Four Redox Isomers of a [3 × 3] Copper-Iron Heterometal Grid.

A mixed-valence heterometallic nonanuclear [3 × 3] grid complex, [CuI2CuII6FeIII(L)6](BF4)5·MeOH·9H2O (1; MeOH = methanol), was synthesized by a one-pot reaction of copper and iron ions with multidentate ligand 2,6-bis[5-(2-pyridinyl)-1H-pyrazol-3-yl]pyridine (H2L). 1 showed five quasi-reversible one-electron redox processes centered at +0.74, +0.60, +0.39, +0.27, and -0.13 V versus SCE, assignable to four CuI/CuII processes and one FeII/FeIII couple, respectively. The two-electron-oxidized species [CuII8FeIII(L)6](PF6)7·4MeOH·7H2O (12eOx), the two-electron-reduced species [CuI4CuII4FeIII(L)6](PF6)3·2H2O (12eRed), and the three-electron-reduced species [CuI4CuII4FeII(L)6](PF6)2·5MeOH·H2O (13eRed) were isolated electrochemically. The four redox isomers were characterized by single-crystal X-ray analysis, SQUID magnetometry, and Mössbauer spectroscopy.

Selective electrochemical CO2 conversion with a hybrid polyoxometalate.

A multi-component coordination compound, in which ruthenium antenna complexes are connected to a polyoxotungstate core is presented. This hybrid cluster effectively promotes the electrochemical conversion of CO2 to C1 feedstocks, the selectivity of which can be controlled by the acidity of the media.

Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII 4 FeII 2 } Compound.

Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)FeIII (CN)3 ]4 [FeII (Ppmp)]2 ⋅2 CH3 OH (1, Tp*=hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Ppmp=2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine), which has a rhombic core structure abbreviated as {FeIII 2 FeII 2 }. Magnetic susceptibility measurements and single-crystal X-ray diffraction analyses revealed that 1 underwent thermally-induced spin transition with the thermal hysteresis. The FeII site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the FeII centers actuated anisotropic deformation of the rhombic {FeIII 2 FeII 2 } core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.

Tetranuclear [Cu3Ln] complexes derived from a tetraketone-type ligand.

A series of tetranuclear [Cu3Ln] complexes, [Cu3Gd(L)3(NO3)2(H2O)3](NO3)·H2O (1), [Cu3Tb(L)3(NO3)2(H2O)3](NO3) (2) and [Cu3Dy(L)3(NO3)3(H2O)2]·1.5(H2O) (3), were synthesized by a one-pot reaction using a simple tetraketone-type ligand (H2L = (3Z,5Z)-4,5-dihydroxy-3,5-octadiene-2,7-dione). X-ray structural analyses revealed that each complex has a planar tetranuclear core of [Cu3Ln] (Ln = Gd, Tb, and Dy), in which the Ln ion is accommodated in the centre of a Cu3O6 metallocycle. A cryomagnetic study revealed that all complexes show intramolecular ferromagnetic interactions between Cu(II) and Ln(III) ions. The [Cu3Gd] complex (1) has an ST = 5 spin ground state and shows a magneto-caloric effect with a maximum magnetic entropy change (-ΔSm) of 16.4 J kg-1 K-1 (5 T, 2.4 K). On the other hand, the [Cu3Tb] complex (2) shows a slow magnetic relaxation behavior under a zero magnetic field. The analysis of an Arrhenius plot reveals that the effective energy barrier of spin reversal is 13.1 K. The [Cu3Dy] complex (3) also shows a slow magnetic relaxation under 1300 Oe dc magnetic field with an effective energy barrier of 6.82 K.

Elucidating the exchange interactions in a {GdIIICuII4} propellor.

The multinucleating ligand 2,2'-(propane-1,3-diyldiimino)bis[2-(hydroxymethyl)-propane-1,3-diol] (bis-tris propane, H6L) is used in the design of a new family of 3d-4f complexes that display an unusual {LnCu4} four-blade propeller topology. We report the synthesis, structure and magnetic characterisation of [LnCu4(H4L)4](Cl)2(ClO4)·6CH3OH, where Ln = Gd (1), Tb (2), Dy (3), La (4). Previously we have used CH3COO- and NO3- as co-ligands with bis-tris propane, but here the use of Cl- and ClO4- leads to coordination of four {Cu(H4L)} units around the central Ln ion. A magneto-structural analysis reveals that the geometrical arrangement of the Cu(II) centres defined by the H4L2- ligands controls the magnetic communication between the different metal centres. DFT calculations performed on the isotropic (Gd) and diamagnetic (La) systems 1 and 4 help to unravel the intriguing exchange interactions.

