Thermal and Light-Induced Spin Transitions in 3D Hofmann-type Frameworks Built on Nonlinear 3-Substituted Pyridine and Pyrimidine Pillaring Ligands

Integration of spin crossover (SCO) properties in 3D frameworks made up of cyano-bimetallic layers connected through pillaring organic ligands, the so-called Hofmann-type coordination polymers (HCPs)- represents an important source of multifunctional advanced materials. Typically, these 3D structures are constituted by 4-substituted pyridine-based linear pillars which afford HCPs with regular pcu topology. Here, we have investigated the suitability of the 3-substituted pyridine and pyrimidine bis-monodentate ligands 2,5-di(pyridin-3-yl)aniline (3-dpyan) and 2,5-di(pyrimidin-5-yl)aniline (bpmdan) as alternative building blocks to explore new structural topologies and functionalities. In this context, we have prepared the compounds Fe(3-dpyan)[Ag(CN)2]2·2MeOH (1Ag·2MeOH), Fe(3-dpyan)[Ag(CN)2]2···0.35NO2Bz·MeOH (1Ag·0.35NO2Bz·MeOH), Fe(3-dpyan)[Au(CN)2]2·NO2Bz (1Au·NO2Bz), and Fe(bpmdan)[Ag(CN)2]2·CH3Bz (2Ag·CH3Bz) (MeOH = methanol, NO2Bz = nitrobenzene, CH3Bz = toluene). Our structural studies have revealed that 1Ag·2MeOH and 1Ag·0.35NO2Bz·MeOH exhibit isomorphous doubly interpenetrated 3D structures strongly differing from the unusual noninterpenetrated ones exhibited by 1Au·NO2Bz and 2Ag·CH3Bz. Temperature-dependent magnetic susceptibility measurements have shown that all the reported compounds exhibit thermal-induced SCO properties, and moreover, three of them display Light Induced Excited Spin State Trapping at low temperatures (LIESST effect). The studied compounds show a wide diversity of SCO behaviors, ranging from abrupt complete one-step SCO centered at 253 K (1Au·NO2Bz) to gradual and incomplete multistepped SCO centered at 120 K (1Ag·0.35NO2Bz·MeOH). This assorted SCO properties are discussed and correlated to the acquired chemical and structural information.

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(II) complexes

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength-for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)-is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands-ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH2OCOMe, L2)-is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching-with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle-is observed for complex 1·CH3CN composed of L1 and BF4- counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH3CN-complex 2a, composed of L2 and BF4- counter anions-was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH3CN-2b·CH3CN-Y, Y denotes yellow colour of the crystals-composed of L2 and ClO4- counter anions, a gradual and non-hysteretic SCO is observed with T1/2 = 234 K. Complexes 1·CH3CN and 2b·CH3CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH3CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH3CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH3CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T1/2 centred at room temperature for 1·CH3CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.

Hysteretic Room-Temperature Magnetic Bistability of the Crystalline 4,7-Difluoro-1,3,2-Benzodithiazolyl Radical

The title radical R⋅, synthesized by reduction of the corresponding cation R+, is thermally stable up to ~380 K in the crystalline state under anaerobic conditions. With SQUID magnetometry, single-crystal and powder XRD, solid-state EPR and TG-DSC, reversible spin-Peierls transition between diamagnetic and paramagnetic states featuring ~10 K hysteretic loop is observed for R⋅ in the temperature range ~310-325 K; ΔH=~2.03 kJ mol-1 and ΔS=~6.23 J mol-1 K-1. The transition is accompanied by mechanical movement of the crystals, i. e., by thermosalient behavior. The low-temperature diamagnetic P-1 polymorph of R⋅ consists of R⋅2 π-dimers arranged in (…R⋅2…)n π-stacks; whereas the high-temperature paramagnetic P21/c polymorph, of uniform (…R⋅…)n π-stacks. With the XRD geometries, CASSCF and broken-symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R⋅2 and weak between R⋅2 for the (…R⋅2…)n stacks; and moderate AF interactions between R⋅ for the (…R⋅…)n stacks. The fully hydrocarbon archetype of R⋅ does not reveal the aforementioned properties. Thus, the fluorinated 1,3,2-benzodithiazolyls pave a new pathway in the design and synthesis of metal-less magnetically-bistable materials.

