Metal ions, ionic liquids

Metal-based ionic liquid crystals containing a tetrahedral tetrahalometalate ion (MX X = Cl, Br, I M = Co, Ni, Cu, Pd, etc.) 7 [41, 54] and 8 [55-57] were studied (Figure 25.1). They provide various coordination geometries. For example, Al-aUcylpyridinium salts 8 often denoted as [C Py]2[CuCl4] (n= 12-18) show a variety of liquid crystalline phases from smectic to columnar or even cubic phases... 

Keywords metals metal ions heavy elements solution salvation speciation solubility s-elements p-elements d-elements f-elements complex ions ionic liquids... 

Abstract Ionic liquids as green solvents have shown important application in extraction and separation of metals. In this chapter, the new application perspective and the important fundamental and applied studies of the extraction and separation of metals in ionic liquids which include metal oxide processing, mineral processing, electrodeposition of metals (especially reactive metals such as Al, Mg, and Ti), and extraction of metal ions are presented. 

Preliminary studies have shown that ionic liquids have potential as solvents and electrolytes for metal recovery, and the feasibility of these solvents has been demonstrated for the extraction of gold and silver from a mineral matrix [7], the recovery of uranium and plutonium from spent nuclear fuel [8], and the electrodeposition and electrowinning of metals (especially, for active metals such as Li, Na, Al, Mg, and Ti) from ionic liquids [9-11], Ionic liquids as green solvents and electrolytes have shown important and potential application in extraction and separation of metals. In this chapter, the new applications and the important fundamental and appUed studies on the extraction and separation of metal in ionic liquids including metal oxides and minerals or ores processing, electrodeposition of metals (mainly for active metals), and extraction and separation of metal ions are described. 

Among his research interests are investigation of the cathode materials for the lithium-ion batteries, physical-chemical properties of low-temperature ionic liquids and electrochemistry of refractory metals in ionic liquids. Recently he has been taking part in the research and optimization on the industrial scale of processes of Zn, Zn-Fe and Zn-Ni alloy deposition. 

Wu W, Lu Y, Liu Y, et al. Structures and electronic properties of transition metal-containing ionic liquids insights from ion pairs, J Phys Chem A 118(13) 2508—2518, 2014. 

Coordination numbers of 2-6 are found for mononuclear metal complex ionic liquid anions, mostly with halide ions. 

In the meantime, we believe that the best prediction of the toxicity of an ionic liquid of type [cation] [anion] can be derived from the often well known toxicity data for the salts [cation]Cl and Na[anion]. Since almost all chemistry in nature takes place in aqueous media, the ions of the ionic liquid can be assumed to be present in dissociated form. Therefore, a reliable prediction of ionic liquids HSE data should be possible from a combination of the loiown effects of the alkali metal and chloride salts. Already from these, very preliminary, studies, it is clear that HSE considerations will be an important criterion in selection and exclusion of specific ionic liquid candidates for future large-scale, technical applications. 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

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. 

However, a number of limitations are still evident when tetrafluorohorate and hexafluorophosphate ionic liquids are used in homogeneous catalysis. The major aspect is that these anions are still relatively sensitive to hydrolysis. The tendency to anion hydrolysis is of course much less pronounced than that of the chloroalu-minate melts, hut it still occurs and this has major consequences for their use in transition metal catalysis. For example, the [PF ] anion of l-hutyl-3-methylimida-2olium ([BMIM]) hexafluorophosphate was found (in the author s laboratories) to hydrolyze completely after addition of excess water when the sample was kept for 8 h at 100 °C. Gaseous HF and phosphoric acid were formed. Under the same conditions, only small amounts of the tetrafluorohorate ion of [BMlMjjBFJ was converted into HF and boric acid [10]. The hydrolytic formation of HF from the anion of the ionic liquid under the reaction conditions causes the following problems with... 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts, while different levels of proton impurities in chloroalumi-... 

Some enzymes require metal ions - such as cobalt, manganese or zinc - for their activity if these are removed by the ionic liquid by complexation, enzyme inactivation may occur. 

Visser AE, Swatloski RP, Reichert WM, Mayton R, Sheff S, Wierzbicki A, Davis JH, Rogers RD (2001) Task-specific ionic liquids for the extraction of metal ions from aqueous solutions. Chem Commun 1 135-136... 

Visser, A.E., Swatloski, R.P., Reichert, W.M. et al. (2001) Task-Specific Ionic Liquids for the Extraction of Metal Ions from Aqueous Solutions. Chemical Communications, 1, 135-136. 

Among the potential impurities in ionic liquids water, halide ions and organic starting material are of great importance for transition metal chemistry while the colour of an ionic liquid is not a critical parameter in most applications. 

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