Pressure Effect Studies on the Spin Transition of Microporous 3D

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Pressure Effect Studies on the Spin Transition of Microporous 3D Polymer [Fe(pz)Pt(CN)4] Georgiy Levchenko,*,‡,§ Ana Beleń Gaspar,*,† Gennadiy Bukin,∥ Ludmila Berezhnaya,∥ and Jose ́ Antonio Real*,†

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†

Institut de Ciència Molecular/Departament de Química Inorgànica, Universitat de València, Catedràtric Beltrán 2, E-46980 Paterna València, Spain ‡ International Centre of Future Science, State Key Laboratory of Superhard Materials of Jilin University, Changchun 130012, Chine § Donetsk Physical − Technical Institute named after A.A. Galkin NANU, Kiiv 03028, Ukraine ∥ Donetsk Institute of Physics and Engineering named after A.A. Galkin, Donestk 83114, Ukraine S Supporting Information *

ABSTRACT: Pressure effects on the spin transition of the three-dimensional (3D) porous coordination polymer {Fe(pz)[Pt(CN)4]} have been investigated in the interval 105 Pa− 1.0 GPa through variable-temperature (10−320 K) magnetic susceptibility measurements and spectroscopic studies in the visible region at room temperature. These studies have disclosed a different behavior of the compound under pressure. In the magnetic experiments, a temperature independent paramagnetic behavior has been observed under 0.4 GPa. In contrast, at room temperature and at 0.8 GPa, a complete HS-to-LS transition has been evidenced. The differences in the magnetic behavior are strongly related with the porous structure of the compound and its capability to adsorb the oil used as pressure transmission media in the magnetic experiments.

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(CN)2]2} (L = 2,6-naphthyridine,30 pyrimidine,31−33 5-Brpyrimidine,34 3X-pyridine,35 and M(I) = Au, Ag, and Cu). The former complexes change color upon spin transition at room temperature (from purple (LS) to white (HS)) while the latest change from red (LS) to yellow (HS). Pressure induced spin transitions (PIST) or the effect of pressure in thermally induced spin transitions (TIST) in Fe(II) coordination compounds has been the focus of study of several research groups.36−44 For the majority of the SCO complexes with temperature independent interaction parameter investigated so far, the critical/characteristic temperature of the ST is shifted upward with increasing pressure; the hysteresis and steepness (cooperativity) of the ST decrease and vanish at a critical pressure. Nevertheless, there are examples of SCO compounds for which the shape of the ST curve remains essentially unaltered in a reasonable interval of pressures, the hysteresis increases/decreases or a nonlineal behavior of the Tc(p) versus p plot is exhibited.31,45−52 In rare cases stabilization of the HS state under pressure has also been reported.48,49 Recently, it has been demonstrated that the change of the elastic and inelastic forces in the crystal as a function of pressure or temperature determines the behavior of the spin transition.53,54 In a previous Raman study conducted on the 3D porous polymer {Fe(pz)[Pt(CN)4]}17 (pz = pyrazine) (Figure 1) an unexpected stabilization of the HS state under pressure at

INTRODUCTION The technology market needs novel materials exhibiting electronic bistability.1 Bistable materials are those that have the ability to switch between two electronic states when interacting with their environment. In this regard, the spin crossover phenomenon (SCO) exhibited by a number of iron coordination compounds is one of the best examples of molecular electronic bistability.2−10 The switching between the spin states can be induced by an external perturbation such as variation of temperature, application of pressure, or light irradiation.3 The molecular Fe(II) switches produce outputs such as changes in absorbance, refractive index, crystal and molecular structure, magnetic and dielectric responses. It then becomes possible to associate a piece of information with each of the low spin (LS) and high spin (HS) states. In others words, these changes of color and/or other physical properties of Fe(II) SCO complexes can be exploited to store and display information using the binary coding. Two families of Fe(II) coordination compounds have been widely investigated because they exhibit abrupt spin transitions (ST) located at room temperature or in the interval of 220− 280 K accompanied by 25−40 K of thermal hysteresis width. The one-dimensional triazole based polymers, [Fe(R-trz)3]X211,12 (R-trz: 4-substituted-1,2,4-triazole, X: anions) or [Fe(Htrz)2(trz)](BF4)13−16 and the cyanide-based two-dimensional and three-dimensional polymers of formulas [Fe(L)xM(CN)4] (x = 1 or 2, L: pyrazine,17−21 pyridine,22,23 bispyridine,24−29 and M(II): Ni, Pd, and Pt) and {Fe(L)[M© XXXX American Chemical Society

