FeII Spin Transition Materials Including an AminoâEster 1,2,4

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FeII Spin Transition Materials Including an Amino−Ester 1,2,4Triazole Derivative, Operating at, below, and above Room Temperature Marinela M. Dîrtu,† Anil D. Naik,† Aurelian Rotaru,‡ Leonard Spinu,§ Dirk Poelman,∥ and Yann Garcia*,† †

Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity, Université catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium ‡ Department of Electrical Engineering and Computer Science & MANSiD Research Center, “Stefan cel Mare” University, University Street, 13, Suceava 720229 Romania § Advanced Materials Research Institute, Department of Physics, University of New Orleans (UNO), New Orleans, Louisiana 70148 United States ∥ Lumilab Department Solid State Sciences, Ghent University, Krijgslaan 281, S1, B-9000 Gent, Belgium S Supporting Information *

ABSTRACT: A new family of one-dimensional FeII 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 ](NO 3 ) 2 ·H 2 O (3); [Fe(αEtGlytrz) 3 ](NO 3 ) 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 57Fe Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and differential scanning calorimetry (DSC). The temperature dependence of the high-spin molar fraction derived from 57Fe 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 twolevel 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.

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INTRODUCTION There is nowadays a great impetus on molecular-based electronic systems integrated with molecular switches,1,2 particularly with those presenting spin transitions,3 related phenomena,4 and spintronic properties,5 which have emerged as promising technological platforms. In particular, recent results on devices containing spin crossover (SCO) nanoparticles5−8 opened new opportunities in the elaboration of SCO-based spintronic and nanoelectronic systems.9 Among switchable materials, bistable molecular complexes exhibiting SCO present indeed numerous assets. This phenomenon has been identified in 3d4−3d7 transition metal ions in octahedral © XXXX American Chemical Society

surroundings and comprehensively investigated for a large number of FeII mononuclear complexes10 and several FeII coordination polymers (CPs),11,12 to such an extent that their potential implementation in binary switching, display devices, and smart sensors,13,14 for example, for the cold channel control for food and drugs,15 is currently considered. In an archetypical FeII SCO material, the electron repositioning via singlet− quintet transitions is substantiated to be significant.3 Indeed, the reversible intraionic electron transfer from a diamagnetic Received: January 6, 2016

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DOI: 10.1021/acs.inorgchem.6b00015 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

teases,52 oxide synthesis with shape and phase selectivity.53 As a continuation of this work, ethyl-4H-1,2,4-triazol-4-yl acetate (αEtGlytrz) was used as a prospective precursor for the preparation of 1D FeII chains (Scheme 1) of formula

low-spin (LS, 1A1g) state to a thermally populated paramagnetic high-spin (HS, 5T2g) state is recognized as an entropy-driven process and could be addressed thermally, optically, electrically, and under pressure with highly profound spectroscopic, optical, magnetic, dielectric readout signal.3 In the solid state, the presence of short- and long-range interactions mediated by phonons and solvent and/or anion occupancy in the crystal lattice can drastically affect the cooperative communication among iron centers. Indeed, a SCO compound meeting display and data processing requirements preferably should have a good shelf life, large hysteresis width (>50 K), easily detectable response, and abrupt spin transition.16,17 Depending on the targeted application its operating temperature could be tuned to below, above, and around room temperature. Accordingly to the need of current nano size reduction of SCO materials, nanoparticles,7,18−22 thin films,23,24 gels,25,26 nanocomposites,27−29 liquid crystals,30 monoliths,31 patterned nanostructures,24,32 hybrid SCO materials made of SiO2,22 polystyrene,33 and deposited on a biomembrane,34 to name few examples, were successfully produced. The family of thermochromic one-dimensional (1D) FeII 1,2,4-triazole SCO CPs of formula [Fe(4R-1,2,4-triazole)3]anion·nSolvent has attracted great attention in recent years,11,12a due to their soluble ability,35 driven by their thermochromic,16,29 fluorescent36 and photochromic properties.37 Only very few FeII molecular systems were found, however, to present SCO around and above room temperature.38 Depending upon their ligand field strength, introducing sterical constraints on the 1D chains or varying noncoordinated species (anions, solvent) to modulate elastic interactions through the crystal lattice, several systems presenting different transition temperatures and different shapes of hysteresis loops were observed.11a,b,12a A remarkable example is described for [Fe(hyetrz)3]I2 (hyetrz = 4-(2′hydroxyethyl)-1,2,4-triazole) that displays a sharp SCO centered at room temperature with a 12 K bistability domain.39 Such a hysteresis width is, however, not large enough in case an unwanted overheating/cooling switches the spin state and associated color for a given prototype made of this material. Only three other examples exhibiting a hysteretic spin transition (ST) above room temperature are reported for [Fe(Htrz)3](CF3SO3)2 (Htrz = 4-H-1,2,4-triazole),40 [Fe(Htrz)2trz]BF4 (trz = triazolato),41,42 and [Fe(NH2trz)3](NO3)2 (NH2trz = 4-amino-1,2,4-triazole)43 with 50, 40, and 35 K widths, respectively, all obtained from commercially available precursors. These are the largest hysteresis widths ever reported for this substance class in the crystalline state. Except [Fe(Htrz)3](CF3SO3)2, these hysteresis widths are, however, too weak for an application in display and data recording devices as recommended by Kahn et al.16 For these 1D cationic chain compounds, the prominent role of hydrogen bonding involving the counteranion in both intra- and intermolecular interactions in the crystal lattice has been recently comprehensively demonstrated;44−46 after that, the importance of hydrogen bonds on the observation of wide hysteresis SCO loops was underlined.47 In this frame, we recently introduced an uncommon class of 1,2,4-triazole functionalized amino acid derivatives by transamination.48,49 These molecules, which present an electronic asymmetry, offer two possible coordination sites: a carboxylic group and a N1,N2 heterocycle.50 Our interest was fueled by promising results shown by these molecules in synthetic chemistry,48 nanoporous metal−organic frameworks design,49−51 biological activities in metallopro-

Scheme 1. Schematic Representation of αEtGlytrz (right) and of the [Fe(αEtGlytrz)3]2+ One-Dimensional Chain (left)

[Fe(αEtGlytrz)3](anion)2·nSolvent, which are discussed in the present paper. A preliminary account on the ST properties of two of these FeII materials was communicated.54 In parallel, we recall that we have introduced a β-amino acid ester 1,2,4triazole (βEtAltrz), which was used to produce 1D FeII SCO chains exhibiting abrupt,55 gradual,56 and even stepwise spin transitions.57 In the present work, we highlight the ST properties of an FeII 1D 1,2,4-triazole chain presenting an exceptionally large hysteresis width for this substance class (60 K) located at room temperature. We also study the influence of annealing on the ST properties of these complexes evidencing unusual “kinks” close to the room-temperature region.

2. RESULTS 2.1. Synthesis and General Characterization. αEtGlytrz was prepared by a transamination reaction,48 which proved to be a relevant synthetic strategy to build a 1,2,4-triazole on a primary amine. Coordination polymers were obtained as pink and off-white powders by reaction of the corresponding FeII inorganic precursor, Fe(H 2O) 6(anion)2, in air, with a methanolic solution of αEtGlytrz, except for [Fe(H2O)6](NO3)2, which was prepared in situ.43 The workup conditions were varied, which led to different solvates. In this respect, after filtration in air, powders were dried either in air, under a N2(g) flow or under vacuum and stored under a N2(g) atmosphere. All these complexes were successfully characterized by elemental analysis, thermogravimetric analysis (TGA) and differential thermal analysis, atomic absorption, X-ray powder diffraction (XRD), IR, Raman, scanning electron microscopy (SEM), and 57Fe Mössbauer spectroscopy. Thermogravimetric and elemental analyses revealed the presence of methanol or water guest molecules, affording the following formula [Fe(αEtGlytrz) 3 ](ClO 4 ) 2 (1); [Fe(αEtGlytrz) 3 ](ClO 4 ) 2 · CH 3 OH (2); [Fe(αEtGlytrz) 3 ](NO 3 ) 2 ·H 2 O (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). Isostructurality among 1 to 6 was concluded from XRD patterns that revealed similarity among the principal characteristic peaks in the diffractogram of this series, located at 2θ = 0.8°, 5.5°, 6.7°, 7.6°, 9.4°, 15.6°, 16.7°, and 20.3° (Figure 1a). Compound 7 shows a less-resolved diffractogram. Although a similar molecular organization around iron chains is expected for other members of the series, the asymmetric nature of the CF3SO3− anion may induce a preferential chain organization, which could influence on direction and strength of H-bonds, which can impact the resulting XRD pattern.55 The crystallinity B


