure was found to upward shift the spin-transition temperature by 23 K per kbar. During the dynamic pressure pulse, a decrease in n HS is observed, with an irreversible (reversible) character in the descending (ascending) branch of the hysteresis loop. In this respect, pressure has a ‘mirror effect’ compared to the application of an intense and pulsed magnetic field, for which – as reported previously – an increase in n HS is observed, with an irreversible (reversible) character in the ascending (descending) branch of the hysteresis loop. To cite this article: A. Bousseksou et al., C. R. Chimie 6 (2003).

Photo-induced spin transition for iron(III) compounds with [formula omitted] interactions

The magnetic susceptibilities of the spin-crossover compound [Fe(pap) 2]PF 6·MeOH ( 1) have been measured and compared with that of the previously reported compound [Fe(pap) 2]ClO 4·H 2O ( 2). Compound 1 is a spin-crossover compound with spin transition temperature T 1/2=288 K, and does not show a hysteresis loop around the spin transition temperature, in contrast with compound 2. Magnetic susceptibilities measured after illumination prove that 1 exhibits the LIESST effect, so does compound 2. The crystallographic data on the molecular packing show the presence of π stacking between the aromatic rings of pap ligands, which appears to be responsible for the LIESST effect.

Control of charge-transfer-induced spin transition temperature on cobalt-iron Prussian blue analogues.

The electronic and spin states of a series of Co-Fe Prussian blue analogues containing Na(+) ion in the lattice, Na(x)()Co(y)()Fe(CN)(6) x zH(2)O, strongly depended on the atomic composition ratio of Co to Fe (Co/Fe) and temperature. Compounds of Co/Fe = 1.5 and 1.15 consisted mostly of the Fe(III)(t(2g)(5)e(g)(0), LS, S = 1/2)-CN-Co(II)(t(2g)(5)e(g)(2), HS, S = 3/2) site and the Fe(II)(t(2g)(6)e(g)(0), LS, S = 0)-CN-Co(III)(t(2g)(6)e(g)(0), LS, S = 0) site, respectively, over the entire temperature region from 5 to 350 K. Conversely, compounds of Co/Fe = 1.37, 1.32, and 1.26 showed a change in their electronic and spin states depending on the temperature. These compounds consisted mainly of the Fe(III)-CN-Co(II) site (HT phase) around room temperature but turned to the state consisting mainly of the Fe(II)-CN-Co(III) site (LT phase) at low temperatures. This charge-transfer-induced spin transition (CTIST) phenomenon occurred reversibly with a large thermal hysteresis of about 40 K. The CTIST temperature (T(1/2) = (T(1/2) descending + T(1/2) ascending)/2) increased from 200 to 280 K with decreasing Co/Fe from 1.37 to 1.26. Furthermore, by light illumination at 5 K, the LT phase of compounds of Co/Fe = 1.37, 1.32, and 1.26 was converted to the HT phase, and the relaxation temperature from this photoproduced HT phase also strongly depended on the Co/Fe ratio; 145 K for Co/Fe = 1.37, 125 K for Co/Fe = 1.32, and 110 K for Co/Fe = 1.26. All these phenomena are explained by a simple model using potential energy curves of the LT and HT phases. The energy difference of two phases is determined by the ligand field strength around Co(II) ions, which can be controlled by Co/Fe.

Synthesis, Stereochemistry, and Spin Transition of Heterometal Dinuclear Fe(II)–Cr(III) Complex Bridged by 3,5-Bis(pyridin-2-yl)-pyrazolate

Abstract We have synthesized and characterized a new heterometal dinuclear complex, [(nta)CrIII(μ-bpypz)FeII(picen)]BF4 (1), where nta = nitritotriacetate; bpypz− =3,5-bis(pyridin-2-yl)-pyrazolate; and picen = N,N′-bis-(2-pyridylmethyl)ethylenediamine. The X-ray structure of the Fe(II)–Cr(III)·H2O·3dmf demonstrated that this has a dinuclear structure with a highly distorted octahedron around a high-spin Fe(II) and intramolecular bifurcated three-centered hydrogen bonds between the carboxylate oxygens of the nta and an amine proton of the picen with an abnormal conformation. A magnetic susceptibility measurement and/or the Mössbauer spectra of 1 substantiated a gradual spin-transition from 130 K to 300 K in a heating process centered around 250 K with a small hysteresis dip in the Fe(II) complex. The spin-transition temperature of 1 is lower than that of the mononuclear complex, [FeII(picen)(Hbpypz)](BF4)2(2), resulting from the weaker ligand field of 1 with a distorted octahedron than that of 2.