Spin Transition Of Iron Research Articles

Cover Picture: Structural and Magnetic Resolution of a Two‐Step Full Spin‐Crossover Transition in a Dinuclear Iron(II) Pyridyl‐Bridged Compound (Chem. Eur. J. 32/2006)

New structural data on the nature of the spin transition in dinuclear iron(II) spin‐crossover materials has been obtained from studying [Fe2(ddpp)2(NCS)4]⋅4 CH2Cl2. In their Full Paper on page 8220 ff., C. J. Kepert, K. S. Murray et al. describe the synthesis and characterisation of this compound, particularly in regard to two‐step transitions, for which it provides the first structure of high‐spin–low‐spin molecules at the plateau region.

Valence state and spin transitions of iron in Earth's mantle silicates

Using ab initio simulations, we investigated the valence and spin states of iron impurities in the perovskite ( Pv) and post-perovskite ( PPv) polymorphs of MgSiO 3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe 2+ → 2Fe 3+ Fe 0 metal. This exothermic reaction results in the predominance of Fe 3+ impurities in lower mantle silicates and produces free metallic iron. It occurs both in Pv and PPv, Al-free and Al-rich, at all lower mantle pressures. This reaction provides a possible mechanism for the growth of the Earth's core and core-mantle chemical equilibration. In the presence of Al 3+, iron forms Fe 3+–Al 3+ coupled substitutions in Pv, but separate Fe 3+-Fe 3+ and Al 3+-Al 3+ substitutions in PPv. Only the high-spin state is found for Fe 2+ impurities at all mantle pressures, while Fe 3+ impurities on the Si-site are low-spin at all pressures in both phases. Fe 3+ impurities on the Mg-site are in the high-spin state in PPv at all mantle pressures, but in Pv we predict a high-spin–low-spin transition. The pressure at which this transition occurs strongly depends on the Al 3+ content and according to our calculations increases from 76 GPa for Al-free to 134 GPa for aluminous Pv; this reconciles many of the previous experimental results. Our findings have implications for the chemical evolution of the Earth and for the radiative conductivity and dynamics of the D” layer.

Is low-spin Fe2+ present in Earth's mantle?

Abstract Recently, X-ray spectroscopy indicated that the low-spin (LS) electronic configuration of Fe 2+ and Fe 3+ ions substituting in silicate perovskite of various compositions, and also in magnesiowustite is stable at room temperature above pressures of ∼49 to 120 GPa [J. Badro, G. Fiquet, F. Guyot, J.-P. Rueff, V.V. Struzhkin, G. Vanko, G. Monaco, Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity, Science 300 (2003), 789–791.; J. Badro, J.-P. Rueff, G. Vanko, G. Monaco, G. Figuet, F. Guyot, Electronic transition in perovskite: possible non-convecting layers in the lower mantle, Science 305 (2004) 383–386.; J. Li, V.V. Struzhkin, H.K. Mao, J. Shu, R.J. Hemley, Y. Fei, B. Mysen, P. Dera, V. Prakapenka, G. Shen, Electronic spin state of iron in lower mantle perovskite, Proc. Natl. Acad. Sci. 101 (2004) 14027–14030.; J.M. Jackson, W. Sturhahn, G. Shen, J. Zhao, M.Y. Hu, D. Errandonea, J.D. Bass, Y. Fei, A synchrotron Mossbauer study of (Mg,Fe)SiO 3 perovskite up to 120 GPa, Am. Mineral. 90 (2005) 199–205.; J.F. Lin, V.V. Struzhkin, S.D. Jacobsen, M.Y. Hu, P. Chow, J. Kung, H. Liu, H.K. Mao, R.J. Hemley, Spin transition of iron in magnesiowustite in the Earth's lower mantle, Nature 436 (2005) 377–380.]. Simple thermodynamic relationships combined with crystal field theory provide a minimum Clausius–Clapeyron slope of ∼0.23 to 0.4 GPa/K for the high-spin (HS) to low-spin transition of Fe ions in perovskite, consistent with lower mantle temperatures stabilizing the disordered HS state. This computation of ∂ P / ∂ T utilizes experimentally determined parameters only ( P and T at the transition), and is supported by microstructural analysis of the change in volume and entropy considerations. Similarly, (∂ P / ∂ T ) min ∼0.18 to 0.31 GPa/K for magnesiowustite. High spin Fe 2+ should be stable in silicates throughout Earth's mantle for compositions of Fe / (Mg + Fe) ∼0.1, consistent with disorder prevailing at high temperature, although partial conversion of octahedral Fe 3+ to LS may occur. Similarly, partial conversion of octahedral Fe 2+ in magnesiowustite to the LS is possible for Fe-rich compositions, which transform at relatively low pressures.

Iron spin transition in Earth's mantle.

