Task Specific Ionic Liquids

Fluorohexane floats on top of a conventional ionic liquid (left). However, adding a small amount of a task-specific ionic liquid that acts like a detergent enables the fluorohexane and the conventional ionic liquid to mix into a yellow emulsion (right).

9Fluorohexane floats on top of a conventional ionic liquid (left). However, adding a small amount of a task-specific ionic liquid that acts like a detergent enables the fluorohexane and the conventional ionic liquid to mix into a yellow emulsion (right).

IL are not, as is sometimes asserted, simply a dispersive matrix; rather, they can and do interact with dissolved molecules - as was recently reemphasized by recent findings by Rogers and others that hard, coordinating anions like chloride can be critical in facilitating the dissolution by IL of biomolecules like cellulose and other oligosaccharides.

Increasing the capacity of ILs to interact with dissolved molecules in specific ways is the focus of several research programs. New IL are being introduced in which a functional group is incorporated as a part of the cation or anion structure. These functional groups can impart a particular reactivity pattern to the IL, enhancing its capacity for interaction with specific solute types.
For example:

  • safe-to-handle Brönsted acidic IL with appended sulfonic acid groups were recently reported, as was their use as solvent-catalysts for esterification and other organic reactions;
  • IL bearing appended amines, can separate CO2 from gas streams;
  • IL with large aromatic head groups show enhanced activity for the extraction of aromatics in aqueous biphasic systems;
  • IL with tethered carboxylate groups have been used as supports for “IL-phase synthesis,” a versatile extension of the solid-phase synthesis concept.

While the development of such “task-specific” IL has been largely a process of design, there is a huge role for discovery to play as well.2

An example of TSIL:
4Task-Specific Ionic Liquids incorporating novel cations for the coordination and extraction of Hg2+ and Cd2+

The advent of moisture stable IL and their unique chemical and physical properties has brought about the widespread application of IL with significant contributions from their use as alternatives to traditional organic solvents and unique reaction and as synthesis media.
In most cases, the IL characteristic that has been exploited is their ability to dissolve a variety of solutes. For example, IL can be used in place of traditional organic solvents in liquid/liquid extractions where hydrophobic molecules such as simple benzene derivatives will partition to the IL phase.

Conversely, for metal ions in [Cnmim][PF6]/aqueous systems, the distribution ratios are below 1, indicating their hydrated nature and preference for the aqueous phase (e.g., the distribution ratios for Hg2+ and Cd2+ in [C4mim][PF6]/ water systems are 0.84 and 0.03, respectively).
In traditional solvent extraction, increasing the metal ion partitioning to the more hydrophobic phase is accomplished by adding extractants that reside quantitatively in the extracting phase. The extractant molecules serve to dehydrate the metal ions and to offer a more hydrophobic environment that enables their transport to the extracting phase. To that end have been used:

  • crown ethers to aid in the extraction of Sr2+ and Cs+ from NO3- media,
  • azo molecules for the extraction of transition metals,
  • halides and pseudohalides for Hg2+ extraction in liquid/liquid systems with [Cnmim][ PF6],
  • crown ethers for Sr2+ extraction in liquid/liquid systems using IL composed of the bis(trifluoromethylsulfonyl)imide anion.

Structured of some TSIL cationsThe drawbacks associated with this approach lie in finding extractant molecules that remain exclusively in the IL and also understanding the increased complexity of the system upon the addition of solutes, prompting the investigation of the task-specific ionic liquid (TSIL) concept for metal ion extraction in IL. Attaching a metal ion coordinating group directly to the imidazolium cation makes the extractant an integral part of the hydrophobic phase and greatly diminishes the chance for loss to the aqueous phase.
Despite the added cost of TSIL, they can be used in a mixture with less expensive IL to achieve the same result.
Ligands employed in metal ion extraction have appended functional groups (e.g., carboxylic acids) or contain donor atoms (e.g., crown ethers) that interact to form complexes with metal ions. Ligands containing soft donor atoms such as N or S have been explored in other systems for the extraction of heavy metals through the use of thioether, thiourea, and urea functional groups. Other reports confirm that acidic conditions can be sufficient to induce Hg2+ and Cd2+ stripping from thiourea and thioether-based extractants. Hence, IL with thioether, thiourea, and urea functional groups incorporated in the alkyl chains of the imidazolium cations have been prepared (figure on the right).

Mechanism for mercury cation removal from water1

The results indicate that appending thioether, urea, and thiourea functional groups to imidazolium cations does produce TSIL cations that can be considered either a new class of IL or novel class of IL extractants. The TSIL cations, in combination with PF6- and used alone or in a mixture with [C4mim][PF6], result in significant distribution ratios for Hg2+ and Cd2+ in liquid/liquid separations while minimizing the reliance on traditional organic solvents for this process.
The concept of TSIL illustrates howfunctional groups can be introduced into the scheme of IL synthesis with the inherent potential for achieving desirable properties tuned to specific applications.


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