Intramolecular Hydroamination Reactions Catalyzed by Group IV Dianionic "Pincer" Complexes

The control of single-site catalyst efficiency by tailoring ligand frameworks is an ultimate goal and a challenging matter for many inorganic and organometallic chemists. Indeed, the ability to vary the steric and electronic properties of a given ligand provides a powerful tool to tune reactivity, stability, catalytic performance, and other important features of the metal center. Research in our group has a long-lasting tradition in the design and synthesis of ligands that combine hard donating atoms (N and C) with additional coordinating (hard/soft) donor groups. Their combination with a large variety of transition-metal and rare-earth metal centers has provided a number of catalyst precursors for the efficient olefin upgrading (oligomerization, polymerization and copolymerization) and aminoalkene hydroamination. By starting from our recent interest in Group IV pyridylamido catalysts a rethink of their molecular framework, while maintaining the same donor atom set, has resulted in a step forward towards more thermally stable and catalytically active systems. Neutral ZrIV and HfIV diamido complexes stabilized by unsymmetrical dianionic N,C,N' pincer ligands have been prepared through the simplest and convenient direct metal-induced CAryl-H bond activation. A simple ligand modification has contributed to highlight the non-innocent role played by the donor atom set in controlling the cyclometallation kinetics. The as-prepared bis-amido catalysts were found to be good candidates for the intramolecular hydroamination/cyclization of primary aminoalkenes, providing, for selected issues, fast and complete substrate conversions at room temperature. The complexes syntheses and their catalytic performance in hydroamination reactions will be discussed in details.