Low-spin Fraction Research Articles

Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

We investigated the interplay between the thermomechanical and spin-crossover (SCO) properties of a series of stimuli-responsive polymer composites consisting of [Fe(NH2trz)3]SO4 and [Fe(Htrz)(trz)2]BF4 particles (trz = 1,2,4-triazolato) embedded in thermoplastic polyurethane, TPU, and poly(vinylidene fluoride–trifluoroethylene), P(VDF-TrFE), matrices. The effective thermoelastic coefficients and transformation stress and strain in the composites were assessed utilizing dynamical mechanical analysis (DMA), differential scanning calorimetry (DSC), thermal expansion, and thermal stress measurements. Remarkably, the composites display a characteristic elastic softening and increased mechanical damping around the spin transition temperature, which arise from the significant spin state–volume strain coupling in the particles and scale semiquantitatively with the pressure derivative of the low spin fraction (∂n∂P)T. Crucially, for a given particle volume fraction, the transformation strain (respectively stress) substantially increases (respectively decreases) in soft matrices, which was rationalized by micromechanical simulations. The results provide a fundamental understanding and a quantitative guideline for the design of SCO@polymer composites for applications in mechanical transducers.

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Intermolecular interactions and spin states of complexes [Fe(3-MeO-Qsal)2]Y (Y = PF6, BF4)

Compounds [Fe(3-MeO-Qsal)2]Y (Y = PF6, BF4) have been prepared by diffusion method and studied in temperature range 5–300 K by EPR and magnetic susceptibility methods. The coexistence of spatially separated high-spin (solvated) and low-spin (unsolvated) fractions in the studied compounds has been established. It has been shown that change in the type of outer-sphere anion leads to change in the character of intermolecular interactions in the high-spin fraction and has no effect on the parameters and character of interactions of paramagnetic centers in the low-spin fraction.

Femtosecond optical reflectivity measurements of lattice-mediated spin repulsions in photoexcited LaCoO3thin films

We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO3 thin film under tensile strain. Probing spin-polarized transitions into the antibonding e(g) band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction (similar to 63%-64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap Delta. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film LaCoO3, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in LaCoO3 thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO3.

Mixed-Spin [2 × 2] Fe4 Grid Complex Optimized for Quantum Cellular Automata

The new pyrazolate-bridged proligand 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazole ((Me)LH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent ((H)LH) it forms a robust Fe(II)4 grid complex, [(Me)L4Fe(II)4](BF4)4. The molecular structure of [(Me)L4Fe(II)4](BF4)4·2MeCN has been elucidated by X-ray diffraction, revealing two high-spin (HS) and two low-spin (LS) ferrous ions at opposite corners of the rhombic metal ion arrangement. SQUID and (57)Fe Mössbauer data for solid material showed that this [HS-LS-HS-LS] configuration persists over a wide temperature range, between 7 and 250 K, while spin-crossover sets in only above 250 K. According to Mössbauer spectroscopy a [1HS-3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [(Me)L](-) leads to a stronger ligand field compared to parent [(H)L](-) and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [(Me)L4Fe(II)4](BF4)4 reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [(Me)L4Fe(II)2Fe(III)2](6+) (K(C) = 3.35 × 10(8)). The particular [HS-LS-HS-LS] configuration as well as the di-mixed-valence configuration, both with identical spin or redox states at diagonally opposed vertices of the grid, make this system attractive as a molecular component for quantum cellular automata.

Spin transition of Fe3+ in Al-bearing phase D: An alternative explanation for small-scale seismic scatterers in the mid-lower mantle

Among dense-hydrous magnesium silicates potentially transporting H2O into Earthʼs deep interior, phase D (MgSi2H2O6) exhibits the highest P–T stability range, extending into the lower mantle along cold slab geotherms. We have studied the compressibility and spin state of Fe in Al-bearing phase D up to 90 GPa using synchrotron X-ray diffraction and X-ray emission spectroscopy. Fe–Al-bearing phase D was synthesized at 25 GPa and 1400 °C with approximate composition MgSi1.5Fe0.15Al0.32H2.6O6, where nearly all of the Fe is ferric (Fe3+). Analysis of Fe-Kβ emission spectra reveals a gradual, pressure-induced high-spin (HS) to low-spin (LS) transition of Fe3+ extending from 40 to 65 GPa. The fitted equation of state for high-spin Fe–Al-bearing phase D results in a bulk modulus KT0=147(2) GPa with pressure derivative K′=6.3(3). An equation of state over the entire pressure range was calculated using the observed variation in low-spin fraction with pressure and a low-spin bulk modulus of KT0=253(30) GPa, derived from the data above 65 GPa. Pronounced softening in the bulk modulus occurs during the spin transition, reaching a minimum at 50 GPa (∼1500 km) where the bulk modulus of Fe–Al phase D is about 35% lower than Fe–Al-bearing silicate perovskite. Recovery of the bulk modulus at 50–65 GPa results in a structure that has a similar incompressibility as silicate perovskite above 65 GPa. Similarly, the bulk sound velocity of Fe–Al phase D reaches a minimum at ∼50 GPa, being about 10% slower than silicate perovskite. The potential association of Fe–Al phase D with subducted slabs entering the lower mantle, along with its elastic properties through the Fe3+ spin transition predicted at 1200–1800 km, suggests that phase D may provide an alternative explanation for small-scale mid-lower mantle seismic scatterers and supports the presence of deeply recycled sediments in the lower mantle.

