posted on 2004-03-15, 00:00authored byWerner Janse van Rensburg, Cronjé Grové, Jan P. Steynberg, Klaus B. Stark, Johan J. Huyser, Petrus J. Steynberg
A theoretical study of a mechanism for ethylene trimerization, catalyzed by Cr-pyrrolyl
complexes, is proposed and investigated with density functional theory methods. The selective
formation of 1-hexene is generally accepted to follow a metallacycle mechanism. A detailed
spin state analysis for active species in the mechanism shows that the triplet spin state
represents the ground spin state for all stationary points. Complete Gibbs free energy (298.15
K) surfaces are mapped for both η5- and σ-bonding modes of pyrrole, as well as a stripped-down Cl anion model and a full ClAlMe3 anion model. From the calculated results it is shown
that the proposed metallacycle mechanism is energetically favorable, with metallacycle
growth identified as the rate-determining step. In addition, it is demonstrated that different
bonding modes of pyrrole are preferred at different stages in the proposed mechanism,
effectively suggesting that ring slippage of the pyrrole occurs on the minimum energy path
on the potential energy surface. From the calculated results important insight is gained
into the hemilabile nature of the pyrrole ring in the mechanism, which in turn sheds light
on the general requirements for an effective ligand in Cr-catalyzed ethylene trimerization.