High-Temperature Spin Crossover in Iron(II) Complexes with 2,6-Bis(1H-imidazol-2-yl)pyridine

Novel iron(II) coordination compounds containing a ligand 2,6-bis(1H-imidazol-2-yl)pyridine (L), having such a composition as [FeL2]SO4·0.5H2O, [FeL2]Br2·H2O, [FeL2](ReO4)2, [FeL2]B10H10∙H2O, [FeL2]B12H12∙1.5H2O had been synthesized and studied using UV-Vis (diffuse reflectance), infrared, extended X-ray absorption fine structure (EXAFS), and Mössbauer spectroscopy methods, as well as X-ray diffraction and static magnetic susceptibility methods. The analysis of the μeff(T) dependence in the temperature range of 80–600 K have shown that all the obtained complexes exhibit a high-temperature spin crossover 1A1 ↔ 5T2.


Introduction
Searching for novel coordination compounds wherein the spin state of the central atom can be switched by an external action is an urgent problem. This is indicated by regularly appearing publications in the literature devoted to this topic [1][2][3][4][5][6][7][8]. These compounds include iron(II) complexes with spin crossover (SCO) 1 A 1 ↔ 5 T 2 . The change in spin multiplicity occurs owing to affecting temperature, pressure, irradiation with light of a certain wavelength (LIESST effect), high-frequency magnetic or electric field, and other factors. The transition from a low-spin (LS) to a high-spin (HS) state causes a change in the magnetic, optical, and vibrational properties of the complexes. An important property associated with the spin transition consists in changing metal-donor interatomic distance amounting to 0.2 Å in the case of Fe(II) complexes. Owing to the universal properties of complexes with SCO they have a wide range of potential applications in making optoelectronic, molecular electronic, and spintronic devices [9][10][11][12][13][14][15]. At present, polyfunctional materials combining SCO and other properties are under active study [16][17][18][19].

Materials
Commercial metal salts and solvents without further purification were used synthesis. 2,6-Bis(imidazol-2-yl)pyridine was synthesized as described in [30] (NM tra of ligand given in the Supplementary Materials, Figures S1-S4); K2B10H1 K2B12H12 were obtained according to the procedure [31].

Synthesis of [FeL2]SO4 0.5H2O (1·0.5H2O)
A 0.28 g (1 mmol) sample of FeSO4·7H2O with the addition of 0.1 g of ascorb was dissolved in 10 mL of water under heating; a 0.51 g (2 mmol) L was dissolve mL of ethanol, and the solutions were then heated and mixed. The resulting soluti evaporated until a red-violet precipitate began to form. After the solution with the itate was cooled in a crystallizer with ice, the precipitate was filtered off, washe with small portions of water, and dried in air. Yield

Synthesis of [FeL2]Br2·H2O (2·H2O) and [FeL2](ReO4)2 (3)
A 0.28 g (1 mmol) sample of FeSO4·7H2O was dissolved in 5 mL of distilled acidified with 0.1 g of ascorbic acid. An excess (0.43 g, 1.5 mmol) of KReO4 or KBr 3 mmol) in 10 mL of water and a solution of L (0.51 g, 2 mmol) in 10 mL of ethan successively added to the resulting solution under stirring. The resulting soluti evaporated until a red-violet precipitate began to form. After the solution with the itate was cooled in a crystallizer with ice, the precipitate was filtered off, washe with small portions of water, and dried in air. Yield: 70% (2·H2O), 30% (3 A 0.14 g (0.5 mmol) sample of FeSO4·7H2O was dissolved in 3 mL of distilled acidified with 0.05 g of ascorbic acid. An excess (0.23 g, 1 mmol) of K2B10H10·2 K2B12H12 (0.22 g, 1 mmol) in 10 mL of water and a solution of L (0.21 g, 1 mmol) in ethanol were added to the resulting solution under stirring. Red-brown precipitate to form immediately. Each precipitate was filtered off, washed twice with small p Scheme 1. 2,6-Bis(1H-imidazol-2-yl)pyridine (L).

