Spin Crossover—An Overall Perspective

Proton-Induced Switching of Paramagnetism: Reversible Conversion between a Low and High Spin CoIII Center within a Heterobimetallic Core

The development of molecular species with switchable magnetic properties has been a long-standing challenge in chemistry. One approach involves binding an analyte, such as protons, to a compound to trigger a change in magnetism. Transition metal complexes have been targeted for this type of magnetic modulation because they can undergo changes in their spin states. However, heterobimetallic complexes have had limited utility because of a lack of ligands that create differentiated structures around each metal center that are often necessary to regulate the electronic and magnetic properties. To circumvent this problem, we have used a tripodal ligand with phosphinic amido groups to prepare a complex with a discrete [CoIII(μ-OH)FeIII] core and an overall system spin of ST = 5/2. Deprotonation readily produces a species with a unique [CoIII(μ-O)FeIII] core and an ST = 1/2 system spin. X-ray diffraction studies, electron paramagnetic resonance spectroscopy, and Mössbauer spectroscopy pinpoint the hexacoordinate CoIII center as the cause of this spin change: the typical SCo = 0 spin state of the CoIII center in the [CoIII(μ-OH)FeIII] complex switches to a rare SCo = 2 spin state in the [CoIII(μ-O)FeIII] analogue; this change turns on antiferromagnetic coupling between the two metal centers. Computational studies link an increase in π bonding within the Co-oxido unit to the change in the CoIII spin state. The conversion is reversible and provides a blueprint for using oxido/hydroxido ligands within a heterobimetallic core to regulate the spin state of a metal site and thus modulate the paramagnetism of a system.

Cobalt Complexes with Aminophenolate Ligands: Spin Crossover and Ligand Versatility

A search for switchable molecules has afforded a family of cobalt complexes featuring derivatives of 2-aminophenol: 4,6-di-tert-butyl aminophenol (H2LH) and 2-anilino-4,6-di-tert-butyl aminophenol (H2LPh). The heteroleptic cobalt complexes incorporate a Mentpa ligand (tpa = tris(2-pyridylmethyl)amine; n = 0-3), which involves the methylation of the 6-position of the pyridine ring). Eight members of this family have been synthesized and characterized: [CoII(HLPhAP)-(tpa)](BPh4) (1), [CoII(HLPhAP)-(Me2tpa)](BPh4) (2a), [CoII(LPhISQ)•-(Me2tpa)](BPh4) (2b), [CoII(HLPhAP)-(Me3tpa)](BPh4) (3a), [CoII(LPhISQ)•-(Me3tpa)](BPh4) (3b), [CoIII(HLHAP)-(tpa)] (BPh4)(ClO4) (4a), [CoIII(LHISQ)•-(tpa)](BPh4)(ClO4) (4b) and [CoII(HLHAP)-(Me2tpa)](BPh4) (5), where the aminophenol-derived ligands are monoanionic in either the open shell radical iminosemiquinonate (LISQ)•- or the closed shell protonated aminophenolate (HLAP)-. We isolate high-spin Co(II) and low-spin Co(III) compounds with no thermal interconversions evident in the solid state. In contrast, variable temperature electronic spectra for solutions of 3b suggest Co(II) spin crossover. Density functional theory calculations employing the M06L-D4/def2-TZVPP and TD-CAM-B3LYP/def2-TZVPP levels of theory support the experimental findings. This work presents the first example of a cobalt-iminosemiquinonate complex to exhibit spin crossover. The electronic structure versatility of complexes that integrate redox-active metal ions with both low- and high-spin states, alongside ligands that can exist in various redox and protonation states, holds significant promise for applications in molecular materials and chemical reactivity.

Force-Activated Spin-Crossover in Fe2+ and Co2+ Transition Metal Mechanophores

Transition metal mechanophores exhibiting force-activated spin-crossover are attractive design targets, yet large-scale discovery of them has not been pursued due in large part to the time-consuming nature of trial-and-error experiments. Instead, we leverage density functional theory (DFT) and external force explicitly included (EFEI) modeling to study a set of 395 feasible Fe2+ and Co2+ mechanophore candidates with tridentate ligands that we curate from the Cambridge Structural Database. Among nitrogen-coordinating low-spin complexes, we observe the prevalence of spin crossover at moderate force, and we identify 155 Fe2+ and Co2+ spin-crossover mechanophores and derive their threshold force for low-spin to high-spin transition (FSCO). The calculations reveal strong correlations of FSCO with spin-splitting energies and coordination bond lengths, facilitating rapid prediction of FSCO using force-free DFT calculations. Then, among all Fe2+ and Co2+ spin-crossover mechanophores, we further identity 11 mechanophores that combine labile spin-crossover and good mechanical robustness that are thus predicted to be the most versatile for force-probing applications. We discover two classes of mer-symmetric complexes comprising specific heteroaromatic rings within extended π-conjugation that give rise to Fe2+ mechanophores with these characteristics. We expect the set of spin-crossover mechanophores, the design principles, and the computational approach to be useful in guiding the high-throughput discovery of transition metal mechanophores with diverse functionalities and broad applications, including mechanically activated catalysis.

Light-induced spin-state switching in heterospin complexes of Cu(hfac)2 with pyridine-based nitroxides

Copper(II)-nitroxide based molecular magnets exhibit spin-crossover-like spin state switching, which is topical in field of molecular magnetism. However, establishing reliable structure-property relationships in these systems is still challenging, especially regarding the light-induced switching of spin states. In this paper, we report the investigation of photoswitching and relaxation in a series of heterospin Cu(hfac)2LR complexes with pyridine-based nitroxide ligands (LR), which belong to this family of materials. Using electron paramagnetic resonance (EPR) for detection, we demonstrate very long lifetimes of photoinduced spin states at liquid helium temperatures (<15 K), where relaxation to the ground state does not exceed 15% within two hours. At the same time, the efficiency of photoswitching strongly depends on the structure of the radical ligand in this series: the bulkier the ligand, the smaller the fraction of heterospin clusters that undergo photoswitching. These findings expand the understanding of mechanisms and factors behind photoswitching and relaxation in copper(II)-nitroxide molecular magnets and aid in further research aiming to optimize their functional properties.

