Themed collection Recent progress and perspectives on spin transition compounds

Guest editors Shinya Hayami, Birgit Weber and Malcolm Halcrow introduce the Dalton Transactions themed collection on spin transitions.

An effective experimental method is presented for preparing PVP-coated NPs presenting stable aqueous colloidal dispersions and hysteretic SCO behavior close to RT, which is essential for future drug delivery and MRI-bioimaging perspectives.

We report a room-temperature synthesis of a {[Fe(atrz)₃](OTs)₂}_n monolith following the green chemistry principles. This material exhibits a gradual spin crossover transition compared to bulk material, aligning with observations in smaller particles.

We have synthesized Fe(quinazoline)₂[Au(CN)₂]₂ (1). Complex 1 shows the Soma–Iwamoto-type bilayer and a spin-crossover (SCO) phenomenon. Mononuclear complex with a hydrogen-bonding network Fe(H₂O)₂(quinazoline)₂[Au(CN)₂]₂ has also been obtained.

The work presents a rationally designed cyanide bridged [Fe₂Co₃] pentanuclear complex integrated with protonation sites, where the electronic states can be triggered by both thermal and chemical stimuli. This provides a promising approach for multi-stimuli responsive switchable molecular materials.

Single-crystal X-ray diffraction studies of a spin crossover coordination polymer under simultaneous control of temperature, pressure, and light.

A tetravalent praseodymium complex with field-induced slow magnetic relaxation is presented.

Four Fe^III complexes of the Hqnal-5-Br_q ligand have been characterized, marking them as the first examples of Fe^III SCO complexes with a quinoline-substituted Hqnal ligand. The effects of anions and solvents on their SCO properties were assessed.

By varying the substituents on the pyrenylhydrazone ligands, fluorescent Fe(II)N₄O₂ complexes show high spin, low spin or spin crossover (SCO) properties.

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

Oriented external electric fields induce spin crossover in Fe(II) complexes, offering a novel pathway for temperature tuning, with potential applications in molecular electronics and sensing.

Electronic structure calculations on dinuclear Fe(II) spin-crossover systems unravel the effects that guest molecules have on tuning the transition temperature and the single or two-step behavior transition in such systems.

Spin transition induced by chloride in a mononuclear tripodal Fe(II) complex.

The alkylated derivatives of bidentate pyridyl-benzimidazole ligands were employed in the synthesis of SCO active ferrous complexes, which were specifically designed for the preparation of molecular films.

High spin Cu–V units with orthogonal magnetic orbitals and anisotropy of a coupled system.

Spin crossover Fe(II) molecules deposited on graphene/SiO₂ as a submonolayer present a switching behavior within the whole temperature range from 350 to 4 K.

The outstanding challenge of the calculation of the energy difference between different spin states is targeted with a new family of KTBM functionals.

By applying the IET to the spin dynamics of Ce porphyrinoid-based complexes, it is possible to investigate the effects of dynamic disorder on SMM properties.

Heterocycle choice in four structurally characterised solvent-free [Fe^II(L^heterocycle)(NCS)₂] complexes of easily accessed Schiff base ligands strongly influences solid state spin crossover behaviour.

Studies on iron(II) complexes composed of isomeric ligands reveal that one of the complexes—1·CH₃CN—undergoes bi-stable spin-state switching with T_1/2 = 298 K and ΔT_1/2 = 44 K.

Four enantiopure spin-crossover complexes have been prepared in which the unexpected incorporation of acetone leads to an additional chiral center.

Three homoleptic Fe(II) complexes with bidentate thiazole-based ligands exhibit spin-crossover (SCO) at the metal center. Abrupt temperature-driven and light-induced spin transitions are observed due to cooperative interactions between SCO cations.

It is discussed why for the Fe(II) complexes [Fe(L^{Tetra-Py/Ph-ODA})(NCE)] with E = S, Se and BH₃ only the latter revealed SCO.

Magnetoelectric coupling and magnetic-field-induced spin state trapping (MIESST) phenomena are discussed in the framework of a model based on the Landau theory for spin crossover materials with polar phases.

Two bipyridine-based bisbidentate ligands with an ethynyl- and a dimethoxy-phenylene bridge, respectively, are used to bridge two [Fe(H₂B(pz)₂)₂] units (pz = pyrazolyl), leading to charge-neutral dinuclear iron(II) compounds.

A new cobalt(II) complex featuring a terpyridine ligand functionalized with a thioether group has been prepared, structurally and magnetically characterized, and the thermodynamics of its spin crossover behaviour investigated in solution.

A new mononuclear iron(II) SCO compound featuring H-bonding donor and acceptor units provides an entry to a purely supramolecular switchable {Fe₄} tetrahedron that exhibits a singular magnetic behavior for each of the four Fe(II) centers.

Terpyridine-based cobalt(II) complexes with dynamic amine substituents have been found to exhibit not only solid-state spin-crossover behavior but also spectral changes associated with spin-state change in various organic solvents.

