Redox-flow batteries (RFBs) are promising electrochemical energy storage devices to load-level intermittent power from renewable energy. In particular, aqueous RFBs using aqueous electrolytes possess several advantages over nonaqueous ones, such as low fabrication cost, nontoxicity, safety, and environmental benignity. Therefore, developing high-performance, abundant, less-expensive iron-based catholytes for aqueous RFBs is essential toward their wide deployment in a power grid. In this Perspective, we summarize the recent progress of iron-based catholytes for aqueous RFBs. We emphasize that iron-based catholytes possess widely ranged redox potentials (−1.0 to 1.5 V vs standard hydrogen electrodes) and solubility in water (0.2–4.0 mol L−1), thereby providing a wide range of cell performance. The molecular design, such as ligand functionalization, counter ion mixing, and asymmetrization, allows for rationally improving solubility, redox potential, and energy density. Furthermore, we demonstrate a simple evaluation method of the redox potential of iron-based catholytes using the calculated energy levels of the lowest unoccupied molecular orbital of ligand molecules. Finally, we rationalize the design strategy of iron-based catholytes for advanced aqueous RFBs.

1,2-Dithiooxalate (dto) can be employed as a bridging ligand and gives symmetric (O,S-chelation) or asymmetric (O,O- and S,S-chelation) coordination modes. In this study, we have prepared a novel dto-bridged diiron(II)...

Multiple proton transfer (PT) controllable by external stimuli plays a crucial role in fundamental chemistry, biological activity, and material science. However, in crystalline systems, controlling multiple PT, which results in a distinct protonation state, remains challenging. In this study, we developed a novel tridentate ligand and iron(II) complex with a short hydrogen bond (HB) that exhibits a PT-coupled spin transition (PCST). Single-crystal X-ray and neutron diffraction measurements revealed that the positions of the two protons in the complex can be controlled by temperature and photoirradiation based on the thermal- and photoinduced PCST. The obtained results suggest that designing molecules that form short HBs is a promising approach for developing multiple PT systems in crystals.

Molecular-basedmagnetoelectric materials are among themost promisingmaterials for next-generation magnetoelectric memory devices. However,practical application of existing molecular systems has proven difficultlargely because the polarization change is far lower than the practicalthreshold of the ME memory devices. Herein, we successfully obtainedan [FeCo] dinuclear complex that exhibits a magnetic field-inducedspin crossover process, resulting in a significant polarization changeof 0.45 & mu;C cm(-2). Mo''ssbauer spectroscopyand theoretical calculations suggest that the asymmetric structuralchange, coupled with electron redistribution, leads to the observedpolarization change. Our approach provides a new strategy toward rationallyenhancing the polarization change.

A mixed-valence iron(II,III) coordination polymer, (Ph₄P)[Fe^IIFe^III(dto)₃] (2; Ph₄P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), exhibits a thermal hysteresis loop and low-temperature shift of the ferromagnetic phase transition temperature, with an increasing...

Using light as a local heat source to induce a temporary pyroelectric current is widely recognized as an effective way to control the polarization of crystalline materials. In contrast, harnessing light directly to modulate the polarization of a crystal via excitation of the electronic bands remains less explored. In this study, we report an Fe-II spin crossover crystal that exhibits photoinduced macroscopic polarization change upon excitation by green light. When the excited crystal relaxes to the ground state, the corresponding pyroelectric current can be detected. An analysis of the structures, magnetic properties and the Mossbauer and infrared spectra of the complex, supported by calculations, revealed that the polarization change is dictated by the directional relative movement of ions during the spin transition process. The spin transition and polarization change occur simultaneously in response to light stimulus, which demonstrates the enormous potential of polar spin crossover systems in the field of optoelectronic materials.

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

The development and the photophysical behavior of a transparent ion-exchange membrane based on a pH-sensitive polypyridyl ruthenium(II) complex, [(bpy)2RuII(H2bpib)RuII(bpy)2](ClO4)4 (bpy = 2,2'-bipyridine, H2bpib = 1,4-bis([1,10]phenanthroline[5,6-d]-imidazol-2-yl)benzene), are experimentally and theoretically reported. The emission spectra of [(bpy)2RuII(H2bpib)RuII(bpy)2]@Nafion film were observed between pH 2 and pH 11 and showed the highest relative emission intensity at pH 5 (λmaxem = 594.4 nm). The relative emission intensity of the film significantly decreased down to 75% at pH 2 and 11 compared to that of pH 5. The quantum yields (Φ) and lifetimes (τ) showed similar correlations with respect to pH, Φ = 0.13 and τ = 1237 ns at pH 5, and Φ = 0.087 and τ = 1014 ns and Φ = 0.069 and τ = 954 ns at pH 2 and pH 11, respectively. These photophysical data are overall considerably superior to those of the solution, with the radiative- (kr) and non-radiative rate constants (knr) at pH 5 estimated to be kr = 1.06 × 105 s-1 and knr = 7.03 × 105 s-1. Density functional theory calculations suggested the contribution of ligand-to-ligand- and intraligand charge transfer to the imidazolium moiety in Ru-H3bpib species, implying that the positive charge on the H3bpib ligand works as a quencher. The Ru-Hbpib species seems to enhance non-radiative deactivation by reducing the energy of the upper-lying metal-centered excited state. These would be responsible for the pH-dependent "off-on-off" emission behavior.

One of the most attractive research in the field of molecular solids is multifunctional molecular magnets coupled with spin, charge and photon, and single-molecular magnet/single-chain magnet toward spintronics. In this chapter, we focus on the molecular magnetism and its related light-induced phase transitions from the viewpoint of Mossbauer spectroscopy. In the Sect. 6.1, the outline of this chapter is described. In the Sect. 6.2, static and dynamic spin crossover phenomena between the high-spin and low-spin states, and the spin frustration system induced by dynamic spin crossover phenomena for [MnIIFeIII(C2O3S)(3)] complex are described. In the Sect. 6.3, metal complexes showing charge transfer phase transitions such as Prussian blue analogous complexes and [FeIIFeIII(C2O2S2)(3)] complexes and their photoinduced phase transitions are described. In the Sect. 6.4, various kinds of molecular magnets including single molecular chain magnets are described.

