Two-Step Thermoinduced Metal-to-Metal Electron Transfer and ON/OFF Photoswitching in a Molecular [Fe2Co2] Square Complex

Reversible light-induced spin state switching in a dinuclear Fe(II) spin crossover complex

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe2L5(NCS)4]·2DMF·2H2O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

Light-Induced, Structural Matrix Guided Stepwise Spin-State Switching in 3d-5d Molecular Assembly

A new iron(II) molecular complex {[W(CN)8][Fe(bik*)3]2}BF4·7H2O·1.5CH3OH (1.7H2O·1.5CH3OH) was synthesized using a versatile octacyanotungstate(V) building block and N-donor bidentate ligand (bik* = bis(1-ethyl-1H-imidazol-2-yl)ketone) and detailed characterizations were carried out. The crystal structure of 1.7H2O·1.5CH3OH is composed of an ionic salt from one anionic [W(CN)8]3- unit, two isolated cationic [Fe(bik*)3]2+ units, and one BF4- counteranion in the asymmetric unit. Magnetic studies of 1.7H2O·1.5CH3OH display interesting two-step reversible thermo-induced spin-state switching and the partially desolvated form 1.7H2O shows a photomagnetic effect at low temperatures. Additionally, the physical properties of 1.7H2O·1.5CH3OH were compared with the monomeric unit of {[Fe(bik*)3]2}·4ReO4·H2O (2.H2O) and detailed photophysical investigations were also performed to study the effect of a structural matrix {[W(CN)8]3- and ReO4- unit} on the spin-state switching properties of the [Fe(bik*)3]2+ unit in both systems (1.7H2O·1.5CH3OH and 2.H2O).

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.

Electrically Detectable Photoinduced Polarization Switching in a Molecular Prussian Blue Analogue

Light, a nondestructive and remotely controllable external stimulus, effectively triggers a variety of electron-transfer phenomena in metal complexes. One prime example includes using light in molecular cyanide-bridged [FeCo] bimetallic Prussian blue analogues, where it switches the system between the electron-transferred metastable state and the system's ground state. If this process is coupled to a ferroelectric-type phase transition, the generation and disappearance of macroscopic polarization, entirely under light control, become possible. In this research, we successfully executed a nonpolar-to-polar phase transition in a trinuclear cyanide-bridged [Fe2Co] complex crystal via directional electron transfer. Intriguingly, by exposing the crystal to the wavelength of light─785 nm─without any electric field─we can drive this ferroelectric phase transition to completely depolarize the crystal, during which a measurable electric current response can be detected. These discoveries signify an important step toward the realization of fully light-controlled ferroelectric memory devices.

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.

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.

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.

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.

Impact of Counteranion on Reversible Spin-State Switching in a Series of Cobalt(II) Complexes Containing a Redox-Active Ethylenedioxythiophene-Based Terpyridine Ligand

The self-assembly of a redox-active ethylenedioxythiophene (EDOT)-terpyridine-based tridentate ligand and cobalt(II) unit with different counteranions has led to a series of new cobalt(II) complexes [Co(L)₂](X)₂ (X = BF₄ (1), ClO₄ (2), and BPh₄ (3)) (L = 4'-(3,4-ethylenedioxythiophene)-2,2':6',2″-terpyridine). The impact of various counteranions on stabilization and spin-state switching of the cobalt(II) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical studies. All three complexes 1-3 consisted of an isostructural dicationic distorted octahedral CoN₆ coordination environment offered by the two L ligands in a bis-meridional fashion and BF₄^-, ClO₄^-, and BPh₄^- as a counteranion, respectively. Complex 2 with ClO₄^- counteranion showed a reversible, gradual, and nearly complete spin-state switching between low-spin (LS) (S = 1/2) and high-spin (HS) (S = 3/2) states, while an incomplete spin-state switching behavior was observed for complexes 1 (BF₄^-) and 3 (BPh₄^-) in the measured temperature range of 350-2 K. The non-covalent cation-anion interactions played a significant role in stabilizing the spin-state in 1-3. Additionally, complexes 1-3 also exhibited interesting redox-stimuli-based reversible paramagnetic HS cobalt(II) (S = 3/2) to diamagnetic LS cobalt(III) (S = 0) conversion, offering an alternate way to switch the magnetic properties.

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.

