Fe[4-(3-Phenylpropyl)Pyridine]2[Fe(CN)5NO]: A 2D Coordination Polymer with Thermally-Induced Spin Transition and Nature of Its Asymmetric Hysteresis Loop

The titled material crystallizes with an orthorhombic unit cell, in the P21212 space group (Nr. 18). Its crystal structure was solved and refined from powder XRD patterns. The solid framework is formed by stacked undulated sheets of inorganic nature, Fe[Fe(CN)5NO], separated by bimolecular organic pillars, [4-(3-Phenylpropyl)pyridine]2, which remain coordinated to the axial coordination sites for the iron atom. The molecules forming these pillars remain coupled through C–H⋯π and dispersive interactions between neighboring molecules. When this solid is cooled and then warmed, a reversible spin transition, from high to low spin (HS → LS), and vice versa, is observed. This transition occurs in the temperature interval of 135–165 K, with a hysteresis between them of about 30 K. That hysteresis loop appears with a pronounced asymmetry when the slopes for the HS → LS and LS → HS transitions are compared. This effect is discussed in terms of the related structural changes in the solid structure during the spin transitions. The transition was also monitored from IR and Raman spectra recorded at 80 and 300 K. Relevant information on the electronic structure for both, the LS and HS phases of this material, was derived from the corresponding XPS spectra recorded at 114 and 270 K. This contribution emphasizes the role of the nitrosyl group (NO) as an electron buffer for tuning the bonding properties of the inorganic layer at the CN 5σ orbital to make possible the observed thermally-induced SCO behavior. The thermally-induced spin transition in this solid shows an asymmetric hysteresis loop, which was ascribed to the nature of the pillar molecule.


Introduction
A reversible induced spin transition, known as spin crossover (SCO), is commonly observed in octahedral complexes with 3d 4 -3d 7 transition metals. Within them, iron(2 +) has received the greatest attention because the spin transition involves two electrons: e g 2 t 2g 4 (S = 2; high spin: HS) ↔ e g 0 t 2g 6 (S = 0; low spin: LS). In consequence, the related changes in the physical properties are quite pronounced. In the high spin (HS) state the sample color has two contributions, the light absorption by the metal d-d transitions and the one originated by the metal-ligand charge transfer (MLCT). For a complete HS → LS transition, the first contribution disappears, and the sample suffers a pronounced color change, which could be purple if the MLCT absorbs light below 500 nm; this is the case of the nitroprusside ion, for instance [1]. These changes in the sample color are easily detected by simple visual inspection of the sample or recording the corresponding UV-vis-NIR spectra. Another contrasting change in the physical properties of iron(2 +) complexes is observed for their magnetic susceptibility related to the paramagnetic character of the sample in the HS state, which becomes diamagnetic on the HS → LS transition. The e g 2 t 2g 4 → e g 0 t 2g 6 charge transfer results in a stronger metal-ligand bonding interaction because now the anti-bonding e g orbitals are empty. This leads to a shorter metal-ligand distance, a volume reduction for the solid, and higher thermodynamic stability. These last two effects are detected in the corresponding XRD pattern, and from a DSC experiment as the appearance of an exothermic peak, respectively. Such a stronger metal-ligand interaction on the HS → LS transition is accompanied by an electron density redistribution in the solid and modification of its vibrational pattern, which is detected from IR and Raman spectra. These are the most evident changes in a given material with SCO behavior and support many of the techniques used to probe the spin transition (UV-vis-NIR, SQUID measurements, XRD, DSC, IR, Raman) complemented with the recording of 57 Fe Mossbauer spectra [2][3][4][5][6][7][8]. The obtained experimental evidence can be complemented with computational calculation using a periodic DFT algorithm to obtain information on the LS phase, relaxing the structure of the parent HS analog [9,10]. This supposes that the spin transition does not involve drastic structural changes, e.g. a change in the unit cell symmetry [10]. The mentioned modification in the sample properties on the spin transition also supports the functional properties and potential applications of those materials with SCO behavior [11][12][13][14][15][16][17][18].