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and yr electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

To investigate the interactions of Fe-57 atoms with sulfur hexafluoride SF6 molecules, in-beam Mossbauer spectra were measured for Mn-57 implanted SF6 at 9 K and 65 K. Isolated Fe-57 atoms or ions produced by beta-decay were not trapped within the SF6 matrix. At 9 K, monomeric FeF2 and FeF3 molecules were produced by reaction of Fe-57 with F atoms released by decomposition of the SF6 molecules. When the temperature of the SF6 solid was increased to 65 K, FeF4 was formed in addition to FeF2 and FeF3. Density functional calculations were performed to confirm the assignments of the candidate species identified in the Mossbauer spectra.

<p>Biradicaloid HDIP derivatives show that the Δ<italic>E</italic>_S–T gap does not follow the reverse trend of the biradical character but depends more on the delocalization of the radical centres at the outer rings.</p>

<p>Finely size-controlled ultrasmall iron oxide particles (IOPs) on graphene nanosheets were usefully prepared by using pulsed arc plasma deposition. The IOPs are composed of a high crystallinity single γ-Fe₂O₃ component without annealing.</p>

<p>A two-dimensional honeycomb layer composed of tetraoxolene-bridged iron subunits whose charge-ordered states are multiply variable <italic>via</italic> thermal treatments and solvation/desolvation is present.</p>

The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp-, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10-8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

© 2019, Springer Nature Switzerland AG. Dilute Fe and Nb codoped SrTiO3-δ (STO) perovskites were prepared by a complex formation and thermal decomposition method. The structure, magnetization properties and chemical states of Fe and Nb codoped STO perovskites were characterized by XRD, VSM and Mössbauer spectrometry. Mössbauer spectra of 57Fe and Nb codoped STO perovskites consist of paramagnetic doublet and magnetic relaxation components. The 1% Fe + 5% Nb codoped STO sample showed the smallest lattice constant and the largest ferromagnetism among 0.5–4% Fe + 5% Nb codoped STO perovskites. The most distortion of cubic STO lattice and the largest magnetic relaxation in Mössbauer spectrum were observed for the 0.5% Fe and 5% Nb codoped STO perovskite. It is considered that diluted magnetism in this system is related with the deformation of cubic structure due to defects.

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

We successfully prepared and crystallographically characterized the first intermolecular charge-transfer (CT)-based vapochromic compound, (EV)(H3O)2[Fe(CN)6] (1-Wet, EV2+: 1,1′-diethyl-4,4′-bipyridine-1,1′-diium), an ethyl viologen-containing CT salt. 1-Wet, which is purple in color, is transformed into a brown powder (1-Dry) upon exposure to methanol vapor, drying over silica gel, or heating
1-Dry returns to 1-Wet upon exposure to water vapor. These color changes are induced by hydration and dehydration, and gravimetric analyses suggest that 1-Dry is the dehydrated form of 1-Wet, namely, (EV)(H)2[Fe(CN)6]. Interestingly, desorption of water molecules from the oxonium ions in 1-Wet produces isolated protons (H+) that remain in 1-Dry as counter cations. Powder X-ray crystal structure analysis of 1-Dry reveals the presence of very short contacts between the nitrogen atoms of adjacent [Fe(CN)6]4- anions in the crystal. The isolated protons are trapped between the nitrogen atoms of cyanido ligands to form very short N···H···N hydrogen bonds. A detailed comparison of the crystal structures of 1-Wet and 1-Dry reveals that hydration and dehydration induce changes in crystal packing and intermolecular CT interactions, resulting in reversible color changes.

A molecular salt with a bulky cation and anion, (H2bipip)-(H)2[Fe(CN)6] (1) (H2bipip2+: 4,4¤-bipiperidine-1,1¤-diium), has been prepared. The salt comprises protons (H+) as counter cations. Single-crystal X-ray structure analysis revealed that the proton is trapped between two cyanido ligands of [Fe(CN)6]41 anions, forming a very short N£H+£N hydrogen bond.

&lt;B&gt;[Back Cover]&lt;/B&gt;

The iron mixed-valence complex (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)] (dto = C2O2S2) exhibits a novel type of phase transition according electron transfer between FeII and FeIII called charge transfer phase transition (CTPT). The existence of the CTPT highly depends on the 3d metal substitution with Zn, Mn, Cr, or Co for corresponding Fe site in (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)]. Here, we prepared a series of magnetically diluted complexes (n-C3H7)(4)N[Fe(II)Fe(III)1-xV(III)x(dto)(3)] (x = 01), and carried out investigation of magnetic properties. Judging from the magnetic susceptibility measurement, there is no suppression of the CTPT unlike other 3d metal substituted cases. Moreover, 57Fe Mossbauer spectroscopy revealed that about 60% of FeII sites changed their spin states from high spin to low spin in case of substitution rate for x = 1.00. Since the difference of hardness and softness between metallic ions (Fe-II, Fe-III and V-III) and coordination sites of ligand (O or S in dto), this behavior is explained by the linkage isomerization of bridging ligand in formation process of the complex. (C) 2017 Elsevier Ltd. All rights reserved.

Manganese (Mn)-doped black iron oxide (Fe3O4) magneticuids in the system of MnxFe(3-x)O(4) were successfully synthesized from natural magnetite (iron sand) by using co-precipitation method at room temperature. The analyses of the small angle neutron scattering (SANS) data by applying a log-normal sphere with a mass fractal models for x = 0 and x = 0: 25 and two lognormal spheres with a single mass fractal models for x 0: 5, 0.75 and 1 revealed that the primary particles of the MnxFe(3-x)O(4) fluids tended to decrease from 3.8 nm to 1.5nm along with the increasing fraction of Mn contents. The fractal dimension (D) increased from about 1.2 to 2.7 as the Mn contents were increasing; which physically represents an aggregation of the MnxFe(3-x)O(4) particles in the uids growing up from 1 to 3 dimensions to consolidate a more compact structure. The magnetization curves of the fluids exhibited an increasing saturation magnetization from x = 0 to x = 0: 25, and a decreasing on x = 0.5 and 0.75, with the maximum achievement of x = 1. These phenomena may probably be due to the combined e r ects, arising from cationic and dopant distributions, aggregation and its size, and also fractal dimension. Furthermore, the decrease of blocking temperature of the MnxFe(3-x)O(4) magneticuids could be associated with the reduced particle sizes, while the freezing temperature had its highest peak intensity when it collectively occurred with the blocking temperature at a similar point of about 270 K.