Received: April 26, 2018

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DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

molecules in the HS state and T is the temperature. At 300 K and 105 Pa (ambient pressure) γHS is equal to 1. On lowering the temperature this value remains constant until the vicinity of Tc(down) ≈ 285 K where it abruptly diminishes to 0.09 due to the HS−LS transition. In the warming mode, the characteristic temperature of the spin transition Tc(up) is 310 K which defines the characteristic 25 K wide thermal hysteresis loop. In contrast, application of a pressure as small as 0.13 GPa has a strong effect on the spin transition. The magnetic properties of the compound were measured at this pressure starting from the low temperature region (LS state). The compound was warmed up from 10 to 320 K and the γHS value of 0.09 remained constant up to 290 K. At this temperature it increased and reached a value of 0.6 at the highest temperature measured (320 K). For safety reasons this is the upper working temperature of the pressure cell. From this γHS value it is inferred that the LS−HS transition at 320 K is not complete, around 50% of the Fe(II) centers have switched to the HS state. The slope of the γHS vs T curve and the γHS value indicates that the critical temperature of the transition in the warming mode is in the interval of 320−325 K. On lowering the temperature, the high spin fraction continues to be 0.6 until Tc(down) = 180 K, where it diminishes progressively down to 0.4. Further cooling of the compound below 100 K does not provoke any change in the high spin fraction. In the subsequent warming cycle, the compound discloses a thermal hysteresis loop as wide as 140 K. The spin transition curve is more continuous and γHS increases from 0.4 to 0.6 within the interval 270−320 K. Further increase of pressure up to 0.4 GPa evidences stabilization of the HS state and suppression of the spin transition. Indeed at 0.4 GPa the compound exhibits paramagnetic behavior in the whole temperature range investigated 320−10 K. After releasing the pressure, the compound recovers essentially the original spin transition properties without any modification in its characteristic temperatures Tc(down) and Tc(up). Absorption Spectroscopy in the Visible Region under Hydrostatic Pressure. Figure 3(a) shows the optical spectrum of {Fe(pz)[Pt(CN)4]} recorded at atmospheric pressure (105 Pa) within the interval of 200−900 nm at 293 K (HS state) and 77 K (LS state). A detailed analysis of the spectrum in the LS state is depicted as well (Figure 3(b)). At 77 K, where the compound is in the LS state, the spectrum exhibits three bands, one humped band is observed in the interval of 500−550 nm while the other two bands are located at 320 and 260 nm. Since the band that appears at lower wavelengths is also observed in the spectrum of the HS state (293 K) one can conclude that the band at λ = 320 nm can be tentatively attributed to the 1A1 → 1T2 transition and the twohumped band could be the result of a combination of MLCT (see below) and the 1A1 → 1T1 d-d electronic transitions of the Fe(II) ions in the LS state. In contrast to the magnetic experiments, the cell press employed in the UV experiments uses as a pressure transmission medium NaCl(s) instead of silicone oil. A thin film of {Fe(pz)[Pt(CN)4]} is placed between two thin plates of the crystalline salt. The spectrometer adapted for such type of experiments, PGS-2 (Carl Zeiss), works in the range of 400−750 nm. For convenience of measuring the absorption changes in the spectrum caused by pressure variation, the spectra of the HS and LS states were normalized to zero value at 400 nm by subtraction of the background signal of the equipment (spectrometer with pressure cell). Two cycles of

Figure 1. View of the crystal structure of the 3D porous coordination polymer {Fe(pz)[Pt(CN)4]}. The {Fe[Pt(CN)4]}∞ sheets (002 plane) are connected by pyrazine organic linkers which are rotationally disordered around the C4 axis. Fe(red), Pt(pink).

