On the nature of the plateau in two-step dinuclear spin-crossover complexes

A remarkable feature of the spin-crossover process in several dinuclear iron(II) compounds is a plateau in the two-step transition curve. Up to now, it has not been possible to analyse the spin state of dinuclear pairs that constitute such a plateau, due to the relative high temperatures at which the transition takes place in complexes investigated so far. We solved this problem by experimentally studying a novel dinuclear spin-crossover compound [[Fe(phdia)(NCS)(2)](2)(phdia)] (phdia: 4,7-phenanthroline-5,6-diamine). We report here on the synthesis and characterisation of this system, which exhibits a two-step spin transition at T(c1)=108 K and T(c2)=80 K, displaying 2 K and 7 K wide thermal hysteresis loops in the upper and the lower steps, respectively. A plateau of approximately 20 K width centred at about 90 K, which corresponds to the 50 % of the spin conversion, separates the two transitions. The composition of the plateau was identified in metastable state after quenching to 4.2 K by means of M?ssbauer spectroscopy in an external magnetic field. Such experiments revealed that the plateau consists mainly of [HS-LS] pairs (HS=high spin, LS=low spin) and confirmed the hypothesis that the spin conversion in dinuclear entities proceeds through [LS-LS]<-->[HS-LS]<-->[HS-HS] pairs. The results are discussed in terms of a thermodynamic model based on the regular solution theory adapted for dinuclear spin-crossover compounds.     

Structural and magnetic resolution of a two-step full spin-crossover transition in a dinuclear iron(II) pyridyl-bridged compound

A dinuclear iron(II) complex containing the new pyridyl bridging ligand, 2,5-di(2',2'-dipyridylamino)pyridine (ddpp) has been synthesised and characterised by single-crystal X-ray diffraction, magnetic susceptibility and M?ssbauer spectral methods. This compound, [Fe(2)(ddpp)(2)(NCS)(4)]4 CH(2)Cl(2), undergoes a two-step full spin crossover. Structural analysis at each of the three plateau temperatures has revealed a dinuclear molecule with spin states HS-HS, HS-LS and LS-LS (HS: high spin, LS: low spin) for the two iron(II) centres. This is the first time that resolution of the metal centres in a HS-LS ordered state has been achieved in a two-step dinuclear iron(II) spin-crossover compound. Thermogravimetric data show that the dichloromethane solvate molecules can be removed in two distinct steps at 120 degrees C and 200 degrees C. The partially de-solvated clathrate, [Fe(2)(ddpp)(2)(NCS)(4)]CH(2)Cl(2), undergoes a one-step transition with an increased transition temperature with respect to the as synthesised material. Structural characterisation of this material reveals subtle changes to the coordination geometries at each of the iron(II) centres and striking changes to the local environment of the dinuclear complex. The fully de-solvated material remains high spin over all temperatures. Interestingly, the solvent can be re-introduced into the monosolvated solid to achieve complete conversion back to the original two-step crossover material, [Fe(2)(ddpp)(2)(NCS)(4)]4 CH(2)Cl(2).     

First dicyanamide-bridged spin-crossover coordination polymer: synthesis, structural, magnetic, and spectroscopic studies

We report here on the synthesis and characterisation of a first iron(II) spin-crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4.MeOH (aqin=8-aminoquinoline, dca=dicyanamide), which displays a two-step complete spin transition. Variable-temperature magnetic susceptibility measurements and M?ssbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and M?ssbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin-state structures belong to the triclinic P1 space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe-Fe distances through the dca ligand are 8.119(1) and 7.835(1) A in the high-spin and low-spin structures, respectively. The polymeric chains extend along a (1, 0, -1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by pi-stacking interactions and H-bonding between the H-donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two-step spin-crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity.     

On the role of intermolecular interactions on structural and spin-crossover properties of 2D coordination networks [Fe(bbtr)3]A2 (bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO4(-), BF4(-))

A series of complexes [M(bbtr)₃]A₂ (M=Fe^II, Zn^II; bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO₄^?, BF₄^?) and [Fe_xZn_1?x(bbtr)₃](ClO₄)₂ (0<x<1) dilute systems was synthesized and characterized. Earlier studies on [Fe(bbtr)₃](ClO₄)₂ ( 1?ClO₄ ), which crystallizes in space group P$\bar 3A series of complexes [M(bbtr)(3)]A(2) (M=Fe(II), Zn(II); bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO(4)(-), BF(4)(-)) and [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) (0相似文献   

Photoexcitation and relaxation dynamics of catecholato-iron(III) spin-crossover complexes.

