| Summary: | The pyridylamido hafnium complex (<b>I</b>) discovered at Dow is a flagship catalyst among postmetallocenes, which are used in the polyolefin industry for PO-chain growth from a chain transfer agent, dialkylzinc. In the present work, with the aim to block a possible deactivation process in prototype compound <b>I</b>, the corresponding derivatives were prepared. A series of pyridylamido Hf complexes were prepared by replacing the 2,6-diisopropylphenylamido part in <b>I</b> with various 2,6-R<sub>2</sub>C<sub>6</sub>H<sub>3</sub>N-moieties (R = cycloheptyl, cyclohexyl, cyclopentyl, 3-pentyl, ethyl, or Ph) or by replacing 2-iPrC<sub>6</sub>H<sub>4</sub>C(H)- in <b>I</b> with the simple PhC(H)-moiety. The isopropyl substituent in the 2-iPrC<sub>6</sub>H<sub>4</sub>C(H)-moiety influences not only the geometry of the structures (revealed by X-ray crystallography), but also catalytic performance. In the complexes bearing the 2-iPrC<sub>6</sub>H<sub>4</sub>C(H)-moiety, the chelation framework forms a plane; however, this framework is distorted in the complexes containing the PhC(H)-moiety. The ability to incorporate α-olefin decreased upon replacing 2-iPrC<sub>6</sub>H<sub>4</sub>C(H)-with the PhC(H)-moiety. The complexes carrying the 2,6-di(cycloheptyl)phenylamido or 2,6-di(cyclohexyl)phenylamido moiety (replacing the 2,6-diisopropylphenylamido part in <b>I</b>) showed somewhat higher activity with greater longevity than did prototype catalyst <b>I</b>.
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