Incomplete Spin Research Articles

Iron(iii) complexes of 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol as spin-crossover molecular materials.

Four new mononuclear Fe(iii) complexes 1-4 of 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol (H2L) have been synthesized and characterized by magnetic susceptibility measurements and single-crystal X-ray diffraction analysis. Complexes 1-3 are ionic compounds with the formula [Fe(HL)2]X·nsolvent (X = ClO4- (1), BF4- (2) and Cl- (3)), while complex 4 ([Fe(HL)(L)]) is neutral. The central Fe(iii) ion in complexes 1-4 is coordinated by four nitrogen and two oxygen atoms of two Schiff base ligands HL- or L2-, and the coordination geometries of FeIII are all distorted octahedral. Magnetic measurements indicate that complex 1 shows an abrupt spin transition with a thermal hysteresis of 32 K (T1/2↓ = 158 K and T1/2↑ = 190 K), and complex 4 exhibits a gradual incomplete spin transition. Complexes 2 and 3 are high spin with a room-temperature χmT value of 4.32 cm3 K mol-1 and 4.39 cm3 K mol-1, respectively.

Nonanuclear Spin-Crossover Complex Containing Iron(II) and Iron(III) Based on a 2,6-Bis(pyrazol-1-yl)pyridine Ligand Functionalized with a Carboxylate Group.

The synthesis and magnetostructural characterization of [Fe(III)3(μ3-O)(H2O)3[Fe(II)(bppCOOH)(bppCOO)]6](ClO4)13·(CH3)2CO)6·(solvate) (2) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe(II)(bppCOOH)2](ClO4)2 (1). The single-crystal X-ray diffraction structure of 2 shows that it contains the nonanuclear cluster of the formula [Fe(III)3(μ3-O)(H2O)3[Fe(II)(bppCOOH)(bppCOO)]6](13+), which is formed by a central Fe(III)3O core coordinated to six partially deprotonated [Fe(II)(bppCOOH)(bppCOO)](+) complexes. Raman spectroscopy studies on single crystals of 1 and 2 have been performed to elucidate the spin and oxidation states of iron in 2. These studies and magnetic characterization indicate that most of the iron(II) complexes of 2 remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the Fe(III) trimer shows the expected antiferromagnetic behavior. From the structural point of view, 2 represents the first example in which bppCOO(-) acts as a bridging ligand, thus forming a polynuclear magnetic complex.

Preparation and magnetic characteristics of mesoporous nickel oxide–silica composites

Mesoporous NiO–SiO2 (MCM-41) silica-matrix composites with various nickel oxide concentrations (NiO : SiO2 = 0.025 : 1 to 0.2 : 1) have been produced by oxide cocondensation under hydrothermal synthesis conditions in the presence of cetyltrimethylammonium bromide as a template and (2-cyanoethyl) triethoxysilane as an organosubstituted trialkoxysilane additive. X-ray diffraction data have been used to evaluate the maximum nickel(II) oxide concentration (NiO : SiO2 = 0.1 : 1) that allows the ordered mesopore structure of MCM-41 to persist in the silica-matrix composites. We have studied the magnetic properties of this material as functions of temperature and magnetic field. The results demonstrate that the magnetic properties of the nanocomposite with NiO : SiO2 = 0.1 : 1 at low temperatures (T < 20 K) are determined by incomplete spin compensation in the matrix and on the surface of the NiO nanoparticles.

Elastic Frustration Triggering Photoinduced Hidden Hysteresis and Multistability in a Two-Dimensional Photoswitchable Hofmann-Like Spin-Crossover Metal-Organic Framework.

