Spin State Of Iron Research Articles

Interplay between spin-crossover and luminescence in a multifunctional single crystal iron(ii) complex: towards a new generation of molecular sensors.

We present a new example of a mononuclear iron(ii) complex exhibiting a correlated spin-crossover (SCO) transition and strong fluorescence, whose coordination sphere is saturated, for the first time, by six phosphorescent ligands. The interplay between SCO and light emission properties in the thermal region of the spin transition was investigated by means of magnetic, fluorescence, optical absorption and optical microscopy measurements on a single crystal. Overall, the results show an excellent correlation between fluorescence and magnetic data of the present gradual transition, indicating an extreme sensitivity of the optical activity of the ligand to the spin state of the active iron(ii) ions. These results open the way for conceiving new prototypes of pressure and temperature sensors based on this synergy between SCO and luminescence properties. In particular, the fact that cooperative SCO material is not a prerequisite for obtaining such synergetic effects, is useful for the design of thin films or nanoparticles, in which the cooperativity is reduced, for appropriate implementation in nanosized devices to enhance the sensing properties at the nanoscale.

Equatorial ligand plane perturbations lead to a spin-state change in an iron(iii) porphyrin dimer.

A complete reversal of the spin state of iron(iii) is observed upon a small change to the diporphyrin bridge from ethane to ethene by keeping all other factors intact. Combined analysis using single crystal X-ray structure determination, Mössbauer, variable-temperature magnetic, 1H NMR and EPR studies has confirmed the spin states of iron(iii) complexes both in solid and solution phases.

Benchmarking quantum chemistry methods for spin-state energetics of iron complexes against quantitative experimental data.

The accuracy of relative spin-state energetics predicted by selected quantum chemistry methods: coupled cluster theory at the CCSD(T) level, multiconfigurational perturbation theory (CASPT2, NEVPT2), multireference configuration interaction at the MRCISD+Q level, and a number of DFT methods, is quantitatively evaluated by comparison with the experimental data of four octahedral iron complexes. The available experimental data, either spin-forbidden transition energies or spin crossover enthalpies, are corrected for relevant environmental effects in order to derive the quantitative benchmark set of iron spin-state energetics. Comparison of theory predictions with the resulting reference data: (1) validates the high accuracy of the CCSD(T) method, particularly when based on Kohn-Sham orbitals, giving the maximum error below 2 kcal mol-1 and the mean absolute error (MAE) below 1 kcal mol-1; (2) corroborates the tendency of CASPT2 to systematically overstabilize higher-spin states by up to 5.5 kcal mol-1; (3) confirms that the latter problem is partly remedied by the recently proposed CASPT2/CC approach [Phung et al., J. Chem. Theory Comput., 2018, 14, 2446-2455]; (4) demonstrates that NEVPT2 performs worse than CASPT2, by giving errors up to 7 kcal mol-1; (5) shows that the accuracy of MRCISD+Q spin-state energetics strongly depends on the size-consistency correction: the Davidson-Silver and Pople corrections perform best (MAE < 3 kcal mol-1), whereas the standard Davidson correction is not recommended (MAE of 7 kcal mol-1). Only a few DFT methods (including the best performing ones identified in this study: B2PLYP-D3 and OPBE) are able to provide a balanced description of the spin-state energetics for all four studied iron complexes simultaneously, corroborating the non-universality problem of approximate density functionals.

The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{5CpFe}2(μ-H)4] Revisited by Hard X-Ray Spectroscopy.

High energy resolution fluorescence detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (VtC-XES) are introduced as powerful tools to investigate hydride-iron interaction, the possible iron-iron bond, and iron spin state of the dinuclear tetra-hydrido complex [{5CpFe}2(μ-H)4] (1H, 5Cp = η5-C5 iPr5) by thoroughly accessing the geometric and electronic structure of this complex in comparison to the nonhydride reference [5CpCpFe] (1, Cp = C5H5). The so far observed most intense hydride induced signals in the pre-edge feature of the HERFD-XANES and in the VtC-XES spectra at the iron K-edge allow a precise analysis of the LUMO and HOMO states, respectively, by application of time-dependent density function theory (TD-DFT) and density functional theory (DFT) calculations. The results of these calculations are further employed to understand the oxidation state, spin states, and potential Fe-Fe bonds in this complex.

