Gaikwad Anil V, Holuigue Alexandre, Thathagar Mehul B, ten Elshof Johan E, Rothenberg Gadi
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
Chemistry. 2007;13(24):6908-13. doi: 10.1002/chem.200700105.
Leaching of palladium species from Pd nanoparticles under C--C coupling conditions was observed for both Heck and Suzuki reactions by using a special membrane reactor. The membrane allows the passage of palladium atoms and ions, but not of species larger than 5 nm. Three possible mechanistic scenarios for palladium leaching were investigated with the aim of identifying the true catalytic species. Firstly, we examined whether or not palladium(0) atoms could leach from clusters under non-oxidising conditions. By using our membrane reactor, we proved that this indeed happens. We then investigated whether or not small palladium(0) clusters could in fact be the active catalytic species by analysing the reaction composition and the palladium species that diffused through the membrane. Neither TEM nor ICP analysis supported this scenario. Finally, we tested whether or not palladium(II) ions could be leached in the presence of PhI by oxidative addition and the formation of [Pd(II)ArI] complexes. Using mass spectrometry, UV-visible spectroscopy and 13C NMR spectroscopy, we observed and monitored the formation and diffusion of these complexes, which showed that the first and the third mechanistic scenarios were both possible, and were likely to occur simultaneously. Based on these findings, we maintain that palladium nanoparticles are not the true catalysts in C--C coupling reactions. Instead, catalysis is carried out by either palladium(0) atoms or palladium(II) ions that leach into solution.
通过使用一种特殊的膜反应器，在Heck反应和Suzuki反应的C-C偶联条件下，均观察到钯纳米颗粒中有钯物种的浸出。该膜允许钯原子和离子通过，但不允许大于5 nm的物种通过。为了确定真正的催化物种，研究了三种可能的钯浸出机理。首先，我们研究了在非氧化条件下，钯(0)原子是否能从簇中浸出。通过使用我们的膜反应器，我们证明了这种情况确实会发生。然后，我们通过分析反应组成和透过膜扩散的钯物种，研究了小的钯(0)簇是否实际上是活性催化物种。TEM分析和ICP分析均不支持这种情况。最后，我们测试了在PhI存在下，钯(II)离子是否能通过氧化加成和形成[Pd(II)ArI]配合物而被浸出。使用质谱、紫外-可见光谱和13C NMR光谱，我们观察并监测了这些配合物的形成和扩散，结果表明第一种和第三种机理都是可能的，并且可能同时发生。基于这些发现，我们认为钯纳米颗粒在C-C偶联反应中不是真正的催化剂。相反，催化作用是由浸出到溶液中的钯(0)原子或钯(II)离子进行的。
