# Lattice Solvent- and Substituent-Dependent Spin-Crossover in Isomeric Iron(Ii) Complexes

Senthil Kumar Kuppusamy, Asato Mizuno, Lea Kämmerer, Soma Salamon, Benoît Heinrich, Corinne Bailly, Ivan Šalitroš, Heiko Wende, Mario Ruben

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Senthil Kumar Kuppusamy, Asato Mizuno, Lea Kämmerer, Soma Salamon, Benoît Heinrich, et al. Lattice Solvent- and Substituent-Dependent Spin-Crossover in Isomeric Iron(Ii) Complexes. Dalton Transactions, 2024, 53 (26), pp.10851-10865. 10.1039/D4DT00429A. hal-04994084

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# Dalton Transactions

An international journal of inorganic chemistry

rsc.li/dalton

<figure><img src="image_2.png" /></figure>

ISSN 1477-9226

<figure><img src="image_3.png" /></figure>

**PAPER**
Senthil Kumar Kuppusamy et al.
Lattice solvent- and substituent-dependent spin-crossover
in isomeric iron(ii) complexes
<figure><img src="image_4.png" /></figure>

Cite this: *Dalton Trans.*, 2024, **53**,
10851

Lattice solvent- and substituent-dependent
spin-crossover in isomeric iron(ii) complexes†

Senthil Kumar Kuppusamy, ID *a Asato Mizuno,‡ b Lea Kämmerer, ID c
Soma Salamon, ID c Benoît Heinrich, ID d Corinne Bailly, e Ivan Šalitroš, ID *f,g
Heiko Wende ID *c and Mario Ruben*a,b,h

Spin-state switching in iron(ii) complexes composed of ligands featuring moderate ligand-field strength—
for example, 2,6-bi(1*H*-pyrazol-1-yl)pyridine (BPP)—is dependent on many factors. Herein, we show that
spin-state switching in isomeric iron(ii) complexes composed of BPP-based ligands—ethyl 2,6-bis(1*H*-
pyrazol-1-yl)isonicotinate (BPP-COOEt, **L1**) and (2,6-di(1*H*-pyrazol-1-yl)pyridin-4-yl)methylacetate
(BPP-CH₂OCOMe, **L2**)—is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-
state switching—with a thermal hysteresis width (ΔT₁/₂) of 44 K and switching temperature (T₁/₂) = 298 K
in the first cycle—is observed for complex 1·CH₃CN composed of **L1** and BF₄⁻ counter anions.
Conversely, the solvent-free isomeric counterpart of 1·CH₃CN—complex 2a, composed of **L2** and BF₄⁻
counter anions—was trapped in the high-spin (HS) state. For one of the polymorphs of complex
2b·CH₃CN—2b·CH₃CN-Y, Y denotes yellow colour of the crystals—composed of **L2** and ClO₄⁻ counter
anions, a gradual and non-hysteretic SCO is observed with T₁/₂ = 234 K. Complexes 1·CH₃CN and
2b·CH₃CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited
spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH₃CN
revealed that spin-state switching goes hand-in-hand with pronounced distortion of the *trans*-N(pyridyl)-
Fe-N(pyridyl) angle (φ), whereas such distortion is not observed for 2b·CH₃CN-Y. This observation points
that distortion is one of the factors making the spin-state switching of 1·CH₃CN hysteretic in the solid
state. The observation of bi-stable spin-state switching with T₁/₂ centred at room temperature for
1·CH₃CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(ii)
complexes can be obtained.

Received 14th February 2024,
Accepted 15th May 2024

DOI: 10.1039/d4dt00429a

rsc.li/dalton

Introduction

Molecular materials that show switching of a physical property —for example, magnetic 1⁻¹² or electric 1³⁻¹⁷—accompanied by hysteresis are candidates desirable for applications. Several classes of magnetic-molecular systems such as organic

radicals,¹⁸⁻²⁰ single-molecule magnets (SMMs),³,²¹⁻²³ and spin-
crossover (SCO) complexes²⁴⁻³¹ are known to show bistable
switching characteristics. However, to the best of our knowl-
edge, SCO complexes are the only class of molecules that fre-
quently show bistable switching near room-temperature
(RT).⁴,¹² For realistic applications, SCO complexes showing a

<footnote>ªInstitute of Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail: senthil.kuppusamy2@kit.edu</footnote>

<footnote>ᵇInstitute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany</footnote>

<footnote>ᶜUniversity of Duisburg-Essen, Faculty of Physics and Center for Nanointegration
Duisburg-Essen (CENIDE), Lotharstraße 1, 47057 Duisburg, Germany</footnote>

<footnote>ᵈInstitut de Physique et Chimie des Matériaux de Strasbourg (IPCMS),
CNRS-Université de Strasbourg, 23, rue du Loess, BP 43, 67034 Strasbourg Cedex 2,
France</footnote>

<footnote>ᵉService de Radiocrystallographie, Fédération de Chimie Le Bel UAR2042
CNRS-Université de Strasbourg, 1 rue Blaise Pascal, BP 296/R8, 67008 Strasbourg
cedex, France</footnote>

<footnote>fCentral European Institute of Technology, Brno University of Technology, Purkyňova
123, 61200 Brno, Czech Republic</footnote>

<footnote>Department of Inorganic Chemistry, Faculty of Chemical and Food Technology,
Slovak University of Technology in Bratislava, Bratislava SK-81237, Slovakia</footnote>

<footnote>hCentre Européen de Sciences Quantiques (CESQ), Institut de Science et d'Ingénierie,
Supramoléculaires (ISIS), 8 allée Gaspard Monge, BP 70028, 67083 Strasbourg
Cedex, France</footnote>

<footnote>†Electronic supplementary information (ESI) available. CCDC 1560719, 1913041,
1953349, 1953350, 1988543, 2330938, 2330939 and 2340087. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/
d4dt00429a</footnote>

<footnote>‡Current address: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.</footnote>
sizable thermal hysteresis width (ΔT₁/₂)—around 70 K—with the switching temperature (T₁/₂) centred around RT are desirable.⁴ However, preparing an SCO complex that can undergo hysteretic switching around RT is not a trivial task because spin-state switching in the solid state is controlled by system-dependent factors, such as lattice solvent and disorder. Therefore, it is necessary to conduct a systematic search starting from a basic ligand skeleton featuring a moderate ligand field allowing spin-state switching of a coordinated metal ion, iron(II) in the context of this study. 2,6-Bis(1H-pyrazol-1-yl)pyridine (BPP)-based ligands (Fig. 1a) are a class of tridentate nitrogen-donor ligands that facilitate spin-state switching of the coordinated iron(II) centre, as first reported by Halcrow and co-workers in 2001.³² The propensity to modify the BPP skeleton with functional groups at the pyridine and pyrazole rings allows the systematic study of spin-state switching of the iron(II)-BPP complexes, useful for establishing structure-SCO property correlations.³³ Recently, technologically relevant spin-state switching characteristics of iron(II) complexes (complexes a–d, Fig. 1b) composed of a BPP-based ligand—ethyl 2,6-bis (1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1)—have been reported. Complex a underwent above room-temperature low-spin (LS)-to-high-spin (HS) switching and remained trapped in the HS-state after the loss of lattice acetone solvent.³⁴ On the

