Conducting Anilate-Based Mixed-Valence Fe(II)Fe(III) Coordination Polymer: Small-Polaron Hopping Model for Oxalate-Type Fe(II)Fe(III) 2D Networks

Two-Dimensional Cobalt(II) Benzoquinone Frameworks for Putative Kitaev Quantum Spin Liquid Candidates

The realization and discovery of quantum spin liquid (QSL) candidate materials are crucial for exploring exotic quantum phenomena and applications associated with QSLs. Most existing metal-organic two-dimensional (2D) quantum spin liquid candidates have structures with spins arranged on the triangular or kagome lattices, whereas honeycomb-structured metal-organic compounds with QSL characteristics are rare. Here, we report the use of 2,5-dihydroxy-1,4-benzoquinone (X2dhbq, X = Cl, Br, H) as the linkers to construct cobalt(II) honeycomb lattices (NEt4)2[Co2(X2dhbq)3] as promising Kitaev-type QSL candidate materials. The high-spin d7 Co2+ has pseudospin-1/2 ground-state doublets, and benzoquinone-based linkers not only provide two separate superexchange pathways that create bond-dependent frustrated interactions but also allow for chemical tunability to mediate magnetic coupling. Our magnetization data show antiferromagnetic interactions between neighboring metal centers with Weiss constants from -5.1 to -8.5 K depending on the X functional group in X2dhbq linkers (X = Cl, Br, H). No magnetic transition or spin freezing could be observed down to 2 K. Low-temperature susceptibility (down to 0.3 K) and specific heat (down to 0.055 K) of (NEt4)2[Co2(H2dhbq)3] were further analyzed. Heat capacity measurements confirmed no long-range order down to 0.055 K, evidenced by the broad peak instead of the λ-like anomaly. Our results indicate that these 2D cobalt benzoquinone frameworks are promising Kitaev QSL candidates with chemical tunability through ligands that can vary the magnetic coupling and frustration.

Tuning the slow magnetic relaxation with the substituents in anilate bridged bis(dysprosium) complexes

Dinuclear lanthanide complexes [((HB(pz)3)2Dy)2(μ-Th2An)] (1Dy) and [((HB(pz)3)2Dy)2(μ-ClCNAn)] (2Dy), based on the hydrotris(pyrazol-1-yl)borate (HBpz3-) scorpionate capping ligand and anilate (An2-) bridging linkers, namely homosubstituted dithiophene- and heterosubstituted chlorocyanoanilate, bearing electron-donating and withdrawing substituents at the 3,6-positions of the benzoquinone core, are reported. 1Dy shows an octacoordinated {N6O2} DyIII ion within a D4h distorted square antiprismatic coordination, an ideal geometry for Single-Molecule Magnet (SMM) behavior, given its oblate nature, whereas in 2Dy the octacoordinated DyIII ion adopts a D2d triangular dodecahedron geometry, while maintaining the same {N6O2} coordination sphere. Both complexes show field-induced single molecule magnet (SMM) behaviour, with tuning of the slow magnetic relaxation as a function of the nature of the substituents at the 3,6-positions of the anilate moiety. A comparison of the Arrhenius fitting parameters for 1Dy and 2Dy supports the hypothesis that square antiprismatic DyIII complexes, as 1Dy, exhibit higher energy barriers. This interpretation is supported by ab initio calculations that also shed light on the crucial role of intermolecular dipolar interactions.

Bimetallic Coordination Polymers: Synthesis and Applications in Biosensing and Biomedicine

Bimetallic coordination polymers (CPs) have two different metal ions as connecting nodes in their polymer structure. The synthesis methods of bimetallic CPs are mainly categorized into the one-pot method and post-synthesis modifications according to various needs. Compared with monometallic CPs, bimetallic CPs have synergistic effects and excellent properties, such as higher gas adsorption rate, more efficient catalytic properties, stronger luminescent properties, and more stable loading platforms, which have been widely applied in the fields of gas adsorption, catalysis, energy storage as well as conversion, and biosensing. In recent years, the study of bimetallic CPs synergized with cancer drugs and functional nanomaterials for the therapy of cancer has increasingly attracted the attention of scientists. This review presents the research progress of bimetallic CPs in biosensing and biomedicine in the last five years and provides a perspective for their future development.

