Spin crossover in Fe(4X-isonicotinate)₂[Fe(CN)₅NO] with X = methyl and ethyl: the effect of the 4X substituent size on the spin transition

Abstract

This contribution reports the spin-crossover behavior of two 2D ferrous nitroprussides formed by incorporating 4-methyl isonicotinate and 4-ethyl isonicotinate as organic pillars (L), labeled MIso and EIso, respectively. The pillars between adjacent layers are formed by pairs of molecules (L) linked to axial coordination sites of the iron atom: Fe(L)₂[Fe(CN)₅NO]. These two Hofmann-like frameworks show thermally induced spin-crossover (SCO) in the 80–150 K temperature range. The corresponding crystal structures for their high-spin (HS) and low-spin (LS) phases were solved and refined from powder XRD patterns. The solids formed with these two pillar molecules crystallize in a monoclinic unit cell in the P2₁ space group. For MIso, where SCO involves pronounced magnetic hysteresis, the HS → LS transition is accompanied by a unit cell volume contraction of approximately 4.94%. When L = EIso, having a narrow magnetic hysteresis loop, there is a resulting unit cell volume contraction of 1.45%. The magnetic and structural study of SCO in these two solids was complemented by DSC curves and IR, Raman, and Mössbauer spectra. The observed thermally induced spin transition shows a marked dependence on the size of the substituent present in the pillar molecule. The nature of this dependence is discussed in this contribution.

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1. Introduction

The changes in the physical properties of solids induced by external stimuli support many of their functional applications, including in sensors and actuators, information storage, and devices utilizing their magnetic and optical properties. A reversible spin transition, known as spin-crossover (SCO), induced by an external stimulus forms part of such effects. SCO is observed in solids with 3d metal ions (3d⁴–3d⁷) in an octahedral or pseudo-octahedral coordination environment. The effect can be induced through an external stimulus that can overcome the energy splitting (10 Dq) between the metal t_2g and e_g orbitals.^1–6 A temperature or pressure change can induce such a spin transition,^3,7–11 as can the application of an electric or magnetic field,^12–16 the incidence of electromagnetic radiation,^8,9,17–19 or the introduction of guest species into the host framework.^20–25 The role of the material dimensions in SCO behavior has also been considered.^26–28 Of these metals, the ferrous ion (3d⁶) has garnered attention for SCO studies because its spin transition involves six electrons, resulting in pronounced changes in the physical properties of the host solid.

The high-spin (HS) to low-spin (LS) transition, and the reverse (LS → HS), modifies several physical properties of the solid, including its magnetic behavior, which changes due to the number of paramagnetic centers; its optical response, associated with the d–d transitions and metal–ligand charge transfer; its structural features, resulting from variations in bond lengths and angles; and its thermal and vibrational properties.^{1–6,29–31} This explains that SCO is usually monitored through structural (XRD), magnetic (SQUID), optical (color change), spectroscopic (IR, Raman, UV-vis, and Mössbauer), and thermal (DSC) studies.^3,4,32–38 Such changes in physical properties support the potential applications of SCO materials, including in smart windows and displays,^5,40 digital information devices,^5,40 sensors and actuators,^41–44 and thermal energy harvesting.⁴⁵

Cyanide-based coordination polymers form an interesting family of solids where SCO has been observed.^{9,10,20–22,24,25,32–39,46–58} Within these solids, pillared ferrous tetracyanides—Fe(L)_n[M(CN)₄] with M = Ni, Pd, Pt, and L being a pyridine derivative (ditopic: n = 1; and monotopic: n = 2) acting as a pillar molecule—also known as Hofmann-like coordination polymers, have been intensively studied as SCO materials.^{9,10,20–22,24,25,32–39} Recently, ferrous nitroprussides, Fe(L)_n[Fe(CN)₅NO], also with a Hofmann-like structure, have emerged as an interesting series of SCO materials.^59–67 In this last series, the nitrosyl group (NO) behaves as an electron buffer, modulating the value of 10 Dq and, as a consequence, tuning the spin transition.⁶⁷

In this contribution, we report a study of the SCO behavior of Fe(4X-isonicotinate)₂[Fe(CN)₅NO], with X = methyl or ethyl, to shed light on the effect of the 4X substituent size on the spin transition. In Hofmann-like coordination polymers, Fe(L)_n[M(CN)₄], the substituent nature and position in the molecule modify the coordination geometry of the iron atom, Fe(N_CN)₄(L)₂, the value of 10 Dq and the related SCO properties of the solid, including the inhibition of the spin transition.³⁹ Such a substituent effect remains poorly documented for pillared ferrous nitroprussides. The SCO behavior in the titled solids was established through magnetic (MPMS-3 SQUID) measurements, IR, Raman, and Mössbauer spectra, DSC curves, and crystal structures corresponding to the HS and LS phases. A more voluminous substituent (ethyl versus methyl) results in a narrower hysteresis loop and a minor unit cell volume change during the spin transition (1.45% (ethyl) vs 4.94% (methyl)). The nature of such differences is discussed herein based on the recorded structural and spectroscopic information. No similar study has been reported for these two pillared ferrous nitroprussides.