Kinetics of electron transfer in the complex of cytochrome P450 3A4 with the flavin domain of cytochrome P450BM-3 as evidence of functional heterogeneity of the heme protein

We used a rapid scanning stop-flow technique to study the kinetics of reduction of cytochrome P450 3A4 (CYP3A4) by the flavin domain of cytochrome P450-BM3 (BMR), which was shown to form a stoichiometric complex ( K D = 0.48 μM) with CYP3A4. In the absence of substrates only about 50% of CYP3A4 was able to accept electrons from BMR. Whereas the high-spin fraction was completely reducible, the reducibility of the low-spin fraction did not exceed 42%. Among four substrates tested (testosterone, 1-pyrenebutanol, bromocriptine, or α-naphthoflavone (ANF)) only ANF is capable of increasing the reducibility of the low-spin fraction to 75%. Our results demonstrate that the pool of CYP3A4 is heterogeneous, and not all P450 is competent for electron transfer in the complex with reductase. The increase in the reducibility of the enzyme in the presence of ANF may represent an important element of the mechanism of action of this activator.

Two Novel Iron(II) Materials Based on Dianionic N4O2Schiff Bases: Structural Properties and Spin-Crossover Characteristics in the Series [Fe(3-X,5-NO2-sal-N(1,4,7,10)] (X = H, 3-MeO, 3-EtO)

Two spin-crossover (SCO) complexes [Fe(II)(3-MeO,5-NO2-sal-N(1,4,7,10)] (1) and [Fe(II)(3-EtO,5-NO2-sal-N(1,4,7,10)] (2) have been prepared and studied (3-MeO,5-NO2-sal-N(1,4,7,10) and 3-EtO,5-NO2-sal-N(1,4,7,10) are deprotonated 2-[12-(hydroxy-3-methoxy-5-nitrophenyl)-2,5,8,11-tetraazadodeca-1,11-dien-1-yl]-2-methoxy-4-nitrophenol and 2-[12-(hydroxy-3-ethoxy-5-nitrophenyl)-2,5,8,11-tetraazadodeca-1,11-dien-1-yl]-2-ethoxy-4-nitrophenol, respectively). The X-ray diffraction analysis of complex 1 (C22H26N6O8Fe) evidenced the same Pbnb orthorhombic system at 160 K (high-spin (HS) state) and 100 K (low-spin (LS) state). At 160 K, a = 8.4810(9) A, b = 14.7704(14) A, c = 18.769(2) A, V = 2351.2(4) A3, and Z = 4. At 100 K, a = 8.5317(8) A, b = 14.4674(15) A, c = 18.814(2) A, and V = 2322.2(4) A3. Complex 2 (C28H38N6O9Fe) crystallizes in the P1 triclinic system. At 160 K (HS state), a = 10.265(4) A, b = 10.861(4) A, c = 14.181(5) A, alpha = 84.18(4) degrees, beta = 70.53(5) degrees, gamma = 88.95(5) degrees, V = 1482.6(10) A3, and Z = 2. The iron(II) coordination sphere is distorted octahedral in 1 and 2 with a cis-alpha arrangement of the N4O2 donor set of the hexadentate ligand. The molecules are connected into 1D infinite chains through hydrogen contacts involving the secondary amine functions and O(nitro) atoms of the ligands in adjacent molecules. Investigation of their magnetic properties and Mossbauer spectra has revealed very different SCO behaviors: complex 1 exhibits a cooperative SCO without residual LS or HS fraction; complex 2 shows a LS <--> HS SCO involving approximately 5% of the Fe(II) ions in the 30-150 K range. The phenomenological cooperative interaction parameter J = 138 K evaluated from the area of the hysteresis loop indicates a cooperative effect weaker in 1 than in [Fe(II)(5NO2-sal-N(1,4,7,10))]. The theoretical approach to the SCO in 2 indicates a HS ground state and a LS first excited level 53 K above: the thermal dependence of the system occurs through population of vibrational states. Comparison of the structural and electronic properties of the ferrous SCO materials with parent N4O2 ligands shows that the properties of SCO are closely related to intermolecular interactions and crystal packing.