room temperature was observed.55 Since it is the first cyanidebased Fe(II) SCO polymer that exhibits such a particular magnetic behavior, we decided to undertake a detailed investigation of the effect of pressure on the SCO properties of {Fe(pz)[Pt(CN)4]}. Two types of studies have been performed: pressure influence on the thermal spin crossover behavior employing magnetic measurements and pressure induced spin crossover at constant temperature monitored by means of visible spectroscopy, taking advantage of the pronounced change of color from pale yellow (HS) to red (LS) which accompanies the ST of this compound.

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RESULTS Magnetic Susceptibility Measurements under Hydrostatic Pressure. The study of the pressure influence on the thermal spin crossover behavior of {Fe(pz)[Pt(CN)4]} has been performed measuring the magnetic susceptibility of the compound at variable temperature on a SQUID magnetometer. The pressure cell used is described in the Experimental Section and works in the pressure range 105 Pa < p < 1.4 GPa with an accuracy of 0.025 GPa. Figure 2 gathers a collection of spin transition curves at different hydrostatic pressures expressed as γHS versus T, where γHS is the HS fraction that accounts for the number of

Figure 2. Thermal variation of the high spin fraction at different pressures for {Fe(pz)[Pt(CN)4]}: ▲, 105 Pa before measurements (red); □, 0.13 GPa first heating and cooling (violet); left pointing arrows, 0.13 GPa second heating (green); ◇, 0.4 GPa cooling (dark blue); ●, 105 Pa after releasing pressure (black). B

DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

atmospheric pressure the spectrum denotes only one band located at around 480 nm, which corresponds to the metal-toligand charge transfer (MLCT). As pressure increases the optical intensity of this band increases and a second band corresponding to the 1A1 → 1T1 d−d electronic transition of the LS state appears. This last band evolves with pressure reaching its maximum in intensity at 0.8 GPa. The optical spectrum recorded at 77 K and atmospheric pressure matches that obtained at 0.8 GPa and 293 K, which is evidence that the HS−LS transition is complete at this pressure (Figure S1). As pressure decreases the intensity of this band diminishes and practically disappears at 0.033 GPa. Clearly, the variation in the optical intensity of the 1A1 → 1T1 transition band reflects the course of the HS−LS transition. Therefore, it has been used to calculate the pressure dependence of the molar LS fraction, γLS (γLS = 1 − γHS)[3] (Figure 5 and Figure S2). At 105 Pa, γLS is

Figure 5. Dependence of the LS fraction for {Fe(pz)[Pt(CN)4]} under increasing (blue line) and decreasing (red line) pressure at 293 K.

Figure 3. (a) Dependence of the optical density for {Fe(pz)[Pt(CN)4]} at room (bottom curve) and nitrogen (top curve) temperatures in the interval of 200−900 nm. (b) Analysis of the optical spectrum for {Fe(pz)[Pt(CN)4]} at 77 K (LS state) and at atmospheric pressure. ΔD = DLS − DHS is the difference in the optical density of the spectra in the HS and LS states. For convenience of measuring the absorption changes in the spectrum caused by temperature variation, the spectra of the HS and LS states were normalized to zero value at 400 nm by subtraction of the background signal of the equipment (spectrometer with pressure cell).

equal to zero. Application of relatively small pressures provokes a notable increase in γLS, being 0.5 at 0.062 GPa (P1/2up). At 0.3 GPa 90% of the molecules are in the LS state and γLS approach 1 at 0.8 GPa. At decreasing pressures P1/2down is found very similar to P1/2up. However, a narrow nontypical hysteresis is observed at pressures slightly higher than P1/2. After several increasing/decreasing pressure cycles it has been found that the compound does not suffer any fatigue since the spectrum at 105 Pa measured, after releasing pressure, has been found identical to the initial spectrum. The reproducibility of the nontypical piezo-hysteresis loop has also been confirmed. Thermodynamic Parameters of the TIST and PIST. In order to evaluate the interaction parameters for the temper-

experiments have been made at 293 K increasing and decreasing the pressure progressively. Every pressure was fixed and kept during 24 h for achieving the steady state. The dependences of the optical density D with the wavelength λ have been calculated from the penetration spectra measured at each pressure. The results are presented in Figure 4. At

