Octacyanidotungstate(IV) Coordination Chains Demonstrate a Light-Induced Excited Spin State Trapping Behavior and Magnetic Exchange Photoswitching

Asymmetric Coordination Toward a Photoinduced Single-Chain Magnet Showing High Coercivity Values

The production of photo-switchable molecular nanomagnets with substantial coercivity, which is indispensable for information storage and process applications, is challenging. Introducing photo-responsive spin-crossover units provides a feasible means of controlling the magnetic anisotropy, interactions, and overall nanomagnet properties. Herein, we report a cyanide-bridged chain 1⋅12H₂ O ({[(^Pz Tp)Fe^III (CN)₃ ]₂ Fe^II (Pmat)₂ }_n ⋅12 H₂ O) generated by linking the Fe^II -based spin-crossover unit with the [(^Pz Tp)Fe(CN)₃ ]^- (^Pz Tp: tetrakis(pyrazolyl)borate) building block in the presence of asymmetric ditopic ligand Pmat ((4-pyridine-4-yl)methyleneamino-1,2,4-triazole). Structural characterization revealed that the introduction of this asymmetric ligand led to a distorted coordination environment of Fe^II ions, which were equatorially coordinated by four cyanide N atoms, and apically coordinated by one pyridine N atom and one triazole N atom. Upon 808-nm light irradiation, 1⋅12H₂ O underwent photoinduced spin-crossover and exhibited single-chain magnet behavior with a coercive field of up to 1.3 T. This represents a 3d-based photoinduced single-chain magnet exhibiting pronounced hysteresis.

Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids

Octacyanometalates K₄[Mo(CN)₈] and K₄[W(CN)₈] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)₈]⁴⁺ [SbF₆]⁻₄ and [W(CNH)₈]⁴⁺ [SbF₆]⁻₄ are the first examples of eight‐coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen‐bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low‐temperature NMR measurements in HF confirmed rapid proton exchange even at −40 °C. Upon protonation, ν(C≡N) increases of about 50 cm⁻¹ which is in agreement with DFT calculations.

Photoinduced Mo-CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping

The photoinduced properties of the octacoordinated complex K₄ Mo^IV (CN)₈ ⋅2 H₂ O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)₇ species originating from the light-induced cleavage of one Mo-CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]₃ [Mo^IV (CN)₇ ]⋅3 CH₃ CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.

Humidity-A Powerful Tool to Customize the Physical Properties of Molecular Magnets

Since the dawn of the century, people have been fascinated by the mysterious force that draws two pieces of lodestone (magnetite, Fe₃ O₄ ) to each other as well as by property of stilbite (natural zeolite, NaCa₄ [Si₂₇ Al₉ O₇₂ ]⋅28 H₂ O) to generate huge amounts of steam by rapidly heating the material. Nowadays, we know the first effect is addressed by magnetic attraction whereas the second one is related to the reversible sorption/desorption of water from humid air inside porous materials. For a long time, it was thought that these two, at first glance, were disjunctive properties that cannot be combined into one material. Nevertheless, the scientists have once again proved that there are no impossible things. Based on the discovery of the first molecular magnets, the idea to combine magnetic properties with other functionalities such as porosity gained great consideration by scientists from different research fields. Very soon, we witnessed numerous reports of novel multifunctional materials among which we can distinguish humidity-responsive magnets. In this manuscript, the most outstanding results for such systems working at normal temperature and pressure (NTP) will be presented to motivate in-depth research on this topic.

Humidity-Induced Switching between Two Magnetic and Structural Phases in a CoII -[WV (CN)8 ] Molecular Magnet

The self-assembly of cobalt(II) with purine and octacyanidotungstate(V) results in the formation of the three-dimensional Co₃ [W(CN)₈ ]₂ (purine)₂ ⋅8.5H₂ O (1) coordination polymer. This compound exhibits humidity-induced variation of the number of water molecules of crystallisation leading to a reversible structural phase transition and the alternation of the long-range ferromagnetic ordering temperature from T_C =29 K for the pristine assembly (1) to T_C =49 K for the sample stored in a low-humidity atmosphere (1-deh). This phenomenon can be attributed to a reversible change in the hydrogen-bonding network resulting in the modification of the local geometries of cobalt(II) as well as the cyanido bridges.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different Fe^II complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled Fe^II-[M^IV(CN)₈]^4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {Fe^II₂(3-OAcpy)₅(3-OHpy)₃[M^IV(CN)₈]}· nH₂O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe₃M₂} _n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)₄(μ-NC)₂], [Fe2(3-OHpy)₄(μ-NC)₂], and [Fe3(3-OAcpy)₃(3-OHpy)(μ-NC)₂], which lead to the thermal two-step Fe^II SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and ⁵⁷Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep Fe^II SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

Reversible on-off switching of both spin crossover and single-molecule magnet behaviours via a crystal-to-crystal transformation

The on-off switching of spin-crossover (SCO) and single-molecule magnetism (SMM) remains highly attractive, especially if it involves dynamic crystal-to-crystal transformation. Herein we report the first molecule, a mononuclear cobalt(ii) complex, that exhibits on-off switching between SCO and SMM reversibly during crystal-to-crystal transformation. Subtle structural transformation triggered by a simple dehydration-rehydration process induces significant geometrical changes of the CoII center and modification of the supramolecular interactions and switches its colour and magnetic properties (dark red/SCO-on/SMM-off ↔ orange/SCO-off/SMM-on). This work suggests that modification of the weak supramolecular interactions could be very effective in achieving switchable materials involving both SCO and SMM properties.