Magnetic Sponge with Neutral-Ionic Phase Transitions

Densely Packed CO2 Aids Charge, Spin, and Lattice Ordering Partially Fluctuated in a Porous Metal-Organic Framework Magnet

Partial charge fluctuations in the charge-ordered state of a material, often triggered by structural disorders and/or defects, can significantly alter its physical characteristics, such as magnetic long-range ordering. However, it is difficult to post-chemically fix such accidental partial fluctuations to reconstruct a uniform charge-ordered state. Herein, we report CO2 -aided charge ordering demonstrated in a CO2 -post-captured layered magnet, [{Ru2 (o-ClPhCO2 )4 }2 {TCNQ(OMe)2 }] ⋅ CO2 (1⊃CO2 ; o-ClPhCO2 - =ortho-chlorobenzoate; TNCQ(OMe)2 =2,5-dimethoxy-7,7,8,8-tetracyanoquinodimethane). Pristine porous layered magnet 1 had a partially charge-fluctuated ordered state, which provided ferrimagnetic ordering at TC =65 K. Upon loading CO2 , 1 adsorbed one mole of CO2 , forming 1⊃CO2 , and raising TC to 100 K. This was because of the vanishing charge fluctuations without significantly changing the framework structure. This research illustrates the post-accessible host-guest chemistry delicately combined with charge, spin, and lattice ordering in a spongy magnet. Furthermore, it highlights how this innovative approach opens up new possibilities for technology and nanoscale magnetism manipulation.

Homo-valent diruthenium(II,II) carbonates Na4[Ru2(CO3)4]·10H2O: synthesis, structure, properties, and calculation

Homo-valent diruthenium(II,II) carbonates have been underexplored hitherto. This paper reports the synthesis and crystal structure of a diruthenium(II,II) compound, Na₄Ru₂(CO₃)₄·10H₂O (1). It has a two-dimensional structure in which the paddle-wheel diruthenium units of Ru₂(CO₃)₄^4- are cross-linked through the carbonate groups. The investigation of the primary magnetic properties and theoretical studies with density functional theory (DFT) reveal that weak antiferromagnetic interactions are propagated between the Ru₂ units which contain two unpaired electrons in an electron configuration σ²π⁴δ²π*²δ*² with a ground state S = 1. According to Raman spectrum measurements combined with theoretical calculations, the strong peak at 356 cm^-1 in the small-wavenumber region was assigned to the stretching of the Ru-Ru double bonds.

Structural Transformable Coulomb Lattice of n-Type Semiconductors for Guest Sorption

In recent years, highly designable organic porous materials have attracted considerable attention in the development of new types of molecular adsorption-desorption materials. The adsorption-desorption process also changes the electronic structure via the existence of guest molecules. Therefore, it is possible to change the physical property during the guest adsorption-desorption cycle using an appropriate chemical design of the host crystal lattice. As the development of n-type organic semiconductors has been limited, we focused on designing an n-type organic semiconductor material to control the host crystal lattice, electronic dimensionality, chemical stability, and high electron mobility using an ionic naphthalenediimide (NDI) derivative. Low symmetrical dianionic bis(benzene-m-sulfonate)-naphthalenediimide (m-BSNDI^2-) forms various types of single-crystal (M⁺)₂(m-BSNDI^2-)·n(guest) with a combination of M⁺ = Na⁺, K⁺, Rb⁺, and guest = H₂O, CH₃OH. Four crystals of (K⁺)₂(m-BSNDI^2-)·n(H₂O), (K⁺)₂(m-BSNDI^2-)·n(CH₃OH), α-(K⁺)₂(m-BSNDI^2-), and β-(K⁺)₂(m-BSNDI^2-) were transformable using the guest adsorption-desorption cycle. Two kinds of single-crystal (K⁺)₂(m-BSNDI^2-)·n(CH₃OH) with n = 0 and 2.0 showed a single-crystal to single-crystal (SCSC) transformation through CH₃OH desorption. On the contrary, five kinds of single crystals with n = 0, 3.0, 3.3, 4.75, and 5.5 were identified in the single-crystal X-ray structural analyses of (K⁺)₂(m-BSNDI^2-)·n(H₂O). Systematic change of the ionic radii in (M⁺)₂(m-BSNDI^2-) modified the crystal lattice flexibility for the guest adsorption-desorption cycles.

