Research Article Cite This: ACS Catal. 2019, 9, 3070−3081
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Phantom Reactivity in Organic and Catalytic Reactions as a Consequence of Microscale Destruction and ContaminationTrapping Effects of Magnetic Stir Bars Evgeniy O. Pentsak, Dmitry B. Eremin, Evgeniy G. Gordeev, and Valentine P. Ananikov* Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47, Moscow 119991, Russia
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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. KEYWORDS: catalysis, contamination effect, metal nanoparticles, PTFE destruction, irreproducible syntheses, phantom reactivity, magnetic stir bars, cross-coupling reaction in commercially available sodium carbonate.6 The Sonogashira reaction is extremely sensitive to palladium, an infinitesimal amount of which may lead to quantitative yields of the product.9 The possible role of contamination in the initiation of bimetallic catalysis has been discussed in the literature.10 Nanoparticle contamination of catalyst precursors, especially labile metal complexes,11 can significantly influence outcomes of catalytic reactions.12 Recent mechanistic studies have put clear emphasis on the dynamic nature of catalytic reactions resulting in facile initiation of the “cocktail-type” systems.13,14 Dynamic catalytic systems, particularly of the “cocktail-type,” comprise a variety of interconvertible metal centers including monometallic species, clusters, and nanoparticles.13−15 With the dynamic character of the system and catalytic activity of at least one of the components, chemical transformations can be readily initiated with virtually any source of the metal. The dynamic
1. INTRODUCTION Many catalytic reactions have been shown to proceed with ppm and even ppb amounts of metal loading.1,2 There are no doubts that trace levels of metal species in the reaction mixtures may have a paramount importance on the outcome of the reaction. The influence is especially significant when studying the reactions that are particularly sensitive to the presence of transition-metal catalysts, including cross-couplings, the Heck reaction, C−H functionalization, atomeconomic addition reactions, etc. Not surprisingly, several examples have described how catalytic processes can be influenced by the impurities contained in reaction mixtures. Buchwald and Bolm have demonstrated that the catalytic activity of iron salts in a Fecatalyzed cross-coupling reaction depends on extremely small impurities defined by commercial sources.3 Metal impurities can play important roles in other reactions, such as the chromium-mediated Nozaki−Hiyama−Kishi reaction,4 olefinations and Simmons−Smith reactions with organozinc reagents,5 and “metal-free” Suzuki or Sonogashira coupling.6−8 It has been shown that the “transition-metal-free Suzuki reaction” is essentially catalyzed by trace amounts of palladium © XXXX American Chemical Society
Received: January 22, 2019 Revised: February 21, 2019
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DOI: 10.1021/acscatal.9b00294 ACS Catal. 2019, 9, 3070−3081
Research Article
ACS Catalysis
Figure 1. Representative set of selected 17 stir bars with most visible defects and contaminations from a regular research lab (a); examples of different types of damage (b); representative examples from randomly selected laboratories (c). Statistical overview: a combined set of 60 stir bars was considered, and all types of defects present were taken into account for each stir bar (d).
similar effect using a PTFE surface as a catalyst support was described by Han and co-workers for reduction of 4nitrophenol to 4-aminophenol by NaBH4.21 Finke and coworkers have made an important contribution toward determining the role of contaminations on the outcome of catalytic transformations and pointed out the use of new stir bars.22,23 However, in spite of some warnings, plastic consumables are ubiquitously utilized in research. Moreover, due to its low cost and convenient handling, the use of PTFE-coated labware has tremendously increased in recent decades. PTFE-coated magnetic stir bars are the most employed experimental tools in chemistry and biochemistry laboratories. In this work, we investigate changes that occur with the PTFE surface during use. Significant changes in the texture of the PTFE surface, with formation of scratches and dents, typically occur, and the aging process promotes the adhesion of metal-containing contaminants. We demonstrate that a randomly selected in-use stir bar in a catalysis lab may comprise a “blend” of metal particles including catalytically active species of palladium, platinum, gold, cobalt, and iron. We show that the impurities accumulated on the surface of stir bars do exhibit high catalytic activity. The study shows how strongly the unnoticed residual species from previous experiments can change the paths of future catalytic reactions.
