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Tuning Charge-State Localization in a Semiconductive Iron(III)–Chloranilate Framework Magnet Using A Redox Active Cation
Abstract
The elucidation of mechanisms to modulate the properties of multifunctional electroactive, conductive and magnetic porous materials is desirable to aid their future application. The synthesis and characterization of a 2D mixed-valence metal-tetraoxolene coordination polymer containing a redox active dication, (PhenQ)[Fe2(can)3]·solvent (1; cann− = deprotonated 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone; PhenQ2+ = 5,6-dihydropyrazino[1,2,3,4-lmn][1,10]-phenanthrolindiium) is reported. The PhenQ2+ cation in 1 introduces additional accessible framework redox states, and effectively directs the localization of ligand valence states. Static and dynamic magnetic susceptibil-ity measurements demonstrated that the DMF solvate, 1b, undergoes spontaneous magnetization below TN = 31 K, with variable-temperature electrical conductivity measurements revealing that 1b is a modest semiconductor with a conductivity of σ295 K = 4.9 × 10−4 S cm−1 (Ea = 0.249(2) eV). In concert, these results demonstrate that introducing non-covalent interactions between anionic metal-tetraoxolene frameworks and redox active cations is an effective method to alter the electronic structure and properties of these porous frameworks. Moreover, they forecast the synthesis of new anionic metal-tetraoxolene compounds with diverse electronic and magnetic properties using this hitherto un-used strategy.
... Solid-state EPR spectra of NKU-123 demonstrated the formation of radicals after exposure to an 808 nm laser or Xe lamp light (Figure 5a and S17). [42,43] To investigate the influence of light-generated radicals on structure of NKU-123, a comparison of the crystal parameters before and after irradiation was performed. The crystal parameters remained essentially unchanged because the generation of radicals only adds additional electron spins, without altering the coordination structure of NKU-123. ...
Solar‐thermal water evaporation is a promising strategy for clean water production, which needs the development of solar‐thermal conversion materials with both high efficiency and high stability. Herein, we reported an ultra‐stable cobalt(II)‐organic assembly NKU‐123 with light‐generated radicals, exhibiting superior photothermal conversion efficiency and high stability. Under the irradiation of 808 nm light, the temperature of NKU‐123 rapidly increases from 25.5 to 215.1 °C in 6 seconds. The solar water evaporator based on NKU‐123 achieves a high solar‐thermal water evaporation rate of 1.442 and 1.299 kg m⁻² h⁻¹ under 1‐sun irradiation with a water evaporation efficiency of 97.8 and 87.9 % for pure water and seawater, respectively. A detailed mechanism study revealed that the formation of light‐generated radicals leads to an increase of spin density of NKU‐123 for enhancing the photothermal effect, which provides insights into the design of highly efficient photothermal materials.
... But the key design lies in the two wings of 1,4-dithiin sulfur units, as these 8πelectron, antiaromatics offer rich reactivities. For example, in various reactions on the ethene and S functions, ring contraction can occur to form the 6e − aromatic thiophenes 38,39 ; moreover, the alternative, 1,3dithiole radical 40,41 product ( Fig. 1) has not been generated from dithiin precursors, but would enrich radical-containing MOF solids [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] . ...
Organic radicals feature unpaired electrons, and these compounds may have applications in biomedical technology and as materials for solar energy conversion. However, unpaired electrons tend to pair up (to form chemical bonds), making radicals unstable and hampering their applications. Here we report an organic radical system that is stable even at 350 °C, surpassing the upper temperature limit (200 °C) observed for other organic radicals. The system reported herein features a sulfur-rich organic linker that facilitates the formation of the radical centers; on the solid-state level, the molecules are crystallized with Eu(III) ions to form a 3D framework featuring stacks of linker molecules. The stacking is, however, somewhat loose and allows the molecules to wiggle and transform into sulfur-stabilized radicals at higher temperatures. In addition, the resulting solid framework remains crystalline, and it is stable to water and air. Moreover, it is black and features strong broad absorption in the visible and near IR region, thereby enhancing both photothermal conversion and solar-driven water evaporation.
... Besides various reactions on the ethene and S functions, ring contraction can occur to form the 6earomatic thiophenes; 26,27 the alternative, 1,3-dithiole radical 28,29 product (Scheme 1) has not been generated from dithiin precursors, but would enrich radical-containing MOF solids. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] The tailorable spatial order of solid state frameworks offers unique opportunity to trap reactive species and to channel reaction pathways. 46-54 Notably, Matsuda and Ma reported spatially isolated carbene sites at the midpoints of the long linkers in a rigid open MOF scaffold, 55 and their higher decomposing temperature (170 K), relative to the control carbene species (80 K; in a frozen matrix of 2methyltetrahydrofuran). Also, similarly anchored -SI and -SH groups on porous MOF grids had been explored to prevent, respectively, disproportionation (into -SS-and I 2 ), 56 and poisoning the Pd centers of the leach-free heterogeneous MOF catalyst. ...
The strong, stable radical signal is forged from two levels of structural design. On the molecular level, we fused terephthalic acid with two 1,4-dithiin (-S-CH = CH-S-) units to afford a polycyclic (3-ring), sulfur-rich linker (TTA) with versatile reactivity— e.g. , for generating stable radicals. On the solid-state level, the linker was crystallized with Eu(III) to form EuTTA as a 3D metal-organic framework (MOF) featuring stacks of TTA molecules along the Eu-carboxyl rod domain. As revealed by single-crystal X-ray diffraction, the slightly loosened packing of the TTA molecules allows wiggle room for uniquely selective thermal reactions: among the repeat of three TTA molecules along the stack, one first transforms into benzodithiophene (by extruding one S atom from each of the dithiin wings) at 230°C, while the other two TTA molecules only react at higher temperatures ( e.g. , 300–350°C)—to form bis(dithiole) and other sulfur-stabilized radicals, as well as covalent crosslinks thereof. Aside from the highest thermal stability observed of its radicals (above 300°C), the black MOF product ( i.e. , EuTTA-350; obtained from heating at 350°C) remain crystalline, and water/air-stable, and features strong broad absorption in the visible and near IR region. As a result, EuTTA-350 achieves highest efficiencies in both photothermal conversion and solar-driven water evaporation among MOF materials, holding promise in solar steam generation.
Six new coordination polymers (CPs) of cadmium based on an extended anilate-type ligand - 4,4'-(1,4-phenylenebis(azanylylidene))bis(3,6-di-tert-butyl-2-hydroxycyclohexa-2,5-dienone) (H2L) have been synthesized. The series of coordination polymers includes [Cd(L)(DMF)] (1), 1D-CP [Cd(L)(dipy-1)]·2DMF (2A)...
Solar‐thermal water evaporation is a promising strategy for clean water production, which need the development of solar‐thermal conversion materials with both high efficiency and high stability. Herein, we reported an ultra‐stable cobalt(II)‐organic assembly NKU‐123 with light‐generated radicals, exhibiting superior photothermal conversion efficiency and high stability. Under the irradiation of 808 nm light, the temperature of NKU‐123 rapidly increases from 25.5 to 215.1 °C in 6 seconds. The solar water evaporator based on NKU‐123 achieves a high solar‐thermal water evaporation rate of 1.442 and 1.299 kg m‐2 h‐1 under 1‐sun irradiation with a water evaporation efficiency of 97.8 and 87.9 % for pure water and seawater, respectively. A detailed mechanism study revealed that the formation of light‐generated radicals leads to an increase of spin density of NKU‐123 for enhancing the photothermal effect, which provides insights into the design of highly efficient photothermal materials.