Simultaneous Photo-Induced Magnetic and Dielectric Switching in an Iron(II)-Based Spin-Crossover Hofmann-Type Metal-Organic Framework.

Molecular materials possessing photo-tunable polarization switching is promising for optical switches, smart sensors, and data storage devices. However, it is challenging to devise a molecular material featuring simultaneous switchable magnetic and dielectric properties with regard to non-invasive and convenient light stimulus. Herein, we report a new Hofmann-type metal-organic framework (MOF) {Fe(bpt)[Pt(CN)4 ]} ⋅ 0.5anth (1, bpt=2,5-bis(4-pyridyl)thiophen; anth=anthracene), which displays thermo- and photo-switchable magnetic and dielectric properties. Photo-monitored structural analyses revealed that it was the photo-induced deformation of FeII coordination sphere and relative movement of guest anthracene that resulted in the variation of the local electric dipoles. These findings provide a new strategy to realize polarization switching through the light-induced spin crossover, and would be of fundamental significance for future photo-switchable and multifunctional materials.

Spin-Crossover Tuned Rotation of Pyrazolyl Rings in a 2D Iron(II) Complex towards Synergetic Magnetic and Dielectric Transitions.

Materials showing synergy of magnetic and dielectric transitions are promising candidates for future molecular devices. The challenge is how to realize synergy between spin and dielectric transitions with responses to external stimuli. Herein, we design a 2D spin crossover (SCO) complex, [FeII (dpa)][(pzTp)FeIII (CN)3 ]2 (1) (dpa=1,2-bis(4-pyridyl)ethyne and pzTp=tetrakis(pyrazolyl)borate). The local structural changes about the FeII ion were propagated to the whole crystal through the rigid bridging ligands (dpa), leading to elastic interactions to realize the abrupt SCO and rotational movements of polar apical pyrazolyl rings in the [(pzTp)FeIII (CN)3 ]- units. Dielectric measurements confirmed a substantial dielectric change (Δϵ'=2.3) upon the spin transition. This work provides a rational strategy to couple the spin transition and rotation of polar components, which is crucial for the synergetic switch of magnetism and dielectricity.

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2 Co]-Assembled Mixed-Valence Chain.

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although it is currently difficult. Herein, we assembled the asymmetric {Fe2 Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3 ]2 Co(BIT)} ⋅ 2CH3 OH (1) (Tp=hydrotris(pyrazolyl)borate and BIT=3,4-bis-(1H-imidazol-1-yl)thiophene), which showed reversible multi-phase transitions accompanied by photo-switchable single-chain magnet properties and a dielectric anomaly. Variable-temperature X-ray structural studies revealed thermo- and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off single-chain magnet behavior by alternating 946-nm and 532-nm light irradiation. A substantial anomaly in the dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

A ring of grids: a giant spin-crossover cluster.

Mononuclear and icosanuclear spin-crossover complexes, [FeII(HL)2](BF4)2 (1) and [FeII20(L)24](BF4)16 (2), were synthesized using an asymmetric multidentate ligand (HL). 1 has a bis-chelate structure with two protonated ligands, while 2 has a ring-shape structure comprising four [2 × 2] grid moieties and four mononuclear units.

Switching the magnetic hysteresis of an [Feii-NC-Wv]-based coordination polymer by photoinduced reversible spin crossover.

Magnetic bistable materials that feature magnetic hysteresis are comparable to elementary binary units and promising for application in switches and memory devices. In this work, we report a material that consists of parallel cyanide-bridged [Feii-Wv] coordination chains linked together through rigid bis(imidazolyl)-benzene ligands and displays multiple magnetic states. The paramagnetic high-spin and diamagnetic low-spin states of the spin-crossover Feii ions can be interconverted by reversible light-induced excited spin state trapping (LIESST) by alternating between light irradiation of 808 and 473 nm. At 1.8 K, under 808-nm-light irradiation, magnetic interactions between the photogenerated paramagnetic high-spin Feii centres and the Wv centres lead to long fragments that exhibit single-chain magnet behaviour, with a wide magnetic hysteresis and a large coercive field of 19 kOe; under a 473 nm light, isolated Feii-Wv fragments behave as single-molecule magnets instead. At 3.3 K, the high-spin form still displays magnetic hysteresis, albeit narrower, whereas the low-spin one does not.

