Thermal Spin Crossover Research Articles

Nanocrystals of Fe(phen)2(NCS)2 and the size-dependent spin-crossover characteristics.

We report on the size reduction of the neutral Fe(phen)2(NCS)2 prototypical compound exhibiting a cooperative spin-crossover associated with a first-order phase transition (at ca. 176 K). We use the [Fe(phen)3](NCS)2 ionic precursor and the solvent-assisted precipitation technique to prepare an array of crystalline objects with sizes varying over two orders of magnitude (from 15 up to 1400 nm). TEM, X-ray diffraction and IR measurements provide evidences for the formation of particles of neutral and ionic species, which results from the interplay between the relevant chemical equilibrium and the reaction kinetics (ligand extraction, complex precipitation), and the modulation of the latter by physico-chemical parameters. A thermal transformation of diamagnetic nanocrystals of [Fe(phen)3](NCS)2 leads to spin-crossover particles of Fe(phen)2(NCS)2 of a comparable size. Powders of nano-, micro- and polycrystals of Fe(phen)2(NCS)2 present X-ray diffractograms typical of the so-called polymorph II. The importance of size effects on the cooperative spin-crossover process was probed with magnetic, Mössbauer, Raman and IR spectroscopic measurements. Each sample exhibits spin-state switching of the Fe(ii) ions. The salient features are: a cooperativity preserved at the micrometric scale, a very limited downshift of the transition temperature and an asymmetric spreading of the thermal process (over ca. 100 K) with the size reduction. At temperatures close to room temperature, the process appears to be quasi complete whatever the size of the samples. This result, extracted from Raman data, was confirmed by Mössbauer measurements in the case of the largest objects (LS residue <5-10% for bulk and microparticles). Below 150 K, a very efficient low-spin to high-spin photoexcitation was induced by the Raman laser beam in all the samples which prevents the extraction of the high-spin fraction in this temperature range. However variable temperature IR spectra of the 29 nm particles indicate that the HS residue, that is close to zero in the case of microparticles, does not drastically increase (<30%) for the smallest particles. The processing of a number of related spin-crossover compounds in the form of nanoparticles may be achieved with this general approach.

Influence of selenocyanate ligands on the transition temperature and cooperativity of bapbpy-based Fe(II) spin-crossover compounds.

Coordination of the ligand bapbpy (1, bapbpy = N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine), of one of its four dimethyl-substituted analogues 2-5 (R2bapbpy = N,N'-di(methylpyrid-2-yl)-2,2'-bipyridine-6,6'-diamine), or of one of its three bis(iso)quinoline analogues 6-8 (R2bapbpy= N,N'-di(quinolyl)-2,2'-bipyridine-6,6'-diamine), to Fe(NCSe)2, afforded eight new iron(II) compounds of the type [Fe(R2bapbpy)(NCSe)2] (9-16). Three of these compounds (11, 13, and 16) were structurally characterized by single crystal X-ray diffraction, which showed similar molecular geometry and packing compared to their thiocyanate analogues. Magnetic susceptibility measurements were carried out for all iron compounds and revealed thermal spin-crossover (SCO) behavior for compounds 9, 11, 13, 15, and 16. Compounds 11, 13, 15, and 16 show an increased transition temperature compared to the thiocyanate analogues. [Fe(bapbpy)(NCSe)2] (9) shows a gradual, one-step SCO, whereas its thiocyanate analogue [Fe(bapbpy)(NCS)2] is known for its cooperative two-step SCO. To discuss the influence of S-to-Se substitution on the cooperativity of the SCO, heat capacity measurements were carried out for compounds 9, 11, 13, 15, and 16, and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is a quantitative measure of the cooperativity of the spin transition, was found to be high for compounds 11 and 15, and low for compounds 9, 11, and 13. Compound 15 is one of the few known SCO compounds that is more cooperative than its thiocyanate analogue. Altogether, X-ray diffraction, calorimetry, and magnetic data give a consistent structure-property relationship for this family of compounds: hydrogen-bonding networks made of intermolecular N-H···Se interactions are of paramount importance for the cooperativity of the SCO.

