Substituent Effects on the Stability of Pd– NHC Complexes: Switching From Molecular to “Cocktail‐Type” Catalysis

ABSTRACT The evolution of catalytic systems based on transition metal complexes under catalytic reaction conditions leads to the formation of a catalyst cocktail, i.e., an ensemble of species with different structures, each capable of exhibiting catalytic activity. Transformation of molecular Pd(II)–NHC complexes into a catalytically active cocktail form involves R–NHC coupling, which results in the formation of low‐coordinated metal centers and [R–NHC] (+) [ X ] (−) salt ( X = halogen). In this work, using DFT methods, it is demonstrated that one factor influencing the R–NHC coupling process is substituent effects within the pyridine or phosphine ligand of the Pd(II) complex ( R = Me, Ph; NHC = IMe, IPr, IMes). The presence of electron‐donating (–OMe) or electron‐withdrawing (–CN; –F) substituents in the ligand molecule significantly influences both the activation energy and the reaction energy of the R–NHC coupling process. In particular, electron‐accepting substituents reduce the potential barrier for this process and completely shift the equilibrium toward the product of the R–NHC coupling, whereas electron‐donating substituents possess the opposite effect. Thus, the effect of substituents can act as a “switch” facilitating the transition from molecular catalysis to “cocktail‐type” catalysis.