Solvent-Induced Hysteresis Loop in Anionic Spin Crossover (SCO) Isomorph Complexes

: Reaction of Fe(II) with the tris-(pyridin-2-yl)ethoxymethane (py 3 C-OEt) tripodal ligand in the presence of the pseudohalide ancillary NCSe − (E = S, Se, BH 3 ) ligand leads to the mononuclear complex [Fe(py 3 C-OEt) 2 ][Fe(py 3 C-OEt)(NCSe) 3 ] 2 · 2CH 3 CN ( 3 ), which has been char-acterised as an isomorph of the two previously reported complexes, Fe(py 3 C-OEt) 2 ][Fe(py 3 C-OEt)(NCE) 3 ] 2 · 2CH 3 CN, with E = S ( 1 ), BH 3 ( 2 ). X-ray powder diffraction of the three complexes ( 1 – 3 ), associated with the previously reported single crystal structures of 1 – 2 , revealed a monomeric isomorph structure for 3 , formed by the spin crossover ( SCO ) anionic [Fe(py 3 C-OEt)(NCSe) 3 ] − complex, associated with the low spin ( LS ) [Fe(py 3 C-OEt) 2 ] 2+ cationic complex and two solvent acetonitrile molecules. In the [Fe(py 3 C-OEt) 2 ] 2+ complex, the metal ion environment involves two py 3 C-OEt tridentate ligands, while the [Fe(py 3 C-OEt)(NCSe) 3 ] − anion displays a hexacoordinated environment involving three N-donor atoms of one py 3 C-OEt ligand and three nitrogen atoms arising from the three (NCSe) − coligands. The magnetic studies for 3 performed in the temperature range 300-5-400 K, indicated the presence of a two-step SCO transition centred around 170 and 298 K, while when the sample was heated at 400 K until its complete desolvation, the magnetic behaviour of the high temperature transition ( T 1/2 = 298 K) shifted to a lower temperature until the two-step behaviour merged with a gradual one-step transition at ca. 216 K.


Structural Characterization and Magnetic Properties
In contrast to complexes 1 and 2, for which the crystal structures were determined using single crystal X-ray diffraction, complex 3, which was expected to be isomorph to the structure observed for 1 and 2, showed poor quality single crystal diffraction patterns that clearly precluded any correct single crystal structural characterization. However, after several attempts at 100 K, we succeeded in collecting some intensities, which led to unit cell parameters depicted in Table S1. Comparison of these parameters to those of the two isomorph complexes 1 and 2, indicated that the structure of complex 3 was isomorphic to complexes 1 and 2 (Table S1). This conclusion was supported by the experimental X-ray powder diffraction pattern observed for the polycrystalline powder of complex 3, which was very similar to the one observed for complex 1, as well as to the simulated pattern derived from the single crystal structure of complex 1 (Figure 1 and Figure S1).