The ability of molecular switches to reversibly interconvert between different forms promises potential applications at the scale of single molecules up to bulk materials. One type of molecular switch comprises cobalt–dioxolene compounds that exhibit thermally-induced valence tautomerism (VT) interconversions between low spin Co(iii)-catecholate (LS-CoIII-cat) and high spin Co(ii)-semiquinonate (HS-CoII-sq) forms. Two families of these compounds have been investigated for decades but have generally been considered separately: neutral [Co(diox)(sq)(N2L)] and cationic [Co(diox)(N4L)]⁺ complexes (diox = generic dioxolene, N2L/N4L = bidentate/tetradentate N-donor ancillary ligand). Computational identification of promising new candidate compounds prior to experimental exploration is beneficial for environmental and cost considerations but requires a thorough understanding of the underlying thermochemical parameters that influence the switching. Herein, we report a robust approach for the analysis of both cobalt–dioxolene families, which involved a quantitative density functional theory-based study benchmarked with reliable quasi-experimental references. The best-performing M06L-D4/def2-TZVPP level of theory has subsequently been verified by the synthesis and experimental investigation of three new complexes, two of which exhibit thermally-induced VT, while the third remains in the LS-CoIII-cat form across all temperatures, in agreement with prediction. Valence tautomerism in solution is markedly solvent-dependent, but the origin of this has not been definitively established. We have extended our computational approach to elucidate the correlation of VT transition temperature with solvent stabilisation energy and change in dipole moment. This new understanding may inform the development of VT compounds for applications in soft materials including films, gels, and polymers.
Three new cyanide-bridged compounds {[Mn((S,S)-Dpen)]3[Mn((S,S)-Dpen)(H2O)][Mo(CN)7]2·4H2O·4C2H3N}n (1-SS), {[Mn((R,R)-Dpen)]3[Mn((R,R)-Dpen)(H2O)][Mo(CN)7]2·4.5H2O·4C2H3N}n (1-RR), and {[Mn(Chxn)][Mn(Chxn)(H2O)0.8][Mo(CN)7]·H2O·4C2H3N}n (2) (SS/RR-Dpen = (S,S)/(R,R)-1,2-diphenylethylenediamine and Chxn = 1,2-cyclohexanediamine) have been successfully synthesized from the self-assembly reaction of the [MoIII(CN)7]4- unit, the MnII ions, and two chiral bidentate chelating ligands. Single-crystal structure determinations show that compounds 1-SS and 1-RR containing ligands SS/RR-Dpen are enantiomers and crystallize in the chiral space group P21. On the other hand, compound 2 crystallizes in the achiral centrosymmetric space group P1̄ due to the racemization of the SS/RR-Chxn ligands during the growth of the crystals. Despite their different space groups and ligands, all three compounds exhibit similar framework structures consisting of cyano-bridged MnII-MoIII two-dimensional layers separated by the bidentate ligands. The circular dichroism (CD) spectra have further demonstrated the enantiopure character of compounds 1-SS and 1-RR. Magnetic measurements revealed that all three compounds display ferrimagnetic ordering with similar critical temperatures of about 40 K. The chiral enantiomers 1-SS and 1-RR exhibit the magnetic hysteresis loop with a coercive field of about 8000 Oe at 2 K, which is by far the highest for all known MnII-[MoIII(CN)7]4- magnets. Analyses of their structures and magnetic properties indicated that their magnetic properties depend on the anisotropic magnetic interactions between the MnII and MoIII centers, which are closely related to the C-N-M bond angles.
Redox-active tetraoxolene ligands such as 1,4-dihydroxybenzoquinone provide access to a diversity of metal-organic architectures, many of which display interesting magnetic behavior and high electrical conductivity. Here, we take a closer look at how structure dictates physical properties in a series of 1D iron-tetraoxolene chains. Using a diphenyl-derivatized tetraoxolene ligand (H2Ph2dhbq), we show that the steric profile of the coordinating solvent controls whether linear or helical chains are exclusively formed. Despite similar ligand environments, only the helical chain displays temperature-dependent valence tautomerism, switching from (FeII)(Ph2dhbq2-) to (FeIII)(Ph2dhbq3˙-) at temperatures below 203 K. The stabilization of ligand radicals leads to exceptionally strong magnetic exchange coupling (J = -230 ± 4 cm-1). Meanwhile, the linear chains are more amenable to oxidative doping, leading to Robin-Day class II/III mixed-valency and an increase in electrical conductivity by nearly three orders of magnitude. While previous studies have focused on the effects of changing metal and ligand identity, this work highlights how altering the metal-ligand connectivity can be a similarly powerful tool for tuning materials properties.
Since the characterization of the extended coordination network Prussian blue as a mixed‐valence material last century, the phenomenon has continued to be a source of inspiration for understanding fundamental aspects of electron transfer in three‐dimensional coordination space. Over the past 20 years, burgeoning interest in the field of coordination polymers and metal–organic frameworks has delivered a new suite of exquisite materials that have shed further light on structure–function relationships underpinning mixed valency in extended solids. This chapter provides an overview of the historical context for mixed‐valence materials and discusses the very latest developments in the field, along with new avenues for the application of the phenomenon.
Porous crystalline materials (PCMs) have attracted widespread attention due to their high porosity and chemical tunability. To solve the problem of the low electrical conductivity of traditional PCMs, a guest-promoted approach has been developed to impart electrical conductivity, whereas microscopic understanding of this process from experiments is largely lacking. Here we use in-situ electrochemical surface-enhanced Raman spectroscopy (EC-SERS) to investigate the microscopic mechanism of the enhanced electrical conductivity in metal-cyanide frameworks, in Prussian Blue (PB), induced by alkali metal ions. The EC-SERS result demonstrates that the charge is localized around the iron atom in PB and becomes delocalized on the CN bond after insertion of the alkali metal ions, verified by density functional theory (DFT) calculations. The enhanced electrical conductivity of PCMs promoted by the guest via the through-bond mechanism instead of the through-space hopping mechanism in pristine PB, offers a new approach to develop conductive PCMs.
The X-ray structure of the title compound as the monohydrate has been determined and shows a complex structure with two independent molecules each of the compound and water and eleven distinct hydrogen bonding interactions. Its melting point has also been recorded for the first time.
The syntheses and structures of a pair of neutral one-dimensional (1D) Fe-anilate based coordination polymers, Fe(Fan)(4,4'-bipy)2 (Fann- = deprotonated 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone; 4,4'-bipy = 4,4'-bipyridine) and Fe(Clan)(OPPh3)2 (Clann- = deprotonated 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone; OPPh3 = triphenylphosphine oxide), are reported. In the case of Fe(Fan)(4,4'-bipy)2, the Fe centre is in the +2 oxidation state and the Fan ligand is present in its quinoidal, dianionic form. In contrast, the structurally similar Fe(Clan)(OPPh3)2 chain contains Fe centres and chloranilate ligands in oxidation states close to +3 and -3 respectively at low temperature. It is suggested that intrachain π-π interactions aid electron transfer from the Fe centres to the bridging ligands.