Mixed-Valence CuI /CuIII Metal-Organic Frameworks with Non-innocent Ligand for Multielectron Transfer

We report two novel three-dimensional copper-benzoquinoid metal-organic frameworks (MOFs), [Cu4 L3 ]n and [Cu4 L3  ⋅ Cu(iq)3 ]n (LH4 =1,4-dicyano-2,3,5,6-tetrahydroxybenzene, iq=isoquinoline). Spectroscopic techniques and computational studies reveal the unprecedented mixed valency in MOFs, formal Cu(I)/Cu(III). This is the first time that formally Cu(III) species are witnessed in metal-organic extended solids. The coordination between the mixed-valence metal and redox-non-innocent ligand L, which promotes through-bond charge transfer between Cu metal sites, allows better metal-ligand orbital overlap of the d-π conjugation, leading to strong long-range delocalization and semiconducting behavior. Our findings highlight the significance of the unique mixed valency between formal Cu(I) and highly-covalent Cu(III), non-innocent ligand, and pore environments of these bench stable Cu(III)-containing frameworks on multielectron transfer and electrochemical properties.

Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Rapid, Selective Extraction of Silver from Complex Water Matrices with a Metal-Organic Framework/Oligomer Composite Constructed via Supercritical CO2

Every year vast quantities of silver are lost in various waste streams; this, combined with its limited, diminishing supply and rising demand, makes silver recovery of increasing importance. Thus, herein, we report a controllable, green process to produce a host of highly porous metal-organic framework (MOF)/oligomer composites using supercritical carbon dioxide (ScCO2 ) as a medium. One resulting composite, referred to as MIL-127/Poly-o-phenylenediamine (PoPD), has an excellent Ag+ adsorption capacity, removal efficiency (>99 %) and provides rapid Ag+ extraction in as little as 5 min from complex liquid matrices. Notably, the composite can also reduce sliver concentrations below the levels (<0.1 ppm) established by the United States Environmental Protection Agency. Using theoretical simulations, we find that there are spatially ordered polymeric units inside the MOF that promote the complexation of Ag+ over other common competing ions. Moreover, the oligomer is able to reduce silver to its metallic state, also providing antibacterial properties.

Light-Induced Intramolecular Electron Transfer in a 1D [CuFe] Coordination Polymer Containing the [Fe(C2O4)3]3- Core

A one-dimensional (1D) ladder-like coordination polymer {NH4[{Cu(bpy)}2(C2O4)Fe(C2O4)3]·H2O}n (1; bpy = 2,2'-bipyridine) containing [Cu(bpy)(μ-C2O4)Cu(bpy)]2+ cationic units linked by oxalate groups of [Fe(C2O4)3]3- building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to [CuII(bpy)(μ-C2O4)CuII(bpy)]2+, leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO2.

Dimensional Control of Highly Anisotropic and Transparent Conductive Coordination Polymers for Solution-Processable Large-Scale 2D Sheets

Controlling the dimensional aspect of conductive coordination polymers is currently a key scientific interest. Herein, solution-based dimension control strategies are proposed for copper chloride thiourea (CuCl-TU) coordination polymers that enable centimeter-scale, 2D nanosheet formation for use as transparent electrodes. Despite the wide bandgap of CuCl-TU polymers (4.33 eV), through polaron-mediated electron transfer, the electrical conductivity of the 2D sheet at room temperature is able to reach 4.45 S cm^-1 without intentional doping. This leads to a highly anisotropic electronic conductivity of up to the order of ≈10³ differences, depending on the material orientation. Furthermore, by substituting alternative thiourea candidates, it is demonstrated that it is possible to predesign CuCl-TU structures with the desired functionality, stability, and porosity through dimensional control. These findings provide a blueprint to design next-generation transparent conducting materials that can operate at room temperature, thereby expanding their applicability to different fields.

Geometry-dependent valence tautomerism, magnetism, and electrical conductivity in 1D iron–tetraoxolene chains

Here we show how a simple change in the geometry of 1D iron–tetraoxolene chains dramatically alters the observed physical properties, including the presence of valence tautomerism, strong magnetic coupling, and electrical conductivity.