Effect of Ligand Chain Length for Tuning of Molecular Dimensionality and Magnetic Relaxation in Redox Active Cobalt(II) EDOT Complexes (EDOT = 3,4-Ethylenedioxythiophene)

Four cobalt(II) complexes, [Co(L1)2(NCX)2(MeOH)2] (X = S (1), Se (2)) and {[Co(L2)2(NCX)2]}n (X = S (3), Se (4)) (L1 = 2,5dipyridyl-3,4,-ethylenedioxylthiophene and L2 = 2,5diethynylpyridinyl-3,4-ethylenedioxythiophene), were synthesized by incorporating ethylenedioxythiophene based redox-active luminescence ligands. All these complexes have been well characterized using single-crystal X-ray diffraction analyses, spectroscopic and magnetic investigations. Magneto-structural studies showed that 1 and 2 adopt a mononuclear structure with CoN4O2 octahedral coordination geometry while 3 and 4 have a 2D [4 x 4] rhombic grid coordination networks (CNs) where each cobalt(II) center is in a CoN6 octahedral coordination environment. Static magnetic measurements reveal that all four complexes displayed a high spin (HS) (S = 3/2) state between 2 and 280 K which was further confirmed by X-band and Q-band EPR studies. Remarkably, along with the molecular dimensionality (0D and 2D) the modification in the axial coligands lead to a significant difference in the dynamic magnetic properties of the monomers and CNs at low temperatures. All complexes display slow magnetic relaxation behavior under an external dc magnetic field. For the complexes with NCS- as coligand observed higher energy barrier for spin reversal in comparison to the complexes with NCSe- as coligand, while mononuclear complex 1 exhibited a higher energy barrier than that of CN 3. Theoretical calculations at the DFT and CASSCF level of theory have been performed to get more insight into the electronic structure and magnetic properties of all four complexes.

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.

Heterobimetallic cyanide-bridged FeIII(μ-CN)MII complexes (M = Mn and Cu): synthesis, structure and magnetism

Systematic magneto-structural studies reveal ‘[FeIII(Tp)(CN)2(μ-CN)MnIICl(L1/L2)]′ (both antiferromagnetic) and [FeIII(Tp)(CN)2(μ-CN)CuII(L1/L2)]+′(L1 ferro- and L2 antiferromagnetic).

Out-of-equilibrium dynamics driven by photoinduced charge transfer in CsCoFe Prussian Blue Analogue nanocrystals

In this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced...

Molecular Approach to Alkali-Metal Encapsulation by a Prussian Blue Analogue FeII/CoIII Cube in Aqueous Solution: A Kineticomechanistic Exchange Study

The preparation of a series of alkali-metal inclusion complexes of the molecular cube [{Co^III(Me₃-tacn)}₄{Fe^II(CN)₆}₄]^4- (Me₃-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), a mixed-valent Prussian Blue analogue bearing bridging cyanido ligands, has been achieved by following a redox-triggered self-assembly process. The molecular cubes are extremely robust and soluble in aqueous media ranging from 5 M [H⁺] to 2 M [OH^-]. All the complexes have been characterized by the standard mass spectometry, UV-vis, inductively coupled plasma, multinuclear NMR spectroscopy, and electrochemistry. Furthermore, X-ray diffraction analysis of the sodium and lithium salts has also been achieved, and the inclusion of moieties of the form {M-OH₂}⁺ (M = Li, Na) is confirmed. These inclusion complexes in aqueous solution are rather inert to cation exchange and are characterized by a significant decrease in acidity of the confined water molecule due to hydrogen bonding inside the cubic cage. Exchange of the encapsulated cationic {M-OH₂}⁺ or M⁺ units by other alkali metals has also been studied from a kineticomechanistic perspective at different concentrations, temperatures, ionic strengths, and pressures. In all cases, the thermal and pressure activation parameters obtained agree with a process that is dominated by differences in hydration of the cations entering and exiting the cage, although the size of the portal enabling the exchange also plays a determinant role, thus not allowing the large Cs⁺ cation to enter. All the exchange substitutions studied follow a thermodynamic sequence that relates with the size and polarizing capability of the different alkali cations; even so, the process can be reversed, allowing the entry of {Li-OH₂}⁺ units upon adsorption of the cube on an anion exchange resin and subsequent washing with a Li⁺ solution.

