Iron(III) spin-crossover compounds with a wide apparent thermal hysteresis around room temperature

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.

Four-step electron transfer coupled spin transition in a cyano-bridged [Fe2Co2] square complex

The design of molecular functional materials with multi-step magnetic transitions has attracted considerable attention. However, the development of such materials is still infrequent and challenging. Here, a cyano-bridged square Prussian blue complex that exhibits a thermally induced four-step electron transfer coupled spin transition (ETCST) is reported. The magnetic and spectroscopic analyses confirm this multi-step transition. Variable-temperature infrared spectrum suggested the electronic structures in each phase and a four-step transition model is proposed.

Effects of mono- or di-fluoro-substitution on spin crossover behavior in a pair of Schiff base-like FeII-coordination polymers

Spin crossover (SCO) has long been a hot topic in the field of molecular magnetism owing to its unique bistability character. Rational control of thermal hysteresis and transition temperature (T1/2) is crucial for their practical applications, which rely on precise manipulation of the substituents of SCO coordinating ligands and molecular packing interactions. In this study, we designed two different bridging ligands (2-FDPB: 4,4'-(2-fluoro-1,4-phenylene)dipyridine; 2,3-FDPB: 4,4'-(2,3-difluoro-1,4-phenylene)dipyridine) featuring one and two fluoro substitution on the central benzene ring and applied a Schiff base-like equatorial tetradentate ligand {diethyl(E,E)-2,2'-[4,5-difluoro-1,2-phenyl-bis(iminomethylidyne)]bis(3-oxobutanoate)-(2-)-N,N',O3,O3'} (H2L) to coordinate with the FeII ion. Two FeII-coordination chain polymers [FeII(L)(2,3-FDPB)]·0.25CH2Cl2 (1) and [FeII(L)(2-FDPB)]·0.5CH3OH (2) were obtained. 1 crystallizes in the monoclinic P21/n space group with only one FeII center, while 2 crystallizes in the triclinic P1̄ space group with two independent FeII centers. Unlike the identical 2D layer stacking in 1, 2 exhibited alternating stacking of the extending 2D layers and meshed chains. Magnetic measurements revealed the typical thermally induced spin crossover behavior (SCO): 1 exhibited a 41 K-wide thermal hysteresis with transition temperatures of T1/2↑ = 245 K and T1/2↓ = 204 K, while 2 showed a higher transition temperature (T1/2 = 330 K) with no thermal hysteresis. Magneto-structural correlation studies suggest that the electron-withdrawing effect present in the fluoro substituents does not have a significant impact on the SCO behaviors. Despite the fluoro substituents having a similar atomic radius of hydrogen atoms, variations in the number of these substituents can alter the crystallization behavior of these complexes, which in turn affects the solvents, molecular stacking patterns, and intermolecular interactions, ultimately influencing the SCO behaviors.

Synthesis, mixed-spin-state structure and Langmuir-Blodgett deposition of amphiphilic Fe(iii) quinolylsalicylaldiminate complexes

Designing and integrating Fe(iii)-based spin crossover (SCO) complexes onto substrates remains a challenging goal with only a handful of examples reported. In this work, we successfully synthesized and characterized three [Fe(qsal-OR)2]NO3 (qsal-OR = 5-alkoxy-2-[(8-quinolylimino)methyl]phenolate) complexes, in which R = C12H251, C16H332, and C22H453 to explore the impact of alkyl chain on the modulation of SCO activity and potential for self-assembly on a glass surface. The SCO is found to be gradual and incomplete in all cases, with the LS state more stabilised as the alkyl group shortens. We also demonstrate that all complexes form stable Langmuir films and achieve good transfer ratios to the glass surface, with 2 being the best in terms of stability. This paves the way for the SCO modulation of complexes in this class and the development of SCO devices.

