Spin Crossover Research Articles

Spin crossover of a Fe(II) mononuclear complex induced by intermolecular factors involving chloride and solvent ordering.

Two new salts of a mononuclear tripodal Fe(II) complex were prepared, using ClO4- and Cl-. The ClO4- sample (1) remained HS at low temperatures, similar to the previously reported BF4- analogue. Crystallising with the Cl- anion (2) led to a markedly different crystal packing arrangement, and engendered SCO activity. This has been correlated to the lower crystal packing density in 2 and the coordination complex conformational differences arising due to the packing motifs of 1 and 2. Further, solvent ordering effects have been proposed to facilitate spin transition behaviour in 2.

A mononuclear Fe(III) complex showing thermally induced spin crossover and slow magnetic relaxation with reciprocating thermal behaviour.

AC susceptibility measurements of [FeIII(L5)(NCSe)] reveal a field supported slow magnetic relaxation. On cooling, the relaxation time of the high-frequency fraction decreases which is a sign of reciprocating thermal behaviour. The relaxation time for the low-frequency mode at T = 2.0 K is as high as τ(LF) = 2.0 s.

Multielectron transfer and field-induced slow magnetic relaxation in opto-electroactive spin crossover cobalt(ii) complexes: structure–function correlations

A unique series of spin-crossover cobalt(ii)-pyridine-2,6-dimine molecular nanomagnets as prototypes of molecular spin quantum transitors and capacitors exhibiting a dual metal- and ligand-centred multielectron redox behaviour is reported.

Binucleating Jäger-type {(N2O2)2}4- ligands: magnetic and electronic interactions of Fe(II), Ni(II) and Cu(II) across an in-plane TTF-bridge.

The simultaneous presence of different electrophores provides an interesting playground for responsive materials. Herein, we present the incorporation of a twice-reversibly oxidizable tetrathiafulvalene (TTF) unit into a binucleating ligand, bridging two metal centers in a fully conjugated plane. A two-step synthesis scheme gave the D2h symmetric Schiff base-like ligand H4L in moderate yields from which the corresponding copper(II) [Cu2L], nickel(II) [Ni2L], [Ni2L(py)4] and iron(II) complexes [Fe2L(py)4], [Fe2L(dmap)4] and [Fe2L(bpee)2]·1 Tol could be obtained. Characterization was performed through 1H-NMR, IR, UV-vis and 57Fe-Mössbauer spectroscopy, SQUID magnetometry and cyclic voltammetry, supported by density functional theory (DFT) calculations. Single crystal X-ray analysis of [Ni2L(py)4] revealed six-coordinate paramagnetic centers, whereas [Ni2L] underwent gradual coordination induced spin state switching (CISSS) in solution. The magnetic independence of both metal centers is echoed by close-to-ideal Curie-plots of the [Cu2L] system and the gradual spin crossover of all iron(II) compounds. By contrast, cyclic voltammetry measurements in solution indicated oxidation-dependent TTF-metal interactions, as well as metal-metal interactions. The reversible TTF-borne events in H4L and [Ni2L] are overlaid with metal-borne events in the case of [Fe2L(py)4], as is corroborated by an analysis of the frontier orbital landscapes and through diagnostic spectral features upon chemical oxidation.

Regulating the thermal expansion of a [FePt(CN)4] layer by axial coordination and dimensional reduction.

Thermal expansion regulation by chemical decoration at a molecular level is of great technological value for materials science. Herein, we show that the spin crossover active compound Fe(pyz)Pt(CN)4 (pyz = pyrazine) shows a rare 2D negative thermal expansion (NTE) in the ab-plane. By introducing axial coordination iodine ions or reducing the framework dimension from 3D to 2D, the NTE behavior can be effectively switched to positive thermal expansion (PTE) or even zero thermal expansion (ZTE). Moreover, it is found that different spin states of Fe2+ also influence the magnitude of NTE. Compared with the low-spin (LS) sate, the high-spin (HS) state tends to enhance the magnitude of NTE. Combined in situ structural and Raman spectral analyses revealed that the NTE mainly originates from the transverse vibration of a bridging cyano group and the tailorable thermal expansion is closely related to the state of the Fe-CN-Pt linkage. The present study shows how the rational regulation of the building unit and framework dimensions can effectively control thermal expansion behaviors. This insight can serve as guidance for designing and synthesizing novel NTE materials.