Figure 4. Dependence of the optical density for {Fe(pz)[Pt(CN)4]} at room temperature and at different pressures. Left: Increasing pressure; Right: decreasing pressure. For convenience of measuring the absorption changes in the spectrum caused by pressure variation, the spectra of the HS and LS states were normalized to zero value at 400 nm by subtraction of the background signal of the equipment (spectrometer with pressure cell). C

DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Thermodynamic Parameters of the Thermal (TIST) and Pressure Induced Spin Transition (PIST) in Cyanide Based Fe(II) SCO Polymers Γ(P) (kJ/mol)

Γ(T) (kJ/mol)

ΔH(P) (kJ/mol)

Pc (GPa)

ΔP (GPa)

ΔH(T) (kJ/mol)

Tc (K)

ΔT (K)

{Fe(3-F-py)2[Ni(CN)4]} {Fe(3-F-py)2[Pd(CN)4]}54 {Fe(3-F-py)2[Pt(CN)4]}54 {Fe(3-Cl-py)2[Pd(CN)4]}56

7.9 7.8 6.8 7.5

7.6 8.3 7.5 7.41

18.6 20.6 20.8 22

0.33 0.33 0.3 0.61

0.1 0.11 0.07 0.08

19.1 21.9 20.8 15.2

24.7 30.3 22.9 4.7

{Fe(pz)[Pt(CN)4]}this work {Fe(phpy)2[Ni(CN)4]}52 {Fe(pmd)(H2O)[Ag(CN)2]2}·H2O31

5 4.7 8.9

7.2 4.72 8.9

21.2 18.82 14.5

0.06 1.45 0.66

0 0.3 0.2

21 7.8 12

220 233 226 162.3 144.9 290 130 217

Compound 54

as well as its host−guest chemical properties.18 The porous structure results from the connection of the square planar [Pt(CN)4]2− anions with four adjacent octahedral Fe(II) atoms that form a 2D {Fe[Pt(CN)4]} grid through the Pt-CNFe linkages. The pyrazine ligand occupies the axial positions of the Fe(II) octahedrons and connects consecutive {Fe[Pt(CN)4]} layers along the (001) direction. This assembly originates channels parallel to the (100) and (101) directions with a gate size of 3.92 × 4.22 Å2 and 3.43 × 3.94 Å2 for the HS and LS states, respectively. The Fe−N bond distances are approximately 0.2 Å shorter in the LS state in comparison with the HS state. Consequently, upon the spin transition the unit cell volume experiences a contraction of 53.5 Å3 per Fe(II) atom. The solvent-accessible void of the LS state and HS state is 18.1 and 22.4%, respectively. The host−guest chemistry of this porous polymer has recently been studied.18−21 The compound adsorbs in a reversible manner hydroxylic solvents, gases (CO2, N2, H2, SO2, CS2), organic molecules (pyrazine (pz), pyridine (py), furan, pyrrole, and thiophene) as well as halogens (I2, Br2, and Cl2). The spin transition properties are strongly influenced by the chemical nature and size of the molecules hosted in the pores. In fact, the clathrates {Fe(pz)[Pt(CN)4]}·nS (S = H2O, CH3OH and n = 5) and {Fe(pz)[Pt(CN)4]}·L (L = pz or py) do not exhibit spin transition if not paramagnetic behavior. The guest molecules induce the stabilization of the HS state since they prevent the contraction of the network. By contrast, in the clathrates {Fe(pz)[Pt(CN)4]}·CS2 and {Fe(pz)[Pt(CN)4(X)p]} [X = Cl− (p = 1), Br− (p = 1), I− (0 ≤ p ≤ 1)] the interaction of the guest molecules with the Pt atoms ends up in a stronger field strength at the Fe(II) centers and hence the LS state is stabilized. In the present work it has been found that {Fe(pz)[Pt(CN)4]} exhibits a paramagnetic behavior at 0.4 GPa. This finding is in line with previous reported pressure experiments performed using the Raman spectroscopy technique on this compound.55 In these experiments, carried out at room temperature and 0.025 GPa, it was found that the compound remains practically in the HS state. The stabilization of the HS state under pressure has already been reported for several spin crossover complexes; nevertheless, none of these complexes have a porous structure.48,49 The hypotheses formulated to explain this phenomenology have always been the pressureinduced structural transformations involving severe distortion of the Fe(II) coordination core. However, taking into account recent studies on host−guest chemistry of the title compound, we are more in favor to propose a distinct interpretation of the experimental results. The pressure cells used in the magnetic and Raman spectroscopy studies use silicon oil as pressure transmission media. Seemingly, application of pressure favors

