Ab Initio Calculations of Propylene Addition to the Metal-Methyl Bond in Titanium and Aluminum Alkyl Complexes
Kinetics and Catalysis
, E-ISSN: 1608-3210
||OLEFIN POLYMERIZATION; ETHYLENE
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
The reaction of propylene addition to the aluminum-methyl and titanium-methyl bonds in the H2AlCH3 and H2TiCH3 complexes has been examined by the ab initio SCF MO LCAO method in the valence-split Gaussian basis set. The structures of the transition state (TS) of the insertion reaction, the heats of the pi-complex formation, and the activation energies for the insertion of the coordinated propylene via the mechanisms of 1,2- and 2,1-cis-insertion were calculated. It is shown that a lower value of the free activation energy of the 1,2-addition in comparison with that of the 2,1-addition is determined by lower values of both the activation energy and entropy. It is suggested that the driving force for 1,2-insertion is the polarization of the C=C bond in propylene, due to which the electrostatic interaction in the four-center transition state stabilizes TS-1,2 more strongly than TS-2,1. The calculated results indicate that propylene is characterized by a decrease in reactivity (as compared to ethylene) in the reaction of addition to the M-R bond. This fact may be attributed to a higher position of the pi* orbital of propylene (the stronger pi bond) as compared to ethylene. It is shown that the energy of the antibonding pi* orbital of monomers (acetylene, ethylene, and propylene) can serve as a reactivity index for the reaction of addition to the metal-alkyl bond.