Liquids transition metals

The intermediate range of concentrations between those at which resonant scattering and s-d transitions are appropriate has not been fully explored, except in the CPA approximation (Stocks et al 1973), which does not give the mean free path. For liquid transitional metals the present author (Mott 1972d) has suggested that one must introduce two mean paths, /s for the s-electrons and /d for the d-electrons, that / afor the latter (as in the alloy) and that s-d transitions are appropriate to describe the resistance. Other authors have described the resistance in terms of a single mean free path, determined by the resonant scattering of the s-electrons by the d-shells (Evans et al 1971). 

B.C. Allen. The Surface Tension of Liquid Transition Metals at Their Melting Points. Trans. Metall. Soc. AJME 221. 15 (1963). 

Gel] Geld, P.V., Gertman, Y.M., About the First Heat of Solubility of Liquid Transition Metals in Liquid Silicon (in Russian), Fiz. Met. Metalloved., 10, 299-300 (1960) (Thermodyn., ExperimentaL 8)... 

As in the nuclear case, only a summary of the relevant results from the theory of magnetic neutron scattering wiU be given there are numerous texts that cover the topic in detail [3,48]. In the context of this thesis, magnetic neutron scattering is only considered in terms of correcting for paramagnetic effects in liquid transition metals and rare earths. 

See Ballentine 1988.) Pl for several liquid transition metals and for liquid La have been calculated, and the dd contribution to p was found to constitute about 80%-90%. This was due to the fact that the magnitude of n Ef) more than compensated for the reduced d-diffusivity. The calculated value for Pi for La (151 p42-cm) compares reasonably well with experiment (Ballentine and Ham-... 

Evans, R., B.L. Gyorffy, N. Szabo and J.M. Ziman, 1973, On the Resistivity of Liquid Transition Metals, in The Properties of Liquid Metals, ed. S. Takeuchi (Taylor and Francis, London) pp. 319-331. 

Waseda, Y., 1977, The Structure of Liquid Transition Metals and their Alloys, in Inst. Phys. Conf. Ser. No. 30, eds R. Evans and D.A. Greenwood (Institute of Physics, Bristol) p. 230. 

Fig. 11. Ionic liquid-transition metal catalyst recycle schemes for coupling reactions (a) conventional product isolation via solvent extraction, (b) Organic Solvent Nanofiltration (OSN) used with a biphasic IL/ organic system, and (c) Organic Solvent Nanofiltration (OSN) used with a single phase IL-organic solvent system [Wong et al., 2006].

Kofman R ef a/1989 Solid-liquid transition of metallic clusters occurrence of surface melting Physica A 157 631 Kofman R ef al 1994 Surface melting enhanced by curvature effects Surf. Sc/. 303 231... 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

The Acetaldehyde Oxidation Process. Liquid-phase catalytic oxidation of acetaldehyde (qv) can be directed by appropriate catalysts, such as transition metal salts of cobalt or manganese, to produce anhydride (26). Either ethyl acetate or acetic acid may be used as reaction solvent. The reaction proceeds according to the sequence... 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

Deprotonation of H2O2 yields OOH , and hydroperoxides of the alkali metals are known in solution. Liquid ammonia can also effect deprotonation and NH4OOH is a white solid, mp 25° infrared spectroscopy shows the presence of NH4+ and OOH ions in the solid phase but the melt appears to contain only the H-bonded species NH3 and H202. " Double deprotonation yields the peroxide ion 02 , and this is a standard route to transition metal peroxides. 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

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