The spin transition can be induced by a temperature change, HS → LS on the sample cooling and the inverse LS → HS, on its warming [19]; under an applied pressure (HS → LS) [20]; light incidence (LS → HS) [21,22]; and by application of electric [23] and magnetic [24] fields. The introduction of guest species inside the solid framework can induce or inhibit the possible spin transition using some of the mentioned external stimuli [25].
Probably the simplest route to obtain a material with SCO behavior is the use of a 2D coordination polymer.  [26]. These coordinated water molecules stabilize a network of hydrogen-bonded water molecules in the interlayer region, which supports a 3D framework for that family of solids. The coordinated water molecules can easily be displaced by an organic ligand with a high affinity for the iron atom [27]. If that axial ligand contributes to generating a splitting between the energy of the e g and t 2g orbitals of the order of kT, a thermally-induced spin transition can be induced. The SCO in that family of coordination polymers has been reported for pyrazine and pyridine, and some of its derivatives as axial ligands (pillars) for the iron atom [20,[27][28][29]. 2D ferrous nitroprussides, Fe(L) x [Fe(CN) 5 NO] where x = 1, 2, with the organic ligand (L) occupying the axial coordination sites for the iron atom can be prepared in situ disrupting the axial Fe-NC coordination bond, sonicating an aqueous suspension of its 3D analog in the presence of the organic molecule (L) [30]. These pillared coordination polymers can also be formed by the precipitation reaction mixing aqueous solutions of an iron(2 +) salt, and sodium nitroprusside in the presence of the organic ligand in an appropriate molar ratio. For pyrazine, pyridine and several of its derivatives, materials with SCO behavior have been obtained [31][32][33][34][35].
In pillared 2D ferrous nitroprussides, the nitrosyl group (NO) behaves as an electron buffer modulating the electron density found at the CN 5σ orbital, through the 2b 2 (xy) → 7e(π*NO) (Fe → NO) electron transfer. This charge transfer mechanism has the capability of tuning the electron density in that orbital (5σ) according to the bonding properties of the organic axial ligand. Both, the electron density at the CN 5σ orbital and σ-donating ability of the pillar molecule determine the crystal field splitting (Δ or 10Dq) between the e g and t 2g orbitals of the iron atom, and the possibility to have a reversible spin transition. Such a tuning process is probed through a frequency shift for the ν(NO) IR band, which depends on the nature of the axial ligand before and on the spin transition [31][32][33][34][35]. This opens the possibility to observe the SCO effect in ferrous nitroprusside with a large number of pillar molecules. Such a feature of the NO group as an electron buffer results in certain flexibility for the coordination environment of the iron atom, which appears as an appropriate mechanism to accommodate the structural changes related to the spin transition. The LS → HS is favored in terms of a higher entropy contribution (ΔS) to the free energy (ΔG) due to both, the electronic configuration for the iron atom, e g 2 t 2g 4 , and the possibility of activating new vibrational states in the solid. To this last contribution to the value of ΔS, the nature, size, and intermolecular interactions for the organic ligand found in the interlayer region could be contributing to the driving forces of that spin transition (discussed below).
In this contribution, we are reporting the preparation, crystal structure, and thermally-induced SCO for the titled material. The involved organic ligand, 4-(3-Phenylpropyl) pyridine, in the following labeled as PhPPy, is a relatively large molecule, with a distorted configuration in the interlayer region, which could be an unfavorable condition to make possible the observed spin transition. It seems the mentioned role of the NO group, as an electron buffer, could be contributing to tuning the crystal field splitting in the iron atom to make possible the observed thermally-induced spin transition in this solid.

