Polymorphism-Dependent Spin-Crossover: Hysteretic Two-Step Spin Transition with an Ordered [HS–HS–LS] Intermediate Phase

Magnetoelectric and MIESST effects in spin crossover materials exhibiting symmetry-breaking

Giant magnetoelectric coupling and magnetic-field-induced spin state trapping (MIESST) were recently reported in spin crossover materials with polar phases. We discuss these phenomena considering the distinct contributions of the change of the molecular spin state, driven by the magnetic field, and the coupled structural symmetry-breaking during the stepwise change of electric polarisation or MIESST.

Design and characterization of Fe(II) complexes with tetradentate ligands exhibiting spin-crossover near room temperature

Construction of spin-crossover (SCO) materials is very appealing for applications such as molecular switches and information storage. This study focuses on the design of Fe(II) complexes using N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine-based ligands with an N4 structure for SCO material development. By incorporating para-substituted benzene groups into the ligand's pyridine moiety, two polymorphs, α and β, were obtained, both exhibiting SCO activity. Notably, the β polymorph displayed a spin crossover temperature of 270 K, which is approaching room temperature. Structural analyses were conducted to compare the differences between the polymorphs, along with a literature review of related complexes, providing insights into the characteristics of SCO behavior.

Hexacoordinate high-spin Fe(III) complexes composed of pentadentate amino-type ligand and pseudohalido coligands

A new series of Fe(III) mononuclear complexes of [Fe(Lam)(X)] type {where X = Cl (1), NCSe (2), NCS (3), N3 (4), NCO (5)} have been prepared and characterized in detail....

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn³⁺ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn^III(3,5-diCl-sal₂(323))]BPh₄ reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1_(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn³⁺ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1_(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1_(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Thermal and Magnetic Field Switching in a Two-Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings

A Mn^III spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where 1 / 4 are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Spin crossover in mononuclear Fe(II) complexes based on a tetradentate ligand

Three isostructural complexes with the formula [Fe(L_5Me)(NCE)₂]: L_5Me  =  N,N'-bis(5-methyl-2-pyridylmethyl)ethane-1,2-diamine and E  =  S (1-S), E  =  Se (1-Se), E  =  BH₃ (1-BH ₃ ) have been synthesized and characterized by single-crystal x-ray diffraction, magnetic susceptibility and DSC studies. All the three derivatives are spin crossover (SCO) active, showing complete one-step spin conversion. The SCO midpoint temperatures (T _1/2) are 193 K for 1-S, 226 K for 1-Se, and 330 K for 1-BH ₃ , which are among the highest values for the homologous Fe(II)-NCE complexes with comparable tetradentate ligands. The almost linear Fe-N  ≡  C(E) angles are consistent with the strong ligand field (LF) strength imposed by these NCE^- co-ligands. Strong hydrogen-like bonding N-H…E was observed to connect the molecules into 2D supramolecular sheets parallel to the bc plane. However, such supramolecular interaction is not sufficient enough to transmit strong cooperativity. A discussion on the factors governing the LF strength and the cooperativity has been made, based on the comparison of analogous complexes and also based on UV-vis spectroscopy studies of the Ni(II) complexes.

Structure–property relationships and the mechanisms of multistep transitions in spin crossover materials and frameworks

Structure–property relationships are identified and applied to explain multi-step transitions and the different antiferroelastic patterns found in spin crossover frameworks.

Slow Dynamics of the Spin-Crossover Process in an Apparent High-Spin Mononuclear FeII Complex

A mononuclear Fe^II complex that shows a high-spin (S=2) paramagnetic behavior at all temperatures (with standard temperature-scan rates, ≈1 K min^-1 ) has, in fact, a low-spin (S=0) ground state below 100 K. This low-spin state is not easily accessible due to the extremely slow dynamics of the spin-crossover process-a full relaxation from the metastable high-spin state to the low-spin ground state takes more than 5 h below 80 K. Bidirectional photo-switching of the Fe^II state is achieved reproducibly by two selective irradiations (at 530-590 and 830-850 nm). The slow dynamics of the spin-crossover and the strong structural cooperativity result in a remarkably wide 95-K hysteresis loop induced by both temperature and selected light stimuli.

Iron(II) and 1,1,1-Tris(2-pyridyl)nonadecane Complex Showing an Order-Disorder-Type Structural Transition and Spin-Crossover Synchronized over Both Conformers

The title compound (Me₄N)[Fe{(2-py)₃C- n-C₁₈H₃₇}(NCS)₃] having a C₁₈ chain underwent spin-crossover (SCO). There are two crystallographically independent molecules in a triclinic cell at 100 K but only one in a monoclinic cell at 300 K. The complex showed a gradual one-step SCO in the same temperature region as that of the structural transition. The SCO is supported with the ⁵⁷Fe Mössbauer spectroscopy. The molecules are differentiated on cooling, but the SCO was synchronized.