Linker Redox Mediated Control of Morphology and Properties in Semiconducting Iron-Semiquinoid Coordination Polymers

The emergence of conductive 2D and less commonly 3D coordination polymers (CPs) and metal-organic frameworks (MOFs) promises novel applications in many fields. However, the synthetic parameters for these electronically complex materials are not thoroughly understood. Here we report a new 3D semiconducting CP Fe₅ (C₆ O₆ )₃ , which is a fusion of 2D Fe-semiquinoid materials and 3D cubic Fe_x (C₆ O₆ )_y materials, by using a different initial redox-state of the C₆ O₆ linker. The material displays high electrical conductivity (0.02 S cm^-1 ), broad electronic transitions, promising thermoelectric behavior (S² σ=7.0×10^-9  W m^-1  K^-2 ), and strong antiferromagnetic interactions at room temperature. This material illustrates how controlling the oxidation states of redox-active components in conducting CPs/MOFs can be a "pre-synthetic" strategy to carefully tune material topologies and properties in contrast to more commonly encountered post-synthetic modifications.

Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

Metal-Organic Framework vs. Coordination Polymer—Influence of the Lanthanide on the Nature of the Heteroleptic Anilate/Terephtalate 3D Network

Metal-organic frameworks (MOFs), whose definition has been regularly debated, are a sub-class of coordination polymers (CPs) which may feature both an overall 3D architecture and some degree of porosity. In this context, MOFs based on lanthanides (Ln-MOFs) could find many applications due to the combination of sorption properties and magnetic/luminescent behaviors. Here we report rare examples of 3D Ln-CPs based on anilate linkers, obtained under solvothermal conditions using a heteroleptic strategy. The three compounds of formula [Yb2(μ-ClCNAn)2(μ-F4BDC)(H2O)4]·(H2O)3 (1), [Er2(μ-ClCNAn)2(μ-F4BDC)(H2O)4]·(H2O)4 (2) and [Eu2(μ-ClCNAn)2(μ-F4BDC)(H2O)6] (3) have been characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and optical measurements. Structural characterization revealed that compounds 1 and 2 present an interesting MOF architecture with extended rectangular cavities which are only filled with water molecules. On the other hand, compound 3 shows a much more complex topology with no apparent cavities. We discuss here the origins of such differences and highlight the crucial role of the Ln(III) ion nature for the topology of the CP. Compounds 1 and 2 now offer a playground to investigate the possible synergy between gas/solvent sorption and magnetic/luminescent properties of Ln-MOFs.

Mixed valency in a neutral 1D Fe-chloranilate coordination polymer

A neutral Fe-chloranilate chain, with triphenylphosphine oxide co-ligands, represents a rare example of a one-dimensional chain in which there is a temperature dependent electron transfer from the Fe(ii) centre to the bridging chloranilate ligand.

Combined Experimental/Theoretical Study on the Luminescent Properties of Homoleptic/Heteroleptic Erbium(III) Anilate-Based 2D Coordination Polymers

The synthesis, structural and photophysical characterization, and theoretical studies on homo/heteroleptic neutral 2D-layered coordination polymers (CPs), obtained by combining the Er^III ion with chlorocyananilate (ClCNAn) and/or tetrafluoroterephthalate (F₄BDC) linkers, are herein reported. The structure of the heteroleptic Er^III-based CP, formulated as [Er₂(ClCNAn)₂(F₄BDC)(DMSO)₆]_n (1) is also reported. 1 crystallizes in the triclinic P1̅ space group, and the structure consists of neutral 2D layers formed by Er^III ions linked through the two linkers oriented in such a way that the neighboring 2D layers are eclipsed along the a axis, leading to parallelogram-like cavities. Photophysical measurements highlight the prominent role of chlorocyananilate linkers as optical antennas toward lanthanide ions, while wave-function-theory analysis supports the experimental findings, providing evidence for the effect of ligand substitution on the luminescence properties of homo/heteroleptic 2D CPs.

