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Water-Soluble Palladium, Copper, and Nickel Catalysts and their Formation in Ligand-Free Suzuki-Miyaura Cross-Coupling Reactions

CP 114
Speaker(s) / Presenter(s):
Priya Karna

Karna Graphic  Karna Photo

"Transition-metal catalyzed Suzuki-Miyaura (SM) cross coupling is a powerful synthetic method for constructing carbon-carbon and carbon-heteroatom bonds in designing organic compounds, agrochemicals, pharmaceuticals, and precursors for materials.  However, the nature of catalysis and identity of the transition metal catalysts used in these reactions remain under debate or unknown. This work reports the studies of three metals: Pd, Cu, and Ni. Pd-nanocluster catalysts and their formation in ligand-free SM reactions with Pd(II) nitrate as a precatalyst was investigated. The catalysts are water-soluble neutral Pd tetramer and trimer in their singlet electronic states as identified by UV-Vis absorption spectroscopy and are formed by leaching of spherical Pd(0) nanoparticles with an average diameter of about three nanometers. The Pd(0) nanoparticles are produced by reducing Pd(II) nitrate and characterized with transmission electron microscopy (TEM) and Pd-K edge extended x-ray fine structure spectroscopy (EXAFS). The Pd(II) reduction is induced by ethanol and enhanced by potassium hydroxide and monitored with x-ray photoelectron spectroscopy (XPS). For the Cu-catalyzed SM coupling, a water-soluble active molecular catalyst, and its formation in the ligand-free SM cross-coupling reactions with copper iodide as the precatalyst in aqueous solutions has been reported. The SM coupling is homogeneous in nature, and the molecular catalyst is Cu(OH) in its singlet electronic state also identified by experimental and computational UV-Vis absorption spectroscopy. The Cu(OH) catalyst is generated through the leaching of oval-shaped Cu2O nanoparticles, which are characterized with X-ray Auger electron spectroscopy, X-ray absorption spectroscopy (XAS), and TEM. The soluble Cu(OH) species is stable for at least four weeks under ambient conditions. Similarly, for Ni-catalyzed ligand-free SM coupling, the active Ni catalyst is reported as Ni(0) species with Ni(0) powder as the precatalyst. The SM coupling is also homogeneous in nature. The water-soluble active Ni(0) catalyst is generated through the leaching of Ni(0) nanoparticles, which are characterized with XPS, XAS, and TEM. The water-soluble active Ni(0) catalyst species is stable for at least eight weeks under ambient conditions. Thus, this talk showcases the nature of catalysis and the identity of catalytically active species in ligand-free SM reactions catalyzed by Pd, Cu, and Ni transition metals."