Experimental
The material herein considered was prepared from an aqueous suspension of polycrystalline ferrous nitroprusside, Fe[Fe(CN) 5 NO]⋅xH 2 O (3D cubic phase) in the presence of the organic ligand with an excess of this last one, over the ideal 1:2 molar ratio. That suspension is maintained in an ultrasonic bath for about 3 h and then left to rest in the darkness for three days when a fine precipitate is formed. That precipitate was separated by centrifugation, washed several times with distilled water, and finally dried in the darkness until it had constant weight. That hybrid inorganic-organic solid is obtained as a polycrystalline material (powder), which was characterized by IR, Raman, XPS, and TG data, and powder XRD patterns. The 3D ferrous nitroprusside sample was previously prepared by the precipitation method from aqueous solutions of sodium nitroprusside and Mohr salt.
IR spectra were recorded at room temperature (300 K) and 80 K (with an N 2 liquid cell), using an ATR device. Raman spectra were collected at 300 K and 80 K, in the 3500-50 cm −1 spectral range, using a 785 nm laser wavelength, at 1 mW power, with 5 s of recording time and 20 scans, with a DRX Raman microscope (from ThermoScientific Co.). XPS spectra were recorded at 114 and 270 K using a monochromatic AlK α X-ray source, with 20 eV as pass energy and a step size of 0.1 eV. Under such conditions, the Au(0) 4f 7/2 line was found at 84.0 eV of binding energy (BE), with 0.73 eV of FWHM. The spectra were fitted with a Lorentzian-Gaussian (Voight) type line shape combined with the background removal using the Shirley method, using the Avantage software, V5.9925 Version (from Thermo-Scientific Co). The C 1 s core-level peak from adventitious carbon, set at 284.6 eV, was used as a charge reference.
Powder XRD patterns used for the structural study were recorded with CuK α radiation and a D8-Advance diffractometer (from Bruker) at 300 K, in the Bragg-Brentano geometry. These patterns were indexed with the DicVol algorithm [36]. The global optimization process in the direct space (simulated annealing) implemented in the EXPO09 program from an initially proposed asymmetric unit cell [37] was used to obtain the structural model to be refined. The space group assignment was corroborated using the Le Bail method [38]. The structural refinement was carried out by the Rietveld method implemented in the FullProf program [39]. Peak profiles (of pseudo-Voigt type) were calculated up to ten times the full width at half maximum (FWHM). A third-order polynomial was used to model the pattern background. Additional details on the XRD data collection and processing are available in Supplementary Information (Table S1).
The spin-crossover transition was evaluated from magnetic data, recording zero-field-cooling (ZFC) and fieldcooling (FC) curves in the 2-300 K temperature range, with 100 Oe of an applied magnetic field, using an MPMS-3 magnetometer (from Quantum Design). The effective magnetic moment (μ eff ) was calculated according to μ eff = 2.828 sqrt(χT) [40], from the experimental magnetic susceptibility (χ) values corrected for the diamagnetic contribution according to the reported Pascal constant for the involved elements [40]. The enthalpy (ΔH) and entropy (ΔS) change related to the spin transition on the sample cooling and warming were evaluated from DSC curves recorded using Q 2000 equipment from TA Instruments operated under He flow (50 mL/min). TG curves were recorded in the 30-800 °C temperature range under a nitrogen atmosphere (100 mL/min) using a TA Instruments high-resolution thermobalance (TGA Q 5000), at a heating rate of 5 °C/min. Information on the crystal structure for the LS phase was obtained by relaxing the room temperature refined crystal structure, through a DFT procedure using the VASP package [41][42][43][44], considering that during the HS → LS transition the unit cell symmetry and the molecule orientation are preserved [9,10]. These are reasonable hypotheses because the structural changes in 2D ferrous nitroprusside with bimolecular pillars involve non-drastic structural changes, with a volume contraction below 1% [31][32][33], which is ascribed to their high flexibility. We have used such a structural relaxing procedure to calculate the structure for the LS phase in ferrous nitroprusside with 4,4´-Azopyridine as a pillar molecule with reliable results [35].

