Pressure‐Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis

Thermal and protonation-induced electron transfer coupled spin transition in a discrete [Fe2Co3] Prussian blue analogue

Multi-stimuli responsive switchable molecules have garnered interest for their potential applications in the field of magnetic materials. However, the persistent challenge lies in isolating these properties within the same material. Herein, we report a discrete [Fe2Co3] pentanuclear cyanide bridged complex, [Co(L)2]3[Fe(CN)6]2·20H2O, (1) (L = (Z)-N'-(4-(trifluoromethyl)phenyl)picolinimidamide) which undergoes electron transfer coupled spin transition (ETCST) from [FeII2CoIIHSCoIII2] to [FeIII2CoII3] configurations through thermal activation and upon protonation.

3 nm-wide Cyanometallate Fe-Co Tape Exhibiting Single-Chain Magnet Behavior

Treatment of Co(OTf)2·6H2O, Li[(pzTp)FeIII(CN)3], and H3PMo12O40·nH2O in protic solvents afforded two structurally related Fe-Co cyanometallate complexes: [{(pzTp)Fe(CN)3}3Co3(MeOH)10][PMo12O40]·H2O·11MeOH (1, pzTp- = tetra(pyrazolyl)borate) and {[(pzTp)Fe(CN)3]4Co3(MeOH)5(H2O)3}n[HPMo12O40]n·3 nMeOH·6.5nH2O (2). Complex 1 consists of a cyanide-bridged hexanuclear [Fe3Co3] cage, characterized by the fused conjunction of two mutually perpendicular trigonal bipyramids (TBPs, [Fe2Co3] and [Co2Fe3]), while complex 2 showcases an intricate cyanide-bridged Fe-Co tape comprising a central chain backbone of vertex-sharing [Fe2Co3] TBPs alongside peripheral [Fe2Co2] squares. Complex 2 is among the widest one-dimensional coordination assemblies characterized by the single-crystal X-ray diffraction technique. Magnetic studies revealed that complex 2 behaved as a single chain magnet with an effective energy barrier (Ueff/kB) of 46.8 K. Our findings highlight the possibilities in the development of cyanometallate-POM hybrid materials with captivating magnetic properties.

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.

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.

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.

Molecular Magnetic Materials Based on {Co III (Tp*)(CN) 3 } − Cyanidometallate: Combined Magnetic, Structural and 59 Co NMR Study

The cyanidocobaltate of formula fac‐PPh₄[Co^III(^Me2Tp)(CN)₃] ⋅ CH₃CN (1) has been used as a metalloligand to prepare polynuclear magnetic complexes (^Me2Tp=hydrotris(3,5‐dimethylpyrazol‐1‐yl)borate). The association of 1 with in situ prepared [Fe^II(bik)₂(MeCN)₂](OTf)₂ (bik=bis(1‐methylimidazol‐2‐yl)ketone) leads to a molecular square of formula {[Co^III{(^Me2Tp)}(CN)₃]₂[Fe^II(bik)₂]₂}(OTf)₂ ⋅ 4MeCN ⋅ 2H₂O (2), whereas the self‐assembly of 1 with preformed cluster [Co^II₂(OH₂)(piv)₄(Hpiv)₄] in MeCN leads to the two‐dimensional network of formula {[Co^II₂(piv)₃]₂[Co^III(^Me2Tp)(CN)₃]₂ ⋅ 2CH₃CN}_∞ (3). These compounds were structurally characterized via single crystal X‐ray analysis and their spectroscopic (FTIR, UV‐Vis and ⁵⁹Co NMR) properties and magnetic behaviours were also investigated. Bulk magnetic susceptibility measurements reveal that 1 is diamagnetic and 3 is paramagnetic throughout the explored temperature range, whereas 2 exhibits sharp spin transition centered at ca. 292 K. Compound 2 also exhibits photomagnetic effects at low temperature, selective light irradiations allowing to promote reversibly and repeatedly low‐spin⇔high‐spin conversion. Besides, the diamagnetic nature of the Co(III) building block allows us studying these compounds by means of ⁵⁹Co NMR spectroscopy. Herein, a ⁵⁹Co chemical shift has been used as a magnetic probe to corroborate experimental magnetic data obtained from bulk magnetic susceptibility measurements. An influence of the magnetic state of the neighbouring atoms is observed on the ⁵⁹Co NMR signals. Moreover, for the very first time, ⁵⁹Co NMR technique has been successfully introduced to investigate molecular materials with distinct magnetic properties.