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vibrational spectroscopic data (Fourier transform infrared (FTIR) and Raman) to confirm the identity of the ligand framework upon complexation, because the ester functionality is susceptible to hydrolysis, as well as to ascertain coordination mode of the ligand and presence of noncoordinated counteranions in the complexes (Figure 2). Three sharp bands at 3112(s), 3012(m), and 2933(m) cm−1 are assigned to the νCH2 modes in the IR spectrum of αEtGlytrz. The carboxylic group of ester shows two stretching vibrations corresponding to CO and C−O at 1743(s) and 1236(s) cm−1, respectively, which are almost unchanged (1746 and 1240 cm−1) in complexes, ruling out the possible ester hydrolysis and also confirming a noninvolvement of carboxylic ester group in coordination. For αEtGlytrz the band assigned to ring torsion of 1,2,4-triazole at ν = 637(m) cm−1, the νCN stretching vibration at 1539(m) cm−1, and the N−N stretching band at νN−N = 1018(s) cm−1 are all shifted upon complexation, for example, in 1 to 625 cm−1, νCN = 1560 cm−1, and νN−N = 1026 cm−1, respectively. These values confirm the coordination of the iron to the 1,2,4-triazole ring.41,58 The νC−N and νC−C vibrations are identified at 1431(m) and 1165(m) cm−1, respectively. The absence of ester hydrolysis for αEtGlytrz was supported by a comparison with the IR bands of 4H-1,2,4triazol-4-yl acetic acid (αHGlytrz),49 which are slightly different. The −COO− vibrations (asymmetric and symmetric) are found as a strong band at 1583 cm−1 and as a weak band at 1411 cm−1. Carboxylic acids are known to present a strong band due to the stretching vibration of the C−O between 1200−1000 cm−1.59 In αHGlytrz this band is identified at 1197 cm−1. The other bands, characteristics of the triazole ring, attributed to the ring torsion of triazole, to the νCN stretching vibration, to the N−N stretching band, and to the C−N band, are observed at ν = 629(m), 1568(m), 1024(s), and 1018(s) cm−1, respectively. A set of bands are also identified at 3123, 2889, and 2837 cm−1, which are assigned to the CH2 stretching. For a better understanding of the molecular structure of these complexes, Raman spectra were collected (Figure 2). The band at 1737 cm−1 for αEtGlytrz is assigned to a CO vibration. In complexes this band is retained and appears around 1749 cm−1. The C−C stretching is also active in Raman as a medium band at 1104 cm−1 for αEtGlytrz and appears in the same wavenumber range, however, being more intense, for the complexes. The Fe−N stretching vibrations could also be identified by Raman spectroscopy in the range of 200−400

Figure 1. (a) X-ray powder diffraction patterns at 293 K for 1 to 7. Diffraction patterns for 3 and 6 are identical to those of 4 and 5 and were thus omitted for clarity; (b) SEM images of powder particles of 1, 4, 5, and 7 at 293 K.

of the samples was investigated by SEM analysis (Figure 1b). Aggregates were identified for 5, rods of dimensions (2.5 μm × 180 nm) for 4, and small blocks for 7 were identified. Attempts to crystallize these complexes proved unsuccessful due to the slow hydrolysis of αEtGlytrz in solution. In view of the difficulty to obtain single crystals for this class of coordination compounds, attention was focused on the

Figure 2. IR spectra of (left) αEtGlytrz (L1) and complexes 1 to 7 and (right) Raman spectra of αEtGlytrz (L1) and complexes 1, 4, 5, and 7. IR and Raman spectra of 3 and 6 are identical to those of 4 and 5 and were thus omitted for clarity. C


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Figure 3. UV−vis diffuse reflectance spectra of selected complexes showing d−d transitions at 293 K (left) and at 400 K (right).

cm−1. The weak band at 349 cm−1 (1), 358 cm−1 (3 and 4), 351 cm−1 (5 and 6), and 350 cm−1 (7) is due to the stretching vibration ν(FeLS−N).60 The spectrum of 7 also contains a low intensity band at 317 cm−1, which is characteristic for ν(FeHS− N) stretching vibration, in agreement with its expected decrease upon a LS to HS transition as a consequence of a lower force constant due to an increase of the Fe−N bond elasticity.61 Such HS species at room temperature60 were also detected by Mössbauer spectroscopy in 7 (vide infra). Presence of noncoordinated monovalent counter-anions originating from the inorganic precursors was also confirmed by IR and Raman as listed in the synthesis section: IR (cm−1) ν(Cl−O) ≈ 1105 (s), (1), (2); ν(N−O) ≈ 1384 (s), (3); ν(N−O) ≈ 1384 (s), (4); ν(B−F) ≈ 522 (w), 1083 (vs) (5); ν(B−F) ≈ 522 (w), 1083 (vs) (6); ν(S−O) ≈ 1251 (vs), (7). Raman (cm−1) ν(Cl−O) ≈ 932 (s), 460 (m) (1), (2); ν(N−O) ≈ 1042 (s), (3); ν(N−O) ≈ 1042 (s), (4); ν(B−F) ≈ 767 (s) (5); ν(B−F) ≈ 767 (s) (6); ν(S−O) ≈ 1036 (s), 770 (m) (7). 57 Fe Mössbauer spectroscopy indicates the presence of only one FeIIN6 site, for this series of complexes, as well as the absence of oxidation product. Hyperfine parameters are typical for 1D FeII polymeric chain compounds62 (see Tables S1−S5). On the basis of IR, Raman, and Mössbauer spectroscopy data, a chain configuration in which the FeII ions are linked by triple N1,N2-1,2,4-triazole bridges, can be safely suggested. The color of complexes 1 to 7, being either pale pink or white at room temperature, can be reversibly changed to white or pink, on warming and cooling, respectively. For instance, 1 exhibits reversible pronounced thermochromism from white to pink on quenching of the sample in liquid nitrogen. However, cooling in ice is also sufficient to reveal a deep color change precluding a spin state change just below room temperature. These colors depend on the spin state of the FeII centers, which were analyzed by UV−vis diffuse reflectance spectroscopy (DRS) at 293 and 400 K (Figure 3). The pink color is due to the 1A1g →1T1g d−d transition of LS FeII sites observed at 18 400 cm−1 (for 1), at 18 200 cm−1 (for 4 and 5), and at 18 700 cm−1 (for 7).45 A second band identified at 26 800 cm−1 (for 1) and 25 100 cm−1 (for 4 and 5) is attributed to the 1A1g →1T2 d−d transition.45 For 7, another band is found at 11 800 cm−1, which is attributed to the transition 5T2(g) →5Eg,45 suggesting the presence of a fraction of HS FeII ions at room temperature (Figure 3). The white color is indeed due to the location of the spin-allowed lowest-energy d−d transition, 5 T2(g) → 5Eg, for the HS sites in the near-infrared region and which is clearly observed at 400 K, for example, for 5.63,64 A supplementary band is identified at ∼18 700 cm−1, for 5 and 7, indicating the presence of LS FeII ions (Figure 3). The ligand field strengths for the HS and LS states, given by eqs 10DqHS =

E(5E) − E(5T2) and 10DqLS = E(1T1) − E(1A1) + (E(1T2) − E(1T1))/4,63,64 allowed to estimate 10DqHS and 10DqLS to be ∼12 000 and 25 100 cm−1, respectively. These values are characteristic for SCO complexes.63,64 2.2. Thermal Spin Crossover Properties. 2.2.1. 57Fe Mössbauer Spectroscopy. Mössbauer spectra for 1−7 were recorded in transmission mode on cooling and warming over the temperature range of 78−368 K. Associated parameters are provided in the Supporting Information (Tables S1−S5). At 78 K, the spectrum of [Fe(αEtGlytrz)3](ClO4)2 (1) consists of a single quadrupole doublet corresponding to an LS FeII ion, with an isomer shift δLS = 0.49(1) mm·s−1 and a low quadrupole splitting ΔEQLS = 0.19(1) mm·s−1 (Figure 4). This quadrupole splitting is characteristic for a lattice contribution to the electric field gradient, which indicates a distortion of the LS octahedron, as expected for a constrained chain of iron atoms triply bridged by 1,2,4-triazole ligands.65 When warmed to 293 K, no change is observed in the spectra, but at 298 K, a new

Figure 4. Selected 57Fe Mössbauer spectra of 1 on warming (left) and cooling modes (right). Gray and dark gray correspond to the HS and LS doublets, respectively. D


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Inorganic Chemistry quadrupole doublet characteristic of a HS FeII ion is detected, indicating the onset of a spin state crossover from LS to HS, which progresses further at 300 K. Indeed, at this temperature, a minor fraction of LS FeII ions (9%) is detected in agreement with the pale pink color of the powder. At 308 K, a single HS quadrupole doublet is observed, with δHS = 1.03(1) mm/s and ΔEQHS = 2.74(4) mm/s, indicating a complete and very abrupt SCO around room temperature. The asymmetry of the lines found in the HS state is attributed to a texture effect. When cooled, a hysteretic behavior is clearly revealed by comparing Mössbauer spectra recorded at 298 and 293 K (Figure 4). Isomer shifts and quadrupole splittings for both LS and HS species, listed in Table S1, are typical of an FeIIN6 chain in which metallic centers are bonded by a bridge made of three N1,N2-triazole ligands.15 The temperature dependence of the HS molar fraction, γHS, was plotted, assuming equal Debye− Waller factors for HS and LS ions (Figure 5a), which is

Figure 6. 57Fe Mössbauer spectra of 2 recorded on warming from room temperature to 300 K and on cooling to 78 K. Gray and dark gray correspond to HS and LS doublet, respectively.

