Complete Spin Crossover Research Articles

Co‐crystallization as a Strategy for Tuning Spin Crossover Properties of Mononuclear Mn(III) Schiff‐base Complexes

AbstractWe present co‐crystallization as an approach to tuning spin crossover (SCO) properties of mononuclear Mn(III) Schiff‐base complexes. Complexes [Mn(5‐F‐sal‐N‐1,5,8,12)]ClO4 ⋅ 0.5carbazole (cocrystal 1 a) and [Mn(5‐F‐sal‐N‐1,5,8,12)]PF6 ⋅ 0.5carbazole (cocrystal 2 b) are synthesized by the co‐crystallization of their parent complexes ( [Mn(5‐F‐sal‐N‐1,5,8,12)]Y, Y=ClO4− (1); Y=PF6− (2) ) with the carbazole and they both exhibit HS electronic configurations between 2 and 300 K. While, the parent complex 1 shows a nearly complete SCO with T1/2=100 K and complex 2 maintains a high‐spin (HS) state. The introduction of carbazole as the third component has a great influence on cation‐anion crystal packing and finally the cooperative effects of the SCO complexes.

Spin Crossover Quenching by "Racemization" in a Family of trans-1,2-Di(tetrazol-1-yl)cyclopentane-Based Fe(II) 1D Coordination Polymers.

Optically pure (RR)- and racemic (RR/SS)-trans-1,2-di(tetrazol-1-yl)cyclopentane were synthesized and used to prepare homo- and heterochiral Fe(II) coordination compounds. [Fe((RR/SS)-C7H10N8)2(CH3CN)2](BF4)2 (1A), [Fe((RR/SS)-C7H10N8)2(C2H5CN)2](BF4)2 (2A), [Fe((RR)-C7H10N8)2(CH3CN)2](BF4)2·2CH3CN (1B·solv), and [Fe((RR)-C7H10N8)2(C2H5CN)2](BF4)2 (2B) form a family of one-dimensional coordination polymers. Fe(II) cations in these complexes are characterized by a heteroleptic coordination environment: the neighboring metal centers are bridged by two 1,2-di(tetrazol-1-yl)cyclopentane molecules, while the nitrile molecules (acetonitrile or propionitrile, respectively) occupy the axial positions. Independently of the kind of nitrile coligands, an ability to thermally induce spin crossover (SCO) is governed by chirality. 1B·solv and 2B exhibit abrupt and complete SCO occurring at T1/2 = 144 K and T1/2 = 228 K, respectively. Desolvated form, 1B (of the same stoichiometry as 2B), also exhibits SCO (T1/2 = 215 K). In contrast, an exchange within the polymeric chain of half of the RR molecules with the SS enantiomeric form results in formation of 1A and 2A, which remain in stable high-spin (HS) form down to 10 K. It has been shown that moving from a homochiral to a heterochiral system changes the structure of the polymeric unit (while maintaining the same polymer dimensionality and bridging fashion) that leads to the deep reorganization of the further coordination spheres, including the anion network.

Thermally and Photoinduced Spin-Crossover Behavior in Iron(II)-Silver(I) Cyanido-Bridged Coordination Polymers Bearing Acetylpyridine Ligands.

We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy)2[Ag(CN)2]2} 1 and {Fe(3acpy)[Ag(CN)2]2} 2 (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in 1 made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties. Changing the acetyl group to a 3-position, a completely different structure is obtained for 2. The unexpected coordination of the carbonyl group to the Fe(II) centers induces a three-dimensional (3D) structure, leading to statistical disorder around the Fe(II) with three different coordination spheres, [N6], [N4O2], and [N5O]. This disorder gives rise to an incomplete thermally induced SCO with a poor photoswitchability. These results demonstrate that the choice of the acetyl position on the pyridine dictates the structural characteristics of the compounds with a direct impact on the SCO behavior. Remarkably, this work opens interesting perspectives for the future design of Fe-Ag cyanido coordination polymers with judiciously substituted pyridine ligands to tune the thermally and photoinduced SCO properties.