Unusual Solvent Dependence of a Molecule‐Based FeII Macrocyclic Spin‐Crossover Complex

AbstractThis work illustrates in detail the reversible hydration–dehydration process of a molecule‐based material, which involves a drastic change of the switchable magnetic properties of the sample. Concretely, the complex [FeL222N3O2(CN)2]·H2O exhibits a spin crossover (SCO) accompanied by a change of the coordination number. The dehydration and rehydration of this SCO complex was carried out and monitored by different methods, which include X‐ray diffraction analysis. The corresponding samples display a thermal spin transition as well as a thermal quenching of the metastable high‐spin state at low temperature. In particular it was observed that the rehydrated material exhibits a particularly high temperature of relaxation, T(TIESST), of the metastable high‐spin state. On the other hand, it was shown that this compound exhibits a uncommon correlation between T(TIESST) and the thermal SCO, as compared with other spin‐crossover compounds that do not exhibit a change in coordination number.

A homologous series of [Fe(H₂Bpz₂)₂(L)] spin-crossover complexes with annelated bipyridyl co-ligands.

Four new iron(II) complexes [Fe(H2Bpz2)2(L)] were prepared (pz = pyrazolyl), where L is dipyrido[3,2-f:2',3'-h]quinoxaline (dpq), dipyrido[3,2-a:2'3'-c]phenazine (dppz), dipyrido[3,2-a:2'3'-c]benzo[i]-phenazine (dppn), and dipyrido[3,2-a:2',3'-c](6,7,8,9-tetrahydro)phenazine (dppc). Crystal structures of [Fe(H2Bpz2)2(dpq)], [Fe(H2Bpz2)2(dppz)], and [Fe(H2Bpz2)2(dppn)] all reveal stacks of complex molecules formed through π-π stacking between interdigitated bipyridyl chelate ligands, often with additional intercalated toluene or uncoordinated bipyridyl ligand (dpq). Molecules of [Fe(H2Bpz2)2(dppc)] form a different stacking motif in the crystal, with weaker contacts between individual molecules. Many of the structures also contain channels of disordered solvent, running between the molecular stacks. Despite their different stacking motifs, all these compounds exhibit very gradual thermal spin-crossover (SCO) on cooling, which occur over different temperature ranges but are otherwise quite similar in form. Weak thermal hysteresis in one of these spin equilibria can be attributed to the effects of a change in bipyridyl ligand conformation in the molecular stacks around 150 K, which was observed crystallographically. These results demonstrate that strong mechanical coupling between molecules in a crystal is not sufficient to engineer cooperative SCO switching, if other regions of the lattice are less densely packed.

Computational approach to the study of thermal spin crossover phenomena.

The key parameters associated to the thermally induced spin crossover process have been calculated for a series of Fe(II) complexes with mono-, bi-, and tridentate ligands. Combination of density functional theory calculations for the geometries and for normal vibrational modes, and highly correlated wave function methods for the energies, allows us to accurately compute the entropy variation associated to the spin transition and the zero-point corrected energy difference between the low- and high-spin states. From these values, the transition temperature, T1/2, is estimated for different compounds.

3d‐ and 4d‐Metal(II) Complexes of a Tris(pyridyl)ethane‐­Derived N4 Ligand – A Structural Study and Reactivity ­Remarks

AbstractA series of complexes of the new N4 chelate ligand L (L = 1‐{6‐[1, 1‐bis(pyridin‐2‐yl)ethyl]pyridin‐2‐yl}‐N,N‐dimethylmethanamine) with intermediate to late divalent transition metal ions M was obtained by the reaction between L and the respective chloride salt or similar precursor in methanol: [MCl2L] (M = Mn, Fe, Co, Ni, Zn; Ru) and [CuClL]Cl. The stereochemical characteristics of the chelate ligand were studied by means of single‐crystal X‐ray diffraction, and quantified on the basis of several geometric parameters, including the tetragonal distortion Σ and the continuous symmetry measure S(Oh). The overall distortion of the coordination environment is predominantly determined by the steric demand of the central ion, while electronic or other subtler influences essentially contribute to the distortion of the ligand L. Unlike similar complexes, [MnIICl2L] cannot be oxidized to a manganese(III) complex by dioxygen, hydrogen peroxide, or iodosylbenzene. In [RuIICl2L], one chlorido ligand can be exchanged against small π‐accepting molecules such as acetonitrile or dinitrogen. L offers an environment ideal for small metal ions (0.4–0.6 Å), such as low‐spin iron(II), which rationalizes the late onset of thermal spin crossover in the complex [FeIIL(NCS)2].