drawn on warming and cooling, confirms the hysteretic behavior associated with the ST at Tc↑ = 273 K and Tc↓ = 263 K, which is shifted to lower temperatures compared to that of 1 (Figure 5a). Interestingly, the hysteresis loop of 10 K width is dissymmetric and may preclude a structural phase transition.66 57 Fe Mössbauer spectrum of [Fe(αEtGlytrz)3](NO3)2·H2O (3) at room temperature reveals a single doublet characteristic of FeII LS (δ = 0.51(1) mm/s and ΔEQ = 0.23(1) mm/s). This sample, which contains water molecules, was not investigated on warming due to the noticed water release (Figure 12), which implies a modification of its composition during the measurement. Selected 57Fe Mössbauer spectra of [Fe(αEtGlytrz)3](NO3)2 (4) are shown in Figure 7. At 298 K, a single quadrupole doublet with an isomer shift δLS = 0.39(1) mm·s−1 and a quadrupole splitting ΔEQLS = 0.19(1) mm·s−1 refers to an LS FeII ion. 57Fe Mössbauer measurements were recorded on warming to 368 K to track the emergence of a HS quadrupole doublet, which would be the signature of an LS to HS transition. This new signal was detected at 344 K, with δHS = 1.01(1) mm·s−1 and ΔEQHS = 2.43(1) mm·s−1, typical of an HS Fe(II) ion. At 368 K the spectrum consists of only one HS doublet δHS = 0.98(1) mm·s−1, ΔEQHS = 2.47(1) mm·s−1, indicating a complete spin transition. When cooled, the reverse HS to LS transition is observed, with a complete character identified at 78 K with one quadrupole doublet corresponding to the LS state (δLS = 0.46(3) mm·s−1, ΔEQLS = 0.19(1) mm· s−1). The spectra shown on cooling and warming modes at 328 and 323 K, respectively, clearly evidence a hysteretic effect of width ΔT = 55 K, which is better seen on Figure 5b with Tc↓ = 280(5) K and Tc↑ = 335(5) K.

Figure 5. HS molar fraction vs T on cooling and warming modes for (a) 1 and 2, (b) 4, (c) 5 and 6, and (d) 7 as deduced from 57Fe Mössbauer measurements. The lines are guide to the eyes.

reasonable due to the abrupt character of the spin transition. The ST curve reveals an abrupt transition centered at room temperature with a hysteresis width of 5 K. The transition temperatures, on warming and cooling, are Tc↑ = 296(2) K and Tc↓ = 291(2) K, respectively. The methanol solvate [Fe(αEtGlytrz)3](ClO4)2·CH3OH (2) was also studied by Mössbauer spectroscopy over the 78−300 K range. At 78 K, the compound is fully in the LS state (Figure 6) with identical hyperfine parameters compared to 1 (Table S1). This situation suggests that lattice methanol molecules are located in a remote fashion from the neighboring environment of chains and have therefore no effect on the local environment of the FeII ions. When warmed further, another quadrupole doublet, which is typical of the HS state, is only detected at 270 K, and no more LS doublet is detected at 300 K indicating a complete LS to HS transition (Figure 6). A hysteresis effect is also evidenced by comparing the spectra recorded at the same temperature (270 or 280 K) either on warming or on cooling, which display a different HS/LS population. The ST profile, E


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Inorganic Chemistry

Figure 7. 57Fe Mössbauer spectra of 4 between 78 and 368 K. Gray and dark gray correspond to the HS and LS doublet, respectively.

= 1.16(1) mm·s−1 and ΔEQHS = 3.39(1) mm·s−1 and δLS = 0.55(3) mm·s−1, ΔEQLS = 0.31(1) mm·s−1. When warmed the HS molar fraction increases to 76% 100 K, followed by a slight increase to 80% at 298 K. When cooled from room temperature to 78 K, the reverse situation is observed. The SCO behavior thus occurs below 100 K in the warming and cooling modes, respectively. The completeness of the spin conversion could not be studied below 78 K due to the temperature limitation of our Mössbauer cryostat but a SQUID measurement down to liquid He temperature was performed instead (vide infra). The spectrum recorded at 78 K, for [Fe(αEtGlytrz)3](CF3SO3)2·2H2O (7), (Figure 8b) shows two quadrupole doublets attributed to HS FeII ions (δ = 1.15(1) mm·s−1, ΔEQ = 3.36(1) mm·s−1) and LS FeII ions (δ = 0.49(1) mm·s−1, ΔEQ = 0.21(1) mm·s−1) in population LS/HS: 20/80, indicating an incomplete spin transition (Table S5). When warmed to room temperature, the relative intensity of the HS doublet increases to reach ∼31%. Above 310 K and up to 335 K, a dramatic change of the spectra indicates that an LS to HS transition takes place. In this temperature interval, the TGA on warming performed in air, keeping the same conditions as for the Mössbauer measurements, indicates a water molecule release (Figure 9). This behavior is typical for spin state transition driven by lattice water molecules release.62,67 Interestingly, a clear water uptake is also recorded when cooled, which is better seen when the mass gain is recorded at room temperature (Figure 9).

The Mössbauer spectrum of [Fe(αEtGlytrz)3](BF4)2·0.5H2O (5) reveals at 78 K a single quadrupole doublet attributed to LS FeII ions (δLS = 0.47(1) mm·s−1, ΔEQLS = 0.19(1) mm·s−1; Figure 8). The LS state is preserved on warming to 298 K without any sign of HS ion (Table S3). At 318 K, a quadrupole doublet with parameters δ = 1.04(1) mm/s and ΔEQ = 2.73(1) mm typical of the HS state clearly appears with a minor fraction (2%) of LS FeII ions indicating an abrupt spin state change (Figure 5c). This change is thought to be attributed to dehydration because of the long acquisition time to record the Mössbauer spectrum at 318 K for almost two weeks, which should be sufficient to remove the 0.5 noncoordinated water molecules from the sample, as demonstrated by optical reflectivity at 320 K for a related 1D chain which contained three non coordinated water molecules.67 Mössbauer spectra which were recorded in the cooling mode down to 77 K (Figure 5c) indicate that the HS molar fraction only decreases below 100 K reaching 72% at 77 K. The abrupt change of the spin population above room temperature along with the stabilization of the HS state which occurs on cooling strongly suggests that dehydration occurred on warming. This hypothesis is confirmed by studying the SCO properties of the non solvated compound [Fe(αEtGlytrz)3](BF4)2 (6) on cooling. The thermal dependence of the HS molar fraction shown in Figure 5c, clearly evidence a very gradual SCO starting from 100 K to reach 51% at 77 K (Table S4). At 77 K the spectrum shows two quadrupole doublets characteristic of HS and LS FeII, with δHS F


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being in the LS state as detected by Mössbauer spectroscopy at room temperature. When cooled further, χMT decreases abruptly to reach 0.36 cm3 K mol−1 at 260 K, corresponding to the LS state. When warmed, a sharp ST is detected delineating a hysteresis loop with Tc↓ = 291 K and Tc↑ = 296 K (Figure 10a), in good agreement with the outcome from 57Fe

Figure 10. Thermal variation of χMT of compounds 1, 2 (a), 3, 4 (b), 5, 6 (c), and 7 (d).

Mössbauer spectroscopy (Figure 5a). The ST curve of the methanol solvate (2) is identical to the one of 1 but shifted to lower temperatures with Tc↓ = 264(1) K and Tc↑ = 270(1) K (Figure 10a). The SQUID data of 2 differ quite considerably from the Mössbauer-derived HS fraction in Figure 5. This situation may arise from different Debye−Waller factors for the HS and LS states in 2. The magnetic properties of [Fe(αEtGlytrz)3](NO3)2·H2O (3) show a similar behavior with a hysteresis loop of ∼20 K width shifted upward above the room temperature region with transition temperatures on cooling and warming of 333 and 353 K, respectively (Figure 10b). Given the presence of a lattice water molecule in 3, the ST is probably induced by a reversible dehydration/hydration process, which is known to influence the spin state of 1D chains.62,67 TGAs confirm a weight loss on warming to 110 °C followed by a weight gain on cooling back to room temperature, thus supporting this hypothesis (Figure 12). The magnetic properties of [Fe(αEtGlytrz)3](NO3)2 (4) also reveal a hysteresis loop, which is centered on the roomtemperature region with Tc↓ = 278 K and Tc↑ = 338 K, and much wider (60 K). Given the absence of solvent molecules, the SCO properties are not associated with any solvent release (Figure 10b). The hysteresis loop was cycled five times and further confirmed even after storing the sample for four months in a desiccator. The ST profile of [Fe(αEtGlytrz)3](BF4)2· 0.5H2O (5) resembles the one of 3 with a hysteresis loop of 15 K width shifted downward with transition temperatures 305 and 320 K on cooling and warming modes (Figure 10c). As discussed in the Mössbauer section, the warming process releases the lattice water molecules. On immediate cooling, water can reenter the sample, which provokes the HS to LS transition detected by SQUID measurements. When warmed to 350 K with a standing period, full dehydration occurs within the

Figure 8. 57Fe Mössbauer spectra of (a) 5 and (b) 7 between 78 and 345 K. Gray and dark gray correspond to the HS and LS doublets, respectively.

