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We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy)2[Ag(CN)2]2} 1 and {Fe(3acpy)[Ag(CN)2]2} 2 (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in 1 made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties. Changing the acetyl group to a 3-position, a completely different structure is obtained for 2. The unexpected coordination of the carbonyl group to the Fe(II) centers induces a three-dimensional (3D) structure, leading to statistical disorder around the Fe(II) with three different coordination spheres, [N6], [N4O2], and [N5O]. This disorder gives rise to an incomplete thermally induced SCO with a poor photoswitchability. These results demonstrate that the choice of the acetyl position on the pyridine dictates the structural characteristics of the compounds with a direct impact on the SCO behavior. Remarkably, this work opens interesting perspectives for the future design of Fe-Ag cyanido coordination polymers with judiciously substituted pyridine ligands to tune the thermally and photoinduced SCO properties.
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The attractive electronic properties of metal-pyrazine materialsâelectrical conductivity, magnetic order, and strong magnetic couplingâcan be tuned in a wide range depending on the metal employed, as well as its ligand-imposed redox environment. Using solvent-directed synthesis to control the dimensionality of such systems, a discrete tetranuclear chromium(III) complex, exhibiting a rare example of bridging radical pyrazine, has been prepared from chromium(II) triflate and neutral pyrazine. The strong antiferromagnetic interaction between CrIII (S = 3/2) and radical pyrazine (S = 1/2) spins, theoretically estimated at about -932 K, leads to a thermally isolated ST = 4 ground state, which remains the only populated state observable even at room temperature.
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In 2019, our groups described a unique FeII complex, [Fe(2MeL)(NCBH3)2] (2MeL = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine) possessing a low-spin ground state that is not easily accessible due to the extremely slow dynamics of the high-spin to low-spin phase transition. Herein, we report the successful chemical manipulation of this spin-crossover (SCO) process through controlled metal-ion dilutions. The emergence or suppression of the thermally induced SCO behavior was observed depending on the radius of the metal ion used for the dilution (NiII or ZnII). Reversible photo-switching has been confirmed in all mixed-metal complexes whether the low-spin state is thermally accessible. Remarkably, the dilution with ZnII metal ions stabilizes HS FeII complexes with complete suppression of the thermally induced SCO process without destroying the reversible photoswitchability of the material.
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The switching properties of a cyanido-bridged Fe/Co square molecule were investigated by single-crystal X-ray diffraction and X-ray absorption spectroscopy at both Fe and Co K-edges. Combining these two techniques, a complete picture of the thermal-, light- and X-ray-induced metal-to-metal electron transfer is obtained, illustrating the concerted role played by the Fe and Co sites.
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Electronic synergy between metal ions and organic linkers is a key to engineering molecule-based materials with a high electrical conductivity and, ultimately, metallicity. To enhance conductivity in metal-organic solids, chemists aim to bring the electrochemical potentials of the constituent metal ions and bridging organic ligands closer in a quest to obtain metal-d and ligand-π admixed frontier bands. Herein, we demonstrate the critical role of the metal ion in tuning the electronic ground state of such materials. While VCl2(pyrazine)2 is an electrical insulator, TiCl2(pyrazine)2 displays the highest room-temperature electronic conductivity (5.3 S cm-1) for any metal-organic solid involving octahedrally coordinated metal ions. Notably, TiCl2(pyrazine)2 exhibits Pauli paramagnetism consistent with the specific heat, supporting the existence of a Fermi liquid state (i.e., a correlated metal). This result widens perspectives for designing molecule-based systems with strong metal-ligand covalency and electronic correlations.
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We describe herein the self-assembly synthesis of an octanuclear CoII [2]catenane {[Co4 (H2 L)6 ]2 16+ } formed by the mechanical interlocking of two {[Co4 (H2 L)6 ]8+ } rectangles of unprecedented topology. Subtle manipulation of the synthetic conditions allows the isolation of a mixed-valence [Co2 III /Co2 II ]10+ non-catenated rectangle. The CoII centers in the [2]catenane exhibit slow relaxation of their magnetic moment, i. e. single-molecule magnet properties, dominated by quantum tunneling and Raman relaxation processes. This work shows that metallo-supramolecular chemistry can precisely control the organization of single-molecule magnets in topologically complex arrangements.