Solvent Dependent Spin-State Behaviour via Hydrogen Bonding in Neutral FeII Diimine Complexes

We report the syntheses, structures, and magnetic properties of cis-[Fe(pizR)2(NCS)2] complexes based on the pyridyl imidazoline ligands 2-(2′-pyridinyl)-4,5-dihydroimidazole (pizH, 1) and 2-(2′-pyridinyl)-4,5-dihydro-1-methylimidazole (pizMe, 2). The ligands, complexes, and magnetic measurements are chosen to separate hydrogen-bonding and intrinsic ligand field properties, so as to improve our understanding of the effect of hydrogen-bonding interactions on spin-state switching. In the solid state, both complexes are high spin between 5 and 300 K. In deuterated methanol and acetonitrile solutions, both complexes show gradual thermal spin crossover. Complex 1, capable of hydrogen bonding, shows solvent-sensitive spin crossover, whereas spin crossover in the methylated analogue 2 is insensitive to solvent identity.

Predicting paramagnetic 1H NMR chemical shifts and state-energy separations in spin-crossover host-guest systems.

The behaviour of metal-organic cages upon guest encapsulation can be difficult to elucidate in solution. Paramagnetic metal centres introduce additional dispersion of signals that is useful for characterisation of host-guest complexes in solution using nuclear magnetic resonance (NMR). However, paramagnetic centres also complicate spectral assignment due to line broadening, signal integration error, and large changes in chemical shifts, which can be difficult to assign even for known compounds. Quantum chemical predictions can provide information that greatly facilitates the assignment of NMR signals and identification of species present. Here we explore how the prediction of paramagnetic NMR spectra may be used to gain insight into the spin crossover (SCO) properties of iron(II)-based metal organic coordination cages, specifically examining how the structure of the local metal coordination environment affects SCO. To represent the tetrahedral metal-organic cage, a model system is generated by considering an isolated metal-ion vertex: fac-ML3(2+) (M = Fe(II), Co(II); L = N-phenyl-2-pyridinaldimine). The sensitivity of the (1)H paramagnetic chemical shifts to local coordination environments is assessed and utilised to shed light on spin crossover behaviour in iron complexes. Our data indicate that expansion of the metal coordination sphere must precede any thermal SCO. An attempt to correlate experimental enthalpies of SCO with static properties of bound guests shows that no simple relationship exists, and that effects are likely due to nuanced dynamic response to encapsulation.

Photoinduced Single-Molecule Magnet Properties in a Four-Coordinate Iron(II) Spin Crossover Complex

The four-coordinate Fe(II) complex, PhB(MesIm)3Fe-N═PPh3 (1) has been previously reported to undergo a thermal spin-crossover (SCO) between high-spin (HS, S = 2) and low-spin (LS, S = 0) states. This complex is photoactive below 20 K, undergoing a photoinduced LS to HS spin state change, as determined by optical reflectivity and photomagnetic measurements. With continuous white light irradiation, 1 displays slow relaxation of the magnetization, i.e. single-molecule magnet (SMM) properties, at temperatures below 5 K. This complex provides a structural template for the design of new photoinduced mononuclear SMMs based on the SCO phenomenon.