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py)2{Pt(CN)4}]·3H2O, built from [FePt(CN)4] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent FeII sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS-HS) and intermediate high-spin low-spin (HS-LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS-HS and fully low-spin (LS-LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)4] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent FeII sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

EDTA-assisted hydrothermal synthesis and magnetic properties of urchin-like YbMn2O5 microspheres

Urchin-like YbMn2O5 microspheres have been fabricated by EDTA (ethylene diamine tetra acetic acid)-assisted hydrothermal method. The results demonstrated that EDTA played key roles in growth of urchin-like YbMn2O5 microspheres. Magnetic measurement indicated that YbMn2O5 showed a weak ferromagnetic at low temperature due to incomplete spin compensation on the surface. The divagation between field cooling and zero field cooling curves could be reasonably ascribed to the collaboration result of spin-glass behavior and the antiferromagnetic (AFM) ordering of the Yb3+ magnetic moments. Exchange bias (a shift in the hysteresis loop toward negative axis) could be observed in both YbMn2O5 microspheres and nanorods (without EDTA). With the increasing particle size, the exchange bias field and the coercivity increased.

Synthesis and magnetic properties of iron(II) closo-borate complexes with tris(3,5-dimethylpyrazol-1-yl)methane

Methods are developed for preparing iron(II) complexes with tris(3,5-dimethylpyrazol-1-yl)methane {HC(3,5-dmpz)3} and closo-borate(2-) ions, namely [Fe{HC(3,5-dmpz)3}2]B10H10 · H2O (I) and [Fe{HC(3,5-dmpz)3}2]B12H12 · H2O (II). These complexes were characterized by static magnetic susceptibility measurements (2–325 K), electronic spectroscopy (diffuse reflectance spectra), and IR spectroscopy. The complexes have a distorted octahedral geometry of the coordination polyhedron; the coordination core is FeN6. μeff(T) studies show low-temperature incomplete spin crossover in complex I and antiferromagnetic exchange coupling among iron(II) unpaired electrons at temperatures below 90 K in complex II. Complex I shows thermochromism; the color transition upon cooling is white ↔ pink.

Syntheses, hydrogen-bonded assembly structures, and spin crossover properties of [FeIII(Him)2(n-MeOhapen)]PF6 (Him = imidazole and n-MeOhapen = N,N′-bis(n-methoxy-2-oxyacetophenylidene)ethylenediamine); n = 4, 5, 6)

Three iron(III) complexes, [FeIII(Him)2(n-MeOhapen)]PF6 (1: n=4, 2: n=5, 3: n=6), where Him=imidazole and n-MeOhapen=N,N′-bis(n-methoxy-2-oxyacetophenylidene)ethylenediamine, were synthesized. Each FeIII ion was coordinated by N4O2 donor atoms of equatorial n-MeOhapen and two axial Him. The saturated FeC2N2 chelate ring involving the ethylenediamine moiety adopted a gauche conformation and the two phenylidene planes of [FeIII(Him)2(n-MeOhapen)]+ were oriented opposite to the FeO2N2 coordination plane. Two adjacent [FeIII(Him)2(4-MeOhapen)]+ cations in 1 were connected via PF6− ions by hydrogen bonds between the imidazole group and PF6− to give a one-dimensional chain. Two adjacent cations in 2 were connected via hydrogen bonds between the phenylidene oxygen and imidazole atoms to form a cyclic dimer structure. Two EtOH molecules were involved in a hydrogen-bonded cyclic dimer structure in 3. Compounds 1 and 2 showed spin crossover behavior, but 3 showed incomplete spin crossover.

Structural Chemistry of Tetracyanopyrrolide (Part 2): Spin Crossover in a Bis‐Tripodal Iron(II) Complex

AbstractThe mononuclear iron(II) complex [Fe(tpm)2]2+(tcp–)2 (1) containing the neutral tripodal N3‐donor ligand tris(pyrazol‐1‐yl)methane (tpm) and the mononegative tetracyanopyrrolide (tcp) as counter‐anion was characterized by X‐ray diffraction on twinned crystals and investigated in the solid state by magnetic susceptibility measurements and differential scanning calorimetry. Structural investigations at 123 K reveal Fe–N distances typical for low‐spin d6 iron(II). Compound 1 undergoes a gradual, incomplete spin crossover in the examined temperature range 1.9–400 K. Comparisons are made with known analogous [Fe(tpm)2]2+(X–)2 compounds bearing different anions X, which display similar crossover behavior.

