Marinela M. Dîrtu

Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH2trz)3](NO3)2: the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH2trz)3](NO3)2·H2O

The thermally induced hysteretic spin transition (ST) that occurs in the polymeric chain compound [Fe(NH(2)trz)(3)](NO(3))(2) (1) above room temperature (T(c)(upward arrow) = 347 K, T(c)(downward arrow) = 314 K) has been tracked by (57)Fe Mössbauer spectroscopy, SQUID magnetometry, differential scanning calorimetry (DSC), and X-ray powder diffraction (XPRD) at variable temperatures. From the XRPD pattern indexation, an orthorhombic primitive cell was observed with the following cell parameters: a = 11.83(2) A, b = 9.72(1) A, c = 6.361(9) A at 298 K (low-spin state) and a = 14.37(2) A, b = 9.61(4) A, c = 6.76(4) A at 380 K (high-spin state). The enthalpy and entropy variation associated to the ST of 1, have been evaluated by DSC as DeltaH = 23(1) kJ mol(-1) and DeltaS = 69.6(1) J mol(-1) K(-1). These thermodynamic data were used within a two-level Ising like model for the statistical analysis of First Order Reversal Curve (FORC) diagram that was recorded for 1, in the cooling mode. Strong intramolecular cooperative effects are witnessed by the derived interaction parameter of J = 496 K. The crystal structure of [Cu(NH(2)trz)(3)](NO(3))(2).H(2)O (2) was obtained thanks to high quality single crystals prepared by slow evaporation after hydrothermal pretreatment. The catena poly[mu-tris(4-amino-1,2,4-triazole-N1,N2) copper(II)] dinitrate monohydrate (2) crystallizes in the monoclinic space group C2/c, with a = 16.635(6) A, b = 13.223(4) A, c = 7.805(3) A, beta = 102.56(3) degrees, Z = 4. Complex 2 is a 1D infinite chain containing triple N1,N2-1,2,4-triazole bridges with an intra-chain distance of Cu...Cu = 3.903(1) A. A dense H-bonding network with the nitrate counteranion involved in intra-chain and inter-chain interactions is observed. Such a supramolecular network could be at the origin of the unusually large hysteresis loop displayed by 1 (DeltaT approximately 33 K), as a result of an efficient propagation of elastic interactions through the network. This hypothesis is strengthened by the crystal structure of 2 and by the absence of crystallographic phase transition for 1 over the whole temperature range of investigation as shown by XRPD.

Prediction of the Spin Transition Temperature in Fe(II) One-Dimensional Coordination Polymers: an Anion Based Database.

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by (57)Fe Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T(1/2) of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 <or= V (nm(3)) <or= 0.09) and for divalent anions (above V >or= 0.11 nm(3)) with saturation around T(1/2) = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.

Spin-state ordering on one sub-lattice of a mononuclear iron(III) spin crossover complex exhibiting LIESST and TIESST

The two-step spin crossover in mononuclear iron(III) complex [Fe(salpm)2 ]ClO4 ⋅0.5 EtOH (1) is shown to be accompanied by a structural phase transition as concluded from (57) Fe Mössbauer spectroscopy and single crystal X-ray diffraction, with spin-state ordering on just one of two sub-lattices in the intermediate magnetic and structural phase. The complex also exhibits thermal- and light-induced spin-state trapping (TIESST and LIESST), and relaxation from the LIESST and TIESST excited states occurs via the broken symmetry intermediate phase. Two relaxation events are evident in both experiments, that is, two T(LIESST) and two T(TIESST) values are recorded. The change in symmetry which accompanies the TIESST effect was followed in real time using single crystal diffraction. After flash freezing at 15 K the crystal was warmed to 40 K at which temperature superstructure reflections were observed to appear and disappear within a 10 000 s time range. In the frame of the international year of crystallography, these results illustrate how X-ray diffraction makes it possible to understand complex ordering phenomena.

