Iron Spin Crossover Research Articles

A Ferroelectric Iron(II) Spin Crossover Material

AbstractA dual‐function material in which ferroelectricity and spin crossover coexist in the same temperature range has been obtained. Our synthetic strategy allows the construction of acentric crystal structures in a predictable way and is based on the high directionality of hydrogen bonds. The well‐known iron(II) spin crossover complex [Fe(bpp)2]2+ (bpp=2,6‐bis(pyrazol‐3‐yl)pyridine), a four‐fold noncentrosymmetric H‐bond donor, was combined with a disymmetric H‐bond acceptor such as the isonicotinate (isonic) anion to afford [Fe(bpp)2](isonic)2⋅2 H2O. This low‐spin iron(II) compound crystallizes in the acentric nonpolar I space group and shows piezoelectricity and SHG properties. Upon dehydration, it undergoes a single‐crystal to single‐crystal structural rearrangement to a monoclinic polar Pc phase that is ferroelectric and exhibits spin crossover.

Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

Selective manipulation of spin states in iron(II) complexes by thermal or photonic energy is a desirable goal in the context of developing molecular functional materials. As dynamic spin-state equilibration in isolated iron(II) complexes typically limits the lifetime of a given spin state to nanoseconds, synthetic strategies need to be developed that aim at inhibited relaxation. Herein we show that modulation of the reaction coordinate through careful selection of the ligand can indeed massively slow down dynamic exchange. Detailed structural analysis of [FeL]2+ and [ZnL]2+ (L: tris(1-methyl-2-{[pyridin-2-yl]-methylene}hydrazinyl)phosphane sulfide) with crystallographic and computational methods clearly reveals a unique trigonal-directing effect of the extended-tripod ligand L during spin crossover, which superimposes the ubiquitous [FeN6] breathing with trigonal torsion, akin to the archetypal Bailar twist. As a consequence of the diverging reaction coordinates in [FeL]2+ and in the tren-derived complex [Fe(tren)py3]2+, their thermal barriers differ massively, although the spin crossover energies are close to identical. As is shown by time-resolved transient spectroscopy and dynamic 1H-NMR line broadening, reference systems deriving from tren (tris-(2-aminoethyl)amine), which greatly lack such trigonal torsion, harbor very rapid spin-state exchange.

Heteroleptic and Homoleptic Iron(III) Spin-Crossover Complexes; Effects of Ligand Substituents and Intermolecular Interactions between Co-Cation/Anion and the Complex

The structural and magnetic properties of a range of new iron(III) bis-tridentate Schiff base complexes are described with emphasis on how intermolecular structural interactions influence spin states and spin crossover (SCO) in these d5 materials. Three pairs of complexes were investigated. The first pair are the neutral, heteroleptic complexes [Fe(3-OMe-SalEen)(thsa)] 1 and [Fe(3-MeOSalEen)(3-EtOthsa)] 2, where 3-R-HSalEen = (E)-2-(((2-(ethylamino)ethyl)imino)methyl)-6-R-phenol and 3-R-H2thsa = thiosemicarbazone-3-R-salicylaldimine. They display spin transitions above room temperature. However, 2 shows incomplete and gradual change, while SCO in 1 is complete and more abrupt. Lower cooperativity in 2 is ascribed to the lack of π–π interactions, compared to 1. The second pair, cationic species [Fe(3-EtOSalEen)2]NO3 3 and [Fe(3-EtOSalEen)2]Cl 4 differ only in the counter-anion. They show partial SCO above room temperature with 3 displaying a sharp transition at 343 K. Weak hydrogen bonds from cation to Cl− probably lead to weaker cooperativity in 4. The last pair, CsH2O[Fe(3-MeO-thsa)2] 5 and Cs(H2O)2[Fe(5-NO2-thsa)2] 6, are anionic homoleptic chelates that have different substituents on the salicylaldiminate rings of thsa2−. The Cs cations bond to O atoms of water and the ligands, in unusual ways thus forming attractive 1D and 3D networks in 5 and 6, respectively, and 5 remains HS (high spin) at all temperatures while 6 remains LS (low spin). Comparisons are made to other literature examples of Cs salts of [Fe(5-R-thsa)2]− (R = H and Br).