Figure 9. TGA recorded at 1 K/min, when warmed to 350 K and cooling modes back to room temperature for 7. Note that a period of time was allocated at room temperature showing a clear mass uptake.

2.2.2. Superconducting Quantum Interference Device Magnetometry. After having delineated the ST range by Mössbauer spectroscopy, magnetic susceptibility measurements were recorded over the temperature range of 4−380 K. For [Fe(αEtGlytrz)3](ClO4)2 (1), a decrease of the χMT product from 3.42 cm3 K mol−1 at 315 K to 3.25 cm3 K mol−1 at 298 K, is observed, in good agreement with a fraction of few spins G


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Inorganic Chemistry

relevant thermodynamic parameters reported in Table 1. An endothermic peak is observed on warming 1 at Tmax↑ = 297 K and an exothermic peak is recorded on cooling at Tmax↓ = 292 K. These peaks correspond to a first-order phase transition associated with the ST behavior of 1. The hysteresis loop width of 5 K exactly matches the one found by 57Fe Mössbauer spectroscopy. The slight temperature shift in the hysteresis loop is related to the higher heating and cooling rates used in the calorimetric measurement. The enthalpy and entropy associated with the ST were derived: ΔHHL = 14.4 kJ mol−1 and ΔSHL = 41 J·mol−1 K−1. Compound 2 displays a hysteresis loop width of 11 K (Tmax↑ = 272 K, Tmax↓ = 261 K), which matches well with the Mössbauer spectroscopy result. The sample is air sensitive, and the measurement had to be realized on a freshly prepared compound because the noncoordinated methanol molecule is susceptible to partial desolvation, in air. A similar heat capacity profile is observed for 3 above room temperature (Tmax↑ = 358 K and Tmax↓ = 338 K) with much higher enthalpies and entropy values (see Table 1). The DSC curves of 4 show an endothermic peak around Tmax↑ = 340 K on warming and an exothermic peak at Tmax↓ = 280 K, in the cooling mode. The peaks are separated by a wide temperature domain, suggesting the presence of a broad hysteresis loop of 60 K, as detected in SQUID measurements (Figure 10). The entropy gain associated with the ST, ΔSHL = 88.4 J mol−1 K−1, is much larger than the characteristic value for the electronic contribution to the entropy, R·ln(5) = 13.4 J/mol·K, typical for FeII SCO compounds.68 This is due to a high extent for the vibrational entropy evaluated as ΔSvib = 75 J mol−1 K−1, which always dominates. The presence of another transition type could be thought. Compound 5 and 7 present first-order phase transitions as observed for the other compounds of the series. 2.2.4. Annealing Investigation. a. Thermal, Powder X-ray Diffraction, and Scanning Electron Microscopy Studies. The reversibility of the dehydration/hydration process of the framework structure with possible change in crystallinity could play an important role on the SCO behavior of these materials.62,67 This process was investigated by comparing the results of XRD and SEM before and after annealing at a temperature corresponding to complete dehydration given by TGA measurements. Two compounds containing lattice water molecules, namely, [Fe(αEtGlytrz)3](NO3)2·H2O (3) and [Fe(αEtGlytrz)3](BF4)2·0.5H2O (5), were selected for this purpose. Accordingly, each sample was deposited on a TGA crucible and slowly heated at 1 °C/min in air atmosphere (air flow 100 mL/min) to the temperature corresponding to a complete dehydration (110 °C for 3 and only 77 °C for 5). The samples were subsequently slowly cooled at 1 °C/min to 25 °C and kept at this temperature for 2 h, to study the rehydration process. Sample 3 reabsorbed appreciable amount of water molecules reaching roughly its original amount on waiting for a sufficient period of time (Figure 12a) like 5 (Figure 12b). The three consequent TGA runs on 3 revealed that rehydration capacity is altered after each heating stage even after respecting a standing period of 25 min, but a rehydration time of 2 h was enough to regain the original hydrated composition. Thus, these experiments confirm that the dehydration−rehydration process in 3 is fully reversible. The framework thermal robustness of 3 and 5 was studied by XRD, after and before TGA analyses. The XRD patterns of 3 before and after TGA confirm that the integrity of the sample was not altered, because no peak position change is observed, although

SQUID cavity, which enables the stabilization of the HS state on cooling thus leading to [Fe(αEtGlytrz)3](BF4)2 (6), which was thus prepared in situ within the magnetometer. A smooth decrease of χMT was first observed on cooling from 3.5 cm3 K mol−1 at 350 K to 3.3 cm3 K mol−1 at 270 K, where a blunt centered at 300 K was observed, presumably due to a fractional amount of 5. When cooled further, the χMT product slightly decreases, until 118 K where a sharp and incomplete ST to the LS state takes place with Tc↓ = 92 K. When warmed back to room temperature, a hysteresis loop of 14 K width was delineated with Tc↑ = 106 K. Below 62 K, a plateau is observed, which decreases progressively below 25 K as a consequence of both zero field splitting of remaining HS FeII ions and/or an overall anti-ferromagnetic coupling of these ions within a chain. The incomplete character of the ST on cooling to low temperature may be assigned to crystal defects, for example, a distribution of different chain lengths, with the presence of shorter chains with end of chains containing coordinated water molecules or to HS FeN6 nonactive sites.67b Warming [Fe(αEtGlytrz)3](CF3SO3)2·2H2O (7) to room temperature revealed a gradual change of the magnetic moment, which was followed by sharp increase and decrease on warming and cooling modes. Again, given the loss of lattice water molecules on warming above 300 K identified by TGA (Figure 9), a ST assisted by water release is expected to occur. The transition temperatures on warming and cooling modes are 325 and 322 K as shown in Figure 10d. 2.2.3. Differential Scanning Calorimetry. Compounds presenting an SCO behavior over the temperature range of 100−400 K (for 1 to 5 and 7) were also investigated by differential scanning calorimetry (DSC) on both warming and cooling modes at a 10 K min−1 rate. Corresponding heat capacity temperature profiles are displayed on Figure 11 and

Figure 11. DSC profiles for [Fe(αEtGlytrz)3](anion)2·nSolvent, in the 230−390 K in the cooling and warming modes for 1; 2; 3; 4; 5; 7. H


Article

Inorganic Chemistry

Table 1. Phase Transition Temperatures on Warming and Cooling As Deduced from DSC Measurements and Thermodynamic Parameters for 1−5 and 7a sample

anion

n H2O

1 2 3 4 5 7

ClO4− ClO4− NO3− NO3− BF4− CF3SO3−

0 1 MeOH 1 0 0.5 2

a

anion volume (nm3) 0.082 0.082 0.064 0.064 0.073 0.129

± ± ± ± ± ±

0.01369 0.01369 0.01169 0.01169 0.00969 0.00770

Tmax↑ (K)

Tmax↓ (K)

ΔT (K)

as (%)a

ΔHHL (kJ mol−1)

ΔSHL (J mol−1 K−1)

ΔSvib (J mol−1 K−1)

297 272 358 340 320 330

292 261 338 280 299 320

5 11 20 60 21 10

100 100

14.4 13 27 27.4 17.3 8

41 48.8 77.7 88.4 60 24.6

27.7 35.4 64.3 75 46.6 11.2

100

as: switching sites as deduced by Mössbauer spectroscopy. ΔSvib: vibrational entropy.

Figure 12. TGAs (recorded at 1 °C/min) for 3 (a) and 5 (b) on warming and cooling. A standby period of 25 min at maximum temperature (110 °C for 3 and 77 °C for 5) was included in the temperature program of both compounds. Three heating and cooling cycles are shown for 3.

the lattice constant increases slightly (∼1%) presumably due to water removal (Figure 13a). The diffractogram of 5 is altered

Figure 13. Comparison of XRD pattern of 3 (a) and 5 (b) thermally treated in TGA and freshly prepared. Figure 14. SEM analysis of sample (a) 3 warmed to 110 °C and (b) 5 warmed to 77 °C. The effect of thermal treatment on framework integrity is evidenced.

after the heating process due to a phase change and a loss of crystallinity of the sample (Figure 13b), which was confirmed by SEM analyses, which show modification of the particle morphology (Figure 14b). Next, variable-temperature XRD patterns were recorded on 5 over the range of 292−352 K on a different diffractometer compared to the one used for routine room-temperature experiments. Focus was done on the 2θ range of 15−35°, where principal peaks are observed (Figure 15a). No dramatic changes were observed indicating the absence of structural phase transition on warming. It was however possible to track the spin state change by following the temperature dependence of the summed intensity of selected peaks (Figure 15b), in the range between 16° and 20° by analysis of the false color plot (Figure 15c). An abrupt change in intensity around 315 K was noticed matching the transition temperature of 320 K recorded by SQUID magnetometry (Figure 10c). When cooled back to 292 K, a non-reversible behavior was observed (green curve), possibly due to the non-immediate rehydration character of the sample after water release. Actually, an amorphization of the

sample and phase change is also identified when comparing the preannealing XRD pattern (at 292 K) and postannealing (measured after heating to 393 K and rapid cooling to 292 K; Figure 14d) under helium flux, which shows severe change in intensity and position of signals. The change is more pronounced compared to Figure 13b presumably due to rapid cooling conditions used in the present experiments. b. SQUID Magnetometry Studies on Thermally Treated 3, 4, 5, and 6. Sample 3 was loaded in the SQUID and maintained at 390 K for 15 min. After the annealing treatment, SQUID measurements were recorded between 10 and 390 K on warming and cooling modes (Figure 16). Between 10 and 100 K, χMT remains constant to 0.16 cm3 K mol−1, which is typical for an FeII complex in the LS state. When heated above 100 K, a slight increase of χMT is observed until 260 K, where the magnetic moment raises to 1.79 cm3 K mol−1 indicating a partial ST to the HS state. Above this temperature, the χMT I


