Toward the Prediction of Activity in the Ethylene Polymerisation of ansa-Bis(indenyl) Zirconocenes: Effect of the Stereochemistry and Hydrogenation of the Indenyl Moiety.

Abstract

A combined experimental and quantum chemical study has been performed on rac- and meso-[Zr{1-Me2 Si(3-η5 -C9 H5 Et)2 }Cl2 ] (rac- and meso-1) and their hydrogenated forms (rac- and meso-2) to understand ligand effects and guide ligand design for more active ansa-bis(indenyl) zirconocenes for the polymerisation of ethylene. The rac-ansa-zirconocene rac-[Zr(1-Me2 Si{3-Et-(η5 -C9 H9 )}2 )Cl2 ] (rac-2) has been prepared and fully characterised by NMR spectroscopy and elemental analysis. The molecular structure of rac-2 has also been determined by single-crystal XRD. The behaviour of the catalysts was analysed in the polymerisation of ethylene and higher activities were obtained for rac-1 and its hydrogenated form rac-2. The influence of the stereochemistry and hydrogenation of the indenyl ligand on the experimental activities has been evaluated by computational studies. The differences along the reaction pathway are dominated by changes in the relative stabilities of the catalytic intermediates. A hybrid density functional B3LYP study, in the presence of toluene as the solvent, indicates that the rac forms give rise to more active species than their meso counterparts. The hydrogenation of the rac forms is a very promising approach to increase activities in polymerisation, in contrast to the meso forms. Finally, the global mechanism rate constants for the polymerisation reaction for each metallocene were calculated by using the thermodynamic formulation of transition-state theory to complement the computational study.