Results and Discussion
The ethylene oligomerization process adopted into the calculation here is based on a well-accepted metallacyclic mechanism, which was first proposed by Manyik[9] at Union Carbide at 1977. The cycle starts with the oxidative coupling of two coordinated ethylene to form a metallacyclopentane intermediate, followed by a β -H transfer to a third coordinated ethylene, from which 1-hexene was released. This mechanism was soon modified by Briggs[10] at 1989, proposing that a third coordinated ethylene was inserted into the metallacyclopentane to give a metallacycloheptane, from which β -H eliminate or β -agostic 3,7-H shift take place to yield 1-hexene. This metallacyclic mechanism was experimentally verified using deuterium labeling by Agapie and coworkers[11], and applied to ethylene tetramerization as well, which was also tested with isotopomers by Sasol[12]. The probing into the active center of the catalysts, however, remained complicated because of the paramagnetic nature of the various oxidation states of chromium formed during the catalytic cycle[13], but there are more evidences supporting Cr(I)/Cr(III) redox in the tetramerization catalytic cycle[11a, 14] using electron paramagnetic resonance and X-ray absorption spectroscopy[15], or self-activating catalysts crystallization[16], so Cr(I)/Cr(III) redox cycle was applied in this study.
Based on the mechanism given before, the production of α -olefin may occur via β -H eliminate or β -agostic 3,7-H shift from the corresponding metallacycloalkane, through which hydrogen might modify the energy barriers of the reactions only by coordination and not necessarily involve into the reaction, as Liu stated in the previous studies[8]. However, we do believe it is possible to include hydrogen into the reaction process that leads to the production of alkene, in which hydrogen is inserted into the Cr-C bond, offering an hydrogenolysis reaction to give an alkane chain, before aβ -H transfer to liberate α -olefin. The selectivity of the catalyst depends on the relative equilibrium between decomposition and ring expansion of the metallacycloalkane, so the energy required for ring expansion is also calculated. Here we present the energy profiles of these reactions on metallacycloheptane and metallacyclononane with or without hydrogen, attempting to illustrate how hydrogen might affect the reactions. All Gibbs free energies are relative to the starting material plus the corresponding number of free ethylene molecules.