other hand, complex **b**, co-crystallized with lattice acetonitrile, underwent bi-stable spin-state switching with T₁/₂ = 233 K and ΔT₁/₂ = 100 K in the first cycle.⁹ However, in the subsequent cycles spin-state switching of complex **b** is rendered unstable due to self-grinding of the crystals. In a subsequent report, some of us have reported on the stable spin-state switching characteristics of complexes **c** and **d** and elucidated on the role of pronounced molecular distortion as a factor contributing to the occurrence of hysteretic spin-state switching.³⁵ In this contribution, we have studied iron(II) complexes composed of BPP-COOEt (L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl) methylacetate (BPP-CH₂OCOMe, L2) ligands (Fig. 1a). Complex 1·CH₃CN (Fig. 1b), composed of the BPP-COOEt ligand and BF₄⁻ counter anions, undergoes bi-stable spin-state switching (ΔT₁/₂ = 44 K) with T₁/₂ centred at RT (298 K) in the first cycle and the switching is not stable upon repeated cycling. To study the impact of substituents on the switching characteristics, we have prepared a new ligand L2, which features the same molecular formula as L1—the ligands L1 and L2 are structural isomers. Complex 2a (Fig. 1c) composed of L2 crystallized without the lattice solvent and showed no spin-state switching from the HS state upon cooling from 300 K. Conversely, for complex 2b·CH₃CN-Y (Fig. 1c), a gradual SCO without thermal hysteresis is observed. Remarkably, complexes

<figure><img src="image_5.png" /><figcaption>**Fig. 1** Molecular structures of ligands and iron(II) complexes discussed in this study. (a) Structures of parent BPP ligand and its functional variants L1 and L2. The ligands L1 and L2 are structural isomers; red and blue colours highlight functional group isomerism. The aim of this study is to elucidate how the nature of the substituent controls spin-state switching in iron(II) complexes composed of L1 and L2. (b) Structures of L1-based complexes a, b, c, d, and 1·CH₃CN. (c) Structures of L2-based complexes 2a and 2b·CH₃CN. For 2b·CH₃CN, two different polymorphs—2b·CH₃CN-Y and 2b·CH₃CN-R—were obtained; Y and R denote yellow and red, respectively, colours of the crystals. Iron(II) complexes a, b, c, and d have been studied previously.⁹,³⁴,³⁵</figcaption></figure>
<figure><img src="image_6.png" /></figure>

1·CH3CN and 2b·CH3CN-Y also underwent light-induced LS-to-HS switching at 5 K following the light-induced excited spin-state trapping (LIESST) mechanism. The molecular structures of the LS and HS forms of 1·CH3CN obtained from single-crystal X-ray diffraction (SC-XRD) studies revealed a pronounced angular distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (φ) in the HS-form relative to the LS-form. Additionally, the conformation of the ethyl substituent of 1·CH3CN is also switched upon spin-state switching. In contrast, no significant variation of φ was observed for the LS and HS forms of 2b·CH3CN-Y. These results indicate the contributory role of molecular distortion in spurring the bi-stable spin-state switching observed for 1·CH3CN. Overall, the contrasting magnetic characteristics of the complexes show a complex interplay between structural- and spin-isomerism, and a structural control over SCO is achievable following molecular engineering principles. Crucially, complex 1·CH3CN is one of the remarkable examples of a mononuclear iron(II) complex that undergoes temperature-induced bistable SCO with a T₁/₂ centred at RT. Importantly, our attribution that pronounced molecular distortion and a change in substituent conformation contribute to the opening of the thermal hysteresis loop could enable future designs of iron(II)-based SCO complexes with application-oriented spin-state switching characteristics.

## Experimental

### Materials and methods

Anhydrous solvents, [Fe(BF₄)₂·6H₂O], and [Fe(ClO₄)₂·6H₂O] were purchased from commercial sources and used as received. Glassware was dried in a vacuum oven at 423 K prior to the experiments. The complexation reactions were performed under an argon (Ar) atmosphere. Experimental protocols used to study the physical properties of the ligands and complexes (section S1), and synthesis procedures used to prepare the ligands and complexes (section S2) are described in the ESI.†

Caution: perchlorate salts are potentially hazardous; care should be taken while handling them. We have encountered no difficulties during the preparation, characterization, and magnetic studies of perchlorate containing complexes—2b·CH₃CN-Y and 2b·CH₃CN-R—discussed in this study.

## Results and discussion

### Synthesis of the ligands and complexes

Ligand L1 was synthesized following a previously reported procedure.³⁶ Ligand L2 was synthesized from (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methanol, as detailed in section S2 of the ESI.† Treatment of [Fe(X)₂·6H₂O] (X = BF₄ or ClO₄) as a solid with the acetonitrile solution of the respective ligand resulted in the formation of complexes 1·CH₃CN, 2a, or 2b·CH₃CN (Scheme 1). Slow diffusion of diethyl ether into the acetonitrile

<figure><img src="image_7.png" /><figcaption>**Scheme 1** Preparation of complex 1·CH₃CN from L1 and complexes 2a and 2b·CH₃CN from L2. Two different polymorphs coloured yellow (2b·CH₃CN-Y) and red (2b·CH₃CN-R) were obtained for 2b·CH₃CN.</figcaption></figure>

solutions of the reaction mixtures kept inside a fridge at 4 °C over a period of 2–3 weeks yielded a crop of wine-red crystals of complex 1·CH₃CN and pale-yellow crystals of 2a, suitable for single-crystal X-ray diffraction (SC-XRD) studies. In the case of 2b·CH₃CN, crystallizing with lattice acetonitrile, two different polymorphs coloured yellow (2b·CH₃CN-Y) and red (2b·CH₃CN-R) were obtained. After harvesting from the mother liquor, the crystals of 1·CH₃CN were washed with diethyl ether and dried under vacuum for 4 h. Elemental analysis of the dried crystals revealed that lattice acetonitrile solvent is retained after the drying process (section S2†). In the case of 2b·CH₃CN-Y, drying under vacuum resulted in the loss of the lattice solvent. Therefore, the yellow crystals were left to dry inside a fume-hood for an hour after washing with diethyl ether. We have also noted slow release of the lattice solvent from the crystals of 2b·CH₃CN-Y over the course of a few days while storing under ambient conditions. The crystals of 2b·CH₃CN-R lost the lattice solvent rapidly after removing from the mother liquor with the consequent cracking and yellow-brown coloration of the crystals. A complete loss of the lattice solvent was inferred from the elemental analysis data of 2b·CH₃CN-R. See section S2 and Fig. S1–S6 of the ESI† for the characterization data of the ligands and complexes.
<figure><img src="image_8.png" /><figcaption>**Fig. 2** Electronic absorption spectroscopic studies. UV-Vis electronic absorption spectra of (a) ligands and (b) complexes measured in acetonitrile solution. Functionalization of the parent BPP skeleton with COOEt substituent caused bathochromic shifting of the absorption maxima of L2, whereas OCH₂COCH₃ substituent caused no such shift in L2.</figcaption></figure>