Role of intramolecular hydrogen bonding in the redox chemistry of hydroxybenzoate-bridged paddlewheel diruthenium(II,II) complexes

The synthesis of a series of trans-heteroleptic paddlewheel diruthenium(II,II) complexes with various hydroxy-substituted benzoate ligands, [Ru₂((OH)_xPhCO₂)₂(2,6-(CF₃)₂PhCO₂)₂(THF)₂] ([RuII,II2]) as tetrahydrofuran (THF) adducts is reported, where (OH)_xPhCO₂^- stands for o-hydroxybenzoate (o-OH), m-hydroxybenzoate (m-OH), p-hydroxybenzoate (p-OH), 2,3-dihydroxybenzoate (2,3-(OH)2), 2,4-dihydroxybenzoate (2,4-(OH)2), 2,5-dihydroxybenzoate (2,5-(OH)2), 2,6-dihydroxybenzoate (2,6-(OH)2), or 3,4-dihydroxybenzoate (3,4-(OH)2), and 2,6-(CF₃)₂PhCO₂^- represents 2,6-bis(trifluoromethyl)benzoate. In this heteroleptic series, the redox potential (E_1/2) of the [RuII,II2]/[RuII,III2]⁺ couple in THF varies over a wide range, from -18 mV (vs. Ag/Ag⁺) for p-OH to 432 mV for 2,6-(OH)2. The redox properties are linearly dependent on the acidity (pK_a) of the OH-substituted benzoic acids, but do not depend on the number of ortho-substituted hydroxy (o-OH) groups. This indicates that the steric effect of o-substituents is irrelevant in the case of hydroxyl groups, owing to the formation of intramolecular hydrogen bonds between the o-OH group and carboxylate oxygens. The value of the Hammett constant σ_o for the o-OH substituent was determined to be 0.667, indicating a strongly electron-withdrawing character, contrary to the expectation of electron-donating character for an OH group. The redox properties of the compounds were well explained in a framework of Hammett analyses and were also consistent with their HOMO energy levels evaluated by DFT calculations based on the atomic coordinates. The unexpected electron-withdrawing character of the o-OH groups could be attributed to the direct effect of intramolecular hydrogen bonding on the charge density on the carboxylate oxygen.

Magnet Creation by Guest Insertion into a Paramagnetic Charge-Flexible Layered Metal-Organic Framework

Changing nonmagnetic materials to spontaneous magnets is an alchemy-inspiring concept in materials science; however, it is not impossible. Here, we demonstrate chemical modification from a nonmagnet to a bulk magnet of either a ferrimagnet or antiferromagnet, depending on the adsorbed guest molecule, in an electronic-state-flexible layered metal-organic framework, [{Ru₂(2,4-F₂PhCO₂)₄}₂TCNQ(EtO)₂] (1; 2,4-F₂PhCO₂^- = 2,4-difluorobenzoate; TCNQ(EtO)₂ = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane). The guest-free paramagnet 1 undergoes a thermally driven intralattice electron transfer involving a structural transition at 380 K. This charge modification can also be implemented by guest accommodations at room temperature; 1 adsorbs several organic molecules, such as benzene (PhH), p-xylene (PX), 1,2-dichloroethane (DCE), dichloromethane (DCM), and carbon disulfide (CS₂), forming 1-solv with intact crystallinity. This induces an intralattice electron transfer to produce a ferrimagnetically ordered magnetic layer. According to the interlayer environment tuned by the corresponding guest molecule, the magnetic phase is consequently altered to a ferrimagnet for the guests PhH, PX, DCE, and DCM or an antiferromagnet for CS₂. This is the first demonstration of the postsynthesis of bulk magnets using guest-molecule accommodations.

A metal-organic framework that exhibits CO2-induced transitions between paramagnetism and ferrimagnetism

With adequate building blocks, metal-organic frameworks (MOFs) can combine magnetic ordering and porosity. This makes MOFs a promising platform for the development of stimuli-responsive materials that show drastically different magnetic properties depending on the presence or absence of guest molecules within their pores. Here we report a CO₂-responsive magnetic MOF that converts from ferrimagnetic to paramagnetic on CO₂ adsorption, and returns to the ferrimagnetic state on CO₂ desorption. The ferrimagnetic material is a layered MOF with a [D⁺-A^--D] formula, produced from the reaction of trifluorobenzoate-bridged paddlewheel-type diruthenium(II) clusters as the electron donor (D) with diethoxytetracyanoquinodimethane as the electron acceptor (A). On CO₂ uptake, it undergoes an in-plane electron transfer and a structural transition to adopt a [D-A-D] paramagnetic form. This magnetic phase change, and the accompanying modifications to the electronic conductivity and permittivity of the MOF, are electronically stabilized by the guest CO₂ molecules accommodated in the framework.