ature induced ST (TIST), the equation of equilibrium of the HS and LS states is written as follows:36,53

From this equation the relation between temperature and high spin molar fraction, γHS, may be written as T (γHS) =

ΔHHL(T ) + Δelast + P·ΔVHL − 2·γHS·Γ

( ) + ΔS

kB·ln

1 − γHS

HL

γHS

(2)

The experimental variation of the enthalpy [ΔHHL(T) = 21 kJ mol−1] and the entropy [ΔSHL = 81 J K−1 mol−1] were previously calculated from the temperature dependence of the heat capacity at atmospheric pressure and have been considered here independent of the temperature.17 ΔV is the unit cell volume variation upon ST. It was experimentally obtained from X-ray crystal structure determination in the HS and LS states and equals 53.5(5) Å3.18 The fitting of this equation to the experimental curve (Figure 2) gives the interaction parameter Γ = 7.2 kJ/mol and the elastic energy Δelast = 10.2 kJ/mol. In order to evaluate the interaction parameter Γ and the change of the enthalpy for pressure induced ST (PIST), the equation of equilibrium (1) has been used. The obtained relation between pressure P and the high spin fraction γHS is as follows:53,54 ÅÄÅ T ·ÅÅÅÅkB·ln ÅÅ P(γHS) = Ç

(

1 − γHS γHS − 0.5

) + ΔS

ÑÉÑ ÑÑ − ΔH (P) + 2·γ ·Γ T HS ÑÖ

HL Ñ ÑÑ

ΔVHL

24 30 8

(3)

In this equation the term ΔHT(P) includes the parameters ΔHHL(P = 0) and Δelast. The values of the enthalpy and the interaction parameter for the PIST are obtained from the fitting of eq 3 to experimental curve γLS(P) (Figure 5). The change of the entropy and volume used for the fitting of the experimental curve are the experimental values given above. The fitting gives Γ = 5 kJ/mol and ΔH = 21 kJ/mol. The change of enthalpy is virtually the same for the TIST and PIST processes. In contrast, the values of the interaction parameter Γ are very different. For PIST the parameter Γ is smaller in 1.5 times and, therefore, the spin transition does not show hysteresis.

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DISCUSSION In this study, it has been found that {Fe(pz)[Pt(CN)4]} exhibits unusual behavior under pressure in both experiments: TIST under pressure and PIST at room temperature. For understanding such behavior one should consider the particular porous tridimensional structure of the compound D

DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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out (300 K). Ongoing PIST studies at higher temperature will support or refute this hypothesis.

