Metal-ligand Synergy in Group-IV Organometallics for the Catalytic Polymerization and Hydroamination of Unactivated Olefins

The work presented aims at providing original insights for a better understanding of the complex structure-activity relationship of Group-IV pyridylamido-based catalysts for the polymerization and hydroamination of unactivated olefins. Re-thinking ligand frameworks, while maintaining the same donor atom set, led to the obtainment of robust "pincer-type" Group IV complexes with remarkable catalytic performance. 1. Introduction Much of the current interest in organometallic chemistry is linked to the academic and industrial quest for novel types of efficient and selective homogeneous catalysts. For the target of improving the catalyst's performance, a precise control of the metal coordination sphere is desired; this is commonly accomplished by a fine tuning of the steric and electronic properties of the ancillary ligands. Group 4 metal alkyls or amides, in combination with pyridylamido ligands, are the basis of nonconventional, highly efficient and selective catalyst precursors of great promise for polyolefin production. Recent studies have unveiled the wealthy organometallic chemistry of these nitrogen-based early transition metal compounds, highlighting at the same time their unique polymerization features and hydroamination performance especially in high temperature solution processes. The long-lasting experience of our group in the preparation and testing of Group-IV pyridylamido catalysts has been put to good use for a new ligand design: a re-thinking of the molecular framework, while maintaining the same donor atoms set, results in a step-forward towards more thermally stable and catalytically active N,C,N pincer-type systems. 2. Experimental Experimental details related to both the ligand and complex syntheses and the general conditions used for polymerization and hydroamination catalyses are provided in the respective full articles. 3. Results and discussion Recent work from our group has unveiled an original and temperature controlled prototropic rearrangement occurring on ZrIV and HfIV dimethylamido complexes stabilized by (potentially) dianionic tridentate {C-,N,N-} amido-pyridinate ligands. This aspect adds additional levels of complexity to the interpretation of their catalytic activity in the polymerization and hydroamination of unactivated olefins as as well as to the identification of the truly catalytic active species. Re-thinking the organic ligand structure (through a "scrambling" of the same donor atom set) led us towards robust unsymmetrical dianionic {N-,C-,N'} pincer-type Group IV complexes. The ability of the latters {either as a neutral M(NMe2)2(L) amido or in a [M(alkyl)(L)]+ cationic alkyl form} at promoting the intramolecular hydroamination reaction of model aminoalkenes (providing, for selected substrates, fast and complete conversions already at room temperature) constitutes a remarkable step forward towards catalytic systems capable of operating at relatively low catalyst's loadings (5 mol%) and under mild (time and temperature) reaction conditions. A careful comparison of the catalytic performance of the pincer-type complexes vs. pyridylamido systems for both polymerization and hydroamination of unactivated olefins will be discussed in details. 4. Conclusions Group-IV pyridylamido catalysts are excellent and to some extent unique candidates for the production of polyolefin materials, especially in high temperature solution processes. An overview on the complex structure-activity relationship of these systems together with their ability to promote the olefin polymerization and the intramolecular hydroamination of primary and secondary aminoalkenes efficiently, is provided. Finally, a tailored re-distribution of the ligand donor atoms set led to the obtainment of robust unsymmetrical "pincer-type" complexes featured by remarkable catalytic performance in the intramolecular hydroamination of primary and secondary aminoalkenes. Their ability at promoting olefin polymerization in high temperature solution processes will also be matter of discussion.