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Spin Crossover in [Fe(qsal-5-Brq)2]+ Complexes with a Quinoline-Substituted Qsal Ligand

Using a quinoline substituted Qsal ligand, Hqsal-5-Brq (Hqsal-5-Brq = N-(5-bromo-8-quinolyl)salicylaldimine), four FeIII complexes, [Fe(qsal-5-Brq)2]A·CH3OH (Y = NO3- (1NO3), BF4- (2BF4), PF6- (3PF6), OTf- (4OTf), were prepared and characterized. Structure analysis revealed that complex 2BF4 contained two species (2BF4(P1̅) and 2BF4(C2/c)). In these compounds except 3PF6, the [Fe(qsal-5-Brq)2]+ cations form 1D chains through π-π interactions and other weak interactions. Adjacent chains are connected to form the 2D "Chain Layer" structures and 3D structures through various supramolecular interactions. For 3PF6, a "Dimer Chain" structure is formed from the loosely connected dimers. Magnetic studies revealed that compounds 1NO3 and 2BF4(P1̅) displayed abrupt hysteretic SCO with the transition temperature T1/2↓ = 235 K, T1/2↑ = 240 K for 1NO3 and T1/2↓ = 230 K, T1/2↑ = 235 K for 2BF4(P1̅), while compounds 3PF6 and 4OTf are in the HS state. Desolvation of the complexes significantly modifies their SCO properties: the desolvated 1NO3 and 2BF4 show a gradual SCO, desolvated 3PF6 undergoes a two-step SCO, and desolvated 4OTf exhibits a hysteretic transition. Overall, this work reported the FeIII-SCO complexes of the quinoline-substituted Hqsal ligand and highlighted the potential of these ligands for the development of interesting FeIII-SCO materials.

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.

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.

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

The impact of solvent on spin crossover (SCO) behaviour is reported in two solvates [Fe(qsal-I)₂]NO₃·2ROH (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate; R = Me 1 or Et 2) which undergo abrupt and gradual SCO, respectively. A symmetry-breaking phase transition due to spin-state ordering from a [HS] to [HS-LS] state occurs at 210 K in 1, while T_1/2 = 250 K for the EtOH solvate, where complete SCO occurs. The MeOH solvate exhibits LIESST and reverse-LIESST from the [HS-LS] state, revealing a hidden [LS] state. Moreover, photocrystallographic studies on 1 at 10 K reveal re-entrant photoinduced phase transitions to a high symmetry [HS] phase when irradiated at 980 nm or a high symmetry [LS] phase after irradiation at 660 nm. This study represents the first example of bidirectional photoswitchability and subsequent symmetry-breaking from a [HS-LS] state in an iron(iii) SCO material.

Ä-Coupling Dielectric Functionality with Magnetic Properties in Coordination Metal Complexes

Significant research has been conducted on molecular ferroelectric materials, including pure organic and inorganic compounds; however, studies on ferroelectric materials based on coordination metal complexes are scarce. Ferroelectric materials based on coordination metal complexes have tunable structures and designs, with coexistence or synergy between the ferroelectric behavior and magnetic properties. Compared to inorganic compounds, few coordination metal complexes exhibit coupling between the magnetic and dielectric properties. Coordination metal complexes with strong coupling between the magnetic and dielectric properties exhibit dielectric permittivity variations under external magnetic fields. Therefore, they have attracted substantial interest for their potential use in magnetoelectric devices. In this review, we discuss recent advances in coordination metal complexes, that exhibit coupled magnetic functionalities and ferroelectricity or dielectric properties, including single-molecule magnets, electron delocalization systems, and external stimuli responsive compounds.

Anion Modified Spin Crossover in [Fe(qsal-4-F)]+ Complexes with a 4-Position Substituted Qsal Ligand

Four iron(III) complexes, [Fe(qsal-4-F)₂]Y·sol (Hqsal-4-F = 4-fluoro-N-(8-quinolyl)salicylaldimine; Y = NO₃^-, sol = 0.91MeOH·0.57H₂O (1_NO3); Y = PF₆^- (2_PF6); Y = BF₄^- (3_BF4); Y = OTf^-, sol =1.5MeOH (4_OTf)), with a new 4-position substituted qsal type ligand Hqsal-4-F have been synthesized and structurally and magnetically characterized. Complexes 1_NO3-3_BF4 consist of 1D chains formed by the [Fe(qsal-4-F)₂]⁺ cations connected by π-π and C-H···O interactions, which are further linked by more weak interactions to form 2D layers and 3D networks. On the other hand, complex 4_OTf has a structure of nearly isolated 1D column where the [Fe(qsal-4-F)₂]⁺ cations are connected by π-π, C-H···π, and C-F···π interactions. Magnetic studies revealed the occurrence of two-step symmetry-breaking SCO in 1_NO3 and two-step gradual SCO in 2_PF6. Complex 3_BF4 undergoes a gradual SCO, whereas 4_OTf remains almost high-spin. The smaller anions tend to stabilize the low-spin state, while larger anions tend to stabilize the high-spin state. In addition, the intermediate spin state of 1_NO3 could be thermally trapped by quenching from the high temperature, thereby kinetically suppressing the spin transition to the full low-spin state. This work represents a good example that the position of the substituent and the anions plays critical roles in the preparation of SCO materials with tunable properties.