the diffusion of silicon oil inside the pores and, consequently, the spin transition is inhibited as observed for several clathrates of this compound. The adsorption/desorption process is reversible and the compound recovers the original properties when pressure is released. Table 1 gathers the thermodynamic parameters of the TIST and PIST for several related two- and three-dimensional Hofmann like Fe(II) SCO polymers previously reported by us. The three derivatives {Fe(3-Fpy)2[MII(CN)4]} (MII = Ni, Pd, Pt; 3-Fpy = 3-fluoropyridine)22,54 are isostructural twodimensional coordination polymers which crystallize in the C2/m space group and display one step cooperative ST characterized by hysteresis loops 23−30 K wide. The related two-dimensional compound {Fe(3-Clpy)2[Pd(CN)4]} (3Clpy = 3-chloropyridine)56 undergoes a cooperative two-step ST with symmetry breaking showing the Pnma space group in the ordered LS-HS intermediate phase (IP) and the Pnc2 space group in the fully populated LS and HS states. Although no structure was solved for {Fe(phpy)2[Ni(CN)4]} (phpy = 4phenylpyridine),52 its crystal structure must be related to the above-mentioned two-dimensional compounds in which the axial pyridine-like ligands coordinated to the Fe(II) centers belonging to adjacent layers interdigitate giving a path for increasing cooperativity. In contrast the compounds {Fe(pz)[Pt(CN)4]}17 and {Fe(pmd)(H2O)[Ag(CN)2]2}·H2O31 are three-dimensional coordination polymers. The former is made up of a single network while the latter is constituted by triple interpenetration of identical networks. In addition the nature of the bridges defining the frameworks are quite different. It is interesting to note that both the enthalpy change and the interaction parameters are very similar for most compounds in this series despite their differences in dimensionality and chemical nature. However, the exception is compound {Fe(pz)[Pt(CN)4]} that presents the smaller interaction parameter and, consistently, no piezo-hysteresis is observed for it. One can conclude that the absence of hysteresis in PIST studies is connected with decrease of the interactions between HS centers. This result could be, at first sight, surprising since the more cooperative thermal induced SCO of {Fe(pz)[Pt(CN)4]} displays the less cooperative pressure induced SCO behavior of the aforementioned series of Hofmann-type compounds. A reasonable explanation to justify this fact might reside in the structure of these compounds. Indeed, the used low hydrostatic pressure can be absorbed by the two-dimensional derivatives {Fe(L)2[MII(CN)4]} by getting closer the interdigitated layers without affecting too much the coordination core of the Fe(II) centers. The triple interpenetrated threedimensional networks of {Fe(pmd)(H2O)[Ag(CN)2]2}·H2O can also absorb the increase of pressure by getting closer. This fact explains the presence of piezohysteresis in these systems because, in principle, cooperativity has to be increased due to reinforcement of the interaction between layers or networks. However, this cannot be the case for the three-dimensional system {Fe(pz)[Pt(CN)4]} because the two-dimensional {Fe[Pt(CN)4]} layers are pillared by the pz ligands, which link the Fe(II) centers of consecutive layers through rigid coordinative bonds. Then, the increase of pressure is essentially absorbed by the Fe(II) centers. Finally, it has to be considered that the divergence of compound {Fe(pz)[Pt(CN)4]} in regard with the cooperativity in PIST may also arise from the fact it is in a metastable state at the temperature where the experiments were carried

