Trigonal antiprismatic mononuclear Cr(II) spin-crossover complexes

Abstract

The engineering of Cr(II) complexes for spin crossover (SCO) phenomena remains poorly understood, primarily due to the limited number of reported examples and the associated challenges in their synthesis. In the context of Cr(II) complexes with octahedral geometry, the pronounced Jahn–Teller distortion in the high-spin (HS) state significantly restricts the selection of co-ligands necessary for SCO to occur. In this study, we successfully synthesized a series of air-stable and solvent-free Cr(II) complexes, [Cr(Tpm^R)₂]X₂ (Tpm^R = Tpm: X = [BF₄]⁻, 1; X = [ClO₄]⁻, 2; Tpm^R = Tpm*: X = [BF₄]⁻, 3; X = [ClO₄]⁻, 4), where the Cr(II) ions adopt a trigonal antiprism (aTPR) geometry rather than the conventional octahedral configuration. Notably, the Tpm-based complexes 1 and 2, with Cr(II) ions in a nearly ideal aTPR geometry, exhibit complete SCO behavior with transition temperatures (T_1/2) of 190 K for 1 and 194 K for 2, respectively. In contrast, the Tpm*-based complexes 3 and 4 display considerable axial elongation within the aTPR structure, leading to abrupt but incomplete SCO transition at a much lower temperature for 3 (T_1/2 = 142 K) while 4 remains predominantly in the HS state. Further structural analysis reveals that substituting [BF₄]⁻ with [ClO₄]⁻ in 4 results in increased elongation and a denser packing arrangement that disfavour the spin transition. Our findings suggest that the aTPR configuration is advantageous for facilitating spin crossover, while the dynamics of spin transitions in these complexes are closely linked to the structural distortions of the [Cr(Tpm^R)₂]²⁺ cations. Furthermore, all complexes demonstrate remarkable resistance to oxygen and maintain air stability, which can be attributed to their compact space-filling structures that effectively impede the diffusion of oxygen into the interstitial regions of the crystal lattice.

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Introduction

Spin crossover (SCO) complexes involving 3d transition metal ions with d⁴–d⁷ electronic configurations have attracted significant attention due to their potential applications in molecular switches, sensors, and information storage devices.^1–4 These molecules undergo transitions between low spin (LS) and high spin (HS) states through electron pairing and dissociation, exhibiting magnetic bistability in response to various stimuli like temperature, light, magnetic field, or pressure.⁵ In addition to significant alterations in magnetism, the SCO process is also correlated with substantial changes in volume, colour, and optics, endowing these materials with additional synergistic functionalities, such as conductivity,⁶ catalysis,^7,8 thermal expansion,^9,10 and luminescence.^11–13 Among the most extensively studied systems are Fe(II)-based SCO complexes,^14–16 which have showcased considerable advantages in synthesis, chemical modification, and systematic regulation towards the development of high-performance SCO complexes with room-temperature transitions,^17–19 multi-stepped transitions,^20–22 or wide thermal hysteresis.²³ These performances are highly desirable for practical applications. Furthermore, SCO complexes based on other transition metal ions, such as Fe(III),^24–26 Co(II/III),^27–30 and Mn(III),^31–33 have also been comprehensively investigated, except for Cr(II).

Indeed, the synthesis of Cr(II, 3d⁴) complexes has long been a challenge due to their inherent instability (extreme air sensitivity), difficulty in isolation, and high reactivity.³⁴ Despite these obstacles, Cr(II) complexes have garnered significant attention with vast potential in molecular magnets,^35–37 especially for developing high-temperature magnetic semiconductors.^38–40 However, progress in Cr(II)-based SCO complexes remains very limited, with only four examples reported to date.^41–45 From a thermodynamic perspective, the modest spin-entropy change (ΔS = 1) between the LS (S = 1) and HS (S = 2) states for a d⁴ system disfavours the spin transition. Additionally, for an octahedral geometric system, its SCO process is essentially associated with a severe Jahn–Teller distortion (Scheme 1a), which imposes further constraints on the conditions conducive to the transition.^46–48 Moreover, the Cr(II) ion exhibits strong MLCT (metal-to-ligand charge transfer) ability.^49–51 Consequently, the ligands suitable for constructing Cr(II)-SCO complexes are poorly selective, and strategies to simply replicate the preparation and regulation of other SCO complexes are not feasible.