Achieving large thermal hysteresis in an anthracene-based manganese(II) complex via photo-induced electron transfer

Achieving magnetic bistability with large thermal hysteresis is still a formidable challenge in material science. Here we synthesize a series of isostructural chain complexes using 9,10-anthracene dicarboxylic acid as a photoactive component. The electron transfer photochromic Mn²⁺ and Zn²⁺ compounds with photogenerated diradicals are confirmed by structures, optical spectra, magnetic analyses, and density functional theory calculations. For the Mn²⁺ analog, light irradiation changes the spin topology from a single Mn²⁺ ion to a radical-Mn²⁺ single chain, further inducing magnetic bistability with a remarkably wide thermal hysteresis of 177 K. Structural analysis of light irradiated crystals at 300 and 50 K reveals that the rotation of the anthracene rings changes the Mn1–O2–C8 angle and coordination geometries of the Mn²⁺ center, resulting in magnetic bistability with this wide thermal hysteresis. This work provides a strategy for constructing molecular magnets with large thermal hysteresis via electron transfer photochromism.

Achieving magnetic bistability with large thermal hysteresis is still a challenge in material science. Here, the authors report a Mn(II) chain complex that enables light-induced magnetic bistability with a 177 K thermal hysteresis loop.

Three Novel Thiazole-Arm Containing 1,3,4-Oxadiazole-Based [HS-HS] Fe(II) Dinuclear Complexes

Novel synthesis of 2,5-bis[(1,3-thiazol-2-ylmethyl)amino]methyl-1,3,4-oxadiazole (LThiazole) is reported, along with the preparation of three new dinuclear Fe(II) complexes with different counterions [FeII2(μ2-LThiazole)2](BF4)4·2CH3CN (1), [FeII2(μ2-LThiazole)2](ClO4)4 (2) and [FeII2(μ2-LThiazole)2](CF3SO3)4·2CH3CN (3). The obtained complexes were characterized by single-crystal X-ray crystallography, SQUID magnetometry and IR spectroscopy. The structure of the crystalline material was determined at 120 K for 1 and 3. The magnetic properties of all three complexes (1–3) were measured between 2–300 K and clearly show that all three complexes stay in the high-spin state over the measured temperature range.

One-Step versus Two-Step Valence Tautomeric Transitions in Tetraoxolene-Bridged Dinuclear Cobalt Compounds

The syntheses of valence tautomeric compounds with multistep transitions using new redox-active ligands are the long-term goal of the field of bistable materials. The redox-active tetraoxolene ligand, 2,7-di-tert-butylpyrene-4,5,9,10-tetraone (pyrene^Q-Q), is now developed to synthesize a pair of dinuclear compounds {[CoL₂]₂(pyrene^Sq-Sq)}[Co(CO)₄]₂·xCH₂Cl₂·2C₆H₅CH₃ (1, x = 2, L = 1,10-phenanthroline, phen; 2, x = 1.5, L = 2,2'-bipyridine, bpy). Variable-temperature magnetic susceptibilities and single-crystal X-ray diffraction measurements indicate a partial one-step valence tautomeric transition for 1 and a rare two-step valence tautomeric transition for 2, respectively. DFT calculation results are consistent with the experimental data, revealing the correlation between thermodynamic parameters and the one-step/two-step valence tautomeric behaviors.

Mixed-valence {FeII2FeIII4} hexanuclear complexes with thermally induced Fe(III) spin crossover behavior

Cyano-bridged mixed-valence {Fe6} hexanuclear complexes {[Tp4-MeFeII(CN)3]2[FeIII(Tpa)]2[FeIII(OR)(Tpa)]2}·6ClO4·S {Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate, Tpa = Tris(2-pyridylmethyl)amine; R = -CH3 S = 8MeOH 1, R = -C2H5 S = 6EtOH 2 } have been obtained...