Critical behavior of Mg1–xFexO at the pressure-induced iron spin-state crossover

We present a high-pressure study of $\mathrm{M}{\mathrm{g}}_{1--x}\mathrm{F}{\mathrm{e}}_{x}\mathrm{O}$ (ferropericlase, Fp) single crystals $0.04(1)\ensuremath{\le}x\ensuremath{\le}0.33(1)$ with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ of the ``FeO high spin lattice contribution'' in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter ${a}_{C}$ (defined as the minimum ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of $\mathrm{F}{\mathrm{e}}^{2+}$.

Influence of the iron spin crossover in ferropericlase on the lower mantle geotherm

AbstractThe iron spin crossover in ferropericlase introduces anomalies in its thermodynamics and thermoelastic properties. Here we investigate how these anomalies can affect the lower mantle geotherm using thermodynamics properties from ab initio calculations. The anomalous effect is examined in mantle aggregates consisting of mixtures of bridgmanite, ferropericlase, and CaSiO3 perovskite, with different Mg/Si ratios varying from harzburgitic to perovskitic (Mg/Si ∼ 1.5 to 0.8). We find that the anomalies introduced by the spin crossover increase the isentropic gradient and thus the geotherm proportionally to the amount of ferropericlase. The geotherms can be as much as ∼200 K hotter than the conventional adiabatic geotherm at deep lower mantle conditions. Aggregate elastic moduli and seismic velocities are also sensitive to the spin crossover and the geotherm, which impacts analyses of lower mantle velocities and composition.

Development of picosecond time-resolved X-ray absorption spectroscopy by high-repetition-rate laser pump/X-ray probe at Beijing Synchrotron Radiation Facility.

A new setup and commissioning of transient X-ray absorption spectroscopy are described, based on the high-repetition-rate laser pump/X-ray probe method, at the 1W2B wiggler beamline at the Beijing Synchrotron Radiation Facility. A high-repetition-rate and high-power laser is incorporated into the setup with in-house-built avalanche photodiodes as detectors. A simple acquisition scheme was applied to obtain laser-on and laser-off signals simultaneously. The capability of picosecond transient X-ray absorption spectroscopy measurement was demonstrated for a photo-induced spin-crossover iron complex in 6 mM solution with 155 kHz repetition rate.

Variety of spin transition temperatures of iron(II) spin crossover complexes with halogen substituted Schiff-base ligands, HqsalX (X = F, Cl, Br, and I)

The preparation and magnetic properties of four Fe(II) Schiff-base complexes, [Fe(qsalX)2] (HqsalX=N-(8′-quinolyl)-2-hydroxy-5-halogeno-1-salicylaldimine; X=F(1), Cl(2), Br(3), I(4)) are reported. X-ray single crystal structure analyses of 1 and 2 reveal that Fe(II) ions in both complexes are coordinated by two qsalX ligands in meridional fashion. Molecular packing of 1 shows that a non-planar qsalF ligand interacts with neighboring two qsalF ligand’s through π–π interactions, while that of 2 shows that two planar qsalCl ligands interact each other through π–π interaction. Although the magnetic property of 1 shows a high-spin state at all the temperature range measured, the χT–T plots of 2, 3, and 4 show gradual spin crossover behaviors with T1/2 of 308K, 341K, and 340K, respectively.