Confirming the Nature of the Formed Material
The material herein considered is prepared under the hypothesis that during the 3D phase of ferrous nitroprusside sonication in the presence of the organic ligand, this last one promotes the rupture of the Fe-NC coordination bond to occupy the axial coordination sites for the iron atom. In the structure of 3D ferrous nitroprusside, the iron atom has a mixed coordination sphere formed by water molecules and the N end of the axial CN. If such a rupture and ligand replacement are possible, an anhydrous solid where the axial CN remains unlinked must be obtained. Figure 1 shows the IR spectra for 3D ferrous nitroprussides, the organic ligand, and the formed material. The IR spectrum of this last one corresponds to an anhydrous solid, with an absence of ν(OH) and δ(HOH) stretching and bending IR absorption bands. This spectrum results from the superposition of the spectra from the organic molecule and of the inorganic block, Fe[Fe(CN) 5 NO], with certain frequency shifts in their absorption bands. These frequency shifts are revealing the bonding interaction between the two building blocks. The organic ligand coordination to the iron atom supposes a charge donation to this last one through the e g orbitals inducing a charge density redistribution in its coordination environment. This includes a reduction in the electron density located at the CN 5σ orbital. The charge density found at this orbital is contributing to the equatorial CNs coordination with the iron atom. At the same time, the rupture of the Fe-NC axial bond leads to higher electron density on the iron atom in the nitroprusside ion, resulting in an enhanced π-back donation interaction with both, the equatorial CNs and the axial NO group. Since the CN 5σ orbital has a certain anti-bonding character for the C≡N bond, these two effects, the rupture of the Fe-NC axial bond and organic molecule coordination to the iron atom leads to a negative frequency shift for both, the ν(CN) and ν(NO) bands, of -5 and -25 cm −1 , respectively. A higher electron density returned to the iron atom in the nitroprusside ion reinforces the Fe → NO π-back bonding interaction. Of course, the ν(Fe-C), ν(Fe-N), δ(Fe-C≡N), and δ(Fe-N = O) vibrations, in the low-frequency spectral region, are sensing the mentioned charge density redistribution. The charge donation from the organic ligand to the metal modifies the molecule vibrational pattern, mainly within the ring containing the N-pyridyl atom, which is appreciated in the recorded IR spectra (Fig. 1). These changes in the IR spectra for both, the inorganic and organic building blocks are conclusive clues on the organic ligand coordination to the iron atom.
The expected hybrid solid stoichiometry, Fe[4-(3-Phenylpropyl)pyridine] 2 [Fe(CN) 5 NO], was confirmed by recording TG curves under a nitrogen flow and using a high-resolution equipment configuration where the sample heating rate is controlled by its decomposition rate. Fortunately, in this type of hybrid inorganic-organic solid, the organic fraction decomposes and evolves at a relatively lower temperature than the thermal effects corresponding to the inorganic block. This makes it possible to obtain the mass fraction corresponding to the two blocks from the TG experiment. Figure 2 shows the recorded TG curves for the material herein under study. The sequence of weight losses was properly assigned to finally confirm the expected solid formula unit.