Structural and theoretical insights into solvent effects in an iron(III) SCO complex

Alcohol effects in a series of iron(III) spin crossover complexes [Fe(qsal-Cl)2]NO3·ROH (R = Me 1, Et 2, 1-Pr 3) are explored. Despite the solvents differing from each other by only...

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

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

Thermodriven, Acidity-Driven, and Photodriven Spin-State Switching in Pyridylacylhydrazoneiron(II) Complexes at or above Room Temperature

The magnetic bistability of spin-crossover (SCO) materials is highly appealing for applications as molecular switches and information storage. However, switching of the spin state around room temperature remains challenging. In this work, we reported the successful manipulation of the spin states of two iron(II) complexes (1-Fe and 2-Fe) based on pyridylacylhydrazone ligands in manifold ways. Both complexes are stabilized in the low-spin (LS) state at room temperature because of the strong ligand-field strength imposed by the ligands. 2-Fe shows thermoinduced SCO above room temperature with a very large and reproducible hysteresis (>50 K), while 1-Fe remains in the LS state up to 400 K. Acidity-driven spin-state switching of the two complexes was achieved at room temperature as a result of the complex dissociation and release of iron(II) in its high-spin (HS) state. Recovery of the complex is feasible upon further alkalization treatment in the case of 1-Fe, allowing bidirectional modulation of the spin state of the metal center. Light-driven one-way switching from LS to HS is also achieved by virtue of E-to-Z isomerization at the C═N double bond, which results in dissociation of the complex because of the poor binding affinity in the Z configuration.

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

Halogen-substituted Fe(III) compounds, [Fe(Hphsalpm^X)₂]PTFB (Hphsalpm^X = 5-X-(R,S)-((phenyl(2-pyridyl)methylimino)methyl)phenol, PTFB = phenyltrifluoroborate; X = F for 1, Cl for 2, Br for 3) and [Fe(Hphsalpm^X)₂]PTFB·MeOH (X = I for 4·MeOH), were synthesized. Compounds 1, 4·MeOH, and its desolvated form 4 exhibited an invariant high-spin state in the whole temperature range, while 2 and 3 underwent gradual, nonhysteretic, and incomplete spin crossover (SCO) with transition temperatures (T_C) of 153 and 220 K, respectively. Interestingly, the SCO-active compounds 2 and 3 showed light-induced excited spin-state trapping (LIESST) effects at 10 K, and light-induced reversible ON/OFF switching behaviors were realized by alternately using 880 and 1064 nm light, while the thermally inert compound 4·MeOH unexpectedly showed a reverse-LIESST effect. These results may help to design and synthesize new photoresponsive SCO Fe(III) compounds for the development of switchable materials.

Valence tautomerism in a [2 × 2] Co4 grid complex containing a ditopic arylazo ligand

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

Hydrogen bond-induced abrupt spin crossover behaviour in 1-D cobalt(II) complexes - the key role of solvate water molecules

The magnetic properties and structural aspects of the 1-D cobalt(ii) complexes, [Co(pyterpy)Cl2]·2H2O (1·2H2O; pyterpy = 4'-(4'''-pyridyl)-2,2':6',2''-terpyridine) and [Co(pyethyterpy)Cl2]·2H2O (2·2H2O; pyethyterpy = 4'-((4'''-pyridyl)ethynyl)-2,2':6',2''-terpyridine) are reported. In each complex the central cobalt(ii) ion displays an octahedral coordination environment composed of three nitrogen donors from the terpyridine moiety, a nitrogen donor from a pyridyl group and two chloride ligands which occupy the axial sites. 1·2H2O exhibits abrupt spin-crossover (SCO) behaviour (T1/2↓ = 218 K; T1/2↑ = 227 K) along with a thermal hysteresis loop, while 2·2H2O and the dehydrated species 1 and 2 exhibit high-spin (HS) states at 2-300 K as well as field-induced single-molecule magnet (SMM) behaviour attributed to the presence of magnetic anisotropic HS cobalt(ii) species (S = 3/2). 1·2H2O exhibited reversible desorption/resorption of its two water molecules, revealing reversible switching between SCO and SMM behaviour triggered by the dehydration/rehydration processes. Single crystal X-ray structural analyses revealed that 1·2H2O crystalizes in the orthorhombic space group Pcca while 2 and 2·2H2O crystallize in the monoclinic space group P2/n. Each of the 1-D chains formed by 1·2H2O in the solid state are bridged by hydrogen bonds between water molecules and chloride groups to form a 2-D layered structure. The water molecules bridging 1-D chains in 1·2H2O interact with the chloride ligands occupying the axial positions, complementing the effect of Jahn-Teller distortion and contributing to the abrupt SCO behaviour and associated stabilization of the LS state.