Materials
Commercial metal salts and solvents without further purification were used in the synthesis. 2,6-Bis(imidazol-2-yl)pyridine was synthesized as described in [30]  A 0.28 g (1 mmol) sample of FeSO 4 ·7H 2 O with the addition of 0.1 g of ascorbic acid was dissolved in 10 mL of water under heating; a 0.51 g (2 mmol) L was dissolved in 10 mL of ethanol, and the solutions were then heated and mixed. The resulting solution was evaporated until a red-violet precipitate began to form. After the solution with the precipitate was cooled in a crystallizer with ice, the precipitate was filtered off, washed twice with small portions of water, and dried in air. Yield: 55%. Anal. Calc. for C 22  A 0.28 g (1 mmol) sample of FeSO 4 ·7H 2 O was dissolved in 5 mL of distilled water acidified with 0.1 g of ascorbic acid. An excess (0.43 g, 1.5 mmol) of KReO 4 or KBr (0.36 g, 3 mmol) in 10 mL of water and a solution of L (0.51 g, 2 mmol) in 10 mL of ethanol were successively added to the resulting solution under stirring. The resulting solution was evaporated until a red-violet precipitate began to form. After the solution with the precipitate was cooled in a crystallizer with ice, the precipitate was filtered off, washed twice with small portions of water, and dried in air. Yield A 0.14 g (0.5 mmol) sample of FeSO 4 ·7H 2 O was dissolved in 3 mL of distilled water acidified with 0.05 g of ascorbic acid. An excess (0.23 g, 1 mmol) of K 2 B 10 H 10 ·2H 2 O or K 2 B 12 H 12 (0.

Measurement and Characterization
The data for elemental analysis of the complexes was acquired on a EURO EA 3000 analyzer (EuroVector, Pavia, Italy).
X-ray absorption spectra (XAS) in the Fe K edge region (150 eV before and 800 eV after) were measured in the transmission mode with the use of synchrotron radiation on the 8 beamline, VEPP-3 storage ring at the Siberian Synchrotron and Terahertz Radiation Center (Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia) [32]. A Si(111) cut-off crystal was used as a two-crystal monochromator. The operating mode of the storage ring during measurement: energy-2 GeV; current-70-140 mA. For measurements, the samples were mixed with cellulose powder as a filler and pressed into tablets. The mass of the sample was calculated so that the absorption jump at the Fe K-edge was 0.8-1. The preprocessing of the absorption spectra (selection of the oscillating part-EXAFS spectra) was performed with the use of the VIPER 10.17 program [33]. The "radial pair distribution function" (Figure 1

Measurement and Characterization
The data for elemental analysis of the complexes was acquired on a EURO EA 3000 analyzer (EuroVector, Pavia, Italy).
X-ray absorption spectra (XAS) in the Fe K edge region (150 eV before and 800 eV after) were measured in the transmission mode with the use of synchrotron radiation on the 8 beamline, VEPP-3 storage ring at the Siberian Synchrotron and Terahertz Radiation Center (Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia) [32]. A Si(111) cut-off crystal was used as a two-crystal monochromator. The operating mode of the storage ring during measurement: energy-2 GeV; current-70-140 mA. For measurements, the samples were mixed with cellulose powder as a filler and pressed into tablets. The mass of the sample was calculated so that the absorption jump at the Fe K-edge was 0.8-1. The preprocessing of the absorption spectra (selection of the oscillating part-EXAFS spectra) was performed with the use of the VIPER 10.17 program [33]. The "radial pair distribution function" (Figure 1) was obtained by the Fourier transform of the k 3 -weighted EXAFS function in the range of wave vectors k = 2.0-11.0 Å −1 . The local environment of the Fe ion [interatomic distances (Ri) and angles (Qi), coordination numbers (Ni), and Debye-Waller factors (σ 2 )] was modeled using the EXCURVE 98 cod [34]. In this program phase and amplitude characteristics were calculated in the von-Bart and Hedin approximation. The amplitude suppression factor S0 2 due to multielectron processes was determined for the crystallographically characterized compound (S0 2 = 0.85) and fixed during further modeling of the studied compounds spectra. The Debye-Waller factor was the same separately for nitrogen and carbon atoms.
IR spectra were taken on a Scimitar FTS 2000 in the range of 4000-400 cm −1 and a Vertex 80 in the range of 600-100 cm −1 . Compound samples were prepared as suspensions in vaseline and fluorinated oils and in polyethylene.
The Kubelka-Munk diffuse reflectance spectra were obtained on a Shimadzu UV-3101 PC scanning spectrometer.
The XRD investigation of polycrystalline samples was performed using a Shimadzu XRD 7000 diffractometer (CuKα radiation). The local environment of the Fe ion [interatomic distances (R i ) and angles (Q i ), coordination numbers (N i ), and Debye-Waller factors (σ 2 )] was modeled using the EXCURVE 98 cod [34]. In this program phase and amplitude characteristics were calculated in the von-Bart and Hedin approximation. The amplitude suppression factor S 0 2 due to multielectron processes was determined for the crystallographically characterized compound (S 0 2 = 0.85) and fixed during further modeling of the studied compounds spectra. The Debye-Waller factor was the same separately for nitrogen and carbon atoms.
IR spectra were taken on a Scimitar FTS 2000 in the range of 4000-400 cm −1 and a Vertex 80 in the range of 600-100 cm −1 . Compound samples were prepared as suspensions in vaseline and fluorinated oils and in polyethylene.
The Kubelka-Munk diffuse reflectance spectra were obtained on a Shimadzu UV-3101 PC scanning spectrometer.
The XRD investigation of polycrystalline samples was performed using a Shimadzu XRD 7000 diffractometer (CuK α radiation).
The static magnetic susceptibility was measured using Faraday balance type setup equipped with electromagnetic compensating torsion quartz microbalance. The Delta DTB9696 temperature controller (Delta Electronics Inc., Taipei, Taiwan) was used for the investigated compounds temperature stabilization (~1 K) in the range of temperatures 80-600 K. The temperature scanning rate for the process of heating or cooling samples was 1 K/min. The magnetic field strength (7300 Oe) stabilization precision was~2%. The compounds studied were sealed in quartz cellules filled with atmospheric air at 760 Torr. In order to study the effect of crystallization water, the samples were placed in open quartz ampoules and vacuumed at 10 −2 Torr, after which the helium atmosphere at 5 Torr was formed.
The effective magnetic moment of studied compounds was calculated as: k-Boltzmann constant, N A -Avogadro constant, and µ B -Bohr magneton. In the above formula, the diamagnetic contribution in total magnetic susceptibility (χ) was taken into account using the method of Pascal's constants. The direct (T c ↑) and reverse (T c ↓) transitions temperatures were obtained using condition the magnetic moment second derivative zero value (d 2 (µ eff (T))/dT 2 = 0). The Mössbauer spectra were collected with a spectrometer NP-610 (KFKI, Budapest, Hungary), using 57 Co in a metal Rh matrix as a radioactive source. The spectra were measured at a room temperature. The spectra were processed to find the values of isomer shift δ and quadrupole splitting ε. The isomer shifts are given relative to metal iron.

