Computational Studies of Reactions of Insertion of Rhodium(I) and Iridium(I) into N–H, N–CH3, and NCH2–H Bonds of the Diarylamine-Based PNP Pincer Ligands

Abstract

This work presents the investigation by DFT methods of the mechanism of N–Me and N–H oxidative addition in reactions of the secondary amine form of the PNP pincer ligand 4-Me-2-(iPr2P)-C6H3)2NH (or PN(H)P), its N-methylated derivative 4-Me-2-(iPr2P)-C6H3)2NMe (or PN(Me)P), and a version of the latter whose aromatic rings are “tied” with a CH2CH2 linker (or TPN(Me)P) with Rh(I) and Ir(I). Reactions were considered by starting from (κ3-PN(H)P)MCl, (κ3-PN(Me)P)MCl, and (κ3-TPN(Me)P)MCl (M = Rh, Ir). Oxidative addition from (κ3-PN(H)P)MCl to give (PNP)M(H)(Cl) is predicted to proceed with essentially no barrier via direct migration of H from N to the metal. The analogous direct migration of Me from N to the metal is predicted to be the dominant mechanism for both Rh systems, with the calculated barrier for (κ3-PN(Me)P)RhCl of 21.8 kcal/mol being in reasonable agreement with the experimental value of 24.0(18) kcal/mol. For Ir, an alternative pathway that involves initial NCH2–H oxidative addition, followed by CH2 extrusion and C–H recombination, is calculated to be competitive with direct Me transfer, especially for the “tied” ligand where it is preferred. This alternative pathway entails prohibitively high barriers for both Rh systems (>35 kcal/mol), which can be traced to the high energy of the intermediate in which a CH2 carbene is bound to a RhIII center. In general, the energies of all barriers and intermediates are lower with the “tied” ligand. DFT calculations also evaluate the energetics of the NCH2–H oxidative addition intermediates. These were observed experimentally for only the “tied” ligand system (for both Rh and Ir), and the DFT energies are consistent with these observations.