Steric Effects in the Binding of Hindered Dibenzothiophene Ligands in [Cp‘Ru(CO)₂(η¹(<i>S</i>)-DBTh)]⁺ Complexes

Structural studies of several complexes of the type [Cp‘Ru(CO)2(η1(S)-DBTh)]+, where Cp‘ = Cp or Cp* and DBTh = DBT, 4-MeDBT, 4,6-Me2DBT, or 2,8-Me2DBT, show that only in [Cp*Ru(CO)2(η1(S)-4,6-Me2DBT)]+ is there evidence for steric crowding between the Cp‘ and DBTh ligands. However, relative equilibrium constants (K‘) for the binding of the DBTh ligands in both the Cp and Cp* complexes show evidence of steric effects in those that contain 4-MeDBT and 4,6-Me2DBT as the K‘ values increase in the order 4,6-Me2DBT < 4-MeDBT < DBT < 2,8-Me2DBT. Kinetics studies of the substitution of the DBTh in [CpRu(CO)2(η1(S)-DBTh)]+ by phosphorus donor ligands establish the following order of DBTh lability, 4,6-Me2DBT > 4-MeDBT > DBT > 2,8-Me2DBT, which is consistent with the trend in K‘ values. The most labile DBTh ligand in both series of complexes is 4,6-Me2DBT in [Cp*Ru(CO)2(η1(S)-4,6-Me2DBT)]+, where both steric crowding and electron donation by the Cp* ligand accelerate the rate of 4,6-Me2DBT dissociation.