NO]
As above-mentioned, this material is obtained as a fine powder, with an average crystallite size of about 58(5) nm according to the XRD pattern evaluation using the Halder-Wagner model [45]. This hybrid solid crystallizes with an orthorhombic unit cell in the P2 1 Table 1). The unit cell accommodates four formula units (Z = 4), for a unit cell volume per formula unit of 757.7 Å 3 , 1.45 times the value found for 4-methylpyridine-contained nitroprusside, for instance [33]. The structural model to be refined was derived by simulating annealing considering that neighboring organic ligands from adjacent layers must be interacting through physical interactions to support the ordered packing of the neighboring layers and resulting in a 3D framework. This model also supposes that all the ligand molecules are found coordinated to the axial positions of the iron atom. Figure 3 shows the experimental and fitted powder XRD patterns and their difference, using the derived structural model. Figure 4 illustrates that the refined crystal structure is formed by pillared layers of the inorganic 2D ferrous nitroprusside. The refined atomic positions, isotropic thermal, and occupation factors are available in Table S2. The calculated bond distances and angles are summarized in Table S3. The refined structural information for this solid was deposited in the CCDC database with the CIF file number 2156406. According to the refined crystal structure (Fig. 4), in the interlayer region, neighboring molecules remain coupled through C-H⋯π and dispersive interactions. The C-rings distance between neighboring molecules is 3.37 Å, which corresponds to a relatively strong C-H⋯π interaction. Figure 4 shows the orientation relative of neighboring molecules in the interlayer region. According to the nature of the phenylpropyl fragment and the C-C distance between such a fragment of adjacent molecules, in the range of 2.55 to 3.94 Å, the bimolecular pillars are stabilized by dispersive interactions. The iron atom, active for SCO, is found with a distorted coordination environment (Table 2), related to the intermolecular interactions in the interlayer region. The intermolecular interactions between neighboring molecules are responsible for the observed deviation of the Fe-N Pyr bond from the ideal position corresponding to octahedral coordination. As above-mentioned, and evidenced by the changes observed in the IR spectra related to the hybrid solid formation from 3D ferrous nitroprussides, the NO group can receive/donate electron density from/to the CN 5σ orbital. It behaves as an electron buffer for changes in the electronic structure of the iron atom linked at the N end of the equatorial CN ligands. These changes include the deformation of its coordination environment and those related to the temperature-depending spin transition (discussed below). Figure 5 shows the temperature dependence for the effective magnetic moment (μ eff ) on the sample cooling and then on its warming for the solid herein considered. Below 150 K, a progressive decrease for the value of μ eff is observed, which is interpreted as electron migration from the anti-bonding e g orbitals to the non-bonding t 2g orbitals. The curve that describes this transition from high spin (HS) to low spin (LS) saturates below 135 K at a value of the effective magnetic moment of about 2.5 Bohr magneton, indicating that the transition is incomplete. Below that temperature, a sample fraction remains in the paramagnetic state. This corresponds to sample regions where the energy splitting between e g and t 2g energy levels for the iron atom is not enough large to stabilize the LS electronic configuration (e g 0 t 2g 6 ) even at low temperatures when the thermal energy (kT) has suffered a significant decrease. In terms of the crystal field theory, such splitting is described through the parameter Δ o or 10D q and at a large value of this parameter, the LS electronic configuration for the ion is the most stable one. Then, on the sample warming, the inverse transition, LS → HS is detected, through a non-reversible process in terms of the temperature where the two transitions are observed. Such an irreversibility results in the appearance of thermal hysteresis. The solid has a certain memory effect. The electron migration from the e g orbitals to be accommodated in the t 2g orbitals makes possible a stronger metal-ligand interaction and shortening for the corresponding interatomic distance, which induces a structural transition, evident as a solid volume contraction. This structural change is the cause of the   (Fig. 5). Unlike the sample behavior on cooling, which remains in the HS state in the 150 -165 K temperature region, followed by a rapid e g 2 t 2g 4 → e g 0 t 2g 6 electron migration, the inverse transition is activated from the temperature where the first one ends, resulting in an asymmetric hysteresis loop. This is a relevant feature for the thermally-induced SCO in this material, not frequently observed in materials with such a physical effect. The LS → HS is a thermally activated process related to both, a higher entropic contribution (ΔS) due to the e g 2 t 2g 4 electronic configuration, ΔS Spin = Rln[(2S + 1) HS /(2S + 1)) LS ], and the activation of new vibrational modes in the solid framework. The phenyl propyl fragment of the ligand, which has a certain flexibility and occupies the interlayer region, is probably contributing to a rapid increase in the solid entropy on the sample warming. For smaller ligands and with stronger interaction between neighboring molecules, e.g. pyridine [31], 3F-pyridine [32], and 4-methylpyridine [33], symmetric hysteresis loops are observed. For pyrazine [34] and 4,4´-Azopyridine [35], which form bridges between iron atoms from adjacent layers (3D frameworks), symmetric hysteresis loops are also observed. Such asymmetry in the hysteresis loop shows a detectable change in the magnetic data recorded using different scan rate values (Fig. 5, Inset). The asymmetry is more pronounced when the magnetic data are collected far from the thermodynamic equilibrium conditions. In that set of curves, the larger hysteresis corresponds to the data recorded at 0.25 K/min, which suggests that the accommodation of the large ligand tail, could be generating a certain delay in the related structural change. The presence of structural interactions that behave as impediments to the structural changes related to the spin transition is usually detected as an increase in the width of the hysteresis loop for data recorded at low scan rate values [34]. The inverse behavior is expected in the absence of such interactions; a lower scan rate value results in a tighter hysteresis loop.

