Activation of Racemic Ethylene-Bridged Bis(indenyl)-Type Siloxy-Substituted Zirconocenes with Methylaluminoxane. A Combined UV/vis Spectroscopic and ab Initio Hartree−Fock Study

Lowest energy ligand-to-metal charge-transfer (LMCT) transitions of racemic siloxy-substituted ethylene-bridged bis(indenyl)-type zirconocenes reacted with methylaluminoxane (MAO) were studied using UV/vis spectroscopy in combination with the ab initio Hartree−Fock method. The LMCT absorptions were recorded at various [Al]/[Zr] ratios, representing dichloride, methylated, and cationic forms of the zirconocenes. The experimental LMCT absorption energies of each intermediate were compared to the calculated energies of the indenyl ligand-based HOMO and the zirconium-based LUMO. The lowest energy LMCT absorptions were directly proportional to the calculated LUMO−HOMO energy gaps. Furthermore, the LMCT absorption energies were demonstrated to correlate with ethylene polymerization activity of the siloxy-substituted complexes. This correlation was clarified by a frontier orbital analysis, suggesting that decreasing absorption energy is related to the feasibility of the activation of the catalyst.