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

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 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 reactivation,
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.