Substituent Effects on the Stability of Pd-NHC Complexes: Switching From Molecular to "Cocktail-Type" Catalysis

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.