Pressure-induced spin-state crossovers at room temperature in iron(II) complexes: comparative analysis; a XANES investigation of some new transitionsDedicated to the memory of Professor Olivier Kahn.

An attempt to rationalize the behaviour of iron(II) spin-crossover compounds under hydrostatic pressure at room temperature from a comparative analysis of data is reported. The selected pressure-induced spin transitions are those previously described for Fe(phen)2(NCS)2 [polymorphs I (a) and II (b)], Fe(py)2bpym(NCS)2 (c) and Fe(py)2phen(NCS)2 (d), and those presented herein for Fe(Me2bpy)2(NCS)2 (1), [Fe(2-pic)3]Cl2·EtOH (2), [Fe(2-pic)3]Cl2·H2O (3) and [Fe(btr)2(NCS)2]·H2O (4) [Me2bpy = 4,4′-dimethyl-2,2′-bipyridine, 2-pic = 2-(aminomethyl)pyridine, btr = 4,4′-bis-1,2,4-triazole]. All these transitions have been investigated by XANES spectrometry. In this paper, the spectra of compounds 1–4 and the resulting nLSvs. P plots (nLS = low-spin fraction) are discussed. Transition pressures, P1/2, are found to be 0.30 (1), 1.32 (2), 0.60 (3) and 2.04 (4) GPa. For compounds 1, a, b, c, with closely related formulas, P1/2 is found to vary quasi-linearly with the transition temperature at atmospheric pressure, T1/2, and deviations from linearity are discussed in terms of the variations in entropy and lattice volume associated with the spin change. For compounds 2, 3 and d, the values of P1/2 markedly depart from the above trend. This is ascribed either to the change in ligand-field strength resulting from the effects of pressure on the hydrogen-bonding network (2, 3) or the occurrence of a structural phase transition simultaneously with the spin conversion (d). In the polymeric two-dimensional compound 4, pressure is shown to first induce a structural transition to a new high-spin phase, then to produce a very gradual high-spin-to-low-spin transformation of this phase, which results in a comparatively high value of P1/2.

Theory of the photoinduced spin-state transitions in spin-crossover complexes

Nonlinear properties observed during the photoinjection of high-spin (HS) domains into a low-spin (LS) phase in spin-crossover complexes are explained theoretically. As the spin-transition processes, we take into account the LS→HS photoexcitation process and the HS→LS nonradiative decay process. The latter one is sensitive to the LS fraction in the crystal; the lifetime of the metastable HS state becomes longer as the LS fraction decreases. Such sensitivity results in the nonlinear temporal evolution for the LS fraction, which qualitatively explains experimental results.

Role of Long-Range Interaction in Photoinduced Spin-State Transitions in Spin-Crossover Complexes

We explain theoretically the peculiarities of photoinduced low-spin (LS)→high-spin (HS) transitions in spin-crossover complexes, such as the threshold behavior in the excitation light intensity for phase conversion and the existence of the incubation period. Our investigation is based on a model where a single complex is described by the two electronic states and the breathing oscillation mode with the long-range interaction among the complexes. As the spin transition processes, we take into account the LS→HS photoexcitation process and the HS→LS nonradiative decay process. The rate for the latter process is sensitive to the LS fraction in the crystal because the potential barrier for the HS→LS decay is dependent on the LS fraction through the long-range interaction among the complexes; the lifetime of the metastable HS state becomes longer as the LS fraction decreases. Such sensitivity leads to the nonlinear temporal evolution for the LS fraction, which coincides with the experimental results.

Pressure Study of the Thermal Spin Transition and the High-Spin → Low-Spin Relaxation in theR3̄ andP1̄ Crystallographic Phases of [Zn1-xFex(ptz)6](BF4)2Single Crystals (x= 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)