Mononuclear Fe(III) complexes with 2,4-dichloro-6-((quinoline-8-ylimino)methyl)phenolate: synthesis, structure, and magnetic behavior.

Three Fe(III)-based coordination complexes [Fe(dqmp)2](NO3)·H2O (1), [Fe(dqmp)2](BF4)·2CH3COCH3 (2), and [Fe(dqmp)2](ClO4) (3) were synthesized from Fe(NO3)3·9H2O/Fe(ClO4)3·xH2O, NaBF4, and 2,4-dichloro-6-((quinoline-8-ylimino)methyl)phenol (Hdqmp) in methanol/acetone and characterized. The structures of complexes 1-3 were determined via single-crystal X-ray crystallography at 100 K and room temperature, and their magnetic properties in the solid and solution forms were investigated. All complexes showed meridional structures with two tridentate dqmp- ligands coordinated with Fe(III) cations. In the solid state, complex 1 showed an abrupt and complete spin crossover at 225 K, whereas complexes 2 and 3 exhibited an incomplete spin crossover at 135 and 150 K, respectively. In a dimethylformamide solution, the complexes showed counterion-dependent spin transitions. In contrast to the solid state, in solution, complex 1 did not exhibit complete spin crossover. However, complexes 2 and 3 showed more complete spin transitions in solutions than in the solid state. The relaxation times, T1 and T2, for 1 and 2 were determined and both increased with temperature from 220 to 380 K. The T1 of 1 was larger than that of 2 at 380 K, and the T1 values were larger than the T2 values.

The cooperative effects of ligand substituent positions and solvents on spin crossover in Mn(III) complexes based on hexadentate Schiff-base ligands

Spin crossover (SCO), as a molecular bistable phenomenon, has attracted extensive interest. In this paper, the syntheses, crystal structures and magnetic properties of three Mn(III) hexadentate Schiff-base complexes [Mn(3-OMe-sal-N-1,5,8,12)]AsF6·H2O (1), [Mn(4-OMe-sal-N-1,5,8,12)]AsF6·H2O (2) and [Mn(5-OMe-sal-N-1,5,8,12)]AsF6(3) have been reported. The substituent positions of ligands and solvent molecules have a cooperative effect on SCO behavior. Complex 1 shows a complete SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis, and complex 2 shows a gradual incomplete SCO behavior. The transition temperatures of 2 (T1/2 = 241 K) is 98 K higher than that of 1 (T1/2 = 143 K), which indicates that the ligand field strength of (4-OMe-sal-N-1,5,8,12)2– is stronger than that of (3-OMe-sal-N-1,5,8,12)2–. Due to the absence of crystallizing solvent water molecule, complex 3 maintains a HS state in the test temperature range.

Pressure-Induced Hysteretic and Abrupt Spin Transition in Amine Functionalized Isoquinoline-Based Two-Dimensional FeII Hofmann Frameworks

Two two-dimensional (2D) Hofmann-type coordination frameworks are synthesized by employing the square planar tetracyanometallate building blocks and an amine-functionalized isoquinoline ligand with the general formula of [Fe(L)2{M(CN)4}] (L = 5-amino isoquinoline) (M = Pt (1Pt) and Pd (1Pd)) to explore the spin-state switching behavior. The inclusion of the amine functional group in the isoquinoline ligand plays a major role in exhibiting a complete spin crossover (SCO) behavior under ambient atmospheric pressure. The effective host–host supramolecular interaction such as strong π···π stacking and N–H···C interactions between interlayer 2D sheets of {FeII[Pt (CN)4]}n is responsible for the abrupt hysteretic spin transition behavior. Interestingly, the applied external pressure enhances the stabilization of the low spin states revealing a one-step abrupt and hysteretic spin transition near room temperature.

Spin-Crossover Behaviors of Iron(II) Complexes Bearing Halogen Ligands in Solid State and Solution.

Numerous Fe(II) spin-crossover (SCO) compounds have been developed in the past decades, while the reports on the SCO materials with halogen atoms acting as coordinating ligands remain rare. In this study, we synthesize three iron(II) halide complexes with a general formula of [FeII(Py5Me2)X]+ (Py5Me2 = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine, X = Cl- or Br-) that undergo complete SCO transitions at near room temperature. The SCO properties of these compounds are investigated in detail by magnetic measurements, variable-temperature single-crystal X-ray diffractions, and Mössbauer spectra analyses. Because of the good stability of the coordination structures and suitable ligand-field strength, these compounds show robust spin transitions in both solid state and solution.

