Remember the good old days when heterogeneous catalysis was heterogeneous catalysis and homogeneous catalysis was homogeneous catalysis?. Take for example palladium: the metal itself is a well known heterogeneous catalyst and there is tris(dibenzylideneacetone)dipalladium(0), the soluble and homogeneous cat. Well not anymore!. At least according to Zalesskiy & Ananikov in the journal Organometallics (DOI). They found that commercially available Pd₂(dba)₃ contains up to 40% palladium nanoparticles and this is relevant because it a) is a catalyst superstar and b) researchers rarely bother to make their own. Zalesskiy & Ananikov investigated the catalyst in a chloroform solution with proton NMR which contrary to expectation yields very complex spectra. Therefore they looked at the DOSY variation and also at 2D-NMR. This allowed detection and quantification of free dba and two yet to be identified Pd₂(dba)₃ isomers and according to the researchers, if there is free dba then there has to be free Pd (as nanoparticles) as well. In commercial samples the ratio can vary between 1:0.09 and 1:0.56.
The finding may explain why Pd₂(dba)₃ catalysed reactions are notoriously inconsistent. Take for example OS 68:47 with a typical Pd₂(dba)₃ application where the supplier was Strem Chemicals but where The submitters noted that distinctly lower enantioselectivity was obtained when the reagent of other suppliers was used.
Luckily Zalesskiy & Ananikov offer a recipe for fresh and reliable Pd₂(dba)₃. So lazy chemists pay attention! Take palladium acetate, sodium acetate and dibenzylideneacetone in methanol (the reducing agent). Stir at 40°C for 3 hours. Isolate the brown solid.