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Inorganic Chemistry

Figure 15. (a) Enlarged view of XRD pattern around selected 2θ (deg) for 5. (b) Evidence of a modification of XRD patterns due to the spin state change on warming (5). (c) False color plot to select the most intense peaks in the range of 16−20° noted a, b and c for 5. (d) XRD pattern for the pre- and postannealing samples for 5.

plateau is observed around 175 K. For the third run, the sample was maintained at 390 K for 15 min, and the measurement was carried out on cooling and warming modes over the temperature range 390−10 K. Most interestingly, the kink identified around 260 K is now shifted to higher χMT values, and the high-temperature ST shifted downward further to 325 K. When cooled, a HS → LS transition proceeds in one step being shifted upward around 195 K. Finally, the sample was again annealed by staying 25 min at 390 K within the SQUID cavity, and magnetic data were recorded on cooling and warming modes over the range of 390−10 K for the fourth run. A very similar profile is obtained compared to the third run, except for the kink, which is again shifted higher, thus defining a hysteresis loop below room temperature with Tc↑ = 245 K and Tc↓ = 195 K, which should correspond to a fully dehydrated sample. This unusual influence of the thermal treatment of the sample on the magnetic properties may indeed be explained by the dehydration/rehydration ability of 3, which was concluded from TGA (see previous section). This situation led us to consider the biphasic character of 3 with phase I (dehydrated) and phase II (hydrated), which are expected to lead to distinct transition temperatures. Indeed, the sum of these two phases will give rise to a two-step SCO identified by the transition temperature of the two phases.57,71 Phase I is characterized by the transition temperatures Tc↑ = 245 K, Tc↓ = 195 K (orange curve in Figure 16), whereas phase II presents a SCO behavior around 345 K (curves depicted in blue and in red in Figure 16).

Figure 16. Thermal variation of χMT of annealed 3, between 10 and 390 K.

value surprisingly decreases, thus defining a kink, to reach 0.24 cm3 K mol−1 at 289 K. When warmed further, an abrupt increase of χMT is observed to reach 3.1 cm3 K mol−1 at 350 K indicating a ST to the HS state with a transition temperature of 343 K shifted lower compared to the temperature found without annealing (Figure 10b). When cooled back from 390 K, χMT remains constant to 275 K, after which it decreases stepwise with two plateaus identified around 250 and 175 K. In the second run carried out on warming and cooling over the same temperature range, a similar ST profile is observed on warming with a shift by 10 K, downward, from 343 to 333 K, for the high-temperature spin-state transition, in good agreement with a dehydration process, which is expected to shift this temperature downward.67b When cooled from 390 K, only one J


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Inorganic Chemistry

Figure 10c (Figure 18, noted as 1) was found in the first heating mode. The sample was next maintained at 390 K for 15

After the first thermal treatment at 390 K, the sample was cooled to 10 K, which allows partial hydration of the sample within the gel-cap (used in the SQUID measurements). Thus, the kink detected around 260 K on warming from 10 K translates the partial conversion of phase I into hydrated phase II. At the kink maximum, half of the molecules are in the HS and in the LS state, respectively. Half of the HS molecules, formed by the dehydrated phase (phase I), are converted in the presence of water into the LS hydrated phase (phase II) and reach 0.25 cm3 K mol−1 at 298 K. After the third and fourth runs, the percentage of the phase I increases and saturates at 3.22 cm3 K mol−1; while the completeness of the process is not achieved, a small % of phase II is detected upon warming (Figure 16). Given the outstanding properties of compound 4, which shows hysteretic room-temperature spin transition, the sample was left for four weeks in a desiccator, and magnetic properties were recorded again to compare with the fresh sample (Figure 10). No major change was observed (Figure 17) with Tc↑ = 335

Figure 18. Thermal variation of χMT for 5: magnetic data were recorded first from 4 to 390 K, after which the sample was maintained at 390 K for 15 min. The second run was carried out over the range of 4−390 K on warming and cooling modes.

min, to perform the annealing process, and cooled back to 4 K. In the cooling mode (route 2), a two-step transition is observed, first around 294 K, which is due to the presence of the hydrated phase, and around 125 K, which is due to the dehydrated phase. The third run (route 3) confirms this behavior with two hysteresis loops centered at 136 and 307 K on warming. The high-temperature step was attributed to a hydrated phase (∼1/3) and the second step to a dehydrated phase (∼2/3) confirming the biphasic character of 5 upon annealing (Figure 18). No effect of annealing on the magnetic properties was found for 6 (Figure S3). c. Theoretical Description. A theoretical approach was developed to simulate the magnetic properties of annealed 3. The first dehydration/hydration process takes place when the sample is heated to 390 K, when a fraction of complexes loses their non-coordinated water molecules leading to a dehydrated phase characterized by different magnetic properties in comparison with the initial phase. The kink from the warming branch of the major hysteresis loop (Figure 16) appears when the dehydrated phase (phase I) is reconverted into the initial hydrated phase (phase II). The hydration process occurs around 280 K (see Figure 19b). The ratio between phases I and II depend on the hydration degree of the sample. A biphasic model, based on a two-level Hamiltonian Ising-like treatment in the mean-field approximation,71−74 was applied considering independent phases I and II. This model provides the kinetic extension in terms of mean-field macroscopic master equation.

Figure 17. (top) Thermal variation of χMT for 4: the first cycle (noted 1) was carried out over the range 350−220 K on cooling, after which the sample was warmed to 390 K. The sample was maintained at 390 K for 15 min. The sample was then cooled to 200 K, and magnetic data were recorded between 200 and 390 K (second cycle, noted 2). Magnetic data were then recorded to 4 K (cycle 2), and the third cycle (noted 3) was recorded on warming the sample to 390 K. (bottom) SEM analysis of 4 freshly prepared (a) and warmed to 386 K (b).

K and Tc↓ = 281 K. After that, sample 4 was held at 390 K for 15 min, and magnetic properties recorded over the range of 200 → 390 → 4 K. Interestingly, a stepwise spin conversion was detected on cooling, as noticed for 3, whereas a kink was identified around 242 K in the second and third runs (Figure 17). This is much surprising, because 4 does not contain any noncoordinated water molecules (Figure S2). The annealing process, although not having an influence on the solvent content, affects the morphology of the sample, which is seen by SEM imaging after thermal treatment at 390 K (Figure 17). Indeed, elongated rods, which were observed on Figure 17a, are broken after thermal treatment (Figure 17b). Thus, these experiments confirm the complexity associated with the presence of the kink, which in this case cannot be attributed to a hydration/dehydration process. Magnetic properties of 5 were recorded first between 4 and 390 K. Unsurprisingly, a similar magnetic profile with respect to

Figure 19. (a) Magnetic data recorded for 3 during the second run when the sample was heated for a second time to 390 K. (b) Derivative of the magnetic susceptibility dependence on temperature. The negative peak gives the temperature at which the hydration process occurs: around 280 K. K


Article

Inorganic Chemistry The Hamiltonians of the two phases are given by the following equations, (a) and (b):

HI = HII =

Δ1eff σ1̂ − J1σ1̂ ⟨σ1̂ ⟩ 2 2 Δeff σ2̂ − J2 σ2̂ ⟨σ2̂ ⟩ 2

(a)

(b)

where Δeffi = Δi (P = 0) − kBT × ln(gi) + P ΔVi (with i = 1, 2), is the electronic energy gap, and σ̂i are fictitious spin operators taking the Eigen values −1 and +1 when the molecules are in the LS and HS states, respectively. Ji the effective interaction parameter, gi = gHS/gLS is the degeneracy ratio of the two spin states, and ΔVi is the molecular volume increase upon complete SCO. We first intended to reproduce the second run for annealed 3 where, besides the kink on the ascending branch, a two-step transition on the descending branch was experimentally observed (Figure 19a). This behavior can be reproduced by considering the two phases switching independently as shown in Figure 20b.

Figure 21. Simulated behavior assuming that the two phases have different transition temperatures: (a) individual behavior; (b) system with a phase I/II ratio of 3/7; (c) system with a phase I/II ratio of 2/3; (d) system with a phase I/II ratio of 2/1.