Scheme 1 The orbital splitting and electronic configurations for the LS and HS states of a Cr(II) ion (d⁴) within an (a) octahedral and (b) trigonal antiprism geometry.^46–48

It was believed that a Cr(II)-SCO complex might necessitate a strong ligand field, such as using phosphine as coordinated ligands. However, recent research by Becker et al. demonstrated that a reduced “molecular ruby”, [Cr^II(ddpd)₂][BF₄]₂ (ddpd = N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine), exhibits gradual thermal SCO behaviour.⁴⁴ Although the HS structure was not achieved above room temperature due to desolvation, this finding highlights the potential for constructing Cr(II)-based SCO complexes using conventional N-donor ligands. Five-membered N-heterocyclic ligands, known for their moderate ligand field, can be tailored by modifying the substitution groups on the rings, making them suitable for exploring SCO mechanisms.^17,52,53 For instance, the fac-tripodal ligands of trispyrazoyl methane (Tpm) or trispyrazoyl borate (Tp⁻), along with their derivatives, have been extensively studied in the context of SCO complex construction.^54–56 Especially, these ligands can facilitate metal ions in a trigonal antiprism (aTPR) geometry rather than an octahedral one, which may be beneficial for minimizing the Jahn–Teller effect and thus realizing Cr(II)-SCO complexes (Scheme 1b). In this frame, fine-tuning the coordination environment with selective ligands or functionalized electron-donating/withdrawing substituent groups may readily modulate the SCO event across a wide temperature range.

In this study, we investigated Tpm and Tpm* (tris(3,5-dimethylpyrazol)methane) as chelate ligands and prepared a series of air-stable and solvent-free Cr(II) complexes, [Cr(Tpm^R)₂]X₂ (Tpm^R = Tpm: X = [BF₄]⁻, 1; X = [ClO₄]⁻, 2; Tpm^R = Tpm*: X = [BF₄]⁻, 3; X = [ClO₄]⁻, 4). In complexes 1 and 2, Cr(II) ions are situated in nearly perfect aTPR geometries with almost uniform Cr–N bonds, whereas significant axial elongation is observed in complexes 3 and 4. As a result, both 1 and 2 exhibit thermally induced gradual but complete SCO behaviour with transition temperatures of 190 and 194 K, respectively; Complex 3 shows an abrupt but incomplete transition at 142 K, while complex 4 remains in the HS state below 300 K without showing SCO behaviour due to the serious axial elongation. This study highlights the crucial role of coordination geometry in dictating the SCO behaviour of Cr(II) complexes and provides insights into the underlying mechanisms of these transitions.

Results and discussion

Syntheses and structural descriptions

In an inert atmosphere, CrCl₂ and the tripodal ligand of Tpm (or Tpm*) were treated in a 1 : 2 ratio in a MeCN/H₂O mixed solution overnight. Afterward, NaBF₄ (or LiClO₄) was added, and the solution was slowly evaporated at room temperature to yield yellow-green block crystals of complexes 1 to 4 in a range of 53–73% yield. Notably, despite the requirement of an inert atmosphere for sample preparation, the crystals obtained are stable in air. Thermogravimetric analysis (TGA) revealed that the weights for all these solvent-free complexes remained unchanged before decomposing at about 180 °C for 1–3, and 240 °C for 4, confirming the absence of crystallized solvent molecules in the lattice and relatively thermal stability of the complexes (Fig. S1–4†). Notably, the abrupt weight increase followed by a sharp drop for 1 and 2 at about 290 °C is attributed to the sample explosion. This is more likely correlated with the N-rich Tpm ligands (N: 39.23%) rather than the counter anions, as both compounds exhibit such behaviour irrespective of ClO₄⁻ or BF₄⁻ anions. For 3 and 4, the addition of six methyl groups per Tpm* ligand significantly reduce the N percentage (N: 28.17%), which may account for the absence of explosion during the measurements. The phase purity for the bulk samples was verified by powder X-ray diffraction (PXRD) experiments (Fig. S5–8†).

