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Spin Crossover (spin + crossover)

Distribution by Scientific Domains

Chemistry	87%

Selected Abstracts

Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2009
alitro
Abstract A nonsymmetrical triamine, 1,6-diamino-4-azahexane, was Schiff-condensed with (X-substituted) o -salicylaldehyde to yield pentadentate ligands X-L5: salpet and MeBu-salpet. These ligands form mononuclear, dinuclear, and trinuclear FeIII complexes, whose structures were determined by single-crystal X-ray analysis. Of the mononuclear complexes, [FeIII(salpet)Cl] and [FeIII(MeBu-salpet)Cl] are high spin (S = 5/2), whereas [FeIII(salpet)CN]·MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind ofthermally induced spin crossover. The dinuclear complex [L5FeIII(CN)FeIIIL5](ClO4)·2H2O (L5 = salpet) is a mixed-spin assembly: the C -coordinated FeIII center is low spin (L) and the N -coordinated FeIII center is high spin (H) at low temperature; an antiferromagnetic interaction occurs between them. This LH reference state is mixed with the LL one. Upon heating, the system shows an increasing content of the HH state. Also, the dinuclear complex [L5FeIII(CN)FeIIIL5](BPh4)·2MeCN (L5 = MeBu-salpet) exhibits a spin transition between LH and HH spin pairs. The mixed-valence trinuclear complex [L5FeIII{FeII(CN)5(NO)}FeIIIL5]·0.5MeOH·3.75H2O (L5 = salpet) shows spin crossover with a residual high-spin fraction at liquid He temperature owing to the LL + LH ground state. The metallacyanido-bridged complex [L5FeIII{Ni(CN)4}FeIIIL5]·2MeOH (L5 = MeBu-salpet) contains a high-spin pair, HH, over the whole temperature interval with a ferromagnetic exchange interaction. A theoretical model was outlined that allows simultaneous fitting of all available experimental data (magnetic susceptibility, magnetization, high-spin mole fraction obtained from the Mössbauer spectra) on a common set of parameters. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Concerted Spin Crossover and Symmetry Breaking Yield Three Thermally and One Light-Induced Crystallographic Phases of a Molecular Material,

ANGEWANDTE CHEMIE, Issue 49 2009
Nicolas Bréfuel
Vier Kristallphasen des neuartigen molekularen Materials [FeIIH2L2MeH2L2Me](PF6)2 sind durch licht- und thermisch induzierten Spin-Crossover unter Symmetriebruch zugänglich (H2L2Me ist eine acyclische sechszähnige N6 -Schiff-Base). Die ausgerichteten Anordnungen der FeII -Komplexkationen (High-Spin: rote Dreiecke; Low-Spin: blaue Dreiecke) sind durch Schichten der Gegenanionen (grüne Zickzacklinien) voneinander getrennt. [source]

ChemInform Abstract: Mutual Interdependence of Spin Crossover and Metal,Metal Bond Formation in M2Cl93- (M: Fe, Ru, Os).

CHEMINFORM, Issue 14 2001
Timothy Lovell
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

Spin Crossover versus Low-Spin Behaviour Exhibited in 2D and 3D Supramolecular Isomers of [FeII(2,4-bpt)2],Guest,

CHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2010
Xin Bao
To spin or not to spin: Four supramolecular isomers based on a [Fe(2,4-bpt)2],guest were synthesised by judicious choice of solvents under solvothermal conditions (see figure). They can be categorised into a 2D spin crossover (SCO) branch and a 3D low-spin (LS) branch (all of them exhibit NbO topology, but one is non-interpenetrated and the other two are twofold interpenetrated). The differences in the supramolecular interactions play important roles in their magnetic behaviour. [source]

Two New Iron(II) Spin-Crossover Complexes with N4O2 Coordination Sphere and Spin Transition around Room Temperature