Toward chiral iron(II) spin-crossover grafted resins with imidazole Schiff-base ligands

Using chiral imidazole Schiff-base as ligands, two couples of mononuclear iron(II) enantiomeric complexes fac-Λ-[Fe(R-L)3](ClO4)2·H2O (1), fac-Δ-[Fe(S-L)3](ClO4)2·H2O (2), fac-Λ-[Fe(R-L)3](BF4)2·H2O (3), fac-Δ-[Fe(S-L)3](BF4)2·H2O (4) (L=1-(2-naphthyl)-N-((1-methyl-imidazol-2-yl)methylene)ethanamine) have been successfully synthesized and characterized. The CD spectra of 1 and 2, 3 and 4, were basically mirror images confirming their enantiomers. X-ray crystallography revealed that complexes 1–4 all crystallized in the chiral space group P213 with the iron(II) center surrounded by three bidentate ligands. Magnetic measurements revealed incomplete spin transition for 1, and gradual spin crossover (T1/2=150K) for 3. However, the spin-crossover behaviors were vanished after grafting 1–4 to the Merrifield's peptide resin, and the iron(II) centers in the resin remained in a high-spin state in the temperature range of 2–300K.

Electronic effects of ligand substitution on spin crossover in a series of diiminoquinonoid-bridged Fe(II)2 complexes.

A series of four isostructural Fe(II)2 complexes, [(TPyA)2Fe2((X)L)](2+) (TPyA = tris(2-pyridylmethyl)amine; (X)L(2-) = doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone; X = H, Br, Cl, and F), were synthesized to enable a systematic study of electronic effects on spin crossover behavior. Comparison of X-ray diffraction data for these complexes reveals the sole presence of high-spin Fe(II) at 225 K and mixtures of high-spin and low-spin Fe(II) at 100 K, which is indicative of incomplete spin crossover. In addition, crystal packing diagrams show that these complexes are well-isolated from one another in the solid state, owing primarily to the presence of bulky tetra(aryl)borate counteranions, such that spin crossover is likely not significantly affected by intermolecular interactions. Variable-temperature dc magnetic susceptibility data confirm the structural observations and reveal that 54(1), 56(1), 62(1), and 84(1)% of Fe(II) centers remain high-spin even below 65 K. Moreover, fits to magnetic data provide crossover temperatures of T1/2 = 160(1), 124(1), 121(1), and 110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies of ΔH = 11.4(3), 8.5(3), 8.3(3), and 7.5(2) kJ/mol, respectively. These parameters decrease with increasing electronegativity of X and thus increasing electron-withdrawing character of (X)L(2-), suggesting that the observed trends originate primarily from inductive effects of X. Moreover, when plotted as a function of the Pauling electronegativity of X, both T1/2 and ΔH undergo a linear decrease. Further analyses of the low-temperature magnetic data and variable-temperature Mössbauer spectroscopy suggest that the incomplete spin crossover behavior in [(TPyA)2Fe2((X)L)](2+) is best described as a transition from purely [FeHS-FeHS] (HS = high-spin) complexes at high temperature to a mixture of [FeHS-FeHS] and [FeHS-FeLS] (LS = low-spin) complexes at low temperature, with the number of [FeHS-FeHS] species increasing with decreasing electron-withdrawing character of (X)L(2-).

