HS State Research Articles

Perferryl Fe(V)-Oxo Nonheme Complexes: Do They Have High-Spin or Low-Spin Ground States?

Nonheme perferryl Fe(V)═O species are studied herein by means of coupled cluster (CCSD(T)) calculations with a complete basis set limit estimate and density functional B3LYP computations. It is shown that the high-spin/low-spin (HS/LS) energy order in these Fe(V)═O species is highly dependent on the electronic nature of the ligand sphere and the geometric position of ligands relative to the Fe(V)═O moiety. When only σ-donor amines ligate Fe(V)═O, the LS state is slightly lower than the HS states. However, when a strong π-donor ligand such as hydroxyl is cis to Fe(V)═O, the HS state becomes highly favored. And on the contrary, if the π-donor ligand is trans to Fe(V)═O, the LS state is predicted here to be highly favored. This last type of perferryl complex has not yet been made by experimental means. Generally, our findings are consistent with the available experimental data. (4a, 6, 7) Some implications of these findings on the behavior of experimental systems are discussed.

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.

Spin and orbital states in single-layered La2−xCaxCoO4studied by doping- and temperature-dependent near-edge x-ray absorption fine structure

The doping-dependent valence, orbital, and spin-state configurations of single-layered La${}_{2\ensuremath{-}x}$Ca${}_{x}$CoO${}_{4}$ ($x=0$, 0.5, 1, and 1.5) were investigated with temperature-dependent near-edge x-ray absorption fine structure at the Co ${L}_{2,3}$ and O $K$ edges. The spectra show that in La${}_{2}$CoO${}_{4}$, the superexchange between neighboring Co${}^{2+}$ HS states is responsible for the strong antiferromagnetism. With increasing hole doping, the superexchange interactions between Co${}^{2+}$ HS ions are rapidly reduced by interlaced nonmagnetic Co${}^{3+}$ LS. For La${}_{1.5}$Ca${}_{0.5}$CoO${}_{4}$, the low N\'eel temperature of the samples together with the 50% Co${}^{2+}$ HS and 50% Co${}^{3+}$ LS configuration suggests a checkerboard arrangement of these ions. The spin blockade resulting from this arrangement naturally explains the high resistivity of La${}_{1.5}$Ca${}_{0.5}$CoO${}_{4}$. Upon further doping, Co${}^{2+}$ HS ions are replaced by Co${}^{3+}$ HS, and for LaCaCoO${}_{4}$ a mixture of Co${}^{3+}$ LS and Co${}^{3+}$ HS occurs. Superexchange via configuration fluctuation processes between these two species seems to induce long-range ferromagnetism, while the superexchange between adjacent Co${}^{3+}$ HS neighbors may lead to a competing antiferromagnetic exchange. For a doping content beyond $x=1$, Co${}^{4+}$ HS is introduced to the system at the expense of Co${}^{3+}$ LS, and a ${t}_{2g}$ double exchange between Co${}^{3+}$ HS and Co${}^{4+}$ HS is established, which further enhances ferromagnetic interactions and reduces resistivity. No indications for a Co${}^{3+}$ IS state are found throughout the La${}_{2\ensuremath{-}x}$Ca${}_{x}$CoO${}_{4}$ doping series.

Accurate Double Many-Body Expansion Potential Energy Surface for Ground-State HS2Based on ab Initio Data Extrapolated to the Complete Basis Set Limit

A double many-body expansion potential energy surface is reported for the electronic ground state of HS(2) by fitting accurate multireference configuration interaction energies calculated with aug-cc-pVTdZ and aug-cc-pVQdZ basis sets upon separate extrapolation of the complete-active-space self-consistent field and dynamical correlation components of the total energy to the complete basis set limit. The major topographical features of the potential energy surface are examined in detail, and the model function is used for thermalized calculations of the rate constants for the S + SH → H + S(2) reaction at 298 and 400 K. A value of (1.44 ± 0.06) × 10(-11) cm(3) s(-1) is obtained at 298 K, providing perhaps the most reliable estimate of the rate constant known thus far for such a reaction.