The hydrate of a neutral iron(III) complex based on the pyruvic acid thiosemicarbazone ligand with abrupt spin-crossover with T1/2 = 340 K and a wide hysteresis loop of 45 K

The hydrate of a neutral iron(III) complex based on the pyruvic acid thiosemicarbazone ligand [FeIII(Hthpy)(thpy)]·H2O (1) was synthesized and characterized using FT-IR spectroscopy, powder and single-crystal X-ray diffraction, dc magnetic measurements, EPR and 57Fe Mössbauer spectroscopy. The crystal structure of 1 was determined for the first time. Two distinct chelating ligands Hthpy- and thpy2- coordinate the Fe(III) ion to form the FeN2O2S2 octahedron which shows a low spin geometry at 150-350 K. The crystal packing contains infinite chains of the Fe(III) complexes as well as water molecules located in cavities. Along the chain, π-π interacting pairs of the Fe complexes are linked by H-bonding. According to the dc magnetic measurements, the complete abrupt spin-crossover with half-transition temperature T1/2 = 340 K and a hysteresis loop of 45 K occurs in the temperature range of 300-363 K. Based on the X-ray structure of 1, the Bleaney-Bowers equation for the isolated dimer model was used to approximate the temperature dependence of the magnetic susceptibility in the range of 2-50 K. The defined intradimer exchange constant Jexp = -0.498(1) K corresponds to a weak antiferromagnetic exchange between the iron(III) magnetic centers. DFT calculations of H- and π-π bonded fragments of the crystal structure of 1 in the HS and LS states were carried out. Moreover, BS-DFT calculations confirm the presence of antiferromagnetic exchange Jcalc = -0.92 K in the π-π bonded pairs of the ferric complexes and show the exchange pathway between Fe(III) ions by the calculated spin density distribution.

Elucidation of Electronic Structures of Mixed-Valence States Induced by dσ-π Charge Delocalization in Linear-Chain and Discrete Rhodium-Dioxolene Tetrameric Complexes

We have successfully synthesized unique linear-chain and discrete mixed-valence tetrameric complexes, {[Rh(3,6-DBDiox-4,5-S2CO)(CO)2]4·hexane}∞ (4) and [Rh(3,6-DBDiox-4,5-S2CO)(CO)2]4 (5), by carefully choosing the solvent. X-ray photoelectron spectra (XPS) confirm that 4 and 5 are in the Rh(I,II) mixed-valence state. Analyses of the metrical oxidation state (MOS) of dioxolene ligands reveal that in 4 and 5, the electron density corresponding to one electron per tetramer is transferred from Rh(I) ions to semiquinonato ligands, and the transferred charge is delocalized throughout the four dioxolene ligands. Due to their mixed-valence state, 4 and 5 are semiconductors with relatively high electrical conductivity at room temperature. Density functional theory (DFT) calculations of tetrameric complex demonstrated for the first time that the dσ* orbitals of the Rh atoms and the π* orbitals of the semiquinonato ligands, which are originally highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively, strongly hybridize with each other, leading to the Rh(I,II)-semiquinonato/catecholato mixed-valence state. Furthermore, time-dependent (TD)-DFT calculations have also revealed that the low energy absorption band observed centered at 5700 cm-1 is attributed to a charge transfer from [dσ*(Rh)] (HOMO) or [π*(SQ)-dσ*(Rh)] (HOMO-1) to [π*(SQ)-dσ*(Rh)] (LUMO/LUMO+1). Although 4 and 5 are tetramers with nearly identical structures, their magnetic interactions are found to differ significantly depending on their crystal structures.

Molecular Fe(II)-Ln(III) dyads for luminescence reading of spin-state equilibria at the molecular level

Due to the primogenic effect, the valence shells of divalent iron Fe(II) ([Ar]3d6) and trivalent lanthanides Ln(III) ([Xe]4fn) are compact enough to induce spin-state equilibrium for the 3d-block metal and atom-like luminescence for the 4f-block partner in Fe(II)-Ln(III) dyads. In the specific case of homoleptic pseudo-octahedral [Fe(II)N6] units, programming spin crossover (SCO) around room temperature at normal pressure requires the design of unsymmetrical didentate five-membered ring chelating N∩N' ligands, in which a five-membered (benz)imidazole heterocycle (N) is connected to a six-membered pyrimidine heterocycle (N'). Benefiting from the trans influence, the facial isomer fac-[Fe(II)(N∩N')3]2+ is suitable for inducing SCO properties at room temperature in solution. Its connection to luminescent [LnN6O3] chromophores working as non-covalent podates in the triple-stranded [Fe(II)Ln(L10)3]5+ helicates (Ln = Nd, Eu) controls the facial arrangement around Fe(II). The iron-based SCO behaviour of the 3d-4f complex mirrors that programmed in the mononuclear scaffold. Because of the different electronic structures of high-spin and low-spin [Fe(II)N6] units, their associated absorption spectra are different and modulate the luminescence of the appended lanthanide luminophore via intramolecular intermetallic energy transfers. It thus becomes possible to detect the spin state of the Fe(II) center, encoded by an external perturbation (i.e. writing), by lanthanide light emission (i.e. reading) in a single molecule and without disturbance. Shifting from visible emission (Ln = Eu) to the near-infrared domain (Ln = Nd) further transforms a wavy emitted signal intensity into a linear one, a protocol highly desirable for future applications in data storage and thermometry.

Asymmetric conformation of the high-spin state of iron(II)-tris(2,2-bipyridine): Time-resolved x-ray absorption and ultraviolet circular dichroism

We analyze the structures of the low-spin (LS) ground state and the high-spin (HS) lowest excited state of the iron-(II)-tris bipyridine complex ([Fe(bpy)3]2+) using density functional theory PBE methods, modeling the solvent interactions with conductor-like polarizable continuum model. These calculations are globally benchmarked against a wide range of experimental observables that include ultraviolet-visible linear absorption and circular dichroism (CD) spectra and Fe K-edge x-ray absorption near edge spectra (XANES). The calculations confirm the already established D3 geometry of the LS state, as well as a departure from this geometry for the HS state, with the appearance of inequivalent Fe-N bond elongations. The simulated structures nicely reproduce the above-mentioned experimental observables. We also calculate the vibrational modes of the LS and HS states. For the former, they reproduce well the vibrational frequencies from published infrared and Raman data, while for the latter, they predict very well the low-frequency vibrational coherences, attributed to Fe-N stretch modes, which were reported in ultrafast spectroscopic experiments. We further present calculations of the high-frequency region, which agree with recent ultrafast transient infrared spectroscopy studies. This work offers a common basis to the structural information encoded in the excited state CD and the Fe K XANES of the HS state tying together different structural IR, UV-visible, and x-ray observables.