## Electronic absorption spectra of ligands L1 and L2 and complexes 1·CH₃CN, 2a, and 2b·CH₃CN-Y

To get insights into the effect of substituents on the absorption spectroscopic characteristics of ligands and complexes, UV-Vis absorption spectroscopic studies of L1, L2, 1·CH₃CN, 2a, and 2b·CH₃CN-Y were performed in acetonitrile solvent. To make comparisons, spectroscopic studies of the parent BPP ligand and its complex—[Fe(BPP)₂](BF₄)₂—were also performed. In general, two sets of transitions in the 250-to-275 nm and 275-to-400 nm regions were observed for the ligands and their corresponding complexes (Fig. 2 and Table 1). The first and second set of transitions are tentatively attributed to the $π → π^*$ and $n → π^*$ transitions of the ligand skeletons. The absorption maxima of BPP and L2 are comparable, elucidating the negligible electronic effect of the OCH₂COCH₃ substituent on the BPP skeleton. On the other hand, absorption maxima of L1 were bathochromically shifted relative to the parent BPP ligand, reflecting on the significant inductive effect of the COOEt substituent on the BPP skeleton. We tentatively attribute the bathochromic shifting observed for L1, relative to BPP, to the stabilization of the lowest unoccupied molecular orbital (LUMO) of the ligand imparted by the electron withdrawing COOEt substituent. Complexation of the ligands with iron(II) induced negligible changes in the absorption characteristics of the ligands, as inferred from the comparable nature of the absorption maxima of the ligands and

their corresponding complexes (Fig. 2a and b). Remarkably, for 1·CH₃CN, a shoulder with a maximum centred at 461 nm was observed (Fig. S7†), attributed to the singlet metal-to-ligand charge transfer (¹MLCT) transition associated with the LS form of the complex. Such a transition was not observed for [Fe(BPP)₂](BF₄)₂, complexes 2a, and 2b·CH₃CN-Y, indicating the HS state of the complexes in the solution.

## Structural studies of L1; low spin (LS) and high spin (HS) forms of complex 1·CH₃CN; HS form of 2a; LS form of 2b·CH₃CN-R; and LS and HS forms of 2b·CH₃CN-Y

Slow evaporation of the CDCl₃ solution of L1 in an NMR tube resulted in the formation of single crystals of L1 suitable for X-ray diffraction (XRD) studies (Table S1†). In the molecular structure shown in Fig. 3a, the coordinating β-nitrogens of the pyrazole rings are in the *trans* position with respect to the pyridyl nitrogen, in line with the previously reported structures of bpp-based ligands.³⁷⁻³⁹ The *trans* orientation of the nitrogen atoms indicates the need for the rotation of the pyrazole rings to coordinate with iron(II) centres.

Structure determination of wine-red crystals at 173 K (Fig. 3b and Table S2†) and 300 K (Fig. S8 and Table S1†) obtained from the reaction between L1 and iron(II) salt revealed that complex 1·CH₃CN crystallized with one lattice acetonitrile solvent. The iron(II) centres in the crystals are in the LS state as inferred from the Fe–N bond lengths and angular parameters summarized in Tables 2 and S3.† We have also obtained the HS structure of 1·CH₃CN at 345 K (Fig. 3c and Table S2†) from a LS 1·CH₃CN crystal by heating it in the diffractometer at a rate of 0.5 K min⁻¹.

A comparison between the structures of the LS and HS forms 1·CH₃CN indicates that the spin-state switching is accompanied by conformational switching of one of the ethyl substituents (Fig. 3b and c) and pronounced angular distortion (Table 2 and Fig. 3d). Noteworthy are the *trans*-*N*{pyridyl}-Fe–N{pyridyl} angle (*φ*), *N*{pyrazolyl}-Fe–N{pyrazolyl} clamp angle (*ψ*), average of four *cis*-*N*{pyridyl}-Fe–N{pyrazolyl} angles (*α*), distortion index (*Σ*),⁴⁰ angle between the planes of two ligands

<table><caption>**Table 1** Absorption maxima ($λ_{max}$) and molar absorption coefficients ($ε$) of ligands and complexes in CH₃CN</caption><thead><tr><th>Entry</th><th>λ<sub>max</sub>. (CH<sub>3</sub>CN)/nm (ε)/dm<sup>3</sup> mol<sup>-1</sup> cm<sup>-1</sup></th></tr></thead><tbody><tr><td>BPP</td><td>265 (12 567), 272 (11 399), 303 (19 136)</td></tr><tr><td>L1</td><td>268 (15 711), 275 (13 559), 331 (13 559)</td></tr><tr><td>L2</td><td>265 (15 008), 272 (13 884), 304 (19 654)</td></tr><tr><td>[Fe(BPP)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub></td><td>264 (32 791), 271 (30 011), 302 (46 736)</td></tr><tr><td>1·CH<sub>3</sub>CN</td><td>268 (38 147), 274 (33 538), 330 (30 875), 461 (708)</td></tr><tr><td>2a</td><td>265 (34 708), 272 (32 440), 304 (41 893)</td></tr><tr><td>2b·CH<sub>3</sub>CN-Y</td><td>265 (33 216), 272 (31 577), 304 (39 479)</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 3** Structures of L1 and the corresponding complex—1·CH₃CN—obtained from single-crystal X-ray diffraction (SC-XRD) studies. Molecular structures of (a) L1, (b) LS- and (c) HS-forms of complex 1·CH₃CN, and (d) overlay of LS (light blue) and HS (light orange) structures of 1·CH₃CN determined at 300 K and 345 K, respectively. The lattice acetonitrile molecules and BF₄⁻ anions in b-d are not shown for clarity. The most abundant parts of the disordered moieties are shown for b and c. Colour code: white, hydrogen; grey, carbon; blue, nitrogen; red, oxygen; orange, iron. The blue circles in a-c indicate the positions of one of the ethyl groups of the COOEt substituents in L1 and the corresponding complex in the LS and HS states. The LS-to-HS switching of 1·CH₃CN involves pronounced angular distortion and conformational switching of the circled ethyl groups. Such structural variations associated with the LS and HS forms of 1·CH₃CN cause bi-stable spin-state switching of the complex, as discussed below.</figcaption></figure>

<table><caption>**Table 2** Fe–N bond lengths (Å) and angular parameters (°) of the LS and HS forms of 1·CH₃CN, HS form of 2a, and LS and HS forms of 2b·CH₃CN-Y</caption><thead><tr><th>Parameter</th><th>1·CH<sub>3</sub>CN (LS)</th><th>1·CH<sub>3</sub>CN (HS)</th><th>2a (HS)</th><th>2b·CH<sub>3</sub>CN-Y (LS)</th><th>2b·CH<sub>3</sub>CN-Y (HS)</th></tr></thead><tbody><tr><td>T/K</td><td>173</td><td>345</td><td>173</td><td>120</td><td>300</td></tr><tr><td>rFe–N</td><td>1.942</td><td>2.161</td><td>2.177</td><td>1.952</td><td>2.160</td></tr><tr><td>N<sub>3</sub>–Fe<sub>1</sub>–N<sub>8</sub> (φ)</td><td>172.87</td><td>159.20</td><td>168.75</td><td>177.37</td><td>178.68</td></tr><tr><td>N<sub>6</sub>–Fe<sub>1</sub>–N<sub>10</sub> (ψ)</td><td>160.53</td><td>145.15</td><td>146.01</td><td>159.7</td><td>147.63</td></tr><tr><td>N<sub>1</sub>–Fe<sub>1</sub>–N<sub>5</sub> (ψ)</td><td>160.43</td><td>145.32</td><td>145.15</td><td>159.87</td><td>147.29</td></tr><tr><td>α</td><td>80.29</td><td>72.94</td><td>72.79</td><td>79.89</td><td>73.74</td></tr><tr><td>Σ</td><td>84.40</td><td>158.67</td><td>160.10</td><td>89.58</td><td>148.09</td></tr><tr><td>θ</td><td>86.70</td><td>80.73</td><td>84.67</td><td>85.86</td><td>85.00</td></tr><tr><td>Θ</td><td>315.18</td><td>637.22</td><td>540.14</td><td>294.20</td><td>474.74</td></tr></tbody></table>

($\theta$), and trigonal distortion parameter ($\Theta$), as shown in Table 2. Upon LS-to-HS switching, significant deviation from close to octahedral-to-trigonal prismatic structure has been observed for 1·CH₃CN.