Resolving the Ultrafast Changes of Chemically Inequivalent Metal-Ligand Bonds in Photoexcited Molecular Complexes with Transient X-ray Absorption Spectroscopy

Photoactive transition-metal complexes that incorporate heteroleptic ligands present a first coordination shell, which is asymmetric. Although it is generally expected that the metal-ligand bond lengths respond differently to photoexcitation, resolving these fine structural changes remains experimentally challenging, especially for flexible multidentate ligands. In this work, ultrafast X-ray absorption spectroscopy is employed to capture directly the asymmetric elongations of chemically inequivalent metal-ligand bonds in the photoexcited spin-switching Fe^II complex [Fe^II(tpen)]²⁺ solvated in acetonitrile, where tpen denotes N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethylenediamine. The possibility to correlate precisely the nature of the donor/acceptor coordinating atoms to specific photoinduced structural changes within a binding motif will provide advanced diagnostics for optimizing numerous photoactive chemical and biological building blocks.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different Fe^II complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled Fe^II-[M^IV(CN)₈]^4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {Fe^II₂(3-OAcpy)₅(3-OHpy)₃[M^IV(CN)₈]}· nH₂O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe₃M₂} _n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)₄(μ-NC)₂], [Fe2(3-OHpy)₄(μ-NC)₂], and [Fe3(3-OAcpy)₃(3-OHpy)(μ-NC)₂], which lead to the thermal two-step Fe^II SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and ⁵⁷Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep Fe^II SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

Investigation of SCO property–structural relationships in a family of mononuclear Fe(ii) complexes

Six complexes with the formula [FeL(NCE)₂] (E = S, Se, BH₃) have been reported. A survey of the magneto-structural relationship has been conducted.

An Incomplete Spin Transition Associated with a Z'=1→Z'=24 Crystallographic Symmetry Breaking

Crystalline [FeL2 ][BF4 ]2 ⋅Me2 CO (L=N-[2,6-di{pyrazol-1-yl}pyrid-4-yl]acetamide) is high-spin at room temperature, and undergoes an abrupt, hysteretic spin-crossover at T1/2 =137 K (ΔT1/2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P21 /c, Z=4) to phase 2 (P21 , Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high-spin and low-spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL2 ][ClO4 ]2 ⋅Me2 CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P21 /c, Z=8) without a change in spin state.

The influence of NCE− (E = S, Se, BH3) ligands on the temperature of spin crossover in a family of iron(ii) mononuclear complexes

The temperature of spin crossover was systematically tuned by replacing the NCE⁻ (E = S, Se, BH₃) co-ligands in a family of mononuclear complexes.

Mononuclear Fe(II) Complexes Based on the Methylpyrazinyl-Diamine Ligand: Chemical-, Thermo- and Photocontrol of Their Magnetic Switchability

Two new mononuclear Fe(II) complexes, [Fe(^2MeL_pz)(NCX)₂] (L = N,N'-dimethyl-N,N'-bis((pyrazin-2-yl)methyl)-1,2-ethanediamine and X = S (1), BH₃ (2)), have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, optical reflectivity, and photomagnetic measurements. They have similar FeN₆ coordination environments offered by the tetradentate ligand with a cis-α conformation and two NCX^- coligands in cis positions. However, 1 and 2 have different molecular arrangements and crystal packings, and are isolated in orthorhombic Pbnb and monoclinic C2/c space groups, respectively. 1 remains in a high spin state (S = 2) over all temperatures while 2 undergoes a spin transition around 168 K with a small thermal hysteresis of about 0.4 K (at a temperature scan rate of 1.3 K min^-1). This phase transition, which can also be optically detected due to the associated marked change of the sample color, occurs between two structurally characterized phases, which exhibit Fe(II) complexes in their high spin and low spin states at high and low temperatures, respectively. The reversible photoswitching between these two states has also been confirmed at low temperatures using well separated wavelength irradiations.

Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer

The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)₃)₂](4,4'-bipy)₂]_n is reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order.

Polymorphism of a chiral iron(ii) complex: spin-crossover and ferroelectric properties

A chiral iron(ii) complex with two polymorphs, one that undergoes a gradual spin crossover, whilst the other remains in a high-spin state but shows a typical ferroelectric feature, is reported.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group

Two iron(ii) structural isomers with only one methyl position difference in their ligands exhibit different spin states due to the competition of the electronic effect and steric crowding.

Co–Co and Co–Fe cyano-bridged pentanuclear clusters based on a methylpyrazinyl-diamine tetradentate ligand: spin crossover and metal substitution effects

A pentanuclear [Co^II₃Co^III₂] cluster complex has been developed by a solvothermal synthesis. Its highly stable metal-mixed Fe–Co derivatives display robust spin crossover (T_1/2 = 268 K) controlled by the degree of substitution.

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.

First Step Towards a Devil's Staircase in Spin-Crossover Materials

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3 )2 ](4,4'-bipy)2 }n exhibits a thermal high-spin (HS)↔low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction γHS . Between the fully HS (γHS =1) and LS (γHS =0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the γHS ≈0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the γHS ≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of "Devil's staircase"-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

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).

Thiazolylimines as novel ligand-systems for spin-crossover centred near room temperature

A new thiazolylimine ligand system for iron(ii) complexes which stabilises spin-crossover in solution and solid states with T_1/2 temperatures around room temperature has been developed.

Structural phase transition in a multi-induced mononuclear FeII spin-crossover complex

One mononuclear Fe^II complex displays a structural phase transition and a multi-induced spin-crossover behavior mediated by heat, light, pressure and solvent.

Two-step spin crossover behaviour in the chiral one-dimensional coordination polymer [Fe(HAT)(NCS)2]∞

Two-step spin crossover behaviour with large stabilisation of the intermediate state in a chiral one-dimensional coordination polymer.

In air a spin crossover active iron(ii) complex of amine/NCBH3− ligands is converted to a low spin complex of imine/CN− ligands

A spin crossover Fe^II complex is in situ converted to a low spin complex in the presence of air, induced by the oxidative dehydrogenation of the tetradentate ligand and the generation of CN⁻ from NCBH₃⁻.