For the third and fourth runs, we observed on Figure 16 that the descending branch from HS to LS became smooth without any sign of stepwise transition. This situation originates from similar transition temperatures on cooling for both phases I and II. This magnetic behavior can be simulated on Figure 21 using identical fitting parameters as for Figure 20 except that Δ2 = 1750 K. Next, we considered the following cases to describe the kink evolution after several annealing and thermal cycles: (a) the magnetic behavior of individual phases I and II; (b) a population of 70% phase II and 30% phase I, which is converted back into the hydrated phase when the system temperature is higher than 280 K; (c) a population of 30% of phase II and only 20% of phase I, which is hydrated back; and finally (d) a population of 5% of hydrated phase and only 10% of phase I, which is converted back into phase II. Thus, this model reproduces the magnetic behavior observed on Figure 16 with the simulation of the kink around 260 K and its growth as much as the phases I/II ratio increases. d. Differential Scanning Calorimetry. Annealing experiments matching the conditions of the SQUID experiments (see Section 2.2.4.b), which maintain the samples at 390 K for 15 min, were performed. The DSC study focuses on compounds 3 and 4 over the temperature ranges of 100−298 K and of 298− 390 at 10 K min−1. Heating and cooling capacity profiles are depicted in Figure 22. The first DSC run on warming 3 shows an identical endothermic peak at Tmax↑ = 358 K as noticed in Figure 11. After the sample was kept for 15 min at 390 K, DSC data were again recorded on cooling back to 100 K, and an exothermic peak was recorded at Tmax↓ = 178 K. When warmed back, a signal was identified at Tmax↑ = 253 K (Figure 22a), which corresponds to the kink observed in the SQUID experiments (Figure 16). In the second run, after the sample was maintained at 390 K, Tmax↓ is shifted to 195 K in the cooling mode, but when warmed, the transition temperature is the same as for the first run. The third run is identical to the second one on warming, the thermodynamic parameters being in the same range of values for runs 2 and 3 (see Table 2). A shift of T1/2↓ was noticed after the second run was compared to the third one

Figure 20. Simulated behavior assuming that the phases I and II have different transition temperatures considering (a) an individual behavior and (b) a global behavior. (c) First derivative of γHS(T) from Figure 19b. The negative peak indicates the temperature at which the conversion from phase I to II occurs.

Besides this assumption, we considered that, for temperatures higher than 280 K, the dehydrated phase I converts into the hydrated one phase II. The parameter values chosen to reproduce the experimental behavior are J1 = 650 K, J2 = 450 K, ln(g1) = 7.25, ln(g2) = 7.68, Δ1 = 2000 K, Δ2 = 1850 K, and ΔV1 = ΔV2 = 8.9 Å3. In this case, we assumed that only a fraction of 30% of the whole compound is dehydrated, and this entire fraction converts into phase II when the system is heated above 280 K. The plateau width of the kink (Figure 20b) is given by the difference between the transition on warming of phase I and the transition temperature for which dehydrated phase I goes into hydrated phase II. The height of the kink is given by the amount of dehydrated phase that follows the hydration process, as it will be demonstrated below (Figure 21). Interestingly, the stepwise transition observed on cooling around 180 K during the second run (Figure 19a) was reproduced too. L


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Inorganic Chemistry

The presence of a bulky substituent of the 4-position of the triazole ring (−CH2−COO−C2H5) for αEtGlytrz was expected to increase the distance between the FeII chains and thereby reducing cooperative effects between active sites, thus leading to gradual spin conversions, as observed in earlier cases.75,76 The reverse effect was actually observed, the system displaying very abrupt hysteretic spin transitions. In particular, [Fe(αEtGlytrz)3](NO3)2 (4) presents an abrupt ST associated with a hysteresis loop of 60 K width, which is not associated with any solvent release/reentrance. This result exceeds by 10 K the minimum hysteresis requirement recommended by Kahn et al. in their pioneering paper describing the use of this substance class for displays and data processing.16 This value of 60 K is also the highest ever reported for a pure 1D chain compound, if we exclude the hybrid molecular alloy [Fe(NH2trz)3](NO3)1.7(BF4)0.3 pre-encapsulated in a polymer whose synthetic procedure and characterization were not described,17 and the monolith of [Fe(Htrz)2trz]BF4.31b [Fe(αEtGlytrz)3](NO3)2 (4) can thus be a very good candidate as active component of memory devices and also smart displays, given that (i) the LS and HS states are easily distinguishable by their different color within the bistability domain of the material centered at room temperature (Figure 23). (ii) The hysteresis

Figure 22. DSC profiles for 3 (a) and 4 (b) studied on warming and cooling at variable temperatures; the samples were maintained at 390 K for 15 min between each run to match the same annealing conditions as for SQUID experiments.

in the SQUID measurement (Figure 16). This is logical because the second run of Figure 16 did not involve any annealing process. To study the influence of annealing on 4, the sample was heated within the DSC instrument to 390 K for 15 min. Consistently, a steep endothermic peak was observed on warming at Tmax↑ = 340 K (Figure S4), corresponding to the one already detected in Figure 11. When cooled to 100 K, an exothermic peak was identified at Tmax↓ = 197 K, which corresponds to the gradual HS to LS transition detected by SQUID magnetometry (Figure 17). When warmed, an endothermic peak was observed at 255 K, which corresponds to the kink observed at the same temperature by SQUID magnetometry (Figure 17). After the second annealing process at 390 K for 15 min, a thermal anomaly was again detected on cooling at 197 K and on warming at 255 K. These experiments thus demonstrate that the square-shaped hysteresis loop of 4 is completely modified by the annealing process, a modification of physical properties in agreement with the change of morphology detected by SEM (Figure 17b).

Figure 23. (a) UV−vis diffuse reflectivity spectra of 4 showing the transitions at 293 K and at 400 K. (b) Room-temperature bistability for 4 thanks to 57Fe Mössbauer spectroscopy and SQUID magnetometry.

3. DISCUSSION FeII 1D chain compounds with azole-based derivatives have been the interest of many research groups during the past decade,11,17,18,21,22 given their ability to present different ligand field strength and therefore a variety of thermal SCO ranges. In the most favorable cases where supramolecular interactions enable contacts between 1D chains, a bistability domain could be observed, which is expected to be useful for technological applications when located in the room-temperature region (e.g., display devices), provided the hysteresis loop width is sufficiently large.16 In our efforts turned to amino acid functionalization of 1,2,4-triazole building blocks for the synthesis of metal organic frameworks,50 we recently reacted an ester of glycine49,50 by transamination to obtain αEtGlytrz. Absence of ligand hydrolysis and formation of 1D FeII chains [Fe(αEtGlytrz)3](anion)2·nSolvent was confirmed by a set of relevant physical techniques.

loop can be cycled at ease without any solvent dehydration/ rehydration effects. This latter property is particularly interesting since only few studies on the thermal fatigability and cyclability of SCO complexes are available in the literature. Miyazaki et al. reported on self-grinding effect of crystals of a mononuclear SCO complex,77 whereas Varret et al. observed self-cleavage effects of single crystals after thermal quenching of the reference SCO material [Fe(1-propyl-tetrazole)6](BF4)2.78 Recent investigations have focused on microrods of the 1D chain [Fe(Htrz)2trz]BF4 integrated into an electronic device.9a These were found to retain their SCO performances after 3000 switching cycles,9a whereas Guionneau et al. pointed out their structural fatigability at the coherent domain scale identifying reduction of chain lengths.79 Most interestingly, a similar rod rupture was observed for 4 after thermal annealing (Figure 17).

Table 2. Transition Temperatures on Warming and Cooling, and Thermodynamic Parameters As Deduced from Differential Scanning Calorimetry Measurements for 3 and 4 after Annealing sample

anion

n H2O

run no after 15 min at 390 K

Tmax↑ (K)

Tmax↓ (K)

3

NO3−

1

253 253

4

NO3−

0

1 2 3 1 2

178 195 196 197 197

255 255

M

as (%)

100

ΔHHL (kJ mol−1)

ΔSHL (J mol−1 K−1)

ΔSvib (J mol−1 K−1)

15.5 13.5 12 20.5 20.6

72.9 60.4 61.2 90.8 91.5

59.3 47.1 47.8 77.5 78 DOI: 10.1021/acs.inorgchem.6b00015 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 24. Evolution of Mössbauer parameters for [Fe(αEtGlytrz)3](anion)2·nSolvent: (a) δLS (78 K) vs Tc. (b) ΔEQLS (78 K) vs anions volume V. (c) Variation of the transition temperatures as deduced from 57Fe Mössbauer spectroscopy, on cooling (▼) and warming modes (Δ) vs V. The lines are a guide. For 7, the reported transition temperatures are not of critical nature (due to water release), which also explains why it is not located around the straight line.