Iron(III) complexes with pentadentate Schiff-base ligands: Influence of crystal packing change and pseudohalido coligand variations on spin crossover

Two novel iron(III) complexes involving pentadentate Schiff-base ligands, [Fe(LBr)(L1)], show temperature induced incomplete spin crossover of a gradual nature. While for the L1=NCSe− coligand the transition temperature (326K or 317K) lies above room temperature, the N3− coligand causes its drop down to 143K or 140K (two values were obtained by different models). This shift is associated with a significant decrease of the enthalpy of the transition from about ΔH=6.1kJmol−1 to ca. ΔH=1.7kJmol−1, while the entropy of the transition is about ΔS=19JK−1mol−1 and ΔS=12JK−1mol−1, respectively. Two analogous complexes with Cl− and NCS− coligands remain high-spin over the whole temperature range.

A spin-crossover catecholato–iron(III) system: Synthesis, crystal structure and magnetic properties

A new mononuclear Fe(III)–catecholate complex, which exhibits an incomplete spin transition without hysteresis, had been synthesized and structurally characterized. Analyzed from the structural characteristics, the tripodal ligand must play the important role in affecting the spin crossover (SCO) behavior. The larger steric strain effect and weak cooperativity existed in the crystal lattice are inclined to favor the incomplete transition.

Control of the spin state by charge and ligand substitution: two-step spin crossover behaviour in a novel neutral iron(II) complex.

The influence of the charge and steric hindrance on the spin state of a series of four monomeric Fe(II) complexes derived from the tridentate ligands 2-(1H-benzoimidazol-2-yl)-1,10-phenanthroline (Hphenbi) and 2-(1H-benzoimidazol-2-yl)-9-methyl-1,10-phenanthroline (Hmphenbi) and their deprotonated forms (phenbi(-), mphenbi(-)) are investigated. The crystal structure and magnetic properties show that [Fe(Hphenbi)2](BF4)2·1.5C6H5NO2·H2O (1) and its neutral form [Fe(phenbi)2](0)·2CHCl3·H2O (2) are low-spin complexes at 400 K due to the strong ligand field imparted by the terpyridine-like ligand. In contrast, the steric hindrance induced by the methyl group in [Fe(Hmphenbi)2](BF4)2 (3) stabilizes the high-spin state of the Fe(II) ion at all temperatures. Application of a hydrostatic pressure of 0.43 GPa shows that3 displays incomplete thermal-induced spin crossover behaviour. However, upon deprotonation of the ligand the resulting neutral complex [Fe(mphenbi)2]·2CHCl3 (4) shows a complete two-step spin crossover behaviour at ambient pressure.

EPR Detection of Possible Superparamagnetic Polyiron Nanoparticles and Free Radicals in the Blood Serum of Patients with Homozygous β-Thalassemia

In homozygous β-thalassemia blood transfusions and chelating therapy cannot prevent completely hemochromatosis. Labile ‘free’ iron forms in blood serum and evolves into insoluble inorganic deposits in tissues. Using X-band EPR at 163 K, we detected a new species of polynuclear iron in serum of some thalassemic patients. It showed a broad g < 2 line and an unusually high dynamic magnetic susceptibility due to ordered magnetism, which was detected by a decreased Q factor of the resonant cavity at B = 0. To explain the ordered magnetism and large line, we postulated either ferri- or ferromagnetic nanocrystals with non-zero residual magnetization, or superparamagnetic nanoparticles with antiferromagnetic ordering and incomplete spin compensation—or maybe both. While the results were not sufficient to check the first hypothesis, they are fully consistent with the second. The new species is similar to, but distinct from, ferritin’s mineral core, as it does not correlate with the ferritin concentration. The spectra suggest particles of ~1.7–4.1 nm diameter, plausibly containing ~40–300 Fe(III) ions coupled by oxygen bridges, but further confirmation is needed. The nanoparticles apparently formed a hydrophilic colloidal dispersion, being probably decorated with hydrophilic small organic molecules. They are postulated to form by heterogeneous nucleation around the ‘free’ iron, then aggregate in chains and eventually precipitate in the tissues. Stable free radicals detected in serum were tentatively identified as the hydroxyperoxyl and monodehydroascorbate radicals forming adducts with Cu(II)-ceruloplasmin, and the Hb–porphyrin–Fe(IV)=O oxoferryl radical probably stabilized on haptoglobin. They are consistent with oxy-radicals promoted both by ‘free’ iron and by polyiron nanoparticles. Potential medical applications like early assessment of patient’s evolution trend toward hemochromatosis and monitoring of the transfusional suppression of endogenous erythrocyte synthesis are suggested.