Guest-induced pore breathing controls the spin state in a cyanido-bridged framework.

Iron(ii) spin cross-over (SCO) compounds combine a thermally driven transition from the diamagnetic low-spin (LS) state to the paramagnetic high-spin (HS) state with a distinct change in the crystal lattice volume. Inversely, if the crystal lattice volume was modulated post-synthetically, the spin state of the compound could be tunable, resulting in the inverse effect for SCO. Herein, we demonstrate such a spin-state tuning in a breathing cyanido-bridged porous coordination polymer (PCP), where the volume change resulting from guest-induced gate-opening and -closing directly affects its spin state. We report the synthesis of a three-dimensional coordination framework {[FeII(4-CNpy)4]2[WIV(CN)8]·4H2O}n (1·4H2O; 4-CNpy = 4-cyanopyridine), which demonstrates a SCO phenomenon characterized by strong elastic frustration. This leads to a 48 K wide hysteresis loop above 140 K, but below this temperature results in a very gradual and incomplete SCO transition. 1·4H2O was activated under mild conditions, producing the nonporous {[FeII(4-CNpy)4]2[WIV(CN)8]}n (1) via a single-crystal-to-single-crystal process involving a 7.3% volume decrease, which shows complete and nonhysteretic SCO at T1/2 = 93 K. The low-temperature photoswitching behavior in 1 and 1·4H2O manifested the characteristic elasticity of the frameworks; 1 can be quantitatively converted into a metastable HS state after 638 nm light irradiation, while the photoactivation of 1·4H2O is only partial. Furthermore, nonporous 1 adsorbed CO2 molecules in a gated process, leading to {[FeII(4-CNpy)4]2[WIV(CN)8]·4CO2}n (1·4CO2), which resulted in a 15% volume increase and stabilization of the HS state in the whole temperature range down to 2 K. The demonstrated post-synthetic guest-exchange employing common gases is an efficient approach for tuning the spin state in breathing SCO-PCPs.

Bidirectional photoswitchability in an iron(iii) spin crossover complex: symmetry-breaking and solvent effects.

The impact of solvent on spin crossover (SCO) behaviour is reported in two solvates [Fe(qsal-I)2]NO3·2ROH (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate; R = Me 1 or Et 2) which undergo abrupt and gradual SCO, respectively. A symmetry-breaking phase transition due to spin-state ordering from a [HS] to [HS-LS] state occurs at 210 K in 1, while T1/2 = 250 K for the EtOH solvate, where complete SCO occurs. The MeOH solvate exhibits LIESST and reverse-LIESST from the [HS-LS] state, revealing a hidden [LS] state. Moreover, photocrystallographic studies on 1 at 10 K reveal re-entrant photoinduced phase transitions to a high symmetry [HS] phase when irradiated at 980 nm or a high symmetry [LS] phase after irradiation at 660 nm. This study represents the first example of bidirectional photoswitchability and subsequent symmetry-breaking from a [HS-LS] state in an iron(iii) SCO material.

The Paramagnetic or Spin Crossover Iron(III) Complexes Based-on Pentadentate Schiff Base Ligand: Crystal Structure, and Magnetic Property Investigation.