2. Experimental

The two materials under study were prepared by a precipitation method from 0.01 M aqueous solutions of the relevant reagents: Mohr salt ((NH₄)₂(FeSO₄)₂·6(H₂O)), sodium nitroprusside (Na₂[Fe(CN)₅NO]·2H₂O), and the corresponding organic ligand (L), at a 1 : 1 : 2 M ratio. Aqueous solutions of the organic ligand and the Mohr salt are initially prepared and then added to the sodium nitroprusside solution, which is stirred. The obtained mixture is then aged in the dark for three days, after which a fine precipitate appears. The resulting solid is separated from the mother liquor by centrifugation and washed with doubly distilled water, followed by air drying in the dark until it has a constant weight. The nature of the obtained powder was established based on IR, Raman, and Mössbauer spectroscopy, TG data, powder XRD patterns, and chemical analysis (see Table S1).

IR spectra were recorded at 80 and 300 K using an ATR device in the spectral range of 4000–400 cm⁻¹ with a Spectrum One spectrophotometer (PerkinElmer). Raman spectra were recorded at 80 and 300 K in the spectral range of 3500–50 cm⁻¹ using a DXR Raman microscope (Thermo Scientific Co.) with a 532 nm laser at a power of 1 mW. ⁵⁷Fe Mössbauer spectra were recorded at 300 K and 5 K using a SeeCo spectrometer with a ⁵⁷Co/Rh radiation source. To obtain the spectra at 5 K, a Janis closed-cycle He cryostat was used to maintain the sample under isothermal conditions at that temperature. All the samples were prepared under thin-absorber conditions. The Mosswinn software package was used for spectra fitting.⁶⁸ The value for the isomer shift (δ, in mm s⁻¹) is reported relative to sodium nitroprusside at room temperature (300 K). TG curves were collected at a heating rate of 5 °C min⁻¹ under N₂ flow (100 mL min⁻¹) in the temperature range of 30–500 °C using an HR thermobalance (TGA Q 5000, from TA Instruments).

The powder XRD patterns for 4-methyl isonicotinate (4MIso) samples were recorded at BM25-SpLine, the European Synchrotron Radiation Facility, at temperatures of 100 and 300 K, with samples mounted in 0.5-mm inner-diameter polyimide capillaries. For the 4-ethyl isonicotinate (4EIso) samples, the XRD patterns were collected with a Brucker diffractometer equipped with CuK_α radiation and a Linx Eye detector. The integrated XRD patterns were indexed with the DicVol algorithm.⁶⁹ The structural model to be refined was derived from an initially proposed asymmetric cell, using a global optimization process in direct space (simulated annealing) implemented in the EXPO09 program.⁷⁰ The space group assignment was corroborated by using the Le Bail profile-fitting method.⁷¹ Crystal structure refinement was performed using the Rietveld method implemented in the FullProf program.⁷² A third-order polynomial was used to model the pattern background. Peak profiles (of pseudo-Voigt type) were calculated up to ten times the full width at half maximum (FWHM). Details on XRD data collection and processing are available in (Table S2).

Thermally induced SCO in the two ferrous nitroprussides considered herein was evaluated from magnetic (SQUID) data and DSC curves. The magnetic data were recorded using an MPMS-3 magnetometer (from Quantum Design). Zero-field-cooling (ZFC) and field-cooling (FC) curves were recorded in the temperature range of 2–300 K, with an applied magnetic field of 100 Oe. The experimental magnetic susceptibility (χ) values were corrected for the diamagnetic contribution according to the reported Pascal constants for the involved elements.⁷³ From the recorded data, the effective magnetic moment (μ_eff) was calculated according to the formula μ_eff = 2.828 sqrt(χT).⁷³ The DSC curves were obtained using TA Instruments equipment under N₂ flow (50 mL min⁻¹). The value of the involved enthalpy (ΔH) during the spin transition was calculated by integrating the heat flow versus temperature curve, and the value of ΔS was then derived by dividing the value of ΔH by the transition temperature.

3. Results and discussion

A. Nature of the solids under study

When an aqueous solution of the pillar molecules is added to a suspension of a 3D transition-metal (T) nitroprusside, T[Fe(CN)₅NO]·nH₂O, after sonication a 2D analog forms: T(L)₂[Fe(CN)₅NO].^74,75 In this 2D solid, the organic ligand occupies the axial coordination sites of the outer metal (T): T(NC)₄(L)₂. Pyridinic molecules are the ligands (L) of choice to obtain 2D transition-metal nitroprussides.⁷⁵ The 2D phase is the thermodynamic product of that reaction because the same end product is obtained from different preparative routes, including direct precipitation reactions using diverse solvent mixtures and solid-state reactions with reagent milling.^59–64,75

