The Impact of Charge in a Ni(II) Polymerization Catalyst

A direct
comparison between neutral active sites and their corresponding
cationic analogues is enabled by protonation of neutral bis­(imino)­phenoxy
complexes, active for ethylene polymerization. The additional imine
motif compared to parent salicylaldiminato catalysts does not influence
the microstructure of the products in ethylene polymerization, but
allows for the incorporation of a proton right next to the active
center in an N···H+···O bridge
yielding cationic complexes. These show an increased Ni–O bond
length and a drastically reduced electron density on the Ni atom.
In pressure reactor experiments, two different catalysts that produce
linear HDPE or undergo extensive chain walking, respectively, in their
neutral version, both produce short chain oligomers when the catalyst
is charged cationically. A mechanistic analysis by DFT methods reveals
an increased propensity for β-hydride elimination compared to
ethylene insertion chain growth for the cationic complexes. This results
from a higher relative stability of β-agostic species vs olefin-coordinated
species.