Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

Crystal engineering--the planning and construction of crystalline supramolecular architectures from modular building blocks--permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity, ferromagnetism and nonlinear optical properties. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of 'bi-functionality' targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic pi-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties, but also systems that are both superconducting and paramagnetic. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.

Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound

Molecule-based materials with active physical properties, in particular electrical, magnetic, and optical, are a focus of contemporary materials chemistry research. Certainly, one reason for this interest has been the realization that these materials can exhibit cooperative properties typically associated with the inorganic network solids, as for example metallic conduction or even superconductivity,1 ferromagnetism,2 and nonlinear optical properties.3 With respect to the electrical properties, many important achievements were obtained in the 1970s with the discovery of the first molecule-based metal in 1972,4 namely the π-electron donor-acceptor complex [TTF][TCNQ] (TTF ) tetrathiafulvalene, TCNQ ) tetracyano-p-quinodimethane),5 and the report of the first molecule-based superconductors in 1979 based on the Bechgaard salts [TMTSF]2X (X ) PF6, AsF6; TMTSF ) tetramethyltetraselenafulvalene).6 The recognition of molecule-based ferromagnetic compounds is more recent and has emerged only in the past decade with the discovery of the electron-transfer salt [Fe(C5Me5)2][TCNE] (TCNE ) tetracyanoethylene) in 1985.7 Since these pioneering studies, the two areassmolecule-based metals and magnetsshave witnessed rapid development, and many new molecules have been designed which, if assembled in the appropriate manner in the solid, will enable researchers to improve the physical properties by increasing superconducting and ferromagnetic critical temperatures. A current development in the general area of molecule-based materials is to design, from a wise choice of the constituent molecules, new materials that combine properties not normally associated with a single material. Some intriguing applications of this concept would be to couple conductivity or optical phenomena with magnetic properties. In fact, this challenging goal was proposed8 in the mid-1980s but has only recently begun to be explored. Efforts in this direction nucleated with the design of hybrid materials formed by two molecular networks, such as anion/cation salts or host/guest solids, where each network furnishes distinct physical properties. In these new types of materials, each network contributes distinct physical properties to the solid. Examples include hybrid molecule-based materials, combining inorganic metal complexes that act as structural or magnetic components with an organic π-electron donor or acceptor molecule that furnishes the pathway for electronic conductivity.9 Polyoxometalates have been found to be extremely versatile inorganic building blocks for the construction of the aforementioned functionally active solids.10 The present article highlights recent results and provides a perspective of the use of polyoxometalates in the construction of molecule-based materials. We present herein the different classes of polyoxometalate-based hybrid materials that are of considerable interest due to their electrical or/and magnetic properties. These are (i) organic/inorganic hybrid salts in which the electron donors are organic molecules of the TTF type, (ii) organometallic/inorganic salts in which the electron donor is the decamethylferrocene complex, and (iii) organic/inorganic films in 273 Chem. Rev. 1998, 98, 273−296

http://www1.udel.edu/chem/polenova/PDF/Polyoxometalates/POM_materials_ChemRev1998.pdf

Polyoxometalate-Based Molecular Materials

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.

New Concepts in Tetrathiafulvalene Chemistry.

2.2. BETS Salts with Halometalate Anions 5424 3. Magnetic Metals and Semiconductors 5426 3.1. Mononuclear Metal Complexes 5427 3.1.1. Tetrahalometalates 5427 3.1.2. Hexahalo Anions 5431 3.1.3. Pseudohalide-Containing Anions 5431 3.2. Polynuclear Metal Complexes 5433 3.2.1. Dimeric Anions 5433 3.2.2. Polyoxometalate Clusters 5434 3.3. Chain Anions: Maleonitriledithiolates 5439 4. Ferromagnetic Conductors 5441 5. Ferrimagnetic Insulators 5443 6. Conclusions 5445 7. Acknowledgment 5446 8. References 5446

Magnetic Molecular Conductors

We have shown that the hexagonal layer motif [AM{sup III}(C{sub 2} O{sub 4}){sub 3}]{sup n-} containing bridging oxalate groups, which has been shown to form a wide variety of compounds with electronically inactive counter-cations having unusual cooperative magnetic properties, can also stabilize lattices containing the organic {pi}-donor BEDT-TTF. In the compounds whose structures we describe here, (BEDT-TTF){sub 4}AFe(C{sub 2}O{sub 4}){sub 3}.C{sub 6}H{sub 5} CN (A = H{sub 2}O, K, NH{sub 4}), the lattice is stabilized by C{sub 6} H{sub 5}CN molecules included in the hexagonal cavities. The packing of the BEDT-TTF in the A = K, NH{sub 4} phases is of a type not previously observed with spin-paired (BEDT-TTF){sub 2}{sup 2+} separated by closed shell (BEDT-TTF){sup 0}, while that in the superconductor is of {Beta}{double_prime} type. Both the superconducting A = H{sub 2}O and semiconducting A = K, NH{sub 4} phases contain high spin 3d{sup 5} Fe{sup III} with only very weak exchange interaction between them. Additional low temperature and high magnetic field experiments (e.g., of Schubnikov-de Haas oscillatory magnetoresistance) will be needed to delineate the Fermi surface in the superconductor. 36 refs., 11 figs., 4 tabs.

Superconducting and Semiconducting Magnetic Charge Transfer Salts: (BEDT-TTF)4AFe(C2O4)3.cntdot.C6H5CN (A = H2O, K, NH4)

β″-(bedt-ttf)4[(H2O)Fe(C2O4)3]·PhCN [bedt-ttf = bis(ethylenedithio)tetrathiafulvalene] is a superconductor [Tc 8.5(3) K]; from 9–300 K its magnetic susceptibility is dominated by the high-spin FeIII and obeys the Curie–Weiss law (C= 4.375 emu K mol–1, θ=–0.2 K).

Anthony W. Graham

Papers

Superconducting and Semiconducting Magnetic Charge Transfer Salts: (BEDT-TTF)4AFe(C2O4)3.cntdot.C6H5CN (A = H2O, K, NH4)

β″-(bedt-ttf)4[(H2O)Fe(C2O4)3]·PhCN: the first molecular superconductor containing paramagnetic metal ions