Redox Activity as a Powerful Strategy to Tune Magnetic and/or Conducting Properties in Benzoquinone-Based Metal-Organic Frameworks

Multifunctional molecular materials have attracted material scientists for several years as they are promising materials for the future generation of electronic devices. Careful selection of their molecular building blocks allows for the combination and/or even interplay of different physical properties in the same crystal lattice. Incorporation of redox activity in these networks is one of the most appealing and recent synthetic strategies used to enhance magnetic and/or conducting and/or optical properties. Quinone derivatives are excellent redox-active linkers, widely used for various applications such as electrode materials, flow batteries, pseudo-capacitors, etc. Quinones undergo a reversible two-electron redox reaction to form hydroquinone dianions via intermediate semiquinone radical formation. Moreover, the possibility to functionalize the six-membered ring of the quinone by various substituents/functional groups make them excellent molecular building blocks for the construction of multifunctional tunable metal-organic frameworks (MOFs). An overview of the recent advances on benzoquinone-based MOFs, with a particular focus on key examples where magnetic and/or conducting properties are tuned/switched, even simultaneously, by playing with redox activity, is herein envisioned.

Engineering of the XY Magnetic Layered System with Adeninium Cations: Monocrystalline Angle-Resolved Studies of Nonlinear Magnetic Susceptibility

An original example of modular crystal engineering involving molecular magnetic {Cu^II[W^V(CN)₈]}^- bilayers and adeninium cations (AdeH⁺) toward the new layered molecular magnet (AdeH){Cu^II[W^V(CN)₈]}·2H₂O (1) is presented. 1 crystallizes within the monoclinic C2 space group (a = 41.3174(12), b = 7.0727(3), c = 7.3180(2) Å, β = 93.119(3)°, and V = 2135 ų). The bilayer topology is based on a stereochemical matching between the square pyramidal shape of Cu^II moiety and the bicapped trigonal prismatic shape of [W^V(μ-CN)₅(CN)₃], and the separation between bilayers is significantly increased (by ∼50%; from ca. 9.5 to ca. 14.5 Å) compared to several former analogues in this family. This was achieved via a unique combination of (i) a 1D ribbonlike hydrogen bond system {AdeH⁺···H₂O···AdeH⁺···}_∞ exploiting planar water-assisted Hoogsteen···Sugar synthons with (ii) parallel 1D π-π stacks {AdeH⁺···AdeH⁺}_∞. In-plane 2D XY magnetism is characterized by a T_c close to 33 K, H_c,in-plane = 60 Oe, and H_{c,out-of-plane} = 750 Oe, high values of in-plane γ critical exponents (γ_b = 2.34(6) for H||b and γ_c = 2.16(5) for H||c), and a Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition, deduced from crystal-orientation-dependent scaling analysis. The obtained values of in-plane ν critical exponents, ν_b = 0.48(5) for H||b and ν_c = 0.49(3) for H||c, confirm the BKT transition (ν_BKT = 0.5). Full-range angle-resolved monocrystalline magnetic measurements supported by dedicated calculations indicated the occurrence of nonlinear susceptibility performance within the easy plane in a magnetically ordered state. We refer the occurrence of this phenomenon to spontaneous resolution in the C2 space group, a tandem not observed in studies on previous analogues and rarely reported in the field of molecular materials. The above magneto-supramolecular strategy may provide a novel means for the design of 2D molecular magnetic networks and help to uncover the inherent phenomena.

UV-vis/X-ray/thermo-induced synthesis and UV-SWIR photoresponsive property of a mixed-valence viologen molybdate semiconductor

A new design strategy through the synergy of Mo(vi)-Mo(v) intervalence charge transfer and π(radical)-π(radical/cation) interactions is proposed to obtain semiconductors with photoresponsive ranges covering the whole UV-SWIR (ultraviolet-shortwave near-infrared; ca. 250-3000 nm) region. With this strategy, a viologen-based molybdate semiconductor with a UV-SWIR photoresponsive range was obtained through UV/X-ray irradiation or thermal annealing. The thermally annealed semiconductor has the highest conversion and the best photocurrent response in the range of 355-2400 nm.

Water-assisted spin-flop antiferromagnetic behaviour of hydrophobic Cu-based metal-organic frameworks

Solvent-dependent magnetism in Cu-based metal-organic frameworks (MOFs) is reported. Spin-flop magnetic behaviour occurs at different dehydrated states of MOFs. The oxygens of guest and coordinated water molecules are responsible as water removal tunes the coordination geometry around the Cu centre and the electronic structure of the framework.