The IR spectrum has proved to be a fingerprint for probing the incorporation of pillar molecules between 2D cyanide-based coordination polymers.^65–67,75 T–L coordination bond formation results in a frequency shift for the ν(CN) band related to a variation in the electron density at the CN5σ orbital.⁷⁶ Concomitant changes in the intensity and frequency of the molecule's IR absorption bands are observed. Fig. S1 and S2 show the IR spectra for 3D ferrous nitroprusside, 4-methyl isonicotinate (MIso) and 4-ethyl isonicotinate (EIso) molecules, and the hybrid inorganic–organic solids, Fe(L)₂[Fe(CN)₅NO], formed by the precipitation reactions involving these two molecules. Relative to 3D ferrous nitroprusside, in the formed 2D pillared solids, the ν(CN) band appears with a frequency shift of −8 and −10 cm⁻¹ for MIso and EIso, respectively. The rupture of the axial CN_Ax–T coordination bond in 3D ferrous nitroprusside and 2D structure formation leads to a higher electron density at the Fe atom of the nitroprusside building block, resulting in a stronger π-back-bonding interaction with both equatorial CN ligands. This explains the observed frequency shift for the equatorial ν(CN) stretching band. 3D ferrous nitroprusside contains water molecules coordinated to the outer metal (T), stabilizing hydrogen-bonded water molecules within its porous framework.⁷⁵ In the IR spectra of the formed 2D pillared solids, the broad ν(OH) absorption band at 3340 cm⁻¹ related to these weakly bonded water molecules is absent. The pillared solids have anhydrous character.

Due to vibrations within the pyridinic ring and the substituents, the two organic ligands have rich IR spectra in the spectral region of 1730 to 650 cm⁻¹. Changes in intensity and frequency in that spectral region can be used to probe the formation of the pyridinic N–Fe coordination bond. The formation of the pillared solid is also indicated by frequency shifts, relative to 3D ferrous nitroprusside, in the low-frequency spectral region of the inorganic building block, the region below 670 cm⁻¹, where the ν(Fe–CN), ν(Fe–NO), δ(Fe–C–N), and δ(Fe–N–O) vibrational modes absorb (Fig. S1 and S2).

The IR spectra for the two Hofmann-like solids under study show narrow absorption bands above 3000 cm⁻¹ (Fig. S1 and S2). These narrow bands could be ascribed to the presence of C–H⋯π interactions between neighboring pillar molecules, which was corroborated by the structural study discussed below.

The information provided by the recorded TG curves (Fig. S3 and S4) corresponds with the evidence obtained from the IR spectra. Below 100 °C, where the structural water molecules in transition-metal nitroprussides evolve,⁷⁵ the TG curves are free of weight loss. Above 150 °C, during sample heating, progressive thermal decomposition is observed, initially dominated by the evolution of the two organic molecules, followed by the decomposition of the inorganic block. The thermal decomposition of the inorganic fraction and the CN-based block is well documented.^75,77

The above-discussed IR and TG data, along with the results from chemical analysis (Table S1), support the nature of the materials under study being 2D pillared ferrous nitroprussides, specifically with the formula unit Fe(4X-isonicotinate)₂[Fe(CN)₅NO] with X = methyl or ethyl. The structural study to be discussed is congruent with the nature of these two inorganic–organic hybrid solids.

B. Structural features of Fe(4X-isonicotinate)₂[Fe(CN)₅NO] with X = methyl or ethyl

Fig. 1 shows the recorded powder XRD patterns of representative samples of the materials under study. Tables S2 and S3 summarize details about XRD data recording and the figures of merit corresponding to the indexing process for the two patterns. These patterns correspond to monoclinic unit cells in the P2₁ space group. The unit cell volume accommodates two formula units (Z = 2). Table 1 reports the unit cell parameters derived from the indexing process for the patterns recorded at 100 and 300 K. The presence of ethyl instead of methyl as a substituent results in a slightly larger unit cell volume and monoclinic deformation of the unit cell. Such differences are related to the 4X substituent nature, particularly its size and the intermolecular interactions in the interlayer region (Fig. 2), which are discussed below. The powder XRD patterns recorded at 300 K for the two materials were of sufficient quality to be used to solve their crystal structures in direct space by simulated annealing and then to refine the structure using the Rietveld method. The resulting structural information was deposited in the CCDC database through the following CIF files: 2346684 (4MIso, at 300 K), 2346685 (4MIso, at 100 K), 2346683 (4EIso, at 300 K), and 2363385 (4EIso, at 80 K). The refined atomic positions and isotropic thermal factors are summarized in Tables S2 and S3. Tables S4 and S5 show the calculated interatomic distances and bond angles. Table 2 summarizes some relevant interatomic distances and the corresponding distortion indices , ,⁷⁸ and CShM (continuous shape measures)⁷⁹ for the iron atom susceptible to SCO. The values of these distortion indexes can show the deviation of the iron coordination polyhedron from ideal octahedral geometry. Significant deviation from the ideal geometry hinders the possibility of observing SCO.^3,39,66 According to the obtained values for Σ, Θ, and CShM (Table 2), a complete spin transition is expected upon cooling the sample for the two pillared ferrous nitroprussides considered herein (as discussed below). In the interlayer region, neighboring pillar molecules continue interacting through CH⋯π and O⋯H weak bonds, which are stronger with 4EIso as a pillar molecule (Fig. 2). The presence of these weak intermolecular interactions between neighboring pillar molecules is supported by the recorded IR spectra (Fig. S1 and S2). Such a structural feature could explain the slightly larger monoclinic distortion observed for 4EIso: 98.76° versus 99.23° for 4MIso (Table 1).