Iron(II) spin crossover complexes of tetradentate Schiff-bases: tuning T1/2 by choice of formyl-heterocycle component.

Four new tetradentate Schiff-base ligands were prepared in situ from the 1 : 2 condensation of 1,3-diaminopropane and either 2-thiazolecarboxaldehyde (L2thiazole), 4-thiazolecarboxaldehyde (L4thiazole), 4-oxazolecarboxaldehyde (L4oxazole), or 5-bromopyridine-2-aldehyde (L5Br-pyridine), and complexed with [Fe(NCS)2(pyridine)4] to give four monometallic FeII complexes, [Fe(Lheterocycle)(NCS)2]. Structural characterisation shows the expected octahedral FeII centres in all cases, with Lheterocycle occupying the equatorial plane and the two thiocyanate ligands trans to each other, resulting in an N6 coordination sphere. Solid state magnetic measurements showed that the two complexes with the thiazole-based ligands exhibit the beginning of a spin transition above 300 K, with T1/2 = 350 K for [Fe(L4thiazole)(NCS)2] and 400 K for [Fe(L2thiazole)(NCS)2], whereas the 4-oxazole-based ligand gives [Fe(L4oxazole)(NCS)2] which remains high spin at all measured temperatures (50-400 K). Interestingly, [Fe(L5Br-pyridine)(NCS)2] crystallised as two solvent-free polymorphs: magnetic measurements on samples with both polymorphs present showed a two step SCO with an abrupt transition at T1/2 = 245 K assigned to the transition in polymorph A (as this was also seen in a sample of pure polymorph A), and a gradual transition at T1/2 = 304 K assigned to polymorph B. These findings show that the order of increasing ligand field strength for these heterocycles is 4-oxazole ≪ 5Br-pyridine < 4-thiazole < 2-thiazole.

Impact of flexible hexyl chain ordering in a mononuclear spin crossover iron(III) complex.

A novel FeIII complex [Fe(Hex-tnal)2]BPh4 (1) with a tridentate N2O ligand having an n-hexyl chain, Hex-Htnal (=1-((((1-hexyl-1H-1,2,3-triazol-4-yl)methyl)imino)methyl)naphthalen-2-ol), is reported. Temperature-dependent magnetic susceptibility measurements revealed that 1 exhibits a two-step spin crossover (SCO) transition in the 400-10 K temperature range, including an unusual gradual χMT change above RT (300-345 K) and a hysteretic χMT jump in a narrow temperature range of 345-357 K. These behaviors were also characterized by differential scanning calorimetry. Variable-temperature single-crystal X-ray diffraction studies revealed that the order-disorder transition and conformational change of the hexyl chains and the symmetry change associated with the re-entrant structural phase transition, namely triclinic P1̄ (100-275 K) ↔ monoclinic C2/c (296-340 K) ↔ triclinic P1̄ (360 K), are coupled to variations in intermolecular interactions and the N4O2 coordination environment, resulting in the occurrence of the unusual two-step SCO transition of 1. This study demonstrates that the flexible motion of alkyl substituents in the supramolecular lattice influences the occurrence of anomalous magnetic switching properties.

Fano resonance in molecular junctions of spin crossover complexes.

In this paper, we introduce a novel molecular switch paradigm that integrates spin crossover complexes with the Fano resonance effect. Specifically, by performing density-functional theory calculations, the feasibility of achieving Fano resonance using spin crossover complexes is demonstrated in our designed molecular junctions using the complex {Fe[H2B(pz)2]2[Bp(bipy)]} [pz = 1-pyrazolyl, Bp(bipy) = bis(phenylethynyl)(2,2'-bipyridine)]. It is further revealed that the Fano resonance, particularly the Fano dip, is most prominent in the junction with cobalt tips among all the schemes, together with the spin-filtering effect. Most importantly, this junction of cobalt tips is able to exhibit three distinct conductance states, which are controlled by the modulation of Fano resonance due to the spin-state transition of the complex and the applied gate voltage. Such a molecular switch paradigm holds potential for applications in logic gates, memory units, sensors, thermoelectrics, and beyond.

