Abstract
The palladium-catalyzed Suzuki-Miyaura cross-coupling (SMCC) reaction is an essential technique for C-C bond formation. It is considered to occur through two distinct pathways involving homogeneous and heterogeneous mechanisms. However, there is still debate about these mechanisms due to the lack of direct structural evidence in both spatial and temporal terms, especially regarding the conversion of active Pd species. In this study, the Pd single-atom catalyst (Pd SAC) SMCC reaction was monitored in real-time using ambient mass spectrometry (AMS) to study the conversions of Pd species both on the catalyst surface and in the liquid phase. This revealed a leaching-oxidizing-landing path as a contributors to the heterogeneous process which involves heterogeneous oxidative addition, Pd leaching along with transmetallation (rather than oxidative addition), and subsequent oxidation-landing back onto the catalyst surface. The leaching-oxidizing-landing mechanism of Pd active sites during the Pd SAC-catalyzed SMCC reaction provides an explanation for the Pd migration on the catalyst surface. A crucial role of molecular oxygen during the SMCC reaction was revealed and attributed to the re-deposition of active Pd species through coordination with the catalyst support. Overall, the leaching-oxidizing-landing mechanism of the SMCC reaction has been revealed and it not only provides insights into mechanistic studies and catalyst designs but also expands AMS applications.