Spin crossover FeIII complexes with a substituted Hqnal ligand: effects of anions and solvents

A new substituted Hqnal ligand, Hqnal-5-Brq, and four resulting FeIII complexes [Fe(qnal-5-Brq)2]A·sol (A = NO3-, sol = CH3OH 1; A = ClO4-, sol = CH3OH 2; A = OTf-, sol = 2CH3OH·H2O 3; and A = NTf2-, sol = CH2Cl24; Hqnal-5-Brq = N-(5-bromo-8-quinolinyl)-2-hydroxynaphthaldimine), have been synthesized and characterized. All four complexes, despite having different anions, adopt similar 1D [Fe(qnal-5-Brq)2]+ cation chains linked by orthogonal π⋯π interactions. These chains are further connected to form 2D and 3D structures by other supramolecular interactions. Complexes 1-3 all exhibit abrupt spin crossover behaviors, with the transition temperatures being 230, 189, and 185 K, respectively, while complex 4 is in a high-spin state. The influence of solvents on spin crossover properties was assessed via magnetic measurements on the desolvated samples. Following desolvation, while complexes 1 and 2 show slight variations in their transition temperatures, complexes 3 and 4 undergo significant changes in their magnetic profiles. Desolvation in complex 3 leads to either a HS state or a very incomplete hysteretic transition with a low transition temperature for different sample batches. In contrast, the gradual desolvation in complex 4 leads to diminished solvent residues and progressive transition from a stable HS state to an SCO-active state.

Gradual spin crossover behavior encompasing room temperature in an iron(II) complex based on a heteroscorpionate ligand

In this paper, we report the synthesis of six novel triazole-based heteroscorpionate ligands based on heterocycle metathesis reactions and their iron(II) complexes. Single crystal structure analyses were performed, the spectroscopic and magnetic properties of the obtained complexes were studied and their spin crossover-structural relationships were compared to those obtained for their pyrazole-based analogues reported in the literature. In particular, the amino derivative complex bis[hydrobis(pyrazol-1-yl)(3-amino-1,2,4-triazol-1-yl)]iron(II) obtained by post-synthetic catalytic nitro-group reduction under pressure of hydrogen in an autoclave presents a scarce gradual spin crossover behavior at room temperature. The profile of the SCO curve can be explained by the presence of only relatively weak H bonds, spreading only in one dimension. Among the interesting spin transition behaviors observed for the different complexes, such stable, complete and gradual spin crossover at room temperature makes this neutral complex a good candidate for sublimation and future investigation as an active element notably for thermoreflectance-based surface microthermometry applications.

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.

Control of Iron(II)-Tris(2,2'-Bipyridine) Light-Induced Excited-State Trapping via External Electromagnetic Fields

Light-induced excited spin-state trapping reactions in iron pyridinic complexes allow the iron's low-to-high spin transition in a sub-picosecond timescale. Employing a recently developed model for [Fe(2,2'-bipyridine)3]2+ photochemical spin-crossover reaction in conjunction with quantum wavepacket dynamics, we explore the possibility of controlling the reaction through external electromagnetic fields, aiming at stabilizing the initial metal-to-ligand charge transfer states. We show that simple Gaussian-shaped electromagnetic fields have a minor effect on the population kinetics. However, introducing vibrationally excited initial wavepacket representations allows for maintaining the population trapped in the metal-to-ligand charge transfer states. Using optimal control theory, we propose an electromagnetic field shape that increases the lifetime of metal-to-ligand charge transfer states. These results open the route for controlling the iron photochemistry through the action of external electric fields.

Observation of TLIESST above Liquid Nitrogen Temperature and Disclosure of Hidden Hysteresis in Multiresponsive Hofmann-type Coordination Polymers

Photoresponsive spin-crossover (SCO) molecules are an important class of bistable magnetic molecules with intriguing potential in device applications. The light-induced excited spin state trapping (LIESST) and the combined application of light and temperature can provide access to the metastable region of the SCO profile. The primary obstacle in utilizing light stimuli is the manifestation of light-induced trappings at extremely low temperatures. Herein, we report two novel multiresponsive 2D Hofmann-type coordination polymers exhibiting light-induced excited spin state trapping above liquid nitrogen temperature (TLIESST = 82 and 81 K). Stimulating the samples in conjugation with light and temperature successfully unveils hysteresis, which is otherwise concealed. Apart from light and temperature, we found that the SCO phenomenon is also responsive to external hydrostatic pressure and exhibits modulation of the hysteresis width and transition temperature shifts with changes in pressure.

Out-of-equilibrium dynamics of a grid-like Fe(ii) spin crossover dimer triggered by a two-photon excitation

The application of two-photon excitation (TPE) in the study of light-responsive materials holds immense potential due to its deeper penetration and reduced photodamage. Despite these benefits, TPE has been underutilised in the investigation of the photoinduced spin crossover (SCO) phenomenon. Here, we employ TPE to delve into the out-of-equilibrium dynamics of a SCO FeII dimer of the form [FeII(HL)2]2(BF4)4·2MeCN (HL = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole). Optical transient absorption (OTA) spectroscopy in solution proves that the same dynamics take place under both one-photon excitation (OPE) and TPE. The results show the emergence of the photoinduced high spin state in less than 2 ps and with a lifetime of 147 ns. Time-resolved photocrystallography (TRXRD) reveals a single molecular reorganisation within the first 500 ps following TPE. Additionally, variable temperature single crystal X-ray diffraction (VTSCXRD) and magnetic susceptibility measurements confirm that the thermal transition is silenced by the solvent. While the results of the OTA and TRXRD utilising TPE are intriguing, the high pump fluencies required to excite enough metal centres to the high spin state may impair its practical application. Nonetheless, this study sheds light on the potential of TPE for the investigation of the out-of-equilibrium dynamics of SCO complexes.

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.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Creation of spin switching in graphene oxide-based hybrid film materials with an anionic Fe(III) 5Cl-salicyaldehyde-thiosemicarbazone complex

The present article describes the synthesis of hybrid composite film materials formed during the self-assembly process through non-covalent interactions of graphene oxide (GO) nanosheets with salt 1, represented by an anionic spin-crossover complex [FeIII(5Cl-thsa)2]- (5Cl-thsa - 5-chlorosalicylaldehyde thiosemicarbazone) and the organic tetraethylammonium cation [Et4N]+. The insertion of the salt 1 molecules into the interlayer space of GO nanosheets with the subsequent formation of a hybrid material GO-1 was observed. The film of the hybrid material GO-1 was characterized by scanning electron and confocal laser microscopy, EDX and XPS analysis, IR, Raman and 57Fe Mössbauer spectroscopy, dc magnetic measurements, and powder X-ray diffraction. Comparison of the magnetic properties of salt 1 and a film of the hybrid material GO-1 demonstrated a significant influence of the GO nanosheets matrix on the completeness of spin transition and showed a slight shift of the hysteresis loop by 1 K in the temperature range of 200-230 K. DFT calculations showed an important role of the organic cation [Et4N]+ in the process of adsorption of the spin-crossover anion [FeIII(5Cl-thsa)2]- on the GO nanosheet surface.