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Cocrystallization of 7,7',8,8'-tetracyanoquinodimethane radical anion (TCNQ-â¢) and 3-methylpyridinium-1,2,3,5-dithiadiazolyl radical cation (3-MepyDTDA+â¢) afforded isostructural acetonitrile (MeCN) or propionitrile (EtCN) solvates containing cofacial π dimers of homologous components. Loss of lattice solvent from the diamagnetic solvates above 366 K affords a high-temperature paramagnetic phase containing discrete TCNQ-⢠and weakly bound π dimers of 3-MepyDTDA+â¢, as evidenced by X-ray diffraction methods and magnetic susceptibility measurements. Below 268 K, a first-order phase transition occurs, leading to a low-temperature diamagnetic phase with TCNQ-⢠σ dimer and π dimers of 3-MepyDTDA+â¢. This study reveals the first example of cooperative interactions between two different organic radical ions leading to magnetic bistability, and these results are central to the future design of multicomponent functional molecular materials.
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Magnets derived from inorganic materials (e.g., oxides, rare-earth-based, and intermetallic compounds) are key components of modern technological applications. Despite considerable success in a broad range of applications, these inorganic magnets suffer several drawbacks, including energetically expensive fabrication, limited availability of certain constituent elements, high density, and poor scope for chemical tunability. A promising design strategy for next-generation magnets relies on the versatile coordination chemistry of abundant metal ions and inexpensive organic ligands. Following this approach, we report the general, simple, and efficient synthesis of lightweight, molecule-based magnets by postsynthetic reduction of preassembled coordination networks that incorporate chromium metal ions and pyrazine building blocks. The resulting metal-organic ferrimagnets feature critical temperatures up to 242°C and a 7500-oersted room-temperature coercivity.
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The photoinduced properties of the octacoordinated complex K4 MoIV (CN)8 â 2 H2 O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10â 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 70â 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 [MoIV (CN)7 ]â 3 CH3 CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.
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A mononuclear FeII complex that shows a high-spin (S=2) paramagnetic behavior at all temperatures (with standard temperature-scan rates, ≈1â K min-1 ) has, in fact, a low-spin (S=0) ground state below 100â K. This low-spin state is not easily accessible due to the extremely slow dynamics of the spin-crossover process-a full relaxation from the metastable high-spin state to the low-spin ground state takes more than 5â h below 80â K. Bidirectional photo-switching of the FeII state is achieved reproducibly by two selective irradiations (at 530-590 and 830-850â nm). The slow dynamics of the spin-crossover and the strong structural cooperativity result in a remarkably wide 95-K hysteresis loop induced by both temperature and selected light stimuli.
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The synthesis and physico-chemical characterization of an FeII complex [Fe(L1)3 ](ClO4 )2 â CH3 CNâ 0.5H2 O, 1, incorporating a bidentate imidazolylimine-based ligand are reported. Complex 1 crystallises as the mer-isomer and the crystal lattice is replete with hydrogen bonding interactions between ClO4 - anions, solvent molecules and imidazole N-H groups. Variable-temperature structural, magnetic, photomagnetic and optical reflectivity techniques have been deployed to fully characterise the spin-crossover (SCO) behaviour in 1 along with its desolvated phase, 1â desolv. Variable-temperature (1.8-300â K) magnetic-susceptibility measurements reveal a broad two-step full SCO for 1 (T1/2 =158 and 184â K) and photomagnetic experiments at 10â K under white-light irradiation revealed complete photo-induced SCO. 1â desolv displays considerably different magnetic behaviour with sharp single-step SCO accompanied by a thermal hysteresis (T1/2↑ =105â K, T1/2↓ =95â K) in addition to full photo-induced SCO at lower temperatures.