Results and Discussion
Iron(II) complexes with 2,6-bis(1H-imidazol-2-yl)pyridine were isolated from aqueousethanol solutions. To maintain the oxidation state of iron(II), ascorbic acid was added to the solution.
Complex The structure of the whole molecule for all complexes in the LS state (T = 297 • C) was established from the EXAFS data using the multiple scattering approximation within the software package EXCURV (except for [FeL 2 ](ReO 4 ) 2 (3)) excluding hydrogen atoms, and anions that exert only a weak effect on the shape of the EXAFS spectrum owing to their spatial separation from the central iron ion. For complex 3, the signal-to-noise ratio in the EXAFS spectrum is worse than that for the spectra of other complexes owing to the presence of a heavy anion ReO 4 − . Simulation of the whole molecule in the multiple scattering approximation gives too large errors in determining the parameters. Therefore, the simulation of the EXAFS spectrum for complex 3 could be carried out only in a single scattering approximation. The structure of the complexes in the LS state (by the example of complex 4·H 2 O) obtained by means of EXAFS spectra simulation is presented in Figure 2.  (3) in the LS state was obtained from modeling the spectrum filtered in real space (∆R = 0.9 to 3.0 Å). The simulation data are presented in Table 2. The coordination numbers of the nearest spheres of the iron ion environment were fixed in accordance with the data obtained in the simulation of complexes in the LS state in the multiple scattering approximation.
The structure of the coordination site of complex [FeL2](ReO4)2 (3) in the LS state was obtained from modeling the spectrum filtered in real space (ΔR = 0.9 to 3.0 Å). The simulation data are presented in Table 2. The coordination numbers of the nearest spheres of the iron ion environment were fixed in accordance with the data obtained in the simulation of complexes in the LS state in the multiple scattering approximation.  (for the nearest sphere of the environment).  [FeL2](ReO4)2 2.7 Fe-C 8 2.78 0.014 Fe-C 4 3.20 Figure 3 illustrates a comparison of the experimental and simulated radial distribu tion function for [FeL2]B10H10·H2O. The simulation was carried out using a multiple scat tering approximation. are centered at the wav numbers of 2470 (ν(BH)) and 1075 cm −1 (δ(BBH)). In the case of the spectra for 4·H2O 5·1.5H2O they are shifted with respect to those observed in the spectra of the initial salts which could be, to all appearance, caused by the formation of H2O δ− ... δ+ H-В bonds. In th far region of the spectra of all the complexes, Fe(3d 6 )-ligand(π) charge-transfer transitio bands and vibration bands (M-N) are observed. The position of these bands is typical fo the spectra of low-spin octahedral iron(II) complexes [35].  δ(BBH)). In the case of the spectra for 4·H 2 O, 5·1.5H 2 O they are shifted with respect to those observed in the spectra of the initial salts, which could be, to all appearance, caused by the formation of H 2 O δ− . . . δ+ H-B bonds. In the far region of the spectra of all the complexes, Fe(3d 6 )-ligand(π) charge-transfer transition bands and vibration bands (M-N) are observed. The position of these bands is typical for the spectra of low-spin octahedral iron(II) complexes [35].
The diffuse reflectance spectra (DRS, Figures S8-S12) of all the obtained complexes exhibit intense metal-ligand charge transfer bands ν 1 (e g → π * L ) in the wavelength range of 300-350 nm (λ max ≈ 324-326 nm). The DRS of 4·H 2 O and 5·1.5H 2 O exhibit three absorption bands, too, (see Table 4); they correspond to the 1 A 1 → 1 T 2 , 1 A 1 → 1 T 1 and 1 A 1 → 1 A 2 transitions in the strong octahedral field of the ligands. For low-spin axially distorted octahedral iron(II) complexes, the term 1 T 1 is transformed according to the representation of 1 E + 1 A 2 , whereas term 1 T 2 is transformed according to the representation of 1 E + 1 B 2 [36]. Therefore, the 1 A 1 → 1 B 2 , 1 A 1 → 1 A 2 and 1 A 1 → 1 E wide transition bands (see Table 4) observed in the spectra of 1·0.5H 2 O, 2·H 2 O and 3, indicate the fact that an axial distortion of the octahedral coordination of these complexes occur. Low-intensity transitions and overlapping three wide bands do not make it possible to perform reliable quantitative calculations of crystal field parameters in this case. The spectra of all the complexes do not exhibit the 5 T 2 → 5 E band, which is caused by the HS state of iron(II). We were able to calculate the splitting parameters based on the difference between 1 A 1 → 1 T 2 and 1 A 1 → 1 T 1 absorption frequencies [36] for low-spin (LS) forms of complexes with closo-borate anions. The B values were computed using the formula 16B = [ν ( 1 A 1 → 1 T 2 ) − ν ( 1 A 1 → 1 T 1 )]. The values C and ∆ LS (Table 4) were calculated using the equations: ν LS = ∆ LS − C + 86B 2 /∆ LS and C = 4.41·B [36][37][38]. The obtained data show that 2,6-bis(1H-imidazol-2-yl)pyridine is a strong field ligand. In addition, these data, as well as the values obtained for a number of previously synthesized Fe(II) complexes with 2,6-bis(benzimidazol-2-yl)pyridine and 2,6-bis(4,5-dimethyl-1H-imidazole) [22][23][24], obey the inequality that reflects the condition for the manifestation of SCO [37]: 19.000 cm −1 ≤ ∆ LS ≤ 22.000 cm −1 .  