Unravelling Kinetics of Intramolecular NdIII → FeII Energy Transfer in Spin Crossover Single Molecules: Dotting the i's and Crossing the t's

Compared with the ripple of visible EuIII-based emission intensity induced by appended [FeIIN6] spin crossover (SCO) units, as detected in the triple-stranded [EuFe(L1)3]5+ helicate, the lanthanide-based luminescent detection of FeII spin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing EuIII with NdIII in [NdFe(L1)3]5+ (i) maintains the favorable SCO properties in acetonitrile [critical temperature T1/2 = 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233-333 K range, and (iii) boosts the crucial intramolecular NdIII → FeII energy transfer rate processes, which are sensitive to the spin state of the FeII metallic center. Consequently, the steady-state NIR Nd(4F3/2 → 4IJ) emission of the luminophore is amplified and linearly correlated with the low-spin-[FeIIN6] and high-spin-[FeIIN6] mole fractions controlled by the SCO equilibrium. This paves the way for a straightforward and direct NIR luminescent reading/sensing of the FeII spin state in single molecules.

Thermodynamics of spin crossover in a bis(terpyridine) cobalt(II) complex featuring a thioether functionality

In this contribution, a terpyridine-based ligand bearing a thioether functionality is used to prepare a new cobalt(II) spin crossover complex: [Co(TerpyPhSMe)2](PF6)2 (1), where TerpyPhSMe is 4'-(4-methylthiophenyl)-2,2':6',2''-terpyridine. Its structure, determined by single crystal X-ray diffraction, reveals a mer coordination of the tridentate terpyridine ligands, leading to a tetragonally compressed octahedron. Intermolecular interactions in the crystal lattice freeze the complex in the high spin state in the solid state at all temperatures, as indicated by magnetometry and Electron Paramagnetic Resonance (EPR) spectra. When dissolved in acetonitrile, however, temperature dependent electronic, 1H-NMR and EPR spectra highlight an entropy-driven spin crossover transition, whose thermodynamics parameters have been determined. This is the first report of a cobalt(II) SCO complex featuring a thioether group, allowing its implementation in chemically grown bistable monolayers and may open important perspectives for the use of such systems in molecular spintronics.

Self-assembly of a supramolecular spin-crossover tetrahedron

A new mononuclear iron(II) SCO compound featuring H-bonding donor and acceptor units has been synthesized and exploited to produce a purely supramolecular switchable [Fe4] tetrahedron. Magnetic and crystallographic measurements evidence a singular magnetic behavior for each of the four Fe(II) centers of the generated architecture and underscore the potential of this strategy to develop novel SCO materials.

The role of intermolecular interactions in [Fe(X-salEen)2]ClO4 spin crossover complexes

Two new spin crossover (SCO) Fe(III) compounds were prepared, their structures were analysed and their magnetic properties were investigated. An exhaustive analysis of the effects of halogen substitution and aromatic ring functionalisation on the magnetic properties of non-solvated Fe(III) perchlorate complexes has been performed. Through comparative analysis, different magnetic profiles were found for the compounds studied, namely F (1), Cl (2), H (3), Br (4a, 4b), and I (5). Using tools like Hirshfeld analysis, the study revealed patterns in octahedral distortions and deviations from the ideal octahedral geometry. The SCO phenomenon as the conducting wire in this study, emphasises the influence of intermolecular interactions on the low spin (LS) to high spin (HS) transitions in these halogen-substituted complexes. The prevalence of H⋯H contributions has been demonstrated, albeit being the weakest and an inverse strength relationship in H⋯X interactions ranging from F to I. The findings not only interpret the intricate balance between halogen substitution, functionalisation, and intermolecular interactions in modulating magnetic properties but also direct future works in designing similar molecular systems.

Two-dimensional spin-crossover coordination polymers based on the 1,1,2,2-tetra(pyridin-4-yl)ethene ligand

A series of two-dimensional (2D) spin-crossover coordination polymers (SCO-CPs) [FeII(TPE)(NCX)2]·solv (1: X = BH3, solv = H2O·2CH3OH·DMF; 2: X = Se, solv = H2O·2CH3OH·0.5DMF; 3: X = S, solv = H2O·2CH3OH·0.5DMF) were synthesized by employing 1,1,2,2-tetra(pyridin-4-yl)ethene (TPE) and pseudohalide (NCX-) coligands. Magnetic measurements indicated that complexes 1-3 exhibited SCO behaviors with diminishing thermal hysteresis (7/4/0 K) upon decreasing the ligand-field strength. The critical temperatures (Tc) during spin transition were found to be inversely proportional to the coordination ability parameters (a™) with a linear correlation. The guest effect was also investigated in the solvent-exchanged phases 1-SE/2-SE/3-SE wherein the DMF molecules were replaced by methanol molecules. Compared with 1-3, 1-SE/2-SE/3-SE displayed more abrupt and complete single-step SCO behaviors but narrower thermal hysteretic loops. The results reported here demonstrate that the Tc values of these two families were dominated by the ligand-field strength of the NCX- anions (NCBH3 > NCSe > NCS), whereas the guest effect only modulated the kinetic factor of the SCO nature in this system.

Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer

A novel two-dimensional (2D) Hofmann-type coordination polymer, {FeII(PyHbim)2[Pd(CN)4]}·2CH3OH [1·2CH3OH, PyHbim = 2-(4-pyridyl)benzimidazole], has been synthesized, which can undergo a spontaneous guest exchange, transforming to 1·2H2O in a single-crystal-to-single-crystal (SCSC) manner, shifting from orthorhombic Cmmm to monoclinic C2/m involving the displacement of 2D layers. The solvent-induced SCSC transformation process was reversible and verified through powder X-ray diffraction (PXRD) and single-crystal X-ray crystallography analyses. Both 1·2CH3OH and 1·2H2O exhibit complete and abrupt spin crossover (SCO) behaviors in two steps, while their SCO temperature ranges drastically shift by ca.100 K, spanning room temperature, owing to different intermolecular interactions resulting from diverse interlayer packing manners and host-guest interactions. Besides, a structural phase transition is observed in 1·2CH3OH, contributing to the two-step spin transition.