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CONCLUSION Here we have presented the study of TIST at various hydrostatic pressures as well as the studies of PIST at constant temperature for the Hofmann-like porous 3D coordination polymer {Fe(pz)[Pt(CN)4]}. Uncommon behavior of the transition temperature and hysteresis under pressure has been observed and described for both experiments. It has been evidenced that TIST is inhibited at a critical pressure (0.4 GPa) due to the diffusion of the silicon oil inside the porous structure of the compound. In contrast to TIST the PIST in {Fe(pz)[Pt(CN)4]} shows no hysteresis. This fact agrees with the calculated values of the interaction parameter Γ, which is 1.5 times smaller in the case of PIST. Compound {Fe(pz)[Pt(CN)4]} is an exception since several related two- and three-dimensional Hofmann-like Fe(II) SCO polymers all exhibit hysteresis in the PIST. The decrease of cooperativity in the PIST for {Fe(pz)[Pt(CN)4]} may be due to its more rigid structure or to the fact that the experiments have been performed at the temperature where the compound is in a metastable state. In any case, the PIST study demonstrates that compound {Fe(pz)[Pt(CN)4]} can act as a molecular pressure sensor in the room-temperature region, which makes it very attractive in the context of the search for new functional materials.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials. {Fe(pz)[Pt(CN)4]} was synthesized as a microcrystalline powder and characterized as described previously.17,18 Magnetic Susceptibility Measurements under Hydrostatic Pressure. Variable-temperature magnetic susceptibility measurements were performed on the microcrystalline powder by using a Quantum Design MPMS2 SQUID susceptometer equipped with a 5.5 T magnet and operating at 1 T and 1.8−375 K. The hydrostatic pressure cell made of hardened beryllium bronze with silicon oil as the pressure transmiting medium operates in the pressure range 105 Pa < p < 1.4 GPa (accuracy ≈ ±0.025 GPa). Cylindrically shaped powder sample holders 1 mm in diameter and 5−7 mm in length were used. The pressure was measured by using the pressure dependence of the superconducting transition temperature of the built-in pressure sensor made of high-purity tin.57 Experimental data were corrected for diamagnetism by using Pascal’s constants. Absorption Spectroscopy in the Visible Region under Hydrostatic Pressure. Full absorption spectra were recorded between 330 and 840 nm by using a Carl Zeiss PGS-2 spectrometer. The sample was in the form of a thin transparent layer of microcrystalline powder placed between colorless Scotch tape layers (with diameters not larger than 0.2 mm). The hydrostatic pressure cell made of hardened beryllium bronze with NaCl as the pressure transmitting medium operates in the pressure range 105 Pa < p < 1.43 GPa (accuracy ≈0.01 GPa). Pressure was monitored by using the pressure dependence of ruby.57 S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01124. Optical density of the {Fe(pz)[Pt(CN)4]} in LS and HS states at atmospheric pressure. Analysis of the UV− visible spectra of the {Fe(pz)[Pt(CN)4]} at room temperature and at 0.8 GPa and at atmospheric pressure and 77 K (PDF) E

DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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Varret, F.; Gonthier-Vassal, A. Spin Transitions and Thermal Hysteresis in the Molecular-Based Materials [Fe(Htrz)2(trz)](BF4) and [Fe(Htrz)3](BF4)2·H2O (Htrz = 1,2,4−4H-triazole; trz = 1,2,4triazolato). Chem. Mater. 1994, 6, 1404−1412. (17) Niel, V.; Martínez-Agudo, J. M.; Muñoz, M. C.; Gaspar, A. B.; Real, J. A. Cooperative Spin Crossover Behavior in Cyanide-Bridged Fe(II)−M(II) Bimetallic 3D Hofmann-like Networks (M = Ni, Pd, and Pt). Inorg. Chem. 2001, 40, 3838−3839. (18) Ohba, M.; Yoneda, K.; Agustí, G.; Muñoz, M. C.; Gaspar, A. B.; Real, J. A.; Yamasaki, M.; Ando, H.; Nakao, Y.; Sakaki, S.; Kitagawa, S. Bidirectional chemo-switching of spin state in a microporous framework. Angew. Chem., Int. Ed. 2009, 48, 4767−4771. (19) Agustí, G.; Ohtani, R.; Yoneda, K.; Gaspar, A. B.; Ohba, M.; Sánchez-Royo, J. F.; Muñoz, M. C.; Kitagawa, S.; Real, J. A. Oxidative Addition of Halogens on Open Metal Sites in a Microporous SpinCrossover Coordination Polymer. Angew. Chem., Int. Ed. 2009, 48, 8944−8947. (20) Aravena, D.; Arcís-Castillo, Z.; Muñoz, M. C.; Gaspar, A. B.; Yoneda, K.; Ohtani, R.; Mishima, A.; Kitagawa, S.; Ohba, M.; Real, J. A.; Ruiz, E. Guest Modulation of Spin-Crossover Transition Temperature in a Porous Iron(II) Metal−Organic Framework: Experimental and Periodic DFT Studies. Chem. - Eur. J. 2014, 20, 12864−12873. (21) Arcís-Castillo, Z.; Muñoz-Lara, F. J.; Muñoz, M. C.; Aravena, D.; Gaspar, A. B.; Sánchez-Royo, J. F.; Ruiz, E.; Ohba, M.; Matsuda, R.; Kitagawa, S.; Real, J. A. Reversible Chemisorption of Sulfur Dioxide in a Spin Crossover Porous Coordination Polymer. Inorg. Chem. 2013, 52, 12777−12783. (22) Martínez, V.; Gaspar, A. B.; Muñoz, M. C.; Bukin, G.; Levchenko, G.; Real, J. A. Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin-Crossover 2D Metal−Organic Frameworks. Chem. - Eur. J. 2009, 15, 10960−10971. (23) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Villain, F.; Verdaguer, M.; Gü tlich, P. Thermal- and Light-Induced Spin Crossover in Novel 2D Fe(II) Metalorganic Frameworks {Fe(4PhPy)2[MII(CN)x]y}·sH2O: Spectroscopic, Structural, and Magnetic Studies. Inorg. Chem. 2009, 48, 6130−6141. (24) Agustí, G.; Cobo, S.; Gaspar, A. B.; Molnár, G.; Moussa, N. O.; Szilágyi, P. A.; Pálfi, V.; Vieu, C.; Muñoz, M. C.; Real, J. A.; Bousseksou, A. Thermal and Light-Induced Spin Crossover Phenomena in New 3D Hofmann-Like Microporous Metalorganic Frameworks Produced As Bulk Materials and Nanopatterned Thin Films. Chem. Mater. 2008, 20, 6721−6732. (25) Bartual-Murgui, C.; Akou, A.; Shepherd, H. J.; Molnár, G.; Real, J. A.; Salmon, L.; Bousseksou, A. Tunable Spin-Crossover Behavior of the Hofmann-like Network {Fe(bpac)[Pt(CN)4]} through Host−Guest Chemistry. Chem. - Eur. J. 2013, 19, 15036− 15043. (26) Muñoz-Lara, F. J.; Gaspar, A. B.; Muñoz, M. C.; Ksenofontov, V.; Real, J. A. Novel Iron(II) Microporous Spin-Crossover Coordination Polymers with Enhanced Pore Size. Inorg. Chem. 2013, 52, 3−5. (27) Niel, V.; Muñoz, M. C.; Gaspar, A. B.; Levchenko, G.; Real, J. A. Thermal-, Pressure-, and Light-Induced Spin Transition in Novel Cyanide-Bridged FeIIAgI Bimetallic Compounds with Three-Dimensional Interpenetrating Double Structures {FeIILx[Ag(CN)2]2}·G. Chem. - Eur. J. 2002, 8, 2446−2453. (28) Sciortino, N. F.; Neville, S. M.; Desplanches, C.; Lètard, J. F.; Martínez, V.; Real, J. A.; Moubaraki, B.; Murray, K. S.; Kepert, C. J. An Investigation of Photo- and Pressure-Induced Effects in a Pair of Isostructural Two-Dimensional Spin-Crossover Framework Materials. Chem. - Eur. J. 2014, 20, 7448−7457. (29) Southon, P. D.; Liu, L.; Fellows, E. A.; Price, D. J.; Halder, G. J.; Chapman, K. W.; Moubaraki, B.; Murray, K. S.; Lètard, J. F.; Kepert, C. J. Dynamic Interplay between Spin-Crossover and Host− Guest Function in a Nanoporous Metal−Organic Framework Material. J. Am. Chem. Soc. 2009, 131, 10998−11009. (30) Piñeiro-López, L.; Valverde-Muñoz, F. J.; Seredyuk, M.; Bartual-Murgui, C.; Muñoz, M. C.; Real, J. A. Cyanido-Bridged

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ana Belén Gaspar: 0000-0002-5784-9350 José Antonio Real: 0000-0002-2302-561X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Spanish Ministerio de Economiá y Competitividad (MINECO) and FEDER funds CTQ2016-78341-P and Unidad de Excelencia Mariá de Maeztu MDM-2015-0538, Generalitat Valenciana (PROMETEO/2016/147) and the Ukrainian State Fund of the Fundamental Investigations F71 (project F-71/61-2017). “The Thousand Talents Program for Foreign Experts”, Project WQ20162200339 (Chine).

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DOI: 10.1021/acs.inorgchem.8b01124 Inorg. Chem. XXXX, XXX, XXX−XXX