Angewandte Chemie International Edition
Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (22), 2494 - 2497
Gelling Agent for Catalysis?
New prospects for catalysis:
a low-molecular organometallic gelling compound
We encounter gels every day: on our plates as raspberry jelly, jello, or aspic; in the bathroom as shower gel, body lotion, or cool sunburn-relieving gel. The gelatinous state guarantees that cosmetics and pharmaceuticals flow easily out of their tubes or bottles while remaining viscous enough to be rubbed in with as little "waste" as possible. Gels are also used in technological applications. A research team from Germany and the Netherlands has now developed an organometallic complex that acts as a novel gelling agent. It converts organic solvents to stable gels and promises new approaches to catalysts.
What is a gel? Gels look like solid liquids -- this is self-contradictory, but accurate. In order for a gel to form, the liquid must contain a gelling agent, whose molecules aggregate into a continual three-dimensional network. This network holds the liquid together and hinders its flow. From the chemical point of view, gels can be made of very different systems. Those gelling agents found in the kitchen, for example, are gelatin, a polypeptide, and preserving sugar, a long-chain compound made of sugar components. In the lab, silica gels or polymers are used. In recent years, interest has been mounting in a different type of gelling agent, consisting of small organic molecules. In contrast to the "classic" agents, these low-molecular gelling agents form networks that are held together by weak attractive forces between the molecules, rather than by "normal", strong, covalent bonds. Such gel systems can easily be converted back to liquids.
The low-molecular gelling agent developed by the team headed by Karl Heinz Dötz at the University of Bonn and Roeland Nolte at the Univesity of Nijmegen is tailored for organic solvents. Slight heating and cooling suffice to turn the solvent into a stable gel at room temperature, heating the gel up causes it to dissolve again. The gelling agent consists of a sugar-like "head" and a carbohydrate "tail"; however, the special thing about it is its "neck". This consists of a catalytically active organometallic fragment -- in this case, part of a chromium complex bound to a carbon atom. "The catalytically relevant group is surrounded by a cage of solvent molecules built up and stabilized by the gel network," explains Dötz. "This allows us to combine the advantages of solid and dissolved catalysts." Because the sugar "head" of the gelling agent contains chiral centers, it may even be possible to generate chiral gels, by means of lefthand or righthand wound spiral strands of the gel network, for example, an interesting prospect for material properties as well as for stereoselective syntheses.