Four Fe(III) spin-crossover (SCO) compounds, [Fe(HL1)2](HCl4TPA) (1-Cl), [Fe(HL1)2](HBr4TPA) (1-Br), [Fe(HL2)2](HCl4TPA) (2-Cl), and [Fe(HL2)2](HBr4TPA) (2-Br) (HL1 = 4-chloro-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate
HL2 = 4-bromo-2-nitro-6-(1-(2- (pyridine-2-yl)hydrazono)ethyl)phenolate
HCl4TPA = 2,3,5,6-tetrachloro-4-carboxybenzoate
and HBr4TPA = 2,3,5,6-tetrabromo-4-carboxybenzoate), were synthesized to investigate the halogen substituent change effect in salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids in SCO complexes to the spin-transition temperature. Crystal structure analyses showed that these compounds were isostructural. In addition, a one-dimensional hydrogen-bonded column was formed by the dicarboxylic acid anion and weak hydrogen bonds between the Fe(III) complexes. From Mössbauer spectroscopy and magnetic property measurements, these compounds were confirmed to exhibit gradual SCO. The spin-transition temperature can be shifted by changing the halogen substituent in the salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids without changing the molecular arrangement in the crystal packing.

The spin-crossover (SCO) phenomenon between a high-spin and a low-spin state has attracted much attention in the field of materials science. Among the various kinds of SCO complexes, the triazole-bridged iron(II) polymeric chain system, [Fe(II)(R-trz)3]X2.xH2O (where trz is triazole and X is the anion), exhibiting the SCO phenomenon with thermal hysteresis around room temperature, has been extensively studied from the viewpoint of molecular memory and molecular devices. In connection with this system, we have controlled the SCO phenomenon according to the characteristic properties of counter ions. In the case of X being CnH2n+1SO3 -, the spin transition temperature (T1/2) increases with increasing the length (n) of the alkyl chain of the counter ion and saturates above n = 5, which is attributed to the increase in the intermolecular interaction of the alkyl chains of CnH2n+1SO3 -, called the fastener effect. The hysteresis width of T1/2 decreases with increasing n, showing the even-odd, also known as parity, effect. In the cases where X is toluenesulfonate (tos: CH3C6H4SO3 -) and aminobenzenesulfonate (abs: NH2C6H4SO3 -), T1/2 and its hysteresis width vary drastically with the structural isomerism (ortho-, metha-, and para-substitution) of counter ions, which implies the possibility of photoinduced spin transition by means of the photoisomerization of counter ions. From this strategy, we have synthesized [Fe(II)(NH2-trz)3](SP150)2·2H2O (SP150 = N-alkylsulfonated spiropyran) and investigated the SCO phenomenon. Moreover, we have developed [Fe(II)(R-trz)3]@Nafion films exhibiting spin transition around room temperature, where the Nafion membrane behaves as a counter anion as well as a transparent substrate, and investigated the photogenerated high-spin state below 35 K. The lifetime of the photogenerated high-spin state strongly depends on the intensity of irradiated light.

To develop room-temperature molecule-based magnets, metal-radical approach is one of the most promising strategies. Strong magnetic interactions beyond a thermal energy of room temperature are essential for achieving such coveted materials. We have developed 2-pyridyl tert-butyl nitroxide (2pyNO) derivatives as a chelating radical ligand with a copper(II) or nickel(II) ion. The torsion angle (φ) around M-O-N-C2py is a useful metric in describing correlation between the structure and metal-radical exchange coupling 2J (defined as H = -2JΣSi •Si +1). Although we previously reported such a magneto-structural correlation, in this paper we updated the plot of 2J versus φ by dividing into two categories: the hexacoordinatenickel(II) and copper(II) complexes. An empirical linear correlation was formulated for both Cu and Ni categories, as 2J/k B = a + b|φ| with a = 503(112) K and b = -36(3) K deg-1 and with a = 449(46) K and b = -21.0(17) K deg-1, respectively. In the latter plot we confirm that the relationship can be generalized for the oxoverdazyl and dithiadiazolyl systems as well as the 2pyNO systems. The results are supported by the calculations based on the density functional theory.

A synchrotron small-angle X-ray scattering (SAXS) study on PVA/Fe3O4 magnetic hydrogels has been performed to investigate the effect of clustering on their magnetic properties. The hydrogels were prepared through freezing-thawing (F-T) processes. The structure, morphology and magnetic properties of magnetite (Fe3O4) nanoparticles (NPs) were investigated using X-ray diffractometry (XRD), transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer, respectively. In this study, SAXS data were used to reveal the structural dimensions of the magnetite and its distribution in the polymer-rich PVA and magnetic hydrogels. As calculated using the Beaucage and Teubner-Strey models, the average of the structural dimensions of the PVA hydrogels was 3.9 nm (crystallites), while the average distance between crystallites was approximately 18 nm. Further analysis by applying a two-lognormal distribution showed that the magnetite NPs comprised secondary particles with a diameter of 9.6 nm that were structured by primary particles (similar to 3.2 nm). A two-lognormal distribution function has also been used in describing the size distributions of magnetite NPs in magnetic hydrogels. The clusters of magnetite NPs in the magnetic hydrogels are significantly reduced from 30.4 nm to 12.8 nm with decreasing concentration of the NPs magnetite from 15 wt.% to 1 wt.%. The saturation magnetization values of the magnetite NPs, the 15% and 1% magnetic hydrogels were 34.67 emu/g, 6.52 emu/g and 0.37 emu/g, respectively.