By combining 3,6-N-ditriazolyl-2,5-dihydroxy-1,4-benzoquinone (H2trz2An) with NIR-emitting ErIII ions, two different 3D neutral polymorphic frameworks (1a and 1b), differing in the number of uncoordinated water molecules, formulated as [Er2(trz2An)3(H2O)4] n ·xH2O (x = 10, a; x = 7, b), have been obtained. The structure of 1a shows layers with (6,3) topology forming six-membered rings with distorted hexagonal cavities along the bc plane. These 2D layers are interconnected through the N4 atoms of the two pendant arms of the trz2An linkers, leading to a 3D framework, where neighboring layers are eclipsed along the a axis, with hexagonal channels filled with water molecules. In 1b, layers with (6,3) topology in the [101] plane are present, each ErIII ion being connected to three other ErIII ions through bis-bidentate trz2An linkers, forming rectangular six-membered cavities. 1a and 1b are multifunctional materials showing coexistence of NIR emission and field-induced slow relaxation of the magnetization. Remarkably, 1a is a flexible MOF, showing a reversible structural phase transition involving shrinkage/expansion from a distorted hexagonal 2D framework to a distorted 3,6-brickwall rectangular 3D structure in [Er2(trz2An)3(H2O)2] n ·2H2O (1a_des). This transition is triggered by a dehydration/hydration process under mild conditions (vacuum/heating to 360 K). The partially dehydrated compound shows a sizeable change in the emission properties and an improvement of the magnetic blocking temperature with respect to the hydrated compound, mainly related to the loss of one water coordination molecule. Theoretical calculations support the experimental findings, indicating that the slight improvement observed in the magnetic properties has its origin in the change of the ligand field around the ErIII ion due to the loss of a water molecule.
A two-dimensional (2D) mixed-valence iron(II,III) coordination polymer, (Ph4P)[FeIIFeIII(dto)3] (1; Ph4P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), was prepared, and its crystal structure was revealed. Compound 1 exhibited a 2D honeycomb structure and a rigid layered framework, due to a hydrogen bond between the cation and anion layers. The temperature dependence of the magnetic susceptibilities for 1 under high pressure showed a thermal hysteresis loop corresponding to a charge-transfer phase transition, although there was no charge-transfer phase transition under ambient pressure.
Exploiting redox activity in supramolecular frameworks such as coordination polymers, metal-organic frameworks and related nanostructures is of paramount importance both at the molecular level and for their technological applications, since the modulation of their redox states is an emerging strategy to enhance their physical properties. In the plethora of organic linkers, quinone derivatives are excellent redox-active ligands, widely used for various applications such as electrode materials, flow batteries, pseudocapacitors, etc. Benzoquinones undergo a one-electron reversible reduction to form a semiquinone radical species that can be further reduced to form hydroquinone. Remarkably, the quinoid ring can be functionalized with various functional groups making these systems excellent linkers to construct supramolecular frameworks as well as challenging platforms to tune the redox potential and therefore the stability of radical anions and electrochemical performances of the obtained materials. The recent advances on benzoquinone-based 2D CPs/MOFs and related nanostructures are reported, highlighting the extreme versatility of this class of redox-active linkers in tailoring the physical properties of the obtained materials. The current/future potential of these materials in electrochemical and technologically relevant applications will also be envisioned. This journal is
Metal complexes that can exists in two different charge distributions often exhibit dramatic color changes when switched between them. The underlying spectral changes are fundamentally related to the switchable behavior. In valence tautomeric (VT) systems, the transition is a stimulated, reversible intramolecular electron transfer between a metal center and a ligand, while in so-called charge-transfer-induced spin transition (CTIST) systems, also known as electron-transfer-coupled-spin-transition (ETCST), the electron is transferred between two metal centers. We discuss, herein, the relationships between the switchable behavior of these systems and two related optical phenomena: charge transfer and solvatochromism. The insights gained from analyzing these phenomena can illuminate important aspects of VT or CTIST behavior, for example, the energetic relationship between the electromeric forms, or the effects of molecular environment on a VT or CTIST thermal equilibrium. Such insights may assist efforts to employ these compounds as molecular scale components in data storage, sensor and display devices.
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 1via 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.
Metal-organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the "through-bond" approach, continuous chains of coordination bonds between the metal centers and ligands' functional groups create charge transport pathways. In the "extended conjugation" approach, the metals and entire ligands form large delocalized systems. The "through-space" approach harnesses the π-π stacking interactions between organic moieties. The "guest-promoted" approach utilizes the inherent porosity of MOFs and host-guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.
Materials with switchable magnetic and electrical properties may enable future spintronic technologies, and thus hold the potential to revolutionize how information is processed and stored. While reversible switching of magnetic order or electrical conductivity has been independently realized in materials, the ability to simultaneously switch both properties in a single material presents a formidable challenge. Here, we report the 2D manganese benzoquinoid framework (Me4N)2[MnII2(L2–)3] (H2L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone), as synthesized via post-synthetic counterion exchange. This material is paramagnetic above 1.8 K and exhibits an ambient-temperature electrical conductivity of σ295K = 1.14(3) × 10–13 S cm–1 (Ea = 0.74(3) eV). Upon soaking in a solution of sodium naphthalenide and 1,2-dihydroacenaphthylene, this compound undergoes a single-crystal-to-single-crystal (SC-SC) reduction to give Na3(Me4N)2[Mn2L3]. Structural and spectroscopic analyses confirm this reduction to be ligand-based, and as such the anionic framework is formulated as [MnII2(L3–•)3]5–. Magnetic measurements confirm that this reduced material is a permanent magnet below Tc = 41 K and exhibits a conductivity value of σ295K = 2.27(1) × 10⁻⁸ S cm⁻¹ (Ea = 0.489(8) eV), representing a remarkable 200,000-fold increase over the parent material. Finally, soaking the reduced compound in a solution of [Cp2Fe]⁺ affords Na(Me4N)[MnII2(L2–)3] via a SC-SC process, with magnetic and electrical properties similar to those observed for the original oxidized material. Taken together, these results highlight the ability of metal benzoquinoid frameworks to undergo reversible, simultaneous redox switching of magnetic order and electrical conductivity.
Conductive metal-organic frameworks are an emerging class of three-dimensional architectures with degrees of modularity, synthetic flexibility and structural predictability that are unprecedented in other porous materials. However, engendering long-range charge delocalization and establishing synthetic strategies that are broadly applicable to the diverse range of structures encountered for this class of materials remain challenging. Here, we report the synthesis of K x Fe2(BDP)3 (0 ≤ x ≤ 2; BDP2- = 1,4-benzenedipyrazolate), which exhibits full charge delocalization within the parent framework and charge mobilities comparable to technologically relevant polymers and ceramics. Through a battery of spectroscopic methods, computational techniques and single-microcrystal field-effect transistor measurements, we demonstrate that fractional reduction of Fe2(BDP)3 results in a metal-organic framework that displays a nearly 10,000-fold enhancement in conductivity along a single crystallographic axis. The attainment of such properties in a K x Fe2(BDP)3 field-effect transistor represents the realization of a general synthetic strategy for the creation of new porous conductor-based devices.
Tetraoxolene radical-bridged lanthanide SMM systems were prepared for the first time by reduction of the respective neutral compounds. Magnetic measurements reveal the profound influence of the radical center on magnetic behavior. Strong magnetic couplings are revealed in the radical species, which switch on SMM behavior under zero applied field for DyIII and TbIII compounds. HFEPR spectra unravel the contributions of the magnetic coupling and the magnetic anisotropy. For GdIII this results in much more accurate magnetic coupling parameters with respect to bulk magnetic measurements.