Structural investigation of complex 2a obtained at 173 K indicates that the complex is in the HS form. The complex cation (Fig. 4a and Table S4†) is distorted, as inferred from the angular parameters listed in Table 2. However, the distortion is less pronounced in HS-2a relative to HS-1·CH₃CN. In the case of 2b·CH₃CN-Y, the LS and HS forms of the complex (Fig. 4b and c and Table S4†) showed no appreciable deviation of $\phi$ value from the ideal value of 180° (Table 2). Other angular parameters associated with the LS and HS forms of 2b·CH₃CN-Y are comparable with the respective ones obtained for the LS and HS forms of 1·CH₃CN. A notable exception is the $\Sigma$ value, which is relatively larger for the LS 2b·CH₃CN-Y than the one obtained for the LS 1·CH₃CN. Conversely, the $\Sigma$

value of the HS 1·CH₃CN is found to be greater than the value obtained for the HS 2b·CH₃CN-Y. Overall, the variation of $\Sigma$ upon spin-state switching is larger for 1·CH₃CN relative to 2b·CH₃CN-Y, indicating the fact that the former complex follows a more distorted pathway than the latter upon spin-state switching.

The red polymorph of 2b, 2b·CH₃CN-R, also crystallized with lattice acetonitrile solvent. See Table S5† for the structural data. The polymorph is in the LS state at 173 K as inferred from the molecular structure (Fig. 4d) and angular parameters (Table S6†) obtained from SC-XRD studies. The average of Fe–N bond lengths and angular parameters of 2b·CH₃CN-R are comparable with the values obtained for the LS 2b·CH₃CN-Y, as listed in Table S6.†

For complex 1·CH₃CN in the HS and LS states, the complex cations in the crystal lattice organize in a pseudo 1D-chain like manner along the c-axis (Fig. S9 and S11†). The pseudo
<figure><img src="image_11.png" /><figcaption>**Fig. 4** Molecular structures of complexes composed of L2 obtained from single-crystal X-ray diffraction (SC-XRD) studies. (a) HS form of complex 2a, (b) LS form of complex 2b·CH3CN-Y, (c) HS form of complex 2b·CH3CN-Y, and (d) LS form of 2b·CH3CN-R. The lattice acetonitrile molecules, where appropriate, and counter anions are not shown for clarity. Colour code: white, hydrogen; grey, carbon; blue, nitrogen; red, oxygen; orange, iron.</figcaption></figure>

1D-chains stack (inter-sheet organization) along the crystallographic *b*-plane. Molecular organization in the lattices is governed by intermolecular interactions between the complex cation, counter anion, and acetonitrile solvent entities as depicted in Fig. S10 and S12.† Noteworthy are the short contacts between the lattice acetonitrile solvent and the complex cation (see Table S7† for details).

For complex 2a in the HS form, complex cations are ordered in an alternative manner along the *b*- and *c*-axes (Fig. S13†). Short contacts between F atoms of the counter cation and H atoms of the pyridine/pyrazole moieties along with an O–H contact, involving carbonyl oxygen of the –COOEt substituent and pyrazole-based hydrogen atom, between adjacent complex cations direct the molecular organization in the lattice (Fig. S14 and Table S8†). In the cases of the HS and LS forms of 2b·CH3CN-Y, a zig-zag molecular organization (Fig. S15 and S17†) in the crystal lattice is observed, when viewed along the crystallographic axis *a*. The lattice is held together by inter molecular contacts between the counter anion and complex cation (see Fig. S16 and S18 and Tables S9† for details). Similar molecular organization (Fig. S19†) and intermolecular contacts (Fig. S20 and Table S10†), as in 2b·CH3CN-Y, are observed in the crystal lattice of LS 2b·CH3CN-R.

For the estimation of short contacts, the criterion $d(x...y) = \sum r(vdW)[x, y] - 0.2 Å$ was used, where $d(x...y)$ is the distance between two atoms and $\sum r(vdW)[x, y]$ is the sum of van der Waals radii of atoms *x* and *y*. Note, the short contact estimation following the above criterion is associated with significant pitfalls as discussed by G. P. Schiemenz.⁴¹ Considering the above point, the discussion presented in this study mainly intends to elucidate how intermolecular short contacts direct

the molecular organization in the crystal lattices of complexes discussed in this study.

### Spin-state switching characteristics of 1·CH3CN and 2b·CH3CN-Y and magnetic studies of HS 2a

Magnetic susceptibility measurements of gently ground crystals of 1·CH3CN, 2a, and 2b·CH3CN-Y were performed under 0.1 T applied magnetic field at a scan rate of 3 K min⁻¹. Complex 1·CH3CN exhibited an abrupt SCO with 44 K and 52 K hysteresis loops for the first ($T_{1/2↑}$ = 320 K and $T_{1/2↓}$ = 276 K) and second ($T_{1/2↑}$ = 320 K and $T_{1/2↓}$ = 268 K) cycles, respectively, as depicted in Fig. 5a.

The obtained $\chi T$ products of 3.18 cm³ K mol⁻¹ and 0.18 cm³ K mol⁻¹ at 335 K and 200 K, respectively, for the first cycle (Table 3), indicate the presence of pure HS and predominantly LS states of the complex at those temperatures. The above point elucidates temperature-induced spin state switching of 1·CH3CN. Upon repeated scanning, a steady decrease of $\chi T$ values was observed until the 11th scan (Fig. 5b). Starting from the 12th cycle, a relatively stabilized $\chi T$ versus *T* plots were observed (Fig. 5b). This is attributed to the trapping of the sample in a mixed spin phase composed predominantly of the LS state.⁴² The cycling is also accompanied by a steady increase of the $\chi T$ value in the 5 K-to-275 K region, as recently observed for an iron(II)-BPP system reported by Halcrow and co-workers.⁴³ A fraction of the complex molecules in the mixed phase, obtained after the 11th cycle, undergoes SCO, as depicted in Fig. 5b.

