Catalytic Gel

Gels are commonplace from lime Jell-o to invigorating minty shower gels. Now, a German-Dutch team has developed an organometallic complex that acts as a novel gelling agent for organic solvents. It converts organic solvents into a stable gel, providing an organometallic model for a catalyst be used efficiently in a single phase, homogeneous catalytic reaction. After use, the catalyst can be retrieved for use again by inducing the gel to break down.

An additional feature might be to use a handed, chiral, gelling agent and produce a chiral gel network which would have different separating and catalysing properties to the non-chiral systems. Controlling chirality is an important issue in pharmaceutical synthesis. Many pharmaceuticals are more active, and/or safer, in one handed form than the other. The painkiller ibuprofen, for instance is three times more active in one form than the other.

What is a gel? Gels are liquids that behave like solids. Or perhaps they are solids that look fluid. Either way, gels form when a suitable gelling agent is added to a susceptible liquid. The gelling agent produces a three-dimensional network that provides some structure for the liquid, preventing it from flowing as normal. In the kitchen, flavored gelatin is often dissolved in hot water in a mould and allowed to cool. A fibrous network of strand-like gelatin molecules forms around aggregates of water molecules. The result is Jell-o.

Gels in the laboratory such as silica gel and some polymers have been studied and exploited for years. Recently, however, small molecules have been discovered that can form networks that are held together by weak attractive forces between the molecules, rather than by covalent bonds. Such gel systems can easily be converted back to liquids.

Roeland Nolte

Karl Heinz Dötz

Now, Karl Heinz Dötz at the University of Bonn, Germany, and Roeland Nolte at the University of Nijmegen, The Netherlands, and their teams, have created just such a small molecule gelling agent tailored for organic solvents. Slight heating and cooling are enough to interconvert the gel between its liquid (dissolved) and stable (gel) forms.

The gelling agent itself is composed of a carbohydrate head group - and an alkyl chain tail. The "neck" in between is the special feature of this amphiphile, however, in that it consists of a potentially catalytically active organometallic fragment - a chromium complex; the product - pentacarbonyl[D-gluco-hex(N-n-octylamino)-1-ylidene]chromium is an organometallic analogue of N-octyl-D-gluconamide. "In this type of compound, 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."

The sugary head of the gelling agent carries numerous chiral centers, which the researchers suggest might be exploited in generating chiral gels as left- or right-hand wound spiral strands form within the gel network. Such a development might be useful in stereoselective catalytic syntheses. If they are carried out in a macroscopic chiral neighborhood they are expected to be enantio- and diastereo-selective, explains Dötz. Another aspect arising from the macroscopic chirality of gel networks refers to applications in material science such as sensors or switches, he told Reactive Reports.

Angew. Chem. Int. Ed. 2003, 42, 2494-2497; http://dx.doi.org/10.1002/anie.200351134