The effect of tether groups on the spin states of iron(II)/bis[2,6-di(pyrazol-1-yl)pyridine] complexes

The synthesis of six 2,6-di(pyrazol-1-yl)pyridine derivatives bearing dithiolane or carboxylic acid tether groups is described: [2,6-di(pyrazol-1-yl)pyrid-4-yl]methyl (R)-lipoate (L1), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]ethyl (R)-lipoate (L2), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxy]ethyl (R)-lipoate (L3), N-([2,6-di(pyrazol-1-yl)pyrid-4-ylsulfanyl]-2-aminoethyl (R)-lipoamide (L4), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]acetic acid (L5) and 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]propionic acid (L6). The iron(ii) perchlorate complexes of all the new ligands exhibit gradual thermal spin-crossover (SCO) in the solid state above room temperature, except L4 whose complex remains predominantly high-spin. Crystalline [Fe(L6)2][ClO4]2·2MeCN contains three unique cation sites which alternate within hydrogen-bonded chains, and undergo gradual SCO at different temperatures upon warming. The SCO midpoint temperature (T1/2) of the complexes in CD3CN solution ranges between 208-274 K, depending on the functional group linking the tether groups to the pyridyl ring. This could be useful for predicting how these complexes might behave when deposited on gold or silica surfaces.

Spin-Crossover Molecules on Surfaces: From Isolated Molecules to Ultrathin Films

Molecular spintronics seeks to use single or few molecules as functional building blocks for spintronic applications, directly relying on molecular properties or properties of interfaces between molecules and inorganic electrodes. Spin-crossover molecules (SCMs) are one of the most promising classes of candidates for molecular spintronics due to their bistability deriving from the existence of two spin states that can be reversibly switched by temperature, light, electric fields, etc. Building devices based on single or few molecules would entail connecting the molecule(s) with solid surfaces and understanding the fundamental behavior of the resulting assemblies. Herein, the investigations of SCMs on solid surfaces, ranging from isolated single molecules (submonolayers) to ultrathin films (mainly in the sub-10 nm range) are summarized. The achievements, challenges and prospects in this field are highlighted.

Elastic Origin of the Unsymmetrical Thermal Hysteresis in Spin Crossover Materials: Evidence of Symmetry Breaking

The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.

Sublimable Spin-Crossover Complexes: From Spin-State Switching to Molecular Devices

Spin-crossover (SCO) active transition metal complexes are an important class of switchable molecular materials due to their bistable spin-state switching characteristics at or around room temperature. Vacuum-sublimable SCO complexes are a subclass of SCO complexes suitable for fabricating ultraclean spin-switchable films desirable for applications, especially in molecular electronics/spintronics. Consequently, on-surface SCO of thin-films of sublimable SCO complexes have been studied employing spectroscopy and microscopy techniques, and results of fundamental and technological importance have been obtained. This Review provides complete coverage of advances made in the field of vacuum-sublimable SCO complexes: progress made in the design and synthesis of sublimable functional SCO complexes, on-surface SCO of molecular and multilayer thick films, and various molecular and thin-film device architectures based on the sublimable SCO complexes.

Structural Transformations and Spin-Crossover in [FeL2 ]2+ Salts (L=4-{tert-Butylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine): The Influence of Bulky Ligand Substituents

4-(tert-Butylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L) was obtained in low yield from a one-pot reaction of 2,4,6-trifluoropyridine with 2-methylpropane-2-thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL₂ ][BF₄ ]₂ ⋅solv (1[BF₄ ]₂ ⋅solv) and [FeL₂ ][ClO₄ ]₂ ⋅solv (1[ClO₄ ]₂ ⋅solv; solv=MeNO₂ , MeCN or Me₂ CO) exhibit a variety of structures and spin-state behaviors including thermal spin-crossover (SCO). Solvent loss on heating 1[BF₄ ]₂ ⋅x MeNO₂ (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a "reverse-SCO" step in its warming cycle at a scan rate of 5 K min^-1 . The reverse-SCO is not observed in a slower 1 K min^-1 measurement, however, confirming its kinetic nature. The final product [FeL₂ ][BF₄ ]₂ ⋅0.75 MeNO₂ was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF₄ ]₂ ⋅y Me₂ CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low-spin 1[ClO₄ ]₂ ⋅0.5 Me₂ CO is not isostructural with its BF₄ ^- congener, and undergoes single-crystal-to-single-crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P 1 ‾ space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low-temperature phases when the materials are fully low-spin.

