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.

character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced. Keywords: spin crossover; aminoacid triazoles; iron(II); coordination polymers

Introduction
Bistable molecular systems, particularly materials exhibiting captivating scenario of spin crossover (SCO) [1], are versatile switchable units in the thriving field of molecular electronics [2]. In a typical SCO material, the electron repositioning via singlet-quintet transitions is substantiated to be technologically significant [2]. Indeed, the reversible electron transfer from a diamagnetic low-spin (LS, 1 A 1g ) state to a thermally populated paramagnetic high-spin (HS, 5 T 2g ) state is recognized as an entropy driven process and could be addressed thermally, optically, electrically and under pressure/shock with highly profound spectroscopic, optical, magnetic, dielectric readout signal [3]. In the solid state, the presence of intra and intermolecular interactions acts as communication media between iron centers promoting cooperative first order spin transitions leading to a large memory domain [4] that can be suitable for potential applications [5]. Indeed, a SCO compound meeting display and data processing requirements would, in addition, have a good shelf life and an easily detectable optical response, and would ideally operate near room temperature [6,7].
Applications envisioned in these fields largely depend on molecular conformations precursors adopt during the coordination process which directs structure-properties relationships. In some cases, magnetic properties can even be modified by a structural perturbation as demonstrated on a 1D polyelectrolyte system [8][9][10][11]. Our interest in amino acid derivatization was fuelled by promising results shown by 1,2,4-triazole-carboxylate derivatives in synthetic chemistry [12], spin crossover area [13], nanoporous MOFs [14], biological interest in several metallo-proteases [15] and 'soft' sacrificial precursors to produce CdO with shape and phase selectivity [16]. As a continuation of our work on amino acid functionalization, [12] ethyl-4H-1,2,4-triazol-4-yl-acetate was used as a prospective precursor for magnetic sensors [13]. Here we introduce a new tailored 4-R-1,2,4-triazole from β-amino-acid, namely 4H-1,2,4-triazol-4-yl-propionate (βAlatrz) (Chart 1). The reason for introducing β-Alanine ethyl ester substituted on the 4 position of a 1,2,4-triazole core was to use an appropriate length of substituent which compromises the distance between 1D chains in complexes which can introduce supramolecular interactions with H-acceptors substituent. Thus, the spin transition (ST) properties of two one-dimensional Fe II

