A DFT Quantum‐Chemical Study of Ion‐Pair Formation for the Catalyst Cp2ZrMe2 /MAO |
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Authors: | Ivan I. Zakharov Vladimir A. Zakharov |
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Abstract: | 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 r[Zr–(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 r[Zr–(MeMAO)] = 3.88 Å and strong Cγ‐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. |
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Keywords: | calculations density functional theory (DFT) ion pair metallocene catalysts methylaluminoxane polyolefins |
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