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.

Field-induced single ion magnet behaviour of discrete and one-dimensional complexes containing [bis(1-methylimidazol-2-yl)ketone]-cobalt(II) building units

We describe herein the first examples of six-coordinate Co^II single-ion magnets (SIMs) based on the β-diimine ^Mebik ligand [^Mebik = bis(1-methylimidazol-2-yl)ketone]: two mononuclear [Co^II(^Rbik)₂L₂] complexes and one mixed-valence {CoIII2Co^II}_n chain of formulas [Co^II(^Mebik)(H₂O)(dmso)(μ-NC)₂CoIII2(μ-2,5-dpp)(CN)₆]_n·1.4nH₂O (3) [L = NCS (1), NCSe (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine (3)]. Two bidentate ^Mebik molecules plus two monodentate N-coordinated pseudohalide groups in cis positions build somewhat distorted octahedral surroundings around the high-spin cobalt(II) ions in 1 and 2. The diamagnetic [CoIII2(μ-2,5-dpp)(CN)₈]^2- metalloligand coordinates the paramagnetic [Co^II(^Mebik)(H₂O)(dmso)]²⁺ complex cations in a bis-monodentate fashion to afford neutral zigzag heterobimetallic chains in 3. Ab initio calculations, and cryomagnetic dc (2.0-300 K) and ac (2.0-12 K) measurements as well as EPR spectroscopy for 1-3 show the existence of magnetically isolated high-spin cobalt(II) ions with D values of 59.84-89.90 (1), 66.32-93.90 (2) and 70.40-127.20 cm^-1 (3) and field-induced slow relaxation of the magnetization, being thus new examples of SIMs with transversal magnetic anisotropy. The analysis of their relaxation dynamics reveals that the relaxation of the magnetization occurs by the Raman (with values of the n parameter covering the range 6.0-6.8) and direct spin-phonon processes.

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.

Spin-crossover iron(ii) long-chain complex with slow spin equilibrium at low temperatures

A mononuclear complex with long alkyl chains, [Fe^II(H₂Bpz₂)₂(C₉bpy)] (1; H₂Bpz₂ = dihydrobis(1-pyrazolyl)borate, C₉bpy = 4,4'-dinonyl-2,2'-bipyridine), was synthesized. Single-crystal X-ray crystallographic studies revealed that the Δ- and Λ-forms of the complex co-crystallized in the lattice asymmetric unit, while magnetic measurements unveiled that this complex underwent incomplete one-step spin crossover (SCO) with the transition completeness and temperature depending on the measurement velocity because of slow spin equilibrium. Multivariable approaches such as varying scan rate, annealing the sample, light irradiation and pressure have been adopted to effectively overcome the slow spin equilibrium and thus improve the SCO completeness.

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

Halogen-substituted Fe(III) compounds, [Fe(Hphsalpm^X)₂]PTFB (Hphsalpm^X = 5-X-(R,S)-((phenyl(2-pyridyl)methylimino)methyl)phenol, PTFB = phenyltrifluoroborate; X = F for 1, Cl for 2, Br for 3) and [Fe(Hphsalpm^X)₂]PTFB·MeOH (X = I for 4·MeOH), were synthesized. Compounds 1, 4·MeOH, and its desolvated form 4 exhibited an invariant high-spin state in the whole temperature range, while 2 and 3 underwent gradual, nonhysteretic, and incomplete spin crossover (SCO) with transition temperatures (T_C) of 153 and 220 K, respectively. Interestingly, the SCO-active compounds 2 and 3 showed light-induced excited spin-state trapping (LIESST) effects at 10 K, and light-induced reversible ON/OFF switching behaviors were realized by alternately using 880 and 1064 nm light, while the thermally inert compound 4·MeOH unexpectedly showed a reverse-LIESST effect. These results may help to design and synthesize new photoresponsive SCO Fe(III) compounds for the development of switchable materials.