Thermally-Induced Spin Transition
The spin transition was also monitored by recording the DSC curves on the sample cooling and warming (Fig. 6). The change for the free energy (ΔG) during the spin transition receives contributions from both, the enthalpy (ΔH) and entropy (ΔS) changes, according to ΔG = ΔH -TΔS. As already mentioned, the HS phase is favored by ΔS, while ΔH dominates the HS → LS transition due to the stronger metal-ligand interaction and the higher thermodynamic stability of the LS phase at lower temperatures. The involved values for ΔH and ΔS during the HS → LS and LS → HS transitions were determined from DSC curves. The higher thermodynamic stability for the LS phase is appreciated as the appearance of an exothermic peak on the sample cooling, which peaks at 139 K, and then, when the sample is warmed it absorbs thermal energy (kT), where a characteristic endothermic peak appears, with its maximum at 160 K (Fig. 6), for a hysteresis loop of 22 K, quite similar to the one observed from the magnetic data (20 K). The value of ΔH was calculated by integrating the heat flow versus the temperature curve, while the value of ΔS was obtained from the value of ΔH and the temperature maximum heat flow. Table 3 collects the obtained values for ΔH and ΔS. Within the expected experimental error, the values of ΔH↓ and ΔH↑ are similar, as expected for an overall reversible process. The same is valid for the values ΔS↓ and ΔS↑, but the observed rapid increase for the value of μ eff on the sample warming suggests that such behavior is determined by the contribution of ΔS↑, favored by the large size and flexibility for the ligand phenyl propyl fragment. The spin transition has been associated with a contraction (HS → LS) and an extension (LS → HS) for the metal-ligand distances and a modification for the solid electronic structure, and from these facts, such transitions modify the sample IR and Raman spectra. Figure 5 (insets) shows the observed changes for the ν(CN) and ν(NO) stretching bands in the IR spectra recorded at 300 and 80 K for the solid herein considered. The HS → LS transition is accompanied by a positive frequency shift of 15 cm −1 for the ν(CN) band corresponding to a reduction in CN π-back bonding interaction with the iron atom in the nitroprusside ion, diminishing the charge density in the CN 5σ orbital, which has a certain antibonding character for the C≡N bond. Such a reduction in the charge density in that orbital is induced by a stronger metal interaction with the organic ligand on the HS → LS transition. The frequency shift for the ν(NO) band is practically indetectable probably because the spin transition is incomplete (Figures S1), as reveals the presence of two ν(CN) bands in the IR spectrum for the low-temperature phase (Fig. 5, Insets). The frequency shift for the vibrational modes involving the ν(CN) stretching vibration in the Raman spectra also shows a positive frequency shift (Fig. 5, Inset; Figures S2, and S3). That Raman spectral region shows three well-resolved vibrational modes, ν(CN) eq,A´, ν(CN) eq,A´´ and ν(CN) ax,A´, where the intermediate band splits into three bands. This splitting is probing the deformation of the iron atom coordination environment, already observed from the refined crystal structure.