High-pressure Mössbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowüstite solid solution confirms that ferrous iron (Fe(2+)) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4-5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowüstite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.

Tuning of Spin Transition in Radical-Containing Iron(III) Complexes by Remote Ligand Substituents

Two new iron(III) complexes, Fe(III)(LF*)3 (1) and FeIII(L(t-Bu*))3 (2), of remote substituted o-aminophenol-based ligands are reported; complexes 1 and 2 contain three O,N-coordinated o-iminobenzosemiquinonate(1-) radical anions with ferric centers in high-spin and low-spin configurations. The crystal structures of 1 and 2 were determined by X-ray diffraction at 100 and 293 K, and the electronic structures were established by various physical methods including Mössbauer (4-290 K) and variable-temperature (2-290 K) susceptibility measurements. Electrochemical measurements (cyclic and square-wave voltammetry) indicate primarily ligand-centered redox processes. Complex 1, with the more electron-withdrawing fluoro substituents, retains the high-spin character of the ferric ion throughout the temperature range studied (2-290 K) and exhibits, as expected, strong antiferromagnetic coupling operating between three radicals (SR = 1/2) and the high-spin Fe(III) center (SFe = 5/2) yielding an St = 1 as the ground state. In contrast, the occurrence of a thermally induced spin crossover process (SFe = 5/2 <--> SFe = 1/2) is observed for complex 2 FeIII(L(t-Bu*))3, in which more electron donating tert-butyl substituents in the ligand are present. A rationale for the control of the electronic state of ferric ions in 2 together with spin-coupling schemes for 1 and 2 are provided.

Spin transition of iron in magnesiowüstite in the Earth's lower mantle

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by approximately 35 percent and bulk sound velocity increases by approximately 15 percent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

Synthesis and characterisation of new ditetrazole-ligands as more rigid building blocks of envisaged iron(II) spin-crossover coordination polymers

As the ligand system plays the most important role in the behaviour of the spin-transition of iron(II) spin-crossover compounds a series of eight new mainly bridging di-tetrazole ligands were synthesised and produced new insights into spacer modifications as well as geometric prerequisites of the ligand and their impact on spin-crossover behaviour. The focus laid on aryl-spaced tetrazole ligands, which were interesting analogues to the well-known alkyl-di-tetrazoles due to expected enhanced interaction within the molecular structure through π–π-stacking. The results of this fundamental study yielded further guidelines to optimize and fine-tune the ligand design, which are envisaged to be used for spin-crossover iron(II) coordination polymers of high T ½-values with abrupt spin transition behaviour. Additionally, one new SCO compound [μ-Tris(1-[1,1-dimethyl-2-(1 H-tetrazol-1-yl)ethyl]-1 H-tetrazole- N4, N4′)iron(II)] bis(tetrafluoroborate)—[Fe( dtmp) 3](BF 4) 2—is presented. The compound features a spin transition around 160 K with a small thermal hysteresis of 5 K.

Study of Intermolecular Interaction for the Spin-Crossover Iron(II) Compounds

Abstract The thermal spin transitions of iron(II) spin-crossover compounds [Fe(PM-iPA)2(NCS)2] (1) and [Fe(PM-iPA)2(NCSe)2] (2) (PM-iPA = N-(2′-pyridylmethyl)-isopropylamine) have occurred at T1/2 = 267 K and 376 K without thermal hysteresis. No light-induced excited spin state trapping (LIESST) effect was observed for compounds 1 and 2 even at 5 K. The iron(II) spin-crossover compounds [Fe(PM-L)2(NCX)2] (PM-A = N-(2′-pyridylmethyl)-aniline, PM-BiA = N-(2′-pyridylmethyl)-4-aminobiphenyl, PM-TeA = N-(2′-pyridylmethyl)-4-aminoterphenyl, PM-PEA = N-(2′-pyridylmethyl)-4-(phenylethynyl)aniline, and PM-AzA = N-(2′-pyridylmethyl)-4-(phenylazo)aniline; X = S and Se) with ligands having π-systems have exhibited the LIESST effect, and the critical LIESST temperature, Tc(LIESST), has been observed. The compounds 1 and 2 are crystallized at Pnna and C2/c at 298 K, respectively, and the space groups of the compounds remained unchanged until 100 K, although the compounds show the spin transition. The molecular packing structure and thermodynamic parameters of the spin transitions calculated from the magnetic susceptibility curves suggest that compounds 1 and 2 have no strong intermolecular interactions between the complexes, while the compounds with π-system ligands form π–π intra- and intermolecular interactions between the ligands. Our conclusion is that the intermolecular interactions play an important role in trapping a light-induced metastatble state.