Nanoscaled Metal-Organic Frameworks: Challenges Towards Biomedical Applications

Achieving metal-organic frameworks (MOFs) in the form of nanoparticles (NanoMOFs) represents a recent challenge due to the possibility to combine the intrinsic porosity of these materials with the nanometric dimension, a fundamental requirement for strategic biomedical applications. In this outlook we envision the current/future opportunities of the NanoMOFs in the field of biomedicine, with particular emphasis on (i) biocompatible MOFs composition; (ii) MOFs miniaturization and (iii) nanoMOFs applications.

Magnetic relaxation in two chain-like Zn2Dy2 Schiff base coordination polymers bridged by tetraoxolene and its one-electron reduced radical

Two chain-like Zn–Dy anilate radical coordination polymers with Schiff base ligands show magnetic relaxation behaviors.

Nitrochloranilic acid: a novel asymmetrically substituted quinoid bridging ligand for design of coordination polymers

A series of alkali salts and transition metal complexes of a novel asymmetrically substituted quinoid ligand, 3-nitro-6-chloro-2,5-dihydroxyquinone (nitrochloranilic acid, H₂NCA) was prepared and characterised.

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.

Designing Magnetic NanoMOFs for Biomedicine: Current Trends and Applications

Metal–organic frameworks (MOFs) have shown a great potential in biomedicine due to their promising applications in different fields, including drug delivery, thermometry, theranostics etc. In this context, the development of magnetic sub-micrometric or nanometric MOFs through miniaturization approaches of magnetic MOFs up to the nanoscale still represents a crucial step to fabricate biomedical probes, especially in the field of theranostic nanomedicine. Miniaturization processes have to be properly designed to tailor the size and shape of particles and to retain magnetic properties and high porosity in the same material, fundamental prerequisites to develop smart nanocarriers integrating simultaneously therapeutic and contrast agents for targeted chemotherapy or other specific clinical use. An overview of current trends on the design of magnetic nanoMOFs in the field of biomedicine, with particular emphasis on theranostics and bioimaging, is herein envisioned.

Anilate Tethered Neutral Tetrahedral Pd(II) Cages Exhibiting Selective Encapsulation of Xylenes and Mesitylene

The design of porous materials for the recognition of multiple hydrocarbons is highly desirable for the energy-efficient separation and recognition of chemical feedstock. Herein, three new iso-structural porous discrete metal-organic cages of formula {[Pd₃ (NiPr)₃ PO]₄ (R-AN)₆ } (R-AN=anilate linkers) for the selective recognition of substituted aromatic hydrocarbons are reported. The tetrahedral cages 1, 2, and 3 containing anilate, chloranilate, and bromanilate linkers exhibited selective encapsulation of mesitylene, o-xylene, and p-xylene, respectively, over other analogous aromatic hydrocarbons. These selective encapsulations were driven by the variations in the portal diameters present at each of these cages and their interactions with the hydrocarbon guests. These observations are supported by mass spectrometry, NMR studies, and theoretical binding-energy calculations.

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.

A Redox-Innocent Uranium(IV)-Quinoid Metal-Organic Framework

Quinoid-based ligands constitute the most common class of redox-active ligands used to construct electrically conductive and magnetic metal-organic frameworks (MOFs). Whereas this chemistry is intensively explored for transition-metal and lanthanide ions, any related actinide compound has not received attention. In particular, the MOF chemistry of actinide ions in the lower oxidation states is underexplored. We herein report the synthesis, and structural and physical property characterization of a uranium(IV) quinoid-based MOF, [U(Cl₂dhbq)₂(H₂O)₂]·4H₂O (1, Cl₂dhbq^2- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone). 1 is a rare example of a U(IV)-based coordination solid and the first material to incorporate bona fide reducible bridging ligands. Despite the anticipated thermodynamic driving force, no indications of valence tautomerism are evident from magnetometry, near-IR spectroscopy, and X-band electron paramagnetic resonance measurements. These initial results suggest that reduction potentials alone are insufficient as guidelines for the prediction of the occurrence of electron transfer in uranium-quinoid-based materials.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

Two-dimensional square-grid iron(ii) coordination polymers showing anion-dependent spin crossover behavior