A series of Mn (x) Fe3-x O-4 (0 a parts per thousand currency sign x a parts per thousand currency sign 1) nanoparticles was successfully synthesized via a simple coprecipitation method. The starting material was a natural magnetite purified from local iron sand. Crystallite nanoparticles were produced by drying without using a high calcination temperature. Rietveld analysis of the X-ray diffractometry (XRD) data for all samples demonstrated that the Mn ions partially substituted the Fe ions in the spinel cubic structure of the Fe3O4 to form Mn (x) Fe3-x O-4 phases. We applied two lognormal spherical and single mass fractal models to the analysis of the small-angle neutron scattering (SANS) data and revealed that the primary Mn (x) Fe3-x O-4 particles ranged in size from 1.5 to 3.8 nm and formed three-dimensional clusters as secondary structures. The samples displayed superparamagnetic behavior, having the saturation magnetization which was most likely influenced by the competing contribution from Mn, the sizes of the primary particles, and their clusters. Further analysis revealed that the zero-field-cooled and field-cooled curves of the Mn (x) Fe3-x O-4 nanoclusters exhibited a superparamagnetic phenomenon with the lowest magnetic blocking temperature approximately 145 K.

To develop an organic inorganic hybrid photomagnet, we performed the intercalation of sulfonate-substituted spiropyran anions into layered cobalt hydroxides, obtaining the photoresponsive compound, Co(OH)(1.84)(nSPSO(3))(0.16)center dot 0.8H(2)O (CoLHSP; nSPSO(3): 1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-indoline]-5'-sulfonate). After UV irradiation (313 nm), the optical and magnetic properties of CoLHSP clearly changed. A new band emerged in the absorption spectrum at 564 nm, corresponding to the pi-pi* transition in the open form of nSPSO(3). Moreover, the photoisomerization of nSPSO(3) induced a new ferrimagnetic phase transition at 10K in the temperature dependence of DC and AC magnetic susceptibilities in addition to the ferrimagnetic phase transition at 15K for the unirradiated sample. Analyzing the magnetic measurements before and after UV irradiation, we estimated the phototransformation ratio of the magnetic phase in CoLHSP at approximately 40%.

Mg oxides doped with 1 % Fe-57 were prepared by a sol-gel method, and annealed at various temperatures. Nano-size Mg oxides were characterized by Mossbauer spectrometry, magnetization and XRD measurements. The crystalline size of MgO increases with increase of annealing temperature. Samples annealed at 600 A degrees C and 800 A degrees C gave only doublet peaks of paramagnetic Fe3+ in Mossbauer spectra although Fe3+ doping into MgO induced a distorted structure and showed weak ferromagnetism. It is considered that the magnetic property is due to defect induced magnetism by doping Fe3+ into MgO. For a sample heated at 1000 A degrees C, it is found from low temperature Mossbauer spectra that Fe3+ species are located at the core and shell of fine MgFe2O4 grains and diluted in MgO matrix.

For fabricating photoresponsive multifunctional materials, we have developed the potassium salt of a new anionic spiropyran, 1',3',3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2,2'-indoline]-5'-sulfonate (1(-)), with the sulfonate group present on the indoline moiety and whose crystal structure was revealed by single-crystal X-ray diffraction analysis. The photochromism of 1.K was demonstrated at room temperature in the KBr-diluted state, as well as in solution. The protonated spiropyran 1H shows negative photochromism.

The solvent-free spin crossover iron(III) complex [Fe-III(Him)(2)(hapen)]AsF6 (Him = imidazole, H(2)hapen = bis(2-hydroxyacetophenylidene)ethylenediamine), exhibiting thermal hysteresis, was synthesized and characterized. The Fern ion has an octahedral coordination geometry, with N2O2 donor atoms of the planar tetradentate ligand (hapen) and two nitrogen atoms of two imidazoles at the axial positions. One of two imidazoles is hydrogen-bonded to the phenoxo oxygen atom of hapen of the adjacent unit to give a hydrogen-bonded one-dimensional chain, while the other imidazole group is free from hydrogen bonding. The temperature dependencies of the magnetic susceptibilities and Mossbauer spectra revealed an abrupt spin transition between the high-spin (S = 5/2) and low-spin (S = 1/2) states, with thermal hysteresis.

The first iron complexes of high-spin iron(II) species directly coordinated to verdazyl radicals, [Fe-11(vdCOO)(2)(H2O)(2)].2H(2)O (1; vdCOO(-) = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate) and [Fe-11(vdCOO)(2)(D2O)(2)].2D(2)O (2), were synthesized. The crystal structure of 1 was investigated by single-crystal X-ray diffraction at room temperature and at 90 K. The compound crystallizes in the P1 space group with no phase transition between 300 and 90 K. The crystals are composed of discrete [Fe-11(vdCOO)(2)(H2O)(2)] complexes and crystallization water molecules. In the complex, two vdCOO(-) ligands coordinate to the iron(II) ion in a head-to-tail arrangement and two water molecules complete the coordination sphere. The Fe-X (X = O, N) distances vary in the 2.069-2.213 A range at 300 K and in the 2.0679-2.2111 A range at 90 K, indicating that the iron(II) ion is in its high-spin (HS) state at both temperatures. At 300 K, one of the coordinated water molecules is H-bonded to two crystallization water molecules whereas the second one appears as loosely H-bonded to the two oxygen atoms of the carboxylate group of two neighboring complexes. At 90 K, the former H-bonds remain essentially the same whereas the second coordinated water molecule reveals a complicated behavior appearing simultaneously as tightly H- bonded to two oxygen atoms and non-H-bonded. The Fe-57 Mossbauer spectra, recorded between 300 K and 10 K, give a clue to this situation. They show two sets of doublets typical of HS iron(II) species whose intensity ratio varies smoothly with temperature. It demonstrates the existence of an equilibrium between the high temperature and low temperature forms of the compounds. The solid-state magic angle spinning H-2 NMR spectra of 2 were recorded between 310 K and 193 K. The spectra suggest the existence of a strongly temperature- dependent motion of one of the coordinated water molecules in the whole temperature range. Variable- temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction (J(Fe-vd) = -27.1 cm(-1); H = -J(ij)S(i)S(j)) of the HS iron(II) ion and the radical spins with high g(Fe) and D-Fe values (g(Fe) = 2.25, D-Fe = +3.37 cm(-1)) for the HS iron(II) ion. Moreover, the radicals are strongly antiferromagnetically coupled through the iron(II) center (J(vd-vd) = -42.8 cm(-1)). These last results are analysed based on the framework of the magnetic orbitals formalism.

