Mechanistic investigation of the effect of S-based poisons on Pd-catalyzed cross-coupling reactions
Date
2024
Authors
Pitipana Achchige, Nadini Thushara
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Abstract
Catalyst poisons are unwanted components in a reaction mixture that lead to the partial or total deactivation of a catalyst. Most previous work on homogeneous catalyst poisoning has focused on identifying the poisons in a catalytic cycle and their effects on catalyst performance, therefore, chemists can take measures to avoid or exclude these types of poison from the system. The chemical behavior of catalyst poisons and how the poisons bind to the catalyst are under-explored areas. Hence, an improved understanding of homogeneous catalyst poisoning is crucial because it can be applied to the systems where the reaction needs to be terminated at a certain point in a catalytic cycle, by deliberately introducing the poison into the reaction mixture. Furthermore, such studies will give molecular insight into the poisoning in homogeneous catalysts, providing the necessary understanding of the catalyst poisoning behavior.
In this work, pressurized sample infusion - electrospray ionization - mass spectrometry (PSI-ESI-MS) was used to introduce a poison into a reaction flask containing a Pd cross-coupling reaction solution, which was monitored in real-time. The combination of mass-to-charge ratio (m/z), isotope pattern, and fragmentation behavior was used to characterize the newly formed Pd-poisoned species. Tetrakistriphenylphosphine palladium(0) was used as the catalyst, and three S-based poisons: 1,2-benzenedithiol, thiourea, and N-acetylcysteine were used in this analysis. Poisoning experiments were conducted on the precatalyst Pd(0) solution and on the complexes generated after oxidative addition of an aryl halide to make Pd(II) complexes. All three of the poisons reacted rapidly with Pd(0), all via oxidation of the Pd to Pd(II) and deprotonation of the poisons, revealing significant changes in the Pd complexes. Newly formed Pd-poisoned species were identified using tandem mass spectrometry (MS/MS).
Mass spectrometry quantification of these poisoned species was a significant challenge during these studies, since MS quantification is complex due to the frequent occurrence of non-linear responses with increasing analyte concentrations. Therefore, a key part of these investigations was ensuring the instrument used was always well-calibrated.
Developing calibration curves for quantification is a time-intensive task. A novel project designed to make calibration faster and easier: A one-experiment approach to calibration, details real-time continuous calibration method was the subject of Chapter 3. The same PSI-ESI-MS technique was used in this collaboration project to construct a highly accurate and precise continuous calibration curve for achieving high-quality analytical results in both mass spectrometry and UV-Vis spectroscopy. The results were sufficiently encouraging that this method has wide scope for any analytical method that allows continuous monitoring of a solution.
Overall, this work provides the molecular insight into the field of catalyst poisoning, as well as providing a novel continuous calibration method for future use in analytical chemistry.
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Keywords
Catalyst, Poison, Pd, Cross-coupling, Mass spectroscopy, Real-time