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

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

Rotational order-disorder and spin crossover behaviour in a neutral iron(II) complex based on asymmetrically substituted large planar ionogenic ligand

Octahedrally coordinated spin crossover (SCO) FeII complexes represent an important class of switchable molecular materials. This study presents the synthesis and characterisation of a novel complex, [FeII(ppt-2Fph)2]0·2MeOH, where ppt-2Fph is a new asymmetric ionogenic tridentate planar ligand 2-(5-(2-fluorophenyl)-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl)pyridine. The complex exhibits a hysteretic thermally induced SCO transition at 285 K on cooling and at 293 K on heating, as well as light induced excited spin state trapping (LIESST) at lower temperatures with a relaxation T(LIESST) temperature of 73 K. Single crystal analysis in both spin states shows that the compound undergoes an unusual partial (25%) reversible order-disorder of the asymmetrically substituted phenyl group coupled to the thermal SCO. The highly cooperative SCO transition, analysed by structural energy framework analysis at the B3LYP/6-31G(d,p) theory level, revealed the co-existence of stabilising and destabilising energy variations in the lattice. The observed antagonism of intermolecular interactions and synchronous rotational disorder, which contributes to the overall entropy change, is suggested to be at the origin of the cooperative SCO transition.

How the spin state tunes the slow magnetic relaxation field dependence in spin crossover cobalt(II) complexes

A novel family of cobalt(II) compounds with tridentate pyridine-2,6-diiminephenyl type ligands featuring electron-withdrawing substituents of general formula [Co(n-XPhPDI)2](ClO4)2·S [n-XPhPDI = 2,6-bis(N-n-halophenylformimidoyl)pyridine with n = 4 (1-3) and 3 (4); X = I (1), Br (2 and 4) and Cl (3); S = MeCN (1 and 2) and EtOAc (3)] has been synthesised and characterised by single-crystal X-ray diffraction, electron paramagnetic resonance, and static (dc) and dynamic (ac) magnetic measurements combined with theoretical calculations. The structures of 1-4 consist of mononuclear bis(chelating) cobalt(II) complex cations, [CoII(n-XPhPDI)2]2+, perchlorate anions, and acetonitrile (1 and 2) or ethyl acetate (3) molecules of crystallisation. This unique series of mononuclear six-coordinate octahedral cobalt(II) complexes displays both thermally-induced low-spin (LS)/high-spin (HS) transition and field-induced slow magnetic relaxation in both LS and HS states. A complete LS ↔ HS transition occurs for 1 and 2, while it is incomplete for 4, one-third of the complexes being HS at low temperatures. In contrast, 3 remains HS in all the temperature range. 1 and 2 show dual spin relaxation dynamics under the presence of an applied dc magnetic field (Hdc), with the occurrence of faster- (FR) and slower-relaxing (SR) processes at lower (Hdc = 1.0 kOe) and higher fields (Hdc = 2.5 kOe), respectively. On the contrary, 3 and 4 exhibit only SR and FR relaxations, regardless of Hdc. Overall, the distinct field-dependence of the single-molecule magnet (SMM) behaviour along with this family of spin-crossover (SCO) cobalt(II)-n-XPhPDI complexes is dominated by Raman mechanisms and, occasionally, with additional temperature-independent Intra-Kramer [LS or HS (D > 0)] or Quantum Tunneling of Magnetisation mechanisms [HS (D < 0)] also contributing.

Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Spin-state switching: chemical modulation and the impact of intermolecular interactions in manganese(III) complexes

A series of mononuclear manganese(III) complexes [Mn(X-sal2-323)](ReO4) (X = 5 Cl, 1; X = 5 Br, 2; X = 3,5 Cl, 3; X = 3,5 Br, 4; and X = 5 NO2, 5), containing hexadentate ligands prepared using the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and 5- or 3,5-substituted salicylaldehyde, has been synthesized. Variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical analyses, and theoretical calculations have been used to explore the role of various ligand substituents in the spin-state switching behavior of the prepared manganese(III) complexes. All five complexes consist of an analogous distorted octahedral monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand and ReO4- as the counter anion. However, a disordered water molecule was detected in complex 4. Complexes 1 (X = 5 Cl) and 5 (X = 5 NO2) show gradual and complete spin-state switching between the high-spin (HS) (S = 2) and the low-spin (LS) (S = 1) state with T1/2 values of 146 and 115 K respectively, while an abrupt and complete transition at 95 K was observed for complex 2 (X = 5 Br). Alternatively, complex 3 (X = 3, 5 Cl) exhibits an incomplete and sharp transition between the HS and LS states at 104 K, while complex 4 (X = 3, 5 Br) (desolvated) remains almost LS up to 300 K and then displays gradual and incomplete SCO at a higher temperature. The nature of the spin-state switch and transition temperature suggest that the structural effect (cooperativity) plays a more significant role in comparison with the electronic effect coming from various substituents (Cl, Br, and NO2), which is further supported by the detailed structural, electrochemical, and theoretical studies.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

