The poisonous truth about the mercury drop test: the effect of elemental mercury on Pd(0) and Pd(II)ArX intermediates

Abstract

The mercury drop test is a widely used method for distinguishing between homogeneous and heterogeneous catalysis in organometallic systems. However, recent research has highlighted the limitations of this test due to the intrinsic reactivity of some organometallic compounds towards elemental mercury. In this study, we used real-time mass spectrometry with charge-tagged substrates to investigate the effect of elemental mercury on LnPd0 and PdIIArX intermediates common in palladium-catalysed cross-coupling reactions. Our findings demonstrate that mercury can interact with both species through redox-transmetallation and amalgamation processes, leading to a decrease in catalytic activity. This result further calls into question the fundamental assumption of mercury selectivity towards heterogeneous catalytic species. These results highlight the importance of careful consideration of the results the mercury drop test provides and encourages further research to gain a more comprehensive understanding of catalyst poisoning mechanisms.