Structural Characterization and Magnetic Properties
In contrast to complexes 1 and 2, for which the crystal structures were determined using single crystal X-ray diffraction, complex 3, which was expected to be isomorph to the structure observed for 1 and 2, showed poor quality single crystal diffraction patterns that clearly precluded any correct single crystal structural characterization. However, after several attempts at 100 K, we succeeded in collecting some intensities, which led to unit cell parameters depicted in Table S1. Comparison of these parameters to those of the two isomorph complexes 1 and 2, indicated that the structure of complex 3 was isomorphic to complexes 1 and 2 (Table S1). This conclusion was supported by the experimental X-ray powder diffraction pattern observed for the polycrystalline powder of complex 3, which was very similar to the one observed for complex 1, as well as to the simulated pattern derived from the single crystal structure of complex 1 (Figure 1 and Figure S1). It is also worth mentioning that the elemental analyses of complex 3 agreed with the chemical formula, [Fe(py3C-OEt)2][Fe(py3C-OEt)(NCSe)3]2·2CH3CN, expected for a complex isomorph to 1 and 2. Therefore, all the data strongly support that the crystal structure of complex 3 is isomorphic to those of complexes 1 and 2 [28]; and therefore its crystal structure consists of a low spin (LS) [Fe(py3C-OEt)2] 2+ cationic complex (Figure 2b  It is also worth mentioning that the elemental analyses of complex 3 agreed with the chemical formula, [Fe(py 3 C-OEt) 2 ][Fe(py 3 C-OEt)(NCSe) 3 ] 2 ·2CH 3 CN, expected for a complex isomorph to 1 and 2. Therefore, all the data strongly support that the crystal structure of complex 3 is isomorphic to those of complexes 1 and 2 [28]; and therefore its crystal structure consists of a low spin (LS) [Fe(py 3 C-OEt) 2 ] 2+ cationic complex (Figure 2b  As previously described, complex 1 exhibited an incomplete SCO transition (T1/2 = 205 K), while 2 displayed a complete two-step transition at 245 K and 380 K ( Figure 3). Their magnetic properties were studied in warming and cooling modes (300-2-400-2 K for 1; 300-2-500-2 K for 2), but no significant hysteretic effects or any change due to possible desolvation were detected. Also, irradiation at 10 K with a green light for several hours revealed no noticeable increase of the thermal variation of the product of the molar magnetic susceptibility times the temperature ( mT) for both compounds. As for complex 1, susceptibility measurements for complex 3 were performed in the 300-5 K and 5-400 K temperature ranges. The thermal variation of the mT product for the three complexes (1-3) are shown in Figure 3. For compound 3, the mT value per formula at 400 K (≈6.71 cm 3 K mol −1 ) was in agreement with the value expected for two isolated Fe(II) ions (S = 2 and g ≈ 2.1), revealing the presence of two magnetically isolated HS Fe(II) ions [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. On cooling, the mT value of 3 showed an initial abrupt drop, at around 298 K, reaching a value of ca. 3.40 cm 3 K mol −1 at 265 K. On further cooling, we observed a second drop, at around 170 K, to reach a plateau at ca. 0.56 cm 3 K mol −1 below 116 K. This low temperature mT value implies the presence of a residual HS fraction of ca. 8 %. This behaviour indicates the presence of an incomplete HS to LS two-step transition centred at around 170 and 298 K.
In contrast to isomorphs 1 and 2, where the magnetic properties did not show any change after successive cooling and heating scans in the ranges 2-400 K for 1 and 2-500 K for 2, the two-step behaviour described above for complex 3 ( Figure 3) was irreversible due to a gradual desolvation of the sample at 400 K, as previously observed in several solvated systems [32][33][34][35][36][37][38][39][40]. As a matter of fact, when the sample was maintained at 400 K until its complete desolvation, the second cycle (400-50-400 K) in 3 produced a shift to lower temperatures for the high temperature step, while the transition temperature of the low temperature step remained unchanged (See Figure 4). Similar trends were observed for the third and fourth cycles, until the fifth cycle where the initial high temperature step merged with the low temperature step (Figure 4), to lead to the gradual one-step transition depicted in Figure 5a. There were no hysteretic effects and the HS fraction of ca. 8% remained unchanged after the different heating and cooling cycles.  [28]. Codes of equivalent positions: (a) = -x, -y, -z.
