Effects of Ligand, Metal, and Solvation on the Structure and Stability of Contact Ion Pairs Relevant to Olefin Polymerization Catalyzed by Rare-Earth-Metal Complexes: A DFT Study

A full account of
theoretical analyses at the DFT level has been
reported, focusing on the formation and reactivity of a family of
cationic [R-(CH2)n-Py-Sc­(CH2SiMe3)]+ catalysts and the effects of
counterion and solvation. Two sets of model systems have been considered:
(a) structures having identical bridging unit (n =
1) but having varying cyclopentadienyl groups (R = Cp′ (1), R = Ind (2), and R = Flu (3))
and (b) systems with the identical cyclopentadienyl moiety (Flu) but
with varying bridging groups (n = 1 (3), n = 0 (4), and n = 2 (5)). For complex 3, various metal
ions (Sc, Y, La, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu) were considered
to investigate the effect of central metals on the contact ion pairs
(CIP). The formation and separation of CIP were found to be influenced
by the steric hindrance of the ligand, the electron-donating ability
of the cyclopentadienyl group, and the rare-earth-metal ion radius.
The separation enthalpy of the CIPs decreases with increasing dielectronic
constant of the solvent. The solvation hardly affects the energy barrier
for styrene insertion into the Sc–C17H19 bond of the CIP [(Flu-CH2-Py)­Sc-(C17H19)]­[B­(C6F5)4]. A bulkier
and more electron donating ancillary ligand, a smaller ion radius
of the rare-earth metal, and a greater polarity of the solvent are
more beneficial to the separation of CIP and thus to the monomer coordination,
which could contribute to the improvement of polymerization activity.