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

A process of ion‐pair formation in the system Cp₂ZrMe₂/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 Cp₂ZrMe₂ with a MAO of the composition (AlMeO)₆ results in the formation of a stable molecular complex of the type Al₅Me₆O₅Al(Me)O–Zr(Me)Cp₂ with an equilibrium distance r(Zr–O) of 2.15 Å. The interaction of Cp₂ZrMe₂ with “true” MAO of the composition (Al₈Me₁₂O₆) proceeds with a tri‐coordinated aluminum atom in the active site (OAlMe₂) and yields the strongly polarized molecular complex or the μ‐Me‐bridged contact ion pair ( d ) Cp₂(Me)Zr(μMe)Al≡MAO] with the distances r(Zr–μMe) = 2.38 Å and r(Al–μMe) = 2.28 Å. The following interaction of the μ‐Me contact ion pair ( d ) with AlMe₃ results in a formation of the trimethylaluminum (TMA)‐separated ion pair ( e ) Cp₂Zr(μMe)₂AlMe₂]⁺–MeMAO]^– with rZr–(MeMAO)] equal to 4.58 Å. The calculated composition and structure of ion pairs ( d ) and ( e ) are consistent with the ¹³C NMR data for the species detected in the Cp₂ZrMe₂/MAO system. An interaction of the TMA‐separated ion pair ( e ) with ethylene results in the substitution of AlMe₃ by C₂H₄ 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 Cp₂ZrCH₂CH₂CH₃]⁺–Me‐MAO]^– named as the propyl‐separated ion pair. Ion pair ( f ) exhibits distance rZr–(MeMAO)] = 3.88 Å 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.