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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2018, Vol.47 (8), p.2662-2669
• Main Authors: Qin, H.-L, Leng, J, Zhang, W, Kantchev, E. A. B
• Format: Article
• Language: English
• Subjects: Atomic properties; Chemical bonds; Critical point; Free energy
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c7dt04759b

DFT calculations confirmed that the rearrangement of a PCP-Rh-H pincer to a CCP-Rh-phosphane pincer occured by C-P oxidative addition (Δ G ‡ = 29.5 kcal mol −1 , rate-determining step), followed by P-H reductive elimination (Δ G ‡ = 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 Δ G ‡ and 2.76 for Δ H ‡ 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 (CH 3 , Ph, t -Bu, OH, F, Cl, and CN) results in a decrease in the OA barrier only for CH 3 , 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. DFT calculations confirmed that the rearrangement of a PCP-Rh-H pincer to a CCP-Rh-phosphane pincer occured by C-P oxidative addition (Δ G ‡ = 29.5 kcal mol −1 , rate-determining step), followed by P-H reductive elimination (Δ G ‡ = 4.8 kcal mol −1 ).