Iron Spin Crossover Research Articles

A Multi-Objective Approach to Force Field Optimization: Structures and Spin State Energetics of d(6) Fe(II) Complexes.

The next generation of force fields (FFs), regardless of the accuracy of the potential energy representation, will always have parameters that must be fitted in order to reproduce experimental and/or ab initio data accurately. Single objective methods have been used for many years to automate the obtaining of parameters, but this leads to ambiguity. The solution depends on the chosen weights and is therefore not unique. There have been few advances in solving this problem, which thus remains a major hurdle for the development of empirical FF methods. We propose a solution based on multi-objective evolutionary algorithms (MOEAs). MOEAs allow the FF to be tuned against the desired objectives and offer a powerful, efficient, and automated means to reparameterize FFs, or even discover the parameters for a new potential. Here, we illustrate the application of MOEAs by reparameterizing the ligand field molecular mechanics (LFMM) FF recently reported for modeling spin crossover in iron(II)-amine complexes (Deeth et al. J. Am. Chem. Soc.2010, 132, 6876). We quickly recover the performance of the original parameter set and then significantly improve it to reproduce the geometries and spin state energy differences of an extended series of complexes with RMSD errors in Fe-N and N-N distances reduced from 0.06 Å to 0.03 Å and spin state energy difference RMSDs reduced from 1.5 kcal mol(-1) to 0.2 kcal mol(-1). The new parameter sets highlight, and help resolve, shortcomings both in the non-LFMM FF parameters and in the interpretation of experimental data for several other Fe(II)N6 amine complexes not used in the FF optimization.

A Hydrogen Bond Motif Giving a Variety of Supramolecular Assembly Structures and Spin-Crossover Behaviors

A series of spin-crossover (SCO) iron(II) compounds, fac-[Fe(II)(HL(R))(3)]Cl·PF(6) [R = methyl (Me, 1), ethyl (Et, 2), n-propyl (n-Pr, 3), n-butyl (n-Bu, 4), and n-pentyl (n-Pen, 5)], were synthesized, where HL(R) denotes a series of [(2-methylimidazol-4-yl)methylidene]monoalkylamines. The cations fac-[Fe(II)(HL(R))(3)](2+) and chloride anions associate through 3:3 imidazole···chloride hydrogen bonding. This hydrogen-bonding motif gives rise to a variety of assembly structures consisting of a one-dimensional ladder for 3 and 4, two kinds of two-dimensional networks for 1 and 2, and a cubane-like structure for 5. The compounds exhibit various types of SCO transitions between high-spin (S = 2) and low-spin (S = 0) states as a result of their intermolecular interactions.

Spin crossover, structural change, and metallization in NiAs-type FeO at high pressure

We report high-spin to low-spin crossover of iron in NiAs (B8)-type Fe${}_{0.96}$O (FeO hereafter) at high pressure by a combination of in-situ x-ray emission spectroscopy (XES) and x-ray diffraction (XRD) measurements. The XES spectra show a loss of spin in B8-type FeO approximately above 119 GPa at 300 K, consistent with the \ensuremath{\sim}2.5% volume reduction around 120 GPa. The high-temperature XRD study also demonstrates a similar volume decrease at 1560--1780 K under the same pressure condition. While the crystal structure of FeO remains to be B8 type across such volume reduction, the atomic arrangements of Fe and O change from inverse to normal NiAs form with considerable decrease in $c$/$a$ axial ratio. With recent electrical resistance measurements, these suggest that iron spin crossover, inverse-normal structural change, and insulator-metal transition occur concurrently in B8 FeO around 120 GPa.

Theoretical Analysis of Spin Crossover in Iron(II) [2 × 2] Molecular Grids

We use quantum-chemical density functional theory calculations to elucidate the origin of spin-crossover pathways in two iron(II) [2×2] molecular grids with carbohydrazide-based bridging ligands. The complexes are characterized energetically and structurally in five available spin states. Special attention is paid to analysis of the structural distortion induced on each iron center by spin transition on any of its neighbors. The evolution of coordination polyhedra is monitored using the Continuous Shape Measures. It is demonstrated that a succession of spin transitions on different centers depends on the character of the induced distortion, either approaching or getting them away from a more regular low-spin geometry. These effects, resulting from the elasticity of bridging ligands, can be modulated by weak perturbations such as a change of the positions of the hydrogen atoms.

