C−S and C−H Bond Activation of Thiophene by Cp*Rh(PMe₃):  A DFT Theoretical Investigation

A theoretical analysis of the C−S and C−H bond-activation pathways involving thiophene and Cp*Rh(PMe 3 ) is presented in which B3LYP density-functional theory is utilized. In addition to the traditional pathway which connects the η 2 -coordinated intermediate with the C−H bond activation products, a new pathway is discussed which connects the η 1 S-bound intermediate with these products. Calculations are performed in the basis of molecular orbitals of the interacting fragments for the reactive η 1 - and η 2 -coordinated intermediates to examine the orbital interactions and density transfer between the fragments. The calculated binding energy of the η 1 -coordinated intermediate is 15.5 kcal/mol weaker than that for the η 2 -coordinated intermediate due to the increased energy separation and reduced density transfer between the molecular orbitals of the interacting fragments. The structure of the transition state connecting the η 1 -coordinated intermediate with the C−S bond-activated product involves the strong overlap of the HOMO of the metal fragment with the LUMO of thiophene which results in the close proximity of the distal carbons on thiophene with the methyl groups of the Cp* ligand. Substitution of bulky groups at the 3 and 4 positions on thiophene may result in a significant steric component to the C−S activation barrier. Substitution of bulky groups on all carbons results in a significant thermodynamic component to the instability of the ring-opened product. Preliminary results indicate that the lack of reactivity toward C−S bond activation in Cp*Re(CO) 2 is due to the participation of the electron withdrawing carbonyl groups in the transfer of density between the interacting fragments.