Kepert, Cameron John; (1996) Molecular charge-transfer salts with transition metal-containing anions: Synthesis, structure and physical properties. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the synthesis, structure and physical properties of molecular charge-transfer salts of the BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene, also ET) donor molecule with transition metal-containing anions. Crystal structures are presented for a diverse array of salts: α-ET₄[ReCl₆].C₆H₅CN, ET₂[IrCl₆], ET[MoOCl₄(H₂O)], ET₂[Cr(NCS)₄(NH3)₂], ET₃[Re₂(NCS)₁₀].2CH₂Cl₂, ET₂[Re₂(NCS)₁₀].C₆H₅CN, β"-ET₄[Mo₆Cl₈]Br₆.2THF, ET₄[Mo₆Cl₈]Cl₆.2CH₂Cl₂, α-ET₄[Mo₆Cl₈]Cl₆.2DMF and β-ET₃[ReO₄]₂. Several other materials have been shown to suffer from crystallographic twinning. Methods of physical characterisation include the measurement to low temperature of electrical conductivity (at ambient and high pressures) and magnetic susceptibility, and the measurement of electron paramagnetic resonance and infrared reflectance at room temperature. Electronic band structures are calculated according to the tight-binding model, using transfer integrals generated by the extended-Hückel treatment of the ET dimer. Properties encountered range from metallic through to insulating. A variety of temperature- and pressure-dependent metallic conductivities have been seen in similar phases of the [Mo₆Cl₈]X₆²⁻ (X=Br, Cl) anion, highlighting great subtleties in the relationship between structure and properties. For materials where electronic localisation dominates, semiconducting and insulating behaviours have, in many cases, been interpreted as being characteristic of electronic hopping. Magnetic measurements have indicated that antiferromagnetic-ordering occurs in many of the localised salts, with ET lattices that range from discrete dimers through to uniform 1-D chains. A systematic study of the intramolecular geometry of the ET molecule has yielded the first accurate experimental technique for evaluating the charge of discrete ET molecules. Application of this method has suggested that a few important materials may be misformulated. A strong spatial correlation between cation and anion layer charge distributions has emerged, suggesting the electrostatic interaction may have a significant influence on the degree of ionicity within the ET cation layer. Low temperature X-ray crystallographic experiments have been performed on five salts: ET₃Cl₂.2H₂O, ET₄Cl₂.6H₂O, ET₃[CuBr₂Cl₂], α'-ET₂[AuBr₂] and ET₄[Mo₆Cl₈]Cl₆.xCH₂Cl₂. Important findings include the observation of a*/2 satellite reflections in ET₃Cl₂.2H₂O at 15 K, the confirmed absence of a low temperature structural modulation in ET₃[CuBr₂Cl₂], and the observation and full characterisation of b*/2 satellite reflections in ET₄Cl₂.6H₂O down to 110K.

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