Transition Metal Complex-Based Conducting Systems

3 Transition Metal Complex-Based Conducting Systems 

The first episode of linear chain type molecular conductors was written by Knop and Schnedermann in the middle of the nineteenth century. Although the isolated crystals of K2[Pt(CN)4] oxidised by chlorine or bromine were reported to show metallic luster, the character of conducting compounds was not clarified. 

The first compound widely noticed as a molecular inorganic conductor was KCP, K2[Pt(CN)4]Xo.3 H20 (X = Cl, Br), which was reported in 1968.KCP is a partially oxidised platinum complex, later called the Krogmann salt, and was prepared by oxidising a platinum(II) complex with halogen, or by electrochemical oxidation, or by mixing platinum(II) and platinum(IV) complexes. The KCP structure is characterised by 

The electrical transport of KCP was reported by Zeller et The room-temperature conductivity along the molecular stacking direction is 300-400 S cm , which is 10 greater than the conductivity perpendicular to the chain direction. The temperature dependence of conductivity along the chain direction shows metallic behaviour down to around 250 K and semiconducting behaviour below that temperature. The broad maximum of conductivity around 250 K is due to the one-dimensionality of the linear-chain system. The large anisotropy of conductivity in directions parallel and perpendicular to the chains confirms the onedimensional character of KCP. 

The bis(oxalato)platinate(II) complexes based on [Pt(C204)2] , forms another family of linear chain systems, and the partially oxidised complex series, Cx[Pt(C204)2]-n(H20) (C = K, Rb, Mg , Co, Ni, Zn ), was reported.According to elemental analysis and X-ray diffuse scattering measurements, the formal oxidation state of platinum in this series is estimated in the range of -1-2.19 to -f2.38. The average Pt-Pt distance is 2.84-2.89 A and the conductivity at room temperature is 10 -10 S cm , depending on the associated cation. 

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This process (hetero Diels-Alder reaction leading to a dihydropyran system) may be also conducted in an asymmetric version application of chiral transition-metal catalysts based on BINOL, BDMAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to 99%) [1,6], In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21],... 

Volume 1 contains 15 chapters on the recent developments in charge-transfer salts, fullerenes, photoconductors and molecular conductors. The topics covered include electron acceptor molecules, photo-induced intermolecular electron transfer systems, perylene based conductors, tetrachalcogenafulvalenes, metal 1,2-dichaIcogenoIenes and their conductive salts, conductive hetero-TCNQs (tetracyano-p-quinodi-methane derivatives), molecular metals and superconductors based on transition metal complexes. 

Almost a decade ago the first molecular superconductors based on the transition metal complex molecule (TTF)[Ni(dmit)2]2 was discovered (TTF = tetrathiafiilvalene, dmit = isotrithionedithiolate) [ 1 ]. Since then several M(dmit)2 superconductors have been found. Although most of the molecular metals currently studied are systems based on multi-sulfur (or selenium) 71 molecules, the first example of a metallic molecular crystal was a partially oxidized platinum complex. The discovery of the partially oxidized platinum complex is very old, about 150 years ago, but the physical meaning of the system had not been noticed until the crystal structure (by Krogmann and Hausen) revealed the existence of the extended linear Pt-Pt metal bonds [2]. The diffuse X-ray scattering experiments on this system (K2[Pt(CN)4]Bro.3 3H2O) by Comes et al. [3] convinced many people, who were interested in conducting crystalline molecular solids at that time, of the existence of one-dimensional metal electrons in molecular crystals for the first time, and so a new era of metallic molecular systems had been bom. 

Many polymer-salt complexes based on PEO can be obtained as crystalline or amorphous phases depending on the composition, temperature and method of preparation. The crystalline polymer-salt complexes invariably exhibit inferior conductivity to the amorphous complexes above their glass transition temperatures, where segments of the polymer are in rapid motion. This indicates the importance of polymer segmental motion in ion transport. The high conductivity of the amorphous phase is vividly seen in the temperature-dependent conductivity of poly(ethylene oxide) complexes of metal salts. Fig. 5.3, for which a metastable amorphous phase can be prepared and compared with the corresponding crystalline material (Stainer, Hardy, Whitmore and Shriver, 1984). For systems where the amorphous and crystalline polymer-salt coexist, NMR also indicates that ion transport occurs predominantly in the amorphous phase. An early observation by Armand and later confirmed by others was that the... 

A modern series of new plastics are based on transition metals (e.g. Fe, Ti, Cr, Zn, V) to form polymers and possess unusual properties such as variable oxidation states, and ligand exchange on the metal atom. They have reduced UV absorption and visible radiation and exhibit electrical conductivity. Examples include cyclopentadienyl and arene metal n polymeric complexes that act as electron rich aromatic system and are very reactive to a range of monomers to form polymers. 

In the following we will discuss some unconventional properties of macrocyclic metal complexes, preferentially semiconductive and liquid crystalline behavior. There are several reviews available on low-dimensional conductive compounds based on these macrocycles [21]. A detailed description of newer developments in the field of bridged macrocyclic metal complexes especially with transition metals was published recently [22]. In order to use the 7t-electrons in these macrocyclic systems for a conduction pathway, polymerization of the metallo-macrocycles is necessary. A polymerization can be carried out in three different ways as explained in Sections 1.2.1-1.2.3. 

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