Fig. 1 Experimental and fitted powder XRD patterns at 300 K (RT) and their difference for Fe(4MIso)₂[Fe(CN)₅NO] (A) and Fe(4EIso)₂[Fe(CN)₅NO] (B). Insets: Comparisons between the XRD patterns corresponding to the HS (300 K) and LS (100 and 80 K with 4MIso and 4EIso as pillar molecules, respectively) states.

Table 1 Unit cell parameters for the Fe(4X-isonicotinate)₂[Fe(CN)₅NO] series, with X = methyl or ethyl

4X substituent	Phase, temp (K)	a, in Å	b, in Å	c, in Å	β, in °	V, in Å^3	Unit cell volume contraction (%)
Methyl	HS, 300	11.5223(4)	15.0011(6)	7.2442(5)	99.23(5)	1235.93(2)	4.94
	LS, 100	11.4112(2)	14.7556(6)	7.0769(5)	99.62(5)	1174.85(2)
Ethyl	HS, 300	12.3359(6)	14.5996(5)	7.3867(5)	98.76(3)	1312.12(2)	1.45
	LS, 80	12.2888(6)	14.5058(5)	7.3438(5)	98.98(3)	1293.02(2)

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Fig. 2 Atom packing within the framework for Fe(4MIso)₂[Fe(CN)5NO] (A) and Fe(4EIso)₂[Fe(CN)₅NO] (B) from the corresponding XRD patterns recorded at 300 K.

Table 2 Relevant interatomic distances (in Å) and values for the distortion indexes (Σ, Θ, and CShM)

Metal	Phase	Fe–NPy	Fe–NCN		.	CShM
Methyl	HS, 300	2.6479(2)	2.2609(2)	110.668	176.215	3.1162
	LS, 100	2.6148(2)	2.1584(2)	99.975	197.281	3.5965
Ethyl	HS, 300	2.2347(2)	2.2426(2)	48.109	64.347	0.5109
	LS, 80	2.2234(2)	2.2319 (2)	47.343	64.794	0.5112

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C. Thermally induced spin crossover from the recorded magnetic data

Fig. 3(A) shows the magnetic behavior of a sample of the solid pillared with 4MIso as it is cooled and then warmed. The observed temperature dependence of μ_eff corresponds to a reversible thermally induced spin transition, exhibiting a well-defined hysteresis of about 14 K at a scan rate of 0.25 K min⁻¹, between the two processes HS → LS and LS → HS. The magnetic data recorded using different cooling and heating rates show definite kinetic effects (discussed below). The onset temperature for the HS → LS transition is close to 115 K, which is little affected by kinetic effects. The heating rate affects the onset temperature for the inverse transition (LS → HS). The kinetic effects for SCO in pillared ferrous nitroprussides arise from the repulsive interaction between the O and N ends of NO and axial CN located in the interlayer region. These two axial ligands accumulate electron density at their O and N ends through the NO and CN_ax π-back-bonding interactions with the Fe atom in the nitroprusside building block. The repulsive interactions oppose a reduction in the layer–layer distance during the HS → LS transition, resulting in the observed kinetic effects. The NO–NC distance of 5.77 Å is within the range where such repulsive interactions induce relevant kinetic effects.⁸⁰ These effects are less pronounced upon sample heating because the repulsive interactions favor layer–layer separation. The onset temperature for HS → LS corresponds to the temperature where a decrease in the thermal energy (kT) makes possible e_g → t_2g electron migration in the iron atom. In Hofmann-like coordination polymers with thermally induced SCO, a relatively strong Fe–N_CN coordination bond favors a higher onset temperature.³⁸ In pillared ferrous nitroprussides, upon sample cooling and solid contraction, that coordination bond becomes stronger via ON → Fe → CN electron density redistribution through weakening Fe → NO π-back-donation, making SCO possible despite the layer–layer repulsive interaction.⁶⁷ A high onset temperature for the HS → LS transition corresponds to weakening of Fe → NO π-back donation and a relatively strong Fe–N_CN coordination bond. In this material, the HS → LS transition saturates at about 3.4 B.M., corresponding to a relatively large fraction of residual paramagnetic centers (HS Fe(II) atoms), which could be interpreted as the presence of an incomplete spin transition. Mössbauer spectra recorded at 5 K reveal that the transition is complete, and that feature is a consequence of the previously mentioned kinetic effects (discussed below).