nature of catalysis may therefore aggravate the catalytic activity of trace contaminants.13 One of the least studied factors is the influence of inevitable deterioration and unavoidable contamination of chemical labware. PTFE-coated stir bars are washed and reused for months and years because it is widely assumed that PTFE is an inert and stable material.16 In spite of this assumption, Tölg has shown that treatment with strong acids induces degradation of PTFE surface, which contributes to Hg2+ adsorption.17 Nagy and Bazsa drew attention to the problem of destruction of plastic labware, adsorption of organic and inorganic substances and metal traces on the surface.18 The important role of contamination of the PTFE surface was noted in the situation with irreproducibility of the kinetics of a number of chemical transformations.18 Pojman and co-workers have shown that adsorption of bromine on the PTFE-coated stir bar can influence the course of the Belousov−Zhabotinskyi reaction.19 Some examples have been described where the stir bar PTFE surface has been used as a support for catalytically active nanoparticles. For instance, rhodium nanoparticle deposition on a PTFE surface was applied by Janiak and co-workers for carrying out hydrogenation of neat cyclohexene or benzene to cyclohexane with a quantitative conversion, and the authors noted that rhodium nanoparticles attached to the PTFE surface cannot be removed by conventional cleaning.20 A 3071
DOI: 10.1021/acscatal.9b00294 ACS Catal. 2019, 9, 3070−3081
Research Article
ACS Catalysis
Tangled masses of filaments, apparently of polymeric nature, are frequently observed inside the cracks. These filaments, most likely produced by destruction of the PTFE shell, effectively trap micrometer-sized particles with typical morphologies of common catalyst supports (Figure 2e). Moreover, formation of filaments, which is clearly detectable at some scratches and dents, probably corresponds to the early stage of the magnetic stir bar surface disruption (Figure 2f). The majority of cracks are located at the edges of the facets and junction ribs of the PTFE coating. The contaminant particles are mostly concentrated within these areas. 2.2. Metal Contaminants on the Stir Bar Surface. To determine the nature of the impurities, a systematic EDX analysis of the PTFE shell was carried out (Figure 3). Most of
2. RESULTS AND DISCUSSION 2.1. Types of Damage to the Stir Bar Surface. Stir bars are indispensable for carrying out any solution-phase chemical reaction. A standard stir bar consists of a magnet coated with a PTFE shell. Initially, a uniform surface bearing bright white color is typical for unused stir bars (Figure S1). Visual examination shows that even with thorough cleaning after each use, magnetic stir bars undergo noticeable changes (Figure 1a). A representative sample of 17 stir bars from a regular research lab doing catalysis and synthesis shows a number of characteristic changes: color changes, dents with contaminations, junction rib contaminations, cracks, scrapes, and scratches (Figure 1b). Repeated sampling from three other laboratories shows a similar picture (Figure 1c). Among the analyzed stir bars (total 60 pieces), only one of them can be classified as totally clean (Figure 1d). In addition to the analysis provided above, it should be emphasized that visual appearance of in-use stir bars showed a similar picture in different countries worldwide (Figure S2). Excising laboratory practice in catalysis and synthesis implies continuous usage of stir bars for a long period of time. We have applied the field emission scanning electron microscopy (FE-SEM) to compare the surface of an in-use stir bar with the surface of a brand new stir bar. The PTFE surface of the new stir bar is smooth and flawless (Figure 2a,b). The energy-dispersive X-ray spectroscopy (EDX) elemental analysis shows that the surface does not contain metal impurities. In contrast to the new stir bar, the PTFE surface of the used stir bar shows various localized defects and structural changes. It comprises multiple dents and scratches of varying depth (Figure 2c−f), as well as formidable cracks (Figure 2c).
Figure 3. Identification of various contaminant particles on the stir bar surface: visual appearance of an in-use stir bar (a); a particle of contaminant on the stir bar surface (b); EDX spectrum of the contaminant with Pd signal (c); region of the contaminated surface with a large number of aligned metal nanoparticles (d); histogram of the observed metal nanoparticles distribution by size (e); enlarged region with metal nanoparticles (f).
the particles did not contain transition metals and were composed of silicon, chlorine, sulfur, calcium, and potassium. However, a small fraction of the observed contaminant particles were clearly derived from metals or leftovers of metal/carbon catalysts (Figure 3b). An EDX spectrum corresponding to the surface of this particle comprises a noticeable palladium signal (Figure 3c). Moreover, microscopy at higher magnifications indicates the presence of metal nanoparticles on its surface, which are specifically responsible for the palladium signal (Figure 3d−f). Such impurities containing small amounts of palladium (ca.