1 A 1 ⇄ 5 T 2 Spin Transition in New Thermochromic Iron(II) Complexes with 1,2,4-Triazole and 4-Amino-1,2,4-Triazole

New complexes of iron(II) chloride and bromide with 1,2,4-triazole (Htrz) and 4-amino-1,2,4-tri-azole (NH2trz) of composition Fe(Htrz)3Cl2 · 1.5H2O, Fe(NH2trz)3Cl2 · 2H2O, Fe(Htrz)3Br2 · 2H2O, and the Fe(NH2trz)3SO4 · H2O complex were synthesized and studied by magnetochemical, electronic, IR and Mossbauer spectroscopy methods. Magnetochemical studies showed that these complexes exhibit 1A1 ⇄ 5T2 spin transition accompanied by thermochromism (a reversible pink ⇄ white change of color).

Spin Transition of 1D, 2D and 3D Iron(II) Complex Polymers The Tug-of-War between Elastic Interaction and a Shock-Absorber Effect

The structures of linear chain Fe(II) spin-crossover compounds of α,β- and α,ω-bis (tetrazol-1-yl)alkane type ligands are described in relation to their magnetic properties. The first threefold interlocked 3-D catenane Fe(II) spin-transition system, [μ-tris(1,4-bis(tetrazol-1-yl)butane-N1,N1′) iron(II)] bis(perchlorate), will be discussed. An analysis is made among the structures and the cooperativity of the spin-crossover behaviour of polynuclear Fe(II) spin-transition materials.

Iron(III) spin transition compound with a large thermal hysteresis

The magnetic properties of the spin-crossover compound, [Fe(qsal)2]NCSe·2DMSO (N-(8-quinolyl)-salicyladimine) (1), have been measured. We have discovered that the compound 1 exhibits a wide "thermal hysteresis loop" of 115 K (T1/2↑ = 324 K and T1/2↓ = 209 K) in the first cycle. Thermogravimetric analysis shows that solvent molecules of the compound 1 are removed at around 324 K. This means that the hysteresis loop observed for the first cycle is only an apparent one. Following the first loop, the compound 1 shows a spin-crossover in warming mode. The spin transition occurs at T1/2↑= 285 K and T1/2↓ = 209 K and the hysteresis width can be estimated to be 76 K, which is one of the largest values reported so far for spin-crossover compounds. It is thought that the cooperativity produced in the compound mainly arises from the intermolecular π interactions between the quinoline and the phenyl rings.

Spin transition in iron(II) polymeric complexes and their structure at equilibrium and metastable spin states

Changes in the electronic and spatial structure of polymeric Fe(II) complexes with 1,2,4-triazoles and various anions upon spin transition was studied using EXAFS and XANES spectroscopy. Spin transition and structural changes were induced by the change of temperature or the action of light. In all complexes, the spin transition is accompanied by drastic changes in the local environment of Fe atoms. The existence of long-life optically excited metastable HS states was revealed for the Fe(atrz) 3(ClO 4) 2 at 4.2 K. It was established that the changes in structure of polymeric complex under the action of light at 4.2 K differ from those in the thermally induced spin transition. Such type of differences are determined by mutual influence of neighboring Fe atoms in high and low spin states in polymeric chains.

ChemInform Abstract: Spin Transition in Iron(II) Complexes with 1,2,4‐Triazoles and Tetrazoles

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IR spectral study of the1 A 1 ⇄5 T 2 spin transition in Iron(II) complexes with 1,2,4-triazole

IR spectral study of the1 A 1 ⇄5 T 2 spin transition in Iron(II) complexes with 1,2,4-triazole

Synthesis, Structure, Magnetic Behavior, and Mössbauer Spectroscopy of Two New Iron(II) Spin-Transition Compounds with the Ligand 4-Isopropyl-1,2,4-triazole. X-ray Structure of [Fe(3)(4-isopropyl-1,2,4-triazole)(6)(H(2)O)(6)](tosylate)(6).2H(2)O.

Two new iron(II) compounds with the formula [Fe(3)(iptrz)(6)(H(2)O)(6)]X(6).xH(2)O (with iptrz = 4-isopropyl-1,2,4-triazole and X = p-toluenesulfonate (Tos) or trifluoromethanesulfonate (Trifl)) were synthesized. The crystal structure of [Fe(3)(iptrz)(6)(H(2)O)(6)](Tos)(6).2H(2)O (I) has been solved at room temperature. Crystals of I are triclinic, space group P&onemacr; with a = 12.8820(11) Å, b = 15.580(2) Å, c = 24.445(4) Å, alpha = 79.270(12) degrees, beta = 86.688(11) degrees, gamma = 83.007(8) degrees, Z = 2. The structure of I consists of linear trinuclear cations with a +6 charge and noncoordinated anions and lattice water molecules. The central iron ion is located on an inversion center and is coordinated by 6 nitrogen atoms of 6 iptrz molecules bridging via the nitrogen atoms in the 1,2-positions. Each external iron atom completes its coordination sphere with three coordinated water molecules. The temperature dependent magnetic measurements of compound I and [Fe(3)(iptrz)(6)(H(2)O)(6)](Trifl)(6) (II) show that both compounds exhibit a gradual spin conversion of the central iron ion centered at T(1/2) = 242 K for I and 187 K for II. Temperature dependent Mössbauer spectroscopy measurements on I show the behavior expected for a compound of this nature. The measurements on II indicate a strong influence of the spin conversion of the central iron ion on both external iron ions. The nature of this phenomenon is proposed to be connected to a very rigid lattice structure (ionic, H-bonding) connecting the trinuclear units.