Machine Learning Prediction of the Experimental Transition Temperature of Fe(II) Spin-Crossover Complexes.

Spin-crossover (SCO) complexes are materials that exhibit changes in the spin state in response to external stimuli, with potential applications in molecular electronics. It is challenging to know a priori how to design ligands to achieve the delicate balance of entropic and enthalpic contributions needed to tailor a transition temperature close to room temperature. We leverage the SCO complexes from the previously curated SCO-95 data set [Vennelakanti et al. J. Chem. Phys. 159, 024120 (2023)] to train three machine learning (ML) models for transition temperature (T1/2) prediction using graph-based revised autocorrelations as features. We perform feature selection using random forest-ranked recursive feature addition (RF-RFA) to identify the features essential to model transferability. Of the ML models considered, the full feature set RF and recursive feature addition RF models perform best, achieving moderate correlation to experimental T1/2 values. We then compare ML T1/2 predictions to those from three previously identified best-performing density functional approximations (DFAs) which accurately predict SCO behavior across SCO-95, finding that the ML models predict T1/2 more accurately than the best-performing DFAs. In addition, we study ML model predictions for a set of 18 SCO complexes for which only estimated T1/2 values are available. Upon excluding outliers from this set, the RF-RFA RF model shows a strong correlation to estimated T1/2 values with a Pearson's r of 0.82. In contrast, DFA-predicted T1/2 values have large errors and show no correlation to estimated T1/2 values over the same set of complexes. Overall, our study demonstrates slightly superior performance of ML models in comparison with some of the best-performing DFAs, and we expect ML models to improve further as larger data sets of SCO complexes are curated and become available for model training.

Towards spin-crossover in pseudo-octahedral iron(III) complexes of strongly absorbing phenanthridine-based N^N^O- ligands

Paramagnetic complexes which can undergo spin crossover between different magnetic states that also strongly absorb light are highly attractive for the pursuit of optical magnetic technologies. Against this backdrop, we prepared a library of Fe(III) complexes ligated by monoanionic, tridentate N^N^O- ligands (RL: (2-R1-(4-imino-methyl)-phenanthridinyl)]-5-R2-phenolate where R1 = tBu, F, OMe, R2 = H or R = tBu, R2 = tBu) bearing strongly absorbing π-extended phenanthridine (benzo[c]quinoline) donors: [Fe(tBuL)2]PF6, [Fe(tBuLtBu)2]PF6, [Fe(FL)2]PF6, and [Fe(OMeL)2]PF6. The solid-state structures of three of the series were determined using single-crystal X-ray diffraction, confirming pseudo-octahedral coordination geometries. Electronic absorption spectra shows strong absorbances near 340 nm and 415 nm for the series due to π-π* and ligand-to-metal-charge-transfer (LMCT) transitions, respectively, along with a long absorptive tail that extends well into the visible region, only attenuating close to 850 nm. Magnetic measurements in the solid-state (SQUID magnetometry) and in solution (Evans’ Method) revealed the influence of ligand substituents. For example, the magnetic moment of [Fe(FL)2]PF6 in the solid-state remains more or less constant at a χmT ∼ 2.3 over a large temperature range (25–300 K) indicative of a mixed spin configuration that is maintained in solution at ambient temperature, while [Fe(tBuL)2]PF6 and [Fe(tBuLtBu)2]PF6 only reach values of ∼1.5 at higher temperature. In comparison, the χmT of [Fe(OMeL)2]PF6 undergoes a much more abrupt drop upon cooling, from ∼2.5 to below 0.5 at 100 K, consistent with spin-crossover type behavior.

Spin effect on redox acceleration and regioselectivity in Fe-catalyzed alkyne hydrosilylation.