The Mössbauer spectra of complexes 1·0.5H 2 O, 2·H 2 O, 3 and 5·1.5H 2 O represent quadrupole doublets whose parameters correspond to the LS state of iron(II) (Figure 4). The spectrum of 4·H 2 O also exhibits a broadened doublet related to the HS form of the complex (36%). The parameters of the Mössbauer spectra are presented in Table 5. The Mössbauer spectra of complexes 1·0.5H2O, 2·H2O, 3 and 5·1.5H2O represent quadrupole doublets whose parameters correspond to the LS state of iron(II) (Figure 4). The spectrum of 4·H2O also exhibits a broadened doublet related to the HS form of the complex (36%). The parameters of the Mössbauer spectra are presented in Table 5.  The temperature dependences of the effective magnetic moment for the analyzed complexes are shown in Figure 5. Spin crossover (SCO) is observed for all the compounds under investigation. In the case of complexes 1·0.5H2O, 2·H2O, 3 the μeff values observed in the thermal stability range (2.35, 2.1 and 3.35 μΒ, respectively), are significantly lower than the theoretical spin-only value of 4.9 μΒ for the Fe(II) ion. The 4·H2O and 5·1.5H2O complexes are more stable and thus a temperature of 600 K could be reached. For these compounds, a complete spin-crossover is observed. However, the μeff values observed in the HS state of these compounds are also lower than the theoretical value for Fe(II). It  The temperature dependences of the effective magnetic moment for the analyzed complexes are shown in Figure 5. Spin crossover (SCO) is observed for all the compounds under investigation. In the case of complexes 1·0.5H 2 O, 2·H 2 O, 3 the µ eff values observed in the thermal stability range (2.35, 2.1 and 3.35 µ B , respectively), are significantly lower than the theoretical spin-only value of 4.9 µ B for the Fe(II) ion. The 4·H 2 O and 5·1.5H 2 O complexes are more stable and thus a temperature of 600 K could be reached. For these compounds, a complete spin-crossover is observed. However, the µ eff values observed in the HS state of these compounds are also lower than the theoretical value for Fe(II). It should be noted that the experimental µ eff values for 4·H 2 O and 5·1.5H 2 O complexes are in the range of 4.6-5.7 observed for Fe(II) compounds [39,40]. In the case of 1·0.5H 2 O and 2·H 2 O the residual effective magnetic moment in LS state (0.65 and 0.4 µ B , respectively), is presented by µ eff (T) dependences. This fact could be connected with temperature-independent paramagnetism. Complexes 3 , 4·H 2 O and 5·1.5H 2 O in LS state exhibit diamagnetism with a zero µ eff value. Despite the fact that low µ eff values are achieved in the investigated temperature range for 1·0.5H 2 O and 3, the condition of d 2 (µ eff (T))/dT 2 = 0 could be satisfied and some values of SCO temperature could be determined. pendent paramagnetism. Complexes 3, 4·H2O and 5·1.5H2O in LS state exhibit diamagnetism with a zero μeff value. Despite the fact that low μeff values are achieved in the investigated temperature range for 1·0.5H2O and 3, the condition of d 2 (μeff(T))/dT 2 = 0 could be satisfied and some values of SCO temperature could be determined.     The effect of the crystallization of water has been studied for 4·H 2 O and 5·1.5H 2 O complexes ( Figure 6). It should be noted that in the case of rarefied atmosphere the decomposition of the dehydrated complexes occurs in a lower temperature range than it is observed for initial compounds. Nevertheless, the SCO has been observed in this case, too. The µ eff value of 4.65 µ B achieved in HS state for 4 corresponds to the value observed for the initial complex. In the case of 5 complex, the µ eff value (4.6 µ B ) exhibits an increase after dehydration. Residual µ eff values (~1-1.5 µ B ) have been registered to occur for both complexes in the LS state. The SCO temperature increases after dehydration; however, the 5 complex demonstrates the highest SCO temperature values as it is observed in the case of the initial compound. Thus, the dehydration of 4·H 2 O and 5·1.5H 2 O complexes lead to appearing residual µ eff value and to an increase in the temperature of SCO.