We have investigated the magnetic properties of a photoresponsive organic inorganic hybrid system, (SP-R)[(FeFeIII)-Fe-II(dto)(3)](SP = spiropyran; R = Me, Et, Pr; dto = C2O2S2), under hydrostatic pressures. At ambient pressure, (SP-Et)[(FeFeIII)-Fe-II(dto)(3)] and (SP-Pr)[(FeFeIII)-Fe-II(dto)(3)] undergo one and two ferromagnetic phase transitions corresponding to the high-temperature phase with the spin configuration of Fe-II(S = 2)-Fe-III(S = 1/2), while (SP-Me)[(FeFeIII)-Fe-II(dto)(3)] undergoes two ferromagnetic phase transitions corresponding to the coexistence of the high-temperature phase and the low-temperature phase with the spin configuration of Fe-II(S = 0)-Fe-III(S = 5/2). Under hydrostatic pressures, (SP-R)[(FeFeIII)-Fe-II(dto)(3)] (R = Et, Pr) undergo a new ferromagnetic phase transition corresponding to the low-temperature phase caused by the pressure-induced charge transfer phase transition. (C) 2013 Elsevier Ltd. All rights reserved.

We applied high-frequency electron paramagnetic resonance to trinuclear 4f-3d heterometallic complexes, [{Ln(hfac)(3)}(2){Ni(dpk)(2)(py)(2)}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter J(Ln-Ni) as well as nickel(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as J(Gd-Ni)/k(B) = +0.301(4) K, J(Tb-Ni)/k(B) = +0.216(12) K, and J(Ho-Ni)/k(B) = +0.110(3) K (defined as -J(Ln-Ni)Sigma J(Ln)(z)S(Ni)).

We synthesized pyrimidine-4,6-diyl bis(tert-butyl nitroxide) (pmbNO), and characterized as a biradical by means of ESR spectroscopy. The zero-field splitting parameters were vertical bar D vertical bar/hc = 2.52 x 10(-2) cm(-1) and vertical bar E vertical bar/hc = 2.34 x 10(-3) cm(-1) in frozen toluene at 100 K. Magnetic study clarified the presence of intramolecular ferromagnetic coupling with 2J/k(B) = +18.5(5) K in frozen toluene-ethanol. DFT calculation supports the ground triplet state of pmbNO. The reaction of pmbNO with Cu(hfac)(2) gave [Cu-3(pmbNO)(2)(hfac)(4)], and the structure was determined by X-ray crystallographic analysis. Though the ligand was reduced to be an anion radical, pmbNO is confirmed to work as a rigid bridge, and the nitroxide oxygen atoms can ligate the metal ions. (C) 2013 Elsevier Ltd. All rights reserved.

Coordination polymers have significant potential for new functionality paradigms due to the intrinsic tunability of both their electronic and structural properties. In particular, octacyanometallate-bridged coordination polymers have the extended structural and magnetic diversity to achieve novel functionalities. We demonstrate that [Mn(H2O)][Mn(HCOO)(2/3)(H2O)(2/3)](3/4)[Mo(CN)(8)]center dot H2O can exhibit electrochemical alkali-ion insertion/extraction with high durability. The high durability is explained by the small lattice change of less than 1% during the reaction, as evidenced by ex situ X-ray diffraction analysis. The ex situ X-ray absorption spectroscopy revealed reversible redox of the octacyanometallate. Furthermore, the solid state redox of the paramagnetic [Mo-V(CN)(8)](3-)/diamagnetic[Mo-IV(CN)(8)](4-) couple realizes magnetic switching.

K4Fe4P5O20 shows an interesting natrolite-like structure with a spin-tetrahedron lattice built by mixed valence Fe ions. Single crystals of the title compound are successfully grown by the flux method using KF as flux. Magnetic results combined from magnetic, heat capacity, and Fe-57 Mossbauer spectra measurements show that K4Fe4P5O20 possesses a short-range magnetic ordering at similar to 13 K and a long-range ordering at similar to 7 K. Magnetic anisotropy of K4Fe4P5O20 is observed between H parallel to c and H perpendicular to c, suggesting that the c-axis is the magnetic easy-axis. The spin arrangements in the system are suggested to be ferrimagnetic along the natrolite chains.

Mg2+ intercalation/deintercalation is achieved by using aqueous electrolytes and Prussian blue analog electrodes. Ex situ X-ray diffraction evidenced the solid solution process of Mg2+ intercalation/deintercalation, while the Fe-57 Mossbauer spectroscopy and X-ray absorption near edge structure revealed redox of both Cu and Fe.

The first stable anionic iron(0) complexes bearing an ate-type pentacoordinated germanium(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge-Fe bonds which can be cleaved by oxidation.

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K0.2Ni[Fe(CN)(6)](0.7)center dot 4.7H(2)O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe3+/Fe2+ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature T-C gradually decreases due to reduction of paramagnetic Fe3+ to diamagnetic Fe2+, and the ferromagnetic transition is completely suppressed for Li-0.6(NiFe-PBA). On the other hand, T-C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe2+ to paramagnetic Fe3+, and the ferromagnetic transition is nearly recovered for Li-0(NiFe-PBA). Furthermore, the plots of T-C as a function of the amount of inserted/extracted Li ion x are well consistent with the theoretical values calculated by the molecular-field approximation.

The iron mixed-valence complex (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe-II and Fe-III occurs reversibly at similar to 120 K, in addition to the ferromagnetic phase transition at T-C = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C3H7)(4)N[(Fe1-xZnxFeIII)-Zn-II-Fe-II(dto)(3)] (dto = C2O2S2; x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and Fe-57 Mossbauer spectroscopy. With increasing Zn-II concentration (x), the CTPT is gradually suppressed and disappears at x approximate to 0.13. On the other hand, the ferromagnetic transition temperature (T-C) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn-II ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn-II concentration. This anomalous dependence of T-C on Zn-II concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.