Expanding the dimensionality of bis(tetrazolyl)alkane-based Fe(II) coordination polymers by the application of dinitrile coligands

Reactions between 1,2-di(tetrazol-2-yl)ethane (ebtz), 1,6-di(tetrazol-2-yl)hexane (hbtz) or 1,1'-di(tetrazol-1-yl)methane (1ditz) and Fe(BF4)2 in the presence of adiponitrile (ADN), glutaronitrile (GLN) or suberonitrile (SUN) resulted in the formation of coordination polymers [Fe(μ-ebtz)2(μ-ADN)](BF4)2 (1), [Fe(μ-hbtz)2(μ-ADN)](BF4)2 (2), [Fe(μ-1ditz)2(GLN)2](BF4)2·GLN (3) and [Fe(μ-1ditz)2(μ-SUN)](BF4)2·SUN (4). It was established that the application of dinitriles allows an increase in the dimensionality of the ebtz and hbtz based systems while maintaining the structure of the polymeric units characteristic of previously studied mononitrile based analogues. In 3 and 4, regardless of the type of dinitrile coligand, the motif of 2D polymeric layers constituted by 1ditz molecules remains preserved. However, the dimensionality of 1ditz based networks is governed by the coordination modes of dinitriles. 3, based on a shorter molecule of glutaronitrile, crystallizes as a two-dimensional (2D) coordination polymer. In this compound, dinitriles coordinate monodentately or play the role of guest molecules. The substitution of glutaronitrile with suberonitrile enables the bridging of neighboring polymeric layers, resulting in a 3D network. The intentional selection of bis(tetrazoles) and dinitriles as building blocks has led, as expected, to obtaining systems with the structure of the first coordination sphere consisting of four tetrazole rings and two axially coordinated nitrile molecules. It created the conditions required for the occurrence of thermally induced spin crossover. Magnetic measurements and single crystal X-ray diffraction studies were used for the characterization of the spin crossover properties of 1-4.

Light-Controlled Multiconfigurational Conductance Switching in a Single 1D Metal-Organic Wire

Precise control of multiple spin states on the atomic scale presents a promising avenue for designing and realizing magnetic switches. Despite substantial progress in recent decades, the challenge of achieving control over multiconfigurational reversible switches in low-dimensional nanostructures persists. Our work demonstrates multiple, fully reversible plasmon-driven spin-crossover switches in a single π-d metal-organic chain suspended between two electrodes. The plasmonic nanocavity stimulated by external visible light allows for reversible spin crossover between low- and high-spin states of different cobalt centers within the chain. We show that the distinct spin configurations remain stable for minutes under cryogenic conditions and can be nonperturbatively detected by conductance measurements. This multiconfigurational plasmon-driven spin-crossover demonstration extends the available toolset for designing optoelectrical molecular devices based on SCO compounds.

Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5'-bis(methylthio)-2,2'-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires.

Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition

Materials based on spin crossover (SCO) molecules have centered the attention in molecular magnetism for more than 40 years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here, we report a novel example of this kind. It is based on a neutral iron(II) coordination complex formulated as [Fe(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part, a comprehensive study, which covers the synthesis and magnetostructural characterization of the [Fe(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part, we investigate the suitability of this material to form thin films through high-vacuum sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm.

Perspective from a Hubbard U-density corrected scheme towards a spin crossover-mediated change in gas affinity

By employing a recently proposed Hubbard U density-corrected scheme within density functional theory, we provide design principles towards the design of materials exhibiting a spin crossover-assisted gas release. Small molecular fragments are used as case study to identify two main mechanisms behind the change in binding energy upon spin transitions. The feasibility of the proposed mechanism in porous crystals is assessed by correlating the change in binding energy of CO2, CO, N2, and H2, upon spin crossover, with the adiabatic energy difference associated with the spin state change of the square-planar metal in Hofmann-type clathrates (M = Fe, Mn, Ni). A few promising cases are identified for the adsorption of intermediate ligand field strength molecules such as N2 and H2. The latter stands out as the most original result as the strong interaction in low spin, as expected from a Kubas mechanism, results in a large change in binding energy. This work provides a general perspective towards the engineering of open-metal site frameworks exhibiting local environments designed to have a spin crossover upon adsorption of specific gas molecules.

Understanding Antiferromagnetic and Ligand Field Effects on Spin Crossover in a Triple-Decker Dimeric Cr(II) Complex

Two possible explanations for the temperature dependence of spin-crossover (SCO) behavior in the dimeric triple-decker Cr(II) complex ([(η5-C5Me5)Cr(μ2:η5-P5)Cr(η5-C5Me5)]+) have been offered. One invokes variations in antiferromagnetic interactions between the two Cr(II) ions, whereas the other posits the development of a strong ligand-field effect favoring the low-spin ground state. We perform multireference electronic structure calculations based on the multiconfiguration pair-density functional theory to resolve these effects. We find quintet, triplet, and singlet electronic ground states, respectively, for the experimental geometries at high, intermediate, and low temperatures. The ground-state transition from quintet to triplet at an intermediate temperature derives from increased antiferromagnetic interactions between the two Cr(II) ions. By contrast, the ground-state transition from triplet to singlet at low temperature can be attributed to increased ligand-field effects, which dominate with continued variations in antiferromagnetic coupling. This study provides quantitative detail for the degree to which these two effects can act in concert for the observed SCO behavior in this complex and others subject to temperature-dependent variations in geometry.

Crystal structure of bis-{3-(3,4-di-meth-oxy-phen-yl)-5-[6-(pyrazol-1-yl)pyridin-2-yl]-1,2,4-triazol-3-ato}iron(II)-methanol-chloro-form (1/2/2)

The unit cell of the title compound, [Fe(C18H15N6O2)2]·2CH3OH·2CHCl3, consists of a charge-neutral complex mol-ecule, two methanol and two chloro-form mol-ecules. In the complex, the two tridentate 2-(5-(3,4-di-meth-oxy-phen-yl)-1,2,4-triazol-3-yl)-6-(pyrazol-1-yl)pyridine ligands coordinate to the central FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa-hedral coordination sphere. Neighbouring tapered mol-ecules are linked through weak C-H(pz)⋯π(ph) inter-actions into one-dimensional chains, which are joined into two-dimensional layers through weak C-H⋯N/C/O inter-actions. Furthermore, the layers stack in a three-dimensional network linked by weak inter-layer C-H⋯π inter-actions of the meth-oxy and phenyl groups. The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H 32.0%, H⋯C/C⋯H 26.3%, H⋯N/N⋯H 13.8%, and H⋯O/O⋯H 7.5%. The average Fe-N bond distance is 2.185 Å, indicating the high-spin state of the FeII ion. Energy framework analysis at the HF/3-21 G theory level was performed to qu-antify the inter-action energies in the crystal structure.

