news of the week MARC H 19, 201 2 E D I TE D BY W I L L I A M G. SC H UL Z & C RA I G BE T T E NH AUS E N
NANO NUISANCE FOR PALLADIUM SOURCE ORGANOMETALLIC CHEMISTRY:
Catalyst precursor’s breakdown complicates efficiency estimates
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HEMISTS WHO USE the popular palladium
ORGANOMETALLICS
source tris(dibenzylideneacetone)dipalladium [Pd2(dba)3] right out of the bottle might want to rethink their approach and revisit their data, a Rus sian team reports (Organometallics, DOI: 10.1021/ om201217r). The researchers found that as much as 40% of a given commercially available sample of the palladium complex decomposes to palladium nano particles in a range of sizes, which could lead to errors in calculations of catalyst performance. Laboratories worldwide rely on Pd2(dba)3, an easy-to-make, relatively air-stable compound, to make palladium catalysts for reactions as diverse as crosscouplings and carbene carbonyl ations. Despite the compound’s popularity, its nature in solution hasn’t been fully described. Valentin P. Ananikov and gradu ate student Sergey S. Zalesskiy, both of the Russian Academy of Sciences, determined what Pd2(dba)3 looks like in solution with a battery of NMR tech niques. They learned that the catalyst precursor decomposes to dibenzylideneacetone and Pd nanoparticles. With scanning electron microscopy, the team determined that the nanoparti cles come in a range of sizes. With inductively coupled plasma mass spectrometry, they confirmed that the nanoparticles are made of palladium. The problem with generating nanoparticles is that they, too, can act as catalysts. Catalyst im purities often drive chemistry of their own. But each of the various nanoparticles is likely to have different properties from the desired palladium catalyst, so the sheer complexity could wreak havoc on estimations of turnover
number, catalyst loading, and other important param eters, Ananikov explains. It is possible that some of the many studies in which Pd2(dba)3 was used “as is” contain mistakes and should be corrected, he adds. He and Zalesskiy also report their own route for pre paring Pd2(dba)3 at high purity and identify NMR signals to watch so chemists can check the reagent’s purity right before using it. If Pd2(dba)3 is pure at that point, then it is reasonable to assume Pd2(dba)3 is acting as a homo geneous palladium source, Ananikov says. However, he adds, “partially decomposed Pd2(dba)3 is a much more complicated catalyst precursor.” “The fact that commercial Pd2(dba)3 contains nanoparticle impurities is not “The variable quality totally surprising” because other groups of Pd2(dba)3 is known, have observed signs of this decomposi but it is surprising that tion, says Antonio M. Echavarren, an or ganometallic chemist at Spain’s Institute it varies so much.” of Chemical Research of Catalonia. “What is really remarkable is the fact that certain samples could contain up to 40% palladium nanopar ticles,” he says. “The use of starting complexes of low purity could have a significant effect on the initial rates and reproducibility of catalytic reactions.” “The variable quality of Pd2(dba)3 is known, but it is sur prising that it varies so much,” agrees MIT organometallic chem ist Stephen L. Buchwald, who de scribes the work as “interesting and very well done. “However, things like turnover numbers are most important in process and manufacturing ven ues,” Buchwald adds. “I would expect in these situations that a very high level of quality control, including ascertaining reagent purity, is already carried out.” “I myself have long worked with Pd2(dba)3 and often ob served irreproducible reactivity,” says Vladimir Gevorgyan, who develops palladium-catalyzed reactions at the University of Il linois, Chicago. “From now on, I will make sure that my group members analyze Pd2(dba)3 prior to use.”—CARMEN DRAHL
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Nanoparticles from partially decomposed samples of Pd2(dba)3, shown by SEM at different resolutions, could throw off estimates of catalyst efficiency.
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