Figure 5. Relative Gibbs free energy profiles (kcal/mol) for the formation of active center on Cr/PNP catalyst, where ethylene coordinated both axially.
The process starts with the insertion of one ethylene into the M-C bond via A-TS0-1, overcoming an energy barrier of 5.9 kcal/mol, followed by a conformation change (A1 to A1’) of 5.7 kcal/mol and reduction elimination of 8.7 kcal/mol via A-TS1’-2 to give an active species A2. Note that this conformation change is necessary to bring β -H from propyl group close to methyl group to enable further reaction, and both the conformation change from A1 to A1’ and the reaction over A-TS1’-2 are endergonic, hence the overall energy required for the formation of active center without hydrogen could be regarded as the sum of the two, namely 14.4 kcal/mol.