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 (tBuPNP)­RhCl in THF-d8 is consistent with a polar two-step reaction pathway, and the formation of [(tBuPNP)­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 SN2 addition barriers, which only require access to one side of the square plane.