Factors Impacting Electron Transfer in Cyano-Bridged {Fe2Co2} Clusters

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.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Modeling of the Charge-Transfer-Induced Spin Transition in the Tetranuclear Cyanide-Bridged [Co2Fe2] Complex

The course of the charge-transfer-induced spin transition demonstrated by the cyanide-bridged tetranuclear [Co2Fe2(bpy*)4(CN)6(tp*)2](PF6)2·2CP·8BN complex has been followed by DFT calculations of the single-point energies for different total spin values of the complex in a wide temperature range. With the aid of these calculations, the picture of spin conversion, that the compound undergoes, has been restored. It has been demonstrated that at 100 K the two crystallographically unique tetranuclear Fe2Co2 subunits A and B present in the structure contain diamagnetic low-spin FeII and low-spin CoIII ions. From the three subunits A, A', and B detected crystallographically in the compound at 200 K, only the A ones contain paramagnetic low-spin FeIII and high-spin CoII ions, while at 260 K, a half of all clusters contained in the crystal are in this state. From the DFT calculations, it also follows that at 320 K in the crystal only paramagnetic units are present. The results obtained are in accordance with the experimental data on the magnetic susceptibility. A possibility to predict new materials exhibiting spin transitions on the basis of DFT calculations of single-point energies as functions of temperature is discussed.

Thermo- and photo-induced electron transfer in a series of [Fe2Co2] capsules

Recently, a family of [Fe₂Co₂] molecular capsules that display tunable electron transfer-coupled spin transition (ETCST) behavior were reported via a smart approach through Schiff-base condensation of aldehyde-functionalized 2,2-bipyridines (bpy^CHO) and 1,7-heptanediamine (H₂N(CH₂)₇NH₂). Here, three more capsule complexes {[(Tp^R)Fe(CN)₃]₂[Co(bpy^{CN(CH₂)_nNC}bpy)]₂[ClO₄]₂}·n(solvent) (1, Tp^R = Tp*, n = 5, sol = 8DMF; 2, Tp^R = Tp^Me, n = 9, sol = 5MeCN; and 3, Tp^R = Tp*, n = 11, sol = 5MeCN), where Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate and Tp^Me = hydridotris(3-methylpyrazol-1-yl)borate are reported, demonstrating a successful extension of such an approach with other alkyldiamines of different lengths. Combined X-ray crystallographic, infrared spectroscopic and magnetic studies reveal incomplete electron transfer with either changing temperature or upon light exposure.

Tuning the electron transfer events in a series of cyanide-bridged [Fe2Co2] squares according to different electron donors

It has been recognized that both the ligand fields and intermolecular interactions may greatly impact the electron-transfer-coupled spin transition (ETCST) events in switchable magnetic materials; however, the engineering of these factors within a given system is still challenging. In this article, we chose the 4,4'-substituent 2,2'-bipyridine derivatives as chelating ligands according to their increasing electron-donating strength and incremental potential for forming hydrogen bonds (bpy^CHO,CH₃(L1) < bpy^{CH₂OH,CH₃} (L2) < bpy^{CH₂OH,CH₂OH} (L3)), and prepared three new [Fe₂Co₂] complexes, {[(Tp*)Fe(CN)₃Co(L)₂]₂[ClO₄]₂}·Sol (1, L = L1, Sol = 4MeCN·2H₂O; 2, L = L2, Sol = 3MeCN; 3, L = L3, Sol = 4MeOH; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate). X-ray crystallography studies revealed that all the complexes share similar cyanide-bridged [Fe₂Co₂] square compositions except for the different substituted groups of L ligands, which led to the clearly evidenced intercluster hydrogen bonds between the neighbouring hydroxyl groups in 2 and 3. As a result, 1 remained in the paramagnetic [FeIII,LS2CoII,HS2] state over the whole temperature range, while 2 and 3 showed complete ETCST behaviour with the transition temperatures (T_1/2) being 221 and 294 K, respectively.

Light actuated single-chain magnet with magnetic coercivity

A cyanide-bridged {Fe2Co}-based coordination polymer was synthesized. It showed photo-induced slow relaxation of magnetization and a coercive field of 400 Oe.

Observation of Protonation-Induced Spin State Switching in a Cyanido-Bridged {Fe2Co2} Molecular Square

The self-assembly of [Co(MeTPyA)(CH₃COO)]PF₆ (1) and [Fe(bbp)(CN)₃]^2- affords a cyanido-bridged square-shaped {Fe₂Co₂} tetranuclear complex, [{Co(MeTPyA)(μ₂-NC)₂Fe(bbp)(CN)}₂]·3H₂O (2; MeTPyA = tris((3,5-dimethylpyrazol-1-yl)methyl)amine and H₂bbp = bis(2-benzimidazolyl)pyridine). The possibility of inducing an intramolecular electron transfer coupled spin transition in 2 by employing protonation as an external stimulant is explored. UV-visible spectrophotometric measurements, electrochemical and ¹H NMR studies establish that a reversible intramolecular electron transfer coupled spin transition can be triggered in 2 upon addition of either acid or base.

