Phantom Reactivity in Organic and Catalytic Reactions as a Consequence of Microscale Destruction and Contamination-Trapping Effects of Magnetic Stir Bars

Abstract

Magnetic stir bars are routinely used by every chemist doing synthetic or catalytic transformations in solution. Each bar lasts for months or years, as the regular PTFE (polytetrafluoroethylene) coating is believed to be highly durable, inert, and resistant to multiple washings and cleanings. By using electron microscopy, we found out quite unexpectedly that the surface of magnetic stir bars is susceptible to microscale destruction and forms various types of defects. These microscopic defects effectively trap and accumulate trace amounts of active components from reaction mixtures, most notably metal species. Trapped in surface defects, the impurities escape elimination by washing and cleaning, thus remaining on the surface. FE-SEM/EDX analysis shows that the surface of used stir bars is littered with contaminants representing a variety of metals (Pd, Pt, Au, Fe, Co, Cr, etc.). ESI-MS monitoring corroborates the transfer of the trace metal species to reaction mixtures, while chemical tests indicate their significant catalytic activity. A theoretical DFT study reveals a remarkably high binding energy of metal atoms to the PTFE surface, especially in cases of local mechanical disruption or chemical influence. A plausible mechanism of PTFE surface contamination is suggested, and the results show that metal contamination of reusable polymercoated labware is greatly underestimated. The present study suggests that corresponding control experiments with an unused stir bar (to avoid misinterpretations due to the influence of contamination of magnetic stir bars) are a "must do" for reporting high-performance catalytic reactions, reactions with low catalyst loadings, metal-catalyst-free reactions, and mechanistic studies.

3D Printed Palladium Catalyst for Suzuki‐Miyaura Cross‐coupling Reactions

“…In order to demonstrate the potential of the SLS 3d printing technology to create functional objects of various shapes, 3d printed sleeves for magnetic stirring bars were fabricated (Figure ). Impregnation of Teflon coated magnetic stirrers with transition metals has been suggested as an attractive approach to fabrication of reusable catalysts, and our group has advanced this idea by implementing removable 3d printed sleeves for stirring bars . Such design provides the advantages of constant renewal of the liquid layer around the catalyst during the reaction due to stirring, and easy removal of the catalyst from the reaction mixture by using a magnetic rod.…”

Reliable chiral recognition with an optoelectronic nose

“…A recent paper warned researchers about the underestimated contamination brought by magnetic stir bars, which are often assumed clean after intensive washing [19], but may retain traces of their previous uses. In fact, the same may hold for every material used in chemical experiments.…”

Cobalt‐Catalyzed Cross‐Coupling of Functionalized Alkylzinc Reagents with (Hetero)Aryl Halides

“…At this point we verified that no other metal contaminants are responsible for this catalytic reaction. Using CoCl 2 (99.99 % purity) in combination with a new stir bar and reaction vessel afforded the pyridine derivative 3 a in 82 % yield (entry 13). With these results in hand, the scope of this cross‐coupling reaction was examined.…”

“…However, CoCl 2 also proved to be a suitable catalyst for this transformation affording the desired alkylated heterocycle 3 a in 52 % yield (entry 7). Various ligands were tested to further improve the reaction outcome (entries [8][9][10][11][12]. Thus, using the unsubstituted 2,2'-bipyridine led to the best coupling yield of 66 % (entry 8).…”