As previously described, complex 1 exhibited an incomplete SCO transition (T 1/2 = 205 K), while 2 displayed a complete two-step transition at 245 K and 380 K ( Figure 3). Their magnetic properties were studied in warming and cooling modes (300-2-400-2 K for 1; 300-2-500-2 K for 2), but no significant hysteretic effects or any change due to possible desolvation were detected. Also, irradiation at 10 K with a green light for several hours revealed no noticeable increase of the thermal variation of the product of the molar magnetic susceptibility times the temperature (χ m T) for both compounds. As for complex 1, susceptibility measurements for complex 3 were performed in the 300-5 K and 5-400 K temperature ranges. The thermal variation of the χ m T product for the three complexes (1-3) are shown in Figure 3. For compound 3, the χ m T value per formula at 400 K (≈6.71 cm 3 K mol −1 ) was in agreement with the value expected for two isolated Fe(II) ions (S = 2 and g ≈ 2.1), revealing the presence of two magnetically isolated HS Fe(II) ions [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. On cooling, the χ m T value of 3 showed an initial abrupt drop, at around 298 K, reaching a value of ca. 3.40 cm 3 K mol −1 at 265 K. On further cooling, we observed a second drop, at around 170 K, to reach a plateau at ca. 0.56 cm 3 K mol −1 below 116 K. This low temperature χ m T value implies the presence of a residual HS fraction of ca. 8 %. This behaviour indicates the presence of an incomplete HS to LS two-step transition centred at around 170 and 298 K.
In contrast to isomorphs 1 and 2, where the magnetic properties did not show any change after successive cooling and heating scans in the ranges 2-400 K for 1 and 2-500 K for 2, the two-step behaviour described above for complex 3 ( Figure 3) was irreversible due to a gradual desolvation of the sample at 400 K, as previously observed in several solvated systems [32][33][34][35][36][37][38][39][40]. As a matter of fact, when the sample was maintained at 400 K until its complete desolvation, the second cycle (400-50-400 K) in 3 produced a shift to lower temperatures for the high temperature step, while the transition temperature of the low temperature step remained unchanged (See Figure 4). Similar trends were observed for the third and fourth cycles, until the fifth cycle where the initial high temperature step merged with the low temperature step (Figure 4), to lead to the gradual one-step transition depicted in Figure 5a. There were no hysteretic effects and the HS fraction of ca. 8% remained unchanged after the different heating and cooling cycles.     Irradiation of the sample with a green laser (λ = 532 nm) at 10 K produced an increase of the χmT product, indicative of the presence of a light-induced excited spin state trapping at low temperatures (LIESST effect). After ca. 4 h, the mT product reached saturation at a value of ca. 1.6 cm 3 K mol −1 ( Figure S3). This value indicates that around 1/3 of one of the two LS Fe(II) centres in the [Fe(py3C-OEt)(NCS)3] − anions were excited to the HS state. After switching off the light irradiation, heating the sample further increased the mT value up to ca. 2.0 cm 3 K mol −1 (representing ca. 44 % of one of the two Fe(II) centres). On further heating, the sample relaxed to the LS state at a TLIESST of ca. 58 K (Figure 5b).
One of the major points, deserving special attention in regards to the three isomorphic complexes, concerns the origin of the unexpected and singular magnetic behaviour that only occurred for isomorph 3 (see Figures 3 and 5a). To try to understand the process that occurred at high temperatures, we performed thermogravimetric analysis (TGA) and X-ray powder diffraction on the three isomorphs to know more about the desolvation and solvation processes of this system. Thus, TGA measurements were performed for the three isomorphs, which were heated at 5 °C min −1 , under nitrogen atmosphere, from room temperature to 390 K. In Figure 6, the mass evolution with temperature for the three complexes were gathered, showing clearly that the two isomorphs 1 and 2 remained stable and retained their solvent molecules up to 390 K, while complex 3 started to lose weight from room temperature and lost 4.37 % of its mass when heated up to 370 K, corresponding to two CH3CN solvent molecules per formula unit. These measurements revealed that despite their isomorphic structures, complexes 1 and 2 retained their crystallization solvent molecules while complex 3 lost them even at moderate temperatures, suggesting that the crystal packing in 3 has larger cavities that allow an easy desolvation (see below). Irradiation of the sample with a green laser (λ = 532 nm) at 10 K produced an increase of the χ m T product, indicative of the presence of a light-induced excited spin state trapping at low temperatures (LIESST effect). After ca. 4 h, the χ m T product reached saturation at a value of ca. 1.6 cm 3 K mol −1 ( Figure S3). This value indicates that around 1/3 of one of the two LS Fe(II) centres in the [Fe(py 3 C-OEt)(NCS) 3 ] − anions were excited to the HS state. After switching off the light irradiation, heating the sample further increased the χ m T value up to ca. 2.0 cm 3 K mol −1 (representing ca. 44 % of one of the two Fe(II) centres). On further heating, the sample relaxed to the LS state at a T LIESST of ca. 58 K (Figure 5b).