The aim of the present work is to highlight the unique role of anilato-ligands, derivatives of the 2,5-dioxy-1,4-benzoquinone framework containing various substituents at the 3 and 6 positions (X = H, Cl, Br, I, CN, etc.), in engineering a great variety of new materials showing peculiar magnetic and/or conducting properties. Homoleptic anilato-based molecular building blocks and related materials will be discussed. Selected examples of such materials, spanning from graphene-related layered magnetic materials to intercalated supramolecular arrays, ferromagnetic 3D monometallic lanthanoid assemblies, multifunctional materials with coexistence of magnetic/conducting properties and/or chirality and multifunctional metal-organic frameworks (MOFs) will be discussed herein. The influence of (i) the electronic nature of the X substituents and (ii) intermolecular interactions i.e., H-Bonding, Halogen-Bonding, π-π stacking and dipolar interactions, on the physical properties of the resulting material will be also highlighted. A combined structural/physical properties analysis will be reported to provide an effective tool for designing novel anilate-based supramolecular architectures showing improved and/or novel physical properties. The role of the molecular approach in this context is pointed out as well, since it enables the chemical design of the molecular building blocks being suitable for self-assembly to form supramolecular structures with the desired interactions and physical properties.
the many thousands of new metal-organic frameworks (MOFs) that are now discovered each year, many possess potential redox activity arising from the constituent metal ions and/or organic ligands, or the guest molecules located within their porous structures. Those redox states that can be accessed via postsynthetic redox modulation often possess distinct physical properties; if harnessed, these provide a basis for applications including microporous conductors, electrocatalysts, energy storage devices and electrochemical sensors, amongst others. This feature article highlights latest developments in experimental, theoretical and computational concepts relevant to redox-active MOFs, including new solid state electrochemical and spectroelectrochemical technqieus that have great utiility in this field. A particular emphasis is on current and emerging trends at the fundamental level which underscore the importance of this promising class of electroactive materials for a wide range of tecnologically- and industrially-relevant
Increasing worldwide energy demands and rising CO2 emissions have motivated a search for new technologies to take advantage of renewables such as solar and wind energies. Redox flow batteries (RFBs) with their high power density, high energy efficiency, scalability (up to MW and MWh), and safety features are one suitable option for integrating such energy sources and overcoming their intermittency. However, resource limitation and high system costs of current RFB technologies impede wide implementation. Here, a total organic aqueous redox flow battery (OARFB) is reported, using low-cost and sustainable methyl viologen (MV, anolyte) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO, catholyte), and benign NaCl supporting electrolyte. The electrochemical properties of the organic redox active materials are studied using cyclic voltammetry and rotating disk electrode voltammetry. The MV/4-HO-TEMPO ARFB has an exceptionally high cell voltage, 1.25 V. Prototypes of the organic ARFB can be operated at high current densities ranging from 20 to 100 mA cm2, and deliver stable capacity for 100 cycles with nearly 100% Coulombic efficiency. The MV/4-HO-TEMPO ARFB displays attractive technical merits and thus represents a major advance in ARFBs.
The isotypic compounds Na2(H2O)24[M2(C6H2O4)3] (M = Mn2+, Cd2+) form infinite net-like layers. The transition metal ion is coordinated octahedrally. The dianion of 2,5-dihydroxy-l,4-benzoquinone, consisting of two allyl systems interlinked by two long C—C distances (152—153 pm), acts as a chelating ligand. Adjacent layers are connected by hydrogen bonds. The stacking sequence of these layers, ••.AAA., leads to channels, which are filled by hydrated Na+ ions. The Na+ ions are coordinated in a distorted octahedral fashion, with neighbouring octahedra sharing common faces. © 1986, Verlag der Zeitschrift für Naturforschung. All rights reserved.
The completion of a crystal structure determination is often hampered by the presence of embedded solvent molecules or ions that are seriously disordered. Their contribution to the calculated structure factors in the least-squares refinement of a crystal structure has to be included in some way. Traditionally, an atomistic solvent disorder model is attempted. Such an approach is generally to be preferred, but it does not always lead to a satisfactory result and may even be impossible in cases where channels in the structure are filled with continuous electron density. This paper documents the SQUEEZE method as an alternative means of addressing the solvent disorder issue. It conveniently interfaces with the 2014 version of the least-squares refinement program SHELXL [Sheldrick (2015). Acta Cryst. C71. In the press] and other refinement programs that accept externally provided fixed contributions to the calculated structure factors. The PLATON SQUEEZE tool calculates the solvent contribution to the structure factors by back-Fourier transformation of the electron density found in the solvent-accessible region of a phase-optimized difference electron-density map. The actual least-squares structure refinement is delegated to, for example, SHELXL. The current versions of PLATON SQUEEZE and SHELXL now address several of the unnecessary complications with the earlier implementation of the SQUEEZE procedure that were a necessity because least-squares refinement with the now superseded SHELXL97 program did not allow for the input of fixed externally provided contributions to the structure-factor calculation. It is no longer necessary to subtract the solvent contribution temporarily from the observed intensities to be able to use SHELXL for the least-squares refinement, since that program now accepts the solvent contribution from an external file (.fab file) if the ABIN instruction is used. In addition, many twinned structures containing disordered solvents are now also treatable by SQUEEZE. The details of a SQUEEZE calculation are now automatically included in the CIF archive file, along with the unmerged reflection data. The current implementation of the SQUEEZE procedure is described, and discussed and illustrated with three examples. Two of them are based on the reflection data of published structures and one on synthetic reflection data generated for a published structure.
The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallographic Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as `a CIF') containing embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to extract the .hkl and .ins files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the determination of absolute structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.
The new computer program SHELXT employs a novel dual-space algorithm to solve the phase problem for single-crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. All space groups in the specified Laue group are tested to find which are consistent with the P1 phases. After applying the resulting origin shifts and space-group symmetry, the solutions are subject to further dual-space recycling followed by a peak search and summation of the electron density around each peak. Elements are assigned to give the best fit to the integrated peak densities and if necessary additional elements are considered. An isotropic refinement is followed for non-centrosymmetric space groups by the calculation of a Flack parameter and, if appropriate, inversion of the structure. The structure is assembled to maximize its connectivity and centred optimally in the unit cell. SHELXT has already solved many thousand structures with a high success rate, and is optimized for multiprocessor computers. It is, however, unsuitable for severely disordered and twinned structures because it is based on the assumption that the structure consists of atoms.
The realization of metal-organic frameworks (MOFs) as electronic conductors is a highly sought after goal, which has the potential to revolutionize the areas of catalysis, solid-state sensors and solar energy conversion devices. To date, the design and synthesis of MOFs that exhibit through-framework conduction has been limited; however, significant interest is now emerging owing to the fascinating prospects for integrating multiple functions. This highlight article introduces the field of conducting nanoporous materials and discusses recent specific examples along with key design features that will underlie future developments in the area.