Anion-Dependent Electron Transfer in the Cyanide-Bridged [Fe2Co2] Capsules

A family of molecular capsules, {[(Tp*)Fe(CN)₃Co(bpy^{C═N(CH₂)₇N═C}bpy)]₂[X]₂}·sol (1, X = ClO₄, sol = 6DMF; 2, X = PF₆, sol = 6DMF; 3, X = OTf, sol = 6DMF; 4, X = BPh₄, sol = 2DMF; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; bpy = 2,2'-bipyridine), were prepared via the Schiff-base condensation of the aldehyde-substituted bpy (bpy^CHO) and 1,7-diaminoheptane (H₂N(CH₂)₇NH₂). All the complexes contain the same cyanide-bridged cationic square cores ([Fe₂Co₂]²⁺), which are encapsuled by the flexible alkyl chains. Variable-temperature single-crystal X-ray diffraction, FT-IR spectra, and magnetic studies reveal the abrupt and complete, thermo- and photo-induced electron-transfer-coupled spin transition for 1-3, while the pure high-spin phase for 4. Such distinct behavior is attributed to the effective long-range cooperative interactions mediated by the intercluster π-π couplings in 1-3, which, however, are significantly blocked in 4 due to the steric effect of interstitial BPh₄^- anions. Furthermore, the shift in the thermally induced transition temperatures of 254 K for 1, 233 K for 2, and 187 K for 3, respectively, is likely correlated to the variable H···O and H···F interactions between the solvent molecules, anions, and the bipyridine ligands of the [Fe₂Co₂] squares, suggesting the significant anion-dependent effect in such a system.

High-temperature electron transfer coupled spin transition (ETCST) with hysteresis in a discrete [Fe2Co2] Prussian blue analogue

A thermoresponsive Fe-Co based discrete Prussian blue analogue (PBA) with the general formula [Fe(Tp)(CN)3]2[Co{en(Bn)py}]2(ClO4)2·4MeOH·4H2O i.e. [1·4MeOH·4H2O] is reported, which discloses a sharp and high temperature ETCST along with a wide hysteresis of 10 K observed for its desolvated complex 1.

Evidence of protonation induced intra-molecular metal-to-metal charge transfer in a highly symmetric cyanido bridged {Fe2Ni2} molecular square

The possibility to induce intra-molecular metal-to-metal charge transfer in a cyanido bridged tetranuclear square shaped {Fe₂Ni₂} complex by employing protonation as an external stimulant is explored. Two cyanido bridged square shaped tetranuclear complexes, [{Ni(TPA)(μ₂-NC)₂Ni(CN)₂}₂]·4H₂O (2) and [{Ni(TPA)(μ₂-NC)₂Fe(bbp)(CN)}₂]·10H₂O (3) [TPA = tris(3,5-dimethylpyrazol-1-ylmethyl)amine and H₂bbp = bis(2-benzimidazolyl)pyridine], are synthesized and characterized. Low temperature magnetic measurements reveal that complex 3 has dominant ferromagnetic interactions between low-spin Fe^III (S = 1/2) and high-spin Ni^II (S = 1) ions. UV-visible spectrophotometric measurements and electrochemical studies establish that reversible intra-molecular metal-to-metal electron transfer can be triggered in 3 upon the addition of either an acid or a base.

Charge transfer and slow magnetic relaxation in a series of cyano-bridged FeIII4MII2 (M = FeII, CoII, NiII) molecules

The synthesis, single-crystal structures and magnetic properties of three new cyano-bridged complexes [FeIII4MII2] (M = Fe^II, Co^II, Ni^II) are reported.

Kineticomechanistic Study of the Redox pH Cycling Processes Occurring on a Robust Water-Soluble Cyanido-Bridged Mixed-Valence {CoIII/FeII}2 Square

A kineticomechanistic study of reversible electron-transfer processes undergone by the water-soluble, cyanido-bridged mixed-valence [{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^II(CN)₄}₂]^2- square has been carried out. The oxidation reaction consists of a two-step process with the participation of a solvent-assisted outer-sphere complex, as a result of the establishment of hydrogen bonds that involve the oxo groups of the oxidant (peroxodisulfate) and the terminal cyanido ligands of the tetrametallic square. The formally endergonic reduction reaction of the fully oxidized ([{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^III(CN)₄}₂]) core by water, producing hydrogen peroxide from water even at low pH values, is also a two-step process. Each one of these processes requires a set of two preequilibria involving the association of OH^- and its subsequent deprotonation by a further OH^- anion. The structure of the square compound in its fully protonated form has also been determined by X-ray diffraction and shows the existence of strong hydrogen-bonding interactions, in agreement with the rather high basicity of the terminal cyanido ligands. Likewise, density functional theory calculations on the tetrametallic complex showed zones with negative electrostatic potential around the Fe^II centers of the square that would account for the establishment of the hydrogen bonds found in the solid state. Spectroelectrochemistry experiments demonstrated the singular stability of the {Co^III/Fe^II}₂^2- complex, as well as that of their partially, {Co₂^III/Fe^IIIFe^II}^-, and fully, {Co^III/Fe^III}₂, oxidized counterparts because no hysteresis was observed in these measurements.

Manipulation of successive crystalline transformations to control electron transfer and switchable functions

Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe2Co2} compound were obtained via enhancement of intermolecular π···π interactions inducing successive single-crystal-to-single-crystal transformations, from solvated 1·2CH3OH·4H2O, to desolvated 1 and its polymorph 1a accompanying electron transfer. 1·2CH3OH·4H2O showed thermally induced reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π···π interactions manipulated dynamic crystalline transformation.

Design of 3d–4f molecular squares through the [Fe{(HB(pz)3)}(CN)3]− metalloligand

A new series of cyanido-bridged {FeIII2LnIII2} heterobimetallic molecular squares [Ln = La (1), Gd (2), Tb (3) and Dy (4)] has been prepared and magneto-structurally investigated.

Solvent-induced on/off switching of intramolecular electron transfer in a cyanide-bridged trigonal bipyramidal complex

A cyanide-bridged [Co₃Fe₂] cluster with trigonal bipyramidal geometry shows solvent-driven reversible on/off switching of its thermally induced electron-transfer-coupled spin transition (ETCST) behaviour.

Switchable Fe/Co Prussian blue networks and molecular analogues

Switchable Fe/Co Prussian blue compounds and their low dimensional analogues displaying thermally and photo-induced electron transfer phenomena are reviewed.

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.