FeII(pap-5NO2)2 and FeII(qsal-5NO2)2 Schiff-Base Spin-Crossover Complexes: A Rare Example with Photomagnetism and Room-Temperature Bistability

Investigation of the spin crossover behaviour of a sublimable Fe(II)-qsal complex: from the bulk to a submonolayer on graphene/SiO2

We synthesized a sublimable molecular spin crossover Fe(II) complex based on the Schiff base tridentate ligand qsal-NEt2 (5-diethylamino-2-((quinolin-8-ylimino)methyl)phenol). The compound undergoes a transition in temperature with thermally induced excited spin state-trapping (TIESST) for high-temperature sweep rates, which can be suppressed by reducing the sweep rate. The X-ray absorption spectroscopy (XAS) studies on the microcrystalline powder confirm the TIESST effect. The molecules are deposited under ultra-high vacuum on a graphene/SiO2 substrate as a submonolayer. Investigation of the submonolayer by XAS reveals the molecular integrity and shows a spin crossover for the whole temperature range from 350 to 4 K, with residual HS species at low temperature and no TIESST effect. DFT calculations suggest a distribution of energetically similar adsorption configurations on graphene, i.e., with smooth crossover behaviour and the absence of TIESST, consistent with very weak intermolecular interactions and the absence of large molecular islands within the submonolayer.

Integrating spin-dependent emission and dielectric switching in FeII catenated metal-organic frameworks

Mechanically interlocked molecules (MIMs) including famous catenanes show switchable physical properties and attract continuous research interest due to their potential application in molecular devices. The advantages of using spin crossover (SCO) materials here are enormous, allowing for control through diverse stimuli and highly specific functions, and enabling the transfer of the internal dynamics of MIMs from solution to solid state, leading to macroscopic applications. Herein, we report the efficient self-assembly of catenated metal-organic frameworks (termed catena-MOFs) induced by stacking interactions, through the combination of rationally selected flexible and conjugated naphthalene diimide-based bis-pyridyl ligand (BPND), [MI(CN)2]- (M = Ag or Au) and Fe2+ in a one-step strategy. The obtained bimetallic Hofmann-type SCO-MOFs [FeII(BPND){Ag(CN)2}2]·3CHCl3 (1Ag) and [FeII(BPND{Au(CN)2}2]·2CHCl3·2H2O (1Au) possess a unique three-dimensional (3D) catena-MOF constructed from the polycatenation of two-dimensional (2D) layers with hxl topology. Both complexes undergo thermal- and light-induced SCO. Significantly, abnormal increases in the maximum emission intensity and dielectric constant can be detected simultaneously with the switching of spin states. This research opens up SCO-actuated bistable MIMs that afford dual functionality of coupled fluorescence emission and dielectricity.

FeII spin crossover complexes containing N4O2 donor ligands

Spin crossover (SCO) is one of the most studied magnetic bistable phenomena because of its application in the field of multifunctional magnetic materials. Fe^II complexes in a N₆ coordination environment have been the most well-studied in terms of their SCO behaviour. Other coordination environments, notably the N₄O₂ coordination environment, has also been quite effective in inducing SCO behaviour in the corresponding Fe^II complexes. This review deals with such systems. The three ligand families that are discussed are: Jager type ligands, hydrazone based ligands and tridentate ligands having salicylaldehyde derivatives. These ligands allow the assembly of both mononuclear and multinuclear complexes that exhibit cooperative spin transitions. Also, Fe^II complexes obtained from some of these ligands are multifunctional and exhibit a coupling of optical and magnetic properties. Most of the Fe^II complexes obtained from these families of ligands are charge neutral which allows easy surface deposition. Further, modulation of these ligand families allows a fine tuning of the ligand field strength which results in varying SCO behavior. In addition some of the Fe^II complexes derived from these ligands exhibit a light induced excited spin state trapping (LIESST) effect. All of the above aspects are reviewed in this review.

