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A DFT Quantum‐Chemical Study of Ion‐Pair Formation for the Catalyst Cp 2 ZrMe 2 /MAO
Author(s) -
Zakharov Ivan I.,
Zakharov Vladimir A.
Publication year - 2002
Publication title -
macromolecular theory and simulations
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.37
H-Index - 56
eISSN - 1521-3919
pISSN - 1022-1344
DOI - 10.1002/1521-3919(20020301)11:3<352::aid-mats352>3.0.co;2-2
Subject(s) - methylaluminoxane , chemistry , cationic polymerization , ion , ethylene , atom (system on chip) , crystallography , catalysis , polymer chemistry , metallocene , organic chemistry , polymerization , computer science , embedded system , polymer
A process of ion‐pair formation in the system Cp 2 ZrMe 2 /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 Å. The interaction of Cp 2 ZrMe 2 with “true” MAO of the composition (Al 8 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 Å and r (Al–μMe) = 2.28 Å. 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 Å. 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 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.