Two Fe(ii)-based coordination polymers [Fe(tpmd)₂(NCS)₂]·5.5H₂O (1) and [Fe(tpmd)₂(NCSe)₂]·7H₂O (2) with the framework of square-grid type have been assembled from FeSO₄·7H₂O, N,N,N′,N′-tetrakis(pyridin-4-yl)methanediamine (tpmd), and KNCS/KNCSe in methanol and characterized. By utilizing two pyridine groups of a tpmd ligand, 1 and 2 are formed in two-dimensional layered structures through coordination of octahedral iron(ii) ions with the tpmd to NCS⁻/NCSe⁻ ligands in which they have a supramolecular isomorphous conformation. 1 shows a paramagnetic behavior between 2 and 300 K, while 2 exhibits two-step spin crossover (ca. 145 and 50 K) in the temperature range due to the coordination of NCSe⁻ ligands. At 300 K 2 is fully high-spin state. However, at 100 K 2 becomes ca. 50% high spin and 50% low spin iron(ii) ions, which is verified by magnetic moments. In the structural analysis of 2 at 100 K, two different layers are observed with different bond distances around iron(ii) ions in which the layers are stacked alternately.

Two-dimensional Fe-based coordination polymers with square-grid shapes were prepared by self-assembly and exhibited an interesting spin crossover behavior depending on the coordinated counter ions.

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

This review aims to reassess the progress, issues and opportunities in the path towards integrating conductive and magnetically bistable coordination polymers and metal–organic frameworks as active components in electronic devices.

Magnetic Molecular Conductors Based on Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the Tris(chlorocyananilato)ferrate(III) Complex

Electrocrystallization of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) organic donor in the presence of the [Fe(ClCNAn)₃]^3- tris(chlorocyananilato)ferrate(III) paramagnetic anion in different stoichiometric ratios and solvent mixtures afforded two different hybrid systems formulated as [BEDT-TTF]₄[Fe(ClCNAn)₃]·3H₂O (1) and [BEDT-TTF]₅[Fe(ClCNAn)₃]₂·2CH₃CN (2) (An = anilato). Compounds 1 and 2 present unusual structures without the typical segregated organic and inorganic layers, where layers of 1 are formed by Λ and Δ enantiomers of the anionic paramagnetic complex together with mixed-valence BEDT-TTF tetramers, while layers of 2 are formed by Λ and Δ enantiomers of the paramagnetic complex together with dicationic BEDT-TTF dimers and monomers. Compounds 1 and 2 show semiconducting behaviors with room-temperature conductivities of ca. 6 × 10^-3 S cm^-1 (ambient pressure) and 1 × 10^-3 S cm^-1 (under applied pressure of 12.1 GPa), respectively, due to strong dimerization between the donors. Magnetic measurements performed on compound 1 indicate weak antiferromagnetic coupling between high-spin Fe^III (S_Fe = 5/2) and mixed-valence radical cation diyads (BEDT-TTF)₂⁺ (S_rad = 1/2) mediated by the anilate ligands, together with an important Pauli paramagnetism typical for conducting systems.

Dysprosium Chlorocyanoanilate-Based 2D-Layered Coordination Polymers

A series of two-dimensional (2D)-layered coordination polymers (CPs) based on the heterosubstituted anilate ligand ClCNAn^2- derived from 3-chloro-6-cyano-2,5-dihydroxybenzoquinone and Dy^III are reported. By changes in the synthetic methods (layering technique, solvothermal or conventional one-pot reactions) and conditions (solvent, concentration, etc.), different types of 2D extended networks could be prepared and structurally characterized. Compounds 1 and 1', two polymorphs with the formula [Dy₂(ClCNAn)₃(DMSO)₆]_n·(H₂O)_x [x = 7 (1), 0 (1')], were prepared by a conventional one-pot reaction and recrystallized at different concentrations. Compound 2, formulated as [Dy₂(ClCNAn)₃(DMF)₆]_n, was prepared by a layering technique, while compound 3, formulated as {(Me₂NH₂)₂[Dy₂(ClCNAn)₄(H₂O)₂]·(DMF)₂·(H₂O)₅}_n, was obtained by a solvothermal method. Compounds 1 and 2 are neutral 2D CPs of the ClCNAn^2- ligand and Dy^III ions, while 3 presents 2D anionic layers of [Dy₂(ClCNAn)₄(H₂O)₂]^2- alternating with cationic layers of Me₂NH₂⁺ ions. These compounds show very diverse networks, with compound 1 forming 2D (8,3) and (4,3) topology with eight- and four-membered rings with square cavities, 1' and 2, respectively, a 2D (6,3) topology with six-membered rings (a rectangular cavity for 1' and a regular hexagonal cavity for 2), and 3 a 2D (4,4) topology with distorted square cavities. In this respect, 1 and 1' represent the first examples of polymorphism in the family of anilate-based CPs. Thermal analysis measurements (differential scanning calorimetry and thermogravimetry) show an exothermic polymorphic transformation from the kinetically stable 1' phase to the thermodynamically stable phase 1. The magnetic behavior of 1-3 very likely indicates depopulation of the m_J levels, while the presence of weak antiferromagnetic coupling between the Dy^III centers mediated by the anilate bridge cannot be excluded.