A new small-pore compound K4Fe4P5O20 was obtained by conventional solid-state reaction in a closed crucible. The crystal structure is constructed by Fe4P5O20 units forming chains along the c axis and elliptical eight-ring channels on the a-b plane in which K cations locate inside. Such structural characteristics seem to be quite similar to those seen in the natrolite family. However, Fe ions in K4Fe4P5O20 have trigonal-bipyramidal instead of common tetrahedral coordination. Furthermore, our experimental results combined from magnetic susceptibility and Fe-57 Mossbauer spectrum measurements show mixed valence Fe3+/Fe2+ in the titled material. To the best of our knowledge, this is the first example that contains mixed valence iron ions in a so-called natrolite framework.

Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K1.72Mn[Mn(CN)(6)](0.93)center dot square(0.07).0.65H(2)O (square: a [Mn(CN)(6)](4-) defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li+. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/discharge experiments revealed that the framework can accommodate a large amount of Li-ions.

Heterometallic pentanuclear coordination compounds [Ln(4)(3+)Cu(2+)] involving light lanthanide ions, Ce, Pr, Nd, Sm, and Eu, were synthesized and structurally characterized. The exchange interactions between the Ln and Cu ions were evaluated by high-frequency electron paramagnetic resonance and magnetization studies, clarifying the presence of ferromagnetic exchange couplings in Nd-Cu (J(Nd-Cu)/k(B) = +0.30(6) K) and Sm-Cu (J(Sm-Cu)/k(B) = +2.08(3) K), very small couplings in Ce-Cu and Pr-Cu, and the absence in Eu-Cu. A significant chemical trend is found in the order of the atomic number. (C) 2012 Elsevier B. V. All rights reserved.

Heterometallic coordination compounds [Cu-II(L)(C3H6O)Ln(III)(NO3)(3)] and [(VO)-O-IV(L)(C3H6O)Ln(III)(NO3)(3)] (abbreviated as LnCu and LnV, respectively; H2L = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane; Ln = Gd, Tb, Dy, Ho, and Er) were synthesized, and the X-ray crystallographic analysis shows that their structures are isomorphous for each series. The single-molecule magnet behavior was observed for TbCu and DyCu, and the activation energies of magnetization reversal were 42.3(4) and 11.5(10) K, respectively. The magnetic exchange couplings in LnCu and LnV were precisely evaluated by means of combined high-frequency EPR and pulsed-field magnetization studies, to give J(Tb-Cu)/k(B) &gt;= 3.3 K, J(Dy-Cu)/k(B) = 1.63(1) K, J(Ho-Cu)/k(B) = 1.09(2) K, and J(Er-Cu)/k(B) = 0.24(1) K. A monotonic decrease of ferromagnetic J(Ln-Cu) was found in the order of the atomic number, Gd-64 to Er-68. The corresponding exchange parameters in LnV are smaller than those of the Cu derivatives, and J(Gd-V) was antiferromagnetic (-3.0 K determined from the magnetization jump). A possible mechanism for the exchange coupling and chemical trend is discussed.

Exchange couplings in isomorphous [LnCu(2)] were evaluated by high-frequency electron paramagnetic resonance and magnetization studies. The exchange parameter J(Ln-Cu) was decreased with an increase in the atomic, number; J(Ln-Cu)/k(B) = 4.45(11), 2.27(6), 0.902(10), 0.334(3), and 0.136(8) K for Ln = Gd, Tb, Dy, Ho, and Er, respectively.

We synthesized and characterized a new compound [{Dy(hfac)(3)}(2)Pd(dpk)(2)] ([Dy(2)Pd]; Hdpk = di-2-pyridyl ketoxime), which is isostructural with the known [Dy(2)Cu] and [Dy(2)Ni]. The study of the [Dy(2)Pd] compound containing diamagnetic Pd ion is indispensable to clarify a contribution of the exchange coupling for a 4f-3d single-molecule magnet behavior. From the ac susceptibility measurements on [Dy(2)Pd], we obtained Delta/k(B) = 28.6(11) K and T(B) = 1.1 K. In accordance with the blocking behavior, the pulsed-field magnetization showed the hysteresis behavior below 1.1 K. These parameters on [Dy(2)Pd] having S(Pd) = 0 was compared with those of the derivatives having S(Cu) = 1/2 (Delta/k(B) = 47 K and T(B) = 1.8 K) and S(Ni) = 1 (Delta/k(B) = 62(4) K and T(B) = 2.5 K). The T(B) and Delta steadily increase with the increasing 3d(4d) spin quantum number. (C) 2011 Elsevier Ltd. All rights reserved.

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy)(4)(X)(2)] (stpy = styrylpyridine, X = NCS, NCBH3) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe(trans-stpy)(4)(NCS)(2)], while [Fe(cis-stpy)(4)(NCS)(2)] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH3, due to the stronger ligand field of NCBH3, the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe(trans-stpy)(4)(NCBH3)(2)]. In the case of [Fe(cis-stpy)(4)(NCBH3)(2)], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65-1.09 GPa where [Fe(stpy)(4)(NCBH3)(2)] undergoes LD-LISC at room temperature. (C) 2011 Elsevier Ltd. All rights reserved.

We newly synthesized transparent films composed of pH-sensitive spin-crossover complex [Fe-II(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.

A CuO-based material Cu5V2O10 was successfully grown in a closed crucible using Sr(OH)(2)center dot 8H(2)O as flux. The structure of Cu5V2O10 can be viewed as being composed of two types of zigzag Cu-O chains running along the b- and c-axes, which shows a two-dimensional crosslike framework with 12-column square tunnels along the a-axis. Magnetic measurements show that Cu5V2O10 exhibits unexpected large magnetic anisotropy, which is the first time magnetic anisotropy energy of similar to 10(7) erg/cm(3) in the CuO-based materials has been observed. The origins of large anisotropy are suggested to arise from strong anisotropic exchanges due to the particular bonding geometry and the Jahn-Teller distortion of Cu2+ ions. Further, the band structure investigated by the GGA+U method suggests that Cu5V2O10 is a semiconductor.