Metal-organic frameworks (MOFs) are a class of hybrid materials with unique optical and electronic properties arising from rational self-assembly of the organic linkers and metal ions/clusters, yielding myriads of possible structural motifs. The combination of order and chemical tunability, coupled with good environmental stability of MOFs, are prompting many research groups to explore the possibility of incorporating these materials as active components in devices such as solar cells, photodetectors, radiation detectors, and chemical sensors. Although this field is only in its incipiency, many new fundamental insights relevant to integrating MOFs with such devices have already been gained. In this review, we focus our attention on the basic requirements and structural elements needed to fabricate MOF-based devices and summarize the current state of MOF research in the area of electronic, opto-electronic and sensor devices. We summarize various approaches to designing active MOFs, creation of hybrid material systems combining MOFs with other materials, and assembly and integration of MOFs with device hardware. Critical directions of future research are identified, with emphasis on achieving the desired MOF functionality in a device and establishing the structure-property relationships to identify and rationalize the factors that impact device performance.
A 2-D coordination framework, (NEt4)2[Fe2(fan)3] (1·5(acetone); H2fan = 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone), was synthesized and structurally characterized. The compound is structurally analogous to a formerly elucidated framework, (NEt4)2[Fe2(can)3] (H2can = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone), and adopts a 2-D (6,3) topology with the symmetrical stacking of [Fe2(fan)3]2- sheets that are held in position by the NEt4+ cations between the sheets. The investigation of the dc and ac magnetic properties of 1·5(acetone) revealed ferromagnetic ordering behavior and slow magnetization relaxation, as evinced from ac susceptibility measurements. Furthermore, the exposure of 1·5(acetone) to air led to the formation of a heptahydrate 1·7H
2
O which displayed distinct magnetic properties. The study of the redox state and extent of delocalization in 1·5(acetone) was undertaken via crystallography, in combination with Mössbauer and vis-NIR spectroscopy, to reveal the mixed-valence and delocalized nature of the as-synthesized material. As a result, the conductivity studies conducted on a pressed pellet showed a relatively high conductivity of 1.8 × 10-2 S cm-1 (300 K). In order to compare structurally related anilate-based structures, a relationship among the redox state, spectroscopic properties, and electronic properties was elucidated in this work. A preliminary investigation of 1·5(acetone) as a candidate anode material in lithium ion batteries revealed a high reversible capacity of 676.6 mAh g-1 and high capacity retention.
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 iron-semiquinoid framework materials, (H2NMe2)2Fe2(Cl2 dhbq)3 (1) and (H2NMe2)4Fe3(Cl2 dhbq)3(SO4)2 (Cl2 dhbqn- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO
4
), are shown to possess electrochemical capacities of up to 195 mAh/g. Employing a variety of spectroscopic methods, we demonstrate that these exceptional capacities arise from a combination of metal- and ligand-centered redox processes, a result supported by electronic structure calculations. Importantly, similar capacities are not observed in isostructural frameworks containing redox-inactive metal ions, highlighting the importance of energy alignment between metal and ligand orbitals to achieve high capacities at high potentials in these materials. Prototype lithium-ion devices constructed using 1 as a cathode demonstrate reasonable capacity retention over 50 cycles, with a peak specific energy of 533 Wh/kg, representing the highest value yet reported for a metal-organic framework. In contrast, the capacities of devices using 2-SO
4
as a cathode rapidly diminish over several cycles due to the low electronic conductivity of the material, illustrating the nonviability of insulating frameworks as cathode materials. Finally, 1 is further demonstrated to access similar capacities as a sodium-ion or potassium-ion cathode. Together, these results demonstrate the feasibility and versatility of metal-organic frameworks as energy storage materials for a wide range of battery chemistries.
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 FeIII (SFe = 5/2) and mixed-valence radical cation diyads (BEDT-TTF)2+ (Srad = 1/2) mediated by the anilate ligands, together with an important Pauli paramagnetism typical for conducting systems.
The generation of the paramagnetic radical (Cl2An³⁻) in the framework-edge moieties of an Fe-based two-dimensional honeycomb framework with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate (Cl2Anⁿ⁻), (H3O)2(phz)3[Fe2(Cl2An)3] (phz = phenazine), allowed the formation of long-range magnetic correlations over the network with a relative high magnetic phase transition temperature (Tc) of 128 K. The original compound is a paramagnet with a diamagnetic Cl2An²⁻ linker that significantly separates paramagnetic FeII ions (S = 2), where the [(H3O)2(phz)3]²⁺ cation is located between layered frameworks. Post-synthetic electron-doping, i.e., reduction in the solid state, using a lithium-ion battery (LIB) system, finally produced (Li⁺)3(H3O⁺)2(phz)3[(Fe²⁺)2(Cl2An³⁻)3], which demonstrated a drastic magnetic change.
Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(-tetraoxolene)] (RE = Y and Dy; HBpz3– = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2–; 1-RE) or bromaniliate (ba2–; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3– ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveals a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(-ba•)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied dc field. Exchange coupling between the Dy(III) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium complexes.
Considerable interest lies on organic materials owing to their exceptional performance in electronic and optoelectronic applications. Among these, electrochromic materials (EC) that can be switched between a distinct color to bleached state exhibiting high contrast, multicolor and improved long-term stability seems to be attractive in the fabrication of an electrochromic device (ECD). In particular, ionic materials have persistent attention in the long-run owing to their tunable optical and electronic properties. 4,4ʹ-bipyridinium salts, commonly acknowledged as viologens (V2+), are a well-recognized class of electrochromic materials that exhibit three reversible redox states, viz., V2+ (dication, pale yellow colored/colorless) ↔ V+•(radical cation, violet/blue/green) ↔ V0(neutral, colorless). The electrochromic properties of these materials can be modulated by varying the nitrogen substituents on the pyridyl “N” and besides these varying the counter ions with specific functionalities is shown to enhance their electrochromic behavior such as switching time, cycling stability and device performance. Though the ECDs based on viologens are regarded for their low operational voltages, they exhibit certain demerits such as low cycle life and poor efficiency of the device in the long run. Extensive efforts have been made to fine-tune the EC properties of viologens either through alteration or by adding suitable electrochromic counter electrode materials which include incorporation of conducting polymers in the device set up and/or addition of complementary redox species. Optimization of the device parameters have shown that the addition of such external agents have positive effect on the overall device performance. This review describes recent developments on the synthesis of viologen based electrochromes with co-redox species and their ECD performance.
Viologens, functional organic materials composed of conjugated bi-/multi-pyridyl groups, have attracted much attention for decades owing to their properties such as unique electrochromic and radical-rich features under redox manipulation, biosensitive properties, ionic and localized conjugation. Considerable efforts have been devoted to synthesize novel viologen-based functional materials to study their properties and develop applications. So far, viologens and viologen-based materials have been widely applied in many fields, including electrochromism, molecular machines, memory devices, gas storage and separation, energy storage, and biochemistry. Rapid development of functional materials in the 21st century has inspired extensive integration of viologens into many types of novel and famous materials (e.g., host–guest supramolecules, porous polymers, and metal-organic frameworks), and therefore, viologens seem no longer a key player. However, the features of viologens (or bi-/multi-pyridyl-based materials) have always dominated the properties of target functional materials and long been neglected by researchers. In order to provide an overview of this field, this perspective paper focuses on reasonable design and synthesis of viologen-based functional materials and their representative applications. The remaining challenges and future potential applications are also discussed.