One of the major points, deserving special attention in regards to the three isomorphic complexes, concerns the origin of the unexpected and singular magnetic behaviour that only occurred for isomorph 3 (see Figures 3 and 5a). To try to understand the process that occurred at high temperatures, we performed thermogravimetric analysis (TGA) and X-ray powder diffraction on the three isomorphs to know more about the desolvation and solvation processes of this system. Thus, TGA measurements were performed for the three isomorphs, which were heated at 5 • C min −1 , under nitrogen atmosphere, from room temperature to 390 K. In Figure 6, the mass evolution with temperature for the three complexes were gathered, showing clearly that the two isomorphs 1 and 2 remained stable and retained their solvent molecules up to 390 K, while complex 3 started to lose weight from room temperature and lost 4.37 % of its mass when heated up to 370 K, corresponding to two CH 3 CN solvent molecules per formula unit. These measurements revealed that despite their isomorphic structures, complexes 1 and 2 retained their crystallization solvent molecules while complex 3 lost them even at moderate temperatures, suggesting that the crystal packing in 3 has larger cavities that allow an easy desolvation (see below).
To check for the reversibility of this desolvation process, we performed successive desolvation and resolvation cycles, by heating the solvated sample and by adding two drops of CH 3 CN on the desolvated sample, respectively. After resolvation, magnetic measurements ( Figure S4) and X-ray powder diffraction (Figure 7) showed that the sample recovered its original behaviour, supporting the reversibility of the desolvation/resolvation process. To check for the reversibility of this desolvation process, we performed successive desolvation and resolvation cycles, by heating the solvated sample and by adding two drops of CH3CN on the desolvated sample, respectively. After resolvation, magnetic measurements ( Figure S4) and X-ray powder diffraction (Figure 7) showed that the sample recovered its original behaviour, supporting the reversibility of the desolvation/resolvation process.

Variable Temperature Magnetic Properties and Infrared Spectroscopy
In order to confirm the Fe(II) spin state at high and low temperatures and the presence of the incomplete HS to LS transition for 3 (see Figure 3), we measured the infrared spectrum at 100 and 300 K in the range of the fundamental stretching vibration of the NCSeunits (1975-2130 cm −1 ), since it has been clearly established that the intensities of these stretching vibrations are very sensitive to the spin state of the Fe(II) metal ion [28,29,[41][42][43][44][45][46][47]. We thus recorded the infrared spectra for 3 at 350 and 100 K, according to the thermal evolution of the mT product depicted in Figure 5a. The infrared spectra for 3 in the C≡N frequency region (1975-2130 cm −1 ) at 350 and 100 K are displayed in Figure 8. At 350 K, two νC≡N stretching broad bands, characteristic of the HS state, appeared at 2050 and 2075 cm −1 , while at 100 K, four strong bands, characteristic of the LS state, appeared  To check for the reversibility of this desolvation process, we performed successive desolvation and resolvation cycles, by heating the solvated sample and by adding two drops of CH3CN on the desolvated sample, respectively. After resolvation, magnetic measurements ( Figure S4) and X-ray powder diffraction (Figure 7) showed that the sample recovered its original behaviour, supporting the reversibility of the desolvation/resolvation process.