A Reversible Hydrogen-Bond Isomerization Triggered by an Abrupt Spin Crossover near Room Temperature

The spin crossover salt [Fe(bpp)₂ ](isonicNO)₂ ⋅ 2.4 H₂ O (1⋅2.4 H₂ O) (bpp=2,6-bis(pyrazol-3-yl)pyridine; isonicNO=isonicotinate N-oxide anion) exhibits a very abrupt spin crossover at T_1/2 =274.4 K. This triggers a supramolecular linkage (H-bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behavior reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1, a material that also exhibits spin crossover coupled to H-bond isomerization, leading to strong kinetic effects in the thermomagnetic properties.

High temperature anionic Fe(III) spin crossover behavior in a mixed-valence Fe(II)/Fe(III) complex

Two ion-pair Fe(iii) complexes (PPh4)[FeIII(HATD)2]·2H2O (1, H3ATD = azotetrazolyl-2,7-dihydroxynaphthalene) and [FeII(phen)3][FeIII(HATD)2]2·3DMA·3.5H2O (2, phen = 1,10-phenanthroline, DMA = N,N-dimethylformamide) were synthesized by employing the tridentate ligand H3ATD. Crystal structure analyses reveal that complexes 1 and 2 consist of FeIII ions in an octahedral environment where a FeIII ion is coordinated by two HATD2- ligands forming the [FeIII(HATD)2]- core. The shortest cationanion distance between the phosphorus ion of the (PPh4)+ cation and the ferric ion of the [FeIII(HATD)2]- anion is 13.190 Å in complex 1, whereas that between the ferrous ion of the [FeII(Phen)3]2+ cation and the ferric ion of the [FeIII(HATD)2]- anion is 7.821 Å in complex 2. C-HC and C-HO hydrogen interactions between the [FeII(phen)3]2+ cation and the [FeIII(HATD)2]- anion are observed in 2. Face-to-face π-π stacking interactions between naphthalene rings with the separated interplanar center to center distances of 3.421-3.680 Å were observed, which result in a one-dimensional supramolecular chain in complexes 1 and 2. Magnetic measurements show that complex 1 is in the low-spin (LS) state below 500 K, whereas 2 undergoes a high temperature spin crossover (SCO) between 360 and 500 K. Magneto-structural relationship studies reveal that π-stacking, hydrogen interactions and Coulomb interactions between the [FeIII(HATD)2]- anion and the [FeII(phen)3]2+ cation play a crucial role in the high temperature Fe(iii) SCO behaviour of complex 2.

Light-induced excited spin state trapping in iron(iii) complexes

This review discusses the correlation of the local and whole molecular structure of iron(iii) complexes with the magnetic properties including the light-induced excited spin-state trapping (LIESST) effect.

Spin crossover phenomena in long chain alkylated complexes

This review presents a discussion of soft metal complexes with a focus on spin crossover behaviours that are associated with structural phase transition, including liquid crystal LC transition.

A rare octacoordinated mononuclear iron(III) spin-crossover compound: synthesis, crystal structure and magnetic properties

The chemistry of transition-metal complexes with unusually high coordination numbers has been of interest because of their application in catalytic and biological systems. Deprotonation of the ionogenic tetradentate ligand 6,6′-bis(1H-tetrazol-5-yl)-2,2′-bipyridine [H2bipy(ttr)2] in the presence of iron(III) and tetra-n-butylammonium bromide, [n-Bu4N]Br, in solution resulted in the synthesis of a rare octacoordinated anionic mononuclear complex, tetra-n-butylammonium bis[6,6′-bis(tetrazol-1-id-5-yl)-2,2′-bipyridine]iron(III) methanol hemisolvate dihydrate, (C16H36N)[Fe(C12H6N10)2]·0.5CH3OH·2H2O or [n-Bu4N][Fe{bipy(ttr)2}2]·0.5CH3OH·2H2O (1), which has been structurally characterized by elemental analysis, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. In 1, the coordination sphere of the iron(III) ion is a distorted bis-disphenoid dodecahedron, in which the eight coordination positions are occupied by eight N atoms from two independent tetradentate [bipy(ttr)2]2− anionic ligands, therefore forming the anionic [Fe{bipy(ttr)2}2]− unit, with the negative charge balanced by a free [n-Bu4N]+ cation. An investigation of the magnetic properties of 1 revealed a gradual incomplete spin-crossover behaviour below 150 K.

