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 π 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 σ donation, thereby
weakening the Rh–Cipso bond and
facilitating metal-to-Cipso methyl migration.