Pressure-induced multi-step and self-organized spin states in an electro-elastic model for spin-crossover solids

Spin transition materials are known to exhibit a rich variety of behaviors under several stimuli, among which pressure leads to major changes in their electronic and elastic properties. From an experimental point of view, thermal spin transitions under isotropic pressure showed transformations from (i) hysteretic to continuous transformations where the hysteresis width vanishes beyond some threshold pressure value; this is the conventional case. In several other cases very pathological and unexpected behaviours emerged, like (ii) persistent hysteresis under pressure; (iii) non-uniform behavior of the thermal hysteresis width which first increases with pressure and then decreases and vanishes at higher pressures; (iv) furthermore, double step transitions induced by pressure are also often obtained, where the pressure triggers the appearance of a plateau during the thermal transition, leading to two-step transitions, and finally (v) other non-conventional re-entrant transitions, where the thermal hysteresis vanishes at some pressure and then reappears at higher pressure values are also observed. In the present theoretical study, we investigate this problem with an electro-elastic description of the spin-crossover phenomenon by solving the Hamiltonian using a Monte Carlo technique. The pressure effect is here introduced directly in the lattice parameters, the elastic constants and ligand field energy. By considering spin state-dependent compressibility, we demonstrate that a large panel of experimental observations can be qualitatively described with this model. Among them, we quote (i) the conventional pressure effect decreasing the hysteresis width, (ii) the unconventional cases with pressure causing a non-monotonous behavior of the hysteresis width, (iii) re-entrant, as well as (iv) double step transitions accompanied with various types of spin state self-organization in the plateau regions.

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...

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.

Downsizing of Nanocrystals While Retaining Bistable Spin Crossover Properties in Three-Dimensional Hofmann-Type {Fe(pz)[Pt(CN)4]}-Iodine Adducts

Mastering nanostructuration of functional materials into electronic devices is presently an essential task in materials science. This is particularly relevant for spin crossover (SCO) compounds, whose properties are extremely sensitive to size reduction. Indeed, the search for materials displaying strong cooperative hysteretic SCO properties operative at the nanoscale close near room temperature is extremely challenging. In this context, we describe here the synthesis and characterization of 20-30 nm surfactant-free nanocrystals of the Fe^II Hofmann-type polymer {Fe^II(pz)[Pt^II,IVI_x(CN)₄]} (pz = pyrazine), which affords the first example of a robust three-dimensional coordination polymer, substantially keeping operational thermally induced SCO bistability at such a scale.

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.

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.

Photoswitchable Nonlinear-Optical Crystal Based on a Dysprosium-Iron Nitrosyl Metal Assembly

Nitrosyl metal complexes (M-NO), in which nitrosyl ligands are coordinated to transition-metal ions, have been studied from the viewpoints of physiological activity, catalytic activity, and photosensitivity. The structural flexibility and electric polarization of the nitrosyl ligand are attractive characteristics. Herein we show a photoswitchable nonlinear-optical (NLO) crystal based on a dysprosium-iron nitrosyl assembly. This crystal is composed of a one-dimensional chain structure in the polar Pna2₁ space group. Because of spontaneous electric polarization, it exhibits a NLO effect of second harmonic generation (SHG). The SHG signal reversibly changes by alternate irradiation with 473 and 804 nm laser lights. The observed photoreversible switching effect on SHG is caused by photoinduced linkage isomerization of the metal nitrosyl sites, i.e., M-N⁺═O ↔ M-O═N⁺. Such an optically switchable NLO crystal should be useful for optical devices such as optical filters and optical shutters as well as probes in SHG microscopy.

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.