Preparation and Characterization of 1·0.5H 2 O and 1
The transamination reaction used in the synthesis of ethyl-4H-1,2,4-triazol-4-yl-acetate and 4H-1,2,4-triazol-4-yl acetic acid [12] proved to be a good synthetic strategy to build a β-amino-acid triazole. The synthetic route, starting with β-Alanine ethyl ester hydrochloride and N,N-Dimethylformamide-azine, helped us to obtain a new molecule, βAlatrz, with a good yield. The synthesis time is relatively longer than the synthesis of previous ligands and the final product had to be purified by "flash" column chromatography.
A 1D coordination polymer was obtained as a white powder by reaction of the corresponding Fe II inorganic precursor, prepared in air, [Fe(H 2 O) 6 ](CF 3 SO 3 ) 2 [17] with a methanolic solution of βAlatrz. This complex was successfully characterized by elemental analysis, TGA-DTA analyses, atomic absorption (AAS), X-Ray powder diffraction, IR, Raman, SEM, DSC, SQUID magnetometry and 57 Fe Mössbauer spectroscopy. The thermogravimetric and elemental analyses reveal the presence of guest water molecules, affording the following general formula [Fe(βAlatrz) 3 ](CF 3 (1), which was also characterized to reveal the influence of solvent molecules on the spin state. 1·0.5H 2 O presents a rather crystalline character as revealed from X-ray powder diffraction (XRPD) patterns ( Figure 1). This result is also illustrated by the SEM analysis with blocks of hexagonal shape of about 2 μm widths ( Figure 2). FT-IR and Raman analyses were performed: (i) to confirm the identity of the ligand framework before and upon complexation as the ester functionality is susceptible to hydrolysis; (ii) to ascertain the coordination mode of iron to the ligand and the presence of the trifluoromethane sulfonate in the crystal lattice ( Figure 3).
Two sharp IR bands at 3,118(s) and 2,983(m) cm −1 of βAlatrz, are assigned to the νCH 2 modes [18]. The carboxylic group of ester shows two stretching vibrations corresponding to (C=O) and (C-O) at 1,728(s) and 1,205(s) cm −1 , respectively, which are almost unchanged (1,732 and 1,212 cm −1 ) in 1·0.5H 2 O ruling out the possible ester hydrolysis and also confirming the non-involvement of carboxylic ester group in coordination. For βAlatrz, the band assigned to ring torsion of triazole at ν = 634(m) cm −1 , the ν C=N stretching vibration at 1,535(m) cm −1 and the N-N stretching band at ν = 1,022(s) cm −1 , are all shifted upon complexation in 1·0.5H 2 O to 632 cm −1 , ν C=N = 1,560 cm −1 , and ν N-N = 1,028 cm −1 , respectively.
These values confirm the coordination of the iron to the 1,2,4-triazole ring [19,20]. Indeed, the characteristic bands of the ligand are not only present in the spectra of the complexes but are also shifted towards larger wave numbers, from 3 to 15 cm −1 , respectively. This energy increase is due to the deformation of the ligand upon coordination of the metal, the ligand molecule being indeed more constrained to perform vibration and twisting movements [19,21].   For a better understanding of the molecular structure of these ligands, Raman spectra were also collected ( Figure 3). The band present at 1,728 cm −1 for βAlatrz is assigned to a C=O vibration. In complexes this band is retained and appears around 1,732 cm −1 . The C-C stretching is also active in Raman as a medium band at 1,098 cm −1 for βAlatrz and appears in the same wavenumber in 1·0.5H 2 O. Presence of the monovalent counter-anion is also confirmed by IR and Raman: [22]. 57 Fe Mössbauer spectroscopy confirm for 1·0.5H 2 O the presence of one Fe II N 6 site, and exclude any oxidation product of iron. These spectroscopic data support a linear chain structure with Fe II ions linked by triple N1,N2-1,2,4-triazole bridges [23,24].
1·0.5H 2 O was prepared as a white powders and presents a reversible thermochromism on cooling to pink ( Figure 4). These colors depend on the spin state of the Fe II centers, as the ST involves a change in the electronic configuration which modifies the absorption spectrum of the complex.  600 800 1000 1200 The white color of 1·0.5H 2 O, is due to the location of the spin-allowed lowest energy d-d transition, 5 T 2g → 5 E g , for the HS sites in the near infrared region (~11,500 cm −1 ) [25,26]. Two supplementary bands, corresponding to the 1 A 1g → 1 T 1g and 1 A 1g → 1 T 2g d-d transitions of LS Fe II sites are observed at ~18,727 cm −1 and ~30,300 cm −1 , respectively suggesting that some proportion of Fe II may be in the LS state at room temperature. The ligand field strengths for the HS and the LS state, given by equations [25,26] allowed us to estimate 10D q HS and 10D q LS to be approximately 11,500 cm −1 and 26,302 cm −1 , respectively. These values are characteristic for SCO complexes [25,26]. Presence of LS state at room temperature has been confirmed by 57 Fe Mössbauer spectroscopy (vide infra). The reversibility of the rehydration/dehydration process within a given framework structure with the change of crystallinity can play an important role on the SCO properties [27]. This process was investigated for 1·0.5H 2 O by thermogravimetric analysis, and the XRD powder pattern and SEM imaging recorded after annealing treatment. The sample was first deposited in a crucible and slowly heated at 1 K/min in air atmosphere (air flow 130 mL/min) to the temperature of complete dehydration (390 K) leading to 1. After a short standby period, the sample was cooled (1 K/min) slowly to reach room temperature revealing no rehydration. Thus the dehydration-rehydration process is irreversible and 1 is air stable which will ease any further physical studies on this compound. Comparison of the powder XRD patterns ( Figure 1) and SEM images (Figure 2) of 1·0.5H 2 O and 1 allow to exclude any framework rupture. Indeed, these two compounds are isostructural, 1 showing higher peak intensities indicating a better crystalline character. No deterioration is observed by SEM.