The HS → LS spin transition on the sample cooling is perceptible through a change in its color, from grey at room temperature (300 K), to violet at 77 K. For a complete spin transition, purple color is expected for the material at low temperature (77 K) corresponding to the metal-ligandcharge transfer in the nitroprusside ion, with a strong absorption band at 498 nm due to the 2b 2 (xy) → 7e(π*NO) (Fe → NO) electronic transition [1]. Figure 7 shows the calculated crystal structure for the LS phase, relaxing the refined crystal structure for the HS phase, under the hypothesis that on the spin transition the unit cell symmetry and the molecule orientation in the interlayer are preserved. The calculated unit cell parameters are available in Table 1. The unit cell volume contraction on the HS → LS transition remains below 1% (Table 2), which is congruent with the experimental behavior observed for analog pillared ferrous nitroprussides [31][32][33]. This is an expected result related to the high flexibility of a 3D framework formed by layers stacked through bimolecular pillars with weak intermolecular interactions between them. The obtained atomic positions and occupation factors are available in Table S4. The calculated bond distances and angles are summarized in Table S5. The calculated values for the Fe-N PhPpy and Fe-NC CN correspond to a shortening of these interatomic distances related to a stronger metal-ligand bonding interaction in the LS state for the iron atom ( Table 2). The observed changes in these interatomic distances are similar to those reported from the refined crystal structures for the LS phase with pyridine and 3F-pyrazine as pillar molecules [31,32].

The Calculated Crystal Structure for the LS Phase
The calculated values for the ∠ N PhPpy -Fe-N PhPpy , ∠ N CN -Fe-N CN , and ∠ N PhPpy -Fe-N CN bond angles increase ( Table 2), revealing that the e g 2 t 2g 4 → e g 0 t 2g 6 charge transfer leads to a more symmetric re-accommodation of the ligands around the iron atom. This is an observed regularity for the SCO in this series of coordination polymers. The formation of the thermodynamically more stable LS phase is accompanied by an increase in the symmetry of the iron atom coordination environment [31,32,35].

Changes in the Electronic Structure on the Spin Transition According to the XPS Spectra
Previous studies of transition metal nitroprussides using XPS spectra have revealed that the core-levels binding energy (BE) for the involved atoms contains valuable information on the electronic structure of these coordination polymers [46,47]. In consequence, this technique must be able to probe the changes in the iron atom and its coordination environment related to the spin transition. The Fe2p binding energy is found 2 eV above the value corresponding to the ferrocyanide ion, where the iron atom is found with an LS electronic configuration (Fig. 8, Inset A). Such a difference of about 2 eV is a consequence of the large π-back bonding ability of the NO ligand, removing a great electron density from the iron atom via the 2b 2 (xy) → 7e(π*NO) (Fe → NO) charge transfer [1]. Such a separation between the Fe2p core-level peak in the nitroprusside ion relative to the one corresponding to an iron(II) species in a low spin state supports the hypothesis that the spin transition can be monitored by recording the XPS at different temperatures. The iron atom that is participating in the spin transition remains coordinated to both, the N end of the equatorial CNs and the pyridinic N atom of the organic ligand. This suggests that the N1s binding energy could be sensing the changes in electron density at these first neighbors of the iron atom during the e g 2 t 2g 4 → e g 0 t 2g 6 charge transfer and the corresponding strengthening for the bonding interactions. At least three N1s peaks at different values of binding energy Fig. 7 The calculated crystal structure for the phase of low temperature of Fe[4-(3-Phenylpropyl)pyridine] 2 [Fe(CN) 5 NO] must be observed in the recorded spectra, two from the inorganic building block (Fig. 8, Inset B) and a third peak from the organic ligand. In the 2D solids herein considered, the axial CN remains unlinked at the its N end and an additional weak N 1 s peak must be considered. The recorded spectral regions corresponding to the Fe2p and N1s binding energies were fitted according to these fitting models for the spectra recorded at 114 and 270 K, considering that the spin transition is incomplete for the material under study. Figure 8 shows the obtained fitting and in Table S6, the corresponding values for the binding energy are summarized.