A Mixed-Valence {Fe13 } Cluster Exhibiting Metal-to-Metal Charge-Transfer-Switched Spin Crossover.

It is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal-to-metal charge transfer (MMCT) and spin-crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a FeII -triazole moiety and generates a mixed-valence complex {[(Tp4-Me )FeIII (CN)3 ]9 [FeII 4 (trz-ph)6 ]}⋅[Ph3 PMe]2 ⋅[(Tp4-Me )FeIII (CN)3 ] (1; trz-ph=4-phenyl-4H-1,2,4-triazole). Moreover, MMCT occurs between FeIII and one of the FeII sites after heat treatment, resulting in the generation of a new phase, {[(Tp4-Me )FeII (CN)3 ][(Tp4-Me )FeIII (CN)3 ]8 [FeIII FeII 3 (trz-ph)6 ]}⋅ [Ph3 PMe]2 ⋅[(Tp4-Me )FeIII (CN)3 ] (1 a). Structural and magnetic studies reveal that MMCT can tune the two-step SCO behavior of 1 into one-step SCO behavior of 1 a. Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin-transition materials with accessible multi-electronic states.

Heteroleptic iron(ii) complexes with naphthoquinone-type ligands.

Heteroleptic iron complexes with a naphthoquinone-type ligand, 2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione (HL), were synthesized, and their structures and magnetic properties were investigated. Two spin-crossover complexes, [Fe(L)2(L1)] (1, L1 = ethylenediamine) and [Fe(L)2(L2)] (2, L2 = cyclohexanediamine) were structurally characterized at 100 K and 270 K, and their gradual spin crossover behavior observed by magnetic susceptibility measurements and confirmed by Mössbauer spectroscopic analyses. Two homologous iron(ii) complexes, [Fe(L)2(L3c)] (3, L3c = diimino benzoquinone) and [Fe(L)2(L4c)] (4, L4c = iminosuccinonitrile), were found to exist in the low-spin state in the range of 10-300 K. In contrast, the corresponding complex with two water ligands, [Fe(L)2(H2O)2] (5), and the tris-chelate complex with protonated ligands (HL), [Fe(HL)3](BF4)2 (6), were found to be high-spin.

A Ferromagnetically Coupled Octanuclear Manganese(III) Cluster: A Single-Molecule Magnet with a Spin Ground State of S = 16.

An octanuclear manganese complex, [MnIII8(µ4-O)4(L)4(OMe)4(OAc)4(OCH2CH2NH3)4] [1; H2L = 3-(dimethoxymethyl)-2-hydroxybenzoic acid], was synthesized with an extended cubane core structure consisting of eight Mn ions bridged by O atoms. Cryomagnetic studies revealed that 1 showed a single-molecule-magnet behavior with an S = 16 spin ground state.

A triple-triangle cluster derived from a simple tridentate ligand.

The heptanuclear ferric cluster, [FeIII7(O)6(HL)5(H2L)(H2O)8](BF4)4 (H2L = 2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine), was synthesized by the reaction of the planar tridentate ligand H2L with iron(ii) tetrafluoroborate hexahydrate. The cluster has a corner-shared triple-triangle core structure connected by three µ-O atoms, and it has a spin-frustrated ferrimagnetic spin ground state of S = 19/2.

Asymmetric Hybrid Polyoxometalates: A Platform for Multifunctional Redox-Active Nanomaterials.

Access to asymmetrically functionalized polyoxometalates is a grand challenge as it could lead to new molecular nanomaterials with multiple or modular functionality. Now, a simple one-pot synthetic approach to the isolation of an asymmetrically functionalized organic-inorganic hybrid Wells-Dawson polyoxometalate in good yield is presented. The cluster bears two organophosphonate moieties with contrasting physical properties: a chelating metal-binding group, and a long aliphatic chain that facilitates solvent-dependent self-assembly into soft nanostructures. The orthogonal properties of the modular system are effectively demonstrated by controlled assembly of POM-based redox-active nanoparticles. This simple, high-yielding synthetic method is a promising new approach to the preparation of multi-functional hybrid metal oxide clusters, supermolecular systems, and soft-nanomaterials.

Effect of Intermolecular Interactions on Metal-to-Metal Charge Transfer: A Combined Experimental and Theoretical Investigation.