Variable Temperature Magnetic Properties and Infrared Spectroscopy
In order to confirm the Fe(II) spin state at high and low temperatures and the presence of the incomplete HS to LS transition for 3 (see Figure 3), we measured the infrared spectrum at 100 and 300 K in the range of the fundamental stretching vibration of the NCSeunits (1975-2130 cm −1 ), since it has been clearly established that the intensities of these stretching vibrations are very sensitive to the spin state of the Fe(II) metal ion [28,29,[41][42][43][44][45][46][47]. We thus recorded the infrared spectra for 3 at 350 and 100 K, according to the thermal evolution of the mT product depicted in Figure 5a. The infrared spectra for 3 in the C≡N frequency region (1975-2130 cm −1 ) at 350 and 100 K are displayed in Figure 8. At 350 K, two νC≡N stretching broad bands, characteristic of the HS state, appeared at 2050 and 2075 cm −1 , while at 100 K, four strong bands, characteristic of the LS state, appeared

Variable Temperature Magnetic Properties and Infrared Spectroscopy
In order to confirm the Fe(II) spin state at high and low temperatures and the presence of the incomplete HS to LS transition for 3 (see Figure 3), we measured the infrared spectrum at 100 and 300 K in the range of the fundamental stretching vibration of the NCSe − units (1975-2130 cm −1 ), since it has been clearly established that the intensities of these stretching vibrations are very sensitive to the spin state of the Fe(II) metal ion [28,29,[41][42][43][44][45][46][47]. We thus recorded the infrared spectra for 3 at 350 and 100 K, according to the thermal evolution of the χ m T product depicted in Figure 5a. The infrared spectra for 3 in the C≡N frequency region (1975-2130 cm −1 ) at 350 and 100 K are displayed in Figure 8. At 350 K, two ν C≡N stretching broad bands, characteristic of the HS state, appeared at 2050 and 2075 cm −1 , while at 100 K, four strong bands, characteristic of the LS state, appeared at 2044, 2075, 2082, and 2109 cm −1 . In agreement with the presence of an 8 % HS fraction at low temperatures (see magnetic section), the three bands observed at 2044, 2075, and 2082 cm −1 can be viewed as resulting from the decrease in intensity of the two broad and strong bands observed for the HS state, while the band observed at a higher frequency (2109 cm −1 ) appeared as the specific band of the LS state. at 2044, 2075, 2082, and 2109 cm −1 . In agreement with the presence of an 8 % HS fraction at low temperatures (see magnetic section), the three bands observed at 2044, 2075, and 2082 cm −1 can be viewed as resulting from the decrease in intensity of the two broad and strong bands observed for the HS state, while the band observed at a higher frequency (2109 cm −1 ) appeared as the specific band of the LS state.

Magneto-Spectroscopic Relationships
Based on the magnetic behaviours ( Figure 5a) and on the main bands that were temperature sensitive (Figure 8) of both solvated and desolvated phases of complex 3, we investigated the thermal evolution of the infrared spectra of the stretching vibration of the NCSe − in the range 1975-2130 cm −1 . For both phases (solvated and desolvated), we recorded the infrared spectra in the vicinity of the SCO transitions from 100 to 350 K. First, we recorded the infrared spectra for the freshly prepared complex 3, heating the sample from 100 to 350 K to avoid the partial desolvation of the sample (Figure 9a). Then, the same sample was heated during one hour at 400 K to ensure its complete desolvation, and the corresponding infrared spectra were then recorded cooling the sample from 350 to 100 K (Figure 9b).

Magneto-Spectroscopic Relationships
Based on the magnetic behaviours ( Figure 5a) and on the main bands that were temperature sensitive (Figure 8) of both solvated and desolvated phases of complex 3, we investigated the thermal evolution of the infrared spectra of the stretching vibration of the NCSe − in the range 1975-2130 cm −1 . For both phases (solvated and desolvated), we recorded the infrared spectra in the vicinity of the SCO transitions from 100 to 350 K. First, we recorded the infrared spectra for the freshly prepared complex 3, heating the sample from 100 to 350 K to avoid the partial desolvation of the sample (Figure 9a). Then, the same sample was heated during one hour at 400 K to ensure its complete desolvation, and the corresponding infrared spectra were then recorded cooling the sample from 350 to 100 K (Figure 9b). at 2044, 2075, 2082, and 2109 cm −1 . In agreement with the presence of an 8 % HS fraction at low temperatures (see magnetic section), the three bands observed at 2044, 2075, and 2082 cm −1 can be viewed as resulting from the decrease in intensity of the two broad and strong bands observed for the HS state, while the band observed at a higher frequency (2109 cm −1 ) appeared as the specific band of the LS state.

