Factors Controlling Stability and Reactivity of Dimeric Pd(II) Complexes in C-H Functionalization Catalysis

We have systematically explored the factors impacting the stability and reactivity of the [Pd(O2CR)(2)](2) and [Pd(OAc)(DG-Ar)](2) dimers in Pd(OAc)(2)-catalyzed and directing group (DG)-assisted C-H functionalization using density functional theory (DFT) calculations. It was shown that the palladium acetate dimer stability (vs the monomer) predominantly arises from interactions between the paddlewheel ligands and the Pd centers. R-substitution in Pd(O2CR)(2) leading to an increase (or decrease) in the electron density of the ligand orbitals polarizes the Pd-ligand interaction and weakens (or strengthens) the stability of the [Pd(O2CR)(2)](2) dimer relative to the monomer. The nature of the substrate/ligand-Pd interaction is shown to be another factor contributing to the nuclearity and reactivity of the Pd-acetates in directing group-assisted C-H functionalization. For the [Pd(OAc)(DG-Ar)](2) dimer, we have found that the major interactions contributing to the stability of the dimer are the bridging acetate interactions and interactions between the substrates, such as pi-pi stacking. Our data reveal that, starting from the dinuclear [Pd(OAc)(2)](2) complex, pathways involving either monomerization of [Pd(OAc)(2)](2) or C-H activation by the dimer can compete with each other, and in general, dinuclear complexes require a higher C-H activation barrier than mononuclear complexes. Even if C-H activation of the substrate is initiated by the Pd(OAc)(2) monomer, involvement of the dimeric [Pd(OAc)(DG-Ar)](2) complex in C-H functionalization, especially for electrophiles with small C-X bond formation barriers, cannot be excluded.