Single crystal X-ray diffraction (SCXRD) data for complexes 1–3 were collected at both low (100 K) and high temperatures (340, 360 or 298 K), excluding 4, which was solely collected at 100 K. Complexes 1, 2, and 4 crystallize in the monoclinic space group P2₁/n, while 3 crystallizes in the space group C2/c (Tables S1 and S2†). In all the complexes, the Cr(II) ion is situated at an inversion center and coordinated to two tripodal Tpm^R ligands, resulting in a trigonally distorted octahedral geometry (Fig. 1), with the Bailar twist angles between two staggered triangles being about 60° (Table S5†). At high temperatures, the Cr–N bond distances in 1 and 2 are nearly equal, ranging from 2.170(2)–2.186(2) Å and 2.171(9)–2.234(9) Å, respectively, with the mean values of 2.177(2) and 2.198(9) Å. The intra-ligand bite angles (N_Tpm–Cr–N_Tpm, both N-atoms from the same chelate ligand) are quite acute within the range of 82.39(8)–83.25(8)° and 82.0(3)–83.6(3)°, respectively. Conversely, the inter-ligand cis N–Co–N angles (N-atoms from the different chelate ligand) are obtuse, spanning 96.75(8)°–97.61(8)°and 96.94(13)–97.32(12)° for 1 and 2, respectively. These parameters suggest a significant contraction along the C₃ axis from an ideal octahedron, yielding a trigonal antiprism geometry. At 100 K, the Cr–N bond distances for both complexes decrease significantly to the range of 2.063(2)–2.068(2) Å (Cr–N_avrg. = 2.065(2) Å), characteristic of the LS Cr(II) ion, suggesting the occurrence of an SCO event. The intra- and inter-ligand bite angles at 100 K are 84.44(8)–85.92(8)°, 94.08(8)°–95.56(8)° for 1, and 84.28(6)–86.07(6), 93.96(6)–95.72(6)° for 2, respectively (Table S3†).

Fig. 1 Crystal structures of (a) 1 (100 K), (b) 2 (340 K), (c) 3 (298 K), and (d) 4 (100 K); the bond distance of Cr1–N1 and the intra-ligand bite bond angle of N1–Cr1–N2 are 2.068(2) Å/85.92(8)°, 2.186(2) Å/83.25(8)°, 2.304 Å/83.15(8)°, and 2.453 Å/81.96(8)°, respectively. Thermal ellipsoids are depicted at the 30% probability level. All the hydrogen atoms, counterions, and disorders are omitted for clarity. (e) The photos of the crystal of 3 at 298 and 100 K.

For 3, the intra-ligand bite angles (N_Tpm*–Cr–N_Tpm*) of 82.36(8)–83.15(8)° at 298 K indicate that the Cr(II) ion also maintains within a trigonal antiprism geometry, albeit with significantly inequivalent Cr–N bond distances: Cr1–N1/N1A = 2.304(2) Å (elongated), Cr1–N2/N2A = 2.184(2) Å, and Cr1–N3/N3A = 2.115(2) Å (compressed). The average Cr–N distance of 2.201(2) Å suggests that the Cr(II) ion is in the HS state. At 100 K, the Cr–N bond distances decrease to 2.272(13)/2.188(13), 2.094(11)/2.089(12), and 2.096(11)/2.090(12) Å, averaging at 2.138(13) Å, implying an incomplete SCO transition to an intermediate state with mixed HS and LS ions, which likely contributes to the two-fold disorder of the Tpm* ligands. For 4, the average Cr–N bond distance of 2.231(2) Å and the more acute intra-ligand bite angles (80.75(8)–81.96(8)°) at 100 K demonstrate that the Cr(II) ion stays in the HS state within a more distorted trigonal antiprism geometry: Cr1–N1/N1A = 2.453(2) Å (very elongated), and compressed Cr1–N2/N2A = 2.115(2) Å and Cr1–N3/N3A = 2.126(2) Å. For the packing structures, the [Cr(Tpm^R)₂]²⁺ cations in 1–3 are fairly isolated (Fig. S9–11†), while C–H⋯π couplings (3.58 Å) between adjacent [Cr(Tpm*)₂]²⁺ units are found for 4 (Fig. S12 and S13†). The nearest Cr⋯Cr distances are 8.032, 8.018, 9.898 and 8.319 Å for the HS phases of 1–4, respectively (Table S4†).