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction

Calculation of transition temperatures T1/2 for thermally driven spin-crossover in condensed phases is challenging, even with sophisticated state-of-the-art density functional approximations. The first issue is the accuracy of the adiabatic crossover energy difference ΔEHL between the low- and high-spin states of the bistable metal-organic complexes. The other is the proper inclusion of entropic contributions to the Gibbs free energy from the electronic and vibrational degrees of freedom. We discuss the effects of treatments of both contributions upon the calculation of thermochemical properties for a set of 20 spin-crossover materials using a Hubbard-U correction obtained from a reference ensemble spin-state. The U values obtained from a simplest bimolecular representation may overcorrect, somewhat, the ΔEHL values, hence giving somewhat excessive reduction of the T1/2 results with respect to their U = 0 values in the crystalline phase. We discuss the origins of the discrepancies by analyzing different sources of uncertainties. By use of a first-coordination-sphere approximation and the assumption that vibrational contributions from the outermost atoms in a metal-organic complex are similar in both low- and high-spin states, we achieve T1/2 results with the low-cost, widely used PBE generalized gradient density functional approximation comparable to those from the more costly, more sophisticated r2SCAN meta-generalized gradient approximation. The procedure is promising for use in high-throughput materials screening, because it combines rather low computational effort requirements with freedom from user manipulation of parameters.

Dual-radical-based molecular anisotropy and synergy effect of semi-conductivity and valence tautomerization in a photoswitchable coordination polymer

Organic radicals are widely used as linkers or ligands to synthesize molecular magnetic materials. However, studies regarding the molecular anisotropies of radical-based magnetic materials and their multifunctionalities are rare. Herein, a photoisomerizable diarylethene ligand was used to form {[Co^III(3,5-DTSQ^·-)(3,5-DTCat^2-)]₂(6F-DAE-py₂)}·3CH₃CN·H₂O (o-1·3CH₃CN·H₂O, 6F-DAE-py₂ = 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene), a valence-tautomeric (VT) coordination polymer. We directly observed dual radicals for a single crystal using high-field/-frequency (∼13.3 T and ∼360 GHz) electron paramagnetic resonance (EPR) spectroscopy along the c-axis, which was further confirmed by angle-dependent Q-band EPR spectroscopy. Moreover, a conductive anomaly close to the VT transition temperature was observed only when probes were attached at the ab plane of the single crystal, indicative of synergy between valence tautomerism and conductivity. Structural anisotropy studies and density functional theory (DFT) calculations revealed that this synergy is due to electron transfer associated with valence tautomerism. This study presents the first example of dual-radical-based molecular anisotropy and charge-transfer-induced conductive anisotropy in a photoswitchable coordination polymer.

Dynamic effects on ligand field from rapid hydride motion in an iron(ii) dimer with an S = 3 ground state

Hydride complexes are important in catalysis and in iron-sulfur enzymes like nitrogenase, but the impact of hydride mobility on local iron spin states has been underexplored. We describe studies of a dimeric diiron(ii) hydride complex using X-ray and neutron crystallography, Mössbauer spectroscopy, magnetism, DFT, and ab initio calculations, which give insight into the dynamics and the electronic structure brought about by the hydrides. The two iron sites in the dimer have differing square-planar (intermediate-spin) and tetrahedral (high-spin) iron geometries, which are distinguished only by the hydride positions. These are strongly coupled to give an S total = 3 ground state with substantial magnetic anisotropy, and the merits of both localized and delocalized spin models are discussed. The dynamic nature of the sites is dependent on crystal packing, as shown by changes during a phase transformation that occurs near 160 K. The change in dynamics of the hydride motion leads to insight into its influence on the electronic structure. The accumulated data indicate that the two sites can trade geometries by rotating the hydrides, at a rate that is rapid above the phase transition temperature but slow below it. This small movement of the hydrides causes large changes in the ligand field because they are strong-field ligands. This suggests that hydrides could be useful in catalysis not only due to their reactivity, but also due to their ability to rapidly modulate the local electronic structure and spin states at metal sites.

First crystal structure of an Fe(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with Li+: synthesis, features of magnetic behavior and theoretical analysis

The iron(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with the lithium cation Li[Fe^III(thpy)₂]·3H₂O (1) has been synthesized and characterized by FTIR spectroscopy, powder and single crystal X-ray diffraction, direct current magnetic susceptibility measurements, and ⁵⁷Fe Mössbauer spectroscopy. Moreover, the molecular structure of the [Fe(thpy)₂]^- anion has been determined for the first time. The [Fe(thpy)₂]^- units in the triclinic P1̄ lattice of 1 are assembled into layers parallel to the bc plane. The Li⁺ cations and water molecules are located between the layers and the structure is stabilized by hydrogen bonding. The [Fe(thpy)₂]^- anions form interconnected dimer pairs through hydrogen bonds and short contacts with Fe⋯Fe separation of 6.7861(4) Å. According to dc magnetic measurements, compound 1 demonstrates an incipient spin-crossover transition from the LS (S = 1/2) to the HS (S = 5/2) state above 250 K. The Bleaney-Bowers equation for a model of an isolated LS dimer with a mean-field correction was applied to fit the experimental data of magnetic susceptibility dependence on temperature in the temperature range of 2-250 K. The intra-dimer J₁ = -1.79(1) K and inter-dimer J₂ = -0.24(3) K antiferromagnetic coupling constants were defined. The analysis of the ⁵⁷Fe Mössbauer spectra at 80 K and 296 K confirms the presence of the shortened distances between the iron nuclei. Moreover, the influence of the lithium cation on the stabilization of the LS state was shown for the [Fe(thpy)₂]^- anion. BS-DFT calculations for the optimized structure of two isolated [Fe(thpy)₂]^- anions also correctly predict a weak exchange J₁(calc) = -0.92 K. DFT calculations revealed the OPBE (GGA-type) functional that correctly predicts the spin-crossover transition for the iron(III) thpy compounds. Besides, the effect of the N₂O₄, N₂S₂O₂, and N₂Se₂O₂ coordination environments on the energy stabilization of the LS state of iron(III) anionic thpy complexes was noted as well.