Understanding the effects of intermolecular interactions on metal-to-metal charge transfer (MMCT) is crucial to develop molecular devices by grafting MMCT-based molecular arrays. Herein, we report a series of solvent-free {Fe2 Co2 } compounds sharing the same cationic tetranuclear {[Fe(PzTp)(CN)3 ]2 [Co(dpq)2 ]2 }2+ (PzTp- =tetrakis(pyrazolyl)borate, dpq=dipyrido[3,2-d:2',3'-f]quinoxaline) square units but having anions with different size, including BF4 - , PF6 - , OTf- , and [Fe(PzTp)(CN)3 ]- . Intermolecular π⋅⋅⋅π interactions between dpq ligands, which coordinate to cobalt ions in the {[Fe(PzTp)(CN)3 ]2 [Co(dpq)2 ]2 }2+ units, can be modulated by introducing different counterions, regulating the distortion of the CoN6 octahedron and ligand field around the cobalt ions. This change results in different MMCT behavior. Computational analyzes reveal the substantial role of the intermolecular interactions tuned by the presence of different counteranions on the MMCT behavior.

Intramolecular Electron Transfers in a Series of [Co2Fe2] Tetranuclear Complexes.

Discrete cyanide-bridged Co-Fe multinuclear complexes can be considered as functional units of bulk Co-Fe Prussian blue analogues, and they have been recognized as a new class of switching molecules in the last decade. The switching property of the cyanide-bridged Co-Fe complexes is based on intramolecular electron transfers between Co and Fe ions, and we herein refer to this phenomenon as an electron transfer-coupled spin transition (ETCST). Although there have been numerous reports on the complexes exhibiting ETCST behavior, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here the syntheses and thermal ETCST behavior both in the solid state and solution for a series of tetranuclear [Co2Fe2] complexes, [Co2Fe2(CN)6(L1)2(L2)4]X2 (L1 and L2: tri- and bidentate capping ligands for Fe and Co ions, X: counteranions). All complexes showed thermal ETCST equilibrium between high-spin ([(hs-CoII)2(ls-FeIII)2]) and low-spin ([(ls-CoIII)2(ls-FeII)2]) states in butyronitrile, and the equilibrium temperatures (T1/2) showed systematic shifts by chemical modifications and chemical stimuli. The T1/2 values were correlated with the redox potential differences (ΔE) of the Fe and Co ions in the constituent units, and the larger ΔE values led to the lower T1/2. The present result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules.

Solid-State Hydrogen-Bond Alterations in a [Co2 Fe2 ] Complex with Bifunctional Hydrogen-Bonding Donors.

A hydrogen-bonding donor-acceptor system, [Co2 Fe2 (bpy*)4 (CN)6 (tp*)2 ](PF6 )2 ⋅2ABA⋅4BN⋅2PE (1 solv ), was prepared by co-crystallization of an external stimuli-responsive cyanide-bridged tetranuclear [Co2 Fe2 ] complex and bifunctional hydrogen-bonding donors, p-aminobenzoic acid. Compound 1 solv exhibited a gradual electron-transfer-coupled spin transition (ETCST), and the removal of solvent molecules led to an abrupt thermal ETCST behavior with increased transition temperature. X-ray structural analysis revealed that the modification of ETCST was caused by a significant alteration of a hydrogen-bonding mode between the tetranuclear [Co2 Fe2 ]2+ cations and ABA molecules. Variable temperature IR measurements indicated that the desolvated form, 1 desolv , showed dynamic alteration of hydrogen-bonding interactions coupled with thermal ETCST behavior. These results suggested that the tetranuclear [Co2 Fe2 ] complex shows solid-state modulations of hydrogen-bond strengths by external stimuli.

A Brønsted-Ligand-Based Iron Complex as a Molecular Switch with Five Accessible States.

A mononuclear FeII complex, prepared with a Brønsted diacid ligand, H2 L (H2 L=2-[5-phenyl-1H-pyrazole-3-yl] 6-benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [FeII (H2 L)2 ](BF4 )2 (1A ), exhibits abrupt spin transition at T1/2 =258 K, and treatment with base yields a deprotonated analogue [FeII (HL)2 ] (1B ), which shows gradual SCO above 350 K. A range of FeIII analogues were also characterized. [FeIII (HL)(H2 L)](BF4 )Cl (1C ) has an S=5/2 spin state, while the deprotonated complexes [FeIII (L)(HL)], (1D ), and (TEA)[FeIII (L)2 ], (1E ) exist in the low-spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid-state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.