Thermal conductivity of ferropericlase in the Earth's lower mantle

(Mg, Fe)O ferropericlase (Fp) is one of the important minerals comprising Earth's lower mantle, and its thermal conductivity could be strongly influenced by the iron content and its spin state. We examined the lattice thermal conductivity of (Mg, Fe)O Fp containing 19 mol% iron up to 111 GPa and 300 K by means of the pulsed light heating thermoreflectance technique in a diamond anvil cell. We confirmed a strong reduction in the lattice thermal conductivity of Fp due to iron substitution as reported in previous studies. Our results also show that iron spin crossover in Fp reduces its lattice thermal conductivity as well as its radiative conduction. We also measured the electrical conductivity of an identical Fp sample up to 140 GPa and 2730 K, and found that Fp remained an insulator throughout the experimental conditions, indicating the electronic thermal conduction in Fp is negligible. Because of the effects of strong iron impurity scattering and spin crossover, the total thermal conductivity of Fp at the core–mantle boundary conditions is much smaller than that of bridgmanite (Bdg). Our findings indicate that Bdg (and post-perovskite) is the best heat conductor in the Earth's lower mantle, and distribution of iron and its valence state among the lower mantle minerals are key factors to control the lower mantle thermal conductivity.

Comparison of density functionals for the study of the high spin low spin gap in Fe(III) spin crossover complexes

AbstractA detailed investigation of the accuracy of different quantum mechanical methods for the study of iron(III) spin crossover complexes is presented. The energy spin state gap between the high and low spin states; ΔE(HS‐LS) of nine iron(III) quinolylsalicylaldiminate complexes were calculated with nine different DFT functionals, then compared. DFT functionals: B3LYP, B3LYP‐D3, B3LYP*, BH&HLYP, BP86, OLYP, OPBE, M06L, and TPSSh were tested with six basis sets: 3‐21G*, dgdzvp, 6‐31G**, cc‐pVDZ, Def2TZVP, and cc‐pVTZ. The cations from the X‐ray crystal structures of [Fe(qsal‐OMe)2]Cl·MeCN·H2O, [Fe(qsal‐OMe)2]Cl·2MeOH·0.5H2O, [Fe(qsal‐OMe)2]BF4·MeOH, [Fe(qsal‐OMe)2]NCS·CH2Cl2, [Fe(qsal‐F)2]NCS, [Fe(qsal‐Cl)2]NCS·MeOH, [Fe(qsal‐Br)2]NCS·MeOH, [Fe(qsal‐I)2]OTf·MeOH, and [Fe(qsal)2]NCS⋅CH2Cl2 were used as starting structures. The results show that B3LYP, B3LYP‐D3, OLYP, and OPBE with a 6‐31G**, Def2TZVP, and cc‐pVTZ basis set give reasonable results of ΔE(HS‐LS) compared with the experimental data. The enthalpy of [Fe(qsal‐I)2]+ calculated with an OLYP functional and cc‐pVTZ basis set (1.48 kcal/mol) most closely matches the experimental data (1.34 kcal/mol). B3LYP* yields an enthalpy of 5.92 kcal/mol suggesting it may be unsuitable for these Fe(III) complexes, mirroring recent results by Kepp (Inorg. Chem., 2016, 55, 2717–2727).

Front Cover: Crystal Structures and Spin‐Crossover Behavior of Iron(II) Complexes with Chiral and Racemic Ligands (Eur. J. Inorg. Chem. 7/2017)

The cover picture shows a Japanese New Year’s greeting card symbolizing the dawn of a new era for chiral magnetism. We chose Mt. Aso, which is the symbol of Kumamoto, as the central picture in the background. Structures of two chiral spin-crossover (SCO) iron complexes are depicted to the left and right of the “dragon”. Details are discussed in the article by S. Hayami et al. on page 1049 ff (DOI: 10.1002/ejic.201601232). For more on the story behind the cover research, see the Cover Profile (DOI: 10.1002/ejic.201700131).