The mixed-valence FeIIFeIII 2D coordination polymer formulated as [TAG][FeIIFeIII(ClCNAn)3]·(solvate) 1 (TAG = tris(amino)-guanidinium, ClCNAn2- = chloro-cyanoanilate dianionic ligand) crystallized in the polar trigonal space group P3. In the solid state structure, determined both at 150 K and 10 K, anionic 2D honeycomb layers [FeIIFeIII(ClCNAn)3]- establish in the ab plane, with intralayer metal-metal distance of 7.860 Å, alternating with cationic layers of TAG. The similar Fe-O distances suggest electron delocalization and an average oxidation state of +2.5 for each Fe center. The cation imposes its C3 symmetry to the structure and engages in intermolecular N-H···Cl hydrogen bonding with the ligand. Magnetic susceptibility characterization indicates magnetic ordering below 4 K and the presence of a hysteresis loop at 2 K with a coercive field of 60 Oe. Mössbauer measurements are in agreement with the existence of Fe(+2.5) ions at RT and statistic charge localization at 10 K. The compound shows semiconducting behavior with the in-plane conductivity of 2.10-3 S/cm three orders of magnitude higher than the perpendicular one. A small-polaron hopping model has been applied to a series of oxalate type FeIIFeIII 2D coordination polymers, providing clear explanation on the much higher conductivity of the anilate based systems than the oxalate ones.
A novel tetraoxolene‐bridged Fe two‐dimensional honeycomb layered compound, (NPr4)2[Fe2(Cl2An)3]·2(Acetone)·H2O (1), where Cl2Ann‐ is 2,5‐dichloro‐3,6‐dihydroxy‐1,4‐benzoquinonate and NPr4+ is the tetrapropylammonium cation, was synthesized. Compound 1 revealed a thermally induced valence tautomeric transition at T1/2 = 236 K (cooling)/237 K (heating) between Fem+ (m = 2 or 3) and Cl2Ann‐ (n = 2 or 3), inducing valence modulations of [FeIIHSFeIIIHS(Cl2An2−)2(Cl2An*3−)]2− at T > T1/2 ⇌ [FeIIIHSFeIIIHS(Cl2An2−)(Cl2An*3−)2]2− at T < T1/2. Even in a two‐dimensional network structure, the low‐temperature phase [FeIIIHSFeIIIHS(Cl2An2−)(Cl2An*3−)2]2− valence set can be regarded as a magnetic chain‐knit network, where ferrimagnetic Δ and Λ chains of [FeIIIHS(Cl2An*3−)]∞ are alternately linked by the diamagnetic Cl2An2−, exhibiting slow magnetization behavior attributed to the single‐chain magnet at lower temperatures.
Metal–organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe(1,2,3-triazolate)2(BF4)x (tri– = 1,2,3-triazolate; x = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly-conductive iron(II) framework Fe(tri)2, and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(1,2,3-triazolate)2(BF4)0.33 , displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of eight orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal–organic frameworks. Detailed characterization of Fe(tri)2 and the Fe(1,2,3-triazolate)2(BF4)x materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV-vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin FeII/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized FeII/III species in Fe(1,2,3-triazolate)2(BF4)x at 290 K, one of the first such observations for a metal–organic framework. The electronic structure of valence-pure Fe(tri)2 and the charge transport mechanism and electronic structure of mixed-valence Fe(1,2,3-triazolate)2(BF4)x frameworks are discussed in detail.
Partial oxidation of an iron-tetrazolate metal-organic framework (MOF) upon exposure to ambient atmosphere yields a mixed-valence material with single crystal conductivities tunable over five orders of magnitude and exceeding 1 S/cm, the highest for a three-dimensionally connected MOF. Variable temperature conductivity measurements reveal a small activation energy of 160 meV. Electronic spectroscopy indicates a decreased bandgap upon air exposure and corroborates intervalence charge transfer between Fe2+ and Fe3+ centers. These findings are consistent with low-lying Fe3+ defect states predicted by electronic band structure calculations and demonstrate that inducing metal-based mixed valency is a powerful strategy to-wards realizing high and systematically tunable electrical conductivity in MOFs.
We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally-induced metal-ligand electron transfer. Reaction of FeCl3 with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) in the presence of (NMe4)Cl gave the compound (NMe4)2[LFeCl2]. A combined analysis of variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L²⁻)FeII units to one with (L3−•)FeIII upon decreasing temperature, with a transition temperature of T1/2 = 213 K. Dc magnetic susceptibility measurements show strong metal-radical magnetic coupling within the chain below this transition, with a coupling constant of J = −81 cm⁻¹. Finally, ac magnetic susceptibility reveal the presence of slow magnetic relaxation, with a relaxation barrier of Δτ = 55 cm⁻¹. To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet.
The isostructural, two-dimensional metal–organic frameworks (H2NMe2)2M2(Cl2dhbq)3 (M = Ti, V; Cl2dhbqⁿ⁻ = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) and (H2NMe2)1.5Cr2(dhbq)3 (dhbqⁿ⁻ = deprotonated 2,5-dihydroxybenzoquinone) are synthesized and investigated by spectroscopic, magnetic, and electrochemical methods. The three frameworks exhibit substantial differences in their electronic structures, and the bulk electronic conductivities of these phases correlate with the extent of delocalization observed via UV–vis–NIR and IR spectroscopies. Notably, substantial metal–ligand covalency in the vanadium phase results in the quenching of ligand-based spins, the observation of simultaneous metal- and ligand-based redox processes, and a high electronic conductivity of 0.45 S/cm. A molecular orbital analysis of these materials and a previously reported iron congener suggests that the differences in conductivity can be explained by correlating the metal–ligand energy alignment with the energy of intervalence charge transfer transitions, which should determine the barrier to charge hopping in the mixed-valence frameworks.
Extending the conjugation of viologen by a planar thiazolo[5,4-d]thiazole (TTz) framework and functionalizing the pyridinium with hydrophilic ammonium group yielded a highly water-soluble "π-conjugation extended viologen", 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium) tetrachloride, [(NPr)2TTz]Cl4, as a novel two-electron storage anolyte for AORFB applications. Its physical and electrochemical properties were systematically investigated. The [(NPr)2TTz]Cl4 / NMe-TEMPO AORFB enables a 1.44 V battery voltage with 53.7 Wh/L theoretical energy density and delivered 70 % EE and 99.97 % capacity retention per cycle battery stability.
The design of molecular magnets using the “porosity” concept seen in metal-organic frameworks (MOFs) is a unique direction for potential applications such as magnetic sensors, switches, and non-volatile electromagnets. In addition, the strategy of “magnet + porosity” could allow for the creation of post-synthesized magnets often exhibiting a higher magnetic phase transition temperature (Tc) than a self-assembled magnet. A class of paramagnetic MOFs with electron-acceptor ligands, which can accept electrons from an appropriate donor or electrode to form organic radicals, shows great promise, because its magnetic phase stability is tuned through an external electron-filling control. Here, we demonstrate the electrochemical switching of non-volatile magnetic phases in a well-known honeycomb-layer ferrimagnet (NBu4)[MnIICrIII(Cl2An)3] (H2Cl2An = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) using a Li-ion battery (LIB) system, in which Li+ ions and electrons are simultaneously inserted into or extracted from the material. Ferrimagnetic phase stability is reversibly modulated using in situ LIB discharge/charge cycles.
