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.