Fine tuning of intra-lattice electron transfers through site doping in tetraoxolene-bridged iron honeycomb layers

The precise control of intra-lattice multiple electron transfers was demonstrated in the solvated and desolvated species of the tetraoxolene-bridged Fe honeycomb layer system, (NPr4)2[Fe2(Cl2An)3]·(solv) (Cl2Ann- = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate; NPr4+ = tetrapropylammonium cation), by the site-doping of the Cl2Ann- bridging unit using X2Ann- units with X = Br or F.

Chameleonic layered metal-organic frameworks with variable charge-ordered states triggered by temperature and guest molecules

Molecular materials whose electronic states are multiply varied depending on external stimuli are among the most promising targets for the development of multiply accessible molecular switches. Here, we report a honeycomb layer composed of tetraoxolene-bridged iron (Fe) subunits whose charge-ordered states are multiply variable via thermal treatments and solvation/desolvation with the crystallinity intact. The compound is (NPr4)2[Fe2(Cl2An)3] (1-d; NPr4 + = tetra-n-propylammonium; Cl2An2- = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate), which possesses three charge-ordered states: a low-temperature (LT) phase [(Fe3+)2(Cl2An2-)(Cl2An˙3-)2]2-; an intermediate (IM) phase [(Fe2.5+)2(Cl2An2-)(Cl2An2.5-)2]2-; and a high-temperature (HT) phase [(Fe2+)2(Cl2An2-)3]2- that varies according to temperature. In addition, the LT phase of 1-d is reversibly changeable to another IM phase in its solvated compound 1 via a solvation/desolvation process at room temperature. This example demonstrates a new multiple-switching system based on electron transfer and host-guest chemistry in a charge-flexible metal-organic framework.

Enhancing magnetic hardness by sonication assisted synthesis of heterometallic carbonato spin-glass Na[Ni(H2O)4Ru2(CO3)4]·3H2O

Control of magnetic performances of molecular magnets is essential but few efforts have been documented. A green and efficient sonication assisted synthesis of a new heterometallic diruthenium(ii,iii) carbonate, Na[Ni(H2O)4Ru2(CO3)4]·3H2O (1), was carried out by self-assembling in aqueous solution. Compound 1 exhibits spin-glass behavior below ∼5.0 K, and a systematic investigation of the ultrasonic irradiation influence on the powder samples reveals that their coercivity increases from 50 Oe to 743 Oe with the control of ultrasonic power under appropriate conditions.

Grinding enhances the magnetic hardness of heterometallic diruthenium(ii,iii) carbonates with a kagome lattice structure

A manual grinding strategy promotes the magnetic hardness of Cs₃Cd(H₂O)₆[{Cd(H₂O)₃}₂{Ru₂(CO₃)₄}₃]·10H₂O with a kagome lattice structure.

Temperature-Induced Single-Crystal-to-Single-Crystal Transformations with Consequential Changes in the Magnetic Properties of Fe(III) Complexes

The present article deals with an one-to-one structure-property correspondence of a dinuclear iron complex, [Dipic(H₂O)FeOH]₂·H₂O (1) (Dipic = pyridine-2,6-dicarboxylic acid). Variable-temperature X-ray single-crystal structural analysis confirms a phase transition of complex 1 to complex 2 ([Dipic(H₂O)FeOH]₂) at 120 °C. Further, single-crystal-to-single-crystal (SCSC) transformation was monitored by temperature-dependent single crystal X-ray diffraction, powder X-ray diffraction, time-dependent Fourier-transform infrared spectroscopy, and differential scanning calorimetry. SCSC transformation brings the change in space group of single crystal. Complex 1 crystallizes in the C2/c space group, whereas complex 2 crystallizes in the Pi̅ space group. SCSC transformation brings the change in packing diagram as well. Complex 1 shows two-dimensional network through H-bonding, whereas the packing diagram of complex 2 shows a zigzag-like arrangement. Phase transformation not only fetches structural changes but also in the magnetic properties. Difference in Fe-O-Fe bond angles of two complexes creates notable variation in their antiferromagnetic interactions with adjacent metal centers.