Iron catalysts are ideal transition metal catalysts because of the Earths abundant, cheap, biocompatible features of iron salts. Iron catalysts often have unique open-shell structures that easily undergo spin crossover in chemical transformations, a feature rarely found in noble metal catalysts. Unfortunately, little is known currently about how the open-shell structure and spin crossover affect the reactivity and selectivity of iron catalysts, which makes the development of iron catalysts a low efficient trial-and-error program. In this paper, a combination of experiments and theoretical calculations revealed that the iron-catalyzed hydrosilylation of alkynes is typical spin-crossover catalysis. Deep insight into the electronic structures of a set of well-defined open-shell active formal Fe(0) catalysts revealed that the spin-delocalization between the iron center and the 1,10-phenanthroline ligand effectively regulates the iron center's spin and oxidation state to meet the opposite electrostatic requirements of oxidative addition and reductive elimination, respectively, and the spin crossover is essential for this electron transfer process. The triplet transition state was essential for achieving high regioselectivity through tuning the nonbonding interactions. These findings provide an important reference for understanding the effect of catalyst spin state on reaction. It is inspiring for the development of iron catalysts and other Earth-abundant metal catalysts, especially from the point of view of ligand development.

Naphthalimide-Tagged Iron(II) Spin Crossover Complex with Synergy of Ratiometric Fluorescence for Thermosensing.

Spin crossover (SCO) materials that possess switchable and cooperative fluorescence have long focused interest in photonic sensor devices to monitor the variations in the physicochemical parameters of the external environment. However, the lack of quantified cooperativity for the SCO transition operating in isolated molecules is detrimental to short-term technological applications. In this study, a pretwisted energy D-A system combining the deep-blue naphthalimide fluorophore (donor) and the FeN6 SCO chromophore (switchable acceptor) has been developed with the formula of Fe(naph-abpt)2(NCS)2·2DMF (1), where naph-abpt is N-[3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl]-1,8-naphthalimide. Dual emission from the naphthalimide function based on its vibronic structure exhibits a different synergy effect with SCO, providing a new platform for ratiometric fluorescence thermosensing. Theoretical calculations and optical experimental results demonstrate an excellent correlation between luminescence intensity ratio signals and magnetic data of spin transition, promising a high sensitivity of the optical activity of the ligand to the spin state of the active iron(II) ions, with the maximum relative sensitivity as 0.7% K-1 around T1/2.

Substituent effect on the spin transition of Mixed-Valence Cyanide-Bridged {FeIII2FeII} complexes

We synthesized three isostructural mixed-valence {FeIII2FeII} trinuclear complexes {[(Tp4-Me)FeIII(CN)3]2FeII(1-(4-R-phenyl)-1H-imidazole)}·Sol (1: R = Br, Sol = 10·H2O; 2: R = Cl, Sol = 2 CH3OH; 3: R = F, Sol = 2 CH3OH; Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate) and characterized their structures and magnetic and spectroscopic properties. The linear trinuclear structures of complexes 1–––3 are isomorphic. Magnetic susceptibility measurements and variable-temperature IR analyses revealed that the magnetic behavior of the complexes can be finely tuned by the substituents, which significantly influence the ligand field strength. The complexes exhibit complete one-step spin-crossover (SCO) behavior with T1/2↑ = 146 K, T1/2↓ = 142 K for 1; T1/2↑ = 155 K, T1/2↓ = 150 K for 2; T1/2↑ = 147 K, T1/2↓ = 144 K for 3, respectively. Photomagnetic study indicates that all of them show the light-induced spin state transition from low-spin to high spin states by applying 946 nm light for 1 and 808 nm light for 2 and 3. Variable-temperature structural analysis demonstrates that the three complexes exhibit π···π interactions between the intermolecular benzene ring and the pyrazole ring. Our study demonstrates that the modification of substituent ligand can significantly affect the intermolecular interactions of these trinuclear complexes and the SCO behaviour of them.

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.

Extremely Slow Thermally-Induced Spin Crossover in the Two-Dimensional Network [Fe(bbtr)3 ](BF4 )2.