Two‐Step versus One‐Step Spin Transitions in Iron(II) 1D Chain Compounds

AbstractEleven iron(II) 1D coordination polymers with the general formula [FeLeq(Lax)]·solvent were synthesised and characterised, where Leq = (E,E)‐{diethyl 2,2′‐[1,2‐phenylenebis(iminomethylidyne)]bis(3‐oxobutanato) (2‐)‐N,N′,O3,O3′} (L1) and {[3,3′]‐[1,2‐phenylenebis(iminomethylidyne)]bis(2,4‐pentane‐dionato)(2‐)‐N,N′,O2,O2′} (L2); Lax = 4,4′‐bipyridine (bipy), 1,2‐bis(4‐pyridyl)ethane (bpea) and 1,3‐bis(4‐pyridyl)propane (bppa) and solvent = MeOH, EtOH and toluene (Tol). [FeL1(bpea)]·MeOH (3·MeOH) shows an abrupt one‐step spin crossover with thermal hysteresis(27 K) and [FeL2(bppa)]·MeOH (2·MeOH), [FeL2(bpea)] (4), [FeL2(bpea)]·0.25MeOH (4·0.25MeOH) and [FeL1(bipy)]·MeOH (5·MeOH) show a two‐step spin transition with an IP at γHS ≈ 0.5 (IP is intermediate plateau and γHS is high‐spin mol fraction) and up to 50 K wide hysteresis loops (5·MeOH). [FeL1(bppa)] (1), [FeL2(bppa)]·EtOH (2·EtOH) and [FeL1(bpea)]·1.5Tol (3·1.5Tol) show an abrupt incomplete spin transition that stops at γHS ≈ 0.5; for [FeL1(bppa)]·0.25MeOH (1·0.25MeOH) and [FeL1(bpea)] (3), the spin transition is gradual and incomplete. The X‐ray crystal structures of six complexes were determined (1, 2·MeOH, 3·MeOH, 3·1.5Tol, 4·0.25MeOH and 5·MeOH). In the case of 4·0.25MeOH, the crystal structures for the HS and LS states were dertermined; for compounds 1, 2·MeOH and 3·1.5Tol, the crystal structure of the HS and the IP state was investigated. For all complexes, the iron(II) centre is located in a distorted octahedral coordination sphere. Each axially coordinated ligand “connects” two iron(II) centres, which results in the formation of extended 1D chains with varying structures from linear (bipy) over steplike (bpea) to zigzag (bppa). Analysis of the intermolecular interactions reveals that the hysteresis width depends on both the stiffness of the axial ligand and the number of intermolecular contacts, while zigzag chains support stepwise spin transitions.

Coupled Crystallographic Order–Disorder and Spin State in a Bistable Molecule: Multiple Transition Dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H(4)L, has led to an Fe(II) spin-crossover (SCO) complex, [Fe(H(4)L)(2)][ClO(4)](2)⋅H(2)O⋅2 (CH(3))(2)CO (1), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T(0.5) of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high-spin state may also be thermally trapped, exhibiting a very large T(TIESST) (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO(4)(-) and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe-N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order-disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising-like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.

N,N′‐Bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenylenediamines: A Tuneable Ligand Scaffold for Room Temperature Fe2+ SCO Complexes

AbstractCondensation and subsequent reduction of 2,2′‐diaminobiphenyls 5 with 6′‐ and 5′‐substituted 6‐carbaldehyde‐2,2′‐bipyridines 4 yielded N,N′‐bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenyl‐enediamines 7, which were employed as hexadendate ligands with N6 donor sets in the synthesis of dicationic [Fe2+(7‐κ6N)] complexes 8. Dependent on the substitution pattern the respective complexes are found in the HS state (8b and 8c) or show SCO behaviour. By means of temperature‐dependent susceptibility measurements, usingEvans' method, the thermodynamic parameters ΔH, ΔS and T1/2 for the SCO have been determined. T1/2 as well as ΔH are remarkably susceptible to substitution next to the central C–C bond of the biphenyl bridge.

