Task-specific ionic liquids for solubilizing metal compounds

Doctoral thesis English OPEN
Thijs, Ben (2007)

The main goal of this PhD thesis was to design new task-specific ionic liquids with the ability to dissolve metal compounds. Despite the large quantity of papers published on ionic liquids, not much is known about the mechanisms of dissolving metals in ionic liquids or about metal-containing ionic liquids. Additionally, many of the commercially available ionic liquids exhibit a very limited solubilizing power for metal compounds, although this is for many applications like electrodeposition and catalysis one of the essential features. To enhance the ability of ionic liquids to incorporate metals in ionic liquids, functional groups that are able to coordinate to the metal ions are required. Task-specific ionic liquids combine, in the best case, a solubilizing power that is comparable with organic solvents with the “green” character of ionic liquids, which makes them environmental friendly solvents. Before these compounds will be applicable on larger scale, these functionalized ionic liquids have to be cheap and easily accessible. Firstly, a protonated betaine bistriflimide, [Hbet][Tf2N], was introduced. This is a cheap and easily accessible functionalized ionic liquid with the ability to dissolve large quantities of metal oxides and metal hydroxides. The metal solubilizing power of this compound is selective. Soluble are: oxides of the trivalent rare earths, uranium(VI) oxide, zinc(II) oxide, cadmium(II) oxide, mercury(II) oxide, nickel(II) oxide, copper(II) oxide, palladium(II) oxide, lead(II) oxide and silver(I) oxide. Insoluble or very poorly soluble are iron(III) and manganese(II) oxides, cobalt(II) oxides, as well as aluminium oxide. The metals can be stripped from the ionic liquid by treatment of the ionic liquid with an acidic aqueous solution. After transfer of the metal ions into the aqueous phase, the ionic liquid can be separated and recycled. Betainium bistriflimide forms one phase with water at high temperatures, whereas phase separation occurs below 55.5 °C (“phase switching” behavior). The mixtures of the ionic liquid with water also show a pH-dependent phase switching behavior: two phases occur at low pH, whereas one phase is present under neutral or alkaline conditions. The structures, the energetics and the charge distribution of the betainium cation, the bistriflimide anion, as well as of the cation-anion pairs were studied by density functional theory calculations (DFT). Additionally, the crystal structures of two modifications of this ionic liquid were determined. A range of crystal structures of the metal complexes obtained from the above mentioned functionalized ionic liquid were examined. This study revealed a rich structural variety especially of oligonuclear complexes, whereas in the literature mainly monomeric or polymeric complexes with this type of ligands are known. Dimeric betaine bistriflimide structures were found for the dysprosium(III) compound; the europium(III) compound and for the copper(II) compound; a trimeric betaine bistriflimide structure for the cobalt(II) compound; a tetrameric betaine bistriflimide structure for the manganese(II) compound; a pentameric betaine bistriflimide structure for the nickel(II) compound; , a cluster formation for the lead(II) compound; and polymeric structures for the cadmium(II) compound; and the silver(I) compound. In the crystal structure of cobalt(II) betaine bistriflimide, pure ionic liquid [Hbet][Tf2N] co-crystallized. The cation-anion interaction found in the crystal structure of the pure ionic liquid remained in the metal complex. No hydrogen bonding between the ionic liquid and the metal complex could be observed. It was demonstrated that ionic liquids might be good media for crystal engineering. In order to modify the physical properties of the betainium ionic liquid, structural modifications have been introduced. Different kinds of cations, all bearing a carboxylic functional group, were applied. Six cations with a positively charged nitrogen ion – l-carnitine, pyrrolidinium, morpholinium, pyridinium, piperidinium, imidazolium and one with a positively charged phosphonium ion - tributylphosphonium, were studied. Despite the rich structural variation, surprisingly small differences in physical properties of these ionic liquids were observed. This can be explained by the strong and dominating influence of the carboxylic group on the physical properties. To study the influence of the carboxylic group, an ionic liquid with a hydroxyl group instead of the carboxylic group, choline bistriflimide, was introduced. Due to this hydroxyl group, the ionic liquid showed only a limited solubility towards metal compounds. This hydrophobic ionic liquid also shows a “phase switching” behavior; this occurred at higher temperatures than in the case of the betainium bistriflimide ionic liquid. Due to the absence of a conjugated system, this ionic liquid possesses a high UV-transparency. This can be an interesting property for the application of this choline bistriflimide ionic liquid as medium for photophysical reactions or as a solvent for spectroscopy. Choline saccharinate and choline acesulfamate are two examples of hydrophilic ionic liquids, which can be prepared from easily available starting materials (choline chloride and a non-nutritive sweetener). The (eco)toxicity of these ionic liquids in aqueous solution is very low in comparison with other types of ionic liquids. A general method for the synthesis and purification of hydrophilic ionic liquids has been demonstrated. The method implies a silver-free metathesis reaction, followed by the purification of the ionic liquids by ion-exchange chromatography. The crystal structures show a marked difference in hydrogen bonding between the two ionic liquids [Chol][Sac] and [Chol][Ace], although the saccharinate and the acesulfamate anions show structural similarities. The optimized structures, the energetics and the charge distribution of cation-anion pairs in the ionic liquids were studied by density functional theory (DFT). For a qualitative picture of the Lewis structure the occupation of the non-Lewis orbitals was considered. The calculated interaction energies and the dipole moments for the ion-pairs in the gas phase have been discussed. In the last chapter, a different strategy to incorporate metals into ionic liquids was introduced. A lanthanide thiocyanate anion was synthesized and used as component for an ionic liquid. By using this anion in combination with a 1-butyl-3-methylimidazolium (C4mim) cation the first lanthanide-containing ionic liquids were obtained. These compounds have melting points ranging from 28 °C (Nd) to 39 °C (Y). The general formula for these compounds is [C4mim]
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