Extrinsically conducting MOFs: guest-promoted enhancement of electrical conductivity, thin film fabrication and applications

Conductive metal-organic frameworks are of current interest in chemical science because of their applications in chemiresistive sensing, electrochemical energy storage, electrocatalysis, etc. Different strategies have been employed to design conductive frameworks. In this review, we discuss the influence of different types of guest species incorporated within the pores or channels of metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) to generate charge transfer pathways and modulate their electrical conductivity. We have classified dopants or guest species into three different categories: (i) metal-based dopants, (ii) molecule and molecular entities and (iii) organic conducting polymers. Different types of metal ions, metal nano-clusters and metal oxides have been used to enhance electrical conductivity in MOFs. Metal ions and metal nano-clusters depend on the hopping process for efficient charge transfer whereas metal-oxides show charge transport through the metal-oxygen pathway. Several types of molecules or molecular entities ranging from neutral TCNQ, I2, and fullerene to ionic methyl viologen, organometallic like nickelcarborane, etc. have been used. In these cases, the charge transfer process varies with the guest species. When organic conducting polymers are the guest, the charge transport occurs through the polymer chains, mostly based on extended π-conjugation. Here we provide a comprehensive and critical review of these strategies to add electrical conductivity to the, in most cases, otherwise insulating MOFs and PCPs. We point out the guest encapsulation process, the geometry and structure of the resulting host-guest complex, the host-guest interactions and the charge transport mechanism for each case. We also present the methods for thin film fabrication of conducting MOFs (both, liquid-phase and gas-phase based methods) and their most relevant applications like electrocatalysis, sensing, charge storage, photoconductivity, photocatalysis,… We end this review with the main obstacles and challenges to be faced and the appealing perspectives of these 21st century materials.

Recent Advances in Design and Synthesis of Diselenafulvenes, Tetraselenafulvalenes, and Their Tellurium Analogs and Application for Materials Sciences

The first organic metals were obtained based on tetrathiafulvalene. The most significant advance in the field of organic metals was the discovery of superconductivity. The first organic superconductors were obtained based on tetramethyltetraselenafulvalene. These facts demonstrate great importance of tetraselenafulvalenes and their precursors, diselenafulvenes, for materials sciences. Derivatives of 1,4-diselenafulvene and 1,4,5,8-tetraselenafulvalene are useful building blocks for organic synthesis and donor units for the preparation of charge-transfer complexes and radical ion salts, the construction of organic metals, superconductors, organic Dirac materials, semiconductors, ferromagnets, and other conductive materials. This review covers the literature on the design, synthesis, and application of 1,4,5,8-tetraselenafulvalenes and 1,4-diselenafulvenes and their tellurium analogs over the past 15–20 years. These two classes of compounds are interconnected, since the main part of methods for the synthesis of tetraselenafulvalenes is based on the diselenafulvene derivatives as starting compounds. Special attention is paid to the development of novel efficient synthetic approaches to these classes of compounds. Conducting properties and distinguishing features of materials based on tetraselenafulvalenes and their tellurium analogs as well as examples of materials with high conductivity are discussed.

The Peter Day Series of Magnetic (Super)Conductors

Here, we review the different series of (super)conducting and magnetic radical salts prepared with organic donors of the tetrathiafulvalene (TTF) family and oxalato-based metal complexes (ox = oxalate = C2O42−). Although most of these radical salts have been prepared with the donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF = ET), we also include all the salts prepared with other TTF-type donors such as tetrathiafulvalene (TTF), tetramethyl-tetrathiafulvalene (TM-TTF), bis(ethylenediseleno)tetrathiafulvalene (BEST), bis(ethylenedithio)tetraselenafulvalene (BETS) and 4,5-bis((2S)-2-hydroxypropylthio)-4′,5′-(ethylenedithio)tetrathiafulvalene (DMPET). Most of the oxalate-based complexes are monomers of the type [MIII(C2O4)3]3−, [Ge(C2O4)3]2− or [Cu(C2O4)2]2−, but we also include the reported salts with [Fe2(C2O4)5]4− dimers, [MII(H2O)2[MIII(C2O4)3]2]4− trimers and homo- or heterometallic extended 2D layers such as [MIIMIII(C2O4)3]− and [MII2(C2O4)3]2−. We will present the different structural families and their magnetic properties (such as diamagnetism, paramagnetism, antiferromagnetism, ferromagnetism and even long-range magnetic ordering) that coexist with interesting electrical properties (such as semiconductivity, metallic conductivity and even superconductivity). We will focus on the electrical and magnetic properties of the so-called Day series formulated as β″-(BEDT-TTF)4[A+MIII(C2O4)3]·G, which represents the largest family of paramagnetic metals and superconductors reported to date, with more than fifty reported examples.

The guanidinium t-diaqua-bis(oxalato)chromate(III) dihydrate complex: synthesis, crystal structure, EPR spectroscopy and magnetic properties

A new salt (CH6N3)[t-Cr(C2O4)2(H2O)2]·2H2O (1) (CH6N3 + = guanidinium cation) has been synthesized and characterized by single-crystal X-ray diffraction, FT-IR and UV–Vis spectroscopies, elemental and thermogravimetric analyses. In the crystal structure of 1, the chromate(III) ion lies on an inversion center in the form of an elongated octahedron. The coordination sphere consists of four oxygen atoms of two chelating oxalato ligands in the equatorial plane and two axial oxygen atoms of water ligands. The structural feature of focal interest in the structure of 1 is the formation of pillars of [Cr(C2O4)2(H2O)2]− complex anions and CH6N3 + guanidinium cations, with the next-neighbor cations rotated by an angle of 60° relative to each other. O–H···O and N–H···O hydrogen bonds play an important role in the construction of the three-dimensional network. The electron paramagnetic resonance (EPR) and magnetic properties of 1 have also been investigated.

(BEDT-TTF)2Cu2(HCOO)5: An Organic-Inorganic Hybrid Conducting Magnet

A dual‐functional organic–inorganic hybrid (BEDT‐TTF)₂Cu₂(HCOO)₅ (1) (BEDT‐TTF=bis(ethylenedithio)tetrathiafulvalene) was obtained through the electrochemical oxidation of neutral BEDT‐TTF in the presence of an ammonium salt of the one‐dimensional copper‐formate framework [(C₂H₅)₃NH]₂Cu₂(HCOO)₅ in a C₆H₅Cl–C₂H₅OH solution. Compound 1 was composed of organic donor BEDT‐TTF⁺⁰⋅⁵ in a θ‐phase arrangement and Jahn–Teller distorted (4,4) grid anion sheets [Cu₂(HCOO)₅^‐]_n with S=1/2. We identified the material as a semiconductor with values of σ_300K=10⁻¹ S cm⁻¹. The anion sheet is a coordination isomer of [Cu₂(HCOO)₅^‐]_n and, compared with the starting material, shows antiferromagnetic behavior as the well‐known inorganic Cu−O, Co−O square layers for creating inorganic conducting magnets. Long‐range antiferromagnetic ordering was observed at 8.0 K.