To check for the effect of *in situ* solvent removal on the spin-state switching characteristics of 1·CH3CN, the sample was annealed in the SQUID chamber at 400 K for thirty minutes. A cool-heat cycle performed after the annealing step
<figure><img src="image_12.png" /><figcaption>**Fig. 5** Temperature-induced spin-state switching characteristics of 1·CH₃CN. (a) χT versus T plots of wine-red crystals of 1·CH₃CN measured at a scan rate of 3 K min⁻¹. The sample showed bi-stable—abrupt and hysteresic—spin-state switching with ΔT₁/₂ = 44 K and T₁/₂ = 298 K in the first cycle. (b) Repeated heat-cool cycling resulted in the decrease of χT values with a concomitant increase of thermal hysteresis width due to the stabilization of the HS state in the cooling mode. Spin-state switching in 1·CH₃CN is accompanied by pronounced angular distortion, see Table 2, with concomitant self-grinding of the crystals, causing cycle-dependent evolution of the switching characteristics.</figcaption></figure>

<table><caption>**Table 3** Parameters associated with the SCO of 1·CH₃CN obtained from SQUID and DSC studies. Scan rates of 3 K min⁻¹ and 2 K min⁻¹ were used for the SQUID and DSC measurements, respectively</caption><thead><tr><th rowspan="2"></th><th colspan="3">SQUID (sample 1)</th><th colspan="2">DSC (sample 1)</th></tr><tr><th>χT (HS)/cm³ K mol⁻¹</th><th>T₁/₂/K</th><th>ΔT/K</th><th>T₁/₂/K</th><th>ΔT/K</th></tr></thead><tbody><tr><td>Cycle 1</td><td>3.18</td><td>298</td><td>44</td><td>298</td><td>44</td></tr><tr><td>Cycle 2</td><td>3.02</td><td>294</td><td>52</td><td>294</td><td>56</td></tr><tr><td>Cycle 3</td><td>2.79</td><td>294</td><td>51</td><td>293</td><td>60</td></tr><tr><td>Cycle 4</td><td>2.56</td><td>295</td><td>50</td><td>292</td><td>66</td></tr></tbody></table>

revealed that the sample is trapped in the HS state, as depicted in Fig. S21.† The results obtained from continuous cycling depicted in Fig. 5 elucidate that spin-state switching induces self-grinding in the lattice, causing the spin-state switching to become unstable upon repeated cycling. On the other hand, the purposeful removal of the lattice solvent in the SQUID sample chamber traps the complex in the HS state, demonstrating the helping hand of the lattice solvent in mediating spin-state switching.

To check for the reproducible nature of the switching process and the effect of scan rate on the switching characteristics, we have prepared a second batch of 1·CH₃CN crystals and probed their spin-state switching characteristics. When measured at a scan rate of 3 K min⁻¹, the sample underwent temperature-induced bi-stable spin-state switching (Fig. S22†) with a thermal hysteresis width of 48 K and T₁/₂ = 298 K (T₁/₂ = 322 K and T₁/₂ = 274 K). Such results are comparable with the values obtained from the data shown in Fig. 5a (cycle 1). When a crop of fresh crystals obtained from the second batch was studied at a scan rate of 1 K min⁻¹ (Fig. S23†), a thermal hysteresis width of 41 K and T₁/₂ = 304 K (T₁/₂ = 325 K and T₁/₂ = 284 K) were obtained. While the T₁/₂ obtained for the heating branch is comparable to the previous results, the T₁/₂

obtained for the cooling branch markedly differs from that of others. We attribute this difference to the nature of the sequence employed to collect the data. In previous cases, the data were collected from 340 K after keeping the sample at that temperature for thirty minutes in the SQUID chamber, see the Experimental section (S1†) for details. On the other hand, the first cooling branch at 1 K min⁻¹ scan rate was obtained after the first heating branch (Fig. S23†). Such an experimental protocol could have caused a slightly different grinding of the crystals upon spin-state switching. Overall, the reproducible nature of bi-stable spin-state switching in complex 1·CH₃CN is unambiguously elucidated by studying the switching properties of two different batches of the complex.

To check for the LIESST⁴⁴ active nature of 1·CH₃CN, we have prepared a third batch of crystals. Prior to photomagnetic studies, the sample was cooled to 5 K at a scan rate of 3 K under an applied field of 0.1 T. The subsequent irradiation of the complex using red laser light (λ = 637 nm, laser intensity was adjusted to 10 mW cm⁻²) caused an increase of the magnetic moment (Fig. 6a, grey solid circles). A gradual increase of the χT value was observed under laser irradiation. After three hours, χT = about 1 cm³ K mol⁻¹ was obtained, indicating that about 28% of the iron(II) centres are switched to the HS state caused by the LIESST effect and no further increase in the χT value was observed after that. Temperature-dependent investigation of the photoexcited metastable HS fraction in the absence of the laser irradiation resulted in a gradual decrease of χT value in two steps, ultimately reaching values typical of the LS state of the compound at 100 K. The gradual nature of the LIESST curve precluded the determination of specific T(LIESST) temperatures. The heat-cool cycles of the sample performed after the LIESST study indicate that the cooperative spin-state switching is preserved in the sample (Fig. 6a). The T₁/₂ = 305 K and ΔT₁/₂ = 42 K (Table S11†) obtained for the batch in the first cycle (scan rate = 3 K min⁻¹) are comparable to the values obtained for the second batch of the complex

<figure><img src="image_13.png" /></figure>
in the absence of light irradiation is attributed to the population of the zero-field split high energy stark levels of the HS term corresponding to the photoexcited metastable HS 2b·CH₃CN-Y. Heating the sample above 50 K led to an abrupt decrease of the HS fraction back to the LS state, indicating the decay of the metastable HS state. T(LIESST) = 83 K is calculated for the complex from the minimum in the d(χT)/dT versus T curve (data not shown). For the family of iron(II)-BPP complexes, the relation T(LIESST) = 150 – 0.3T₁/₂ was established. Using the relation T(LIESST) = 80 K is calculated for 2b·CH₃CN-Y, comparable with the experimental value of 83 K.

<figure><img src="image_14.png" /><figcaption>**Fig. 7** Temperature-dependent ⁵⁷Fe Mössbauer spectroscopic studies of 1·CH₃CN. The complex was heated from 300 to 335 K, leading to the observation of partial LS → HS switching: 74.6% HS and 25.4% LS. Cooling the sample back to 300 K resulted in the observation of a LS:HS mixture in a 67.3%:32.7% ratio, indicating the hysteretic nature of the spin-state switching process. Cooling down to 200 K resulted in a pure LS state, in agreement with the magnetometry data. Note: 335 K is the maximum achievable temperature limit with the instrument used to collect the ⁵⁷Fe Mössbauer data.</figcaption></figure>

This elucidates that there exists a linear dependence between T(LIESST) and T₁/₂ for 2b·CH₃CN-Y, as previously observed for iron(II) complexes composed of BPP-based and other ligand systems.⁴⁶

### ⁵⁷Fe Mössbauer spectroscopic studies of 1·CH₃CN

In addition to the magnetometry measurements, ⁵⁷Fe Mössbauer spectroscopy measurements were performed on 1·CH₃CN to obtain the spin state composition at different temperatures. Fig. 7 shows the temperature-dependent ⁵⁷Fe Mössbauer spectra of 1·CH₃CN at 300 K and 335 K (heating) and 300 K and 200 K (cooling). The LS and HS contributions were fitted with two discrete doublets and the fit parameters obtained are given in Table 4. At 300 K, 100% LS state of the complex was inferred after fitting the data (Fig. 7 and Table 4). Upon heating to 335 K, the original LS doublet (300 K) lost intensity, and a second HS doublet with higher quadrupole and isomer shift forms. A composition of 74.6% HS and 25.4% LS was obtained based on the fitted resonant areas shown in Table 4. Upon cooling to 300 K, the spin-state equilibrium shifted towards the LS state and a LS:HS ratio of 67.3%:32.7% was obtained. Further cooling to 200 K resulted in a complete spin state switching and the complex was found to be in 100% LS state.