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2009
Birgit Weber
Abstract The reaction of iron(II) acetate with the tetradentate Schiff base like ligand H2L1 {[3,3,]-[4,5-dihydroxy-1,2-phenylenebis(iminomethylidyne)bis(2,4-pentanedion)]} leads to the formation of the complex [FeL1(MeOH)]. Reaction of this complex with pyridine (py) or N,N,-dimethylaminopyridine (dmap) leads to the two N4O2 -coordinated complexes [FeL1(py)2]·py (1) and [FeL1(dmap)2]·MeOH·0.5dmap (2). Both complexes are spin-crossover compounds that were characterised by using magnetic measurements, X-ray crystallography and temperature-dependent 1H NMR spectroscopy. Special attention was given to the role of the two hydroxy groups on the phenyl ring in the formation of a hydrogen-bonding network and the influence of this network on the spin-transition properties. Although only a gradual spin crossover was observed for both complexes, the transition temperature was shifted to higher temperatures relative to that of the complexes with no additional hydroxy groups at the Schiff base like ligand. The hydrogen-bonding network was responsible for this effect.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges

Six-coordinate Co2+ with imidazole, NH3, and H2O ligands: Approaching spin crossover

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2007
Ann M. Schmiedekamp
Abstract Octahedral, six-coordinate Co2+ can exist in two spin states: S = 3/2 and S = 1/2. The difference in energy between high spin (S = 3/2) and low spin (S = 1/2) is dependent on both the ligand mix and coordination stereochemistry. B3LYP calculations on combinations of neutral imidazole, NH3, and H2O ligands show that low-spin isomers are stabilized by axial H2O ligands and in structures that also include trans pairs of equatorial NH3 and protonated imidazole ligands, spin crossover structures are predicted from spin state energy differences. Occupied Co d orbitals from the DFT calculations provide a means of estimating effective ligand strength for homoleptic and mixed ligand combinations. These calculations suggest that in a labile biological system, a spin crossover environment can be created. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

X-ray study of the light-induced metastable state of a spin-crossover compound

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2000
Joachim Kusz
Iron(II) complexes exhibiting thermal spin crossover may be converted from the 1A1 low-spin (LS) state to the 5T2 high-spin (HS) state by irradiation with green light (light-induced excited spin-state trapping, LIESST). The lifetime of the metastable LIESST state may be sufficiently long for X-ray diffraction study. The lattice parameters of a single crystal of [Fe(ptz)6](BF4)2 (ptz = propyltetrazole) were measured between 300 and 10,K, while the crystal changed from the HS to the LS state near 135,K. Using the green light (514,nm) of an argon-ion laser, the crystal was quantitatively converted to the metastable LIESST state at 10,K; its lattice parameters were measured up to 50,K, at which point the LIESST state begins to decay on a minute timescale. The change of the lattice parameters can be interpreted by a superposition of a normal temperature dependence, for which the isostructural zinc compound served as a reference, and an almost temperature-independent part which is proportional to the fraction of molecules in the HS state. [source]

Supramolecular control of spin-crossover phenomena in lipophilic Fe(II)-1,2,4-triazole complexes

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 17 2006
Keita Kuroiwa
Abstract The spin-crossover properties of lipophilic, supramolecular Fe(II) complexes bridged by 4-(3-dodecyloxy)propyl-1,2,4-triazole [Fe(II)(1)3Cl2] were investigated in chloroform and cast films. A purple low-spin (LS) complex in a powdery form was transformed into pale yellow high-spin (HS) polymers by dissolution in chloroform. The formation of lipophilic molecular wires in chloroform was observed with transmission electron microscopy. The casting of chloroform solutions onto solid supports produced purple, transparent films (LS state). The cast films exhibited sluggish spin-crossover (LS , HS) behavior without thermal hysteresis. On the other hand, the cocasting of equimolar dodecanol or tetradecanol with Fe(II)(1)3Cl2 produced composite films in which alcohol molecules were bound to the complex by ionic hydrogen bonding (ROH···Cl,) and van der Waals interactions. At room temperature, the cast films exhibited regular lamellar structures before and after alcohol doping; this was confirmed by wide-angle X-ray diffraction measurements. Interestingly, the Fe(II)(1)3Cl2/CnOH (n = 12 or 14) ternary films showed a reversible abrupt spin crossover accompanied by thermal hysteresis. The observed bistability was related to dynamic structural transformations between lamellar and hexagonal structures. This study provides a novel supramolecular approach to designing spin-crossover polymer films with controlled thermal bistability. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5192,5202, 2006 [source]