SQUID Magnetometry
The temperature dependent magnetic properties of 1·0.5H 2 O and 1 were determined by temperature-dependent susceptibility measurements using a SQUID magnetometer operating at 1000 Oe ( Figure 5).
For complex 1·0.5H 2 O, χ Μ T is 3.10 cm 3 K mol −1 at 300 K, which is in the region expected for an Fe II complex essentially in the HS state, with a small % of LS species. Upon cooling, χ Μ T remains almost constant until 240 K where it drops sharply to T c ↓ = 228 K down to ~0.14 cm 3 K mol −1 at 77 K, which is typical for a diamagnetic Fe II complex with possibly a few HS ions. Upon warming, the χ Μ T curve differs slightly revealing a small hysteresis width (4 K) at T c ↑ = 232 K for this very sharp spin transition. An hysteretic behavior of width 4 K was observed too for 1 with a ST shifted upwards (T c ↓ = 248 K and T c ↑ = 252 K). The same curve shape is observed too which confirms the preservation of the 1D chain character [27] and does not support the appearance of a structural phase transition following the annealing process. The stabilization of the LS state upon water release is uncommon for this family of complexes [28] and was only observed for the 1D chain compounds [Fe(4-(2'-hydroxyethyl)-1,2,4-triazole) 3 ](anion) 2

57 Fe Mössbauer Spectroscopy
An inspection of electronic and structural features for 1·0.5H 2 O and 1 using 57 Fe Mössbauer spectroscopy was undertaken over the temperature range 77-300 K (Figure 6). At 77 K, the spectrum of 1·0.5H 2 O consists of two quadrupole doublets of different resonance area fractions. The major quadrupole doublet with isomer shift δ LS = 0.51(2) mm/s and quadrupole splitting, ΔE Q LS = 0.23 (2) mm/s correspond to the LS state of Fe II . The presence of an LS quadrupole splitting stems from a lattice contribution to the electric field gradient and therefore reveals a distorted character for the LS octahedron as expected within a chain, where constraints may not be negligible [30]. Another doublet, corresponding to HS Fe II ions of weaker population (7%), with parameters (δ HS = 1.14(1) mm/s and ΔE Q HS = 3.15(2) mm/s) confirms the incomplete nature of the ST at 77 K. Upon warming to 300 K, the intensity of the HS doublet slowly increases to 8% at 225 K, after which it increases dramatically to 87.5% at 300 K, confirming an incomplete thermally induced LS → HS conversion for a single Fe II site. The presence of LS ions at room temperature could be related to crystal defects or end of chains. The hysteresis effect is clearly evidenced at 225 K (Figure 6a). At 80 K, the spectrum of 1 shows a single LS quadrupole doublet (δ LS = 0.52(1) mm/s and ΔE Q LS = 0.25(2) mm/s) indicating a complete spin transition. The compound remains mostly in the LS state on warming up to 250 K, after which a second quadrupole doublet attributed to HS Fe II ions grows in intensity (e.g., at 297 K, δ HS = 1.03(1) mm/s and ΔE Q HS = 2.75(1) mm/s). The asymmetry of the lines observed in the HS state is attributed to a texture effect. Upon cooling to low temperature, the reverse situation is observed with a clear hysteresis effect at 250 K (Figure 6b). The isomer shift (δ LS~0 .51(1) mm/s at 77-80 K and ΔE Q HS = 1.03(1) mm/s at 297-300 K) is not affected by the dehydration process which indicates that non coordinated water molecules are not H-bonded to the triazole ligand [31], but should be located at a remote position to the complex in the crystal lattice, either isolated or hydrogen bonded to the sulfonate group of the non coordinated anions.
Their releases have no influence on the structural organization as demonstrated by the similarity in X-ray powder diffraction data. However, a clear influence on the transition temperatures, i.e., on the respective energy levels of the HS and LS states [25], has been detected.

