Spin Crossover in Tetranuclear Fe(II) Complexes, {[(tpma)Fe(μ-CN)]4}X4 (X = ClO4–, BF4–)

Electron transfer coupled spin transition in cyano-bridged mixed-valence {FeIII2FeII2} molecular squares

The research on electron transfer coupled spin transition regulating the valence state and spin state transition of metal ions is promising and challenging. Herein, we report a cyano-bridged {FeIII2FeII2} molecular square complex, {[Fe(Tp)(CN)3]2 [Fe(bnbpen)]2}(ClO4)2·8CH3OH (1·8CH3OH, bnbpen = N,N'-bis-(2-naphthylmethyl)-N,N'-bis(2-picolayl)-ethylenediamine), and its free of solvents form (1). Combined single-crystal X-ray diffraction, temperature-dependent infrared (IR) spectra, magnetic measurements, and Mössbauer spectra reveal that 1·8CH3OH and 1 exhibit reversible one-step and two-step electron transfer coupled spin transition (ETCST) with temperature change, between the low-temperature state {FeII,LS(μ-CN)FeIII,LS}2 (LS = low spin, HS = high spin) and the high-temperature state {FeIII,LS(μ-CN)FeII,HS}2, respectively.

Stimuli-responsive magnetic materials: impact of spin and electronic modulation

Addressing molecular bistability as a function of external stimuli, especially in spin-crossover (SCO) and metal-to-metal electron transfer (MMET) systems, has seen a surge of interest in the field of molecule-based magnetic materials due to their enormous potential in various technological applications such as molecular spintronics, memory and electronic devices, switches, sensors, and many more. The fine-tuning of molecular components allow the design and synthesis of materials with tailored properties for these vast applications. In this Feature Article, we discuss a part of our research work into this broad topic, pertaining to the recent discoveries in the field of switchable molecular magnetic materials based on SCO and MMET systems, along with some historical background of the area and related accomplishments made in recent years.

Observation of protonation-induced intra-molecular metal-to-metal charge transfer in cyano-bridged [Fe-CN-Mn/Ni] dinuclear complexes

A key challenge in the design of magnetic molecules with intramolecular charge transfer behavior is to obtain reversible magnetic bistability triggered by external stimuli. Here, we show that two dinuclear metal complexes, [(bbp)Fe(CN)3Mn(Py5Me2)]·2.5CH3OH (4) and [(bbp)Fe(CN)3Ni(Py5Me2)]·2.5CH3OH (5) (Py5Me2 = 2,6-bis(1,1-di(pyridine-2-yl)ethyl)pyridine, H2bbp = 2,6-bis(benzimidazole-2-yl)pyridine), were self-assembly synthesized by (Bu4N)2[(bbp)FeIII(CN)3] and [Mn(Py5Me2)(OH2)](ClO4)2 or [Ni(Py5Me2)(OH2)](ClO4)2, respectively. Complexes 4 and 5 exhibited intramolecular metal-to-metal charge transfer with the addition of acids or bases in solution by UV-visible spectrophotometric measurements and electrochemistry studies, and concomitant switching of the {FeIII(μ-CN)MnII/NiII} state to the {FeII(μ-CN)MnIII/NiIII} state.

ON/OFF Photo(switching) along with Reversible Spin-State Change and Single-Crystal-to-Single-Crystal Transformation in a Mixed-Valence Fe(II)Fe(III) Molecular System

A mixed-valence Fe(II)Fe(III) molecular system, {[Fe(pzTp)(CN)₃]₂[Fe(bik)₂]₂}·[Fe(pzTp)(CN)₃]₂·4MeOH (1·4MeOH) (bik = bis-(1-methylimidazolyl)-2-methanone, pzTp = tetrakis(pyrazolyl)borate), exhibits single-crystal-to-single-crystal (SC-SC) transformation while increasing the temperature and is converted into {[Fe(pzTp)(CN)₃]₂[Fe(bik)₂]₂}·[Fe(pzTp)(CN)₃]₂ (1). Both complexes exhibit thermo-induced spin-state switching behavior along with reversible SC-SC transformation, where the low-temperature [Fe^III_LSFe^II_LS]₂ phase transforms into a high-temperature [Fe^III_LSFe^II_HS]₂ phase. 1·4MeOH exhibits an abrupt spin-state switching with T_1/2 at 355 K, whereas 1 undergoes a gradual and reversible spin-state switching with a lower T_1/2 at 338 K. Astonishingly, 1 exhibits ON/OFF photo-induced spin-state switching with T_LIESST = 67 K, whereas 1·4MeOH does not show such an effect.