Magnetic studies

Variable-temperature magnetic susceptibility measurements were conducted on microcrystalline samples of complexes 1–4 in the temperature range of 2–300 K, under an applied direct current field of 1 kOe and a sweep rate of 3 K min⁻¹ (Fig. 2). The χT products for 1–4 at room temperature are 2.90, 2.89, 2.91, and 2.88 cm³ mol⁻¹ K, respectively, in good agreement with the value expected for an isolated HS Cr(II) ion (S = 2, g = 1.97). Upon cooling, the χT products for both 1 and 2 undergo a gradual one-step decrease, centered around 190 and 194 K (T_1/2), respectively, then stabilizing at approximately 1.0 cm³ mol⁻¹ K within the 130 and 30 K range. This value is consistent with the anticipated result for a LS Cr(II) ion (S = 1, g = 2), indicating the occurrence of a complete SCO transition from the HS to the LS state for the Cr(II) ions. The rapid decrease observed below 20 K can likely be attributed to spin–orbital coupling (SOC) as well as possible intermolecular antiferromagnetic interactions. The heating curve displayed a similar trend to that of the cooling one, with no evidence of thermal hysteresis. It should be mentioned that the magnetic transition may be also proceeded via a valence tautomerism mechanism since a Cr(II) compound holds internal metal–ligand electron transfer possibility. However, both high-temperature (HT) and low-temperature (LT) average Cr–N bond distances of 2.177(2)/2.065(2) Å for 1 (HT/LT), 2.198(9)/2.065(2) Å for 2 (HT/LT), and 2.201(2)/2.154(13) Å for 3 (HT/LT) are significantly larger than that of Cr(III) species⁵⁷ (∼2.03 Å). Additionally, the CV measurements (vide infra) do not reveal any redox signals for Tpm or Tpm* ligands (Fig. S14 and S15†). These results support the presence of Cr(II) instead of Cr(III) at low temperatures.

Fig. 2 Variable-temperature magnetic susceptibility data of 1–4 collected in the dark (1 kOe). Solid lines are guides for the eye.

For 3, upon cooling, the χT product remains constant above 145 K, followed by an abrupt drop centered at 142 K (T_1/2), and reaches a quasi-plateau of about 2.0 cm³ mol⁻¹ K at 120–25 K. This value corresponds to a mixture of 50% of HS and 50% of LS Cr(II) ions, demonstrating that only half of the Cr(II) ions undergo an SCO transition. Notably, the crystal data for 3 at 100 K does not reveal a mixture of HS and LS components, suggesting an intermediate average phase typical of SCO systems.¹ The χT product further decreases to 1.14 cm³ mol⁻¹ K at 2 K due to the SOC effect and intermolecular antiferromagnetic interaction. The heating branch coincided with the cooling one without hysteresis. The χT product for 4 kept essentially unchanged before a final drop below 20 K, indicating that 4 remains in the HS state above 2 K. The spin crossover behaviour is also tracked by differential scanning calorimetry measurements. As shown in Fig. S16,† compound 3 exhibited a pair of exothermic and endothermic peaks at about 141 K, consistent with the transition temperature of SCO, indicating a first-order phase transition. However, for 1 and 2, no exothermic or endothermic peaks associated with SCO were observed, likely due to the small entropy change associated with the gradual spin transition.

Magneto-structural correlation

Considering the structures of complexes 1–4, it is noteworthy that all Cr(II) ions adopt a trigonal antiprismatic (aTPR) geometry instead of the typical octahedral configuration. The primary distinction among these complexes lies in the extent of the axial elongation of the Cr(II) centers, which significantly influences their magnetic behaviours.

For a Cr(II) ion (d⁴) in octahedral configuration, the SCO process is readily hindered by the Jahn–Teller distortion, which would lower the energy gap between e_g and t_2g orbitals, effectively decreasing the ligand field strength and stabilizing the high-spin state. However, for the Tpm-based complexes 1 and 2, the Cr(II) ions maintain a nearly ideal aTPR geometry with uniform Cr–N bonds. In this configuration, with the C₃ axis oriented along the z-direction, the high-energy d_xz and d_yz orbitals remain almost degenerate in both HS and LS states. This results in a stable and moderate crystal field that facilitates SCO to occur, as observed in both complexes. Notably, there are no significant intermolecular interactions present in either complex, and variations in the counter anions do not lead to substantial changes in the local coordination environment or packing structures (Fig. S17†). This stability accounts for their closely aligned transition temperatures (1, T_1/2 = 190 K; 2, T_1/2 = 194 K).

