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.