Characterization of spin crossover crystal surface by AFM

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2010
C. Chong
Abstract Imaging nano-domains in spin crossover (SCO) compounds remains so far an unreached goal. We report on the first AFM tapping-mode investigation of SCO single crystals, performed at room temperature with the well known mononuclear compound [Fe(ptz)6](BF4)2 (ptz,=,1-propyl-tetrazole) and the trinuclear supramolecular compound [Fe3(hyetrz)6(H2O)6](CF3SO3)6 (hyetrz,=,4-(2,-hydroxyethyl)-1,2,4-triazole) which shows a gradual spin conversion centred at room temperature. The natural surface of the former crystal revealed a volatile coating of the scanned area attributed to the transport of adsorbed water under the effect of interaction with the AFM tip. The second one showed astonishing leopard-skin patterns assigned to the effect of atmospheric humidity on this hygroscopic compound. Their origin is discussed. We suggest the use of fluid coating layers as a general method for revealing the nano-patterning of physical properties (e.g. like-spin domains) at the surface of dielectric materials. AFM-tapping images of [Fe3(hyetrz)6(H2O)6](CF3SO3)6 at room temperature and ambient atmosphere. [source]

Spin Crossover versus Low-Spin Behaviour Exhibited in 2D and 3D Supramolecular Isomers of [FeII(2,4-bpt)2],Guest,

Structural and Magnetic Resolution of a Two-Step Full Spin-Crossover Transition in a Dinuclear Iron(II) Pyridyl-Bridged Compound

CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2006
Jarrod J. M. Amoore
Abstract A dinuclear iron(II) complex containing the new pyridyl bridging ligand, 2,5-di(2,,2,,-dipyridylamino)pyridine (ddpp) has been synthesised and characterised by single-crystal X-ray diffraction, magnetic susceptibility and Mössbauer spectral methods. This compound, [Fe2(ddpp)2(NCS)4],4,CH2Cl2, undergoes a two-step full spin crossover. Structural analysis at each of the three plateau temperatures has revealed a dinuclear molecule with spin states HS,HS, HS,LS and LS,LS (HS: high spin, LS: low spin) for the two iron(II) centres. This is the first time that resolution of the metal centres in a HS,LS ordered state has been achieved in a two-step dinuclear iron(II) spin-crossover compound. Thermogravimetric data show that the dichloromethane solvate molecules can be removed in two distinct steps at 120,°C and 200,°C. The partially de-solvated clathrate, [Fe2(ddpp)2(NCS)4],CH2Cl2, undergoes a one-step transition with an increased transition temperature with respect to the as synthesised material. Structural characterisation of this material reveals subtle changes to the coordination geometries at each of the iron(II) centres and striking changes to the local environment of the dinuclear complex. The fully de-solvated material remains high spin over all temperatures. Interestingly, the solvent can be re-introduced into the monosolvated solid to achieve complete conversion back to the original two-step crossover material, [Fe2(ddpp)2(NCS)4],4,CH2Cl2. [source]

High-Spin- and Low-Spin-State Structures of [Fe(chloroethyltetrazole)6](ClO4)2 from Synchrotron Powder Diffraction Data

CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2006
Eva Dova Dr.
Abstract The spin-crossover complex [Fe(teec)6](ClO4)2 (teec = chloroethyltetrazole) exhibits a 50,% incomplete spin crossover in the temperature range 300,30 K. Time-resolved synchrotron powder diffraction experiments have been carried out to elucidate its structural behavior. We report crystal structure models of this material at 300 K (high spin) and 90 K (low spin), as solved from synchrotron powder diffraction data by using Genetic Algorithm and Parallel Tempering techniques and refined with Rietveld refinement. During short synchrotron powder diffraction experiments (five minutes duration) two distinguishable lattices were observed the quantities of which vary with temperature. The implication of this phenomenon, that is interpreted as a structural phase transition associated with the high-to-low spin crossover, and the structural characteristics of the high-spin and low-spin models are discussed in relation to other compounds showing a similar type of spin-crossover behavior. [source]

Mixed-Valence, Mixed-Spin-State, and Heterometallic [2×2] Grid-type Arrays Based on Heteroditopic Hydrazone Ligands: Synthesis and Electrochemical Features

CHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005
Lindsay H. Uppadine Dr.
Abstract An extended family of heterometallic [M12M22(L,)4]n+ [2×2] grid-type arrays 1,9 has been prepared. The three-tiered synthetic route encompasses regioselective, redox and enantioselective features and is based on the stepwise construction of heteroditopic hydrazone ligands A,C. These ligands contain ionisable NH and nonionisable NMe hydrazone units, which allows the metal redox properties to be controlled according to the charge on the ligand binding pocket. The 2-pyrimidine (R) and 6-pyridine (R,) substituents have a significant effect on complex geometry and influence both the electrochemical and magnetic behaviour of the system. 1H NMR spectroscopic studies show that the FeII ions in the grid can be low spin, high spin or spin crossover depending on the steric effect of substituents R and R,. This steric effect has been manipulated to construct an unusual array possessing two low-spin and two spin-crossover FeII centres (grid 8). Electrochemical studies were performed for the grid-type arrays 1,9 and their respective mononuclear precursor complexes 10,13. The grids function as electron reservoirs and display up to eight monoelectronic, reversible reduction steps. These processes generally occur in pairs and are assigned to ligand-based reductions and to the CoIII/CoII redox couple. Individual metal ions in the heterometallic grid motif can be selectively addressed electrochemically (e.g., either the CoIII or FeII ions can be targeted in grids 2 and 5). The FeII oxidation potential is governed by the charge on the ligand binding unit, rather than the spin state, thus permitting facile electrochemical discrimination between the two types of FeII centre in 7 or in 8. Such multistable heterometallic [2×2] gridlike arrays are of great interest for future supramolecular devices incorporating multilevel redox activity. Une famille de complexes hétérométalliques [M12M22(L,)4]n+du type grille [2×2] 1,9 a été préparée. Les trois approches synthétiques multiétapes des ligands hydrazone hétéroditopiques A,C possèdent chacune des caractéristiques régio-, rédox-, énantio-sélectives. Ces ligands possèdent un groupement NH ionisable et un groupement hydrazone non-ionisable NMe, qui permettent un contrôle des propriétés rédox du métal complexé en fonction de la charge de la cavité coordinante. Les substituants 2-pyrimidine (R) et 6-pyridine (R,) influencent de manière significative la géométrie des complexes formés ainsi que leurs propriétés électrochimiques et magnétiques. Les études par spectroscopie RMN1H montrent que les ions FeII, dans ces complexes, possèdent une configuration électronique soit de bas spin, soit de haut spin ou encore de transition de spin en fonction de l'encombrement stérique du substituant. Cet effet stérique a été utilisé pour construire des grilles originales possédant deux cations FeIIde bas spin et deux cations FeIIà transition de spin (grille 8). Des études électrochimiques détaillées ont été réalisées pour les assemblages du type grille 1,9 et leurs précurseurs mononucléaires 10,13. Ces grilles fonctionnent comme réservoir à électrons. Jusqu'à huit étapes de réduction monoélectronique réversibles peuvent être observées. Ces transferts d'électrons se font généralement par paire et interviennent sur le ligand à l'exception des deux premières réductions correspondant aux couples CoIII/CoII. Chaque cation métallique d'une grille peut être sollicité sélectivement par voie électrochimique (les ions CoIIIou FeIIpeuvent être ciblés dans les grilles 2 et 5). Le potentiel d'oxydation du FeIIest tributaire de la charge des ligands et non de l'état de spin, permettant une discrimination électrochimique parmi les deux types de FeIIdans 7 ou 8. De telles grilles [2×2] hétérométalliques ont des niveaux de stabilité multiples et présentent un grand intérêt pour des systèmes supramoléculaires possédant une multiplicité de niveaux d'oxydo-réduction. [source]

Nanoparticles of [Fe(NH2 -trz)3]Br2,3,H2O (NH2 -trz=2-Amino-1,2,4-triazole) Prepared by the Reverse Micelle Technique: Influence of Particle and Coherent Domain Sizes on Spin-Crossover Properties

CHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2009
Thibaut Forestier Dr.
Abstract By changing the surfactant/water ratio, nanoparticles of the iron(II) spin crossover material, [Fe(NH2 -trz)3]Br2,3,H2O (with NH2 -trz=4-amino-1,2,4-triazole), have been synthesised from 1,,m down to 30,nm (see figure). Magnetic and reflectivity experiments indicate that the critical size for observing a thermal hysteresis in this 1D polymer family is around 50,nm, and powder X-ray diffraction shows that particles of about 30,nm are constituted by about one coherent domain. This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2 -trz)3]Br2,3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320,K. We find that when the size is reduced down to 50,nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50,nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30,nm individual particles consist of one coherent domain. [source]