Role of MAO in Chromium-Catalyzed Ethylene Tri- and Tetramerization: A DFT Study

The successful [Ph 2 PN( i Pr)PPh 2 ]Cr-catalyzed trimerization and tetramerization of ethylene to 1-hexene and 1-octene requires the presence of a cocatalyst, of which methylaluminoxane (MAO) is particularly relevant. Density functional theory (DFT) calculations are reported on the interaction of various MAO models with chromacycloheptane intermediates. Chromacycloheptanes are well established to be important intermediates during the selective chromium-catalyzed trimerization and tetramerization of ethylene, effectively resembling appropriate models for a study of MAO interaction with chromium complexes during active catalysis. A systematic study is presented evaluating different (AlOMe) n cage structure models for MAO, as both “classic” MAO cages and cages activated by interaction with trimethylaluminium (TMA), comparing methylation aptitudes of TMA versus MAO models and evaluating the interaction of MAO models with chromacycloheptane intermediates. From the results the importance of the use of realistic ligand and large MAO models is shown to be a prerequisite for obtaining accurate catalyst activation data. In particular, use of a “stripped-down” ligand [Me 2 PN(Me)PMe 2 ] on chromacycloheptane in combination with a relatively small MAO cage [(AlOMe) 6 ·(AlMe 3 )] results in the optimization of formally coordinated chromacycloheptane−MAO complexes, even with increased steric congestion on chromium upon coordination of an additional ethylene moiety. In contrast, use of the “full” ligand [Ph 2 PN( i Pr)PPh 2 ] and larger MAO cages [(AlOMe) 9 ·(AlMe 3 )] shows that while the formation of formally coordinated chromacycloheptane−MAO complexes are successfully optimized in the absence of additional ethylene, only dissociated ion-pair complexes are present when an additional ethylene molecule is introduced. From these results important insight is gained on the role of MAO during catalysis, as well as the model requirements for both MAO and chromium complexes to conduct fundamental theoretical studies in selective chromium-catalyzed ethylene oligomerization.