The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF(6) (-), BPh(4) (-); R-Cat=catecholate dianion substituted by R=NO(2), Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below approximately 40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, k(HL)(T-->0), vary in the range from 0.58 to 8.84 s(-1). These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.     

Nanoscale self-hosting of molecular spin-states in the intermediate phase of a spin-crossover material

A new spin-crossover (SC) complex [Fe(II)H(2)L(2-Me)][AsF(6)](2) has been synthesized, in which H(2)L(2-Me) denotes the chirogenic hexadentate N(6) Schiff-base ligand bis{[(2-methylimidazol-4-yl)methylidene]-3-aminopropyl}ethylenediamine. This complex has revealed a rich variety of phases during its two-step thermal crossover, as well as photoinduced spin-state switching. A high-symmetry high-spin (HS, S=2) phase, a low-symmetry low-spin (LS, S=0) phase, an intermediate phase characterized by an unprecedented lozenge pattern of 12 predominantly HS molecular crystallographic sites confining 18 predominantly LS molecular crystallographic sites, and a photoinduced low-symmetry HS phase have been accurately evidenced by temperature-dependent magnetic susceptibility, M?ssbauer spectroscopy, and crystallographic studies. This variety of phases illustrates the multi-stability of this system, which results from coupling between the electronic states and structural instabilities.     

Electronic, vibrational, and structural properties of a spin-crossover catecholato-iron system in the solid state: theoretical study of the electronic nature of the doublet and sextet states

As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form to be identified. In addition, the vibrational progression observed in the NIR absorption of the LS form was assigned to a five-membered chelate ring mode. The X-ray crystal structure solved at two different temperatures, shows the presence of highly distorted pseudo-octahedral ferric complexes that occupy two nonequivalent crystalline sites. The variation of the molecular parameters as a function of temperature strongly suggests that the two-step transition proceeds by a successive transition of the species in the two nonequivalent sites. The thermal dependence of the high-spin fraction of metal ions determined by M?ssbauer experiments is consistent with the magnetic data, except for slight deviations in the high temperature range. The optimized geometries, the electronic transitions, vibrational frequencies, and thermodynamic functions were calculated with the B3LYP density functional method for the doublet and the sextet states. The finding of a ground state that possesses a significant mixture of Fe(III)-catecholate and FeII-semiquinonate configurations is discussed with regard to the set of experimental and theoretical data.     

Interplay between kinetically slow thermal spin-crossover and metastable high-spin state relaxation in an iron(II) complex with similar T1/2 and T(LIESST)

This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL(2)][BF(4)](2).x H(2)O (L=2,6-bis[3-methylpyrazol-1-yl]pyridine; x=0-1/3) at 1 K min(-1) leads to a cooperative spin transition, taking place in two steps centered at 147 and 105 K, that is only 54 % complete by magnetic susceptibility. Annealing the sample at 100 K for 2 h results in a slow decrease in chi(M)T to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL(2)][BF(4)](2).x H(2)O are isostructural (C2/c, Z=8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z=24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110+/-20 K when x=1/3 and 155+/-5 K when x=0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10 K, and in approximately 70 % yield by rapidly quenching the sample to 10 K. This metastable high-spin state relaxes back to the low-spin ground state at 87+/-1 K in one, not two, steps, and without passing through the intermediate phase. This implies that thermal spin-crossover and thermally activated high-spin-low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T(LIESST) being greater than T(1/2).     

Heteroleptic FeII Complexes of 2,2′‐Biimidazole and Its Alkylated Derivatives: Spin‐Crossover and Photomagnetic Behavior

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO₄)₂?0.5H₂O ( 1 ), [Fe(TPMA)(XBIM)](ClO₄)₂ ( 2 ), and [Fe(TPMA)(XBBIM)](ClO₄)₂ ?0.75CH₃OH ( 3 ), were prepared by reactions of Fe^II perchlorate and the corresponding ligands (TPMA=tris(2‐pyridylmethyl)amine, BIM=2,2′‐biimidazole, XBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐biimidazole, XBBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bibenzimidazole). The compounds were investigated by a combination of X‐ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T_1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T_1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high‐spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter‐ and intramolecular interactions in 1 and 2 . The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3 , steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe^II coordination environment, which stabilizes the high‐spin state of the complex. Both 1 and 2 exhibit a photoinduced low‐spin to high‐spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high‐spin state. For 1 , the decay rate constant follows the single‐exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high‐spin state at 50 K are similar to those determined for the high‐spin state at 295 K. This study shows that N‐alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.     

