Attention: Your mixer may need to be washed with aqua regia

Agitators are an indispensable type of equipment in synthetic experiments. Of course, you may say that a shaker can be used instead, but the universality of agitators is broader.

The coupling reaction without metal catalysis, or the reaction catalyzed by copper instead of other precious metals such as palladium catalysis, are both attractive research directions. Because the amount of catalyst required for metal catalyzed reactions is relatively small, even trace residues have the potential to catalyze the reaction. I paid attention to it during my school years, saying that a certain metal could replace a certain metal in catalyzing reactions. However, the final experiment confirmed that the metal purity was insufficient and contained trace amounts of other metals that could catalyze reactions. So later on, for reactions catalyzed by non-metallic catalysts or catalysts such as iron copper instead of palladium, it was necessary to test the purity of the reactants or catalysts, otherwise there would be significant potential risks, including retraction and falsification.

Literature interpretation

Recently, the team of Valentine P. Ananikov from the Russian Academy of Sciences published a paper in the top American Chemical Society journal ACS Catal The article (ACS Catal. 2019, 9, 3070-3081) analyzed contaminated stirrers that may affect your reaction. The following figure compares a new stirrer with a previously used stirrer. The reaction stirred by this stirrer is the Heck reaction catalyzed by Pd/C.

In general, new stirrers are white, while used stirrers have a variety of shapes and colors. The following image shows the morphological characteristics of some used stirrers with wear and tear, including color changes, scratches, dents, and cracks.

Scanning electron microscopy shows that most of these indentations are entangled with polymer filaments, while X-ray spectroscopy suggests that these filaments may be enriched with metal particles, including palladium, gold, platinum, cobalt, or iron. Valentine P. Ananikov stated that almost all stirrers in the laboratory were contaminated to varying degrees.

In order to demonstrate the potential impact of used stirrers on chemical reactions, the author used the Suzuki coupling reaction as an example to illustrate. From the following figure, it can be seen that this type of coupling reaction does not occur in the absence of Pd/C and new stirrers. Under the condition of no catalyst and using a used stirrer, the reaction can be catalyzed.