Hydrogenation of Esters Catalyzed by Bis(N-Heterocyclic Carbene) Molybdenum Complexes

A series of Mo complexes bearing inexpensive bidentate bis(NHC) ligands have been synthesized and characterized by NMR and IR spectroscopy as well as single crystal XRD analysis. These complexes proved to be efficient for the catalytic hydrogenation of aliphatic and aromatic esters (>35 examples) operating at low catalyst loadings (0.5–2 mol %) and temperatures (80–120 °C). Various functional groups, e.g., C═C double bonds, nitriles, alcohols, tertiary amines, halides, and acetals, as well as heteroaromatic substrates, lactones, and diesters, are tolerated by the optimal catalyst system. Based on NMR spectroscopic investigations, control experiments and DFT computations a non-bifunctional outer-sphere hydrogenation mechanism is proposed.