Donor-Ligand-Substituted Cyclopentadienylchromium(III) Complexes: A New Class of Alkene Polymerization Catalyst. 2. Phosphinoalkyl-Substituted Systems

A series of phosphinoalkyl-substituted cyclopentadienylchromium complexes, e.g., (η1:η5-R2PC2H4C5H4)CrCl2, have been synthesized. The complexation of the P atom to the chromium has been confirmed by crystal structure determinations. Activated by methylalumoxane (MAO), these compounds catalyze the oligomerization and polymerization of ethylene. The catalytic activity and the degree of oligomerization are controlled by the steric properties of the substituents on the donor atom:  the smaller the substituent, the greater the activity and the lower the molecular weight of the oligomer. Density functional calculations indicate that propagation and termination through β-hydrogen transfer are competitive for small substituents and that the latter process, which involves spin inversion, becomes selectively disfavored with increasing size of the substituents. For large substituents β-hydrogen elimination to the chromium atom is found to be the preferred route for termination, although this reaction is less facile than insertion

Steric control of the chromium-catalyzed oligomerization of ethylene

Catalysts for the oligomerization and polymerization of ethylene have been obtained by activating phosphinoalkyl-substituted cyclopentadienyl chromium complexes, e.g. (η1,η5-R2PC2H4C5H4)CrCl2, with methylalumoxane (MAO). DFT-calculations indicate that the observed dependence of the degree of oligomerization upon the size of the substituents (R) on the P-donor atom is a result of a steric influence on the stability of the transition state for the chain termination step