Impact of ligand spacer and counter-anion in selected 1D iron(II) spin crossover coordination polymers

A new 1D coordination polymer [Fe(βAlatrz)3](ClO4)2 ∙ H2O (1) with a neutral bidentate ligand, βAlatrz = 4H-1,2,4-triazol-4-yl-propionate, was prepared and its magnetic behavior was investigated by temperature dependent magnetic susceptibility measurements and 57Fe Mossbauer spectroscopy. The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals a gradual single step transition with T1/2 = 173 K between high-spin and low-spin states in agreement with magnetic susceptibility measurements. 1 presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen. The phase transition is of first order as deduced from differential scanning calorimetry, with T1/2 matching the one determined by both SQUID and 57Fe Mossbauer spectroscopy. A brief assessment has been made among closely related 1D coordination polymers to perceive the effect of ligand spacer length and anion effect on the spin crossover behavior of these new materials.

Effect of the Crystal Packing and LIESST Effect of a Two-Dimensional Iron(II) Spin-Crossover Complex: [FeII(HLH,Me)2](CF3SO3)2 (HLH,Me: 8-(Imidazol-4-ylmethylideneamino)-2-methylquinoline)

Abstract A 2D iron(II) spin-crossover (SCO) complex, [FeII(HLH,Me)2](CF3SO3)2, was synthesized, where HLH,Me: 8-(imidazol-4-ylmethylideneamino)-2-methylquinoline, and its structures and magnetic properties were investigated. The complex [FeII(HLH,Me)2](CF3SO3)2 has no crystal solvent and has structure isomorphous to the complex [FeII(HLH,Me)2](ClO4)2·1.5MeCN, and shows a gradual and reversible one-step SCO between the HS (S = 2) and LS (S = 0) states at T1/2 = 215 K without hysteresis. The T1/2 value is higher than that of corresponding desolvated ClO4− compound, [FeII(HLH,Me)2](ClO4)2, about 35 K, although the spin transition is more gradual than that of ClO4− compound and the hysteresis is absent, which is observed in ClO4− compound. The crystal structures were determined at 296 (HS state) and 100 K (LS state), where the crystal system and space group showed no change between these temperatures. The structures at both temperatures have the same 2D layered structure, which is constructed of both the bifurcated NH···O− hydrogen bonds between two CF3SO3− ions and two neighboring imidazole NH groups of the [FeII(HLH,Me)2]2+ cations and the π–π interactions between the two quinolyl rings of the two adjacent cations. This network structure is the same as that of ClO4− compound. 600 nm light irradiation at 10 K induced the LIESST effect.

Time-domain terahertz spectroscopy of spin state transition in [Fe(NH2 − trz)3]2+ spin crossover compounds

We have measured the evolution of the THz spectrum of iron(II) spin crossover compounds within the low-spin/high-spin thermal hysteresis loop in the 0.6–6 THz frequency range. This study enabled to follow both the variations of the refractive optical index and absorption during the spin state transition. Marked absorptions centered ∼2 – 3 THz and ∼5 THz shifting with the spin state are revealed. Our work provides a means to store optically information and to read it out in the THz domain and also offers indications about the structural evolution occurring during the spin state transition.

Development of pH-Sensitive Spin-crossover Iron(II) Complex Films, [FeII(diAMsar)]–Nafion: Manipulation of the Spin State by Proton Concentration

Abstract We newly synthesized transparent films composed of pH-sensitive spin-crossover complex [FeII(diAMsar)] (diAMsar: 1,8-diaminosarcophagine) and an ion-exchange polymer, Nafion, having different colors corresponding to proton concentration. The UV–vis spectroscopic measurements revealed that the films, which were provided under pH 4, 7, and 10 conditions, showed the spin state depending on pH.

Chiral spin crossover iron(II) complex, fac-Λ-[FeII(HLR)3](ClO4)2·EtOH (HLR = 2-methylimidazol-4-yl-methylideneamino-R-(+)-1-methylphenyl)

A chiral spin crossover iron(II) complex, fac-Λ-[FeII(HLR)3](ClO4)2·EtOH was synthesized and its crystal structures in both the high-spin (HS) and low-spin (LS) states were determined, where HLR denotes 2-methylimidazol-4-yl-methylideneamino-R-(+)-1-methylphenyl. The complex assumes octahedral coordination geometry of N6 donor atoms by three bidentate ligands HLR. The complex exists as the facial-Λ-isomer of fac-Λ-[FeII(HLR)3]2+ of the possible geometrical fac- and mer-isomers and the Δ- and Λ-enantiomorphs. The X-ray structural analyses revealed that the R-form of the ligand (HLR) induces the fac-Λ-isomer of fac-Λ-[FeII(HLR)3]2+ and the S-form of the ligand (HLS) induces the fac-Δ-isomer of fac-Δ-[Fe(HLS)3]2+. The complex fac-Λ-[FeII(HLR)3](ClO4)2·EtOH shows a complete steep spin crossover between the HS and the LS states at T1/2=195K.