First-principles study of iron spin crossover in the new hexagonal aluminous phase

The new hexagonal aluminous (NAL) phase, chemical formula $A{B}_{2}{C}_{6}{\mathrm{O}}_{12}$ ($A= {\mathrm{Na}}^{+}, {\mathrm{K}}^{+}, {\mathrm{Ca}}^{2+}$; $B= {\mathrm{Mg}}^{2+}, {\mathrm{Fe}}^{2+}, {\mathrm{Fe}}^{3+}$; $C= {\mathrm{Al}}^{3+}, {\mathrm{Si}}^{4+}, {\mathrm{Fe}}^{3+}$), is considered a major component ($\ensuremath{\sim}20$ vol%) of mid-ocean ridge basalt (MORB) under the lower-mantle condition. As MORB can be transported back into the Earth's lower mantle via subduction, a thorough knowledge of the NAL phase is essential to fully understand the fate of subducted MORB and its role in mantle dynamics and heterogeneity. In this Rapid Communication, the complicated spin crossover of the Fe-bearing NAL phase is revealed by a series of local density approximation + self-consistent Hubbard $U$ (LDA+${U}_{sc}$) calculations. Only the ferric iron (${\mathrm{Fe}}^{3+}$) substituting Al/Si in the octahedral ($C$) site undergoes a crossover from the high-spin (HS) to the low-spin (LS) state at $\ensuremath{\sim}40$ GPa, while iron substituting Mg in the trigonal-prismatic ($B$) site remains in the HS state, regardless of its oxidation state (${\mathrm{Fe}}^{2+}$ or ${\mathrm{Fe}}^{3+}$). The volume/elastic anomalies and the iron nuclear quadrupole splittings determined by calculations are in great agreement with room-temperature experiments. The calculations further predict that the HS-LS transition pressure of the NAL phase barely increases with temperature due to the three nearly degenerate LS states of ${\mathrm{Fe}}^{3+}$, suggesting that the elastic anomalies of this mineral can occur at the top lower mantle.

Heteroleptic iron(iii) Schiff base spin crossover complexes: halogen substitution, solvent loss and crystallite size effects.

The influence of the halogen substituent on the qsal moiety of iron(iii) heteroleptic compounds with the formulae [Fe(qsal-X)(thsa)]·nMeCN, where qsal-X- = X-substituted quinolylsalicylaldimine; thsa2- = thiosemicarbazone-salicylaldiminate; X = F; n = 2.5, 1·2.5MeCN and X = Cl 2, Br 3 and I 4, n = 1 (labelled 2·MeCN, 3·MeCN and 4·MeCN, respectively) has been systematically investigated. Magnetic studies on solid samples show incomplete spin crossover in 1-3 which can be related to MeCN solvent loss. Complex 4·MeCN remains fully LS up to 360 K. Single crystals have been examined at variable temperatures for samples possessing different degrees of solvation. Intermolecular C-XH interactions are present for X = F, Cl and Br while a C-Iπ interaction is uniquely observed in 4·MeCN. These preferential interactions result in different supramolecular packings of the various halogen substituted compounds. However, as the LS stability increases from F to I, the ligand field strength is then also suggested to increase from F to I. Consequently, in this family, the electronic structure resulting from halogen variation is believed to influence the magnetic properties more than crystal packing effects. Mössbauer spectra, at variable temperatures, confirm the presence of Fe(iii) and the magnetic properties in these compounds. The effect of different drying methods as well as the crystal/powder effect on the magnetic properties are discussed in the case of 2·MeCN.

Spin crossover in (Mg,Fe3+)(Si,Fe3+)O3 bridgmanite: Effects of disorder, iron concentration, and temperature

The spin crossover of iron in Fe3+-bearing bridgmanite, the most abundant mineral of the Earth’s lower mantle, is by now a well-established phenomenon, though several aspects of this crossover remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA+Usc calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted configurations in super-cells containing up to 80 atoms. Vibrational/thermal effects on the crossover are addressed within the quasiharmonic approximation. The effect of disorder seems quite small, while increasing iron concentration results in considerable increase in the crossover pressure. Our calculated compression curves for iron-free, Fe2+-, and Fe3+-bearing bridgmanite compare well with the latest experimental measurements. The comparison also suggests that in a closed system, Fe2+ present in the sample may transform into Fe3+ by introduction of Mg and O vacancies with increasing pressure. As in the spin crossover in ferropericlase, this crossover in bridgmanite is accompanied by a clear volume reduction and an anomalous softening of the bulk modulus throughout the crossover pressure range. These effects reduce significantly with increasing temperature. Though the concentration of [Fe3+]Si in bridgmanite may be small, related elastic anomalies may impact the interpretation of radial and lateral velocity structures of the Earth’s lower mantle.

