New Modes for Coordination of Aromatic Heterocyclic Nitrogen Compounds to Molybdenum:  Catalytic Hydrogenation of Quinoline, Isoquinoline, and Quinoxaline by Mo(PMe₃)₄H₄

The heterocyclic nitrogen compounds isoquinoline (iQH), quinoxaline (QoxH) and quinazoline (QazH), abbreviated generally as NHetH, react with Mo(PMe₃)₆ to give (η²-NHet)Mo(PMe₃)₄H as a result of cleavage of the C−H bond adjacent to the nitrogen atom. The C−H bond cleavage is reversible. For example, in the case of isoquinoline and quinoxaline, treatment of (η²-NHet)Mo(PMe₃)₄H with PMe₃ regenerates Mo(PMe₃)₆ and NHetH. Furthermore, at elevated temperatures (η²-NHet)Mo(PMe₃)₄H converts sequentially to isomers of (η⁶-NHetH)Mo(PMe₃)₃ in which the N-heteroaromatic ligand coordinates via either the heterocyclic or carbocyclic rings. Isomers of (η⁶-NHetH)Mo(PMe₃)₃ in which the heterocyclic ring coordinated to molybdenum may be hydrogenated. Thus, (η⁶-C₅N-iQH)Mo(PMe₃)₃ and (η⁶-C₄N₂-QoxH)Mo(PMe₃)₃ react with H₂ at 90 °C to give Mo(PMe₃)₄H₄ and release 1,2,3,4-tetrahydroisoquinoline and 1,2,3,4-tetrahydroquinoxaline, respectively. Furthermore, Mo(PMe₃)₄H₄ serves as a catalyst precursor for the hydrogenation of quinoline, isoquinoline, and quinoxaline to 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroisoquinoline, and 1,2,3,4-tetrahydroquinoxaline, respectively. Mo(PMe₃)₄H₄ is the first simple molybdenum complex to effect catalytic hydrogenation of these heterocyclic nitrogen compounds, a necessary step in hydrodenitrogenation.