Selective Pathway toward Heteroleptic Spin-Crossover Iron(II) Complexes with Pyridine-Based N-Donor Ligands

A new synthetic pathway is devised to selectively produce previously elusive heteroleptic iron(II) complexes of terpyridine and N,N'-disubstituted bis(pyrazol-3-yl)pyridines that stabilize the opposite spin states of the metal ion. Such a combination of the ligands in a series of the heteroleptic complexes induces the spin-crossover (SCO) not experienced by the homoleptic complexes of these ligands or shifts it to lower/higher temperatures respective to the SCO-active homoleptic complex. The midpoint temperatures of the resulting SCO span from ca. 200 K to the ambient temperature and beyond the highest temperature accessible by NMR spectroscopy and SQUID magnetometry. The proposed "one-pot" approach is applicable to other N-donor ligands to selectively produce heteroleptic complexes─including those inaccessible by alternative synthetic pathways─with highly tunable SCO behaviors for practical applications in sensing, switching, and multifunctional devices.

Solvatomorphs of Iron(II) Complex with N,N'-Disubstituted 2,6-Bis(pyrazol-3-yl)pyridine with a Temperature-Induced Spin Transition in Solution

Abstract

The reaction of the tridentate ligand 4-(2,6-bis(5-tert-butyl-1-(2,6-dichlorophenyl)-1H-pyrazol-3-yl)pyridin-4-yl)benzonitrile (L) with iron(II) salt gave the complex [Fe(L)2](BF4)2, which was isolated in a pure state and characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction as two crystal polymorphs differing in the nature of the solvent molecule in the crystal (solvatomorphs I and II). According to the results of X-ray diffraction study (CCDC nos. 2104367 (I), 2104368 (II)), the iron(II) ion in these compounds occurs in different spin states and does not undergo a temperature-induced spin transition, which was previously observed for this complex in solution. The details of supramolecular organization of two solvatomorphs that prevent this transition were studied using the Hirshfeld surface analysis.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete Fe^II neutral complex [Fe^IIL₂]⁰ (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P2₁/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central Fe^II ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large T_LIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

Investigations on the Spin States of Two Mononuclear Iron(II) Complexes Based on N-Donor Tridentate Schiff Base Ligands Derived from Pyridine-2,6-Dicarboxaldehyde

Iron(II)-Schiff base complexes are a well-studied class of spin-crossover (SCO) active species due to their ability to interconvert between a paramagnetic high spin-state (HS, S = 2, 5T2) and a diamagnetic low spin-state (LS, S = 0, 1A1) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L1)2](ClO4)2.CH3OH (1) and [Fe(L2)2](ClO4)2.2CH3CN (2), from two N6–coordinating tridentate Schiff bases derived from 2,6-bis[(benzylimino)methyl]pyridine. The complexes have been characterized by elemental analysis, electrospray ionization–mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), solution state nuclear magnetic resonance spectroscopy, 1H and 13C NMR (both theoretically and experimentally), single-crystal diffraction and magnetic susceptibility studies. The structural, spectroscopic and magnetic investigations revealed that 1 and 2 are with Fe–N6 distorted octahedral coordination geometry and remain locked in LS state throughout the measured temperature range from 5–350 K.

