Gas-Phase Energetics of Reductive Elimination from a Palladium(II) N-Heterocyclic Carbene Complex



Energy-resolved collision-induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N-heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol−1, whereas competitive ligand dissociation requires 47.1(17) kcal mol−1. The resulting three-coordinate palladium complex readily undergoes reductive C[BOND]C coupling to give the phenylimidazolium π complex, for which the binding energy was determined to be 38.9(10) kcal mol−1. Density functional calculations at the M06-L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate-determining step for the direct reductive elimination process switches from the C[BOND]C coupling step to the fragmentation of the resulting σ complex at low activation energy.