Spin‐Crossover Complex on Au(111): Structural and Electronic Differences Between Mono‐ and Multilayers

Submono-, mono- and multilayers of the Fe(II) spin-crossover (SCO) complex [Fe(bpz)2 (phen)] (bpz=dihydrobis(pyrazolyl)borate, phen=1,10-phenanthroline) have beenprepared by vacuum deposition on Au(111) substrates and investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). As evidenced by NEXAFS, molecules of the second layer exhibit a thermal spin crossover transition, although with a more gradual characteristics than in the bulk. For mono- and submonolayers of [Fe(bpz)2 (phen)] deposited on Au(111) substrates at room temperature both NEXAFS and STM indicate a dissociation of [Fe(bpz)2 (phen)] on Au(111) into four-coordinate complexes, [Fe(bpz)2 ], and phen molecules. Keeping the gold substrate at elevated temperatures ordered monolayers of intact molecules of [Fe(bpz)2 (phen)] are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST).

Stimuli Responsive Hybrid Magnets: Tuning the Photoinduced Spin-Crossover in Fe(III) Complexes Inserted into Layered Magnets

The insertion of a [Fe(sal2-trien)](+) complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHCl3 (1·CHCl3), [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHBr3 (1·CHBr3), and [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CH2Br2 (1·CH2Br2). The three compounds crystallize in a 2D honeycomb anionic layer formed by Mn(II) and Cr(III) ions linked through oxalate ligands and a layer of [Fe(sal2-trien)](+) complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and Mössbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.6 K and a spin crossover of the intercalated [Fe(sal2-trien)](+) complexes at different temperatures T1/2. The three compounds present a LIESST effect with a relaxation temperature TLIESST inversely proportional to T1/2. The isostructural paramagnetic compound, [Fe(III)(sal2-trien)][Zn(II)Cr(III)(ox)3]·CH2Cl2 (2·CH2Cl2) was also prepared. This compound presents a partial spin crossover of the inserted Fe(III) complex as well as a LIESST effect. Finally, spectroscopic characterization of the Fe(III) doped compound [Ga0.99Fe0.01(sal2trien)][Mn(II)Cr(III)(ox)3]·CH2Cl2 (3·CH2Cl2) shows a gradual and complete thermal spin crossover and a LIESST effect on the isolated Fe(III) complexes. This result confirms that cooperativity is not a necessary condition to observe the LIESST effect in an Fe(III) compound.

Thermal Spin Crossover and LIESST Effect Observed in Complexes [Fe(LCh)2(NCX)2] [LCh = 2,5-Di(2-Pyridyl)-1,3,4-Chalcadiazole; Ch = O, S, Se; X = S, Se, BH3

Two new complexes belonging to the family [Fe(II)(L)2(NCX)2] have been synthesized and structurally characterized. Complexation of the ligand L(O) = 2,5-di(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe(II)(L(O))2(NCS)2] (1) with the common s-trans and the rarely observed s-cis conformation. No thermal spin crossover (SCO) was observed for the amorphous bulk material of the mixture. Using the new ligand L(Se) = 2,5-di(2-pyridyl)-1,3,4-selenadiazole, compound [Fe(II)(L(Se))2(NCS)2]·1.4DCM·0.6MeOH (2·1.4DCM·0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [Fe(II)(L(Se))2(NCS)2]·MeOH (2·MeOH) exhibits a thermally induced SCO with small hysteresis [T(1/2)(↓) = 91 K, T(1/2)(↑) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [Fe(II)(L(S))2(NCS)2], [Fe(II)(L(S))2(NCSe)2], [Fe(II)(L(S))2(NCBH3)2]·H2O [L(S) = 2,5-di(2-pyridyl)-1,3,4-thiadiazole], and 2·MeOH. All complexes belong to the T0 = 90 K line [T(LIESST) = T0 - 0.3T(1/2)]. [Fe(II)(L(S))2(NCS)2], that exhibits a two-stepped thermal SCO process, has been found to also exhibit two well-separated T(LIESST) temperatures [T(LIESST, 1) = 44 K; T(LIESST, 2) = 53 K].