A Theoretical Assessment of Spin and Charge States in Binuclear Cobalt-Ruthenium Complexes: Implications for a Creutz-Taube Model Ion Separated by a C60-Derivative Bridging Ligand

We investigate the spin-state energetics and the role of ionic charges in the electronic configuration of binuclear complexes of the form [(NH₃)₅Co(py)-X-(py)Ru(NH₃)₅]^q+. In these compounds with q = 4-6, py = pyridine, and X = C≡C and C₆₀, the Co-Ru distance varies from ∼1.4 to ∼2.1 nm. We carry out a systematic electronic structure calculation using different exchange-correlation (xc) approaches within spin-density functional theory, which are largely employed to investigate the properties of a variety of coordination complexes. To evaluate the effects of spin states and type of spacer in the bridging ligand on the valence tautomerism between Co^2+/3+ and Ru^2+/3+, we examine in more detail the case of Creutz-Taube-type ions [(NH₃)₅Co(py)-X-(py)Ru(NH₃)₅]⁵⁺. Our analysis shows that the stabilization of low- and high-spin states critically depends on the total charge of the complex, type of X-bridged ligand, and employed xc approach to calculate the electron spin density. Importantly, the C₆₀-bridged group may result in a blockage of the valence tautomerism of the Creutz-Taube complex, inducing bistable charge configurations. Overall, our results also show that an adiabatic description in terms of the frontier molecular spin-orbitals for analyzing the distinct spin-charge states of these complexes may dramatically depend on the density-functional description.

The Impact of the Next-Nearest Neighbor Dispersion Interactions on Spin Crossover Transition Enthalpy Evidenced by Experimental and Computational Analyses of Neutral π-Extended Heteroleptic Fe(III) Complexes

A neutral heteroleptic Fe(III) complex 1 derived from a π-extension of the parent complex 2 was prepared and characterized. Complex 1 exhibited an abrupt spin crossover (SCO) transition exactly at room temperature (T_SCO = 298 K). A crystal structure analysis of 1 revealed that the Fe(III) complex molecules formed a three-dimensional π-stacking interaction network. To thermodynamically clarify the mechanism of the SCO transition, the thermodynamic parameters of the SCO transitions for 1 and 2 were deduced from the temperature dependence of the magnetic susceptibility in the solid and solution states using the regular solution model. A comparison of the SCO enthalpy difference between the solid and molecule for 1 and 2 revealed that the lattice enthalpy difference would largely contribute to the SCO transition enthalpy difference. A computational evaluation of intermolecular interactions and lattice energies before and after the SCO transitions in 1 and 2 disclosed the significant contribution of the next-nearest neighbor dispersion interactions to the lattice enthalpy differences. This finding indicates that not only conventional nearest neighbor intermolecular interactions but also next-nearest neighbor dispersion interactions should be taken into account to understand the fundamental mechanism of a phase transition in molecular solids.

Heteronuclear Iron(III)-Schiff Base Complexes with the Hexacyanidocobaltate(III) Anion: On the Quest To Understand the Governing Factors of Spin Crossover

Two heteronuclear compounds (1 and 2) containing three ferric centers linked in facial-like mode with the magnetically silent hexacyanidocobaltate(III) anion were prepared and studied. The structural investigation revealed that both compounds are tetranuclear complexes with molecular formulas of [{Fe(L1)NC}₃Co(CN)₃]·2CH₃OH·2.5CH₃CN (1) and [{Fe(L2)NC}₃Co(CN)₃]·2H₂O·1CH₃OH (2). The magnetic properties of both complexes are controlled by the molecular design of the corresponding pentadentate Schiff base anions L1^2- and L2^2-. While compound 2 with a symmetric ligand prepared from salicylaldehyde shows high-spin state properties, compound 1 containing the asymmetric ligand with naphthalene units either is low-spin in its solvated form or shows a gradual but hysteretic spin crossover event when desolvated. The magnetic behavior was analyzed with respect to the Ising-like model and spin Hamiltonian, respectively, and the results were confronted with ab initio calculations. Additionally, the influence of structural features, lattice solvent molecules, the distribution of electronic terms, and active orbitals on the spin state properties of reported complexes is discussed.

