Catalytically Active <i>N</i>‑Acylamidine–Zirconium Complexes: Synthesis, Structures, and Application in Ethylene Polymerization

Three novel N-acylamidines 1a–c with
varying steric bulk and substitution patterns were
synthesized and thoroughly characterized by X-ray diffraction. Compounds 1a and 1b, which contain two additional binding
sites located at two pyridine substituents, were treated with equimolar
amounts of cyclopentadienylzirconium­(IV) trichloride at room temperature.
The X-ray data of the resulting coordination compounds 7a and 7b indicate the formation of five-membered metallacycles
with one of the pyridine nitrogen atoms and the carbonyl oxygen atom
acting as binding sites. For the complexation of ligand 1c, a different route was chosen: 1c was first deprotonated
to yield the polymeric potassium compound 8 with a very
complex substitution pattern based on O , N, and aromatic interactions
with the potassium ions. Transmetalation of 8 with cyclopentadienylzirconium­(IV)
trichloride gave amidinate complex 9, which is dimeric
in the solid state but exists in solution in equilibrium with monomeric
species. After addition of methylaluminoxane (MAO), the three novel
cyclopentadienylzirconium complexes 7a, 7b, and 9 gave active homogeneous single-site catalysts
for the polymerization of ethylene. Of these three systems, 9/MAO turned out to be the most efficient one, showing activities
3–5 times higher than 7a/MAO and 7b/MAO, respectively, and producing polymers with a well-defined “monomodal”
molecular weight distribution. An important feature of these materials
is their broader distribution in molecular weight (PDI > 3), which
is best seen in the products of reactions at 53 °C, with a “monomodal”
main fraction at higher molecular weight and only small fractions
of low molecular weight.