FeIII in the high-spin state in dimethylammonium bis[3-ethoxysalicylaldehyde thiosemicarbazonato(2-)-κ3O2,N1,S]ferrate(III)

The synthesis and crystal structure (100 K) of the title compound, [(CH3)2NH2][Fe(C10H11O2N3S)2], are reported. The asymmetric unit consists of an octahedral [FeIII(L)2]- fragment, where L2- is 3-ethoxysalicylaldehyde thiosemicarbazonate(2-), and a dimethylammonium cation. Each L2- ligand binds with the thiolate S, the imine N and the phenolate O atoms as donors, resulting in an FeIIIS2N2O2 chromophore. The ligands are orientated in two perpendicular planes, with the O and S atoms in cis positions, and mutually trans N atoms. The FeIII ion is in the high-spin state at 100 K. The variable-temperature magnetic susceptibility measurements (5-320 K) are consistent with the presence of a high-spin FeIII ion with D = 0.83 (1) cm-1 and g = 2.

Mean Value Ensemble Hubbard-U Correction for Spin-Crossover Molecules

High-throughput searches for spin-crossover molecules require Hubbard-U corrections to common density functional exchange-correlation (XC) approximations. However, the U_eff values obtained from linear response or based on previous studies overcorrect the spin-crossover energies. We demonstrate that employing a linearly mixed ensemble average spin state as the reference configuration for the linear response calculation of U_eff resolves this issue. Validation on a commonly used set of spin-crossover complexes shows that these ensemble U_eff values consistently are smaller than those calculated directly on a pure spin state, irrespective of whether that be low- or high-spin. Adiabatic crossover energies using this methodology for a generalized gradient approximation XC functional are closer to the expected target energy range than with conventional U_eff values. Based on the observation that the U_eff correction is similar for different complexes that share transition metals with the same oxidation state, we devise a set of recommended averaged U_eff values for high-throughput calculations.

Transient Mesoscopic Immiscibility, Viscosity Anomaly, and High Internal Pressure at the Semiconductor-Metal Transition in Liquid Ga2Te3

Gallium tellurides appear to be promising phase-change materials (PCMs) of the next generation for brain-inspired computing and reconfigurable optical metasurfaces. They are different from the benchmark PCMs because of sp³ gallium hybridization in both cubic Ga₂Te₃ and amorphous pulsed laser deposition (PLD) films. Liquid Ga₂Te₃ also shows a viscosity η(T) anomaly just above melting when η(T) first increases and only then starts decreasing. We used high-energy X-ray diffraction to observe a transient mesoscopic immiscibility that suggested dense metallic liquid droplets in a semiconducting melt. The η(T) shape was consistent with this finding. A vanishing first sharp diffraction peak that also shifts to a higher Q indicates a high internal pressure in the metallic melt, which produces a remarkable asymmetry of the Ga-Te nearest neighbor distances and is reminiscent of high-pressure rhombohedral Ga₂Te₃. The observed phenomena provide a realistic scenario for a fast, multilevel SET-RESET response, which also unravels similar trends in the purported density-driven liquid polyamorphism of water, phosphorus, sulfur, and other materials.

High-Sensitivity Microthermometry Method Based on Vacuum-Deposited Thin Films Exhibiting Gradual Spin Crossover above Room Temperature

We report on the fabrication, characterization, and microthermometry application of high-quality, nanometric thin films, with thicknesses in the range 20-200 nm, of the molecular spin-crossover complex [Fe(HB(1,2,3-triazol-1-yl)₃)₂]. The films were obtained by vacuum thermal evaporation and characterized by X-ray diffraction, UV spectrophotometry, and atomic force microscopy. The as-deposited films are dense and crystalline with a preferred [011] orientation of the monoclinic crystal lattice normal to the substrate surface. The films exhibit a gradual spin conversion centered at ca. 374 K spanning the 273-473 K temperature range, irrespective of their thickness. When deposited on a microelectronic device, these films can be used to enhance the UV-light thermoreflectance coefficient of reflective surfaces by more than an order of magnitude, allowing for high-sensitivity thermoreflectance thermal imaging.

Electro-Elastic Modeling of Thermal Spin Transition in Diluted Spin-Crossover Single Crystals

Spin-crossover solids have been studied for many years for their promising applications as optical switches and reversible high-density memories for information storage. This study reports the effect of random metal dilution on the thermal and structural properties of a spin-crossover single crystal. The analysis is performed on a 2D rectangular lattice using an electro-elastic model. The lattice is made of sites that can switch thermally between the low-spin and high-spin states, accompanied by local volume changes. The model is solved by Monte Carlo simulations, running on the spin states and atomic positions of this compressible 2D lattice. A detailed analysis of metal dilution on the magneto-structural properties allows us to address the following issues: (i) at low dilution rates, the transition is of the first order; (ii) increasing the concentration of dopant results in a decrease in cooperativity and leads to gradual transformations above a threshold concentration, while incomplete spin transitions are obtained for big dopant sizes. The effects of the metal dilution on the spatiotemporal aspects of the spin transition along the thermal transition and on the low-temperature relaxation of the photo-induced metastable high-spin states are also studied. Significant changes in the organization of the spin states are observed for the thermal transition, where the single-domain nucleation caused by the long-range elastic interactions is replaced by a multi-droplet nucleation. As to the issue of the relaxation curves: their shape transforms from a sigmoidal shape, characteristic of strong cooperative systems, into stretched exponentials for high dilution rates, which is the signature of a disordered system.

Crystal structure of bis-{3-(3-bromo-4-methoxyphenyl)-5-[6-(1H-pyrazol-1-yl)pyridin-2-yl]-1,2,4-triazol-3-ato}-iron(II) methano-l disolvate

The unit cell of the title compound, [FeII(C17H12BrN6O)2]·2MeOH, consists of a charge-neutral complex mol-ecule and two independent mol-ecules of methanol. In the complex mol-ecule, the two tridentate ligand mol-ecules 2-[5-(3-bromo-4-meth-oxy-phen-yl)-4H-1,2,4-triazol-3-yl]-6-(1H-pyrazol-1-yl)pyridine coordinate to the FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa-hedral coordination sphere around the central ion. In the crystal, neighbouring asymmetric mol-ecules are linked through weak C-H(pz)⋯π(ph) inter-actions into chains, which are then linked into layers by weak C-H⋯N/C inter-actions. Finally, the layers stack into a three-dimensional network linked by weak inter-layer C-H⋯π inter-actions between the meth-oxy groups and the phenyl rings. The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H 34.2%, H⋯C/C⋯H 25.2%, H⋯Br/Br⋯H 13.2%, H⋯N/N⋯H 12.2% and H⋯O/O⋯H 4.0%. The average Fe-N bond distance is 1.949 Å, indicating the low-spin state of the FeII ion. Energy framework analysis at the HF/3-21 G theory level was performed to qu-antify the inter-action energies in the crystal structure.