Conformational effect of a spin crossover iron(II) complex: Bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamineiron(II) perchlorate

A iron(II) complex of the linear hexadentate N6 ligand H2L2-3-2, [Fe(H2L2-3-2)](ClO4)2, was synthesized and the spin crossover properties were investigated, where H2L2-3-2 denotes bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamine. The complex showed a gradual and reversible one-step spin crossover (SCO) between the high-spin (S=2) and low-spin (S=0) states at T1/2=208K without hysteresis. The crystal structures were determined at 296 and 250K (HS state), 230, 210, and 200K (intermediate between the HS and LS states) and 150 and 110K (LS state). The spin transition from 296 to 150K accompanies with the conformation change of the chelate rings at the triamine moiety and the formation of the hydrogen bond network in the same space group of orthorhombic Pbcn (no. 60). However, in the LS state at 110K, the space group changed from orthorhombic Pbcn at 150K (Pcan when the same axial setting to 110K was used) to monoclinic P21/a (no. 14) at 110K, although no spin transition and no change of assembly structure between 150 and 110K were observed. It give us an idea that the space group transformation is mainly related to the conformational thermodynamic stability of the chelate rings at the triamine moiety and is not directly correlated with the spin transition.

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.

Spin states of Co ions inLa1.5Ca0.5CoO4from first principles

The spin states and electronic structure of layered perovskite La1.5Ca0.5CoO4 are investigated using fullpotential linearized augmented plane-wave method. All the computational results indicate that the Co2+ ion is in a high-spin state and the Co3+ in a low-spin state. The Co2+ t2g orbitals with a small crystal-field splitting are mixed by spin-orbit coupling, which accounts for the observed easy in-plane magnetism. The nonmagnetic LS-Co3+ state, which is stabilized by a strong crystal field, provides a natural explanation for the observed low magnetic ordering temperature and a spin-blockade phenomenon of the electron hopping. Furthermore, we find that the intermediate-spin state of Co3+ has a large multiplet splitting. But the lowest-lying IS state of Co3+ is still higher in energy than the LS ground state by a few hundred millielectron volts and the HS state of Co3+ is even less stable, both in sharp contrast to a recent experimental study which suggested the HS+IS mixed Co3+ ground state. We note that either the IS-Co3+ or HS-Co3+ states or their mixture would produce a wrong out-of-plane magnetic anisotropy and a much higher magneticordering temperature than observed. Thus, the present work sheds light on this material concerning its electronic and magnetic structure, and it would stimulate different experiments to settle this intriguing spin-state issue.

Modification of Symmetric H State Lifetime in Splayed Nematic Cells

We investigate the transformation processes of splayed nematic cells. Experimental results reveal that polymer networks modify the lifetime of the symmetric H (Hs) state of splay nematic cells. Polymer networks that polymerize with voltage application suppress the lifetime of the Hs state, speeding up the formation of the Ha state. In contrast, polymer networks that polymerize without voltage application markedly enhance the lifetime of the Hs state. Under a 7 V bias voltage, the lifetime of the Hs state in a polymer-stabilized splayed nematic cell is ∼600 ms, which is ∼4 times that in a pristine splayed nematic cell.