The influence of substituted methyl groups of Tpm derivatives on the SCO event is intricate, closely correlated with the substitution sites. For example, methylation at 4-position typically enhances the ligand field and elevates the transition temperature. In contrast, substitutions at 3- or 5-positions generally weaken the ligand field and lowers the transition temperatures.⁵⁸ Notably, the additional steric effect, particularly pronounced for 3-position substitutions, cannot be overlooked as it may substantially hinder Cr–N bond contraction during the SCO process, potentially resulting in an incomplete transition or persistence of the HS state. In compounds 3 and 4, the methylations at 3,5-position on Tpm are supposed to weaken the ligand field, leading to a decrease in the spin transition temperature. Meanwhile, the additional steric hindrances induce notable axial elongation within the aTPR geometry of [Cr(Tpm*)₂]²⁺. This distortion is characterized by axial elongation along the Cr–N1 axes, measuring 2.304(2) Å for complex 3 at 298 K and 2.453(2) Å for complex 4 at 100 K. As a consequence, this structural distortion, coupled with the effects of substitution, greatly reduces the ligand field splitting energy, resulting in a lowered transition temperature of T_1/2 = 142 K for complex 3 and an overall HS state for complex 4. Regarding the incomplete spin crossover for 3, it is noteworthy that the [Cr(Tpm)₂]²⁺ units along the nearest intermolecular packing direction in 3 adopt a staggered arrangement, characterized by a torsion angle of 63.472(7)° between the pseudo-C3 axis, differing from the parallel configuration in 1 and 2 (Fig. S18†). The interlaced arrangement of the [Cr(Tpm)₂]²⁺ units may disrupt the effective ferroelastic interactions between SCO centers, leading to an incomplete spin transition in 3. In order to investigate if such supramolecular interactions could be released in the solution to achieve a complete spin transition for 3 or a possible SCO for 4, we have also measured the variable temperature magnetic susceptibility for 2 and 4 in MeCN solution (Fig. S19†). However, both compounds remain in the HS state over the temperature range of 100–350 K (note that the abnormality observed at approximately 227 K is due to the freezing of acetonitrile). This may be attributed to the complicated solution environment such as the polarity of solvents, and the intermolecular interactions between [Cr(Tpm^R)₂]²⁺ and solvent molecules which play an important role on the metal spin states. Nevertheless, we propose that both intramolecular steric effects and interlaced packing arrangement may account for the incomplete SCO in 3. In the case of 4, the more significant structural distortion leads to a weaker ligand field, which may be inadequate to trigger an SCO event.

Moreover, the increased axial elongation observed in 4 compared to 3 can be attributed to the change in counter anions, which significantly alters the crystal packing due to short C–H⋯X contacts (where X = F in 3 and O in 4). Regarding the C–H⋯X contacts below van der Waals radii (3.17 Å for C–H⋯F and 3.22 Å for C–H⋯O), each BF₄⁻ anion in 3 interacts with one adjacent cation (d_C⋯F = 3.178 Å), resulting in an isolated packing structure (Fig. 3a). In contrast, complex 4 has each ClO₄⁻ anion connecting to three adjacent cations via multiple C–H⋯O interactions (d_C⋯O = 3.117–3.204 Å), leading to the formation of a supramolecular network (Fig. 3b). Consequently, complex 4 presents a more compact packing arrangement, as indicated by shorter Cr⋯Cr distances (8.319 Å in 4versus 9.898 Å in 3) and significant C–H⋯π interactions (3.58 Å) between adjacent [Cr(Tpm*)₂]²⁺ cations.

Fig. 3 Supramolecular arrangements of 3 (a) at 298 K and 4 (b) at 100 K. Dashed lines correspond to short C⋯F (3.178 Å) and C⋯O (3.117–3.204 Å) contacts.

To further analyze the intermolecular interactions between complexes 3 and 4, we conducted Hirshfeld surface analyses.⁵⁹ The primary supramolecular interactions in the [Cr(Tpm*)₂]²⁺ units arise from adjacent cations and counter anions, focusing on the dominant H⋯F(O) and H⋯C contacts. As illustrated in Fig. 4 and S20,† both complexes demonstrate weak interactions with their surrounding counter anions, featuring similar contributions for H⋯F(O) contacts – 26.3% for 3 and 24.6% for 4. Notably, the H⋯C interactions are significantly stronger in 4 (12%) compared to 3 (8%). This observation aligns with the packing patterns, showing that the [Cr(Tpm*)₂]²⁺ units in 3 are relatively isolated, whereas 4 exhibits a denser packing arrangement characterized by substantial intermolecular C–H⋯π interactions. These findings suggest that the tighter packing facilitated by ClO₄⁻ ions, in contrast to BF₄⁻ ions, enhances the intermolecular interactions, leading to a greater distortion of the [Cr(Tpm*)₂]²⁺ cation in 4.

Fig. 4 Hirshfeld surface of [Cr(Tpm*)] cations of 3 (a) at 298 K and 4 (b) at 100 K mapped with d_norm function, and percentage contributions (c) of the various close intermolecular contacts to the Hirshfeld surface area.