Femtosecond optical pump–probe reflectivity studies of spin-state photo-switching in the spin-crossover molecular crystals [Fe(PM-AzA)2(NCS)2

We report here on the ultrafast photo-switching dynamics of a Fe(II) molecular material [Fe(PM-AzA)2(NCS)2]. It undergoes a thermal spin-crossover which can be detected by magnetic measurements or by optical reflectivity. We use here femtosecond optical reflectivity to study the ultrafast photo-switching dynamics. Our results indicate that the HS state is reached from the LS state within less than 100fs, through an intermediate MLCT state. This ultrafast relaxation from the electronic excited state towards the structurally relaxed HS state is followed by a vibrational cooling of the hot HS molecules within ≈1ps timescale.

Bis[(E)-2,6-bis(1H-pyrazol-1-yl)-4-styrylpyridine]iron(II) Complex: Relationship between Thermal Spin Crossover and Crystal Solvent

The crystal structures and thermal spin-crossover (SCO) behavior of [Fe(E-dpsp)(2)](BF(4))(2)·X [E-dpsp = (E)-2,6-bis(1H-pyrazol-1-yl)-4-styrylpyridine; X = crystal solvent] are investigated. The titled iron(II) complex features polymorphology induced by crystal solvents, which is identified by means of single-crystal X-ray diffraction analysis: For [Fe(E-dpsp)(2)](BF(4))(2)·acetone and [Fe(E-dpsp)(2)](BF(4))(2)·4MeNO(2), detailed analyses at various temperatures are conducted. The magnetic properties of bulk microcrystalline samples of [Fe(E-dpsp)(2)](BF(4))(2)·X are assessed using a SQUID magnetometer. Among the series, only [Fe(E-dpsp)(2)](BF(4))(2)·acetone undergoes peculiar thermal SCO, such as a precipitous and hysteretic spin-state change (T(1/2↑) = 179 K, T(1/2↓) = 164 K, and ΔT(1/2) = 15 K) and frozen-in effect. All single crystals of [Fe(E-dpsp)(2)](BF(4))(2)·X are free from intermolecular interaction except for [Fe(E-dpsp)(2)](BF(4))(2)·acetone: One of the phenyl rings in [Fe(E-dpsp)(2)](BF(4))(2)·acetone is twisted appreciably and features an intermolecular H-H short contact with one of the neighboring complexes to form a one-dimensional network. The twisted phenyl group also participates in π-π stacking with one of the pyrazolyl rings of another neighboring molecule, constructing a dimeric couple. These intermolecular interactions would induce cooperative effects, which leads to the good thermal SCO phenomenon of [Fe(E-dpsp)(2)](BF(4))(2)·acetone.

Thermal and Photoinduced Spin Crossover in a Mononuclear Iron(II) Complex with a Bis(pyrazolyl)pyridine Type of Ligand

AbstractA mononuclear iron(II) compound 1 of the general formula [Fe(L)2](ClO4)2 {L = 4‐[2,6‐bis(pyrazol‐1‐yl)pyridin‐4‐yl]benzaldehyde} was prepared and structurally characterised. Single‐crystal X‐ray structure analysis revealed the presence of a complex dication [Fe(L)2]2+ and two ClO4– counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN6 indicate the low‐spin state of the central iron(II) metal ion at T = 180 K. Magnetic investigations elucidate spin crossover with T1/2 = 285 K. The experimental magnetic susceptibility data could be satisfactorily fitted with the Curie law in combination with the Ising‐like model. The room‐temperature character of the spin crossover was further studied by variable‐temperature far‐IR, Vis and Mössbauer spectroscopy. Laser irradiation of 1 carried out at 10 K gives rise to a complete low‐spin to high‐spin photoconversion. A subsequent temperature‐dependent investigation revealed the existence of a photoexcited metastable HS state up to TLIESST = 70 K (LIESST = light‐induced excited‐spin‐state trapping) as well as the presence of a light‐induced thermal hysteresis loop with a width of 10 K.