The results obtained from the ⁵⁷Fe Mössbauer spectroscopic studies indicate the existence of hysteretic spin-state switching in 1·CH₃CN, in agreement with the magnetic susceptibility measurements discussed above. However, a 100% HS state at 335 K (heating) and 300 K (cooling) was not observed in the ⁵⁷Fe Mössbauer spectroscopic studies. In line with the thermal behavior of the 1·CH₃CN powder (see below), we attribute this discrepancy to the partial loss of lattice solvent during the Mössbauer studies, where the sample is kept above room temperature for several hours.

### Solution-state spin-state switching characteristic of 1·CH₃CN

In addition to the magnetometry measurements performed on the crystalline form of 1·CH₃CN, magnetometry studies of the complex in the solution phase were performed using a SQUID magnetometer. For the studies, a 7.42 mmol solution of the complex in acetonitrile was used.

Fig. 8 shows the mass magnetization as a function of temperature for three measured cycles. The inset shows the range from 180 to 360 K to highlight the observed thermal hysteresis. The measurement shows a much smaller thermal hysteresis

<table><caption>**Table 4** Fitting parameters obtained from ⁵⁷Fe Mössbauer spectroscopic studies of 1·CH₃CN. Fits were performed with two discrete doublets. δ is the isomer shift relative to α-Fe and ΔE_Q is the quadrupole splitting. The values were rounded to the third decimal</caption><thead><tr><th rowspan="2">Temperature [K]</th><th colspan="3">LS doublet</th><th colspan="3">HS doublet</th></tr><tr><th>δ<sub>LS</sub> [mm s<sup>-1</sup>]</th><th>ΔE<sub>Q(LS)</sub> [mm s<sup>-1</sup>]</th><th>Resonant area [%]</th><th>δ<sub>HS</sub> [mm s<sup>-1</sup>]</th><th>ΔE<sub>Q(HS)</sub> [mm s<sup>-1</sup>]</th><th>Resonant area [%]</th></tr></thead><tbody><tr><td>300</td><td>0.268</td><td>0.548</td><td>100</td><td>—</td><td>—</td><td>0</td></tr><tr><td>335</td><td>0.323</td><td>0.678</td><td>25.4</td><td>0.967</td><td>1.694</td><td>74.6</td></tr><tr><td>300</td><td>0.296</td><td>0.571</td><td>67.3</td><td>0.969</td><td>1.818</td><td>32.7</td></tr><tr><td>200</td><td>0.327</td><td>0.591</td><td>100</td><td>—</td><td>—</td><td>0</td></tr></tbody></table>

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 8** Spin-state switching characteristics of 1·CH₃CN in acetonitrile solution. Mass magnetization of acetonitrile solution of complex 1·CH₃CN as a function of temperature at an applied magnetic field of 0.1 T. For the three heating–cooling cycles presented, the heating branch is shown in red, and the cooling branch is shown in blue, both going to brighter colours with a higher cycle number. Highlights, like the freezing and melting of the solvent, as well as $T_1 = 265$ K, $T_\downarrow = 290$ K, and $\Delta T = 25$ K (all cycle 1) are shown. In contrast to the measurement of the powder form of 1·CH₃CN, the top branch is the heating branch, and the lower branch is the cooling branch, as labelled in the inset: 1. heat and 2. cool.</figcaption></figure>

width than the one observed in the crystalline state with $\Delta T = 25$ K ($T_\uparrow = 265$ K, $T_\downarrow = 290$ K) for the first cycle. In addition, the freezing and melting of the solvent acetonitrile were observed at $T_{freeze} = 203$ K and $T_{melt} = 217$ K, respectively. The thermal hysteresis remains almost the same for all the three cycles performed, indicating the absence of self-grinding in the solution phase. Such a result is contrary to the unstable nature of switching profiles obtained in the solid-state and elucidates the fact that stable hysteretic spin-state switching of the complex can be observed in the solution state.

At present, we are not clear about factors causing hysteretic SCO of 1·CH₃CN in the solution state. Probable ones are aggregation of the complex in the solution and strong intermolecular interactions between the complex entity and acetonitrile solvent molecules.

## Differential scanning calorimetric (DSC) and small- and wide-angle X-ray scattering (SWAXS) studies of 1·CH₃CN

To provide supplementary confirmation for the occurrence of temperature-induced spin-state switching in 1·CH₃CN, differential scanning calorimetric (DSC) studies were performed, as shown in Fig. 9a. The $T_{1/2}$ and $\Delta T_{1/2}$ values obtained from the DSC studies (Table 3) are comparable with the values obtained from the $\chi T$ versus T plots shown in Fig. 5a, confirming the bistable spin-state switching characteristic of 1·CH₃CN.

Enthalpy ($\Delta H$) and entropy ($\Delta S$) variations associated with the LS-to-HS and HS-to-LS switching branches of the first cycle have also been estimated from the DSC plots shown in Fig. 9a. The estimated values of $\Delta H = 13.2$ kJ mol$^{-1}$ (LS-to-HS); 14.4 kJ mol$^{-1}$ (HS-to-LS) and $\Delta S = 41.25$ J mol$^{-1}$ K$^{-1}$ (LS-to-HS);

52.17 J mol$^{-1}$ K$^{-1}$ (HS-to-LS) are comparable with the values reported for iron(II) SCO complexes.$^{47,48}$ The differing values of thermodynamic parameters in the LS-to-HS and HS-to-LS switching branches are attributed to different vibrational modes associated with the LS and HS forms of 1·CH₃CN.

We have also performed SWAXS studies of complex 1·CH₃CN to get a glimpse of lattice variations during the spin-state switching process upon repeated cycling. As shown in Fig. 9b, the SWAXS pattern obtained from a ground sample of 1·CH₃CN is comparable with the calculated pattern obtained from the single-crystal data of the LS crystal. This observation confirming the retainment of crystalline order in the ground sample is remarkable, considering the fact that 1·CH₃CN crystals are ground prior to magnetic measurements. The differences between the SWAXS and calculated patterns obtained at 297 K and 173 K, respectively, are attributed to temperature- and grinding-induced variations of lattice parameters in relation with the displacement of the co-crystallized solvent. Indeed, grinding may strongly impact the structure$^{49}$ and even cause loss of crystallinity and transformation to an amorphous state, as we recently observed for complex c (Fig. 1).$^{35}$ Repeated heat-cool cycles of the ground 1·CH₃CN caused an irreversible transformation of diffraction profiles, serving as the proof of the evolution of the crystalline phases during the repeated heat-cool cycling.