The 4f-3d exchange couplings were definitively and precisely determined in the dinuclear complexes (Ln-M) involving double mu-oxo-bridges, by means of combined high-frequency electron paramagnetic resonance and pulsed-field magnetization techniques, revealing a monotonic decrease of ferromagnetic J(Ln-Cu) in the order of the atomic number, (64)Gd to (68)Er.

Exchange couplings in [{Ln(hfac)(3)}(2)Cu(dpk)(2)] (Hdpk stands for di 2-pyridyl ketoxime) were evaluated by high-frequency electron paramagnetic resonance and magnetization studies, showing ferromagnetic Ln-Cu couplings with J(Ln-Cu)/k(B) = +1 271(7), +0 405(3), and +0 196(13) K for Ln Gd Tb, and Ho, respectively

The ground spin-state of [Cu(phpyNO)(2)(H(2)O)(2)](BF(4))(2) was switched between S(total) = 1/2 and 3/2 across 175 K. On warming, the space group was changed from P2(1)2(1)2(1) to C222(1) in a single-crystal-to-single-crystal manner, and the transient structure could be monitored by means of the crystallographic analysis. The copper-radical exchange coupling changed from 2J/k(B) = -463(3) to +312(6) K with rather small Cu-O-N-C(2py) twisting deformation on one side, while practically no distortion occurred on the other.

Two novel copper(II) complexes with tert-butyl 2-pyridyl nitroxide (2pyNO(center dot)), [Cu(2+)(2pyNO(-))(2PyNO(center dot))](2)(BF(4)(-))(2)(1 center dot BF(4)) and [Cu(2+)(2pyNO(-))(2pyNO(center dot))](2)(ClO(4)(-))(2)(1 center dot ClO(4)), were prepared and structurally characterized. They contained mixed-valent ligands from 2pyNO, whose oxygen atoms were located at equatorial positions of the copper ion. The [Cu(2+)(2pyNO(-))(2pyNO(center dot))] unit was dimerized by mu-oxo bridges of the anion ligand, giving a zigzag linear spin system involving four paramagnetic S = 1/2 centers. The two compounds are isomorphous in an orthorhombic Pbca space group. Magnetic study revealed that 1 center dot ClO(4) showed ferromagnetic copper-radical coupling in all temperature ranges investigated here. On the other hand, 1 center dot BF(4) exhibited a structural phase transition at 64 K, where the magnetic susceptibility was drastically dropped on cooling. The copper-radical magnetic couplings were characterized as ferro- and antiferromagnetic for the high- and low-temperature phases, respectively. The crystallographic analysis clarified that the nitroxide oxygen atom remained at the equatorial position throughout the single-crystal-to-single-crystal phase transition, while the previously known spin-transition-like copper-radical compounds showed conversion of the roles of equatorial and axial positions. The orthogonal arrangement between the copper d sigma and nitroxide pi(star) orbitals is essential for the ferromagnetic coupling, and a slight dislocation of the radical oxygen atom from the chelate plane leads to violation of the orthogonal orbital arrangement, giving a practically diamagnetic low-temperature phase.

We synthesized a mto (= monothiooxalato)-bridged heterometal complex, Ph4P[(ZnFeIII)-Fe-II(mto)(3))] consisting of (FeO3S3)-O-III and (ZnO6)-O-II octahedra and investigated its physical properties. Ph4P[(ZnFeIII)-Fe-II(mto)(3)] has a two-dimensional honeycomb network structure. The ESR and Fe-57 Mossbauer spectra revealed that the rapid spin equilibrium in which the high-spin (HS) state and the low-spin (LS) state exchange in the time scale of 10(-10) &lt; tau &lt; 10(-7)s occurs at the (FeO3S3)-O-III site, which is the first report for an assembled metal complex system.

We successfully isolated a new paramagnetic bidentate ligand tert-butyl 5-methoxy-2-pyridyl nitroxide (meopyNO). Complexation of nickel(II) and copper(II) perchlorates with meopyNO gave the corresponding ML(2)-type bis-chelated compounds. The magnetic studies showed that they were ground high-spin molecules with 2J/k(B) = +288(5) and +178(3) K for [M(meopyNO)(2)(H(2)O)(2)]center dot(ClO(4))(2) (M = Ni and Cu, respectively), where the spin Hamiltonian is defined as H = -2J(S(1)center dot S(2) + S(2)center dot S(3)). From the crystallographic analysis, the torsion angles (phi) around M-O-N-C(2py) were 4.2(3)degrees and 6.87(19)degrees, respectively, being so small that the orthogonality between the magnetic radical pi* and the metal d sigma orbitals would be guaranteed. (C) 2008 Elsevier Ltd. All rights reserved.

&lt;B&gt;[Correction]&lt;/B&gt;

The exchange couplings in [{Dy(hfac)(3)}(2)Cu(dpk)(2)] and [{Dy(hfac)(3)}(2)-Ni(dpk)(2)(py)(2)] (Hdpk = di-2-pyridyl ketoxime) were precisely evaluated by high-frequency electron paramagnetic resonance and pulsed-field magnetization studies, giving J(Dy-Cu)/k(B) = -0.126 K and J(Dy-Ni)/k(B) = +0.031 K.

We have isolated and characterized a new paramagnetic bidentate ligand, tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO). The spin distribution onto the pyridine ring was clarified from the Electron Spin Resonance spectrum. Complexation of phpyNO with nickel(II) and copper(II) ions gave ML2-type chelated compounds. The magnetic measurements of [Ni(phpyNO)(2)(H2O)(2)]center dot(ClO4)(2) and [Cu(phpyNO)(2)(H2O)]center dot(ClO4)(2)center dot C6H4Cl2 revealed that the metalradical exchange couplings were ferromagnetic with 2J/k(B) = +409(10) and +434(12) K, respectively. The torsion angle (phi) around M-O-N-C-2py can be regarded as a reliable indicator for the plane geometry of chelates; namely, highly planar chelates defined by small phi exhibit ferromagnetic coupling. An approximate linear relation. was found in the J versus phi plot using the data of the present complexes and. related known compounds. The critical angle of phi, at which the sign of the metal-radical exchange changes from positive to negative, was 12.6(9)degrees. This finding could be almost reproduced by density-functional theory calculation on a model copper(II)-nitroxide dyad. The exchange couplings in equatorially coordinated copper(II)- and nickel(II)-nitroxide complexes are very strongly ferromagnetic by nature.