Spin Resolved Multiple Scattering Study on Iron Spin Transition in Metmyoglobin

Self-consistent spin polarized calculations of the electronic structure of a Fe-heme complex have been performed. The resulting electron wave function has been used to simulate the Fe K-edge XANES of the high spin and low spin Fe states in Fe(III)-metmyoglobin reported by Oyanagi et al. [1]. By including the Fe-heme out-of-plane movement we reproduce the experimental changes, following the low spin -> high spin transition; the ideal, simple rearrengement of d-electrons (computed without structural changes) contributes to the low energy spectral variations.

Spin transition in iron(II) complexes induced by heat, pressure, light, and nuclear decay

Spin transition in iron(II) complexes induced by heat, pressure, light, and nuclear decay

Nickel(II) and iron(II) complexes of 4-(Pyridin-2-yl)thiazole: Spectral, magnetic and structural studies

Iron(II) and nickel(II) complexes of 4-(pyridin-2-yl)thiazole (L) have been prepared. The electronic spectrum of [NiL3] [ClO2,]2 is similar to that of the corresponding complex of 2,2'-bipyridine (bpy) but indicates that the thiazole provides the weaker field, the actual value of Dq(Ni2+) being within the range which suggests the likely appearance of a spin transition in iron(III). The iron(II) complexes [FeL3] X2 (X = ClO4, BF4) are, however, low-spin both in the solid state and in solution. The structure of [NiL3] [ClO4]2.3H2O has been determined, and this reveals a close similarity in the coordination environment to that in [Ni(bpy),] SO4.7.5H2O. The compound is monoclinic, space group P21/c, with four molecules in a unit cell of dimensions a 9.404(4), b 29.169(8), c 17.900(8)ź, β 137.76(1)�. The disordered structure was refined to a residual R 0.087 for 2307 observed reflections. The pseudo-symmetric ligands are orientationally disordered and one anion was also disordered. Restrained refinement with 194 parameters for 85 atomic positions produced a reliable geometry. The ligands function as bidentates, giving the cation a distorted octahedral structure.

Singlet .dblharw. quintet spin transitions of iron(II) complexes with a P4Cl2 donor set. X-ray structures of the compound FeCl2(Ph2PCH:CHPPh2)2 and of its acetone solvate at 130 and 295 K

Singlet .dblharw. quintet spin transitions of iron(II) complexes with a P4Cl2 donor set. X-ray structures of the compound FeCl2(Ph2PCH:CHPPh2)2 and of its acetone solvate at 130 and 295 K

Interpretation of high spin⇌low spin transition in iron (II) complexes. I. A phenomenological thermodynamic model

A phenomenological thermodynamic model suited for the interpretation of the high spin⇌low spin transition in solid transition metal complexes is described. The model is based on the idea of the spin transition taking place through a significant phonon-assisted coupling between the electronic state of the primary spin change center and n−1 neighboring complex molecules undergoing secondary spin transitions and thus forming a cooperative domain as suggested by Sorai and Seki. The model has been applied to interpret the spin transition in polycrystalline solid solutions of [FexZn1−x(2-pic)3]Cl2⋅C2H5OH(2-pic=2-picolylamine) with 1.0⩾x⩾0.0009. The spin transition temperature Tc may be calculated using the effective changes of enthalpy and entropy, respectively, obtained from 1nK vs 1/T plots. Phenomenological expressions for the transition temperature Tc (x) as well as for the effective enthalpy and entropy changes ΔHeff(x) and ΔSeff(x) have been adjusted to the measured data. The average domain size for the pure iron compound has been calculated to be n≈3.5 which indicates comparatively weak intermolecular coupling strength for the present system. The domain size n depends markedly on the iron concentration x and tends to one at very low iron concentration. The volume expansion of the domain size n approximated by n (x) =1+noxγ, decreases rather strongly with falling iron concentration (γ≈1.7). In the pure iron compond, the sum of the intramolecular enthalpy contributions changes by some 550 cal mol−1 and that of the intermolecular enthalpy contributions, responsible for the cooperative effect, by some 200 cal mol−1 on going from low spin to high spin.