Host-Guest Hydrogen Bonding Varies the Charge-State Behavior of Magnetic Sponges

The electron-donor(D) and -acceptor(A)-assembled D₂ A-layer framework [{Ru₂ (m-FPhCO₂ )₄ }₂ TCNQ(OMe)₂ ]⋅nDCE (1-nDCE; m-FPhCO₂ ^- =m-fluorobenzoate; TCNQ(OMe)₂ =2,5-dimethoxyl-7,7,8,8-tetracyanoquinodimethane; DCE=1,2-dichloroethane) undergoes drastic charge-ordered state variations via three distinct states that are a two-electron-transferred state (2e-I), a charge-disproportionated state (1.5e-I), and a one-electron-transferred state (1e-I), depending on the degree of solvation by nDCE. The pristine form 1-4DCE has a paramagnetic 2e-I state, which eventually produces the solvent-free form 1 in 1.5e-I via an intermediate state 1-nDCE (n≤1) in 1e-I. Resolvation of 1 stabilizes 1-DCE, allowing it to switch between 1.5e-I and 1e-I, and to become ferrimagnetic with a T_c of 30 K (1.5e-I) and 88 K (1e-I). The stabilization of the 1e-I state of 1-DCE is due to the presence of host-guest hydrogen bonding that enables to suppress the electron-donation ability of D even in an identical framework with 1.

Layered ferrimagnets constructed from charge-transferred paddlewheel [Ru2] units and TCNQ derivatives: the importance of interlayer translational distance in determining magnetic ground state

A donor (D)/acceptor (A) assembly reaction using the paddlewheel-type diruthenium(ii,ii) complex [Ru2(3,5-F2PhCO2)4(THF)2] (3,5-F2PhCO2- = 3,5-difluorobenzoate; abbreviated hereafter as [Ru2]) as D and 7,7,8,8-tetracyano-p-quinodimethane derivatives (TCNQRx; 2,5-R-substituted TCNQ) as A in a DCM/TCE or DCE solvent system (DCM = dichloromethane, TCE = 1,1,2,2-tetrachloroethane, DCE = 1,2-dichloroethane) led to the formation of D2A-type two-dimensional layered compounds [{Ru2(3,5-F2PhCO2)4}2{TCNQRx}]·nsolv (Rx = H2 (1), Rx = Me2, (2), and Rx = (OEt)2 (3)). All the compounds had similar two-dimensional fishnet-type structures, where the two [Ru2] units were fully coordinated with the four cyano groups of TCNQRx. The compounds 1-3 were categorized as a one-electron transferred ionic state (1e-I) with D+-A--D formulation (D+[triple bond, length as m-dash][RuII,III2]+; A-[triple bond, length as m-dash]TCNQRx˙-). This subunit state formally derived a ferrimagnetically ordered state composed of up-spins with S = 1 for [RuII,II2] and S = 3/2 for [RuII,III2]+ and a down-spin of S = 1/2 for TCNQRx˙-, with antiferromagnetic superexchange interactions in each layer. Eventually, 1-3 became three-dimensional ferrimagnets with Curie temperatures TC = 93, 93, and 92 K, respectively, owing to the presence of interlayer ferromagnetic interactions. The interlayer translational distances (l2) for the three compounds were 10.56, 10.54, and 10.84 Å, respectively, which agreed with the empirical prediction based on the threshold value of l2 > 10.3 Å for a valid ferromagnetic interaction.

Thermally Induced Valence Tautomeric Transition in a Two-Dimensional Fe-Tetraoxolene Honeycomb Network

A novel tetraoxolene-bridged Fe two-dimensional honeycomb layered compound, (NPr₄ )₂ [Fe₂ (Cl₂ An)₃ ] ⋅2 (acetone)⋅H₂ O (1), where Cl₂ An^n- =2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate and NPr₄⁺ =tetrapropylammonium cation, has been synthesized. 1 revealed a thermally induced valence tautomeric transition at T_1/2 =236 K (cooling)/237 K (heating) between Fe^m+ (m=2 or 3) and Cl₂ An^n- (n=2 or 3) that induced valence modulations between [Fe^II_HS Fe^III_HS (Cl₂ An^2- )₂ (Cl₂ An^.3- )]^2- at T>T_1/2 and [Fe^III_HS Fe^III_HS (Cl₂ An^2- )(Cl₂ An^.3- )₂ ]^2- at T<T_1/2 . Even in a two-dimensional network structure, the low-temperature phase [Fe^III_HS Fe^III_HS (Cl₂ An^2- )(Cl₂ An^.3- )₂ ]^2- valence set can be regarded as a magnetic chain-knit network, where ferrimagnetic Δ and Λ chains of [Fe^III_HS (Cl₂ An^.3- )]_∞ are alternately linked by the diamagnetic Cl₂ An^2- . This results in a slow magnetization behavior attributed to the structure acting as a single-chain magnet at lower temperatures.