Modeling Key Pathways Proposed for the Formation and Evolution of “Cocktail”-Type Systems in Pd-Catalyzed Reactions Involving ArX Reagents

Abstract

Reversible leaching of palladium nanoparticles occurs in a variety of catalytic reactions including cross-couplings, amination, the Heck reaction, etc. It is complemented by capturing of soluble palladium species on the surface of nanoparticles and de novo formation of nanoparticles from Pd precatalysts. We report here a detailed computational study of leaching/capture pathways and analysis of related stabilization energies. We demonstrate the validity of the "cocktail-of-species" model for the description of Pd catalysts in ArX oxidative addition-dependent reactions. Three pools of Pd species were evaluated, including (1) the pool of catalytically active Pd nanoparticles with a high concentration of surface defects, (2) the pool of monomeric and oligomeric L[ArPdX]_nL species, and (3) the pool of irreversibly deactivated Pd. Stabilization by ArX oxidative addition, coordination of base species, and binding of X⁻ anions (derived from salt additives) were found to be crucial for "cocktail"-type systems, and the corresponding reaction energies were estimated. An inherent process of ArX homocoupling, leading to the formation of Pd halides that require re-activation, was considered as well. The pool of irreversibly deactivated Pd comprises nanoparticles with (1 1 1) and (1 0 0) facets and Pd in the bulk form. The study is based on DFT modeling and specifies the role of Pd nanoparticles in (quasi )homogeneous coupling reactions involving ArX reagents.