Light-controlled switching of the spin state of iron(III)

Controlled switching of the spin state of transition metal ions, particularly of FeII and FeIII, is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S = 1/2) and high-spin (S = 5/2) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature. The system is neither oxygen nor water sensitive, and no fatigue was observed after more than 1000 switching cycles. Concomitant with the spin-flip is a change in redox potential by ~60 mV. Besides serving as a simple model for the first step of the cytochrome P450 catalytic cycle, the spin switch can be used to switch the spin-lattice relaxation time T1 of the water protons by a factor of 15.

A Strain-Deformation Nexus within Pincer Ligands: Application to the Spin States of Iron(II) Complexes.

The substitution of a pyrrolide ring for one (or more) pyridyl rings within the ubiquitous terpyridine (tpy, A) scaffold results in more open geometries of the pyridine-pyrrolide chelate ligands. DFT calculations (B3LYP-GD3BJ/6-31G**) demonstrate that the more open geometries of the unbound ligands are mismatched with the "pinched in" geometries required to chelate transition metal ions (e.g., Zn2+). The strain which builds within these ligands (Δ EL(strain)) as they bind transition metal ions can be related to changes in a single geometric parameter: the separation between the two terminal N atoms (ρ). This relationship applies more generally to other three-ringed tridentate pincer ligands, including those with different donor groups. The approach was applied to homoleptic iron(II) complexes to investigate the contribution of the steric effects operating within the ligands to the different magnetic properties, including spin crossover (SCO) activities, of these systems.

Non-covalent control of spin-state in metal-organic complex by positioning on N-doped graphene

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.

Low-Temperature P–T Phase Diagram of the (Mg, Fe)SiO3 Perovskite

The electron spin states of iron in minerals of the Earth’s mantle at high pressures mostly determine the physicochemical properties of deep layers of the Earth and are of great interest not only for geophysics but also for fundamental physics of strongly correlated electron systems. In this work, using Raman and synchrotron Mossbauer nuclear forward scattering (NFS) spectroscopies, iron-containing magnesium–silicate perovskite (Mg, Fe)SiO3 (10% Fe) has been studied in the cryogenic temperature range of 35–300 K and at high pressures up to 48 GPa, which are created in diamond anvil cells. The analysis of NFS spectra has indicated that iron ions are in a nonmagnetic (para- or diamagnetic) state in the entire region of temperatures and pressures and the electronic properties can be controlled by means of the quadrupole splitting parameter. It has been found that an increase in the pressure and a decrease in the temperature are accompanied by a significant increase in the parameter Δ from 2 mm/s to ~4 mm/s, which indicates that the electronic state of Fe2+ ions changes. The maximum Δ value has been observed at P > 20 GPa, but the pressure behavior of a transition strongly depends on the temperature. Possible mechanisms of the transition have been discussed.

Ba3Fe1.56Ir1.44O9: A Polar Semiconducting Triple Perovskite with Near Room Temperature Magnetic Ordering

The crystal chemistry and magnetic properties for two triple perovskites, Ba3Fe1.56Ir1.44O9 and Ba3NiIr2O9, grown as large, highly faceted single crystals from a molten strontium carbonate flux, are reported. Unlike the idealized A3MM2'O9 hexagonal symmetry characteristic of most triple perovskites, including Ba3NiIr2O9, Ba3Fe1.56Ir1.44O9 possesses significant site-disorder, resulting in a noncentrosymmetric polar structure with trigonal symmetry. The valence of iron and iridium in the heavily distorted Fe/Ir sites was determined to be Fe(III) and Ir(V) by X-ray absorption near edge spectroscopy (XANES). Density functional theory calculations were conducted to understand the effect of the trigonal distortion on the local Fe(III)O6 electronic structure, and the spin state of iron was determined to be S = 5/2 by Mössbauer spectroscopy. Conductivity measurements indicate thermally activated semiconducting behavior in the trigonal perovskite. Magnetic properties were measured and near room temperature magnetic ordering (TN = 270 K) was observed for Ba3Fe1.56Ir1.44O9.

