December 23, 2002
Volume 80, Number 51
CENEAR 80 51 p. 8
ISSN 0009-2347



Homogeneous reaction mechanism yields to X-ray absorption method


DISTINCT The structure of a ligand-Pd-allyl complex in solution (top) differs from the crystalline form (bottom) and offers clues about the catalyst's selectivity (Pd = pink, P = blue, C = red, white, yellow).
A new data analysis procedure for extended X-ray absorption fine structure (EXAFS) spectroscopy makes the method more amenable to liquid-phase compounds. The technique, developed by a team of scientists in the Netherlands, has been used to uncover structure-selectivity relations in homogeneous palladium catalysts [J. Am. Chem. Soc., 124, 14814 (2002)].

Discovering relationships between a catalyst's structure and its catalytic performance requires analyzing the material in its active phase. But that can be tough to do because of experimental limitations. X-ray absorption techniques, for example, are notoriously hard to apply to liquid-phase samples because the results are difficult to interpret. Similar limitations exist with other methods. As a result, researchers sometimes isolate homogeneous (solution-phase) catalysts as crystals and study the compounds in the solid state. The Dutch work addresses this problem.

To gain insight into the basis for product selectivity, Utrecht University graduate student Moniek Tromp and chemistry professor Diek C. Koningsberger and their coworkers studied palladium compounds used to catalyze allylic substitution reactions. They compared two cationic ligand-Pd-dimethylallyl complexes that differ only in the nature of their bidentate diphosphine ligands. One of the compounds includes a 1,2-bis(diphenylphosphino)ethane (dppe) ligand described as a "narrow-bite" ligand because of its P-Pd-P angle. The other catalyst contains a related "wide-bite" ligand.

The team finds that, unlike the crystalline structure of the dppe complex, in solution the unsubstituted allyl carbon bends away from Pd, leaving that carbon atom electronically activated and accessible to nucleophilic attack. Attack at that position leads to linear products. In contrast, the wide-bite complex remains much the same in solid and solution phases, they say, and forms branched products.

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Copyright © 2002 American Chemical Society