In the iron(II) spin-crossover compound [Fe(ptz)6](BF4)2, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with Tc↓ = 128 K and Tc↑ = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1982, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R3̄ → P1̄ crystallographic phase transition with a large low-spin fraction stabilizing the P1̄ phase at low temperatures. In the mixed crystal [Zn1-xFex(ptz)6](BF4)2, x = 0.1, with the iron complexes imbedded into the isomorphous zinc lattice, the crystallographic phase transition can be induced by an external pressure [Jeftić, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the P1̄ phase is additionally stabilized by external pressure. The interaction constant Γ, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Γ(P1̄) of 144(8) cm-1 and the volume difference of 29(4) Å3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P1̄ phase. These values are compared to the corresponding values for the R3̄ phase, viz. Γ(R3̄) of 170(9) cm-1 and of 26(3) Å3. Surprisingly Γ(R3̄) is larger than Γ(P1̄) despite the fact that is smaller than The high-spin → low-spin relaxation at temperatures above ∼80 K is thermally activated, while below ∼40 K temperature independent tunnelling takes place. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation exponentially by 1 order of magnitude in the tunnelling region in both crystallographic phases and regardless of x. In the concentrated material the high-spin → low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. Chem. Phys. Lett. 1992, 192, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the 1A1 → 1T1 absorption band.

Crystal structure of tris(4,4′-dimethoxy-2,2′-bipyridine)iron (II) bis (hexafluorophosphate) , solvate with N-methyl-2-pyrrolidone

The crystal study of tris(4,4′-dimethoxy-2,2′-bipyridine)iron(II)bis(hexafluorophosphate) (crystal solvate with N-methyl-2-pyrrolidone) was performed at 295 K. The crystals are triclinic, a = 11.721(4), b = 12.864(6), c =15.664(7) A ̊ , α = 77.14(4),β = 85.03 (3), γ = 81.76(3)° , space group P 1, Z = 2, R = 0.0679 for 4694 reflections. The average FeN distances of 1.967 Å agrees well with those observed in other tris(2,2-bipyridine) complexes of hexacoordinated Fe(II) with a large low-spin fraction. A small difference of the individual FeN distances in the dication (which are shorter for one of the bipyridine ligands) may be explained by crystal packing effects. A very specific orientation of dications [Fe(DMO-bpy) 3] 2+ in the crystal is worthy of note (the vector of one of the moieties N…(Fe)…N of the dication coincides in fact with direction [011] of the crystal). The possible interaction of the trication of tris(4,4′-dimethoxy-2,2′-bipyridine)iron (III) with the active site of glucose oxidase (i.e. the metal complex acting as an oxidant/electron-transfer mediator) is discussed.

Pressure-Induced Spin-State Crossovers in Six-Coordinate FeIILnL‘m(NCS)2Complexes with L = L‘ and L ≠ L‘: A XANES Investigation

The pressure-induced iron(II) high-spin (HS) → low-spin (LS) conversion has been investigated, by near-edge X-ray absorption (XANES) spectroscopy at room temperature, in a number of parent six-coordinate complexes of the type FeLnL‘m(NCS)2, viz. Fe(phen)2(NCS)2 form I (1) and form II (2), Fe(py)2bpym(NCS)2 (3), Fe(py)2phen(NCS)2 (4) and Fe(py)4(NCS)2 (5), where phen = 1,10-phenanthroline, py = pyridine, and bpym = 2,2‘-bipyrimidine. The spectra of the two spin isomers are interpreted. For compounds 1−4, known to exhibit thermally-induced spin transitions, nLS vs P plots (nLS = LS fraction, P = pressure) are centered around Pc = 0.80 (1), 0.65 (2), 1.00 (3) and 1.55 (4) GPa. For 5, which retains the HS form at any temperature under atmospheric pressure, the Pc value is much higher than the previous ones, lying in the range 5−7 GPa. After pressure was released, the spectra were found to be quite similar to those obtained before pressure was applied. The Pc values of 1−4 were analyzed in terms of spin-transition temperatures (Tc), entropy (ΔS) and crystal volume (ΔVSC) variations associated with the spin change, crystal structures, and volumic compressibility coefficients (kv), insofar as these data were available. The alteration of the spin-conversion development, when subjecting a compound (here 2) several times to increasing then to decreasing pressures, was ascribed to a progressive decrease in the number of crystal defects (vacancies in particular).

Cytochrome c peroxidase complexed with cytochrome c has an unperturbed heme moiety.

Transient resonance Raman, Raman difference, circular dichroism (CD), and optical absorption studies have been carried out on the electrostatic complexes formed by yeast cytochrome c peroxidase (CCP) with horse cytochrome c (Cytc) in low ionic strength solutions. In all the complexes examined [e.g., CCP(II)/Cytc(II), CCP(III)/Cytc(II), CCP(III)/Cytc(III)], the local heme environments of both proteins are largely unperturbed upon complexation. Specifically, CCP preserves a completely pentacoordinate high-spin heme in both its ferric and ferrous forms in CCP/Cytc complexes and uncomplexed mixtures. We found no evidence corroborating the previously reported increase in the low-spin fraction of CCP heme upon complexation with Cytc [Hildebrandt et al. (1992) Biochemistry 31, 2384-2392]. Instead, our Raman data strongly suggest that the H-bonding networks in the distal and proximal pockets of CCP are well maintained in the complexes. On the other hand, CD spectra of CCP(III)/Cytc(III) complexes showed substantial variations (relative to the uncomplexed mixtures) in the far-UV region, reflecting some protein conformational rearrangements. In addition, the spectral data suggest that complexation with Cytc affects the previously observed pH-dependent flexibility of the heme structure of CCP and thus influences the photodynamics of the CCP active site.