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Molecular complexes based on Prussian Blue analogues have recently attracted considerable interest for their unique bistable properties combined to ultimately reduced dimensions. Here, we investigate the first dinuclear FeCo complex exhibiting both thermal and photomagnetic bistability in the solid state. Through an experimental and theoretical approach combining local techniques-X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and ligand field multiplet calculations-we were able to evidence the changes occurring at the atomic scale in the electronic and magnetic properties. The spectroscopic studies were able to fully support at the atomic level the following conclusions: (i) the 300 K phase and the light-induced excited state at 4 K are both built from FeLSIII-CoHSII paramagnetic pairs with no apparent reorganization of the local structure, (ii) the 100 K phase is composed of FeLSII-CoLSIII diamagnetic pairs, and (iii) the light-induced excited state is fully relaxed at an average temperature of ≈50 K. In the paramagnetic phase at 2 K, XAS and XMCD reveal that both Fe and Co ions exhibit a rather large orbital magnetic moment (0.65 µB and 0.46 µB, respectively, under an external magnetic induction of 6.5 T), but it was not possible to detect a magnetic interaction between spin centers above 2 K.
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The dinuclear mesocate [Fe2L3](BF4)4, 1, is a supramolecular building block for a microporous material. Structural analysis reveals that extensive noncovalent interactions in the solid state generate a 3D framework with microporous channels. These channels are permanently accessible to incoming guest molecules and adsorption isotherms demonstrate that the material has a high selectivity for CO2 over N2.
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We report a new bimetallic cyanido-bridged trinuclear complex [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1) (enpnen = N,N'-bis(2-aminoethyl)-1,3-propanediamine) that shows reversible photomagnetic effect. The photo-induced increase of magnetization is characterized by the irradiation temperature-dependent shapes of the χM T( T) plots and different magnetization values at low temperature in high magnetic field, suggesting multiple photoexcited states. The photomagnetic effect in 1 is explained through two possible processes simultaneously: the light-induced metal-to-metal charge transfer (MMCT) in the CuII-NC-MoIV pair and the light-induced excited spin-state trapping (LIESST) effect in MoIV center. A numerical model assuming the simultaneous existence of three possible states after irradiation: the MMCT CuI-NC-MoV-CN-CuII state, the LIESST CuII-NC-MoIVHS-CN-CuII state, and the ground-state CuII-NC-MoIVLS-CN-CuII was applied to the data and resulted in Cu-Mo exchange coupling constants J1MMCT = 11 cm-1 and J2LIESST = 109 cm-1 for the MMCT and LIESST mechanisms induced states, respectively. Fractions of respective states after irradiations at different temperatures were also calculated, shedding light on the influence of irradiation temperature on the photomagnetic mechanism. The proposed model can provide the interpretative framework for testing and refinement of the mechanism of photomagnetic effect in other coordination networks with cyanido-bridged Cu-[Mo(CN)8]4- linkages.
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Dinuclear triple-helicate complexes of the formula [Fe2L3](BF4)4·solv (solv = CH3CN, CHCl3, H2O) have been synthesised and structurally characterised. The bis-bidentate ligands, L, present either strong-field 2-pyridylimine (1) or weaker-field 2-imidazolylimine (2) and 4-imidazolylimine (3) coordination spheres about Fe(ii) centres in an octahedral geometry. Whereas 1 is pervasively diamagnetic, spin crossover (SCO) behaviour is observed in 2 and 3 and has been studied using variable-temperature structural, UV-visible spectroscopic, magnetic and photo-magnetic techniques. Variable-temperature (1.8-400 K) magnetic-susceptibility measurements reveal the T1/2 values of 2 and 3 to be strongly dependent upon the solvent and degree of solvation. Photomagnetic studies at 10 K under white-light irradiation revealed an inefficient photo-induced SCO in 2, but full switching in 3.