A series of bi- or mononuclear hexacoordinate iron(III) complexes, [Fe(L)][Fe(bpb)(CN)2]·CH3OH·0.5H2O (1), [Fe(L)][Co(bpb)(CN)2]·CH3OH (2) [(Fe (L))2(4,4'-bipy)](BPh4)2 (3), [Fe(L)(py)](BPh4) (4) and [Fe(L)(dmap)](BPh4) (5) (bpb = 1,2-bis(pyridine-2-carboxamido)benzenate, L = N,N'-bis(2-hydroxybenzyliden)-1,7-diamino-4-azaheptane, dmap = 4-dimethylaminopyridine), have been prepared with the pentadentate Schiff base iron(III) compound as assemble precursor and characterized by element analysis, IR and X-ray diffraction. Single crystal structural determination revealed the neutral cyanide-bridged binuclear entity for complexes 1 and 2 and the cationic di- or mononuclear structure for complexes 3-5 with the positive charge(s) balanced by BPh4- ion(s). The experimental study and theoretical simulation of the magnetic property discovered the ferromagnetic coupling between the Fe(III) ions bridged by cyanide group in complex 1 and the always high spin state of the Fe(III) ion coordinated to the Schiff base ligand in both complexes 1 and 2. The temperature dependent magnetic susceptibility investigation over complexes 3-5 showed the occurrence of the thermo-induced gradual complete spin crossover (SCO) property at about 115, 170 and 200 K, respectively.

Two-Dimensional Coordination Polymer Showing Spin-Crossover Behavior with a 64 K Wide Hysteresis Loop.

A two-dimensional grid-like coordination polymer, [Fe(NCBH3)2(Py2ttz)2]·4CHCl3 (1·4CHCl3, Py2ttz = 2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole), showed one-step complete spin crossover with unexpectedly large hysteresis loop of 64 K wide and temperature-induced excited spin-state trapping effect below 91 K.

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

Halobenzene Clathrates of the Porous Metal-Organic Spin-Crossover Framework [Fe(tvp)2(NCS)2]n. Stabilization of a Four-Step Transition.

Here we show that the porous metal–organic spin crossover (SCO) framework [Fe(tvp)2(NCS)2]@4(CH3CN·H2O) [1@4(CH3CN·H2O)] is an excellent precursor material for the systematic synthesis, via single-crystal to single-crystal transformation, of a series of halobenzene clathrates. Immersion of samples constituted of single crystals of 1@4(CH3CN·H2O) in the liquid halobenzenes PhXn, X = F (n = 1–6), X = Cl (n = 1, 2), and X = Br (n = 1) at room temperature induces complete replacement of the guest molecules by PhXn to afford 1@2PhXn. Single-crystal analyses of the new clathrates confirm the integrity of the porous framework with the PhXn guests being organized by pairs via π-stacking filling the nanochannels. The magnetic and calorimetric data confirm the occurrence of practically complete SCO behavior in all of the clathrates. The characteristic SCO equilibrium temperatures, T1/2, seem to be the result of a subtle balance in the host–guest interactions, which are temperature- and spin-state-dependent. The radically distinct supramolecular organization of the PhCl2 guests in 1@2PhCl2 affords a rare example of four-step SCO behavior following the sequence [HS1:LS0] ↔ [HS2/3:LS1/3] ↔ [HS1/2:LS1/2] ↔ [HS1/4:LS3/4] ↔ [HS0:LS1], which has been structurally characterized.

A Mononuclear Iron(II) Spin-Crossover Molecule Decorated by Photochromic Azobenzene Group.

Aiming at constructing photoresponsive spin crossover (SCO) behavior, herein we designed a new ligand Abtz (Abtz = (E)-N-(4-((E)-phenyldiazenyl)phenyl)-1-(thiazol-4-yl)methanimine) which was decorated by a photochromic azobenzene group. Based on this photochromic ligand, a mononuclear Fe(II) SCO molecule [Fe(Abtz)3](BF4)2·(EAC)2 (1, EAC = ethyl acetate) was successfully synthesized and showed a complete one-step SCO behavior. Under continuous UV light and blue-light exposure, the cis–trans photoisomerization of both ligand Abtz and compound 1 in the liquid phase was confirmed through UV–Vis spectra. Moreover, the 1H-NMR spectra of Abtz reveal a trans–cis conversion ratio of 37%. Although the UV–Vis spectra reveal the photochromic behavior for 1 in the solution phase, the SCO behavior in the liquid state is absent according to the variable-temperature Evans method, suggesting the possible decomposition. Moreover, in the solid state, the cis–trans photoisomerization of both Abtz and 1 was not observed, due to the steric hindrance.

Cation modulated spin state and near room temperature transition within a family of compounds containing the same [FeL2]2- center.