Tuning haptotropic rearrangements of arene-chromium complexes: steric and electronic effects of phosphorus coligands

(Eta6-arene)tricarbonylchromium 2 was synthesised by [3+2+1] benzannulation of the Fischer carbene complex 1 and converted to the thermodynamically more favorable regioisomer 3 by haptotropic metal migration. Photo-induced ligand-exchange reactions in both regioisomers with triphenylphosphine, triphenylphosphite, trimethylphosphine, and trimethylphosphite afforded dicarbonyl(phosphine or phosphite)arene complexes 4-11. The regioisomers were separated by high-performance liquid chromatography (HPLC), and kinetic analyses of the thermo-induced haptotropic metal shift were performed with regioisomers 4, 6, 8, and 10. The kinetic parameters were compared with those obtained for the parent tricarbonyl complex 2 and were discussed in terms of the steric and electronic properties of the phosphorus ligands by applying a quantitative analysis of ligand effects (QALE). The molecular structures of regioisomeric PPh3 and P(OPh)3 complexes 4/5 and 6/7 as well as of P(OMe)3 complex 10 have been established by single-crystal X-ray analysis.     

Molecular statistical model of spin crossover equilibrium in the crystal state taking into account the phenomenon of ordering

Formalism is developed in which contributions of binary and ternary interactions towards free energy of a mixture of low-spin (A) and high-spin (B) isomers of spin crossover compounds as well as effects of ordering are taken into account. Parameters characterising non-ideality in this formalism are the excess free energy (ΔF _ex) and absolute asymmetries (Δ ^A , Δ ^B ) of splittings of free energy levels. The excess free energy characterises the effects of binary interactions whereas asymmetries arise from ternary interactions. According to this model, the plateau in the spin crossover transition curve originates from the phenomenon of ordering taken into account in the Gorsky-Bragg-Williams approximation.     

Solid- and solution-state studies of the novel mu-dicyanamide-bridged dinuclear spin-crossover system {[(Fe(bztpen)]2[mu-N(CN)2]}(PF6)3 x n H2O

The mononuclear diamagnetic compound {Fe(bztpen)[N(CN)2]}(PF6)CH3OH (1) (bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine) has been synthesized and its crystal structure studied. Complex 1 can be considered to be the formal precursor of two new dinuclear, dicyanamide-bridged iron(II) complexes with the generic formula {[(Fe(bztpen)]2[mu-N(CN)2]}(PF6)3 x n H2O (n = 1 (2) or 0 (3)), which have been characterized in the solid state and in solution. In all three complexes, the iron atoms have a distorted [FeN6] octahedral coordination defined by a bztpen ligand and a terminal (1) or a bridging dicyanamide ligand (2 and 3). In the solid state, 2 and 3 can be considered to be molecular isomers that differ by the relative position of the phenyl ring of the two {Fe(bztpen)[N(CN)2]}+ halves (cis and trans, respectively). Depending on the texture of the sample, 2 exhibits paramagnetic behavior or displays a very incomplete spin transition at atmospheric pressure. Complex 3 undergoes a gradual two-step spin transition with no observed hysteresis in the solid state. Both steps are approximately 100 K wide, centered at approximately 200 K and approximately 350 K, with a plateau of approximately 80 K separating the transitions. The crystal structure of 3 has been determined in steps of approximately 50 K between 400 K and 90 K, which provides a fascinating insight into the structural behavior of the complex and the nature of the spin transition. Order-disorder transitions occur in the dicyanamide bridge and the PF6(-) ions simultaneously, with the spin-crossover behavior suggesting that these transitions may trigger the two-step character. In solution, 2 and 3 display very similar continuous spin conversions. Electrochemical studies of 2 and 3 show that the voltammograms are typical of dimeric systems with electronic coupling of the metals through the dicyanamide ligand.     

Lattice Effects on the Spin‐Crossover Profile of a Mononuclear Manganese(III) Cation