Fig. 3 Effective magnetic moment (μ_eff) versus temperature for (A) Fe(4MIso)₂[Fe(CN)₅NO] and (B) Fe(4EIso)₂[Fe(CN)₅NO] from magnetic data recorded at cooling and heating rates of 0.25, 2, 8, 16 and 32 K min⁻¹ and under an external applied magnetic field of 100 Oe. Insets: NO–NC_ax distances in the formed frameworks; changes in sample color when immersed in liquid nitrogen; and Curie–Weiss plots for the low-temperature region of the magnetic data for the solid involving 4EIso.

Fig. 3(B) shows the magnetic curves of a sample of the 2D hybrid ferrous nitroprusside formed with 4EIso as a pillar molecule as it cools and then warms. Relative to the already discussed 4MIso, the HS–LS transition occurs at higher temperatures. The onset of that transition is observed at about 125 K. In this material, SCO is characterized by a narrow hysteresis of less than 2 K. This feature can be attributed to two related factors: (1) the larger separation of the charge centers (O and N ends), about 6.97 Å; and (2) their non-colinear arrangement in the interlayer region, which allows re-accommodation during solid contraction to minimize repulsive forces, thereby influencing cooperativity. Cooperative effects in spin transition materials are primarily due to different variables, such as intermolecular interactions, steric effects, and the mechanical properties of the entire solid. It is the solid intermolecular interactions that play a pivotal role in strong cooperativity, leading to hysteresis behavior and bistability.⁸¹ Behavior similar to that obtained here was reported previously for materials of the same family.^59–67

Weaker repulsive layer–layer interactions explain the observed increase in onset temperature by approximately 10 K. Due to the mentioned kinetic effects, in this material, the HS → LS transition saturates at approximately 2.2 B.M. Upon cooling the sample from approximately 70 K, residual paramagnetic species participate in weak cooperative magnetic interactions of antiferromagnetic nature, characterized by a negative Curie–Weiss temperature (θ_CW) of –3.7 K (Fig. 3B, inset). Such interactions result from HS Fe(II) species in –Fe_HS(II)–N≡C–Fe_LS–C≡N–Fe_HS(II)– chains, with a separation of above 10 Å. This explains the low value for θ_CW. Then, upon sample warming, the fraction of these species decreases, and the magnetic curve is observed to be below the one corresponding to the cooling cycle. The Curie–Weiss constant for the corresponding antiferromagnetic interactions was −2.2 K (Fig. 3(b), inset). Such a difference in the value of θ_CW is related to the population of paramagnetic centers.

Fig. 3 (inset photos) shows the change in the powder color when the sample is immersed in liquid nitrogen. The color of transition-metal nitroprussides has two origins: the d–d transitions in the paramagnetic centers and metal–ligand charge transfer (MLCT) within the nitroprusside building block. Sodium nitroprusside is characterized by a purple-red color ascribed to MLCT, with strong absorption bands below 500 nm.⁷⁵ The absorption spectrum related to the d–d transitions in HS Fe(II) species in an octahedral or pseudo-octahedral environment is characterized by a broad band at about 640 nm. The combination of light absorption in these two spectral regions produces the yellow-brown color shown by the powder at room temperature (Fig. 3). When the sample is immersed in liquid nitrogen, MLCT absorption dominates, and the powder turns red-yellow for 4MIso and close to purple-red for 4EIso. The change in sample color when immersed in liquid nitrogen (77 K) is congruent with the magnetic data and the observed kinetic effects.

D. Thermally induced spin crossover evaluated from DSC curves

Fig. 4 shows the DSC curves for Fe(4MIso)₂[Fe(CN)₅NO] in the temperature region where the spin transition is observed. For Fe(4EIso)₂[Fe(CN)₅NO], no reliable DSC curves were recorded because with the available DSC instrument, only data from 80 K can be recorded, and for this material, the HS → LS transition ends below that temperature. Within the expected experimental error, the obtained values for ΔH and ΔS are pretty similar: 2.07 and 1.71 kJ mol⁻¹, and 18.48 and 14.65 J mol⁻¹ L⁻¹, respectively, for the HS → LS and LS → HS transitions. These values for ΔH suggest that both the e_g → t_2g electron migration and the inverse process are soft electronic transitions involving a relatively small energy change. For Fe(pyridine)₂[Fe(CN)₅NO], the reported ΔH values are 5.30 and 5.25 kJ mol⁻¹.⁵⁹ For that pillar molecule, the values of ΔS were 46.90 and 40.10 J mol⁻¹ K⁻¹, following a trend to that observed for ΔH. The temperature difference between the peak positions in the DSC curve (Fig. 4), the thermal hysteresis, is approximately 5 K, differing from that observed from the magnetic data. Such a difference could be related to the already discussed kinetic effects due to repulsive interactions between NO and axial CN in the interlayer region.

Fig. 4 DSC curves for Fe(4MIso)₂[Fe(CN)₅NO] in the temperature region where the spin transition is observed. Estimated ΔH and ΔS values during sample cooling and heating are indicated.