Impact of counter anions on spin-state switching of manganese(III) complexes containing an azobenzene ligand

Four mononuclear manganese(III) complexes coordinated with photo-active hexadentate azobenzene ligands, [Mn(5azo-sal₂-323)](X) (X = Cl, 1; X = BF₄, 2; X = ClO₄, 3; X = PF₆, 4), were prepared. The impact of various counter anions on the stabilization and switching of the spin state of the manganese(III) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, and spectroelectrochemical studies, along with theoretical calculations. All four complexes consisted of an isostructural monocationic distorted octahedral MnN₄O₂ coordination environment offered by the hexadentate ligand and Cl^-, BF₄^-, ClO₄^-, and PF₆^- as counter anions respectively. Complex 1 with a spherical Cl^- counter anion showed a reversible and gradual spin-state switching between low-spin (LS) (S = 1) and high-spin (HS) (S = 2) states above 400 K, where non-covalent cation-anion interactions played a significant role in stabilizing the LS state. While, irrespective of the shape of the counter anion, complexes 2-4 remained in the HS state throughout the measured temperature range of 300-2 K, where strong π-π interaction between the azobenzene motifs among cationic units played a substantial role in stabilizing the HS state. Furthermore, magnetic data analyses revealed significantly large zero-field splitting in the S = 1 state for 1 (D = 19.4 cm^-1, E/D = 0.008) in comparison with that in the S = 2 state for 2-4 (D = 3.99-4.97 cm^-1, E/D = 0.002-0.195). Spectroelectrochemical investigations revealed the quasi-reversible reduction and oxidation of the manganese(III) center to manganese(II) and manganese(IV), respectively. A detailed theoretical calculation at the DFT and CASSCF level of theory was carried out to better understand the magneto-structural correlation.

Spin-Crossover in Supramolecular Iron(II)-2,6-bis(1H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2'-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework

The interplay of host-guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [Fe^IIPd(CN)₄(L)₂]·1.3MeOH (1·1.3MeOH; L = methyl isonicotinate). Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1·1.3MeOH) and partially desolvated (1·0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the Fe^II sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet (¹Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host-guests interactions provides a great opportunity to design potential chemosensors.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [Fe^II(L^npdtz)₂(NCX)₂] (X = S; Se), with L^npdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-Pyrazol-1-yl)Pyridine) Complexes

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes - [Fe(bpp-COOEt)2 ](X)2 ⋅CH3 NO2 (X=ClO4 , 1; X=BF4 , 2). Stable spin-state switching - T1/2 =288 K; ΔT1/2 =62 K - is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO - T1/2 =331 K; ΔT1/2 =43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations - from the Jahn-Teller-distorted HS state to the less distorted LS state - and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.

High spin iron(ii) complexes based on imidazolyl- and 1,2,3-triazolyl-thione ligands and NCE (E = S, Se or BH3) co-ligands: effect of the S-functional group on the structural and magnetic properties

Differing magnetic behaviour in complexes of the type [FeII(N2S2)(NCE)2]: thione-based complexes are stabilised in the HS-state, whilst a previously reported picolylbenzylthioether–iron(ii) complex shows a hysteretic spin crossover behaviour.

Spin transition and symmetry-breaking in new mononuclear FeII tren-complexes with up to 38 K hysteresis around room temperature

New FeII complexes based on the well-known tripodand ligand type undergo abrupt hysteretic spin transition due to the symmetry-breaking in the room temperature region.

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

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

Chiral Resolution of Spin-Crossover Active Iron(II) [2x2] Grid Complexes

Chiral magnetic materials are proposed for applications in second-order non-linear optics, magneto-chiral dichroism, among others. Recently, we have reported a set of tetra-nuclear Fe(II) grid complex conformers with general formula C/S-[Fe4 L4 ]8+ (L: 2,6-bis(6-(pyrazol-1-yl)pyridin-2-yl)-1,5-dihydrobenzo[1,2-d : 4,5-d']diimidazole). In the grid complexes, isomerism emerges from tautomerism and conformational isomerism of the ligand L, and the S-type grid complex is chiral, which originates from different non-centrosymmetric spatial organization of the trans type ligand around the Fe(II) center. However, the selective preparation of an enantiomerically pure grid complex in a controlled manner is difficult due to spontaneous self-assembly. To achieve the pre-synthesis programmable resolution of Fe(II) grid complexes, we designed and synthesized two novel intrinsically chiral ligands by appending chiral moieties to the parent ligand. The complexation of these chiral ligands with Fe(II) salt resulted in the formation of enantiomerically pure Fe(II) grid complexes, as unambiguously elucidated by CD and XRD studies. The enantiomeric complexes exhibited similar gradual and half-complete thermal and photo-induced SCO characteristics. The good agreement between the experimentally obtained and calculated CD spectra further supports the enantiomeric purity of the complexes and even the magnetic studies. The chiral resolution of Fe(II)- [2×2] grid complexes reported in this study, for the first time, might enable the fabrication of magneto-chiral molecular devices.