Photo-switchable spin-crossover iron(III) compound based on intermolecular interactions

Iron(III) spin-crossover compounds, [Fe(qnal)2]CF3SO3·MeOH (1·MeOH) and [Fe(qnal)2]CF3SO3·acetone (1·acetone) were prepared and their spin transition properties were characterized by magnetic susceptibility measurement, Mossbauer spectroscopy and single crystal analysis. Two iron(III) compounds exhibited abrupt spin transition with thermal hysteresis loop (T 1/2↑ = 115 K and T 1/2↓ = 104 K for 1·MeOH, and T 1/2↑ = 133 K and T 1/2↓ = 130 K for 1·acetone). Single crystal analysis revealed both of the structures in high-spin (HS) and low-spin (LS) states for 1·acetone. The difference of bond length between the HS and LS states for 1·acetone was ~0.10 A, which was corresponding to that of typical iron(III) SCO compounds. Specially, it showed strong intermolecular interactions by π–π stacking formed between the neighbor complexes leading to 2-D sheet. Both 1·MeOH and 1·acetone exhibited LIESST effect when it was illuminated at 1000 nm. We also confirmed that the introduction of strong intermolecular interactions, such as π–π stacking, can play an important role in LIESST effect.

Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region. Recent theoretical calculations combined with estimates of iron partitioning between (Mg,Fe)SiO(3) perovskite and melt at shallower mantle conditions suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ∼76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg(0.95)Fe(0.05))SiO(3) glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO(3) liquid becomes more dense than coexisting solid at ∼1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ∼1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO(3) perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region.

Tetrameric spin crossover iron(II) complex constructed by imidazole···chloride hydrogen bonds

Abstract A tetrameric spin crossover (SCO) FeII complex was synthesized, and the structure and magnetic properties were studied. The tetrameric structure is constructed by four imidazole···Cl− hydrogen bonds centered at one Cl− ion. The compound showed an incomplete gradual SCO behavior due to intra-cluster “anticooperative-like“ interaction.

Iron(II) spin crossover complexes with branched long alkyl chain

The bzimpy iron(II) complexes, 1– 3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T 1/2↑ = 236 K to T 1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.

First observation of light-induced spin change in vacuum deposited thin films of iron spin crossover complexes

Thin films of [Fe(H(2)Bpz)(2)(phen)] (1) and [Fe(H(2)Bpz)(2)(bipy)] (2) are prepared by vacuum deposition and investigated with respect to their spin crossover behaviour. For the first time light-induced excited spin state trapping (LIESST) is observed in such systems. T(1/2) and T(LIESST) in the films are in agreement with the bulk values.

Layered iron(II) spin crossover complex constructed by NH⋯Br− hydrogen bonds with 2 K wide thermal hysteresis, [FeIIH3LMe]Br·CF3SO3 (H3LMe = tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

A series of compounds [FeIIH3LMe]Br·Y·nMeOH (Y=PF6- (1), AsF6- (2), SbF6- (3), CF3SO3- (4); n=0 or 1) were synthesized, where H3LMe is a hexadentate N6 tripodal ligand of the neutral form, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine, and their structures and magnetic properties were investigated. The compounds 1−3 with counter anions Y=PF6-, AsF6-, and SbF6- contain methanol as a crystal solvent, and show no SCO behaviors, while the corresponding Cl− compounds have no crystal solvent and show a variety of SCO behaviors. The compound [FeIIH3LMe]Br·CF3SO3 (4) has no crystal solvent and has isomorphous structure to the Cl− compounds, and shows an abrupt spin transition between the HS (S=2) and LS (S=0) states with a hysteresis about 2K and large frozen-in effect below 72K. The T1/2↑ and T1/2↓ values are 98 and 96K, whose values are higher than those of corresponding Cl− compound about 15K and the width of hysteresis is narrower than that of corresponding Cl− compound about 2K. The crystal structures of 4 were determined at 296 and 93K, where the crystal system and space group showed no change between these temperatures. The structures at both temperatures have a same 2D layered structure, which is composed of NH⋯Br− hydrogen bonds between the Br− ion and the imidazole NH groups of three neighboring cations [FeIIH3LMe]2+. This network structure is the same as that of corresponding Cl− compound. The 600nm light irradiation at 5K induced the LIESST effect.

