Abstract
Sterically very demanding aminopyridinato ligands stabilize thermally robust, highly active and selective zirconium ethylene polymerization catalysts. Small changes in the bulky ligand periphery can be used to fine tune the nature of the formed polymers and even “living” ethylene polymerization at elevated temperatures (50
°C) can be observed.
This paper describes a substantial enhancement of the aminopyridinato ligand stabilized early transition metal chemistry by introducing the sterically very demanding 2,6-dialkylphenyl substituted aminopyridinato ligands derived from (2,6-diisopropylphenyl)-[6-(2,6-dimethylphenyl)-pyridin-2-yl]-amine (
1a–
H, ApH) and (2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridin-2-yl]- amine (
1b–
H, Ap
∗H). The corresponding bis aminopyridinato zirconium dichloro complexes, [Ap
2ZrCl
2] (
3a) and [Ap
2
∗ZrCl
2] (
3b) and the dimethyl analogues, [Ap
2ZrMe
2] (
4a) and [Ap
2
∗ZrMe
2] (
4b) (Me
=
methyl) were synthesized, using standard salt metathesis routes. Single-crystal X-ray diffraction was carried out for the dichloro derivatives. Both zirconium metal centers have a distorted octahedral environment with a
cis-orientation of the chloride ligands in
3a and a closer to
trans-arrangement in
3b. The dimethyl derivatives are proven to be highly active ethylene polymerization catalysts after activation with [R
2N(Me)H][B(C
6F
5)
4] (R
=
C
16H
33–C
18H
37). During attempted co-polymerizations of α-olefins (propylene) and ethylene high activity and selectivity for ethylene and nearly no co-monomer incorporation was observed. Increasing the steric bulk of the ligand going from (2,6-dimethylphenyl) to (2,4,6-triisopropylphenyl) substituted pyridines, switches the catalyst system from producing long chain α-olefins to polymerization of ethylene in a living fashion. In contrast to the dimethyl complexes only [Ap
2
∗ZrCl
2] in the presence of MAO at elevated temperature gave decent polymerization activity. NMR investigations of the reaction of dichloro complexes with 25
equiv. of MAO or AlMe
3 at room temperature revealed, that [Ap
2ZrCl
2] decomposes under ligand transfer to aluminum and formation of [ApAlMe
2], while [Ap
2
∗ZrCl
2] remains almost unreacted under the same conditions. The aminopyridinato dimethyl aluminum complexes, [ApAlMe
2] (
5a) and [Ap
∗AlMe
2] (
5b) were synthesized independently and structurally characterized. The aluminum complexes
5a and
b show no catalytic activity towards ethylene, when “activated” with[R
2N(Me)H][B(C
6F
5)
4].