Spin-crossover (SCO) active compounds have received much attention due to their potential application in molecular devices. Herein, a family of solvent-free FeII compounds, formulated as (A)2[FeL2], (H2L = pyridine-2,6-bi-tetrazolate, A = (Me)4N+1, Et2NH2+2, iPr2NH2+3 and iPrNH3+4), were synthesized and characterized. Single-crystal X-ray diffraction studies reveal that 1-4 are all supramolecular frameworks containing the same [FeL2]2- center, which is arranged into two packing modes via inter-molecular interactions, that is, a 3D architecture in 1 and 1D chain in 2-4. The spin states of 1-4 at different temperatures are assigned on the basis of the single-crystal X-ray diffraction data. Solid state magnetic investigations indicate that 1 and 4 exhibit a low spin state (below 350 K) and high spin state (2-400 K), respectively. 2 and 3 display clear SCO behavior in the measured temperature, but with different profiles and critical temperatures. 2 undergoes a complete gradual SCO with a critical temperature of T1/2 = 260 K. 3 has an abrupt near room temperature transition between T1/2 cooling = 278 K and T1/2 warming = 286, centered at 282 K (9 °C). This study reveals the importance of organic cations in the modulation of SCO behavior and offers a new insight for the design of SCO compounds with near room temperature spin transitions.

Correlation between Supramolecular Connectivity and Magnetic Behaviour of [FeIII(5-X-qsal)2]+-Based Salts Prone to Exhibit SCO Transition

We present an extensive study to determine the relationship between structural features of spin crossover (SCO) systems based on N-(8-quinolyl)salicylaldimine (qsal) ligand derivatives and their magnetic properties. Thirteen new compounds with general formula [FeIII(5-X-qsal)2]+ (X = H, F, Cl, Br and I) coupled to Cl−, ClO4−, SCN−, PF6−, BF4− and BPh4− anions were prepared and magnetically characterized. The structure/properties correlations observed in these compounds were compared to those of salts with the same [FeIII(qsal-X)2]+ cations previously reported in the literature. These cations favour the LS configuration in compounds with the weakest connectivity. As connectivity increases most of them present HS states at room temperature and structures may be described as arrangements of parallel layers of interacting cation dimers. All the compounds based on these cations undergoing complete SCO transitions within the 4–300 K temperature range have high intralayer connectivity. If, however, the interlayer connectivity becomes very strong they remain blocked in the HS or in the LS state. The SCO transition may be affected by the slightest change of solvent molecules content, disorder or even crystallinity of the sample and it remain difficult to predict which kind of ligand substituent should be selected to obtain compounds with the desired connectivity.

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N'-Disubstituted Bis(pyrazol-3-yl)pyridines.

Here, we report a combined study of the effects of two chemical modifications to an N,N′-disubstituted bis(pyrazol-3-yl)pyridine (3-bpp) and of different solvents on the spin-crossover (SCO) behavior in otherwise high-spin iron(II) complexes by solution NMR spectroscopy. The observed stabilization of the low-spin state by electron-withdrawing substituents in the two positions of the ligand that induce opposite electronic effects in SCO–active iron(II) complexes of isomeric bis(pyrazol-1-yl)pyridines (1-bpp) was previously hidden by NH functionalities in 3-bpp precluding the molecular design of SCO compounds with this family of ligands. With the recent SCO-assisting substituent design, the uncovered trends converged toward the first iron(II) complex of N,N′-disubstituted 3-bpp to undergo an almost complete SCO centered at room temperature in a less polar solvent of a high hydrogen-bond acceptor ability.

