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., (eta (1):eta (R2PC2H4C5H4)-R-5)CrCl2, have been synthesized. The comp lexation of the P atom to the chromium has been confirmed by crystal struct ure determinations. Activated by methylalumoxane (MAO), these compounds cat alyze the oligomerization and polymerization of ethylene. The catalytic act ivity and the degree of oligomerization are controlled by the steric proper ties of the substituents on the donor atom: the smaller the substituent, th e greater the activity and the lower the molecular weight of the oligomer. Density functional calculations indicate that propagation and termination t hrough beta -hydrogen transfer are competitive for small substituents and t hat the latter process, which involves spin inversion, becomes selectively disfavored with increasing size of the substituents. For large substituents beta -hydrogen elimination to the chromium atom is found to be the preferr ed route for termination, although this react-ion is less facile than inser tion.