Cooling [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T1/2 ↓ =76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2 ↑ =89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3 ](ClO4 )2 in which spin crossover during cooling is preceded by phase transition at TPT =126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T1/2 (x) and TPT (x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3 ](BF4 )2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X-ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P-3→P-1 phase transition occurs. In this report we have shown that 2D coordination polymer [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) treated to date as spin crossover silent shows thermally induced spin crossover phenomenon. Spin crossover in tetrafluoroborate is extremely slow. Determination of the spin crossover curve required carrying measurement in the settle mode-cooling from 85 to 70 K took about 600 h (average velocity of change of temperature ca. 0.0004 K/min).

A comparative study between Monte Carlo entropic sampling method and local mean field investigations of thermal properties of spin-crossover nanoparticles based on Ising-like model

The thermally induced transitions between low-spin (LS) and high-spin (HS) configurations of spin-crossover (SCO) nanoparticles are simulated, focusing on the effects of localized surface and bulk interactions on the average magnetization of 2D square lattices. The thermal behaviors and hysteresis cycles are investigated within the framework of the Ising model Hamiltonian and are conducted following two approaches: local mean field approximation (LMFA) and Monte Carlo entropic sampling (MCES) techniques. The results obtained by these two methods are compared for the two square lattice sizes, 6 $$ \times $$ 6 and 7 $$ \times $$ 7. Thus, when the bulk-surface interaction term is set to zero, the two approaches lead to identical values of the surface and bulk transition temperatures separated by a long intermediate plateau in both cases. Although hysteresis curves exhibit a similar shape, LMFA shows slightly larger widths $$ \Delta T $$ than MCES. On increasing bulk-surface interaction term, the two methods lead to different shifts in equilibrium temperature values for both bulk and surface components, respectively, to lower and higher values by MCES. In general, it is found that LMFA shifts surface equilibrium temperature differently to lower values and enhances the hysteresis effect, particularly for surface molecules. On the other hand, for the 7 $$ \times $$ 7 square lattice, the equilibrium temperatures are slightly higher by 1.5% and 3.2% for bulk and surface molecules, respectively, with a narrower hysteresis width in the surface. Moreover, with the MCES method, an abrupt transition instead of a hysteresis transition is calculated for surface molecules ($$ \Delta T_{surf}^{MCES}=0 $$ K).

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.

Variability of the Conductance Changes Associated with the Change in the Spin State in Molecular Spin Crossover Complexes

Here, we examine the conductance changes associated with the change in spin state in a variety of different structures, using the example of the spin crossover complex [Fe(H2B(pz)2)2(bipy)] (pz = (pyrazol-1-yl)-borate and bipy = 2,2′-bipyridine) and [Fe(Htrz)2(trz)](BF4)] (Htrz = 1H-1,2,4-triazole) thin films. This conductance change is highly variable depending on the mechanism driving the change in spin state, the substrate, and the device geometry. Simply stated, the choice of spin crossover complex used to build a device is not the only factor in determining the change in conductance with the change in spin state.

Adsorption of Aromatic Molecules in a Microporous Spin Crossover Material, Fe(pyrazine)[Fe(CN)5NO]. Tuning of Its Physical Properties

Adsorption of Aromatic Molecules in a Microporous Spin Crossover Material, Fe(pyrazine)[Fe(CN)₅NO]. Tuning of Its Physical Properties

Detecting Fe(II) Spin-Crossover by Modulation of Appended Eu(III) Luminescence in a Single Molecule.

Multifunctionality in spin-crossover (SCO) devices is limited to macroscopic or nanoscopic materials because of the need for long-range effects for inducing favorable cooperativity, efficient energy migration processes, and detectable magnetization transfer. The difficult reproducibility, control, and rational design of doped materials offer some place to SCO processes, modulating the optical properties of neighboring luminescent probes in single molecules. We report here on the combination of a [FeN6] chromophore, the SCO temperature and absorption spectra of which have been tuned to induce unprecedented room-temperature modulation of Eu(III)-based line-like luminescence in the molecular triple-helical [EuFe(L2)3]5+ complex in solution.