Spin crossover and hyperfine interactions of iron in(Mg,Fe)CO3ferromagnesite

Ferromagnesite, an iron-bearing carbonate stable up to 100--115 GPa, is believed to be the major carbon carrier in the earth's lower mantle and play a key role in the earth's deep carbon cycle. In this paper, we use the local density approximation plus self-consistent Hubbard $U$ (LDA+${U}_{\mathrm{sc}}$) method to study the iron spin crossover in ferromagnesite with a wide range of iron concentration (12.5--100%). Our calculation shows that this mineral undergoes a crossover from the high-spin (HS) ($S=2$) to the low-spin (LS) ($S=0$) state at around 45--50 GPa, regardless of the iron concentration. The intermediate-spin ($S=1$) state is energetically unfavorable and not involved in spin crossover. The anomalous changes of volume, density, and bulk modulus accompanying the spin crossover obtained in our calculation are in great agreement with experiments. Our calculation also predicts that an abrupt change of the iron nuclear quadrupole splitting, from $\ensuremath{\gtrsim}2.8$ mm/s to $\ensuremath{\lesssim}0.3$ mm/s, can be observed in M\ossbauer spectra at 45--50 GPa as a signature of the HS-LS crossover.

Thermoelasticity of Fe3+‐ and Al‐bearing bridgmanite: Effects of iron spin crossover

AbstractWe report ab initio (LDA + Usc) calculations of thermoelastic properties of ferric iron (Fe3+)‐ and aluminum (Al)‐bearing bridgmanite (MgSiO3 perovskite), the main Earth forming phase, at relevant pressure and temperature conditions and compositions. Three coupled substitutions, namely, [Al]Mg‐[Al]Si, [Fe3+]Mg‐[Fe3+]Si, and [Fe3+]Mg‐[Al]Si have been investigated. Aggregate elastic moduli and sound velocities are successfully compared with limited experimental data available. In the case of [Fe3+]Mg‐[Fe3+]Si substitution, the high‐spin (S = 5/2) to low‐spin (S = 1/2) crossover in [Fe3+]Si induces a volume collapse and elastic anomalies across the transition region. However, the associated anomalies should disappear in the presence of aluminum in the most favorable substitution, i.e., [Fe3+]Mg‐[Al]Si. Calculated elastic properties along a lower mantle model geotherm suggest that the elastic behavior of bridgmanite with simultaneous substitution of Fe2O3 and Al2O3 in equal proportions or with Al2O3 in excess should be similar to that of (Mg,Fe2+)SiO3 bridgmanite. However, excess of Fe2O3 should produce elastic anomalies in the crossover pressure region.

Ligand field tuned spin crossover for an iron(II)-di(diamine) system

Investigation of spin-crossover in iron(II)-di(diamine) system, based on ligands L1, 4,5-dimethy-2-(pyridine-2-yl)imidazole, has been presented in this paper. The methyl group at the adjacent sites of the donor atoms in ligand L1 can generate the crossover situation for the iron(II) tris(diimine) system. While for the iron(II)-di(diamine) system with L1, the results are complicated: complex 1 {Fe(L1)2(SCN)2} displays paramagnetic behavior while 2 {Fe(L1)2(SeCN)2} is in crossover situation. The difference was attributed to the various ligand field strength of SCN− and SeCN−, which also resulted to the blue-shift of IR and Raman spectra, as well as red-shift and decrease in molar absorptivity of UV–Vis absorption spectrum.