Abrupt Spin-State Switching in Mn(III) Complexes with BPh4 Anion: Effect of Halide Substituents on Crystal Structure and Magnetic Properties

Three tetraphenylborates of mononuclear Mn(III) cation complexes with hexadentate ligands, the products of the reaction between a N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehydes with the different haloid substitutions at the 5 or 3,5 positions, have been synthesized: [Mn(5-F-sal-N-1,5,8,12)]BPh₄ (1), [Mn(3,5-diCl-sal-N-1,5,8,12)]BPh₄ (2) and [Mn(3,5-Br,Cl-sal-N-1,5,8,12)]BPh₄ (3). Their crystal structure, dielectric constant (ϵ) and magnetic properties have been studied. Ligand substituents have a dramatic effect on the structure and magnetic properties of the complexes. With decreasing temperature, the complex (1) shows a gradual spin crossover from the high-spin state (HS) to the HS:LS intermediate phase, followed by an abrupt transition to the low-spin state (LS) without changing the crystal symmetry. The complexes 2 and 3 are isostructural, but have fundamentally different properties. Complex 2 demonstrates two structural phase transitions related to sharp spin crossovers from the HS to the HS:LS intermediate phase at 137 K and from the intermediate phase to the LS at 87 K, while complex 3 exhibits only one spin transition from the HS to the HS:LS intermediate phase at 83 K.

A ring of grids: a giant spin-crossover cluster

Mononuclear and icosanuclear spin-crossover complexes, [Fe^II(HL)₂](BF₄)₂ (1) and [FeII20(L)₂₄](BF₄)₁₆ (2), were synthesized using an asymmetric multidentate ligand (HL). 1 has a bis-chelate structure with two protonated ligands, while 2 has a ring-shape structure comprising four [2 × 2] grid moieties and four mononuclear units.

Optimization of crystal packing in semiconducting spin-crossover materials with fractionally charged TCNQ δ- anions (0 < δ < 1)

Co-crystallization of the prominent Fe(ii) spin-crossover (SCO) cation, [Fe(3-bpp)2]2+ (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ δ- radical anion has afforded a hybrid complex [Fe(3-bpp)2](TCNQ)3·5MeCN (1·5MeCN, where δ = -0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σ RT = 3.1 × 10-3 S cm-1, and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ- anions, [Fe(3-bpp)2](TCNQ)2·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) to obtain [Fe(bzimpy)2](TCNQ)6·2Me2CO (4) and [Fe(bzimpy)2](TCNQ)5·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σ RT = 9.1 × 10-2 S cm-1 and 1.8 × 10-3 S cm-1, respectively.

Thermo- and photoinduced spin state switching in an iron(II) 2D coordination network associated with large light-induced thermal hysteresis and tuning of dimensionality via ligand modulation

Three iron(ii) complexes, [Fe(L1)2(NCS)2(MeOH)2] (1), [Fe(L1)2(NCSe)2(MeOH)2] (2), and [Fe(L2)2(NCS)2]n (3) (L1 = 2,5-dipyridyl-3,4-ethylenedioxythiophene and L2 = 2,5-diethynylpyridinyl-3,4-ethylenedioxythiophene), have been synthesized using redox-active luminescent ethylenedioxythiophene (EDOT)-based ligands, and characterized by variable temperature single-crystal X-ray diffraction, (photo)magnetic, optical reflectivity, and spectroscopy studies. Magneto-structural investigations revealed that 1 and 2 are mononuclear with a FeN4O2 octahedral coordination geometry and remain in a high-spin (HS) (S = 2) state in a temperature range of 2-280 K. Interestingly, a 2D coordination network structure with FeN6 surrounding each iron center was observed for 3, which exhibits reversible thermo-induced spin-state switching between the paramagnetic high-spin (HS) (S = 2) and diamagnetic low-spin (LS) (S = 0) states at around 105 K (T1/2). Furthermore, optical reflectivity and photomagnetic measurements at low temperature confirmed that 3 shows reversible ON/OFF switching between the photoinduced excited paramagnetic HS metastable state and diamagnetic LS state under light irradiation (ON mode using red light and OFF mode using green light). Finally, the photoinduced excited HS state can be reversibly relaxed back to the diamagnetic ground LS state by heating the system at ca. 88 K (TLIESST = 88 K) (light-induced excited spin state trapping (LIESST) effect). Furthermore, 3 also showed an exciting and unique 18 K wide light-induced thermal hysteresis (LITH) effect above liquid nitrogen temperature (100 K). DFT and CASSCF level theoretical calculations were utilized to better understand the magneto-structural correlations of these complexes.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