E. Evidence for SCO from IR and Raman spectra

For the e²_gt⁴_2g → e⁰_gt⁶_2g electron migration associated with the HS → LS transition, the e_g orbitals are depopulated, facilitating the removal of electron density from the CN5σ orbital. This last orbital on the N side has specific antibonding character for the C≡N bond. In the recorded IR spectra, such electron density redistribution is manifested in a frequency shift of +5 and 3 cm⁻¹ for 4MIso and 4EIso, respectively (Fig. 5, insets). During the sample cooling of 2D ferrous nitroprussides and solid contraction, the related stronger repulsive NO–NC interactions induce the migration of electron density from the Fe atom in the nitroprusside ion towards the CN5σ orbital of equatorial CN via π-back donation.⁶⁷ The shorter the NO–NC distance, the stronger the repulsive force, and the higher the electron density accumulated at the CN5σ orbital. This concerted mechanism accounts for the lower frequency shift (+3 cm⁻¹) observed in the solid with 4EIso as the pillar molecule. Conversely, with 4MIso, the shorter NO–NC distance leads to a higher frequency shift (+5 cm⁻¹).

Fig. 5 The ν(CN) spectral region in Raman spectra recorded at 80 and 300 K (RT) for (A) Fe(4MIso)₂[Fe(CN)₅NO] and (B) Fe(4EIso)₂[Fe(CN)₅NO]. Insets: IR ν(CN) stretching bands for the HS and LS phases of the materials under study.

Three resolved bands appear in the CN spectral region of the Raman spectra of metal nitroprussides, corresponding to the ν(CN_eq)_A′, ν(CN_eq)_A′′, and ν(CN_ax)_A′ vibrational modes (Fig. 5). Similar to that observed in the IR spectra, the bands involving equatorial CN are sensitive to SCO in ferrous nitroprussides. For 4MIso, the frequency shifts for the ν(CN_eq)_A′ and ν(CN_eq)_A′′ bands are +6 and +4 cm⁻¹, respectively (Fig. 5(A)). The frequency shift is minor, about +2 cm⁻¹ for the ν(CN_ax)_A′ vibration. The change in this last vibration is related to an increase in electron density at the Fe atom of the nitroprusside building block due to the weakening of Fe → NO π-back donation during the HS → LS transition. For 4EIso, the frequency shifts for the ν(CN_eq)_A′ and (CN_eq)_A′′ bands are 5 and 3 cm⁻¹, respectively. The band corresponding to ν(CN_ax)_A′ appears unresolved (Fig. 5(B)). These Raman spectra show the same regularity observed for the IR spectra in terms of the spin transition. However, in the Raman spectra, the three bands partially overlap, which increases the uncertainty in determining their positions and frequency shifts.

F. Mössbauer spectra

⁵⁷Fe Mössbauer spectroscopy is a key technique for probing iron in solids. It provides information on the valence and electronic structure of the iron atom, the nature of its first and second neighbors, the relative populations of different iron species in the solid under study, and details of its bonding interactions with ligands, among other insights.⁸² Its energetic resolution is about 10⁻¹², related to the lifetime of the ⁵⁷Fe nuclear-excited state: about 10⁻⁷ s. A typical peak width in ⁵⁷Fe Mössbauer spectroscopy is 2 × 10⁻⁸ eV (equivalent to 0.22 mm s⁻¹). In contrast to the other techniques herein considered, Mössbauer spectroscopy provides information on the state of the iron atom near thermodynamic equilibrium, with minimal kinetic effects, especially when recording times extend to a day or longer per spectrum.

Fig. 6 shows spectra recorded at 295 and 5 K for Fe(4MISo)₂[Fe(CN)₅NO] and Fe(4EISo)₂[Fe(CN)₅NO]. These spectra were fitted as superpositions of two quadrupole splitting doublets, corresponding to iron atoms in the following coordination environments: [Fe(CN)₅NO] and Fe(NC_eq)₄(N_L)₂. According to their isomer shift (δ) values (Table 3), at 295 K all the Fe(NC_eq)₄(N_L)₂ species are found with a HS electronic configuration. These are the iron species susceptible to participating in a spin-crossover transition upon sample cooling. The iron atom in the nitroprusside ion, [Fe(CN)₅NO], is always found with an LS electronic configuration. The Mössbauer spectra recorded at 5 K provide conclusive evidence that, near thermodynamic equilibrium, the HS → LS transition is complete.

Fig. 6 Mössbauer spectra recorded at 295 and 5 K for the materials under study: (A) Fe(4MIso)₂[Fe(CN)₅NO] and (B) Fe(4EIso)₂[Fe(CN)₅NO]. These spectra were fitted as a superposition of two LS quadrupole splitting doublets, corresponding to a complete HS → LS spin transition.