Magneto-Spectroscopic Relationships
Based on the magnetic behaviours ( Figure 5a) and on the main bands that were temperature sensitive (Figure 8) of both solvated and desolvated phases of complex 3, we investigated the thermal evolution of the infrared spectra of the stretching vibration of the NCSe − in the range 1975-2130 cm −1 . For both phases (solvated and desolvated), we recorded the infrared spectra in the vicinity of the SCO transitions from 100 to 350 K. First, we recorded the infrared spectra for the freshly prepared complex 3, heating the sample from 100 to 350 K to avoid the partial desolvation of the sample (Figure 9a). Then, the same sample was heated during one hour at 400 K to ensure its complete desolvation, and the corresponding infrared spectra were then recorded cooling the sample from 350 to 100 K (Figure 9b).  The intensities of the two ν(NCSe) broad bands (2050 and 2075 cm −1 ) attributed to the HS state decreased gradually with decreasing temperature from 350 to 100 K, but persisted even at 100 K, supporting the presence of a fraction of HS Fe(II) centres, as revealed by the magnetic data. In parallel, a new band, characteristic of the LS state, appeared at higher frequencies (2109 cm −1 ), whose intensity gradually increased with decreasing the temperature. However, as can be easily observed in both Figure 9a,b, the infrared spectra did not show any clear difference between the infrared band evolutions of the solvated (Figure 9a) and desolvated (Figure 9b) phases of complex 3, as revealed by the magnetic data. Thus, in order to show more clearly this difference and to appreciate, at least qualitatively, the consistency of the experimental infrared data, we correlated the thermal variation of the χ m T product derived from magnetic study and the thermal evolution of the intensity of the infrared bands. The results, depicted in Figure 10, showed that the intensities of the characteristic infrared bands (2109, 2075, 2050 cm −1 ) fit perfectly with the thermal evolution of the χ m T product, in agreement with the presence of a two-step SCO transition in the solvated sample ( Figure 10a) and a gradual one-step SCO behaviour in the desolvated sample (Figure 10b).
The intensities of the two ν(NCSe) broad bands (2050 and 2075 cm −1 ) attributed to the HS state decreased gradually with decreasing temperature from 350 to 100 K, but persisted even at 100 K, supporting the presence of a fraction of HS Fe(II) centres, as revealed by the magnetic data. In parallel, a new band, characteristic of the LS state, appeared at higher frequencies (2109 cm −1 ), whose intensity gradually increased with decreasing the temperature. However, as can be easily observed in both Figure 9a,b, the infrared spectra did not show any clear difference between the infrared band evolutions of the solvated (Figure 9a) and desolvated (Figure 9b) phases of complex 3, as revealed by the magnetic data. Thus, in order to show more clearly this difference and to appreciate, at least qualitatively, the consistency of the experimental infrared data, we correlated the thermal variation of the χmT product derived from magnetic study and the thermal evolution of the intensity of the infrared bands. The results, depicted in Figure 10, showed that the intensities of the characteristic infrared bands (2109, 2075, 2050 cm −1 ) fit perfectly with the thermal evolution of the χmT product, in agreement with the presence of a two-step SCO transition in the solvated sample ( Figure 10a) and a gradual one-step SCO behaviour in the desolvated sample (Figure 10b). The final question to be answered by this study was why the isomorph based on the NCSe − ligand (complex 3) displayed a reversible solvation/desolvation process with CH3CN while the two other isomorphs (complexes 1 and 2) retained the solvent molecules at temperatures exceeding 390 K. The answer could only come from the crystal packing of this triad of isomorphs. Indeed, examination of the crystal packing revealed that the CH3CN solvent molecules are located in tetragonal-like channels running along the [010] direction, which are generated by eclipsed stacks of the -Fe-NCE…Fe…ECN-Fe-NCE…Fe-metallacycles (see Figure 11). Even though the three isomorphs display, as expected, similar crystal packing, the fact that they differ by the nature of the NCE − ancillary ligands (E = S (1), BH3 (2), Se (3)), induces strong differences in the sizes of the metallacycles, due to the different lengths of the three linear anions (the N-E distance increases from 2.72 Å for E = BH3, to 2.80 Å for E = S and 2.94 Å for E = Se). The largest tetragonal-like channels are expected to be those of the isomorph with the longest ancillary linear ligand (i. e., compound 3). Unfortunately, this difference could not be quantified due to the lack of single crystal structural data of complex 3. However, in order to provide a reasonable estimation of the effect of the NCE − ligands on the size of the metallacycles, we have calculated the relative increase of their dimensions when passing from complex 2 to complex The final question to be answered by this study was why the isomorph based on the NCSe − ligand (complex 3) displayed a reversible solvation/desolvation process with CH 3 CN while the two other isomorphs (complexes 1 and 2) retained the solvent molecules at temperatures exceeding 390 K. The answer could only come from the crystal packing of this triad of isomorphs. Indeed, examination of the crystal packing revealed that the CH 3 CN solvent molecules are located in tetragonal-like channels running along the [010] direction, which are generated by eclipsed stacks of the -Fe-NCE . . . Fe . . . ECN-Fe-NCE . . . Fe-metallacycles (see Figure 11). Even though the three isomorphs display, as expected, similar crystal packing, the fact that they differ by the nature of the NCE − ancillary ligands (E = S (1), BH 3 (2), Se (3)), induces strong differences in the sizes of the metallacycles, due to the different lengths of the three linear anions (the N-E distance increases from 2.72 Å for E = BH 3 , to 2.80 Å for E = S and 2.94 Å for E = Se). The largest tetragonal-like channels are expected to be those of the isomorph with the longest ancillary linear ligand (i.e., compound 3). Unfortunately, this difference could not be quantified due to the lack of single crystal structural data of complex 3. However, in order to provide a reasonable estimation of the effect of the NCE − ligands on the size of the metallacycles, we have calculated the relative increase of their dimensions when passing from complex 2 to complex 1 (see Figure 11 and Table 1 with the different Fe···Fe distances (d 1 -d 4 ) determining the size of the channels). 1 (see Figure 11 and Table 1 with the different Fe···Fe distances (d1-d4) determining the size of the channels). As can be seen in Table 1, when passing from complex 2 (with NCBH3 − and N···B distance of 2.72 Å) to complex 1 (with NCS − and N···S distance of 2.80 Å), there is an increase of +2.9 % in the size of the anion, which led to increases in the Fe···Fe distances in the metallacycle of up to 28.6 % (see d1 to d4 in Table 1). Therefore, if we consider complex 3 based on the NCSe − linear ligand, which corresponds to the biggest linear anion (with N···Se distance of 2.94 Å), the expected metallacycle should be significantly larger than those observed for the isomorphs 1 and 2. These observations explain clearly why complex 3 involves larger tetragonal-like channels, allowing the easy and reversible solvation and desolvation processes.

Synthesis of [Fe(py3C-OEt)2][Fe(py3C-OEt)(NCSe)3]2·2CH3CN (3)
Tris  As can be seen in Table 1, when passing from complex 2 (with NCBH 3 − and N···B distance of 2.72 Å) to complex 1 (with NCS − and N···S distance of 2.80 Å), there is an increase of +2.9 % in the size of the anion, which led to increases in the Fe···Fe distances in the metallacycle of up to 28.6 % (see d 1 to d 4 in Table 1). Therefore, if we consider complex 3 based on the NCSe − linear ligand, which corresponds to the biggest linear anion (with N···Se distance of 2.94 Å), the expected metallacycle should be significantly larger than those observed for the isomorphs 1 and 2. These observations explain clearly why complex 3 involves larger tetragonal-like channels, allowing the easy and reversible solvation and desolvation processes.