Differential Scanning Calorimetry
Compound 1·0.5H 2 O and 1 were investigated by differential scanning calorimetry over the temperature range 100-300 K, at 10 K/min for both cooling and heating modes (Figure 7). An endothermic peak is observed on warming for 1·0.5H 2 O at T max ↑ = 235(1) K and an exothermic peak is recorded at T max ↓ = 233(1) K, on cooling. These peaks correspond to a first-order phase transition in agreement with the transition temperatures determined by both SQUID and Mössbauer spectroscopy. The enthalpy and entropy variations associated to the active SCO centers are ΔH = 12(2) kJ/mol and ΔS = 51(2) J/mol/K. A similar profile, although with more abrupt peaks, was detected for 1 with phase transitions shifted upwards to T max ↑ = 254(1) K and T max ↓ = 247(1) K. The peaks are separated by a narrow temperature domain, which is indicative of the presence of a hysteresis loop. The enthalpy and entropy associated to the ST, considering only the switching sites, were derived as follows: ΔH = 13(1) kJ/mol and ΔS = 59(2) J/mol/K.

Ethyl 4H-1,2,4-Triazol-4-yl-Propionate (βAlatrz)
N,N-Dimethylformamide azine dihydrochloride (I) and its free base (II) were obtained following the reported method [12]. The free base was recrystallized twice from benzene with charcoal and used in transamination reactions [12]. To a suspension of β-alanine ethyl ester hydrochloride (3 g, 19.53 mmol) in benzene (100 mL) at approx. 60 °C was added solid II (2.14 g, 15.04 mmol) with stirring, obtaining a transparent yellow solution. The mixture was refluxed then (at 130 °C) for 93 h with vigorous stirring. The reaction was monitored by NMR analysis at interval of time. Finally, the solvent was removed under vacuum and a chromatographic purification (SiO 2 , CH 2 Cl 2 → 5% isopropanol in CH 2 Cl 2 ) of the yellow oil gave pale yellow oil. Yield 2 g (60%). 1 6 ](CF 3 SO 3 ) 2 was first synthesized as a very pale green powder using a described procedure [16], starting with an aqueous solution (1 mL) containing an iron powder in excess (2 g) carefully mixed to triflic acid (

Physical Measurements
Elemental analyses were performed at University College London (UK) and at S.C.A. CNRS Solaize (France). 1 H and 13 C NMR spectra were recorded at 300 MHz and 75 MHz, respectively, on a Bruker AC300 instrument. The residual solvent peak was used as internal reference. Mass spectral data were obtained on Thermo Finnigan LCQ Ion trap spectrometer (APCI mode). HRMS were carried out on a Micromass Q TOF 2 spectrometer in ESI mode, detecting positive mode. Raman spectra with 1,064 nm excitation were recorded between 2,300 to 400 cm −1 with a Bruker RFS 100/s FT-Raman spectrometer (I = 200 mW) at r.t using a diode-pumped, air-cooled Nd:YAG laser as the excitation source. IR spectra were collected on a Shimadzu FTIR-84005 spectrometer using KBr pellets. Thermogravimetric analyses (TGA) were performed in air (100 mL/min) at the heating rate of 1 °C/min from 293 K 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 X-ray diffraction 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. 57 Fe Mössbauer spectra were recorded in transmission geometry over the temperature range (78-300 K) with a conventional Mössbauer spectrometer equipped with a Cyclotron Ltd 57 Co(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 [32]. All isomer shifts refer to α-Fe at room temperature. Magnetic susceptibilities were measured in the temperature range 4-390 K using a MPMS-XL (7T) SQUID magnetometer. Data were corrected for magnetization of the sample holder and diamagnetic contributions, which were estimated from the Pascal constants. Differential scanning calorimetry measurements were carried out in a He (g) atmosphere using a Perkin-Elmer 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 In (99.99%) [33], and the crystal-crystal transitions of pure cyclopentane (≥99%) [34], over the range 78-298 K. Scanning electron microscopy (SEM) was performed using a Gemini Digital Scanning Microscope 982 with 1 kV accelerating voltage with an aluminum sample holder.

Concluding Remarks
We have presented two novel Fe II 1D ST chain compounds switching in the range 225-250 K. Release of non coordinated water molecules has a paramount effect on the LS state stabilization of 1.
Its air stability and absence of solvent will ease further physical measurements, particularly using hydrostatic pressure [5] so as to shift its hysteresis loop towards the room temperature region.