Effect of Ligand Modulation on Metal-to-Metal Electron Transfer in a Series of [Fe2Co2] Molecular Square Complexes

A series of three new cyanide-bridged [FeCo] molecular square complexes, namely, {[Fe(Tp*)(CN)₃]₂[Co(L)₂]₂}(BF₄)₂·2DMF (L = bik (1), bik* (2), and vbik (3); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bik = bis(1-methyl-1H-imidazol-2-yl)ketone, bik* = bis(1-ethyl-1H-imidazol-2-yl)ketone, and vbik = bis(1-vinyl-1H-imidazol-2-yl)ketone; DMF = dimethylformamide) were synthesized and characterized by single-crystal X-ray diffraction analyses and by magnetic, electrochemical, and spectroscopic measurements. Magnetic studies reveal that all three complexes exhibit temperature-induced metal-to-metal electron transfer (MMET) from a high-spin Co(II) center to a low-spin Fe(III) center, transforming a high-temperature paramagnetic {Fe^III_LS-CN-Co^II_HS} ground state into a low-temperature diamagnetic {Fe^II_LS-CN-Co^III_LS} state with a decrease in the temperature from 300 to 100 K. Complexes 1 and 3 show the interconversion of the paramagnetic {Fe^III_LS-CN-Co^II_HS} ground state into a diamagnetic {Fe^II_LS-CN-Co^III_LS} state in a single-step transition with T_1/2 values of 180 and 186 K, respectively, while a two-step MMET with T_1/2 value of 214 and 178 K was observed for complex 2.

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.

Reversible Photo- and Thermo-Induced Spin-State Switching in a Heterometallic {5d-3d} W2Fe2 Molecular Square Complex

Following the complex-as-a-ligand strategy, self-assembly of [W(CN)₈]^3- and iron(II) with bidentate nitrogen donor ligand bik (bik = bis(1-methyl-1H-imidazol-2-yl)ketone) ligand affords a cyanide-bridged [W₂Fe₂] molecular square complex [HNBu₃]₂{[W(CN)₈]₂[Fe(bik)₂]₂}·6H₂O·CH₃OH (1). The complex was characterized by single-crystal X-ray diffraction analyses, (photo)magnetic studies, optical reflectivity, electrochemical studies, and spectroscopic studies. Structural analyses revealed that in the [W₂Fe₂] square motif tungsten(V) and iron(II) centers reside in an alternate corner of the square and are bridged by the cyanide ligands. Complex 1 exhibits thermo-induced spin crossover (SCO) between {W^V (S = 1/2) - Fe^II_LS (S = 0)} and {W^V (S = 1/2) - Fe^II_HS (S = 2)} pairs near room temperature and photoinduced spin-state switching with T_LIESST = 70 K under light irradiation at low temperature. To the best of our knowledge, 1 represents the first complex containing iron(II) and [W^V(CN)₈]^3- units exhibiting both SCO and photomagnetic effect.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

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.

Cyclic Heterometallic Interactions formed from a Flexible Tripeptide Complex Showing Effective Antiferromagnetic Spin Coupling

Developing tunable motifs for heterometallic interactions should be beneficial for fabricating functional materials based on cooperative electronic communications between metal centers. Reported here is the efficient formation of cyclic heterometallic interactions from a complex containing an artificial tripeptide with metal binding sites on its main chain and side chains. X-ray structural analysis and X-ray absorption spectroscopy revealed that the cyclic metal-metal arrangements arise from the amide groups connecting four square-planar Cu^II centers and four octahedral Ni^II centers in a cyclic manner. UV/Vis spectral studies suggested that this efficient formation was achieved by the selective formation of the square-planar Cu^II centers and a crystallization process. Magnetic measurements using SQUID clarified that the cyclic complex represented the S=2 spin state at low temperatures due to effective antiferromagnetic interactions between the Ni^II and Cu^II centers.

Cyanido-bridged {FeIIILnIII} heterobimetallic chains assembled through the [FeIII{HB(pz)3}(CN)3]− complex as metalloligand: synthesis, crystal structure and magnetic properties

The first isostructural series of cyanido-bridged 3d–4f chains showing a unique zig-zag motif have been obtained by using the heteroleptic [Fe^III{HB(pz)₃}(CN)₃]⁻ complex as a metalloligand towards the preformed [Ln^III(bpdo)(NO₃)₂(H₂O)]⁺ species.