Case Study on a Rare Effect: The Experimental and Theoretical Analysis of a Manganese(III) Spin-Crossover System

The six-coordinated mononuclear manganese(III) complex [Mn(5-Br-sal-N-1,5,8,12)]ClO(4) has been synthesized and isolated in crystalline form. Magnetic measurements and variable-temperature single-crystal X-ray crystallography corroborated with theoretical analysis provided firm evidence for the spin-crossover effects of this system. The monomeric complex cations are made by a hexadentate mixed-donor Schiff base ligand imposing a distorted octahedral geometry and subtle structural effects determining the manifestation of the variable spin properties of the manganese(III) centers. The spin crossover in [Mn(5-Br-sal-N-1,5,8,12)]ClO(4) has resulted in an unprecedented crystallographic observation of the coexistence of high-spin (HS; S = 2) and low-spin (LS; S = 1) manganese(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct manganese centers are both HS. Only one of the two slightly different units undergoes spin crossover in the temperature range ∼250-50 K, whereas the other remains in the HS state down to 50 K. The density functional theory calculations, performed as relevant numerical experiments designed to identify the role of orbital and interelectron effects, revealed unedited aspects of the manganese(III) spin-conversion mechanisms, developed in the conceptual frame of ligand-field models.

Spin crossover in theCsFeII[CrIII(CN)6]Prussian blue analog: Phonons and thermodynamics from hybrid functionals

Solid-state lattice-dynamics calculations within the hybrid density-functional approach are applied to the study of the thermally induced ${\text{Fe}}^{2+}$ $\text{low}\text{ }\text{spin}(\text{LS};S=0)\ensuremath{\leftrightarrow}\text{high}\text{ }\text{spin}(\text{HS};S=2)$ crossover (SCO) in the extended network of the $\text{CsFe}[\text{Cr}{(\text{CN})}_{6}]$ Prussian blue analog. The variations in the thermodynamic parameters defining the SCO transition with the Fock exchange content $({F}_{0})$ of the functional are obtained and discussed, where, in keeping with the findings of previous studies of isolated complexes, it is found that an admixture ${F}_{0}\ensuremath{\approx}14\mathrm{%}$ provides reliable values. The transition is shown to be dominated by the entropy difference, $\ensuremath{\Delta}S$, associated with the softening of low-frequency vibrational (vib) modes in the HS state, as has been suggested previously for a wide range of SCO materials, more than half of $\ensuremath{\Delta}{S}_{\text{vib}}$ deriving from modes with wave numbers of $250\text{ }{\text{cm}}^{\ensuremath{-}1}$ or less. Analysis of the influence of the spectroscopic selection rules upon the apparent SCO thermodynamics reveals that determinations based solely upon infrared or Raman frequencies, or upon their combination, lead to significant errors. The effect upon the SCO transition of the electronic entropy associated with the degenerate ${\text{Fe}}^{2+}$ HS $({e}_{\text{g}}^{2}{t}_{2\text{g}}^{4})$ configurations is also detailed, evidence for the existence of an associated dynamic Jahn-Teller distortion being presented. Optimized structures, bulk moduli, $\ensuremath{\Gamma}$-point vibrational frequencies, and crystal-field energy models are discussed for all relevant spin states.

Quantum chemical study of Co3+ spin states in LaCoO3

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO3. The main concern is to calculate the energy level ordering of different spin states of Co3+, which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T = 5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S = 0) ground state is separated from the first excited high-spin (HS, S = 2) state by a gap <100 meV, while the intermediate-spin (IS, S = 1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co3+ ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200 $\div$ 300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors.

Pressure-induced spin transition in FeCO3-siderite studied by X-ray diffraction measurements

We have collected synchrotron X-ray diffraction patterns of FeCO3-siderite after or in-situ laser heating at high pressures to 66 GPa. Diffraction peaks of FeCO3 in all diffraction patterns obtained can be indexed as a trigonal cell. However, calculated cell volumes show an abrupt decrease (about 6.5%) between 47 and 50 GPa at room temperature. This abrupt change of the cell volume on FeCO3 is possibly due to a pressure-induced spin transition of ferrous Fe (HS: high-spin → LS: low-spin). Because cell parameters obtained at high temperature and at pressures above 50 GPa suggest HS state rather than LS state, the Clapeyron slope of the HS-to-LS transition of FeCO3 should be positive.

