DFT modelling of a diphosphane - N-heterocyclic carbene-Rh(i) pincer complex rearrangement: a computational evaluation of the electronic effects in C-P bond activation.

DFT calculations confirmed that the rearrangement of a PCP-Rh-H pincer to a CCP-Rh-phosphane pincer occured by C-P oxidative addition (ΔG‡ = 29.5 kcal mol-1 , rate-determining step), followed by P-H reductive elimination (ΔG‡ = 4.8 kcal mol-1 ). The oxidative addition proceeded via a 3-centered transition state and is accelerated by electron-withdrawing substituents p- to the reacting C-P bond, resulting in a reaction constant (ρ) of 2.12 for ΔG‡ and 2.76 for ΔH‡ in a Hammett-type linear free energy relationship. AIM wavefunction analyses indicated a decrease in the negative charge on the carbon bonded to Rh with a concomitant increase in the positive charge on the latter. The electronic density at the Rh-P bond critical point and the atomic charge on Rh correlate well with the Hammett constants (σ) of the p-substituents. The replacement of the Rh-bound hydride with other anions (CH3 , Ph, t-Bu, OH, F, Cl, and CN) results in a decrease in the OA barrier only for CH3 , which is in accordance with the experimental results. The reductive elimination occurs via a 3-centered (Rh, H, P) transition state, which adopts a conformation wherein the steric clash between the i-Pr groups is minimized, followed by recomplexation of Rh and the newly formed (i-Pr)2 PH by a conformational twist around the Rh-P axis.