The fundamentally important phenomenon of mixed valency has been discussed in detail over the past 50 years, predominantly in the context of dinuclear complexes, which are used as model systems for understanding electron delocalization in more complex biological and physical systems. Very recently, mixed valency has been shown to be an important mechanism for charge transfer, leading to delocalization and conductivity in two- and three-dimensional framework materials such as metal-organic frameworks and related systems including covalent organic frameworks and semicrystalline semiconducting metal-organic graphenes. This Viewpoint provides a current perspective on the field of mixed-valence frameworks, where the property is either intrinsic or generated postsynthetically via an external stimulus. Aspects of the spectroscopy and applications of these materials are also discussed, highlighting the future potential for exploiting mixed valency in extended solid-state systems.
The two-dimensionally connected metal-organic frameworks (MOFs) Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene) are bulk electrical conductors and exhibit ultraviolet-photoelectron spectroscopy (UPS) signatures expected of metallic solids. Electronic band structure calculations confirm that in both materials the Fermi energy lies in a partially filled delocalized band. Together with additional structural characterization and microscopy data, these results represent the first report of metallic behavior and permanent porosity coexisting within a metal-organic framework.
Organic materials have recently gained considerable attention for electronic applications, improving performance and sustainability to current technologies. Commercialized metal-based systems are generally expensive, toxic and difficult to recycle, however organic materials offer promising solutions. Viologens, N,N' di-quaternized bipyridyl salts, are a well-studied species exhibiting three reversible redox states, possessing valuable electrochromic and electron-accepting properties. These properties can be fine-tuned through synthesis by altering the nitrogen substituents and various counteranions. Currently, viologens have become of great interest as functional materials in a wide array of applications; a few to name include electrochromic devices, molecular machines, and organic batteries. This review highlights representative recent work and advances towards utilizing viologens in practical applications that currently compete with metal-based technologies. Additionally, modified viologens that can be further fine-tuned will be discussed.
The synthesis, structure, and characterization of a pair of coordination networks of composition (NBu4)2[M2(fan)3] (fan = fluoranilate; M = Fe, Zn) is described. In each case there are two interpenetrating enantiomeric networks in which octahedral metal centers are linked by fluoranilate ligands. While the two compounds are structurally similar, there are significant differences in the properties of the two materials that can be attributed to the mixed valency in the semiconducting compound (NBu4)2[Fe2(fan)3].
We report the synthesis, magnetic properties, electrical conductivity and delamination into thin nano-sheets of two anilato-based Fe(II)/Fe(III) mixed valence 2D-MOFs. Compounds [(H3O)(H2O)(phenazine)3][FeIIFeIII(C6O4X2)3]·12H2O; X = Cl (1) and Br (2) present a honey-comb layered structure with an eclipsed packing that generates hexagonal channels containing the water molecules. Both compounds show ferrimagnetic ordering at ca. 2 K coexisting with electrical conductivity (with room temperature conductivities of 0.06 and 0.004 S/cm). Changing the X group from Cl (1) to Br (2) leads to a decrease in the ordering temperature and in the room temperature conductivity that can be correlated with the decrease of the electronegativity of X. Despite the ionic charge of the anilato-based layers, these MOFs can be easily delaminated in thin nano-sheets with a few monolayers thickness.
In the presence of the Et4N⁺ cation the chloranilate dianion (can²⁻) associates with a range of divalent cations, M²⁺, to yield an isomorphous series of crystalline compounds of composition (Et4N)2[M2(can)3] (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn). The fluoranilate dianion (fan²⁻) likewise affords the closely related (Et4N)2[Zn2(fan)3]. The structures of (Et4N)2[Zn2(can)3], (Et4N)2[Fe2(can)3], and (Et4N)2[Zn2(fan)3] were determined by single crystal X-ray diffraction. Powder X-ray diffraction indicates that all the members of the can²⁻ series are isomorphous. The structure of (Et4N)2[Zn2(fan)3] is very closely related to the structures of the can²⁻ compounds. The [M2(can)3²⁻]n component is present as chicken-wire-like sheets with (6,3) topology. The Et4N⁺ cation binds sheet to sheet and aligns them so that the large holes within the sheets are arranged one above another, thereby generating spacious channels running perpendicular to the sheets. The solvent molecules present in the channels are ill-defined and easily removed. The (Et4N)2[M2(can)3] structure remains intact after desolvation. The void spaces are calculated to be ∼39% in the case of the can²⁻ compounds and ∼43% in (Et4N)2[Zn2(fan)3]. Substantial amounts of CO2 are sorbed at 273 K by (Et4N)2[Zn2(can)3] and (Et4N)2[Zn2(fan)3]. Spectroscopic evidence supports the presence of at least some of the chloranilate in the radical trianion form in (Et4N)2[Fe2(can)3].
We report the magnetism and conductivity for a redox isomeric pair of iron-quinoid metal-organic frameworks (MOFs). The oxidized isomer, (Me2NH2)2[Fe2L3]·2H2O·6DMF (LH2 = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) was previously shown to magnetically order below 80 K in its solvated form, with the ordering temperature decreasing to 26 K upon desolvation. Here, we demonstrate this compound to exhibit electrical conductivity values up to σ = 1.4(7) × 10(-2) S/cm (Ea = 0.26(1) cm(-1)) and 1.0(3) × 10(-3) S/cm (Ea = 0.19(1) cm(-1)) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of Cp2Co, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (Cp2Co)1.43(Me2NH2)1.57[Fe2L3]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [Fe(III)2(L(3-•))3](3-). Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal-organic radical coupling to give a magnetically ordered phase below Tc = 105 K, one of the highest reported ordering temperatures for a MOF. This high ordering temperature is significantly increased relative to the oxidized compound, and stems from the overall increase in coupling strength afforded by an additional organic radical. In line with the high critical temperature, the new MOF exhibits magnetic hysteresis up to 100 K, as revealed by variable-field measurements. Finally, this compound is electrically conductive, with values up to σ = 5.1(3) × 10(-4) S/cm with Ea = 0.34(1) eV. Taken together, these results demonstrate the unique ability of metal-quinoid MOFs to simultaneously exhibit both high magnetic ordering and high electrical conductivity.
The development of metal-organic frameworks (MOFs) as microporous electronic conductors is an exciting research frontier that has the potential to revolutionize a wide range of technologically and industrially relevant fields, from catalysis to solid-state sensing and energy-storage devices, among others. After nearly two decades of intense research on MOFs, examples of intrinsically conducting MOFs remain relatively scarce; however, enormous strides have recently been made. This article briefly reviews the current status of the field, with a focus on experimental milestones that have shed light on crucial structure-property relationships that underpin future progress. Central to our discussion are a series of design considerations, including redox-matching, donor-acceptor interactions, mixed valency, and π-interactions. Transformational opportunities exist at both fundamental and applied levels, from improved measurement techniques and theoretical understanding of conduction mechanisms to device engineering. Taken together, these developments will herald a new era in advanced functional materials.
Owing to their outstanding structural, chemical, and functional diversity, metal-organic frameworks (MOFs) have attracted considerable attention over the last two decades in a variety of energy-related applications. Notably missing among these, until recently, were applications that required good charge transport coexisting with porosity and high surface area. Although most MOFs are electrical insulators, several materials in this class have recently demonstrated excellent electrical conductivity and high charge mobility. Herein we review the synthetic and electronic design strategies that have been employed thus far for producing frameworks with permanent porosity and long-range charge transport properties. In addition, key experiments that have been employed to demonstrate electrical transport, as well as selected applications for this subclass of MOFs, will be discussed.