Multiple Bistability and Tristability with Dual Spin-State Conversions in [Fe(dpp)2][Ni(mnt)2]2·MeNO2

The multicomponent system of [Fe(dpp)(2)][Ni(mnt)(2)](2).MeNO(2) (1; dpp = 2,6-bis(pyrazol-1-yl)pyridine and mnt = maleonitriledithiolate) was prepared by the reaction of [Fe(dpp)(2)](BF(4))(2) with (Bu(4)N)[Ni(mnt)(2)] in MeNO(2). Variable-temperature X-ray structural analyses, magnetic susceptibility, and heat capacity measurements confirmed that 1 undergoes multiple spin-state conversions in both the cationic and anionic components. The asymmetric unit in the crystal contains one [Fe(dpp)(2)](2+) cation, two [Ni(mnt)(2)](-) anions ([Ni1](-) and [Ni2](-)), and one solvent molecule. Magnetic susceptibility measurements revealed that a paramagnetic state in the high-temperature region (HT phase) was abruptly converted to a diamagnetic low-temperature (LT) phase below 180 K as the temperature was lowered from 270 K. As the temperature was raised from 125 to 270 K, successive phase transitions occurred to the HT phase via intermediate phases (IM1, IM2, and IM3) at 175.5, 186.5, 194.0, and 244.0 K, respectively. In the HT phase [Fe(dpp)(2)](2+) is in the high-spin state, and each [Ni1](-) and [Ni2](-) moiety is arranged in monomeric form with an S = (1)/(2) spin ground state. In the LT phase [Fe(dpp)(2)](2+) is in the low-spin state and the nickel moieties are dimerized and diamagnetic. In the IM1 and IM2 phases the iron(II) sites are partially in the HS state and both [Ni](-) moieties are dimeric, as suggested by (57)Fe Mossbauer measurements. In the IM3 phase, [Fe(dpp)(2)](2+) is in the HS state and the anions exist in both their monomeric ([Ni1](-)) and dimeric ([Ni2](-)) forms. Rapid thermal quenching from 300 to 5 K yielded a metastable HS phase, which relaxed to the LT phase via the IM1 phase as the temperature was raised to 150 K. A partial light induced spin transition on the iron site was observed at 5 K.

One-Dimensional Spin-Crossover Iron(II) Complexes Bridged by Intermolecular Imidazole−Pyridine NH···N Hydrogen Bonds, [Fe(HLMe)3]X2 (HLMe = (2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine; X = PF6, ClO4, BF4)

2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine (abbreviated as HL(Me)) is the 1:1 condensation product of 2-methyl-4-formylimidazole and 2-aminoethylpyridine and functions as a bidentate ligand to the iron(II) ion to produce the 3:1 complexes together with anions, [Fe(HL(Me))(3)]X(2) (X = PF(6) (1), ClO(4) (2), BF(4) (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and Mossbauer spectral measurements demonstrated that complexes 1, 2, and 3 showed a steep one-step spin crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states with small thermal hysteresis. Three complexes have an isomorphous structure and are crystallized in the same monoclinic space group, C2/c, both in the HS and LS states. The iron(II) ion has the octahedral coordination geometry of a facial isomer with N(6) donor atoms of three bidentate ligands, in which an imidazole and an imine nitrogen atom per ligand participate in the formation of the coordination bond, but the pyridine nitrogen is free from coordination. The complex cation fac-[Fe(HL(Me))(3)](2+) is a chiral species with a Delta or Lambda isomer, and the adjacent Delta and Lambda isomers are linked alternately by an intermolecular imidazole-pyridine NH...N hydrogen bond to produce an achiral 1D chain. The two remaining imidazole moieties per complex are hydrogen-bonded to the anions that occupy the space among the chains. The SCO profile becomes steeper with the decrease of the anion size (73.0 A(3) for PF(6)(-), 54.4 A(3) for ClO(4)(-), and 53.4 A(3) for BF(4)(-)). The SCO transition temperature T(1/2) of the PF(6) (1), ClO(4) (2), and BF(4) (3) salts estimated from the magnetic susceptibility measurements are (T( downward arrow) = 151.8 K, T( upward arrow) = 155.3 K), (T( downward arrow) = 184.5 K, T( upward arrow) = 186.0 K), and (T( downward arrow) = 146.4 K, T( upward arrow) = 148.2 K), respectively, indicating that the T(1/2) value is not in accord with the anion size.

