A DFT Quantum-Chemical Study of Ion-Pair Formation for the Catalyst Cp2ZrMe2 /MAO

Abstract

A process of ion-pair formation in the system Cp 2 ZrMe/methylaluminoxane (MAO) has been studied by means of density functional theory quantum chemical calculations for MAOs with different structures and reactive sites. An interaction of Cp 2 ZrMe 2 with a MAO of the composition (AlMeO) 6 results in the formation of a stable molecular complex of the type Al 5 Me 6 O 5 Al(Me)O-Zr(Me)Cp 2 with an equilibrium distance r(Zr-O) of 2.15 A. The interaction of Cp 2 ZrMe 2 with true MAO of the composition (Al 5 Me 12 O 6 ) proceeds with a tri-coordinated aluminum atom in the active site (OAlMe 2 ) and yields the strongly polarized molecular complex or the μ-Me-bridged contact ion pair (d) [Cp 2 (Me)Zr(μMe)Al]≡ MAO] with the distances r(Zr-μMe) = 2.38 A and r(Al-μMe) = 2.28 A. The following interaction of the μ-Me contact ion pair (d) with AlMe 3 results in a formation of the trimethylaluminum (TMA)-separated ion pair (e) [Cp 2 Zr(μMe) 2 AlMe 2 ] + -[MeMAO] with r[Zr-(MeMAO)] equal to 4.58 A. The calculated composition and structure of ion pairs (d) and (e) are consistent with the 13 C NMR data for the species detected in the Cp 2 ZrMe 2 /MAO system. An interaction of the TMA-separated ion pair (e) with ethylene results in the substitution of AlMe 3 by C 2 H 4 in a cationic part of the ion pair (e), and the following ethylene insertion into the Zr-Me bond. This reaction leads to formation of ion pair (f) of the composition [Cp 2 ZrCH 2 CH 2 CH 3 ] + -[Me-MAO] - named as the propyl-separated ion pair. Ion pair (f) exhibits distance r[Zr-(MeMAO)] = 3.88 A and strong C γ -agostic interaction of the propyl group with the Zr atom. We suppose this propyl-separated ion pair (f) to be an active center for olefin polymerization.