Usteri Marc Eduard, Giannakakis Georgios, Bugaev Aram, Pérez-Ramírez Javier, Mitchell Sharon
Department of Chemistry and Applied Biosciences, Institute of Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, Zurich 8093, Switzerland.
Paul Scherrer Institute, Forschungsstrasse 111, Villigen 5232, Switzerland.
ACS Catal. 2024 Aug 7;14(16):12635-12646. doi: 10.1021/acscatal.4c03531. eCollection 2024 Aug 16.
Using heterogeneous single-atom catalysts (SACs) in the Suzuki-Miyaura coupling (SMC) has promising economic and environmental benefits over traditionally applied palladium complexes. However, limited mechanistic understanding hinders progress in their design and implementation. Our study provides critical insights into the working principles of Pd@CN, a promising SAC for the SMC. We demonstrate that the base, ligand, and solvent play pivotal roles in facilitating interface formation with reaction media, activating Pd centers, and modulating competing reaction pathways. Controlling the previously overlooked interplay between base strength, reagent solubility, and surface wetting is essential for mitigating mass transfer limitations in the triphasic reaction system and promoting catalyst reusability. Optimum conditions for Pd@CN require polar solvents, intermediate base strength, and increased water content. Our investigations reveal that high selectivity requires minimizing competitive coordination of bases and phosphine ligands to the Pd centers, to avoid homocoupling and alternative reductive elimination mechanisms giving rise to phosphonium side-products. Furthermore, in situ XAS investigations probing electronic structures and coordination environments of Pd sites further rationalize the base and ligand coordination, confirming and expanding upon previous computational hypotheses for Pd@CN. This understanding allows for designing a more selective ligand-free reaction pathway using the solvent and base to modulate the chemical environment of the active sites. We highlight the importance of environment-compatible surface tension, the creation of coordinatively available active sites, and the stabilization of partially reduced Pd centers, emphasizing the importance of mechanistic studies to advance the design of SACs in organic liquid phase reactions.
在铃木-宫浦偶联反应（SMC）中使用多相单原子催化剂（SACs）比传统应用的钯配合物具有可观的经济和环境效益。然而，对其作用机理的了解有限阻碍了它们在设计和应用方面的进展。我们的研究为Pd@CN（一种有前景的用于SMC的SAC）的工作原理提供了关键见解。我们证明碱、配体和溶剂在促进与反应介质形成界面、活化钯中心以及调节竞争反应途径方面起着关键作用。控制碱强度、试剂溶解度和表面润湿性之间先前被忽视的相互作用对于减轻三相反应体系中的传质限制和提高催化剂的可重复使用性至关重要。Pd@CN的最佳条件需要极性溶剂、中等碱强度和增加的水含量。我们的研究表明，高选择性要求尽量减少碱和膦配体与钯中心的竞争性配位，以避免均偶联和产生鏻副产物的替代还原消除机制。此外，原位X射线吸收光谱（XAS）研究探测钯位点的电子结构和配位环境，进一步使碱和配体配位合理化，证实并扩展了先前关于Pd@CN的计算假设。这种理解使得能够设计一种更具选择性的无配体反应途径，利用溶剂和碱来调节活性位点的化学环境。我们强调了环境兼容的表面张力、创建配位可用的活性位点以及稳定部分还原的钯中心的重要性，强调了机理研究对于推进有机液相反应中SACs设计的重要性。