Abundant Solvatomorphism-Tuned Spin Crossover in a Dinuclear Fe(II) Compound: Computational Insights on Molecular Distortion and Packing Effects

A series of solvates based on a dinuclear Fe(II) complex [(TPA)2Fe2(μ-DHBQ)][BF4]2·S [S = CH3OH (1·CH3OH), 2CH3OH (2·2CH3OH), 4DMF (3·4DMF), 2Et2O (4·2Et2O), Et2O·MeCN (5·Et2O·MeCN), and 2CH2Cl2 (6·2CH2Cl2)] were synthesized. Upon solvent removal, single-crystal-to-single-crystal (SC-to-SC) transitions could occur for the first four solvates to give 1, 2, 3, and 4·0.5Et2O. Within the temperature range of 400–10 K, these compounds exhibited abundant variations in their spin crossover (SCO) properties. 2,5·Et2O·MeCN and 6·2CH2Cl2 displayed half high-spin (HS) to low-spin (LS) transitions, and 4·2Et2O underwent incomplete LS-to-HS conversion, whereas other solvatomorphs showed complete SCO. As all these solvatomorphs possessed the identical [(TPA)2Fe2(μ-DHBQ)][BF4]2 core, these variations in SCO behavior emphasized the critical role of the crystal lattice contributions. With the aim of deciphering their origin, periodic DFT+U+D3 computations were performed on these solvatomorphs to quantify the importance of all possible intramolecular and intermolecular effects on their spin-state energetics. Computationally, the isolated [(TPA)2Fe2(μ-DHBQ)]2– molecule in the gas phase would undergo SCO around 350 K in one step intrinsically. However, distinctive roles of the crystal-lattice effects in the solid state resulted in varying SCO behaviors. Different reasons were discovered for the incomplete spin transitions of different solvatomorphs. For 2, although serious volume shrinkage of the LS state caused efficient packing of the overall crystal-lattice organization, the originally close-packing SCO cations got loose and thus strongly destabilized its LS state. For 5·Et2O·MeCN and 6·2CH2Cl2, severe molecular distortions kinetically trapped their LS state. These computationally magneto-structural studies on dinuclear solvatomorphs have great importance for designing SCO compounds with selected properties, which is critical for their actual application.

New Spin-Crossover Compounds Containing the [Ni(mnt)] Anion (mnt = Maleonitriledithiolate)

Two novel salts containing the anion [Ni(mnt)2]− (mnt = maleonitriledithiolate) have been synthesized. The counter-ions, [Fe(II)(L1 or L2)2], are cationic complexes where L1 and L2 are methylated derivatives of 2,6-bis(pyazolyl)pyridine or pyrazine, which are similar to ligands found in a series of spin-crossover (SCO) complexes. Both salts are characterized by variable temperature single crystal X-ray diffraction and bulk magnetization measurements. Compound 1, [Fe(II)(L1)2][Ni(mnt)2]2 displays an incomplete and gradual SCO up to 300 K, followed by a more rapid increase in the high-spin fraction between 300 and 350 K. Compound 2, [Fe(II)(L2)2][Ni(mnt)2]2.MeNO2, shows a gradual, but more complete SCO response centered at 250 K. For compound 2, the SCO is confirmed by variable temperature Mössbauer spectroscopy. In both cases, the anionic moieties are isolated from each other and so no electrical conductivity is observed.

Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands.

Two new mononuclear Fe(II) polymorphs, [(C2H5)4N]2[Fe(py3C-OEt)(NCS)3]2 (1) and [(C2H5)4N][Fe(py3C-OEt)(NCS)3] (2) (py3C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized and characterized by single-crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py3C-OEt)(NCS)3]- complex (two units for 1 and one unit for 2) generated by coordination to the Fe(II) metal center of one tridentate py3C-OEt tripodal ligand and three terminal κN-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over the entire studied temperature range (300-10 K), while complex 1 exhibits an abrupt and complete spin crossover (SCO) transition at ca. 132.3 K, the structural characterizations of which, performed at 295 and 100 K, show a strong modification, resulting from the thermal evolutions of the Fe-N bond lengths and of the distortion parameters (∑ and Θ) of the FeN6 coordination sphere, in agreement with the presence of HS and low-spin (LS) states at 295 and 100 K, respectively. This thermal transition has been also confirmed by the thermal evolution of the maximum absorbance for ν(NCS) vibrational bands recorded in the temperature range 200-10 K. In 1 the signature of a metastable photoinduced HS state has been observed using photomagnetic and photoinfrared spectroscopy, leading to a similar T(LIESST) relaxation temperature (LIESST = light-induced excited spin-state trapping) of 70 K.