Syntheses, Structural Variation, and Characterization of a Series of Crystalline Coordination Compounds with 4-Benzene-1,2,4-triazole: Polymorph, Incomplete Spin Transition, and Single Crystal-to-Single Crystal Transformation

Based on the 4-substituted 1,2,4-triazole derivate ligand 4-benzene-1,2,4-triazole (L), a series of crystalline coordination complexes varying from mononuclear to trinuclear species, namely, [Zn(L)2Br2] (1), [Zn(L)2Br2] (2), [Fe(L)4(NCS)2]2 (3), [Fe2(μ2-L)3(L)2(NCS)4]·CH3OH·CH3CH2OH (4), [Fe2(μ2-L)3(L)2(NCS)4]·2CH3CH2OH (5), [Fe2(μ2-L)3(L)2(NCS)4]·2CH3CH2OH·1.5H2O (6), and [Ni3(μ2-L)6(L)4(H2O)2](NO3)6·15.5H2O (7), have been isolated. 1 and 2 present a temperature-induced polymorphic phenomenon of two zinc(II) coordination complexes with L. The solvent effect plays the key role for the self-assembly of these Fe(II) complexes 3–6: 3 contains mononuclear Fe(L)4(NCS)2 units without spin-transition behavior, whereas both 4 and 5 present binuclear Fe(II) complexes with three N1,N2-1,2,4-triazole bridges exhibiting incomplete spin-transition behavior. The low-temperature X-ray structural analysis (100 K) of 4 also confirms that one of the Fe(II) centers is located at the low-spin (LS) state and the other Fe(II) ...

Solution and solid state properties of Fe(III) complexes bearing N-ethyl-N-(2-aminoethyl)salicylaldiminate ligands

The effect of the phenolate ring derivatisation on the magnetic properties of Fe(III) complexes bearing N-ethyl-N-(2-aminoethyl)salicylaldiminate ligands both in solid state and solution have been investigated. Two new complexes [Fe(3,5-Br-salEen)2]ClO4.EtOH (5) and [Fe(3,5-Br-salEen)2]BPh4.DMF (6) have been synthesised. SQUID magnetometry studies on these complexes showed that while complex 5 is in the low-spin (LS) state, complex 6 displays a gradual and incomplete spin crossover (SCO) transition over the temperature measured. Solution measurements on a series of six complexes – [Fe(salEen)2]ClO4 (1), [Fe(salEen)2]BPh4·0.5H2O (2), [Fe(5-Br-salEen)2]ClO4 (3), [Fe(5-Br-salEen)2]BPh4·DMF (4), [Fe(3,5-Br-salEen)2]ClO4·EtOH (5) and [Fe(3,5-Br-salEen)2]BPh4·DMF (6) – were performed by UV–vis and NMR spectroscopies and cyclic voltammetry. Solution studies show that the presence of electron withdrawing groups (bromine atoms) affect the electronic density at the phenolate ring, thus influencing the ligand field strength and the separation between the t2g and eg* energy levels. The presence of two bromide substituents at the phenolate ring has a more pronounced effect on the magnetic behaviour in solution than in the solid state, with both complexes 5 and 6 adopting preferentially the LS state. Electrochemical studies of complexes 1–6 reveal that the reduction of the metallic centres in the complexes with electron withdrawing groups is easier, with E1/2 values of iron moving to more positive potentials with the number of bromide substituents at the phenolate ring.

