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Researchers map out way to better reactions

ChemicalProcessing.com
03/29/2007

A porous crystal enables for the first time the pinpointing of movement of individual atoms during a reaction, say its developers at the University of Liverpool, Liverpool, U.K. The result could be deeper understanding of the reactions and of how to engineer better catalysts, they add.

The team created a porous crystalline solid to contain molecules taking part in a chemical reaction similar to ones by enzymes and proteins, notes Professor Matthew Rosseinsky. The crystal then was analyzed by X-ray diffraction, determining the positions of individual atoms both before and after the reaction.

“We designed a robust structure that remained stable when a chemical reaction occurred inside its walls — a structure with an opening the same size as a single molecule of aspirin. The X-ray experiment then allowed us to see how the entire structure changed during the chemical process,” Rosseinsky explains.

Such knowledge of the movement of atoms in the active site of enzymes, for instance, can aid in understanding what happens in these very large biomolecules and help in the design of catalysts, adds Darren Bradshaw, another member of the Liverpool team.

So far, the work, which is funded by the U.K.’s Engineering and Physical Sciences Research Council, hasn’t looked at catalyzed reactions. “…It’s something we plan to do,” says Bradshaw. “The system is very well set up for this to occur since we have a metal center with a labile coordination site (metal-bound water molecule). A substrate molecule, for example, could displace this water and be activated by binding to the Lewis acid metal center. Alternatively, the metal-bound water could itself act as an active species for nucleophilic-type reactions … We may be able to study such reactions very precisely in our synthetic systems.”

“The first barrier to doing useful catalytic work is the removal of the bipyridine ligands that occupy the channels. While these species nicely demonstrate the ligand substitution properties of this material, they are relatively inert to reaction. We need to be able to replace these channel species with more active ‘substrate” molecules that can actually undergo reaction,” Bradshaw adds.

“It is possible to tailor the porosity in this system … by using extended ligands,” he notes. “This would allow us to include larger guest species than bipy[ridine]. Size of the channels is also important for catalytic reactions, especially if the ‘product’ of the reaction is larger than the starting materials, since we need to be able to get this out of the pores for it to be useful.”

The researchers currently are working to replace bipy with more reactive species. “Analogous materials with other metal ions are also a target, and heterometallic systems where two types of metal ions responsible for holding the framework together and for reactivity are different. This would add a significant new dimension to materials of this type,” says Bradshaw.