Time-Resolved Spectroscopy of Photoinduced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues

The dynamics of the photodriven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) are revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature, the known tetranuclear square-type complex [Co₂Fe₂(CN)₆(tp*)₂(4,4'-dtbbpy)₄](PF₆)₂ (1) exists in two electronic states. In acetonitrile solution, at <240 K, the low temperature (LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [Fe^II_LSCo^III_LS]₂. Temperature rise is the reason behind thermally-induced CTIST toward the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [Fe^III_LSCo^II_HS]₂, being prevalent at >300 K. Photoexcitation into the intervalence charge transfer (IVCT) band of the LT phase at 800 nm induces electron transfer in one Fe-Co edge of PBA 1 and produces a [Fe^III_LSCo^II_LS] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [Fe^III_LSCo^II_HS] species. In contrast, IVCT excitation of the HT phase at 400 nm generates a [Fe^II_LSCo^III_HS] species with a lifetime of 3.6 ps. Subsequent back-electron transfer populates the vibrationally hot ground state, which thermalizes within 8 ps. The newly synthesized dinuclear PBA, [CoFe(CN)₃(tp*)(pz*₄Lut)]ClO₄ (2), provides a benchmark of the HT phase of 1, i.e., [Fe^III_LSCo^II_HS], as verified by variable temperature magnetic susceptibility measurements and ⁵⁷Fe Mössbauer spectroscopy. The photoinduced charge transfer dynamics of PBA 2 indeed are almost identical to that of the HT phase of PBA 1 with a lifetime of the excited [Fe^II_LSCo^III_HS] species of 3.8 ps.

Reversible Spin-State Switching and Tuning of Nuclearity and Dimensionality via Nonlinear Pseudohalides in Cobalt(II) Complexes

The self-assembly of a macrocyclic tetradentate ligand, cobalt(II) tetrafluoroborate, and nonlinear pseudohalides (dicyanamide and tricyanomethanide) has led to two cobalt(II) complexes, {[Co(L)(μ_1,5-dca)](BF₄)·MeOH}_n (1) and [Co₂(L)₂(μ_1,5-tcm)₂](BF₄)₂ (2) (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane; dca^- = dicyanamido; tcm^- = tricyanomethanido). Both complexes were characterized by single-crystal X-ray diffraction, spectroscopic, magnetic, and electrochemical studies. Structural analyses revealed that 1 displays a one-dimensional (1D) coordination polymer containing [Co(L)]²⁺ repeating units bridged by μ_1,5-dicyanamido groups in cis positions, while 2 represents a discreate dinuclear cobalt(II) molecule bridged by two μ_1,5-tricyanomethanido groups in a cis conformation. Both complexes have a CoN₆ coordination environment around each cobalt center offered by the tetradentate ligand and cis coordinating bridging ligands. Complex 1 exhibits a high-spin (S = 3/2) state of cobalt(II) in the temperature range of 2-300 K with a weak ferromagnetic coupling between two dicyanamido-bridged cobalt(II) centers. Interestingly, complex 2 exhibits reversible spin-state switching associated with spin-spin coupling. Complexes 1 and 2 also exhibit interesting redox-stimuli-based reversible paramagnetic high-spin cobalt(II) to diamagnetic low-spin cobalt(III) conversion, offering an additional way to switch magnetic properties. A detailed theoretical calculation was consistent with the stated results.

ON/OFF Photoswitching and Thermoinduced Spin Crossover with Cooperative Luminescence in a 2D Iron(II) Coordination Polymer

A 2D coordination polymer, {[Fe(L)₂(NCSe)₂]·6MeOH·14H₂O}_n (1; L = 2,5-dipyridylethynylene-3,4-ethylenedioxythiophene), has been synthesized based on a redox active luminescence ligand. 1 possesses a 2D [4 × 4] square-grid network where the iron(II) center is in a FeN₆ octahedral coordination environment. 1 displays reversible thermoinduced high-spin (HS; S = 2) to diamagnetic low-spin (LS; S = 0) ON/OFF spin-state switching with a T_1/2 value of 150 K. Interestingly, optical reflectivity and photomagnetic studies at 10 K under light irradiation revealed an efficient conversion to a photoinduced metastable HS excited state from a LS ground state. Remarkably, the photoexcited HS state can be reversibly switched ON and OFF by using 625 and 850 nm light-emitting-diode lights. Intriguingly, the thermal dependence of the luminescence intensity of the maximum emission at 524 nm for 1 shows a minimum at around the spin-crossover (SCO) temperature, indicating a cooperative nature between the SCO and luminescence properties. Theoretical calculations confirmed the above findings.