Use of Ligand Steric Properties to Control the Thermodynamics and Kinetics of Oxidative Addition and Reductive Elimination with Pincer-Ligated Rh Complexes

Oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using (RPNP)RhX complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was studied to deduce the effect of the size of the phosphine substituents. Using (^RPNP)RhCl as the starting material, oxidative addition of MeI was observed to produce (^RPNP)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form (^RPNP)RhI. The thermodynamics and kinetics vary with the identity of the substituent R on phosphorus of the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl, mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An Eyring plot for the oxidative addition of MeI to (^(tBu)PNP)RhCl in THF-d₈ is consistent with a polar two-step reaction pathway, and the formation of [(^(tBu)PNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk affects the reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol⁻¹. The ligand steric bulk is calculated to have a reduced effect (a few kcal mol⁻¹) on S_N2 addition barriers, which only require access to one side of the square plane.