Dutch and Israeli scientists have found a way to induce the chirality usually only found in organic materials in palladium. They say palladium and other so-called chirally imprinted metals will have widespread application in chiral catalysis.
'Usually, when someone claims they have a chiral palladium catalyst, they mean a palladium atom that is modified by chiral organic ligands,' says Gadi Rothenberg, who led the team at the University of Amsterdam. 'But what we have is a chiral palladium metal - no organic material whatsoever. It's just a jumble of metal atoms and it's chiral.'
This new type of catalyst combines metallic properties with the diversity of organic materials. Although a collaborating group previously induced chirality in silver and gold, Rothenberg notes that palladium is a far more interesting metal from a catalytic point of view.
The team tested samples of chiral palladium as catalysts for the hydrogenation of isophorone, proving that they could produce modest excesses of left- and right-handed enantiomers. Results for this specific reaction were not spectacular, Rothenberg admits, but demonstrate that chiral metals do at least work as catalysts.
Chiral palladium is created by reducing ordinary palladium in the presence of specific alkaloid enantiomers that become trapped in the metal. Dissolving the alkaloid leaves pure palladium - a black powder - which is somehow imprinted with the enantiomer it has lost. The researchers think the metal must retain a morphological memory of the chiral organic compounds. But they have also shown that the powder retains its metallic properties - by pressing it into coins.
'The beauty of this work is that the chiral metal surface appears to have a high surface area to weight ratio making it attractive for large-scale industrial processes,' says David Watson, who studies chiral catalysis at the University of Reading. 'Palladium is still a very expensive material to use pure as a catalyst and enantiomeric excesses far greater than those reported here will be required before the process becomes financially viable. But it's an important step in the right direction.'
Rothenberg says chiral metals will have wide-ranging applications, but probably not in the near future. 'We're not just talking about a new catalyst, we're talking about a new type of catalyst, so of course these are not going to be applied in chiral catalysis tomorrow. Think how long it took before organometallics gave us chiral catalysts.'
Another application might be that of chiral sensing. Watson says this could work via a simple 'lock and key' mechanism - one enantiomer would fit the metal's chiral pores better than another.
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