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Incorporating functional molecules into sensor devices is an emerging area in molecular electronics that aims at exploiting the sensitivity of different molecules to their environment and turning it into an electrical signal. Among the emergent and integrated sensors, microelectromechanical systems (MEMS) are promising for their extreme sensitivity to mechanical events. However, to bring new functions to these devices, the functionalization of their surface with molecules is required. Herein, we present original electronic devices made of an organic microelectromechanical resonator functionalized with switchable magnetic molecules. The change of their mechanical properties and geometry induced by the switching of their magnetic state at a molecular level alters the device's dynamical behavior, resulting in a change of the resonance frequency. We demonstrate that these devices can be operated to sense light or thermal excitation. Moreover, thanks to the collective interaction of the switchable molecules, the device behaves as a non-volatile memory. Our results open up broad prospects of new flexible photo- and thermo-active hybrid devices for molecule-based data storage and sensors.
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An iron(II) pyridyl-benzohydrazonate-based complex decorated with long alkyl chains is reported as a rare spin-crossover compound displaying a wide thermal hysteresis spanning room temperature. On heating, this compound exhibits a spin transition between a LS ground state and an ordered HS-LS phase with symmetry breaking from monoclinic P21/n into orthorhombic P21212 space groups. During cooling, the compound first transits into a magnetically distinguishable HS-LS phase with monoclinic P21 symmetry before returning into the LS phase. Interconversion between the two distinct HS-LS phases is the result of subtle structural changes in the alkyl chains and produces a second minor thermal hysteresis that superposes to the large one. This unprecedented result shows that the combination of a conventional cooperative spin transition and ligand-driven magnetic changes can promote magnetic tristability at room temperature.
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The reaction of a multisite coordination ligand, LH3, with Cu(II) salts and Ln(NO3)3·nH2O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu5Ln2(L)2(µ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)x]∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1-3, respectively] coordination polymers. Complexes 1-3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu5Ln2] molecular analogues. The ac susceptibility measurements for complexes 1-3 confirm their SMM behavior with the following characteristics: Δeff/kB = 23.4 K, τ0 = 1.1 × 10-6 s and Δeff/kB = 27.9 K, τ0 = 6.6 × 10-7 s under 0 and 1200 Oe dc fields, respectively for 1; Δeff/kB = 8.3 K, τ0 = 3.1 × 10-6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu5Ln2] analogues are preserved after their assembly in these coordination networks.
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A detailed crystallographic study of [FeII(LN5)(CN)2]·MeOH (LN5 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) shows the light-induced formation and thermally-assisted rupture of a metal-ligand coordination bond concomitantly with a spin-crossover process occurring at the Fe(ii) center between its low spin (LS) and high spin (HS) states. These results illustrate the remarkable reversibility and non-destructive character of this photo-induced magneto-structural process in a single crystal.
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Two new mononuclear Fe(II) complexes, [Fe(2MeLpz)(NCX)2] (L = N,N'-dimethyl-N,N'-bis((pyrazin-2-yl)methyl)-1,2-ethanediamine and X = S (1), BH3 (2)), have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, optical reflectivity, and photomagnetic measurements. They have similar FeN6 coordination environments offered by the tetradentate ligand with a cis-α conformation and two NCX- coligands in cis positions. However, 1 and 2 have different molecular arrangements and crystal packings, and are isolated in orthorhombic Pbnb and monoclinic C2/c space groups, respectively. 1 remains in a high spin state (S = 2) over all temperatures while 2 undergoes a spin transition around 168 K with a small thermal hysteresis of about 0.4 K (at a temperature scan rate of 1.3 K min-1). This phase transition, which can also be optically detected due to the associated marked change of the sample color, occurs between two structurally characterized phases, which exhibit Fe(II) complexes in their high spin and low spin states at high and low temperatures, respectively. The reversible photoswitching between these two states has also been confirmed at low temperatures using well separated wavelength irradiations.