First-principles calculations of elasticity of minerals at high temperature and pressure

The elasticity of minerals at high temperature and pressure (PT) is critical for constraining the composition and temperature of the Earth’s interior and understand better the deep water cycle and the dynamic Earth. First-principles calculations without introducing any adjustable parameters, whose results can be comparable to experimental data, play a more and more important role in investigating the elasticity of minerals at high PT mainly because of (1) the quick increasing of computational powers and (2) advances in method. For example, the new method reduces the computation loads to one-tenth of the traditional method with the comparable precise as the traditional method. This is extraordinarily helpful because first-principles calculations of the elasticity of minerals at high PT are extremely time-consuming. So far the elasticity of most of lower mantle minerals has been investigated in detail. We have good idea on the effect of temperature, pressure, and iron concentration on elasticity of main minerals of the lower mantle and the unusual softening in bulk modulus by the spin crossover of iron in ferropericlase. With these elastic data the lower mantle has been constrained to have 10–15 wt% ferropericlase, which is sufficient to generate some visible effects of spin crossover in seismic tomography. For example, the spin crossover causes that the temperature sensitivity of P wave at the depth of ~1700 km is only a fraction of that at the depth of ~2300 km. The disruptions of global P wave structure and of P wave image below hotspots such as Hawaii and Iceland at similar depth are in consistence with the spin crossover effect of iron in ferropericlase. The spin crossover, which causes anomalous thermodynamic properties of ferropericlase, has also been found to play a control role for the two features of the large low shear velocity provinces (LLSVPs): the sharp edge and high elevation up to 1000 km above core-mantle boundary. All these results clearly suggest the spin crossover of iron in the lower mantle. The theoretical investigations for the elasticity of minerals at the upper mantle and water effect on elasticity of minerals at the mantle transition zone and subducting slab have also been conducted extensively. These researches are critical for understanding better the composition of the upper mantle and water distribution and transport in the Earth’s mantle. Most of these were static calculations, which did not include the vibrational (temperature) effect on elasticity, although temperature effect on elasticity is basic because of high temperature at the Earth’s interior and huge temperature difference between the ambient mantle and the subducting slab. Including temperature effect on elasticity of minerals should be important future work. New method developed is helpful for these directions. The elasticity of iron and iron-alloy with various light elements has also been calculated extensively. However, more work is necessary in order to meet the demand for constraining the types and amount of light elements at the Earth’s core.

Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

This review provides an up to date survey of a singular class of iron(II) spin crossover (SCO) molecular materials that undergo high-spin (HS) ↔ low-spin (LS) phase transitions accompanied by crystallographic symmetry breaking (CSB). Particular interest has been focused on a variety of complexes that exhibit one-step or stepwise SCO behavior and CSB. Most of them afford excellent examples of well-ordered 1HS-1LS, 2HS-1LS or 1HS-2LS intermediate phases (IP) and represent an important platform to disclose microscopic mechanisms responsible for cooperativity and ordering in such multistable materials.

Determination of electronic ground state properties of a dinuclear iron(II) spin crossover complex

The dinuclear complex [(Fe(L-N4Me2))2(BiBzIm)](ClO4)2⋅2EtCN (1) has been investigated by Mossbauer spectroscopy carried out in the temperature range from 5 to 150 K with externally applied magnetic fields of up to B = 5 T. By means of a consistent simulation of all experimental data sets within the Spin Hamiltonian formalism, the zero-field splitting D and the rhombicity parameter E/D of the ferrous high-spin (HS) site in this complex was determined to be D = −15.0 ± 1.0 cm−1 and E/D = 0.33 respectively. The sign of the quadrupole splitting of the HS site is positive which indicates that this iron site of the dinuclear complex 1 has an electronic ground state with the dxy orbital being twofold occupied.

Ligand effects in a heteroleptic bis-tridentate iron(iii) spin crossover complex showing a very high T1/2 value.

We describe the first example of a bis-tridentate heteroleptic iron(iii) spin crossover (SCO) complex, [Fe(3-OMe-SalEen)(thsa)]. Compared to the parent homoleptic compounds, the spin crossover features in the heteroleptic species are enhanced with a gradual-abrupt spin transition at 344 K. Clear correlations between π-π interactions and the T1/2 value have been revealed.