this compound could be used to sense moisture/water as a sensor (the same observation was done for 5). A subtle transformation mechanism between a hydrated and a dehydrated phase was postulated to account for the unusual magnetic properties in the form of kink around 280 K (Figure 16) of postannealed samples of 3. This hypothesis was supported by a biphasic model treated in a mean-field approximation. Noticeably, very similar kinks occurring in the same temperature range were observed for the unsolvated compound 4, thus indicating that its magnetic properties are also dramatically modified after annealing (Figure 17). These magnetic anomalies were also confirmed by DSC on annealed samples (Figure 22). Since compound 4 does not contain any water molecules intrinsically, this is not surprising to record entropy data, which are in the same range before and after annealing, contrary to 3, which show a 10 J mol−1 K−1 difference after annealing on comparing Tables 1 and 2. The origin of the kinks in 4 cannot have the same origin in 3, since 4 does not contain any water molecules, and therefore no propensity for rehydration compared to 3. A close look at SEM imaging data shows a rupture of rod particles for 4 after annealing (Figure 17b), whereas a partial morphological modification is observed for the annealed compound 3 (Figure 14). Thus, morphological shape modification could also contribute to the observation of the kinks, although the mechanism remains at this stage unclear. It is interesting to note that such particle deterioration was also identified on porous MOF nanoballs based on a 1,2,4-triazole ligand derived from an amino acid, which was also subjected to thermal treatment at 423 K.49 Another important aspect of this work concerns the identification of the first complete ST for FeII 1,2,4-triazole chain compounds (1, 2, and 4) by 57Fe Mössbauer spectroscopy. All chain compounds reported earlier show uncomplete ST at high temperatures, as noticed for the reference material [Fe(Htrz)2trz]BF4,42 with a 3% LS rest at 426 K, or below room temperature as found for [Fe(fletrz)3](BF4)2·2H2O (fletrz = (4-(2′-fluoroethyl)-4H-1,2,4-triazole) with 4% HS fraction at 80 K.83 As deduced by comparison of the XRD patterns recorded at 298 K for compounds 1 to 6 (Figure 1a), the insertion of such series of monovalent anions does not affect the structural organization of the chains, and it allows valid comparisons of complementary spectroscopic techniques. In particular, Mössbauer spectroscopy suggests that the inserted anions and solvent molecules modify the local symmetry of the Fe octahedra. This is seen for 5 and 6, which only vary by a 0.5 H2O difference, which is enough to slightly modify the

The cooperative ST behavior of 4 is due to the rigid 1,2,4triazole ligands connecting Fe(II) ions as well as presumably to intermolecular interactions between the substituent on the triazole ring and the counteranion of the complexes. Such kind of interactions were described in similar 1D triazole iron systems, in particular, by both a muon spectroscopy study44 and a single-crystal X-ray structure determination for [Fe(NH2trz)3](NO3)2·2H2O80 as well as for [Fe(Htrz)2trz]BF4.46 Interestingly, 4 includes the nitrate anion, which is also present in [Fe(NH2trz)3](NO3)2, which displays a hysteretic ST of 35 K width located near the room-temperature region. The tripodal and planar nature of the anion allows to establish tridirectional H-bonds, thus leading to an extended H-bonding network. It was remarkably shown to act as a fourth bridge, of supramolecular nature, between the metal centers, in addition to the three N1,N2-1,2,4-triazole linkers in [Fe(NH2trz)3](NO3)2.45 These characteristics allow a better propagation of cooperative effects of elastic nature and therefore lead to a large hysteresis width. The entropy variation associated with the ST of 4 is much higher compared to the one of other 1D 1,2,4triazole-FeII coordination polymers,81,82 but it is found in the same range of values obtained for 1D and two-dimensional systems [FeL1(HIm)2] and [FeL2(azpy)]·MeOH, (L1 = (E,E)-[diethyl-2,2′-[1,2-phenylenebis(iminomethylidyne)]bis(3-oxobutanato)(2-)-N,N′,O 3 ,O 3 ′}], L2 = [3,3′]-[1,2phenylenebis(iminomethylidyne)bis(4-phenyl-2,4butanedionato)(2-)-N,N′,O2,O2′], where Him = imidazole and azpy = 4,4′-azopyridine).47b These latter SCO compounds display large hysteresis widths, for which a thermally induced structural phase transition was inferred by DSC.47b As for these systems, the presence of a structural phase transition accompanying the ST cannot be excluded for 4. Noticeably, using the same αEtGlytrz ligand for the construction of 1D chains, temperature can be sensed optically thanks to thermochromism near (1), at (4), below (6), and above room temperature (3). Thus, 1 and 4 are new examples of FeII chain compounds presenting a ST in close vicinity of room temperature, which has been observed only for rare cases.38 Compared to previous examples of FeII triazole chains, where the inclusion of MeOH molecules stabilizes the HS state,67b here the reverse effect is observed when comparing 1 and 2. Considering potential room-temperature applications, we studied the effect of temperature annealing and water release/ re-entrance on the magnetic properties of two SCO compounds displaying wide hysteresis loops (3 and 4). Lattice water molecules are reversibly removed along with a color change in 3. The quick response of the dehydrated sample for water vapor (obtained in the TGA sample holder) suggests that N


Article

Inorganic Chemistry

like pressure, light irradiation, or electric field. An unprecedented magnetic behavior occurring in the room-temperature region has been also observed for the first time. Finally, we have also presented the first FeII 1,2,4-triazole chain compounds displaying a complete spin transition as unambiguously identified by 57Fe Mössbauer spectroscopy. We anticipate that the use of amino acids and amino−ester 1,2,4-triazole systems will be extended for the preparation of new SCO compounds stimulated by the present report to explore their rich iron chemistry.

symmetry of the octahedron around the iron center with ΔEQHS (298 K) = 2.73(1) mm/s for 5, which is smaller than the one of 6 (2.76(1) mm/s). In the cases of 1 and 2, the presence of a non-coordinated methanol molecule in the crystal lattice does not affect the local distortion of the FeN6 octahedron with ΔEQHS (298 K) = 2.78 mm/s for both compounds. For 1, 4, and 5, ΔEQLS (78 K) is exactly the same at 0.19(1) mm/s (Figure 24b). Because there is no valence contribution to consider in the LS state, the observed quadrupole splitting of 0.19(1) mm/s corresponds only to the lattice contribution. This value indicates that the FeII sites are distorted, as expected for metal centers connected within a chain by three N1,N2-1,2,4-triazole bridges.65 A linear decrease of the isomer shift δLS (78 K) with the transition temperature Tc↑ for nonsolvated complexes (Figure 24a) traduces an increase of the Fe−N covalency and indicates that chains are much closer together, because when Tc increases, the anion volume decreases (Figure 24c). Such linear dependency was also observed for the [Fe(NH2trz)3]2+ series,82,84 as well as for the [Fe(hyetrz)3]2+ series considering in this later work only the anions radii.39 A new relationship between the entropy gap, evaluated from DSC measurements, and the volume of the inserted counteranion is presented in Figure 25. It shows a linear decrease of the entropy with increase in the size of the counteranion volume, although a deviation was noticed for the trifluoromethanesulfonate anion.

5. EXPERIMENTAL SECTION Syntheses. All reagents and solvents were used as received from commercial source: benzene (Fluka analytical), methanol (VWR), SOCl2 (Sigma-Aldrich), glycine ethyl ester hydrochloride (ACROS), Fe(ClO4)2·6H2O (Alfa Aesar), Fe(BF4)2·6H2O (Sigma-Aldrich), FeSO4·7H2O (VWR), Ba(NO3)2 (ACROS), CF3SO3H (ACROS), Fe powder (Merck). Ethyl-4H-1,2,4-triazol-4-yl-acetate (αEtGlytrz) was prepared as described in ref 49. [Fe(αEtGlytrz)3](ClO4)2 (1). Fe(ClO4)2·6H2O (78 mg, 0.21 mmol) was dissolved in CH3OH (2 mL) with a pinch of ascorbic acid and added with stirring to a solution of L1 (100 mg, 0.64 mmol) dissolved in CH3OH (5 mL). The immediate white precipitate, which was obtained, was stirred for 10 min and filtered, washed with CH3OH (2 mL), and dried under vacuum (1 × 10−2 mbar) for one night. Yield: 100 mg, 83%. Anal. for FeC18H27N9O14FeCl2 (720.22 g/mol): calcd. C, 30.02; H, 3.78; N, 17.5; Fe, 7.9%. Found C, 29.06; H, 3.74; N, 16.90; Fe, 7.5%. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1240 (vs), υ(CN) ≈ 1560 (m), υ(C−H out of plane) ≈ 1026 (m), υ(C−H ring torsion) ≈ 625 (m), υ(Cl−O) ≈ 1105 (s, with shoulder). [Fe(αEtGlytrz)3](ClO4)2·CH3OH (2). The same procedure as for 1 was applied except that the precipitate was dried under a slow stream of N2(g). Yield: 100 mg, 83%. Anal. for FeC19H31N9O15FeCl2 (752.26 g/mol): calcd. C, 30.34; H, 4.15; N, 16.76; Fe, 7.4%. Found C, 30.01; H, 3.92; N, 16.60; Fe, 7.30%. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1240 (vs), υ(CN) ≈ 1560 (m), υ(C−H out of plane) ≈ 1026 (m), υ(C−H ring torsion) ≈ 625 (m), υ(Cl−O) ≈ 1105 (s, with shoulder). [Fe(αEtGlytrz)3](NO3)2·H2O (3). An aqueous solution (1 mL) of FeSO4·7H2O (59.5 mg, 0.21 mmol) was mixed with an aqueous solution (1 mL) of Ba(NO3)2 (56.0 mg, 0.21 mmol) and stirred at room temperature and in air for 5 min. A white precipitate was formed instantaneously, which was filtered in air and digested to afford a pale green solution of [Fe(H2O)6(NO3)2. This solution with a pinch of ascorbic acid was added with stirring to a solution of L1 (100.0 mg, 0.64 mmol) dissolved in CH3OH (1 mL). A pink precipitate was obtained after stirring for 2 h under a slow stream of N2(g) and filtered, washed with CH3OH (2 mL), and dried under vacuum (1 × 10−2 mbar) for one night. Yield: 95 mg, 69%. Anal. for FeC18H27N13O13 (663.34 g/mol): calcd. C, 32.57; H, 4.41; N, 23.23; Fe, 8.43%. Found C, 32.68; H, 4.36; N, 22.80; Fe, 8.35. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1226 (vs), υ(CN) ≈ 1560 (m), υ(C−H out of plane) ≈ 1028 (m), υ(C−H ring torsion) ≈ 636 (m), υ(N−O) ≈ 1384 (s, with shoulder). [Fe(αEtGlytrz)3](NO3)2 (4). The synthesis of this compound was identical as for 3 except that the precipitate was dried under vacuum (1 × 10−2 mbar) at 40 °C for one night. Yield: 105 mg, 76%. Anal. for FeC18H27N13O13 (663.34 g/mol): calcd. C, 32.57; H, 4.41; N, 23.23; Fe, 8.43%. Found C, 32.68; H, 4.36; N, 22.80; Fe, 8.35. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1226 (vs), υ(CN) ≈ 1560 (m), υ(C−H out of plane) ≈ 1028 (m), υ(C−H ring torsion) ≈ 636 (m), υ(N−O) ≈ 1384 (s, with shoulder). [Fe(αEtGlytrz)3](BF4)2·0.5H2O (5). Fe(BF4)2·6H2O (72.5 mg, 0.21 mmol) was dissolved in CH3OH (2 mL) with a pinch of ascorbic acid and added to a solution of L1 (100 mg, 0.64 mmol) dissolved in CH3OH (5 mL). The mixture was stirred for 20 min at room temperature after which a pale pink precipitate was obtained. It was filtered, washed with CH3OH (2 mL), and dried under vacuum (1 ×