The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with FeII affords the solid (Me2NH2)2[Fe2L3]·6DMF·2H2O. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of FeIII centers with mixed-valence ligands of the form (L3)8- that result from a spontaneous electron-transfer from FeII to L2-. Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m2/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally-characterized extended solid that contains semiquinone radical ligands.
A three-dimensional network solid composed of Fe(III) centers and paramagnetic semiquinoid linkers, (NBu4)2Fe(III)2(dhbq)3 (dhbq(2-/3-) = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone), is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298 K, one of the highest values yet observed for a metal-organic framework. The origin of this electronic conductivity is determined to be ligand mixed-valency, which is characterized using a suite of spectroscopic techniques, slow-scan cyclic voltammetry, and variable-temperature conductivity and magnetic susceptibility measurements. Importantly, UV-Vis-NIR diffuse reflectance measurements reveal the first observation of Robin-Day Class II/III mixed valency in a metal-organic framework. Pursuit of stoichiometric control over the ligand redox states resulted in synthesis of the reduced framework material Na0.9(NBu4)1.8Fe(III)2(dhbq)3. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are investigated and correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal-semiquinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in metal-organic frameworks.
A tristable [2]catenane, composed of a macrocyclic polyether incorporating 1,5-dioxynaphthalene (DNP) and tetrathia-fulvalene (TTF) units, along with a 4,4'-bipyridinium (BIPY?+) radical cation as three very different potential recognition sites, interlocked mechanically with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), was synthesized by donor-acceptor templation, employing a ?threading-followed-by-cyclization? approach. In this catenane, the movement of the CBPQT4+ ring in its different redox states between these three potential recognition sites, and the corresponding color changes, are achieved by tuning external redox potentials. In the starting state, where no external potential is applied, the ring encircles the TTF unit, displaying a green color. Upon oxidation of the TTF unit, the CBPQT4+ ring moves to the DNP unit, producing a red color. Finally, if all the BIPY2+ units are reduced to BIPY?+ radical cations, the resulting CBPQT2(?+) diradical dication will migrate to the BIPY?+ unit, resulting in a purple color. These readily switchable electrochromic properties render the [2]catenane attractive for use in electro-optical devices.
Metal-organic frameworks (MOFs) are crystalline nanoporous materials comprised of organic electron donors linked to metal ions by strong coordination bonds. Applications such as gas storage and separations are currently receiving considerable attention, but if the unique properties of MOFs could be extended to electronics, magnetics, and photonics, the impact on material science would greatly increase. Recently, we obtained “emergent properties,” such as electronic conductivity and energy transfer, by infiltrating MOF pores with “guest” molecules that interact with the framework electronic structure. In this Perspective, we define a path to emergent properties based on the Guest@MOF concept, using zinc-carboxylate and copper-paddlewheel MOFs for illustration. Energy transfer and light harvesting are discussed for zinc carboxylate frameworks infiltrated with triplet-scavenging organometallic compounds and thiophene- and fullerene-infiltrated MOF-177. In addition, we discuss the mechanism of charge transport in TCNQ-infiltrated HKUST-1, the first MOF with electrical conductivity approaching conducting organic polymers. These examples show that guest molecules in MOF pores should be considered not merely as impurities or analytes to be sensed but also as an important aspect of rational design.
Reaction of FeCl2 and H4DSBDC (2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe2(DSBDC), an analog of M2(DOBDC) (MOF-74, DOBDC(4-) = 2,5-dihydroxybenzene-1,4-dicarboxylate). The bulk electrical conductivity of both Fe2(DSBDC) and Fe2(DOBDC) is approximately six orders of magnitude higher than that of the Mn(2+) analogs, Mn2(DEBDC) (E = O, S). While the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe(2+) β-spin electron. These results provide important insight for the rational design of conductive metal-organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials.
Electrocrystallization of enantiopure (S,S,S,S)- and (R,R,R,R)-tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) donors, as well as the racemic mixture, in the presence of potassium cations and the tris(chloranilato)ferrate(III) [Fe(Cl2An)3](3-) paramagnetic anion afforded a complete series of chiral magnetic molecular conductors formulated as β-[(S,S,S,S)-TM-BEDT-TTF]3PPh4[K(I)Fe(III)(Cl2An)3]·3H2O (1), β-[(R,R,R,R)-TM-BEDT-TTF]3PPh4[K(I)Fe(III)(Cl2An)3]·3H2O (2), and β-[(rac)-TM-BEDT-TTF]3PPh4[K(I)Fe(III)(Cl2An)3]·3H2O (3). Compounds 1-3 are isostructural and crystallize in triclinic space groups (P1 for 1 and 2, P-1 for 3) showing a segregated organic-inorganic crystal structure, where anionic honeycomb layers obtained by self-assembling of the Λ and Δ enantiomers of the paramagnetic complex with potassium cations alternate with organic layers where the chiral donors are arranged in the β packing motif. Compounds 1-3 show a molecular packing strongly influenced by the topology of the inorganic layers and behave as molecular semiconductors with room-temperature conductivity values of ca. 3 × 10(-4) S cm(-1). The magnetic properties are dominated by the paramagnetic S = 5/2 [Fe(Cl2An)3](3-) anions whose high-spin character is confirmed by magnetic susceptibility measurements. The correlation between crystal structure and conducting behavior has been studied by means of tight-binding band structure calculations which support the observed conducting properties.
The burgeoning field of metal–organic frameworks (MOFs) has been marked by numerous key advances over the past two decades. An emerging theme is the incorporation of radical species which may be ligated as an integral structural component of, or simply appended to, the material, or else merely a guest within it. Radical incorporation has been shown to endow MOFs with a plethora of unique and fascinating magnetic, electronic and optical properties, paving the way towards their application as spin probes, and in magnetic/electronic devices, chemical sensing and molecular recognition. In view of the rapid growth of the literature in the area, this review highlights progress over the past three years (since 2011), and seeks to uncover promising ideas that will underscore future advancements at both the fundamental and applied levels.
Electrocrystallization of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in the presence of the tris(chloranilato)ferrate(III) [Fe(Cl2An)3](3-) paramagnetic chiral anion in different stoichiometric ratios and solvent mixtures afforded three different hybrid systems formulated as [BEDT-TTF]3[Fe(Cl2An)3]·3CH2Cl2·H2O (1), δ-[BEDT-TTF]5[Fe(Cl2An)3]·4H2O (2), and α‴-[BEDT-TTF]18[Fe(Cl2An)3]3·3CH2Cl2·6H2O (3). Compound 1 presents an unusual structure without the typical alternating organic and inorganic layers, whereas compounds 2 and 3 show a segregated organic-inorganic crystal structure where layers formed by Λ and Δ enantiomers of the paramagnetic complex, together with dicationic BEDT-TTF dimers, alternate with layers where the donor molecules are arranged in the δ (2) and α‴ (3) packing motifs. Compound 1 behaves as a semiconductor with a much lower conductivity due to the not-layered structure and strong dimerization between the fully oxidized donors, whereas 2 and 3 show semiconducting behaviors with high room-temperature conductivities of ca. 2 S cm(-1) and 8 S cm(-1), respectively. The magnetic properties are dominated by the paramagnetic S = 5/2 [Fe(Cl2An)3](3-) anions whose high-spin character is confirmed by electron paramagnetic resonance and magnetic susceptibility measurements. The correlation between crystal structure and conductivity behavior was studied by means of tight-binding band structure calculations, which support the observed conducting properties.