Bi-stable spin-crossover in charge-neutral [Fe(R-ptp)2] (ptp = 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine) complexes

Bi-stable charge-neutral iron(ii) spin-crossover (SCO) complexes are a class of switchable molecular materials proposed for molecule-based switching and memory applications. In this study, we report on the SCO behavior of a series of iron(ii) complexes composed of rationally designed 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine (ptp) ligands. The powder forms of [Fe²⁺(R-ptp^-)₂]⁰ complexes tethered with less-bulky substituents-R = H (1), R = CH₂OH (2), and R = COOCH₃ (3; previously reported)-at the 4-position of the pyridine ring of the ptp skeleton showed abrupt and hysteretic SCO at or above room temperature (RT), whereas complex 5 featuring a bulky pyrene substituent showed incomplete and gradual SCO behavior. The role of intermolecular interactions, lattice solvent, and electronic nature of the chemical substituents (R) in tuning the SCO of the complexes is elucidated.

Substituent dependence on the spin crossover behaviour of mononuclear Fe(ii) complexes with asymmetric tridentate ligands

Three mononuclear iron(ii) complexes of the formula [Fe^II(H₂L^1-3)₂](BF₄)₂·x(solv.) (H₂L^1-3 = 2-[5-(R-phenyl)-1H-pyrazole-3-yl] 6-benzimidazole pyridine; H₂L¹: R = 4-methylphenyl, H₂L², R = 2,4,6-trimethylphenyl, H₂L³, R = 2,3,4,5,6-pentamethylphenyl) (1, H₂L¹; 2, H₂L²; 3, H₂L³) with asymmetric tridentate ligands (H₂L^1-3) were synthesized and their structures and magnetic behaviour investigated. Significant structural distortions of the dihedral angles between phenyl and pyrazole groups were observed and found to depend on the nature of the substituent groups. Cryomagnetic studies reveal that 1 and 2 show gradual spin crossover behavior, while 3 remains in the high spin state between 1.8 and 300 K.

High-Temperature Cooperative Spin Crossover Transitions and Single-Crystal Reflection Spectra of [FeIII(qsal)2](CH3OSO3) and Related Compounds

New Fe(III) compounds from qsal ligand, [Fe(qsal)2](CH3OSO3) (1) and [Fe(qsal)2](CH3SO3)·CH3OH (3), along with known compound, [Fe(qsal)2](CF3SO3) (2), were obtained as large well-shaped crystals (Hqsal = N-(8-quinolyl)salicylaldimine). The compounds 1 and 2 were in the low-spin (LS) state at 300 K and exhibited a cooperative spin crossover (SCO) transition with a thermal hysteresis loop at higher temperatures, whereas 3 was in the high-spin (HS) state below 300 K. The optical conductivity spectra for 1 and 3 were calculated from the single-crystal reflection spectra, which were, to the best of our knowledge, the first optical conductivity spectra of SCO compounds. The absorption bands for the LS and HS [Fe(qsal)2] cations were assigned by time-dependent density functional theory calculations. The crystal structures of 1 and 2 consisted of a common one-dimensional (1D) array of the [Fe(qsal)2] cation, whereas that of 3 had an unusual 1D arrangement by π-stacking interactions which has never been reported. The crystal structures in the high-temperature phases for 1 and 2 indicate that large structural changes were triggered by the motion of counter anions. The comparison of the crystal structures of the known [Fe(qsal)2] compounds suggests the significant role of a large non-spherical counter-anion or solvate molecule for the total lattice energy gain in the crystal of a charged complex.

