The effect of substituents on dimer formation and cation–anion interaction in silicon bridged bisindenyl zirconocene propylene polymerization catalysts

Abstract

The dimerization and ion-pair formation enthalpies of advanced silicon bridged bisindenyl zirconocenes have been estimated at the density functional (DFT) level of theory. According to our calculations (based on molecular mechanics geometry optimization with subsequent DFT geometry relaxation), a properly designed bisindenyl ligand sphere helps to avoid catalyst dimerization and reduces coordination of methylaluminoxane (MAO) anions to cationic sites. Interestingly, we also found a correlation between the dipole moment of the active species considered here and their catalytic performance. These results suggest that an appropriate substituent pattern favours complexation of propylene monomers as the dominant mechanism to stabilize electron deficient active sites due to a diminution of particular intermolecular forces which yield catalytically inactive resting states.