Hydrothermal synthesis of DyMn2O5 nanorods and their magnetic properties

Abstract Highly pure DyMn2O5 nanorods have been synthesized via hydrothermal route. FT-IR and Raman patterns were employed to determine DyMn2O5. PLE and PL spectra of DyMn2O5 showed UV excitation and near band edge (NBE) emission. Magnetic measurement indicated the crystalline DyMn2O5 showed a weak ferromagnetic ordering due to incomplete spin compensation on the surface. The ferromagnetic transition could be explained by an antiferromagnetic core and a ferromagnetic surface.

Iron(II) closo-borate complexes with 1,2,4-triazole derivatives: Spin crossover in the iron(II) closo-borate complexes with tris(pyrazol-1-yl)methane

Methods of synthesis of iron(II) complexes containing cluster closo-borate anions—[Fe(Htrz)3]B10Cl10 (I) (HTrz is 1,2,4-triazole), [Fe(NH2Trz)3]B10Cl10 · 2H2O (II) (NH2Trz is 4-amino-1,2,4-triazole), [Fe{HC(pz)3}2]B10Cl10 (III), [Fe{HC(pz)3}2]B10H10 (IV), and [Fe{HC(pz)3}2]B12H12 · 2H2O (V) (HC(pz)3 is tris(pyrazol-1-yl)methane)—have been developed. The compounds have been studied by the static magnetic susceptibility method (78–500 K) and electronic, IR, and EXAFS spectroscopy. Complexes I and II in the temperature range under consideration remain in the high-spin state. Low-spin complex III shows incomplete spin crossover and decomposes on heating above 440 K. Complexes IV and V are characterized by reversible spin crossover 1 A 1 ⇔ 5 T 2 accompanied by thermochromism (the pink ⇔ white color change). The crossover temperature (T c) for IV and V is 375 and 405 K, respectively.

Spin-split conduction band in EuB6and tuning of half-metallicity with external stimuli

We report an Eu ${L}_{3}$-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB${}_{6}$. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic ordering temperature and follows the magnetic order parameter. This spectral feature is attributed to the intersite excitation from the local 4$f$ orbital to the spin-split 5$d$ orbital on the neighboring site, illustrating the essential role of exchange splitting of the conducting electrons. Based on our density functional theory calculations and the RIXS data, EuB${}_{6}$ at low temperature can be consistently described with a semimetallic electronic structure with incomplete spin polarization. We propose that half-metallicity in EuB${}_{6}$ can be achieved utilizing the strong tunability of the electronic structure against gate voltage, strain, and magnetic field.

Syntheses, structures and magnetic properties of a series of iron(II)-triazole crystalline coordination compounds: Solvent effect, different substituted groups and incomplete spin transition

Abstract Based on 4-substituted mono- and bis-triazole ligands 4-2-pyridyl-1,2,4-triazole (L1) and 2,6-(1,2,4-triazole-4-yl)-pyridine (L2), a series of iron(II)-triazole complexes, namely [Fe2(μ2-L1)3(L1)2(NCS)4] 2H2O (1), [Fe2(μ2-L1)3(L1)2(NCS)4] CH3OH CH3CH2OH (2) and [Fe(μ2-L2)2(NCS)2]n (3) have been isolated. For the Fe(II)-L1 system, though dinuclear complexes were obtained in both aqueous solution and methanol/ethanol solution for 1 and 2, respectively, anti-ferromagnetic interactions were found in 1 while incomplete spin transition phenomenon was found in 2. Low temperature structural analysis (100 K) of 2 is consistent with one of the Fe(II) centers within the dinuclear complex at low spin (LS) state and the other at high-spin (HS) state resulting in a [LS:HS] species. When L2 is used under the similar preparation method for 1–2, one-dimensional neutral iron (II) chain complex 3 is isolated. The different substituted groups and solvent effect are both important for self-assembly and different magnetic properties of these crystalline iron(II)-triazole complexes 1–3.