Homogeneous catalysis not always what it seems

19 March 2012 -Catalysis

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.