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

Alternative Route Triggering Multistep Spin Crossover with Hysteresis in an Iron(II) Family Mediated by Flexible Anion Ordering

Multistep spin crossover (SCO) compounds have attracted much attention, since they can be great candidates for high-density multinary memory devices. The introduction of substituents, such as methyl (Me), chloro (Cl), bromo (Br), and methoxy (MeO) groups, at para positions to the phenyl-substituted tripodal N₆ ligand-coordinated SCO Fe^II material, [FeL^Ph](NTf₂)₂ [where L^Ph = tris(2-{[(1-phenyl-1H-1,2,3-triazol-4-yl)methylidene]amino}ethyl)amine and NTf₂ = bis(trifluoromethanesulfonyl)imide], affords a new family of solvent-free Fe^II complexes: [FeL^4-R-Ph](NTf₂)₂ {where L^4-R-Ph = tris[2-({[1-(4-R-phenyl)-1H-1,2,3-triazol-4-yl]methylidene}amino)ethyl]amine, where R = Me (1), Cl (2), Br (3), and MeO (4)}. 1 shows temperature invariant high-spin (HS) state, whereas the others show spin transitions with different characteristics, such as half-SCO (4), two-step SCO (3), and unusual three-step SCO with hysteresis (2). Mössbauer and X-ray absorption fine structure (XAFS) spectroscopic studies of them support the magnetic susceptibilities results. Density functional theory calculations indicate that the electronic effect of different substituents on magnetic properties is negligible in this Fe^II family. Single-crystal X-ray diffraction studies reveal that 1-4 has a similar packing arrangement with three-dimensional supramolecular network via intermolecular π-π and CH···π interactions between complex cations, and CH···X (X = O, N, and F) hydrogen bonding interactions between cations and inherently frustrated NTf₂ anions. Variable-temperature structural studies unveil a variety of stepped SCO behaviors of 2-4 and deactivation of SCO in 1 are governed by the regulation of ordering of NTf₂ counteranions through the subtle modification of terminal substituents of complex cations. Quantitative light-induced excited spin-state trapping (LIESST) effect was observed for 2-4 via green light irradiation (532 nm) at 10 K. This study opens up a new way for systematic control of magnetic response from no SCO to half-, two-step, and finally three-step SCO with hysteresis by precise tuning of the ordering of flexible NTf₂ anions included in the supramolecular network with potentially SCO-active complex cations.

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [FeL₂ ][BF₄ ]₂ ⋅2 H₂ O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T_1/2 =205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL₂ ][BF₄ ]₂ ⋅2 H₂ O and [ZnL₂ ][BF₄ ]₂ ⋅2 H₂ O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL₂ ][BF₄ ]₂ involves ordering of its Jahn-Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe_0.97 Cu_0.03 L₂ ][BF₄ ]₂ imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.

Physical stimulus and chemical modulations of bistable molecular magnetic materials

In this Feature Article, we summarize the recent progress made in modulating the multifaceted magnetic behaviour of single-molecule magnets (SMMs) and spin-crossover (SCO) materials based on chemical modifications and external stimuli.

Temperature-dependent hysteretic two-step spin crossover in two-dimensional Hofmann-type compounds

Two 2D Hofmann-type compounds [Fe^II(ppe)₂M^II(CN)₄]·3H₂O [ppe = 1-(2-pyridyl)-2-(4-pyridyl)ethylene; M = Pd for 1 and Pt for 2] have been synthesized. Both of them show complete two-step hysteretic SCO transitions HS^1.0 ⇌ HS^0.6–0.5LS^0.4–0.5 ⇌ LS^1.0.

Manipulating the spin crossover behavior in a series of {FeIII2FeII} complexes

Three cyanide-bridged {Fe₂Fe} complexes are reported to exhibit excellent SCO properties which are highly dependent on the compact degree of the π-π stacking, the loss of lattice solvents as well as the electron-donor strength of Tp^R.