The simultaneous presence of different electrophores provides an interesting playground for responsive materials.

Valence tautomeric interconversion taking place in the solid solutions of the cobalt complex in the inert nickel matrix maintains the hysteresis loop. All the crystals destruct upon cooling down to the transition temperature.

In this study, we synthesize silica-coated core@shell nanoparticles based on the relatively unexplored [Fe(NH₂trz)₃](NO₃)₂. This enables the fabrication of water-stable spin-crossover nanoparticles exhibiting spin transition at room temperature.

Fe(II) complexes with a customized N4 ligand exhibit spin-crossover near room temperature, suitable for molecular switch applications.

A method is proposed to estimate the thermodynamic effects of neighbouring molecules on the spin transition of a mononuclear complex.

Room temperature spin crossover behaviour and LIESST effect synchronised with order–disorder at the 2-fluorophenyl substituent in a new neutral mononuclear Fe^II complex based on an asymmetric ionogenic tridentate planar ligand.

Dehydration induces spin crossover in hydrogen-bonded metal–organic frameworks such as [Fe(bpp)₂](ADC)·9H₂O, a compound showing large channels of a Wheel-of-Mainz shape.

Valence tautomerization in the cobalt bis verdazyl system [Co(dipyvd)₂]²⁺ is associated with large ligand geometry changes. As a result the equilibrium is strongly dependent on intermolecular interaction in both solid and solution phases.

A two-dimensional Hofmann-type coordination polymer [Fe^II(o-NTrz)₂Pt^II(CN)₄]·H₂O (o-NTrz = 4-(o-nitrobenzyl)imino-1,2,4-triazole) shows changes in spin-crossover behavior due to the loss of water molecules and consequent rotation of ligand o-NTrz.

A composite film combining a ferroelectric organic polymer with a spin crossover coordination polymer exhibits memory effect in their ferroelectric properties, with the appearance of thermal hysteresis in electric polarization and coercivity.

Spin-crossover behaviors and guest effects were investigated in three Fe(II) two-dimensional coordination polymers based on the 1,1,2,2-tetra(pyridin-4-yl)ethene ligand.

2D Spectrochemical series for predicting a magnitude of the ligand field splitting in six-coordinated octahedral metal complexes ML₆.

Thin films of the spin crossover compound {Fe(pyrazine)[μ₄-M(CN)₄]} (M = Ni, Pt) have been deposited on transparent fused silica substrates using a sequential assembly method with a scope for photonic and (opto)electronic applications.

[Fe(bpp)₂][ClO₄]₂ (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5–300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral–trigonal prismatic distortion pathway.

In an alternating layered structure of an amphiphilic anion, the electron transfer-coupled spin transition of a [Co₂Fe₂] complex causes a reversible structural change between the hydrogen-bonded 1D chains and discrete complexes.

Di(tetrazolyl)alkanes form with Fe(II) chains or layers which can be extended into spin-crossover 2D and 3D networks by applying dinitriles.

A series of mixed-anion Fe(NH₂trz)₃(BF₄)_2−x(SiF₆)_x/2 complexes presenting abrupt spin crossover behaviour with a narrow hysteresis loop just above room temperature is obtained by coordination reaction or solid–liquid post synthetic modification.

We synthesized three Fe(III) mononuclear complexes and investigated the effects of the functional group of qsal ligands and anions on spin crossover behaviors and the relaxation times of T₁ and T₂ in DMF solution.

Temperature-dependent variation of flexible hexyl groups in an iron(III) complex influences cation-to-cation and cation-to-anion interactions, leading to an unusual two-step SCO transition in the 400–10 K temperature range.

The cobalt(II) compound [Co(MeOphterpy)₂](ClO₄)₂ shows reversible switching between SCO and SMM properties depending on solvation or desolvation.

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds.

Intramolecular electron transfer without accompanying spin-crossover in cobalt bis-dioxolenes has been observed for the first time.

Judicious pairing of ancillary and redox-active ligands affords thermally-induced two-step valence tautomerism for a dinuclear cobalt complex bridged by a conjugated bis(dioxolene) ligand.

We report the discovery of a novel valence tautomeric cobalt mononuclear compound with a chiral ligand, which enhances the efficiency of light-induced polarization switching.

Crystal structure of the low-temperature phase of galvinoxyl consists of a radical dimer with a significant overlap between the SOMOs.

Discrete spin crossover (SCO) tetranuclear cages are a unique class of materials that have potential use in next-generation molecular recognition and sensing.

Four structurally related Co(II) spin crossover compounds were prepared, which revealed that combining a rigid polymeric framework with flexible substituents may constitute to a prototype for cooperative SPT in cobalt(II) SCO systems.

Heteroleptic iron(III) spin crossover complexes are reported with one of the compounds exhibiting a strongly hysteretic spin crossover that is superior to the homoleptic complexes.