CO-Induced Methyl Migration in a Rhodium Thiophosphoryl Pincer Complex and Its Comparison with Phosphine-Based Complexes: The Divergent Effects of S and P Donor Ligands

A cationic Rh(III) aryl-methyl complex, bearing a thiophosphoryl-based SCS-type pincer ligand, undergoes facile migratory insertion upon addition of CO, thereby affording a Rh(III) aryl-acetyl complex. This reactivity diverges from that of structurally analogous Rh(III) aryl-methyl complexes of phosphine-based PCP- and PCN-type pincer ligands, which have been previously shown to undergo C-C reductive elimination upon addition of CO, thereby giving C-C agostic Rh(I) complexes. A comparative DFT study of CO migratory insertion in the SCS and PCP complexes reveals that the difference in reactivity originates from the higher electrophilicity of the sulfur atoms of the SCS ligand relative to the phosphorus atoms of the PCP ligand. This leads to decreased Rh -> CO pi back-donation in the SCS system, resulting in CO labilization and facilitating the metal-to-CO methyl migration. In the PCP system it leads to stronger Rh-CO bonding, which enhances OC -> Rh -> arene sigma donation, thereby weakening the Rh-C-ipso bond and facilitating metal-to-C-ipso methyl migration.