To shed light on the fate of lattice acetonitrile in 1·CH₃CN during the repeated heat-cool cycling, thermogravimetric analyses (TGA) of various forms of the complex were performed. The crystalline 1·CH₃CN underwent a 5% weight loss around 400 K, corresponding to the loss of one molecule of acetonitrile from the lattice (Fig. S27a†). The weight loss obtained from the TGA is comparable with the calculated value of 4.9% for a loss of one molecule of acetonitrile from the lattice. In contrast, no such weight loss is observed for a sample dried at 423 K (Fig. S27a†), revealing the complete removal of acetonitrile molecules from the lattice. TGA analysis of the sample obtained after SWAXS studies also showed a lack of pronounced weight loss (Fig. S27b†) observed for 1·CH₃CN. Moreover, the TGA profiles of the deliberately desolvated sample at 423 K and the sample obtained after the repeated SWAXS measurements are comparable. From the above observations, we infer that repeated spin-state switching of 1·CH₃CN is accompanied by a gradual loss of lattice acetonitrile molecules, forming a 1·xCH₃CN ($x < 1$) lattice with variable solvent content. Note that the solvent removal process during the SWAXS measurements is accelerated due to the *ex situ* heating and cooling of the sample, which is in contrast to the magnetic measurements performed inside a SQUID sample chamber, where the sample is repeatedly scanned under a low-pressure He-atmosphere without exposure to the external conditions. This implies that the switching-induced lattice solvent release could progress differently between magnetometry and SWAXS conditions.

The first synthesis of BPP-COOEt ligand used to prepare 1·CH₃CN discussed in this study was reported almost 20 years ago.$^{36}$ However, the utility of the ligand to prepare iron(II)-

<figure><img src="image_17.png" /></figure>
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Fig. 9** Differential scanning calorimetric (DSC) and small- and wide-angle X-ray scattering (SWAXS) studies of complex 1·CH₃CN. (a) DSC profiles showing temperature-induced spin-state switching in the 210-to-330 K temperature range. (b) SWAXS patterns of 1·CH₃CN showing transitions between crystalline phases in response to repeated heat-cool cycling. The patterns were collected at 297 K after heating and cooling the sample ex situ at 330 K and 77 K, respectively. The irreversible changes in the SWAXS patterns obtained after repeated heat-cool cycling indicates that there exists phase variations contributing to the irreversible alteration of switching parameters, including the hysteresis width, upon repeated heat-cool cycling, as shown in Fig. 5b.</figcaption></figure>

based mononuclear complexes was not reported until 2018.³⁴ The first reported iron(II) complex (a, Fig. 1) of the ligand, showed an irreversible LS-to-HS switching with the switching temperature centred around 330 K. Once heated above 350 K, complex a lost the lattice acetone solvent and the solvent-free version was trapped in the HS-state in the subsequent cooling. Our attempts to study similar complexes as acetonitrile (complex b; Fig. 1) and nitromethane (complexes c and d; Fig. 1) solvates led to the observation of hysteretic spin-state switching.⁹,³⁵ Remarkably, complex c showed stable hysteretic spin-state switching characteristics with T₁/₂ = 288 K; ΔT₁/₂ = 62 K.³⁵

In a different scenario, Sato and co-workers used BPP-COOEt to prepare a mononuclear cobalt(II) complex, which underwent switching of orbital angular momentum due to switching of coordination number from seven-to-six, and *vice versa*,⁵⁰ demonstrating an altogether different switching mechanism contrast to the spin-multiplicity-based switching demonstrated in this study. The above discussion indicates that coordination complexes based on a simple ligand system such as BPP-COOEt can feature exotic magnetic properties.

In a recent study, we observed pronounced molecular distortion upon spin-state switching of complexes **c** and **d** (Fig. 1 and Tables S12 and S13†).³⁵ Such distortion coupled with conformational variation of the Et group stabilizes the complexes either in a LS- or HS-state; creating an energy barrier, whereby thermal hysteresis loops are observed upon spin-state switch-

ing. Recently, Real and co-workers reported a mononuclear iron(II) complex with a wide 105 K thermal hysteresis loop and T₁/₂ = 308 K. They elucidated that molecular distortion and conformational switching cause hysteretic spin-state switching of the complex.⁴ The angular parameters collected in Table 3 reveal that the HS-form of complex 1·CH₃CN features a distorted molecular geometry relative to its LS counterpart showing close to the ideal octahedral geometry. The obtained variations in φ and θ—Δφ = 13.67° and Δθ = 5.97°—indicate that the complex needs to traverse a significant energy barrier imposed by the crystal lattice to switch from one spin-state to another; that is, HS-to-LS and *vice versa*. Moreover, conformation of one of the ethyl groups of 1·CH₃CN varies upon spin-state switching (Fig. 3b and c, blue circles). In the solid-state, positions of the ethyl groups of L1 and LS form of 1·CH₃CN are comparable as can be seen in Fig. 3a and b. To compare the structural variation coupled with spin-state switching in iron(II) complexes composed of BPP-COOEt ligands, we have compiled the difference between the angular parameters of the LS and HS forms of complexes **c**, **d**, and 1·CH₃CN (Fig. 1) in Tables S12–S14.† All the complexes showed pronounced angular variation upon spin-state switching with HS forms of the complexes featuring large Θ values. Remarkably, the ΔΘ(LS-HS) values observed for 1·CH₃CN (322.04°), **c** (302.07°), and **d** (318.22°) are the largest so far observed for BPP-based iron(II) complexes, surpassing the previous best of 215° reported by Halcrow and co-workers.⁵¹
<figure><img src="image_20.png" /></figure>

The successful determination of the structures of LS and HS forms of 1·CH3CN enabled us to compare the angular parameters obtained for the complex with the previously reported iron(II) complexes composed of BPP-based ligands and BPP-COOEt-based complexes (Fig. 10a). The task is made easy by Halcrow and co-workers, who have recently reported a compilation of angular parameters of LS and HS forms of iron(II)-BPP complexes and structural distortion pathways.³⁷ The authors have shown that a select band of HS-complexes (yellow squares in the pale grey region of Fig. 10b) that undergo strong angular distortion upon LS-to-HS switching are capable of showing abrupt and hysteretic spin-state switching.

The angular parameters obtained for the LS form of 1·CH3CN and the related complexes c and d are clustered together in Fig. 10a at the top right corner, as highlighted with a red circle. Similarly, the HS forms of the complexes—1·CH3CN, b, c, and d—are also clustered in closer proximity, as highlighted with a yellow circle. Remarkably, the HS complexes fall in the pale grey region identified by Halcrow and co-workers (Fig. 10b), a position where an iron(II)-BPP complex is expected to show bi-stable SCO with sizable thermal hysteresis loop, albeit rarely. The above points elucidate that iron(II)-BPPCOOEt complexes belong to a not-frequently reported class of SCO systems featuring pronounced angular distortion and the consequent bi-stable spin-state switching.

In the case of HS complex 2a, the angular parameters reveal a less distorted coordination geometry relative to its isomeric HS counterparts—1·CH3CN, b, c, and d. As in Fig. 10a, the φ versus θ point is placed in the dark grey region of the angular landscape (Fig. 10b), where SCO is more probable relative to the structures in the pale grey region. However, complex 2a remains stable in the HS state upon cooling. Such an observation is attributed to the lack of co-crystallized lattice solvent molecules aiding spin-state switching via elastic intermolecular interactions. This attribution is supported by the

SCO active nature of the 2b·CH3CN-Y, crystallizing with lattice acetonitrile. The angular parameters associated with the LS and HS forms of 2b·CH3CN-Y (Table S15†) is placed in the dark grey region (Fig. 10a), where SCO is probable. Crucially, the lack of significant variation of the angular parameters between the HS and LS forms of 2b·CH3CN-Y and the gradual SCO accompanied with no thermal hysteresis serve as an additional proof of the contributing role of distortion in spurring bi-stable SCO in 1·CH3CN.

