Selective C−C vs C−H Bond Activation by Rhodium(I) PCP Pincer Complexes. A Computational Study

A theoretical study of the oxidative addition of C−C vs C−H bonds to a rhodium(I) complex with PCP-type ligands has been carried out. Special attention has been paid to the effect of different bulky substituents at the phosphorus atoms of the chelate ligand. Therefore, B3LYP/lanl2dz+p//B3LYP/lanl2dz and ONIOM(B3LYP/lanl2dz+p:B3LYP/lanl2dz)//ONIOM(B3LYP/lanl2dz:HF/lanl1mb) methods have been utilized. According to the calculations, C−H activation is always the kinetically favored process (ΔΔ E ⧧ 20 kJ·mol - 1 ), though the C−C activation product is more stable (ΔΔ E 20 kJ·mol - 1 ). C−H addition is a reversible process; the product of the C−H activation can interconvert to the C−C activation product via an intermediate structure. Bulky substituents are found to increase the barrier for C−H activation relative to that for C−C activation. With additional ligands (e.g., phosphines), hexacoordinate complexes are formed. This is more favorable for the C−C activation products. Our calculations show that the activation reaction proceeds via complexes with a pentacoordinated rhodium atom. Thus, in the presence of donor ligands, the activation reaction is inhibited.