Facing Unexpected Reactivity Paths with Group-IV Pyridylamido Polymerization Catalysts

Much of the current interest in organometallic chemistry is linked to the academic and industrial quest for novel types of efficient and selective olefin oligomerization and polymerization catalysts. An improvement of their catalytic performance implies a precise control of the metal coordination sphere, commonly accomplished by a fine tuning of the steric and electronic properties of the ancillary ligands. Nitrogen donor ligands (imines, amides) have proven to be versatile components of bi- and polydentate ligands for the preparation of transition and rare-earth metal complexes in polymerization catalysis. A recent work by our group on amino-pyridinate ligands in combination with selected Group IV transition metal precursors [MIV(NMe2)4 and/or MIV(Bn)4; M = Zr, Hf] has unveiled unexpected organometallic paths, ultimately showing how the metal precursor choice plays a non-innocent role towards the preparation of tailored catalysts of unambiguous identity. Selected ZrIV-amidopyridinate complexes from the same series have revealed outstanding polymerization activities in the production of poly(1-hexene). An in-depth study on the nature of the catalytically active species, together with the characteristics of the polymers produced (molecular weights, tacticity and distribution of regioerror in the polymer microstructure) have demonstrated a close dependence of the nature of the active species from the catalyst aging-time. In the light of the well-established efficiency of the ZrIV and HfIV complexes for the production of specialty polyolefins, the results presented in this study provide a useful reference to gain further insight into the wealth (organometallic and polymerization) chemistry of early transition metal complexes containing nitrogen-based ligands.