Density‐Pressure Profiles of Fe‐Bearing MgSiO3 Liquid: Effects of Valence and Spin States, and Implications for the Chemical Evolution of the Lower Mantle

AbstractDensity is a key property controlling the chemical state of Earth's interior. Our knowledge about the density of relevant melt compositions is currently poor at deep‐mantle conditions. Here we report results from first‐principles molecular‐dynamics simulations of Fe‐bearing MgSiO3 liquids considering different valence and spin states of iron over the whole mantle pressure conditions. Our simulations predict the high‐spin to low‐spin transition in both ferrous and ferric iron in the silicate liquid to occur gradually at pressures around 100 GPa. The calculated iron‐induced changes in the melt density (about 8% increase for 25% iron content) are primarily due to the difference in atomic mass between Mg and Fe, with smaller contributions (&lt;2%) from the valence and spin states. A comparison of the predicted density of mixtures of (Mg,Fe)(Si,Fe)O3 and (Mg,Fe)O liquids with the mantle density indicates that the density contrast between the melt and residual‐solid depends strongly on pressure (depth): in the shallow lower mantle (depths &lt; 1,000 km), the melt is lighter than the solids, whereas in the deep lower mantle (e.g., the D″ layer), the melt density exceeds the mantle density when iron content is relatively high and/or melt is enriched with Fe‐rich ferropericlase.

Valence and spin states of iron are invisible in Earth\u2019s lower mantle

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg that Fe3+ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

Correlating Structure and Function of Non-Native Cytochrome C: The Relationship Between Iron Spin State and Peroxidase Activity

Correlating Structure and Function of Non-Native Cytochrome C: The Relationship Between Iron Spin State and Peroxidase Activity

Implementing a Statistical Thermodynamic Model to Describe Functionally Relevant Cytochrome C - Cardiolipin L-Site Binding

The L-site binding of ferricytochrome c to cardiolipin (CL) - containing liposomes at slightly acidic pH has attracted interest owing to its potential role in converting this classical electron transfer protein into a peroxidase. Various lines of evidence presented by Nantes and coworkers suggest that this mode of binding is electrostatic, active below pH 7, and involves residues K22, K25, and K27 as well as H26 and H33. We combined several spectroscopic techniques to characterize L-site binding thermodynamically and to identify concomitant structural changes. To this end we employed and extended a recently presented thermodynamic model that describes cytochrome c binding to CL- containing liposomes as a two-step process, where native-like liposome-bound conformers convert to more unfolded conformers with an increase in CL concentration. These partially unfolded species are characterized by a loss of the M80 ligand, which is likely replaced by H26 and/or H33. At acidic pH, the partially unfolded low-spin species converts into a mixture of penta- and hexacoordinate high-spin species at moderate-to- high CL concentration. This change of the heme iron's spin state is most likely caused by the protonation of the non-native histidine ligand, which is facilitated by low effective pH at the liposome surface. Results of our analysis of binding isotherms obtained in the absence and presence of NaCl confirmed the electrostatic character of L-site binding. The inhibiting effect of sodium ions is attributed to the accumulation of cations in the liposome's double layer, which reduces its surface potential. Thus, L-site binding is clearly distinct from cytochrome c - CL interactions at neutral pH where only the conformational change on the membrane surface is affected by the presence of NaCl.

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry.

Iron tetraphenylporphyrins are prime candidates as catalysts for CO2 reduction. Yet, even after 40 years of research, fundamental questions about the electronic structure of their reduced states remain, in particular as to whether the reducing equivalents are stored at the iron center or at the porphyrin ligand. In this contribution, we address this question by a combination of resonance Raman spectroscopy and quantum chemistry. Analysis of the data allows for an unequivocal identification of the porphyrin as the redox active moiety. Additionally, determination of the spin state of iron is possible by comparing the characteristic shifts of spin and oxidation-state-sensitive marker bands in the Raman spectrum with calculations of planar porphyrin model structures.

Assessment of the ground spin state of iron(i) complexes: insights from DFT predictive models

Factors governing the ground spin state of iron(i) complexes are analyzed by DFT methods. An efficient benchmarking procedure is reported.