Overall, complex 1·CH3CN reported in this study is one of the remarkable mononuclear iron(II) spin-crossover complexes showing hysteretic spin-state switching with T₁/₂ centred at RT. However, the SCO is lattice-solvent-dependent and significant variations in the lattice parameters, as inferred from the SWAXS studies, accompany the spin-state switching. Such aspects render the SCO unstable to thermal cycling, impeding the practical utility of the complex as a molecular switch or memory. On the other hand, the attribution that pronounced angular distortion and conformational variation of a functional group contribute to the opening of thermal hysteresis in 1·CH3CN adds to the extensive knowledge collected on SCO systems over almost a century. Remarkably, a direct proof of molecular distortion causing bi-stable SCO in 1·CH3CN can be obtained from the studies on 2b·CH3CN-Y. The HS form of the latter complex is significantly less distorted relative to the HS 1·CH3CN. Moreover, the gradual SCO of 2b·CH3CN-Y is accompanied with much smaller variations of the angular parameters relative to the ones obtained for 1·CH3CN.

As a perspective, the grand old phenomenon, SCO, continues to evolve keeping abreast with the contemporary developments in molecular magnetism and related topics.⁵,⁵²⁻⁶³ It is our opinion, agreeing with the remarks made by the editors of this special collection, that the topic is fascinating to study, one of the pillars of molecular magnetism, and offers opportunities to perform basic science research,⁶⁴ allowing us chemists to elucidate how molecular structure variation imbues a

<figure><img src="image_21.png" /><figcaption>**Fig. 10** Angular parameters of HS and LS states of iron(II)-BPP complexes. (a) Angular parameters of HS and LS forms of complexes 1·CH3CN, b, c, d, and 2b·CH3CN-Y. For complexes b and 2a only HS structures are available, and the angular parameters obtained from the structures are shown. (b) A compilation of angular parameters reported for iron(II)-BPP complexes. (b) is reproduced from the study by Halcrow and co-workers.³⁷ Copyright © 2024 The authors. Open access licensed under CC-BY 4.0.</figcaption></figure>
physical property change, as exemplified in this script relating
molecular distortion with thermal hysteresis.

Conclusions

In this study, we show how structural isomerism influences
spin isomerism. Complexes 1·CH₃CN and 2a featuring BF₄⁻
counter anions are a case. 1·CH₃CN crystallizes with a lattice
solvent and undergoes bi-stable spin state switching, whereas
lattice solvent-free 2a is trapped in the HS state, elucidating
the helping hand of the co-crystallized lattice solvent in facili-
tating spin-state switching. Pronounced angular distortion and
conformation variation of the ethyl substituent accompany
spin-state switching of 1·CH₃CN, causing a sizable thermal
hysteresis width. As far as the angular parameters are con-
sidered, the HS form of 1·CH₃CN features one of the largest
reported values of the distortion parameter (θ) comparable to
the previously reported isostructural solvates (b–c, Fig. 1b) of
the complex. The difference (Δθ) in the values of θ estimated
from the HS and LS structures of the complex series is also the
largest reported for the iron(u)-BPP family of complexes. The
perchlorate anion containing complex of L2—2b·CH₃CN—crys-
tallizes as two polymorphs—2b·CH₃CN-Y and 2b·CH₃CN-R.
The switching in polymorph 2b·CH₃CN-Y, crystallizing with
lattice acetonitrile, is gradual and no hysteresis is observed.
Analysis of the structures of the LS and HS forms of
2b·CH₃CN-Y reveals that the *trans*-N{pyridyl}-Fe-N{pyridyl}
angles (φ) are 177.37° and 178.68°, respectively. This indicates
that switching in 2b·CH₃CN-Y is accompanied by a small vari-
ation of φ and supports our hypothesis that a strong variation
of φ upon spin-state switching contributes to the opening of
the thermal hysteresis loop. Overall, the picture-perfect center-
ing of the T₁/₂ at 298 K and ΔT₁/₂ = 44 K observed for 1·CH₃CN
places the complex as one of the less frequently encountered
mononuclear iron(u) complexes featuring bi-stable spin-state
switching characteristics. The results are encouraging to
pursue iron(u) complexes with a sizable thermal hysteresis
width with T₁/₂ centered in the vicinity of RT, a fundamental
scientific quest in the topic.

Author contributions

S.K.K.: conceptualization, synthesis and characterization of the
ligands and complexes, magnetic measurements, writing of
the original and revised drafts, review and editing. A.M.:
SC-XRD studies, review and editing. L.K.: ⁵⁷Fe Mössbauer and
solution-phase magnetic studies and writing of the corres-
ponding parts, review and editing. S.S.: solution-phase mag-
netic studies and review and editing. B.H.: SWAXS, TGA, and
DSC studies, writing of the corresponding parts, review and
editing. C.B.: SC-XRD studies, review and editing. I.S.: mag-
netic measurements, LIESST measurements, writing of the
corresponding part, review and editing. H.W.: review and
editing and funding acquisition. M.R.: review and editing and

funding acquisition. All the authors have read and agreed to
the submission of the manuscript.

Data availability

Crystallographic data for complexes 1·CH₃CN and 2 have been
deposited at the CCDC under 1560719 (1·CH₃CN, LS, 173 K);
2330938 (1·CH₃CN, LS, 300 K); 2330939 (1·CH₃CN, HS, 345 K);
1913041 (2a, HS, 173 K); 1953349 (2b·CH₃CN-R, LS, 173 K);
1953350 (2b·CH₃CN-Y, LS, 120 K); 2340087 (2b·CH₃CN-Y, HS,
300 K). The data can be obtained from the Cambridge
Crystallographic Data Center.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank Lydia Karmazin, University of Strasbourg,
for the structure determination of L1. Grant Agency Innovation
FRC is acknowledged for the financial support for the project
self-assembly of spin-crossover (SCO) complexes on
graphene. A. M. acknowledges the Alexander von Humboldt
(AvH) Foundation for a postdoctoral fellowship, a Grant-in-Aid
for the JSPS Research Fellowship program, and the financial
support by JSPS KAKENHI Grant Numbers JP22KJ3097 and
JP23K13723. M. R. thanks the DFG Priority Program 1928
"COORNETS" for generous support. L. K. and
H. W. acknowledge the funding by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) -
Project-ID 278162697 - CRC 1242, Project A05. S. S. and
H. W. acknowledge the financial support by the German
Research Foundation (DFG) via the CRC/TRR 247 (Project-ID
388390466, sub-project B02). The authors thank Ulrich von
Hörsten for his technical assistance with the ⁵⁷Fe Mössbauer
spectroscopy measurements. I. S. acknowledges the Slovak
(Slovak Research and Development Agency: APVV-19-0087,
APVV-22-0172, DS-FR-22-0010 and Scientific Grant Agency of
the Ministry of Education of Slovak Republic VEGA 1/0029/22)
and Czech (Grant Agency of the Czech Republic No. 22-
23760S) grant agencies for the financial support. The authors
thank Karlsruhe Institute of Technology (KIT) for providing
open access of the article through programme DEAL.

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