Air stability

The electrochemical properties of complexes 1 and 3, chosen as the representatives, were investigated using cyclic voltammetry (CV) in a degassed acetonitrile solution at room temperature. As shown in Fig. 5a and b, a set of reversible redox potentials was observed at E_1/2 = −0.79 and −0.90 V (versus ferrocene) for 1 and 3, respectively, which were assigned to the one-electron oxidation to [Cr^III(Tpm^R)₂]³⁺ states, indicating a relatively strong oxidizing ability of [Cr^II(Tpm^R)₂]²⁺ ions. However, the crystals of title complexes demonstrate remarkable oxygen resistance and air stability for over six months, likely attributed to their unique molecular arrangement and steric hindrance. Specifically, in terms of the space-filling pattern, all the complexes form densely packed structures with interstitial space filled by counterions, leaving no accessible voids smaller than 2.7 ų for guest molecules, thereby shielding Cr(II) ions from oxygen. Additionally, despite a more negative redox potential for 3 than 1 due to the presence of methyl groups acting as electron donors, both 3 and 4 exhibit superior air stability compared to 1 and 2. This is attributed to the increased ligands’ spatial hindrance (Tpm* > Tpm) that further improves their resistance to oxygen. To further understand the shielding effect, the buried volume descriptor (%V_Bur) was introduced to quantify the steric hindrance.^60,61 As shown in Fig. 5c–f, the topographic steric maps of the HS [Cr(Tpm^R)₂]²⁺ in 3 and 4 result in a %V_Bur value of 95.9–96.3%, significantly larger than that (87.0–87.3%) found in 1 and 2. This finding underscores the ability of the 3,5-positioned methyl groups to reduce the likelihood of Cr(II) ions encountering oxygen molecules, thereby strengthening the air stability of the [Cr(Tpm*)₂]²⁺ cations.

Fig. 5 Cyclic voltammograms of 1 (a) and 3 (b) in MeCN solution at room temperature; c–f correspond to topographic steric maps of 1 (340 K), 2 (360 K), 3 (298 K) and 4 (100 K), respectively. Red to blue indicate the more- and less-hindered zones.

In comparison with the previously reported Cr(II) SCO compounds that bears phosphine or carbon as coordinated ligands, the Cr(II) ions in 1–4 reside in a weaker ligand field of Cr–N6 coordination environment (Table S6†). Whereas, compounds 1–3 display typical SCO behaviour with comparable transition temperatures, potentially due to the aTPR geometry that provides a stable and moderate crystal field conducive to SCO events. Moreover, the position of substituted groups on the ligands is found to have a substantial effect on the magnetic properties of the complexes, via both electronic and steric effects. In this frame, the number and position of substituents, combined with molecular symmetry constraints, offer extensive opportunities for tuning the magnetic ground state.

Conclusions

In summary, we successfully synthesized a series of air-stable mononuclear Cr(II)-based SCO complexes, in which the Cr(II) ions adopt a trigonal antiprismatic (aTPR) geometry within an all-N sphere. Specifically, the Cr(II) ions in the Tpm-based complexes (1 and 2) exhibit a nearly ideal aTPR geometry, while the Tpm*-based complexes (3 and 4) display significant axial elongation. Consequently, complexes 1 and 2 demonstrate complete SCO at comparable transition temperatures (1, T_1/2 = 190 K; 2, T_1/2 = 194 K). In contrast, complex 3 undergoes an abrupt but incomplete SCO transition at a substantially lower temperature (T_1/2 = 142 K), while 4 remains predominantly in the HS state due to its larger elongation. Further structural analysis reveals that the substitution of [BF₄]⁻ with [ClO₄]⁻ in 4 results in increased elongation and a denser packing arrangement that disfavours the spin transition, as evidenced by shorter C⋯X (X = F for 3, O for 4) contacts. Moreover, all complexes exhibit remarkable resistance to oxygen and maintain air stability, attributed to their compact, space-filling structures that effectively hinder oxygen diffusion into the interstitial regions of the crystal lattice. Our findings provide valuable insights into the design and understanding of Cr(II)-based SCO complexes through geometric control.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22173043, 22473054) and the Shenzhen Science and Technology Program (no. JCYJ20220818100417037).

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Footnote

† Electronic supplementary information (ESI) available: Selected bond lengths and angles, crystal structures, thermal gravimetric analysis data, powder X-ray diffraction data and magnetic details. CCDC 2385013–2385019. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02973a

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