Rare-earth and alkaline-earth metal complexes with alkylidene and imido ligands: synthesis, derivatization, and reactivity

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Fundamental issues regarding structure and bonding (Fischer- versus Schrock-type carbenes) and, moreover, their relevance as reaction intermediates in olefin metathesis, Fischer-Tropsch processes, and numerous organic functional group transformations are pivotal attributs of transition metal carbene/alkylidene complexes. In the same breath, metal-imido (M=NR) moieties engage in a multitude of chemical transformations and are utilized as ancillaries in transition metal alkylidene complexes to promote stereoselective olefin metathesis and polymerization. The stabilization and isolation of metal-ligand multiple bonds poses an ever ongoing challenge in rare-earth (Ln) and alkaline-earth (Ae) metal chemistry and the existence of long-time elusive alkylidene or imido entities has led to new paradigms in 4f element-ligand bonding as well. This project aims at a fundamental understanding of the formation of such Ln/Ae ligand multiple bonds. We and others have recently embarked on the organoaluminum-assisted formation of rare-earth metal methylidene, methine, and imido species via (multiple) methyl group deprotonation. We wish to demonstrate that this organoaluminum-assisted deprotonation pathway has a more general applicability, e.g., by extending the scope of alkylidene and imido ligands, but also to elaborate new reaction protocols, such as the dehalosilylation. The utilization of bulky ancillary ligands is supposed to suppress extensive and uncontrollable hydrogen abstraction and hence counteract cluster formation, thus ensuring the reproducible formation of discrete, ideally monometallic complexes. Assessing the feasibility of Tebbe- and Petasis-like reagents will be a primary target by exploiting the thermal stability of (bi)metallic alkyl complexes. The identification and isolation of rare-earth metal alkylidene and imide complexes with the Ln metal centers in the tetravalent (Ce) and divalent oxidation states (Sm, Eu, Yb) represents maybe the biggest challenge. The overall mechanistic investigations will be supported by isotope labeling techniques and DFT calculations. The reactivity of the obtained complexes will be explored by comparison with routinely employed d-transition-metal reagents, e.g., for carbonyl olefination reactions.