Tetrahydrobiopterin-deficient nitric oxide synthase has a modified heme environment and forms a cytochrome P-420 analogue.

Optical absorption and resonance Raman spectra of neuronal nitric oxide synthase (b-NOS) isolated in the absence of tetrahydrobiopterin demonstrate that the enzyme preparation is very unstable. This unstable form of the enzyme has properties analogous to those of cytochrome P-420cam, an inactive form of cytochrome P-450cam. Although cysteine is preserved as the proximal ligand in both the ferric and ferrous forms of unstable b-NOS, the lack of tetrahydrobiopterin significantly increases the hexacoordinate low-spin fraction of the heme content, resulting in a loss of the enzymatic activity. Upon the addition of CO, the unstable b-NOS converts from a species exhibiting a Soret absorption maximum at 443 nm, as reported for the CO adducts of stable b-NOS and cytochrome P-450cam, to a species with a Soret maximum at 421 nm. The resonance Raman spectrum of the 421-nm form is the same as those of CO-bound myoglobin at low pH and CO-bound cytochrome P-420cam. The heme in this form of the enzyme is coordinated by a weaker ligand than thiolate; histidine coordination in the CO-bound form of the P-420-like species of NOS is consistent with all of the available data. A similar unstable form of the macrophage (i-NOS) enzyme was also detected. Not only does the heme pocket of NOS have the same coordination as cytochrome P-450 in its stable form, but the partially denatured form has the same properties as cytochrome P-420, the inactive form of cytochrome P-450. Among other possible roles, tetrahydrobiopterin may play a significant role in the stabilization of the active enzyme.

Correlations between spin equilibrium shift, reduction rate, and N-demethylation activity in liver microsomal cytochrome P-450 and a series of benzphetamine analogues as substrates

Cytochrome P-450 forms a thermal ferric spin equilibrium which is significantly shifted by substrate binding. Within a series of benzphetamine analogues the liver microsomal enzyme system exhibits a close correlation of the substrate induced spin equilibrium shift towards the high spin state and both the rate of P-450 reduction, and of substrate turnover, as well. The spin equilibrium regulates the first electron transfer by favoured high spin state reduction and rapid pre-equilibration with respect to the low spin fraction.

Electron paramagnetic resonance detectable states of cytochrome P-450cam.

Cytochrome P-450cam is a low-spin Fe3+hemoprotein (g = 2.45, 2.26, and 1.91) which is made 60% high spin (g = 7.85, 3.97, and 1.78) at 12 K by the addition of 1 mol of substrate per mol of enzyme. Low-temperature EPR spectra show that the low-spin fraction of substrate-bound P-450cam contains two magnetic species. The majority species has an unusual EPR spectrum (g = 2.42, 2.24, and 1.97) which connot be simulated by using the range of crystal field parameters known for other heme proteins. The minority species has the same g values as substrate-free enzyme. Both low-spin species show Curie law temperature dependence below 50 K and have similar saturation behavior. Above 50 K the g = 2.42, 2.24, and 1.97 species rapidly loses signal intensity. The distribution of low-spin species is pH dependent (apparent pKa = 6.2) with the g = 2.42, 2.24, and 1.97 magnetic species favored at high pH. The substrate binding stoichiometry and the equilibria observed in the low-spin fraction suggest that there are not multiple protein forms of cytochrome P-450cam. Putidaredoxin and other effector molecules which specifically catalyze hydroxylation convert either the high-spin or the g = 2.42, 2.24, and 1.97 low-spin species to another new magnetic species (g = 2.47, 2.26, and 1.91). This species is only seen in the presence of substrate, and its stability reflects the catalytic potency of the effector molecule. The EPR and UV-visible spectra of cytochrome P-420 depend upon the manner in which the P-420 is generated. Incubation with acetone or reaction with N-ethylmaleimide or diethyl pyrocarbonate generates P-420 with different spectral characteristics. Through identification of active-site amino acids by chemical modification and comparison with porphyrin model complexes, the range of ligands likely to participate in each of the EPR detectable species is assigned. Mechanisms of interconversion of these species and their bearing on catalysis are discussed.