Rapid Cooling Experiments and Use of an Anionic Nuclear Probe to Sense the Spin Transition of the 1D Coordination Polymers [Fe(NH2trz)3]SnF6⋅n H2O (NH2trz=4‐amino‐1,2,4‐triazole)

[Fe(NH2trz)3]SnF6n x H2O (NH(2)trz=4-amino-1,2,4-triazole; n=1 (1), n=0.5 (2)) are new 1D spin-crossover coordination polymers. Compound 2 exhibits an incomplete spin transition centred at around 210 K with a thermal hysteresis loop approximately 16 K wide. The spin transition of 2 was detected by the Mössbauer resonance of the 119Sn atom in the SnF6 (2-) anion primarily on the basis of the evolution of its local distortion. Rapid-cooling 57Fe Mössbauer and superconducting quantum interference device experiments allow dramatic widening of the hysteresis width of 2 from 16 K up to 82 K and also shift the spin-transition curve into the room temperature region. This unusual behaviour of quenched samples on warming is attributed to activation of the molecular motion of the anions from a frozen distorted form towards a regular form at temperatures well above approximately 210 K. Potential applications of this new family of materials are discussed.

Water effect on the spin-transition behavior of Fe(II) 1,2,4-triazole 1D chains embedded in pores of MCM-41

The spin-crossover (SCO) compounds [Fe(Htrz)3](BF4)2·H2O (SCO-1) and [Fe(Htrz)2trz]BF4 (SCO-2) (Htrz = 1,2,4-triazole) were embedded in the pores of mesostructured silica MCM-41 to yield SCO@MCM composites as evidenced by electron microscopy, gas sorption studies, powder X-ray diffractometry, atomic absorption and infrared spectrometry. Studies of the temperature-induced spin crossover behavior of the composites by temperature-variable 57Fe Mossbauer spectroscopy, magnetic and differential scanning calorimetry measurements and optical reflectivity indicate that the spin transition of the composites was significantly shifted for SCO-1@MCM to higher temperature in comparison to bulk SCO-1 compounds while the shift for SCO-2 was negligible. These shifts in the transition temperature for SCO-1@MCM [versus bulk SCO-1] amounted to T↑c = 371/376 K [282/291 K] and T↓c = 340/345 K [276/286 K] (magnetic/optical reflectivity data) with a broadening of the hysteresis by 25–26 K relative to bulk SCO-1 (varying slightly with the used method). The significant difference in the SCO behavior of the similar materials SCO-1 and SCO-2 when embedded in the MCM-41 matrix is assigned to the hydration of the SCO-1@MCM material. Water is apparently crucial in transmitting the confinement pressure or matrix effect on the spin transition when the SCO compound is embedded between the pore walls.

FeII Spin Transition Materials Including an Amino–Ester 1,2,4-Triazole Derivative, Operating at, below, and above Room Temperature