Figure 25. Variation of the entropy difference between HS and LS states, ΔSHL vs the anion volume for [Fe(αEtGlytrz)3](anion)2· nSolvent series.

This information provides a direct experimental evaluation of cooperative effects as a function of the size of counteranion for FeII 1D 1,2,4-triazole SCO compounds: the smaller the anion, the higher is the entropy difference between HS and LS states, and the higher is the transition temperature.

4. CONCLUDING REMARKS In this work, we have carried out a systematic study of a family of 1D FeII coordination polymers, where simple change of anion and solvent leads to SCO compounds that could operate as “optical alerts” in a wide range of temperature, including the room-temperature region. One compound of this family displays a significant hysteresis loop at room temperature, which could lead to various applications given the associated thermochromic effect and thermal stability. In particular, experiment could be thought to switch the spin state at room temperature within the bistability domain, using other stimuli O


Article

Inorganic Chemistry 10−2 mbar) for 2 h. Yield: 90 mg, 60%. Anal. for FeC18H28N9O6.5B2F8 (703.93 g/mol): calcd. C, 30.71, H, 4.01, N, 17.91, Fe, 7.93%. Found C, 30.72; H, 3.83; N, 17.89; Fe, 7.9%. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1226 (vs), υ(CN) ≈ 1562 (m), υ(C−H out of plane) ≈ 1031 (m), υ(C−H ring torsion) ≈ 634 (m), υ(B−F) ≈ 522 (w), 1083 (vs, with shoulders). [Fe(αEtGlytrz)3](BF4)2 (6). The same procedure as for 5 was followed except that the precipitate was dried under vacuum (1 × 10−2 mbar) for 2 d. Yield: 25 mg, 17%. Anal. Calcd for (C18H27N9O6Fe(BF4)2; 694.92 g/mol); C, 31.11, H, 3.92, N, 18.14, Fe, 8.04%; Found C, 31.05, H, 3.80, N, 18.01, Fe, 8.1%. IR (KBr, cm−1): υ(CO) ≈ 1747 (vs), υ(C−O) ≈ 1226 (vs), υ(CN) ≈ 1562 (m), υ(C−H out of plane) ≈ 1031 (m), υ(C−H ring torsion) ≈ 634 (m), υ(B−F) ≈ 522 (w), 1083 (vs, with shoulders). [Fe(αEtGlytrz)3](CF3SO3)2·2H2O (7). [Fe(H2O)6](CF3SO3)2 was first synthesized as a very pale green powder using a described procedure,85 starting with an aqueous solution (1 mL) containing an iron powder in excess (2 g) carefully mixed to triflic acid (5 mL, 56.5 mmol). Yield: 9.8 g, 74%. [Fe(H2O)6](CF3SO3)2 (99.2 mg, 0.21 mmol) was dissolved in CH3OH (2 mL) with a pinch of ascorbic acid and added to the above solution of L1 (100 mg, 0.64 mmol) dissolved in CH3OH (5 mL). The mixture was stirred for 20 min at room temperature, after which a pale pink precipitate was obtained. It was filtered, washed with CH3OH (2 mL), and dried under vacuum (1 × 10−2 mbar) for one night. Yield: 150 mg, 69%. Anal. for FeC20H31N9O14F6S2 (855.48 g/mol): calcd. C, 28.08; H, 3.65; N, 14.74; Fe, 6.53%. Found C, 27.99; H, 3.32; N, 14.26; Fe, 6.2%. IR (KBr, cm−1): υ(CO) ≈ 1749 (vs), υ(C−O) ≈ 1226 (vs), υ(CN) ≈ 1564 (m), υ(C−H out of plane) ≈ 1030 (m), υ(C−H ring torsion) ≈ 632 (m), υ(S−O) ≈ 1251 (vs, with shoulders). Physical Measurements. Elemental analyses were performed at University College London (U.K). Atomic absorption analyses were carried out on a PerkinElmer 3110 spectrometer. Sampling was carried out in aqueous solutions containing nitric acid (65%). Standard solutions were prepared with Mohr’s salt. IR spectra were collected on a Shimadzu FTIR-84005 spectrometer using KBr pellets. TGA were performed in air (100 mL/min) at the heating rate of 1 °C/min from 293 to 400 K using a Mettler Toledo TGA/SDTA 851e analyzer. Diffuse reflectance spectra on solids were recorded with a CARY 5E spectrophotometer using polytetrafluoroethylene as a reference. Powder XRD patterns were recorded on a Siemens D5000 counter diffractometer working with Cu Kα radiation and operating at room temperature. The samples were mounted on the support with silicon grease. 57Fe Mö ssbauer spectra were recorded in transmission geometry over the temperature range (78−300 K) with a conventional Mössbauer spectrometer equipped with a 57Co(Rh) radioactive source operating at room temperature. The samples were sealed in aluminum foil and mounted on an Oxford nitrogen bath cryostat. The spectra were fitted to the sum of Lorentzians by a least-squares refinement using Recoil 1.05 Mössbauer Analysis Software.86 All isomer shifts refer to α-Fe at room temperature. Magnetic susceptibilities were measured in the temperature range of 4−390 K using an MPMS-XL (7T) SQUID magnetometer operating at 1000 Oe. Data were corrected for magnetization of the sample holder and diamagnetic contributions, which were estimated from the Pascal constants. DSC measurements were carried out in a He(g) atmosphere using a PerkinElmer DSC Pyris 1 instrument equipped with a cryostat and operating down to 98 K. Aluminum capsules were loaded with 20−50 mg of sample and sealed. The heating and cooling rates were fixed at 10 K min−1. Temperatures and enthalpies were calibrated over the temperature range of interest (298−400 K) using the solid−liquid transitions of pure indium (99.99%), and the crystal−crystal transitions of pure cyclopentane (≥99%),87 over the range of 78− 298 K. For the annealing experiments, the samples were encapsulated in aluminum holder capsules with holes, loaded with 20.9 mg of each sample. Enthalpy data were obtained by integration of the peaks using the PYRISTM DSC Software 7.0 in specific heat Cp (J mol−1·K−1) format. The transition temperatures were derived by considering the maximum (Tmax) of the thermal anomalies. Entropy data were derived considering the switching sites as deduced by Mössbauer spectroscopy,

when available. SEM was performed using a Gemini Digital Scanning Microscope 982 with 1 kV accelerating voltage with an aluminum sample holder.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00015. Overview of the 57Fe Mössbauer parameters for 2, 4, 5, 6, 7 and selected spectra of 2, 5, 7. TGA of 4, 6, and 7. Magnetic properties of 6. DSC of 4 on warming after annealing. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from FNRS (PDR T.0102.15), Romanian National Authority for Scientific Research, CNCS−UEFISCDI, Project No. PN-II-RU-TE2014-4-2695, FNRS-Academie Roumaine, WBI Roumanie, and COST Action Nos. MP1202, CM1305, and CA15128. The work at AMRI/UNO was supported by the NSF Award No. ECCS-1546650. We thank C. Detavarnier and W. Knaepen for their help during the recording of powder XRD data at variable temperature. M.M.D is a chargé de recherches from the FNRS.

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REFERENCES

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