Crystal structure of bis-{3-(3,4-di-methyl-phen-yl)-5-[6-(1H-pyrazol-1-yl)pyridin-2-yl]-4H-1,2,4-triazol-4-ido}iron(II) methanol disolvate

As a result of the high symmetry of the Aea2 structure, the asymmetric unit of the title compound, [FeII(C18H15N6)2]·2MeOH, consists of half of a charge-neutral complex mol-ecule and a discrete methanol mol-ecule. The planar anionic tridentate ligand 2-[5-(3,4-di-methyl-phen-yl)-4H-1,2,4-triazol-3-ato]-6-(1H-pyrazol-1-yl)pyridine coordinates the FeII ion meridionally through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa-hedral coordination sphere of the central ion. The average Fe-N bond distance is 1.955 Å, indicating a low-spin state of the FeII ion. Neighbouring cone-shaped mol-ecules, nested into each other, are linked through double weak C-H(pz)⋯π(ph') inter-actions into mono-periodic columns, which are further linked through weak C-H⋯N'/C' inter-actions into di-periodic layers. No inter-actions shorter than the sum of the van der Waals radii of the neighbouring layers are observed. Energy framework analysis at the B3LYP/6-31 G(d,p) theory level, performed to qu-antify the inter-molecular inter-action energies, reproduces the weak inter-layer inter-actions in contrast to the strong inter-action within the layers. Inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, showing the relative contributions of the contacts to the crystal packing to be H⋯H 48.5%, H⋯C/C⋯H 28.9%, H⋯N/N⋯H 16.2% and C⋯C 2.4%.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)₂Pd(CN)₄]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H₂O,EtOH (A·H₂O,Et), and H₂O (A·H₂O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H₂O,Et and an abrupt single-step SCO for A·H₂O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H₂O,Et characterized by a unique Fe^II center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H₂O shows a triclinic symmetry with two inequivalent Fe^II centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Pressure-induced multi-step and self-organized spin states in an electro-elastic model for spin-crossover solids

Spin transition materials are known to exhibit a rich variety of behaviors under several stimuli, among which pressure leads to major changes in their electronic and elastic properties. From an experimental point of view, thermal spin transitions under isotropic pressure showed transformations from (i) hysteretic to continuous transformations where the hysteresis width vanishes beyond some threshold pressure value; this is the conventional case. In several other cases very pathological and unexpected behaviours emerged, like (ii) persistent hysteresis under pressure; (iii) non-uniform behavior of the thermal hysteresis width which first increases with pressure and then decreases and vanishes at higher pressures; (iv) furthermore, double step transitions induced by pressure are also often obtained, where the pressure triggers the appearance of a plateau during the thermal transition, leading to two-step transitions, and finally (v) other non-conventional re-entrant transitions, where the thermal hysteresis vanishes at some pressure and then reappears at higher pressure values are also observed. In the present theoretical study, we investigate this problem with an electro-elastic description of the spin-crossover phenomenon by solving the Hamiltonian using a Monte Carlo technique. The pressure effect is here introduced directly in the lattice parameters, the elastic constants and ligand field energy. By considering spin state-dependent compressibility, we demonstrate that a large panel of experimental observations can be qualitatively described with this model. Among them, we quote (i) the conventional pressure effect decreasing the hysteresis width, (ii) the unconventional cases with pressure causing a non-monotonous behavior of the hysteresis width, (iii) re-entrant, as well as (iv) double step transitions accompanied with various types of spin state self-organization in the plateau regions.

Reconciling Local Coupled Cluster with Multireference Approaches for Transition Metal Spin-State Energetics

Spin-state energetics of transition metal complexes remain one of the most challenging targets for electronic structure methods. Among single-reference wave function approaches, local correlation approximations to coupled cluster theory, most notably the domain-based local pair natural orbital (DLPNO) approach, hold the promise of bringing the accuracy of coupled cluster theory with single, double, and perturbative triple excitations, CCSD(T), to molecular systems of realistic size with acceptable computational cost. However, recent studies on spin-state energetics of iron-containing systems raised doubts about the ability of the DLPNO approach to adequately and systematically approximate energetics obtained by the reference-quality complete active space second-order perturbation theory with coupled-cluster semicore correlation, CASPT2/CC. Here, we revisit this problem using a diverse set of iron complexes and examine several aspects of the application of the DLPNO approach. We show that DLPNO-CCSD(T) can accurately reproduce both CASPT2/CC and canonical CCSD(T) results if two basic principles are followed. These include the consistent use of the improved iterative (T₁) versus the semicanonical perturbative triple corrections and, most importantly, a simple two-point extrapolation to the PNO space limit. The latter practically eliminates errors arising from the default truncation of electron-pair correlation spaces and should be viewed as standard practice in applications of the method to transition metal spin-state energetics. Our results show that reference-quality results can be readily achieved with DLPNO-CCSD(T) if these principles are followed. This is important also in view of the applicability of the method to larger single-reference systems and multinuclear clusters, whose treatment of dynamic correlation would be challenging for multireference-based approaches.

Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

Spin-crossover (SCO) materials display many fascinating behaviors including collective phase transitions and spin-state switching controlled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fêted for numerous practical applications, but these remain largely unrealized-partly because of the difficulty of switching these materials at high temperatures. We introduce a semiempirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. For realistic parameters, this model reproduces the key experimental results including thermally induced phase transitions, light-induced spin-state trapping (LIESST), and reverse-LIESST. Notably, we reproduce and explain the experimentally observed relationship between the critical temperature of the thermal transition, T_1/2, and the highest temperature for which the trapped state is stable, T_LIESST, and explain why increasing the stiffness of the coordination sphere increases T_LIESST. We propose strategies to design SCO materials with higher T_LIESST: optimizing the spin-orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy difference between the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativity of the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to low temperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We show that such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate this in silico, and discuss material design rationale to realize this.