A new family of one-dimensional Fe(II) 1,2,4-triazole spin transition coordination polymers for which a modification of anion and crystallization solvent can tune the switching temperature over a wide range, including the room temperature region, is reported. This series of materials was prepared as powders after reaction of ethyl-4H-1,2,4-triazol-4-yl-acetate (αEtGlytrz) with an iron salt from a MeOH/H2O medium affording: [Fe(αEtGlytrz)3](ClO4)2 (1); [Fe(αEtGlytrz)3](ClO4)2·CH3OH (2); [Fe(αEtGlytrz)3](NO3)2·H2O (3); [Fe(αEtGlytrz)3](NO3)2 (4); [Fe(αEtGlytrz)3](BF4)2·0.5H2O (5); [Fe(αEtGlytrz)3](BF4)2 (6); and [Fe(αEtGlytrz)3](CF3SO3)2·2H2O (7). Their spin transition properties were investigated by (57)Fe Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and differential scanning calorimetry (DSC). The temperature dependence of the high-spin molar fraction derived from (57)Fe Mössbauer spectroscopy in 1 reveals an abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (Tc(↑) = 296 K and Tc(↓) = 291 K). The properties drastically change with modification of anion and/or lattice solvent. The transition temperatures, deduced by SQUID magnetometry, shift to Tc(↑) = 273 K and Tc(↓) = 263 K for (2), Tc(↑) = 353 K and Tc(↓) = 333 K for (3), Tc(↑) = 338 K and Tc(↓) = 278 K for (4), T(↑) = 320 K and T(↓) = 305 K for (5), Tc(↑) = 106 K and Tc(↓) = 92 K for (6), and T(↑) = 325 K and T(↓) = 322 K for (7). Annealing experiments of 3 lead to a change of the morphology, texture, and magnetic properties of the sample. A dehydration/rehydration process associated with a spin state change was analyzed by a mean-field macroscopic master equation using a two-level Hamiltonian Ising-like model for 3. A new structural-property relationship was also identified for this series of materials [Fe(αEtGlytrz)3](anion)2·nSolvent based on Mössbauer and DSC measurements. The entropy gap associated with the spin transition and the volume of the inserted counteranion shows a linear trend, with decrease in entropy with increasing the size of the counteranion. The first materials of this substance class to display a complete spin transition in both spin states are also presented.

Single-walled metal-organic nanotube built from a simple synthon.

A conformationally flexible triazole-carboxylic acid ligand derived from an L-amino acid, namely, 4 H-1,2,4-triazol-4-yl-acetic acid (αHGlytrz), has been exploited to synthesize a structurally diverse and functionally intriguing metal-organic framework with CuSiF6. The crystal structure reveals a novel single-walled metal-organic nanotube (SWMONT), namely, {[Cu3(μ3-OH)(H2O)3(Glytrz)3]⋅SiF6⋅8 H2O⋅X}∞ (1), (where X = disordered lattice water molecules) having a pore size as large as zeolites. Compound 1 was synthesized as crystals, as powder, or as layers by precipitation/electrodeposition. Mercury intrusion porosimetry demonstrates the ability of this material to store metallic mercury, after a pressure treatment, contrary to previous literature examples.

Spin Transition Sensors Based on β-Amino-Acid 1,2,4-Triazole Derivative

A β-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (βAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(βAlatrz)3](CF3SO3)2·0.5H2O (1·0.5H2O). The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (Tc↑ = 232 K and Tc↓ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H2O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(βAlatrz)3](CF3SO3)2 (1) was found to exhibit a similar spin transition curve however much temperature was increased by (Tc↑ = 252 K and Tc↓ = 248 K). The removal of lattice water molecules from 1·0.5H2O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced.

Pressure Sensor via Optical Detection Based on a 1D Spin Transition Coordination Polymer

We have investigated the suitability of using the 1D spin crossover coordination polymer [Fe(4-(2′-hydroxyethyl)-1,2,4-triazole)3]I2·H2O, known to crossover around room temperature, as a pressure sensor via optical detection using various contact pressures up to 250 MPa. A dramatic persistent colour change is observed. The experimental data, obtained by calorimetric and Mössbauer measurements, have been used for a theoretical analysis, in the framework of the Ising-like model, of the thermal and pressure induced spin state switching. The pressure (P)-temperature (T) phase diagram calculated for this compound has been used to obtain the P-T bistability region.

Iron(II) spin transition coordination polymers with a zigzag structure

The synthesis and characterisation of seven iron(ii) 1D chain coordination polymers with tetradentate Schiff-base like equatorial ligands and bis(4-pyridylmethyl)sulfide (bpms) as a flexible bridging axial ligand is reported. This new family of materials displays a wide spectrum of spin transition properties in the solid state ranging from gradual, abrupt, incomplete to even step-wise that have all been characterized by SQUID magnetometry. The X-ray structure analysis of two complexes at several temperatures is discussed in the frame of their spin crossover properties.