Ionic liquids — salts that remain in liquid state — show promise as ‘green’ alternatives to the traditional organic solvents used in synthetic chemistry because these charged liquids are reusable and less volatile. Recently, however, scientists have started to rethink their enthusiasm for ionic liquid solvents because of new evidence of their toxicity.

A more sustainable path would be to use ionic liquids as recyclable catalysts in synthetic reactions. Now, researchers Asit K. Chakraborti and Sudipta Raha Roy from the National Institute of Pharmaceutical Education and Research in Punjab, India1 have uncovered key catalytic mechanisms that will make selecting the most effective ionic liquids catalysts even easier.

Ionic liquids contain two parts, a cation and an anion. As the cation for his catalytic ionic liquid, Chakraborti chose an imidazolium-based molecule that includes a positively charged carbon–nitrogen pentagonal ring. Imidazolium is relatively non-toxic, and is known to be a highly active material in synthetic chemistry.

According to Chakraborti, choosing the most suitable anion was difficult. Although the choice of ionic liquid anion is known to affect the reaction rates and products of the catalyzed reactions, few guiding principles exist for selecting the most efficient species, making the anion choice subject to trial and error.

The researchers studied how three different ionic liquid anions — a basic, an acidic and a neutral species — affected a fundamental synthetic reaction: the addition of a carboxyl-based molecule to the end of an aromatic alcohol.

While it was anticipated that the acidic anion would be the best catalyst for this reaction, the basic anion in fact gave the highest synthetic yield. “In many cases the usual reactivity of the substrates was found to be reversed from what one would expect on the basis of classical chemical knowledge,” says Chakraborti.

Fig. 1: Chemical structure of a multi-member ring complex formed during ionic liquid catalysis.

Spectroscopic measurements taken during the reaction revealed that successful catalysis required the anion to form a multi-member ring complex with both the starting materials and the imidazolium cation (Fig. 1). Only the basic anion had the ability to join the complex because of its hydrogen-bonding and charge-donating capabilities.

Chakraborti says that this study gives scientists a predictive guide, based on molecular interactions, for selecting reusable and recyclable catalytic ionic liquids.