A 4f - 3d heterometallic polymeric coordination compound [[Dy(hfac)(2)(CH3OH)}(2)(Cu(dmg)(Hdmg)(2)}](n) [(Dy2Cu](n) Hdmg = dimethylglyoxime; Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) was synthesized, and the X-ray crystallographic analysis shows that the structure is isomorphous to the known ferrimagnetic [Gd2Cu2)](n) polymer. A centrosymmetric diamond-arrayed ferrimagnetic unit involving the oximate bridge, Dy - O - N - Cu, is repeated to form a discrete chain. An onset of the frequency dependence was found in the ac magnetic susceptibility measurements down to 1.8 K. Low-temperature magnetization measurements on [Dy2Cu2](n) exhibited magnetic hysteresis with magnetization steps. To examine the energy level structure and the exchange coupling between the Dy and Cu ions, high-frequency EPR (HF-EPR) spectra of a polycrystalline sample of [Dy2Cu2](n) were recorded at various frequencies (34.7 - 525.4 GHz) and temperatures. We analyzed the spectra by treating the Dy moments as Ising spins and built an exchange-coupling model for a diamond-arrayed tetranuclear macrocycle [Dy2Cu2]. The Dy - Cu exchange couplings were precisely evaluated, owing to the high resolution of HF-EPR; J/k(B) = - 0.895(8) and - 0.061(8) K for two independent Dy - Cu relations, where the exchange parameter is defined as - J. The Cu center dot center dot center dot Cu coupling among the macrocycles was estimated to be ferromagnetic with approximately 1 K. The present study has established the definitive methodology to examine the energy level in 4f - 3d heterometallic systems by using HF-EPR. The magnetic properties of the [Dy2Cu2](n) chain were plausibly described as a perturbed system of a single-molecule magnet. The quantum tunneling of the magnetization is based on a different principle compared with those of the 3d-based single-molecule magnets, since the energy level structure of [Dy2Cu2](n) is regulated by the exchange couplings.

We prepared tetranuclear complexes [{Cu(emg)(Hemg)(ROH)}(2)Dy(hfac)(2)(ROH)}(2)] (abbreviated as [Dy2Cu2]-R; R = Me, Et; H(2)emg=ethylmethylglyoxime) and the corresponding polymer [{Cu(dmg)(Hdmg)}(2){Dy(hfac)(2)(CH3OH)}(2)](n) center dot 2(MeOH) ([Dy2Cu2](n); H(2)dmg = dimethylglyoxime), which were characterized as analogs of the known [Gd2Cu2] monomer and polymer, respectively, by means of X-ray crystallographic analysis. The M-H curves clarified antiferromagnetic coupling between Dy and Cu in the [Dy2Cu2] motif, leading to the Dy magnetic moments aligned parallel ([Cu(down arrow)Dy(up arrow)Cu(down arrow)Dy(up arrow)]). Ac magnetic susceptibility measurements of [Dy2Cu2]-Me and [Dy2Cu2](n), exhibited a chi(ac)' (in-phase) decrease and a concomitant chi('')(ac) (out-of-phase) increase with an increase of frequency. Low-temperature magnetization measurements on [Dy2Cu2], exhibited magnetic hysteresis, which is characteristic of single-chain magnets. (c) 2006 Elsevier Ltd. All rights reserved.

We successfully synthesized and characterized two precursors for paramagnetic chelating ligands, N-p-anisyl-N-2-pyridylhydroxyl-amine and 2,6-bis(N-t-butylhydroxylamino)pyridine. The former can be regarded as a rare case since N,N-diarylhydroxylamines cannot be isolated in general because of facile autoxidation. The oxidation with silver(I) oxide gave radicals showing typical nitroxide ESR signals, but the resultant radicals were unstable under ambient conditions. Theoretical DFT calculation indicated that the bisnitroxide radical, pyridine-2,6-diyl bis(t-butyl nitroxide), had a triplet ground state. (c) 2006 Elsevier Ltd. All rights reserved.

Ferromagnetic exchange coupling was operative between nickel(II) spins in a dinuclear nickel(II) complex [{Ni(hfac)(2)}(2)(py(4)C)] with J/k(B) = +0.57(2)K (H = -2JS(1) . S-2), where hfac and py(4)C Stand for 1,1,1,5,5,5-hexafluoropentane-2,4-dionate and tetrakis(2-pyridyl)methane, respectively. X-ray crystal structure analysis revealed an approximate D-2d symmetry of the [Ni(py4C)Ni] core and accordingly the orthogonal arrangement between the two coordination basal planes. Mechanism of the ferromagnetic coupling was proposed on the basis of the orthogonal d sigma-type magnetic orbitals, which was supported by DFT calculation. (c) 2006 Elsevier B.V. All rights reserved.

We designed spiro-fused dinculear complexes using tetrakis(2-pyridyl)methane (py(4)C) for the development of ground high-spin molecules. We attempted to prepare a dinuclear copper(II) complex [{Cu(hfac)(2)}(2)(py(4)C)], where hfac stands for 1, 1,1,5,5,5-hexafluoropentane-2,4-dionate, but we obtained [Cu(hfac)(2)(py(4)C)] and [Cu(hfac)(py(4)C) (.) Cu(hfac)(3)]. These molecular structures were determined by the X-ray crystal structure analysis. (c) 2005 Elsevier Ltd. All rights reserved.

A dinuclear manganese(II) complex [{Mn(hfac)(2)}(2)(py(4)C)] was synthesized, where hfac and py(4)C denote 1,1,1,5,5,5-hexafluoropentane-2,4-dionate and tetrakis(2-pyridyl)methane, respectively. X-Ray crystallographic analysis revealed that the dihedral angle was 87.4(2)degrees between two chelate rings around the spiro center. Very weak antiferromagnetic interaction is operative between manganese(II) spins.