Reliability and Storage Capacity: a Compromise Illustrated in the Two-Step Spin-Crossover System [Fe(bapbpy)(NCS)2

The design of bistable magnetic systems should enable the storage of information by manipulation of the spin degrees of freedom. However, such a strategy relies on the preparation of target objects, whose environment must be controlled to favor a hysteretic behavior. Here, we report the successful modeling of a highly cooperative two-step spin-crossover iron(II) compound, [Fe(bapbpy)(NCS)(2)]. The magnetic susceptibility measurements and low- and high-temperature hysteretic cycles reflect the presence of an intermediate phase, which controls the memory-storage capacity of this material. It is shown that the hysteresis loop widths can be traced theoretically by evaluating the electrostatic contributions between the transiting units. Despite the apparent reduction of intermolecular interactions upon cooling, it is suggested that the enhanced fluctuations of the Madelung field are responsible for the observed hysteresis width changes. This counterintuitive scenario makes the preparation of information storage devices an even more challenging task, where theoretical inspections are very insightful.

Spin crossover in iron(II) complexes with tris(pyrazol-1-yl)methane

Mononuclear iron(II) coordination compounds with tris(pyrazol-1-yl)methane (HC(Pz)3) described as [Fe{HC(Pz)3}2]A2 × nH2O, where A = Cl−, Br−, I−, 1/2 SO42−, n = 0–7, were synthesized. The compounds were studied by static magnetic susceptibility measurements, IR and UV/Vis spectroscopy, and powder X-ray diffraction. The crystal and molecular structures of all compounds were determined by single crystal X-ray diffraction.

Influence of a thermochromic anion on the spin crossover of iron(II) trinuclear complexes probed by Mössbauer spectroscopy

The insertion of thermochromic anions (1 and 2) into triazole based iron(II) trinuclear spin crossover complexes [FeII3(hyetrz)6(H2O)6](1 or 2)4Cl2 paves the way to a large panel of multifunctional materials. In addition to a gradual thermo-induced spin conversion of the central metal ion on cooling below room temperature, an unexpected split of the Mössbauer signal of the external iron(II) ions as well as pronounced relaxation phenomenon for the central HS iron(II) ion was discovered. This phenomenon is suggested to be induced by the tautomeric equilibrium between enol and keto forms of the anion.

Formation of hybrid spin crossover polymer microspheres

We report the formation of hybrid metallopolymer microspheres, deposited from an acetonitrile solution containing a known iron(3+) spin crossover complex bearing a thienyl substituent. We characterize the metallopolymer using SEM, FT-IR, elemental analysis (including C, H, N, S, Fe analyses), small-angle X-ray scattering (SAXS) techniques, and PXRD. Of greatest significance, the polymeric microspheres also exhibit spin crossover, conferring on metallopolymer 2 the first spin crossover nanoscale material of its kind.

Effect of site degeneracies on the spin crossovers in (Mg, Fe)SiO 3 perovskite

We investigate by first principles several aspects of the spin crossover of ferrous iron (Fe 2+) between the high-spin (HS), intermediate-spin (IS), and low-spin (LS) states in MgSiO 3 perovskite under pressure. First, we investigate the effect of site degeneracy intrinsic to LS and IS states of iron. We show that the entropy owing to this effect (site entropy) significantly increases the fraction of LS irons and reduces the crossover onset pressure, especially at high temperatures. Even including the effect of site entropy, the fraction of IS irons in the lower mantle or in the laboratory is predicted to be negligible. We also show that the effect of the crossover on equation-of-state parameters is probably undetectable at any temperature. Although we address only ferrous iron in only one atomic configuration and results are specific to this situation, our results suggest that the effect of the crossover on the bulk modulus derived from direct sound speed measurements could perhaps be more noticeable below room temperature. However, at mantle conditions the bulk sound speed should hardly be affected by the crossover.