The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients

Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs) show a positive correlation between the X-ray flux at which the low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the following HS state. By analyzing the data from the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux after the source reaches its HS flux peak still correlates with the transition flux during soft X-ray transient (SXT) outbursts. By studying large outbursts or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition, respectively. These time scales correspond to the viscous time scale in a standard accretion disk around a stellar mass black hole or a neutron star at a radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion flow either correlate over a large range of radii at a given time or correlate over a long period of time at a given radius. If the accretion geometry is a two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow in the LH state, the disk flow with a radius up to ~105 Rg would have contributed to the nearly instantaneous non-thermal radiation directly or indirectly, and therefore affects the time when the state transition occurs.

Thermo- and photoswitchable spin-crossover nanoparticles of an iron(ii) complex trapped in transparent silica thin films

The elaboration and study of hybrid nanocomposites based on photoswitchable spin-crossover nanoparticles is reported. A silica polymeric gel is used as the confining medium to control the kinetics of nucleation and growth of a molecular spin-crossover prototype [Fe((mepy)(3)tren)](PF(6))(2). The precipitation of nanoparticles in the matrix is triggered by spin-coating of the doped gel on a convenient substrate. This process leads to spherical particles of controlled size from 730 (+/- 80) to 47 (+/- 10) nm homogeneously dispersed in transparent silica thin films. The chemical integrity of the coordination compound is checked by EDS and Raman spectroscopies. UV-Vis measurements confirm the persistence of a spin-crossover regime for these nanocomposites. Indeed, the MLCT absorption features are typical of the molecules being in the high-spin state at high temperature and in the low-spin state at low temperature. With respect to the microcrystalline parent compound, the spin-crossover curves afforded by the nanoparticles do not significantly vary over the explored size range. In addition, they are strongly shifted toward lower temperatures. This feature is accounted for by the in-silica formation of a quenched product which behaves like a new phase generated by sudden precipitation of [Fe((mepy)(3)tren)](PF(6))(2). Indeed the precipitated bulk phase, characterized by powder XRD, exhibits both magnetic and optical characteristics very close to those of the nanoparticles. The photoswitching properties based on light-induced excited spin-state trapping (LIESST) are probed by UV-Vis and magnetic measurements. The complexes embedded in silica thin films can be efficiently photoexcited and evidences are provided for the formation of a metastable HS state.

Mononuclear Bis(tridentate)-Type and Dinuclear Triple Helicate Iron(II) Complexes Containing 2-Ethyl-5-methylimidazole-4-carbaldehyde Azine

Abstract Mononuclear [Fe(H2L2-Et-5-Me)2]2+ and dinuclear [Fe2(H2L2-Et-5-Me)3]4+ complexes, where H2L2-Et-5-Me denotes 2-ethyl-5-methylimidazole-4-carbaldehyde azine, were prepared and isolated as the perchlorate salts, and their structures and magnetic properties were studied. In the mononuclear complex, the ligand acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom. The complex was in the HS state above 50 K. In the dinuclear complex, each ligand acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate structure. Two types of crystals, plates and blocks, were isolated for the dinuclear complex. The plate crystals exhibited a sharp spin transition, [LS–HS] ↔ [HS–HS], with no [LS–LS] state being observed. The block crystals remained in the [HS–HS] state above 50 K.