A Multi-Stimuli-Responsive Fe(II) SCO Complex Based on an Acylhydrazone Ligand

An Fe(II) SCO complex based on an acylhydrazone ligand with an amino functional group has been prepared. The complex is able to dissociate and regather upon protonation and deprotonation, in both solid state and solution, accompanied by spin state switching, marked change of color, and distinct solubility in water. Moreover, the complex shows distinct magnetic responses toward formaldehyde and protic and nonprotic solvents, as a result of the different affinity of the amino functional site with those chemicals.

Spin-crossover in iron(ii)-Schiff base complexes

A collective overview of iron(ii)-Schiff base complexes, showing abrupt and hysteretic SCO suitable for device applications, and the structure–property relationships governing the SCO of the complexes in the solid-state is presented.

One-Dimensional Thiocyanato-Bridged Fe(II) Spin Crossover Cooperative Polymer With Unusual FeN5S Coordination Sphere

We present here a novel example of spin crossover phenomenon on a Fe(II) one-dimensional chain with unusual N₅S coordination sphere. The [{Fe(tpc-OMe)(NCS)(μ-NCS)} _n] (1) compound was prepared using the tridentate tpc-OMe ligand (tpc-OMe = tris(2-pyridyl)methoxymethane), FeCl₂·4H₂O, and the KSCN salt. Crystallographic investigations revealed that the Fe(II) ions are connected by a single bridging NCS^- ligand (μ-κN:κS-SCN coordination mode) to afford a zigzag neutral chain running along the [010] direction, in which the thiocyanato bridging groups adopt a cis head-to-tail configuration. The (N₅S) metal environment arises from one thiocyanato-κS and two thiocyanato-κN ligands and from three pyridine of the fac-tpc-OMe tripodal ligand. This compound presents a unique extension of Fe(II) binuclear complexes into linear chains built on similar tripodal ligands and bridging thiocyanate anions. Compound 1 shows a spin crossover (SCO) behavior which has been evidenced by magnetic, calorimetric, and structural investigations, revealing a sharp cooperative spin transition with a transition temperature of ca. 199 K. Temperature scan rate studies revealed a very narrow hysteresis loop (∼1 K wide). Photoswitching of this compound was also performed, evidencing a very fast relaxation process at low temperature. Among other factors, the linearity of the N-bound terminal thiocyanato ligand appears as the main structural characteristic at the origin of the presence of the SCO transition in compound 1 and in the two others Fe(II) previous systems involving thiocyanato-bridges and tripodal tris(2-pyridyl)methane ligands.

Synthesis, structure, and spin crossover above room temperature of a mononuclear and related dinuclear double helicate iron(ii) complexes

A mononuclear iron(ii) complex, [Fe^II(L1^Me)₂](PF₆)₂ (1), and a related dinuclear one, [Fe(L2^C2)₂](PF₆)₄·5H₂O·MeCN (2), were prepared, where L1^Me is a tridentate N₃ coordinating Schiff-base ligand, 1-methyl-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and L2^C2 is a dimeric form ligand of L1^Me, 1,1'-(1,2-ethanediyl)bis-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and their magnetostructural relationships were investigated. Mononuclear 1 shows a gradual one-step spin crossover (SCO) from the low-spin (LS) state (S = 0) at room temperature (RT) to the mostly high-spin (HS) state (S = 2) at 470 K spanning a temperature range of more than 170 K, while dinuclear 2 shows an anomalous two-step SCO with a gradual transition in a first step from the LS-LS state at RT to around 432 K and then an abrupt spin transition centred at 432 K with 11 K wide hysteresis in a second step (T_1/2↑ = 437 K and T_1/2↓ = 426 K) reaching the mostly HS-HS state. Single crystal X-ray structure analyses revealed that the iron(ii) centre of mononuclear 1 has an N6 octahedral coordination environment with two tridentate L1^Me ligands, while the complex-cation of 2 has a dinuclear double helicate architecture as though a dimeric form of two complex-cations of 1 replaced their methyl group with a bridging ethylene chain. Both complexes have a cationic two-dimensional (2D) structure in which the 2D layer of mononuclear 1 is constructed of intermolecular CHN hydrogen bonds between adjacent complex-cations, while that of dinuclear 2 is constructed of both intermolecular π-π and CH/π interactions between neighboring helicates.

Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior

The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)₂]⁺ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu₂N/Cu(100) surface is evidenced. The deposited Fe^III spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)₂ molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Heteroleptic Fe(II) Complexes with N4S2 Coordination as a Platform for Designing Spin-Crossover Materials

Heteroleptic complexes [Fe(bpte)(bim)]X₂ and [Fe(bpte)(xbim)]X₂ (bpte = S,S'-bis(2-pyridylmethyl)-1,2-thioethane, bim = 2,2'-biimidazole, xbim = 1,1'-(α,α'-o-xylyl)-2,2'-biimidazole, X = ClO₄^-, BF₄^-, OTf^-) were prepared by reacting the corresponding Fe(II) salts with a 1:1 mixture of the ligands. All mononuclear complexes exhibit temperature-induced spin crossover (SCO) with the onset above room temperature. The SCO is rather gradual, due to low cooperativity of interactions between the cationic complexes, as revealed by crystal structure analyses. These complexes expand the range of the recently discovered Fe(II) SCO materials with {N₄S₂} coordination environment.

Comparison of structural dynamics and coherence of d-d and MLCT light-induced spin state trapping

Light-induced excited spin state trapping (LIESST) in FeII spin-crossover systems is a process that involves the switching of molecules from low (LS, S = 0) to high spin (HS, S = 2) states. The direct LS-to-HS conversion is forbidden by selection rules, and LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in an HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process has been investigated in FeN6 ligand field systems, under MLCT excitation. Herein, we studied LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allowed for quite intense and low-energy shifted d-d bands. By combining ab initio DFT and TD-DFT calculations and fs optical absorption measurements, we demonstrated that shorter intermediates enhanced coherent structural dynamics, and d-d excitation induced faster LS-to-HS switching, compared to MLCT.

Hybrid magnetite-gold nanoparticles as bifunctional magnetic-plasmonic systems: three representative cases

Hybrid systems based on magnetite and gold nanoparticles have been extensively used as bifunctional materials for bio- and nano-technology. The properties of these composites are assumed to be closely related to the magnetite to gold mass ratio and to the geometry of the resulting hetero-structures. To illustrate this, we compare and analyze the optical and magnetic properties of core-shell, dumbbell-like dimers and chemical cross-linked pairs of magnetite and gold nanoparticles in detail. We explore how the combination of gold with magnetite can lead to an improvement of the optical properties of these systems, such as tunability, light scattering enhancement or an increase of the local electric field at the interface between magnetic and plasmonic constituents. We also show that although the presence of gold might affect the magnetic response of these hybrid systems, they still show good performance for magnetic applications; indeed the resulting magnetic properties are more dependent on the NP size dispersion. Finally, we identify technological constraints and discuss prospective routes for the development of further magnetic-plasmonic materials.

Solvent modified spin crossover in an iron(iii) complex: phase changes and an exceptionally wide hysteresis

The impact of solvent on spin crossover is explored with T_1/2 varying by 200 K and hysteresis up to 80 K.

Solvent effects in a series of Fe(iii) spin crossover (SCO) complexes [Fe(qsal-I)₂]OTf·sol (sol = MeOH 1, EtOH 2, n-PrOH 3, i-PrOH 4, acetone 5 and MeCN 6) are explored. SCO is abrupt in 1 (following MeOH loss) and 2, gradual for 3 (T_1/2 = 199 K) and 4 (T_1/2 = 251 K) and incomplete, even up to 350 K, for 5 and 6. In [Fe(qsal-I)₂]OTf SCO occurs at T_1/2↓ = 225 K and T_1/2↑ = 234 K (ΔT = 9 K), while aged samples of 2 exhibit an exceptionally wide hysteresis of 80 K (T_1/2↓ = 139 K and T_1/2↑ = 219 K). In contrast, fresh samples of 2 exhibit stepped SCO with hysteresis varying from 2 to 42 K. VT-PXRD (variable temperature powder X-ray diffraction) studies indicate a new phase, 2b, is formed upon cooling below 180 K along with a minor LS phase 2c. Phase 2c and the HS phase 2a undergo a spin transition at T_1/2↓ = 180 K and T_1/2↑ = 215 K with phase 2b exhibiting two-step SCO. Structural studies in both spin states, except 6, show the cations are linked through extensive π–π interactions to form 1D chains. A combination of P4AE (parallel fourfold aryl embrace) and I···X (X = I, O, π) interactions create tightly packed 3D supramolecular networks. This study emphasizes that while solvent may result in only small structural changes SCO characteristics can be impacted dramatically.