Six solvated salts of a mononuclear manganese(III) complex with a chelating hexadentate Schiff base ligand are reported. One member of the series, [MnL]PF₆^.0.5 CH₃OH ( 1 ), shows a rare low‐spin (LS) electronic configuration between 10–300 K. The remaining five salts, [MnL]NO₃? C₂H₅OH ( 2 ), [MnL]BF₄?C₂H₅OH ( 3 ), [MnL]CF₃SO₃?C₂H₅OH ( 4 ), [MnL]ClO₄?C₂H₅OH ( 5 ) and [MnL]ClO₄?0.5 CH₃CN ( 6 ), all show gradual incomplete spin‐crossover (SCO) behaviour. The structures of all were determined at 100 K, and also at 293 K in the case of 3 – 6 . The LS salt [MnL]PF₆^.0.5 CH₃OH is the only member of the series that does not exhibit strong hydrogen bonding. At 100 K two of the four SCO complexes ( 2 and 4 ) assemble into 1D hydrogen‐bonded chains, which weaken or rupture on warming. The remaining SCO complexes 3 , 5 and 6 do not form 1D hydrogen‐bonded chains, but instead exhibit discrete hydrogen bonding between cation/counterion, cation/solvent or counterion/solvent and show no significant change on warming.     

Protonation of an Oxo‐Bridged Diiron Unit Gives Two Different Iron Centers: Synthesis and Structure of a New Class of Diiron(III)‐μ‐hydroxo Bisporphyrins and the Control of Spin States by Using Counterions

Reported herein is a hitherto unknown family of diiron(III)‐μ‐hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo‐bridged dimer ( 2 ) by using strong Brønsted acids, such as HI, HBF₄, and HClO₄, produce red μ‐hydroxo complexes with I₃^? ( 3 ), BF₄^? ( 4 ), and ClO₄^? ( 5 ) counterions, respectively. The X‐ray structure of the molecule reveals that the Fe? O bond length increases on going from the μ‐oxo to the hydroxo complex, whereas the Fe‐O(H)‐Fe unit becomes more bent, which results in the smallest known Fe‐O(H)‐Fe angles of 142.5(2) and 141.2(1)° for 3 and 5 , respectively. In contrast, the Fe‐O(H)‐Fe angle remains unaltered in 4 from the corresponding μ‐oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4 , whereas the cores are deformed almost equally but to a lesser extent in 5 . Although 3 was found to have nearly high‐spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4 . Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe? N_p, Fe? O, and Fe???Ct_p bond lengths and the ring deformations, are all different between the two Fe^III centers in 3 and 4 , which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the μ‐oxo dimer (J, ?126.6 cm^?1) is attenuated to almost zero in the μ‐hydroxo complex with the I₃^? counterion, whereas the values of J are ?36 and ?42 cm^?1, respectively, for complexes with BF₄^? and ClO₄^? counterions.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.     

Moisture effects on the glass transition and the low temperature relaxations in semiaromatic polyamides

The influence of moisture absorption on the primary (glass) transition (T_a or T_g) and the low temperature relaxations of semiaromatic amorphous polyamides synthesized by isomeric aliphatic diamine and metha or para oriented phthalicdiacids has been investigated by means of differential scanning calorimeter (DSC) and dynamic mechanical thermal analyser (DMTA). The glass transition of semiaromatic polyamides was lowered due to the water absorption, and the β and the γ relaxations were as well. From the observed T_g and the difference in the heat capacity, the calculated T_g depression per 1 wt % water content was 12.3 K and the result was in good agreement with the experimental data. The depression of the glass transition may be expressed by the same manner as the plasticization of nylon 6 by water. The depressed β relaxation observed in the specimen containing a few percent of moisture was splitted into two transitions due to the reduction of water content, of which one was the elevation of the T_β and another was the simultaneous appearance of the T_γ, and then the single T_γ solely was observed for the completely dried specimen. The T_γ seemed to be merged into or not to be observed by the large and broad T_β transition when the sample was governed by a few percent of water, then it was emerged from the T_β due to water desorption. Thus, the T_β is believed to arise from the intermolecular hydrogen bonding between water molecules or between water and amide groups in wet polyamides. In addition, the γ relaxation originated from the peptide groups is attributable to the inter- and intramolecular hydrogen bonding between amide groups. © 1997 John Wiley & Sons, Inc. J Polyn Sci B: Polym Phys 35: 807–815, 1997     

Concurrent symmetries: the interplay between local and global molecular symmetries

We analyze in this article the degree to which different groups of atoms retain local symmetries when assembled in a molecule. This study is carried out by applying continuous symmetry measures to several families of mixed sandwiches, a variety of piano-stool molecules, and several organic groups. An analysis of the local symmetry of the electron density shows that, sandwiched between two regions of different symmetry that correspond to the ligand sets, its symmetry is cylindrical at the central metal atom.     

Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H₄L, has led to an Fe^II spin‐crossover (SCO) complex, [Fe(H₄L)₂][ClO₄]₂ ? H₂O ? 2 (CH₃)₂CO ( 1 ), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T_0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large T_TIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO₄^? and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe? N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.