The Fe2p core-level spectral region (Fig. 8, left), for the spectrum recorded at 270 K, was fitted as two overlapped peaks of similar intensity corresponding to the iron atom in the nitroprusside ion, [Fe(CN) 5 NO], and to the iron atom coordinated at the N end of equatorial CNs and the organic ligand, Fe(N CN ) 4 (N PhPpy ) 2 . These two iron species are found in LS and HS states, respectively, with a difference in 2p 3/2 binding energy of about 1.1 eV. For the spectrum recorded at 114 K, a pronounced shoulder appears at the right side of the main peak revealing that a fraction of the Fe(N CN ) 4 (N PhPpy ) 2 species is now found with an LS electronic configuration. The 2p core-level binding energy for that Fe II LS species is slightly lower than the corresponding value for Fe 2+ HS, in about − 0.4 eV, sensing an enhanced charge donation from the organic ligand to the iron atom concomitant with the e g 2 t 2g 4 → e g 0 t 2g 6 charge transfer. The presence of Fe 2+ HS species in these spectra (Fig. 8, left) explains the appearance of a broad satellite peak at apparently higher values of binding energy (714.4 eV).
The N 1 s core-level spectral region for the pillared 2D ferrous nitroprusside herein considered, in the spectrum corresponding to the HS phase, is composed of four peaks corresponding to the axial CN and equatorial CN ligands, the organic ligand, and the NO group, which are found at 396.6, 397.9, 399.2, and 402.9 eV, respectively (Fig. 8, right). The revealing a difference of about 2 eV between them, related to the Fe → NO charge transfer in the first one. Inset B: N1s spectrum corresponding to the nitroprusside ion HS → LS spin transition induces a reduction for the electron density found at the CN 5σ and the N atom of the equatorial CNs becomes more positive. This is appreciated as a higher value of binding energy for the N 1 s peak, at 398.5 eV, for a shift of about + 0.6 eV relative to the value found when the iron atom is found with an HS state. Such a spin transition also favors a stronger bonding interaction of the organic molecule with the iron atom, involving certain charge donation to this last one. This explains that the N atom in the organic ligand has a more positive character when the iron atom is found with an LS electronic configuration. The energy shift for that N atom is about + 1.8 eV. The original single N 1 s peak for the NO group in the spectrum recorded at 270 K, appears splitted into two peaks when the sample was studied at 114 K, corresponding to the presence of both, the HS and LS phases in the sample at that temperature. The reduction of the CN π-back bonding during the HS → LS transition increases the electron density on the iron atom of the nitroprusside ion and this favors a stronger 2b 2 (xy) → 7e(π*NO) (Fe → NO) charge donation, which is sensing a more negative character for the N atom, by about − 0.7 eV.

Conclusions
The pillared 2D coordination polymer herein considered was prepared by sonicating an aqueous suspension of 3D ferrous nitroprusside in the presence of the organic ligand. The formed solid crystallizes with an orthorhombic unit cell, in the P2 1 2 1 2 space group (Nr. 18). The organic ligand is found occupying the axial coordination sites for the iron atom. The pillars maintain the stacking of adjacent layers through two molecules coupled by C-H⋯π and C⋯C dispersive interactions between neighboring molecules in the interlayer region. This hybrid inorganic-organic solid shows thermally-induced spin transition upon cooling (HS → LS) and then on heating (LS → HS), in the temperature range 135 to 165 K, with an asymmetric hysteresis loop of 30 K for the recorded magnetic data. Such an asymmetrical hysteresis has its origin in the contribution of the organic ligand to the entropy change during the LS → HS transition. The spin transition was also monitored from DSC, IR, and Raman data. XPS spectra recorded at 114 and 270 K shed light on the changes in the electronic structure of the iron atom and its coordination environment on the spin transition. Computational calculations based on the periodic DFT algorithm implemented in the VASP software package provided information on the structure of the LS phase. No similar study has been reported for this material. The results herein discussed reveal new fine changes and effects involved in the thermalinduced spin transition in ferrous nitroprussides, many of them related to the bonding properties of the nitrosyl group.