Table 3 Mössbauer parameters (δ, Δ_QS, and Γ) at 295 and 5 K for Fe(4MIso)₂[Fe(CN)₅NO] and Fe(4EIso)₂[Fe(CN)₅NO]

T, K	L	δ , mm s^−1	Δ QS, mm s^−1	Γ, mm s^−1	Area, %	Assignment
a Isomer shift (δ) value reported relative to sodium nitroprusside at room temperature.
295	4MIso	−0.0182(1)	1.832(1)	0.264	50	LS, [Fe(CN)5NO]
		1.283(1)	0.959(1)	0.305	50	HS, Fe(NC)4(NL)2
	4EIso	−0.0173(1)	1.875(1)	0.283(1)	50	LS, [Fe(CN)5NO]
		1.302(1)	0.961(1)	0.292(1)	50	HS, Fe(NC)4(NL)2
5	4MIso	0.0131(1)	1.623(1)	0.281(1)	50	LS, [Fe(CN)5NO]
		0.7563(1)	0.230(1)	0.272(1)	50	LS, Fe(NC)4(NL)2
	4EIso	0.0231(1)	1.675(1)	0.272(1)	50	LS, [Fe(CN)5NO]
		0.7102(1)	0.241(1)	0.291(1)	50	LS, Fe(NC)4(NL)2

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The value of δ for the [Fe(CN)₅NO] site is close to 0.01 mm s⁻¹, relative to sodium nitroprusside at room temperature. That low value δ is typical of LS Fe(II) in metal nitroprussides and arises from the reduced electron density at the iron atom at that site, a consequence of the π-back-bonding capabilities of both CN and NO ligands at their C and N termini, respectively.⁸³ The δ value for the other structural site, Fe(NC)₄(N_L)₂, of close to 0.78 mm s⁻¹ also corresponds to an LS Fe(II) species but where the ligand properties are dominated by σ-bonding character. Regarding the quadrupole splitting (Δ_QS), the values for the two sites are quite different. In [Fe(CN)₅NO], it is determined by the strong π-back-bonding properties of the NO group relative to the CN ligands, resulting in pronounced axial deformation of the iron atom coordination environment, with a Δ_QS value above 1.40 mm s⁻¹, while for the Fe(NC)₄(N_L)₂ site, the sub-spectrum is a narrow doublet with a Δ_QS value of about 0.38 mm s⁻¹. This small value suggests a slightly elongated coordination octahedron for that site. The relatively small value for the line width supports the used spectral fitting model of two quadrupole doublets. As expected, the relative populations of these two structural sites are close to 1 : 1.

G. Structural changes related to spin crossover

Table 2 contains the refined unit cell parameters from the XRD pattern recorded at 100 K for Fe(4MIso)₂[Fe(CN)₅NO]. At this temperature, the material is found in the LS phase. Relative to the HS phase, the HS → LS transition involves a unit cell contraction, observed as a shift of the diffraction peaks towards slightly higher 2θ values (Fig. 1(a), inset). According to the unit cell parameters, this volume contraction is 4.94%. This is an expected result of forming stronger Fe–NC_eq and Fe–N_L coordination bonds during the HS → LS transition. This structural change in the iron atom coordination environment induces slight changes in the bond distances in both the inorganic [Fe(CN)₅NO] layer and the pillar molecule, including the intermolecular interactions. Fig. 7(A) illustrates the atomic packing for the LS phase, where these structural changes are evident.

Fig. 7 Atom packing within the frameworks of Fe(4MIso)₂[Fe(CN)₅NO] (refined from XRD data recorded at 100 K) (A) and Fe(4EIso)₂[Fe(CN)₅NO] (calculated at 0 K) for the LS phases of this solid (B).

For Fe(4EIso)₂[Fe(CN)₅NO], the XRD pattern for the LS phase was recorded at 80 K. The inset of Fig. 1(B) compares the XRD pattern fitting for the HS and LS phases of the material containing 4EIso as a pillar molecule. For the LS phase of this solid, the crystal structure was calculated based on the provided structural information, including unit cell parameters and space group information, using VASP code^84–87 as detailed elsewhere.^63–66Fig. 7(B) shows details about the interactions between adjacent layers and neighboring pillar molecules. The relative dispositions of the axial CN and NO groups from adjacent layers in the two solids considered herein explain the differences during the HS → LS transition (discussed above). For 4MIso, the CN–ON distance suffers a significant reduction upon this last spin transition, from 5.77 to 5.62 Å (Fig. 1(A) and 7(A)). Such a reduction between charge centers corresponds with the observed contraction in the unit cell volume of about 4.94% (Table 1). For 4EIso, with a unit cell volume reduction of 1.45%, the change in the CN–ON distance is 0.03 Å, from 6.97 to 6.94 Å (Fig. 1(B) and 7(B)).

H. Information derived from Hirshfeld surface analysis

Hirshfeld surface analysis is a computational tool used to obtain information about intermolecular interactions and to visualize the involved electron density. In this study, it was applied to visualize the change in the electron density in the coordination environment of the iron atom susceptible to SCO, including the pillar molecule. The calculations were performed with CrystalExplorer software.⁸⁸Fig. 8 illustrates the changes in the electron density related to the SCO transition in the solids under study.