Two Mononuclear Iron(II) Spin‐Crossover Complexes with a N4O2 Coordination Sphere

AbstractTwo mononuclear spin‐crossover compounds Fe(L1)2 (1) and Fe(L2)2 (2) {HL1 = 2‐hydroxy‐3‐methyl‐N′‐[1‐(pyridine‐2‐yl)butylidene]benzohydrazide, HL2 = 2‐hydroxy‐N′‐[1‐(pyridine‐2‐yl)butylidene]benzohydrazide} were successfully synthesized. The magnetic measurements of the compounds revealed different thermal magnetic behavior. Compound 1 undergoes a gradual complete thermal spin crossover (SCO) with T1/2↑/↓ = 243/240 K and a hysteresis loop of 3 K. Compound 2 shows a gradual incomplete SCO during the measurable temperature region with T1/2 ≈ 330 K. Through the comparison of the magnetic and structural results for 1 and 2 with previously reported related compounds, the alkyl substitutions of the ligand were considered to affect the ligand‐field strength and hydrogen‐bonding interactions. In addition, photomagnetism results showed an 87 % photoconversion and 36 K of T(LIESST) (LIESST = light‐induced excited spin‐state trapping) for compound 1. The results enrich the knowledge on how to control the spin‐crossover behavior in the new system.

Isostructural salts of the same complex showing contrasting thermal spin-crossover mediated by multiple phase changes

Two salts of [FeL2](2+) (L = 2,6-bis[5-methyl-1H-pyrazol-3-yl]pyridine) are isostructural under ambient conditions but show different thermal spin-crossover behaviour, involving a variety of crystallographic phase changes.

Spin-state studies with XES and RIXS: From static to ultrafast

We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) FeII complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS–LS difference spectra measured at thermal spin crossover, and reference HS–LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe 1s pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales.

Heterobimetallic MOFs Containing Tetrathiocyanometallate Building Blocks: Pressure-Induced Spin Crossover in the Porous {FeII(pz)[PdII(SCN)4]} 3D Coordination Polymer

Here we describe the synthesis, structure, and magnetic properties of two related coordination polymers made up of self-assembling Fe(II) ions, pyrazine (pz), and the tetrathiocyanopalladate anion. Compound {Fe(MeOH)(2)[Pd(SCN)(4)]}·pz (1a) is a two-dimensional coordination polymer where the Fe(II) ions are equatorially coordinated by the nitrogen atoms of four [Pd(SCN)(4)](2-) anions, each of which connects four Fe(II) ions, forming corrugated layers {Fe[Pd(SCN)(4)]}(∞). The coordination sphere of Fe(II) is completed by the oxygen atoms of two CH(3)OH molecules. The layers stack one on top of each other in such a way that the included pz molecule establishes strong hydrogen bonds with the coordinated methanol molecules of adjacent layers. Compound {Fe(pz)[Pd(SCN)(4)]} (2) is a three-dimensional porous coordination polymer formed by flat {Fe[Pd(SCN)(4)]}(∞) layers pillared by the pz ligand. Thermal analysis of 1a shows a clear desorption of the two coordinated CH(3)OH molecules giving a rather stable phase (1b), which presumably is a polymorphic form of 2. The magnetic properties of the three derivatives are typical of the high-spin Fe(II) compounds. However, compounds 1b and 2, with coordination sphere [FeN(6)], show thermal spin crossover behavior at pressures higher than ambient pressure (10(5) MPa).

When Stable and Metastable HS States Meet in Spin‐Crossover Compounds

AbstractWith the ultimate goal of rationally inducing overlap between the stable high spin state (HSSS) and the metastable high spin state (HSMS) generated by a rapid frozen effect, a series of [FexMn1–x(bpp)2](BF4)2 [bpp = 2,6‐bis(pyrazol‐3‐yl)pyridine] complexes was prepared. For all the materials, the thermal spin crossover (SCO) properties were measured and the stability of the metastable HSMS state was probed through the determination of the T(TIESST) value associated with the Thermally‐Induced Excited Spin‐State Trapping effect. The overlap between the two systems was observed for x ≤ 0.90 and the consequences on the shape and the completeness of the SCO behavior were discussed.