Nano Nuisance For Palladium Source

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. 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 impurities 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 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 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 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.”

“The variable quality of Pd₂(dba)₃ is known, but it is surprising that it varies so much,” agrees MIT organometallic chemist Stephen L. Buchwald, who describes the work as “interesting and very well done.

“However, things like turnover numbers are most important in process and manufacturing venues,” 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 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 analyze Pd₂(dba)₃ prior to use.”