We investigate giant magnetoelectric coupling at a Mn³⁺ spin crossover in [Mn^IIIL]BPh₄ (L = (3,5-diBr-sal)₂323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη² coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Effect of ligand substituents and tuning the spin-state switching in manganese(iii) complexes

Three mononuclear manganese(iii) complexes based on flexible hexadentate ligands obtained from the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehyde or salicylaldehyde with substitutions at the 5 or 3,5 positions, namely [Mn(X-sal₂-323)](BPh₄) (X = 5 H, 1; X = 5 Br, 2, and X = 3,5 Br, 3) have been synthesized. The impact of ligand substituents has been studied by variable temperature single-crystal X-ray diffraction analyses, and magnetic, spectroscopic and electrochemical investigations. The complexes have an analogous monocationic MnN₄O₂ surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Complex 1 remains in the high spin state over the entire temperature range, while complex 2 shows a reversible and complete two-step thermo-induced spin-state switching. An incomplete spin-state switching from a high spin to an intermediate high-spin low-spin (1 : 1) state was observed for complex 3. Single-crystal X-ray structural studies show the presence of three different spin states in 2 during the occurrence of the spin-state switching process. Electrochemical investigations showed that the reduced state of manganese(iii) centers in 3 is easily accessible in comparison to complexes 1 and 2.

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.

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Δ_O) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T_1/2 above room temperature (300–368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an “inverted” ligand field, with an approximate “two-over-three” splitting of the d-orbitals and a high degree of metal–ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C–Ar or N–Ar rings, which is ascribed to the opposing effect on metal–ligand σ- and π-bonding.

Mapping the cooperativity pathways in spin crossover complexes

Crystal packing energy calculations are applied to the [Fe(PM-L)₂(NCS)₂] family of spin crossover (SCO) complexes (PM-L = 4-substituted derivatives of the N-(2-pyridylmethylene)-4-aminobiphenyl ligand) with the aim of relating quantitatively the cooperativity of observed SCO transitions to intermolecular interactions in the crystal structures. This approach reveals a linear variation of the transition abruptness with the sum of the magnitudes of the interaction energy changes within the first molecular coordination sphere in the crystal structure. Abrupt transitions are associated with the presence of significant stabilising and destabilising changes in intermolecular interaction energies. While the numerical trend established for the PM-L family does not directly extend to other classes of SCO complex in which the intermolecular interactions may be very different, a plot of transition abruptness against the range of interaction energy changes normalised by the largest change shows a clustering of complexes with similar transition abruptness. The changes in intermolecular interactions are conveniently visualised using energy difference frameworks, which illustrate the cooperativity pathways of an SCO transition.

The abruptness of spin crossover (SCO) is related to intermolecular energy changes occurring over the course of an SCO transition. Crossover is abrupt when SCO-induced strain is accommodated synergistically in a few key interactions.

Stepwise spin-state switching in a manganese(III) complex

A mononuclear manganese(iii) complex containing a flexible hexadentate chelating ligand has been prepared and characterized by performing, at various temperatures, single-crystal X-ray diffraction analyses and magnetic, spectroscopic, and electrochemical studies. The complex was shown to consist of an MnN4O2 octahedral coordination environment, and to exhibit reversible two-step thermally induced spin-state switching, a gradual one at 168 K and an abrupt one at 103 K. Structural analyses revealed the existence of three spin-states, namely high-spin, low-spin, and intermediate states, during the spin-state switching process. Electrochemistry studies showed the quasi-reversible reduction and oxidation of the manganese(iii) center with a comparatively easily accessible reduced state.