Fig. 8 Visualization of the electron density in Fe(4MIso)₂[Fe(CN)₅NO] (A) and (Fe(4EIso)₂[Fe(CN)₅NO]) (B). (C) 2D fingerprint plots (in rows) for the main interactions in the interlayer region for these two materials, with changes related to the HS → LS transition.

The HS → LS transition concerns e²_gt⁴_2g → e⁰_gt⁶_2g electron migration within the iron atom, and this has a relatively small impact on the electron density of its ligands and their interactions in the interlayer region. Such expected features are seen in the visualized electron densities for the HS and LS phases of the materials considered herein. The HS → LS transition involves increases of 0.1 and 0.2% for ligand–Fe interactions in Fe(4MIso)₂[Fe(CN)₅NO] and (Fe(4EIso)₂[Fe(CN)₅NO]), respectively. The higher value for the solid with 4EIso as a pillar molecule is congruent with the Fe–N_Pyr bond distance, which is about 2.23 Å, versus 2.65 Å when 4MIso is the pillar (Table 2). Both the calculated electron density and the bond distance suggest that a weaker CN–ON repulsive interaction favors a stronger Fe–N_Pyr coordination bond. The C–H⋯π and O⋯H interactions (Fig. 2 and 7) have a relevant role in determining the intermolecular forces in the interlayer region, contributing to about 25% of the calculated electron density, and this is slightly larger for the LS phases due to the unit cell volume contraction. The ligand–ligand interactions via pyridine rings, herein referred to as π–π cloud overlapping (but not necessarily limited to π clouds), contribute over 41% of the electron density, reaching up to 46% in the solid containing 4EIso. Such a difference was ascribed to stronger intermolecular interactions for this last ligand. The decrease in electron density around the nitroprusside building block upon the HS → LS transition is connected with the above-discussed NO → Fe → CN_eq electron density redistribution upon e²_gt⁴_2g → e⁰_gt⁶_2g electron migration. This suggests that Hirshfeld surface analysis is a powerful tool for visualizing the electron density redistribution during SCO in Hofmann-like coordination polymers.

4. Conclusions

Thermally induced spin-crossover in Fe(4X-isonicotinate)₂[Fe(CN)₅NO], with X = methyl or ethyl, reveals the role of the pyridine substituent (X) in pillared 2D ferrous nitroprussides. The substituent group determines the NO–NC interatomic distance within the interlayer region and the related layer–layer repulsive interaction. This repulsive interaction is responsible for the pronounced kinetic effects observed in ferrous nitroprussides and also determines the onset temperature for the HS → LS transition. Regarding the two hybrid 2D inorganic–organic solids considered herein, for methyl, where the NO–NC distance is 5.77 Å, the magnetic data show pronounced kinetic effects characterized by hysteresis of 15 K, while for ethyl, with a NO–NC distance of 6.97 Å, the observed hysteresis is less than 2 K. The observed kinetic effects are responsible for an apparently incomplete HS → LS transition in the recorded magnetic data, as appreciated through a residual paramagnetic fraction at low temperatures. The residual fraction is larger for 4MISo, where the repulsive interaction is stronger. When the Mössbauer spectra for these solids were recorded at 5 K, the presence of paramagnetic species was not detected in the resulting spectra. The observed HS → LS transition is complete. The results of this study shed light on the role of the pyridine molecule substituent and its nature on the SCO properties of 2D ferrous nitroprussides, an emerging series of spin-crossover materials. Calculations of the involved changes in electron density in the iron atom coordination environment, based on the Hirshfeld surface, give a good visualization of the nature of SCO in pillared transition-metal nitroprussides.

Author contributions

All the authors contributed equally to this work.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

All the available data are contained in the main manuscript and its SI. Supplementary information: Spectroscopic, thermal, and structural information for the Fe(4X-isonicotinate)₂[Fe(CN)₅NO] series with X = methyl or ethyl. See DOI: https://doi.org/10.1039/d5nj02938d.

If additional information is required, the authors can be contacted through the provided email addresses.

CCDC 2346684 (4MIso, 300 K), 2346685 (4MIso, 100 K), 2346683 (4EIso, 300 K) and 2363385 (4EIso, 80 K) contain the supplementary crystallographic data for this paper.^89a–d

Acknowledgements

The authors thank the LNCAE (Laboratorio Nacional de Conversión y Almacenamiento de Energía) for access to its experimental facilities. This study was partially supported by the SECIHTI project ApoyosLN-2023-95. We acknowledge the Spanish Ministerio de Ciencia, Innovación y Universidades and Consejo Superior de Investigaciones Científicas for financial support through the projects 2010 6 0E 013 and 2021 60 E 030 and for provision of synchrotron radiation facilities at BM25-SpLine at The European Synchrotron. Y. Avila and R. Mojica acknowledge support from SECIHTI through the postdoctoral fellowship program.

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