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...

A Family of Heterobimetallic Cubes Shows Spin-Crossover Behaviour Near Room Temperature

Using 4-(4'-pyridyl)aniline as a simple organic building block in combination with three different aldehyde components together with metal(II) salts gave three different Fe8 Pt6 -cubes and their corresponding Zn8 Pt6 analogues by employing the subcomponent self-assembly approach. Whereas the use of zinc(II) salts gave rise to diamagnetic cages, iron(II) salts yielded metallosupramolecular cages that show spin-crossover behaviour in solution. The spin-transition temperature T1/2 depends on the incorporated aldehyde component, giving a construction kit for the deliberate synthesis of spin-crossover compounds with tailored transition properties. Incorporation of 4-thiazolecarbaldehyde or N-methyl-2-imidazole-carbaldehyde yielded cages that undergo spin-crossover around room temperature whereas the cage obtained using 1H-4-imidazolecarbaldehyde shows a spin-transition at low temperatures. Three new structures were characterized by synchrotron X-ray diffraction and all structures were characterized by mass spectrometry, NMR and UV/Vis spectroscopy.

Role of the B′-site metal ion in the framework structures and dielectric transitions in host–guest type cyanometalates (HIm)2[B′Co(CN)6] (HIm = imidazolium cation)

The B′-site alkali metal ion in (HIm)₂[B′Co(CN)₆] plays a key role in the change of the framework structures and the dynamic behaviours of the guest.

Dicyanometalates as Building Blocks for Multinuclear Iron(II) Spin-Crossover Complexes

A synthetic strategy featuring dicyanometalates [M(CN)₂]^- (M = Ag, Au) as N-coordinating ditopic linkers connecting partially blocked Fe^II centers has been employed to produce heterometallic hexanuclear complexes, which exhibit spin-crossover (SCO) behavior at the Fe^II sites. The reaction between tris(2-pyridylmethyl)amine (tpma)-capped Fe^II ions and [Ag(CN)₂]^- proceeded with partial decomposition of the dicyanoargentate and led to the formation of {[Fe(tpma)]₄(μ-CN)₂[μ-Ag(CN)₂]₂}(ClO₄)₄·3H₂O (1), in which both [Ag(CN)₂]^- and CN^- act as bridging ligands, and the opposite [Ag(CN)₂]^- bridges are engaged in a pronounced argentophilic d¹⁰-d¹⁰ interaction. In an analogous synthesis, the more stable [Au(CN)₂]^- species remained intact and furnished the complex {[Fe(tpma)]₂[μ-Au₂(CN)₄]₂} (2), which features two Fe^II centers bridged by two [Au₂(CN)₄]^2- dimers. The use of S,S'-bis(2-pyridylmethyl)-1,2-thioethane (bpte) as a mixed-donor, N₂S₂-coordinating capping ligand yielded {[Fe(bpte)]₂[μ-Au₂(CN)₄]₂} (3), with a structure analogous to that of 2. Variable-temperature magnetic susceptibility measurements revealed that complexes 1-3 exhibit an onset of SCO above 350 K. Measurements above 400 K further confirmed the occurrence of a gradual spin-state conversion for complex 2.

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

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

Spin crossover in discrete polynuclear iron(ii) complexes

A comprehensive review of 127 dinuclear to octanuclear complexes, mostly 2012-present, reveals key design features and future directions for spin crossover active supramolecular assemblies.

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

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

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.

A series of organic–inorganic hybrid materials consisting of flexible organic amine modified polyoxomolybdates: synthesis, structures and properties

A series of hybrid materials consisting of Tris[(2-pyridyl)methyl]amine modified polyoxomolybdates were obtained and some complexes have efficient photocatalytic activities to degrade pararosaniline hydrochloride dye molecules.

Spin crossover and reversible single-crystal to single-crystal transformation behaviour in two cyanide-bridged mixed-valence {FeIII2FeII2} clusters

Two mixed-valence {FeIII2FeII2} clusters exhibit spin crossover behaviour, in which a distorted {Fe₄(μ-CN)₄} core and deviations of bent ∠Fe−NC angles play a key role in tuning the transition temperature.

An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]

Iron(ii) and zinc(ii) complexes of bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide assemble into tetranuclear metallacycles according to ES-MS, DOSY NMR and conductivity data. The iron complex exhibits a thermal spin-state equilibrium in solution, which the iron centres appear to undergo independently of each other.