Conclusions
In this work, we have synthesized and investigated five novel coordination c pounds of various iron(II) salts with 2,6-bis(1H-imidazol-2-yl)pyridine (L). The struc of the coordination core of the complexes has been determined by means of EXAFS spe simulation. Two ligand molecules are coordinated to the iron(II) ion in a tridentate-cy fashion by the nitrogen atom belonging to pyridine and two nitrogen atoms belongin imidazole rings. Thus, the complexes have the distorted-octahedral structure of the c dination polyhedron, the FeN6 core. The studies on the μeff(T) dependence have sho that complexes having such a composition as

Conclusions
In this work, we have synthesized and investigated five novel coordination compounds of various iron(II) salts with 2,6-bis(1H-imidazol-2-yl)pyridine (L). The structure of the coordination core of the complexes has been determined by means of EXAFS spectra simulation. Two ligand molecules are coordinated to the iron(II) ion in a tridentate-cyclic fashion by the nitrogen atom belonging to pyridine and two nitrogen atoms belonging to imidazole rings. Thus, the complexes have the distorted-octahedral structure of the coordination polyhedron, the FeN 6 core. The studies on the µ eff (T) dependence have shown that complexes having such a composition as [ O exhibit an 1 A 1 ↔ 5 T 2 high-temperature spin crossover. A comparison of the data obtained for the synthesized compounds with those obtained by us earlier [21][22][23][24] shows that spin-crossover 1 A 1 ↔ 5 T 2 is observed in all complexes of Fe(II) with 2,6-bis(imidazole-2-yl)pyridines. The temperatures of direct SCO, T c ↑, in most compounds are significantly higher than room temperature.