Zirconium-Catalyzed Imine Hydrogenation via a Frustrated Lewis Pair Mechanism

Zirconium-based frustrated Lewis pairs (FLPs) are active imine hydrogenation catalysts under mild conditions. Complexes of the type [Cp^R ₂ZrOMes][B(C₆F₅)₄] utilize the imine substrate itself as the Lewis base component of the FLP. Catalyst performance is a function of ligand structure; in general more bulky, more electron rich cyclopentadienyl derivatives give the best results. However, Cp∗ derivatives are not catalytically active, being stable after initial heterolytic cleavage of H₂; this allows experimental verification of the competence of the zirconocene-imine pair in FLP-type heterolytic H₂ cleavage. Enamines and protected nitriles are also hydrogenated if an additional internal phosphine base is used.