Pre- and post-synthetic modulation of the ordering temperatures in a family of anilato-based magnets

We report the synthesis and characterization of six novel heterometallic molecule-based 2D magnets with the bromanilato ligand (C₆O₄Br₂^2- = 1,3-dibromo-2,5-dihydroxy-1,4-benzoquinone dianion) and six different benzene derivative molecules. The compounds, formulated as (NBu₄)[MnCr(C₆O₄Br₂)₃]·1.75C₆H₅Br (1), (NBu₄)[MnCr(C₆O₄Br₂)₃]·C₆H₅X with X = Cl (2), I (3) and CH₃ (4) and (NBu₄)[MnCr(C₆O₄Br₂)₃]·2C₆H₅X with X = CN (5) and NO₂ (6), present the classical hexagonal honeycomb-(6,3) lattice with alternating Mn(ii) and Cr(iii) ions. The layers are packed in an eclipsed way along the a direction giving rise to hexagonal channels where the benzene derivative molecules are located with π-π interactions between the benzene and anilato rings. The interlayer space contains the NBu₄⁺ cations needed to compensate the anionic charge of the [Mn^IICr^III(C₆O₄Br₂)₃]^- layers. The Mn-Cr exchange coupling through the bromanilato ligands is antiferromagnetic, leading to a long range ferrimagnetic order in the six compounds with ordering temperatures around 10 K. These ordering temperatures can be slightly modified in the range 9.5-11.4 K by simply changing the benzene-derivative solvent molecule. Here we discuss the possible structural and electronic reasons for this tuning effect of the solvent molecule and the important structural role played by the solvent molecules. We also show that it is possible to exchange the solvent molecules inside the hexagonal channels post-synthetically causing a tiny change in the ordering temperature and coercive field. Furthermore, we also show that it is possible to further change the ordering temperatures by simply removing the solvent molecules by heating the sample at low pressures to obtain a de-solvated phase.

Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures

We report the synthesis and the characterization of six new heterometallic chloranilato-based ferrimagnets formulated as (NBu4)[MnCr(C6O4Cl2)3]·nG with n = 1 for G = C6H5Cl (1), C6H5I (3), and C6H5CH3 (4); n = 1.5 for G = C6H5Br (2) and n = 2 for G = C6H5CN (5) and C6H5NO2 (6); (C6O4Cl2)2− = 1,3-dichloro,2,5-dihydroxy-1,4-benzoquinone dianion. The six compounds are isostructural and show hexagonal honeycomb layers of the type [MnCr(C6O4Cl2)3]− alternating with layers containing the NBu4+ cations. The hexagons are formed by alternating Mn(II) and Cr(III) connected by bridging bis-bidentate chloranilato ligands. The benzene derivative solvent molecules are located in the hexagonal channels (formed by the eclipsed packing of the honeycomb layers) showing π-π interactions with the anilato rings. The six compounds behave as ferrimagnets with ordering temperatures in the range 9.8–11.2 K that can be finely tuned by the donor character of the benzene ring and by the number of solvent molecules inserted in the hexagonal channels. The larger the electron density on the aromatic ring and the larger the number of solvent molecules are, the higher Tc is. The only exception is provided by toluene, where the formation of H-bonds might be at the origin of weaker π-π interactions observed in this compound.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

One-electron oxidation of the compound shown shows no evidence for intervalence charge transfer in the macrocylic ligand radical product. In contrast, related [{Pt(L)}₃(μ₃-ctc˙)]⁺ (H₆ctc = cyclotricatechylene), exhibits class II mixed valency.