Bistable spin-crossover in a new series of [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine; R = CN) complexes

Spin-crossover (SCO) active transition metal complexes are a class of switchable molecular materials. Such complexes undergo hysteretic high-spin (HS) to low-spin (LS) transition, and vice versa, rendering them suitable for the development of molecule-based switching and memory elements. Therefore, the search for SCO complexes undergoing abrupt and hysteretic SCO, that is, bistable SCO, is actively carried out by the molecular magnetism community. In this study, we report the bistable SCO characteristics associated with a new series of iron(ii) complexes-[Fe(BPP-CN)2](X)2, X = BF4 (1a-d) or ClO4 (2)-belonging to the [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine) family of complexes. Among the complexes, the lattice solvent-free complex 2 showed a stable and complete SCO (T1/2 = 241 K) with a thermal hysteresis width (ΔT) of 28 K-the widest ΔT reported so far for a [Fe(BPP-R)2](X)2 family of complexes, showing abrupt SCO. The reproducible and bistable SCO shown by the relatively simple [Fe(BPP-CN)2](X)2 series of molecular complexes is encouraging to pursue [Fe(BPP-R)2]2+ systems for the realization of technologically relevant SCO complexes.

Spin State Behavior of A Spin-Crossover Iron(II) Complex with N,N′-Disubstituted 2,6-bis(pyrazol-3-yl)pyridine: A Combined Study by X-ray Diffraction and NMR Spectroscopy

A series of three different solvatomorphs of a new iron(II) complex with N,N′-disubstituted 2,6-bis(pyrazol-3-yl)pyridine, including those with the same lattice solvent, has been identified by X-ray diffraction under the same crystallization conditions with the metal ion trapped in the different spin states. A thermally induced switching between them, however, occurs in a solution, as unambiguously confirmed by the Evans technique and an analysis of paramagnetic chemical shifts, both based on variable-temperature NMR spectroscopy. The observed stabilization of the high-spin state by an electron-donating substituent contributes to the controversial results for the iron(II) complexes of 2,6-bis(pyrazol-3-yl)pyridines, preventing ‘molecular’ design of their spin-crossover activity; the synthesized complex being only the fourth of the spin-crossover (SCO)-active kind with an N,N′-disubstituted ligand.

Allosteric Spin Crossover Induced by Ligand-Based Molecular Alloying

The spin crossover (SCO) phenomenon represents a source of multistability at the molecular level, and dilution into a nonactive host was originally key to understand its cooperative nature and the parameters governing it in the solid state. Here, we devise a molecular alloying approach in which all components are SCO-active, but with significantly different characteristic temperatures. Thus, the molecular material [Fe(Mebpp)₂](ClO₄)₂ (2) has been doped with increasing amounts of the ligand Me2bpp (Mebpp and Me2bpp = methyl- and bis-methyl-substituted bis-pyrazolylpyridine ligands), yielding molecular alloys with the formula [Fe(Mebpp)_2-2x(Me2bpp)_2x](ClO₄)₂ (4x; 0.05 < x < 0.5). The effect of the composition on the SCO process is studied through single-crystal X-ray diffraction (SCXRD), magnetometry, and differential scanning calorimetry (DSC). While the attenuation of intermolecular interactions is shown to have a strong effect on the SCO cooperativity, the spin conversion was found to occur at intermediate temperatures and in one sole step for all components of the alloys, thus unveiling an unprecedented allosteric SCO process. This effect provides in turn a means of tuning the SCO temperature within a range of 42 K.

Spin Crossover in Bipyridine Derivative Bridged One-Dimensional Iron(III) Coordination Polymer

Herein, the syntheses, solid-state molecular structures, and characterization of two types of one-dimensional FeIII coordination polymers showing thermally induced spin crossover are reported. The reaction of [Fe(acen)Cl] (acen2− = N,N′-ethylenebis(acetylacetonylideneaminate) with 3,3′-bpy or 4,4′-bpy (bpy = bipyridine) produced zigzag and linear one-dimensional chain complexes, [Fe(acen)(3,3′-bpy)][BPh4] (1) or [NEt3H][Fe(acen)(4,4′-bpy)][BPh4]2·0.5(4,4′-bpy) (2), respectively, as confirmed by single crystal X-ray diffraction analysis. Variable-temperature single crystal X-ray diffraction measurements, continuous-wave X-band electron paramagnetic resonance (EPR) spectra, 57Fe Mössßauer spectra, and DC magnetic susceptibility data revealed that complex 1 exhibited a gradual and complete spin crossover at a transition temperature of 212 K, while complex 2 undergoes an incomplete spin crossover even at 400 K.