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.