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Web Date: March 13, 2012

Nano Nuisance for Palladium Source

Organometallic Chemistry: Catalyst precursor’s breakdown complicates efficiency estimates
Department: Science & Technology
News Channels: Nano SCENE, Materials SCENE
Keywords: Catalysis, organometallic chemistry, palladium, nanoparticles, impurity
Nanoparticles from partially decomposed samples of Pd2(dba)3, shown by SEM at different resolutions, could throw off estimates of catalyst efficiency
Credit: Organometallics
SEM images of nanoparticles that result from decomposition of a popular Palladium catalyst precursor
Nanoparticles from partially decomposed samples of Pd2(dba)3, shown by SEM at different resolutions, could throw off estimates of catalyst efficiency
Credit: Organometallics

Chemists who use the popular palladium source tris(dibenzylideneacetone)dipalladium [Pd2(dba)3] right out of the bottle might want to rethink their approach and revisit their data, a Russian 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 nanoparticles 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 cross-couplings and carbene carbonylations. Despite the compound’s popularity, its nature in solution hasn’t been fully described. Valentin P. Ananikov and graduate 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 techniques. They learned that the catalyst precursor decomposes to dibenzylideneacetone and Pd nanoparticles. With scanning electron microscopy, the team determined that the nanoparticles come in a range of sizes. They confirmed the nanoparticles are made of palladium with inductively coupled plasma mass spectrometry.

The problem with generating nanoparticles is that they, too, can act as catalysts. Catalyst impurities often drive chemistry of their own. But each of the various nanoparticles are 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 parameters, 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 preparing 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 homogeneous palladium source, Ananikov says. However, he says, “partially decomposed Pd2(dba)3 is a much more complicated catalyst precursor.”

“The fact that commercial Pd2(dba)3 contains nanoparticle impurities is not totally surprising” because other groups have observed signs of this decomposition, says Antonio M. Echavarren, an organometallic chemist at Spain’s Institute of Chemical Research of Catalonia. “What is really remarkable is the fact that certain samples could contain up to 40% palladium nanoparticles,” he says. “The use of starting complexes of low purity could have a significant effect on the initial rates and reproducibility of catalytic reactions.”

“I myself have long worked with Pd2(dba)3 and often observed irreproducible reactivity,” says Vladimir Gevorgyan, who develops palladium-catalyzed reactions at the University of Illinois, Chicago. “From now on, I will make sure that my group members always analyze Pd2(dba)3 prior to use.”

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Dr Bahij H Sakakini (Tue Jul 31 19:23:32 EDT 2012)
do we know what the Pd nano-particles formed do exactly ....has there been any studies in which the complex has been fully decomposed and the same reaction done under the same conditions to see if a reaction goes and if yes has the products been identified....
It might be worth it to think of the lines of investigating the effect of the extent of decomposition on the catalytic behaviour both in terms of activity and selectivity. I guess this will shed some light into the non-reproducibility of the performance of the catalysts observed

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