A luminescent Pt2Fe spin crossover complex

A heterotrinuclear luminescent [Pt₂Fe] spin crossover (SCO) complex was developed, synthesized, and investigated.

Symmetry breaking above room temperature in an Fe(ii) spin crossover complex with an N4O2 donor set

An halogen-substituted salicylaldimine iron(ii) complex shows an abrupt two-step spin transition above room temperature, variable temperature crystallography showing symmetry breaking.

Iron(iii) complexes of 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol as spin-crossover molecular materials

Four new mononuclear Fe(iii) complexes are obtained by using 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol. The perchlorate complex displays spin crossover with a hysteresis of 32 K, while the neutral complex exhibits a gradual incomplete spin transition.

Influence of Intermolecular Interactions on Valence Tautomeric Behaviors in Two Polymorphic Dinuclear Cobalt Complexes

Two polymorphic structures have been well determined in a valence tautomeric (VT) dinuclear cobalt complex. These polymorphs showed distinct thermal- and photomagnetic behavior, and are thus ideal for studying the "pure" intermolecular factors to VT transitions. In polymorph 1A, the VT cations are arranged head-to-waist with their neighbors and exhibit weak π⋅⋅⋅π interactions, resulting in a gradual and incomplete thermal VT transition. In contrast, the cations in polymorph 1B are arranged head-to-tail and exhibit relatively strong π⋅⋅⋅π interactions, leading to an abrupt and complete thermal VT transition with adjustable hysteresis loop at around room temperature. The VT process for both polymorphs can be induced by light, but the light-excited state of 1B⋅2H₂ O has a higher thermal relaxation temperature than that of 1A⋅3H₂ O.

A polymorphism-dependent T(1/2) shift of 100 K in a hysteretic spin-crossover complex related to differences in intermolecular weak CH···X hydrogen bonds (X = S vs. S and N)

A neutral mononuclear iron(II) complex with a 1,2,3-triazole-containing tetradentate ligand has been obtained as two solvent-free polymorphs. Both polymorphs show hysteretic spin crossover with a polymorphism-dependent T(1/2) shift of 100 K that spans room temperature due to differences in intermolecular weak CH···X hydrogen-bonding interactions (X = S vs. S and N).

Synthesis, magnetism and spectral studies of six defective dicubane tetranuclear {M4O6} (M = NiII, CoII, ZnII) and three trinuclear CdII complexes with polydentate Schiff base ligands

Six defective dicubane face-shared and three trinuclear complexes are synthesized. The magnetic behaviors of Ni^II/Co^II complexes and fluorescence properties of Zn^II and Cd^II are studied.

Water induced spin-crossover behaviour and magneto-structural correlation in octacyanotungstate(iv)-based iron(ii) complexes

The SCO is mainly tuned by the octahedral distortion of the [FeN₆] core caused by intermolecular hydrogen bonds, and there is an exact correlation between SCO behavior and the amount of lattice water molecules existing in the crystal.

Preparation of dihydroquinazoline carbohydrazone Fe(ii) complexes for spin crossover

Mononuclear spin-crossover complexes of dihydroquinazoline derivatives were synthesized to enable the spin crossover phenomenon via the variation of the solvent, anion and substituent.