Characterization of the Materials
Infrared spectra of complex 3 were performed using a platinum ATR Vertex 70 BRUKER spectrometer with variable temperature cell holder (VT Cell Holder typer P/N GS21525). 1 H and 13 C NMR spectra were performed using BRUKER DRX 300 MHz, Advance 400 MHz and Advance III HD 500 MHz equipment. TGA measurements were performed on ATG-LabsysTM, Setaram (see details in Supplementary Information).

Magnetic Measurements
Magnetic susceptibility measurements were performed using a Quantum Design MPMS-XL-5 SQUID susceptometer (San Diego, CA, USA). The susceptibility data were corrected for the diamagnetic contributions using Pascal's constant tables [48]. The photomagnetic studies were performed by irradiating the sample at 10 K with a green Diode Pumped Solid State Laser DPSS-532-20 from Chylas (see details in Supplementary Information).

Conclusions
We have shown that the compound [Fe(py 3 C-OEt) 2 ][Fe(py 3 C-OEt)(NCSe) 3 ] 2 ·2CH 3 CN (3), based on the [Fe(py 3 C-OEt) 2 ] 2+ LS cation and the SCO [Fe(py 3 C-OEt)(NCSe) 3 ] − anion, displays a reversible desolvation process that affects the SCO behaviour. This compound has been prepared as a polycrystalline powder and as single crystals using a similar protocol to that used previously for the syntheses of the two isomorphic complexes [Fe(py 3 C-OEt) 2 ][Fe(py 3 C-OEt)(NCE) 3 ] 2 ·2CH 3 CN (E = S (1), BH 3 (2)) based on the two ancillary linear ligands NCS − and NCBH 3 − [28]. Despite being obtained in the form of prismatic-shaped single crystals, complex 3 could not be characterized by X-ray single crystal diffraction, because of the low stability of its single crystals, in contrast to complexes 1 and 2, which have been structurally characterized. However, combined X-ray powder diffraction, infrared spectra, and CHN elemental analyses clearly revealed that complex 3 exhibits an isomorphic structure to those of complexes 1 and 2. TGA analyses performed on the single crystal samples of the three isomorphs revealed clearly that the two isomorphs 1 and 2, for which the corresponding single crystals are stable, retained their solvent molecules up to 390 K, while complex 3 began to lose its CH 3 CN solvent molecules from room temperature. The magnetic studies for 3 performed in cooling and heating scans in the temperature ranges 300-5 K and 5-400 K, respectively, indicated the presence of an incomplete HS to LS two-step like transition centred around 170 and 298 K, while when the sample was heated at 400 K until its complete desolvation, the magnetic behaviour of the high temperature transition (T 1/2 = 298 K) shifted to a lower temperature until the two-step behaviour merged with a gradual one-step transition at ca. 216 K. Such behaviour, which can be viewed as a solvent-induced hysteresis loop of 50 K [41][42][43][44][45][46][47], was confirmed by infrared spectra recorded in the vicinity of the SCO transition for both solvated and desolvated samples. Furthermore, successive desolvation and solvation cycles, tracked by SQUID measurements and X-ray powder diffraction, showed that the desolvation process is fully reversible. As shown by the TGA analysis, compound 3 exhibited a different magnetic behaviour due to its easy desolvation and solvation process, in contrast to the two other isomorphs, which retained their solvent molecule up to 390 K, despite their isomorphic structures. This unexpected behaviour was elucidated by careful examination of the crystal packing of these isomorph complexes, which clearly revealed that the solvent molecules are located in tetragonal-like channels generated by the eclipsed stacks of the "-Fe-NCE . . . Fe . . . ECN-Fe-NCE . . . Fe-" metallacycles. Therefore, the isomorph based on the bigger NCSe − ancillary ligand, should display the larger tetragonal-like channels, allowing easier solvation and desolvation as observed in isomorph 3.