Spin state and electronic environment of iron in basaltic glass in the lower mantle

The spin states of iron in deep magmas are one of the most important properties that affect the partitioning of iron between magmas and minerals and, thus, the gravitational stability of magmas in the Earth. We investigated the spin state and electronic environments of iron in a basaltic glass containing ~70 Fe 3+ /∑Fe at room temperature and pressures from 1 bar to 130 GPa using a diamond-anvil cell combined with energy domain synchrotron 57 Fe Mossbauer source spectroscopy. The basaltic glass represents an analog of a multi-component magma typical for the Earth. The Mossbauer spectra could be fitted by a two pseudo-Voigt doublet model including a high quadrupole splitting (QS) doublet and a low QS doublet, which were assigned to high-spin Fe 2+ and high-spin Fe 3+ , respectively. The high-spin states of Fe 2+ and Fe 3+ remained up to 130 GPa corresponding to the pressure in the lowermost mantle. The center shift values of high-spin Fe 2+ and Fe 3+ did not show large changes with pressure, ruling out sharp electronic changes in the basaltic glass. Therefore, a sharp and complete spin crossover of Fe 2+ from the high-spin to the low-spin state does not appear to occur in the basaltic glass although the possibility of a partial spin transition cannot be fully excluded. The QS values of Fe 2+ increased slightly at 0–20 GPa and above 100 GPa, and the higher value was preserved after decompression to ambient conditions. This behavior may be related to distortion of Fe 2+ polyhedra due to short-range ordering on compression. Such a distortion of Fe 2+ polyhedra could gradually stabilize Fe 2+ in the basaltic glass with pressure compared to bridgmanite according to the Jahn-Teller effect, and thus could gradually enhance the partitioning of iron into deep magmas in the lower mantle.

Structure and Spin State of Iron(II) Assembled Complexes Using 9,10-Bis(4-pyridyl)anthracene as Bridging Ligand

Assembled complexes, [Fe(NCX)2(bpanth)2]n (X = S, Se, BH3; bpanth = 9,10-bis(4-pyridyl)anthracene), were synthesized. The iron for the three complexes was in temperature-independent high spin state by 57Fe Mössbauer spectroscopy and magnetic susceptibility measurement. X-ray structural analysis revealed the interpenetrated structure of [Fe(NCS)2(bpanth)2]n. In the local structure around the iron atom, the coordinated pyridine planes were shown to be a parallel type, which is in accordance with the results investigated by density functional theory (DFT) calculation. This complex (X = S) has CH–π interactions between the H atom of coordinated pyridine and the neighboring anthracene of the other 2D grid. It was suggested that the interpenetrated structure was supported by the stabilization of CH–π interaction, and this intermolecular interaction forced the relatively unstable parallel structure. That is, the unstable local structure is compensated by the stabilization due to intermolecular interaction, which controlled the spin state as high spin state.

Spin crossover behaviour in Hofmann-like coordination polymer Fe(py)2[Pd(CN)4] with 57Fe Mössbauer spectra

We have prepared the 2D spin crossover complexes Fe(L)2Pd(CN)4 (L = py : 1a; py-D5 : 1b and py-15N :1c). 1a has been characterised by 57Fe Mossbauer spectroscopic measurements, single crystal X-ray determination and SQUID measurements. The Mossbauer spectra for 1a indicate that the iron(II) spin states are in high spin states at 298 K and are in low spin states at 77 K. The crystal structures of 1a at 298 K and 90 K also show the high spin state and the low spin state respectively, associated with the Fe(II)-N distances. The spin transition temperature range of 1a is higher than that of Fe(py)2Ni(CN)4 since Pd(II) ions are larger and heavier than Ni(II) ions. SQUID data indicate isotope effects among 1a, 1b and 1c are observed in very small shifts of the transition temperatures probably due to larger and heavier Pd(II) ions. The delicate shifts would be associated with subtle balances between different vibrations around Fe(II) atoms and electronic factors.

Reduced lattice thermal conductivity of Fe‐bearing bridgmanite in Earth's deep mantle

AbstractComplex seismic, thermal, and chemical features have been reported in Earth's lowermost mantle. In particular, possible iron enrichments in the large low shear‐wave velocity provinces (LLSVPs) could influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core‐mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earth's lower mantle. Importantly, the pressure‐induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe‐bearing bridgmanite to 120 GPa using optical pump‐probe spectroscopy. The conductivity of Fe‐bearing bridgmanite increases monotonically with pressure but drops significantly around 45 GPa due to pressure‐induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid‐lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000 km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

AbstractCeric ammonium nitrate (CAN) or CeIV(NH4)2(NO3)6 is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)FeIII‐O‐CeIV(OH2)(NO3)4]+ (3), a complex obtained from the reaction of [(N4Py)FeII(NCMe)]2+ with 2 equiv CAN or [(N4Py)FeIV=O]2+ (2) with CeIII(NO3)3 in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the FeIV and CeIV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 FeIV in 2 to S=5/2 in 3, which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that FeIV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.