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 quinozoline (QazH), abbreviated generally as NHetH, react with Mo(PMe3)(6) to give (eta(2)-NHet)Mo(PMe3)(4)H 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 (eta(2)-NHet)Mo(PMe3)(4)H with PME3 regenerates Mo(PMe3)(6) and NHetH. Furthermore, at elevated temperature (eta(2)-NHet)Mo(PMe3)(4)H converts sequecially to isomers of (eta(6)-NHEtH)Mo(PMe3)(3) in which the N-heteroaromatic ligand coordinates via either the heterocyclic or carbocyclic rings. Isomers of (eta(6)-NHEtH)Mo(PMe3)(3) in which the heterocyclic ring coordinated to molybdenum may be hydrogenated. Thus, (eta(6)-C5N-iQH)Mo (PMe3)(3) and (eta(6)-C4N2-QoxH)Mo(PMe3)(3) react with H-2 at 90 degrees C to give Mo(PMe3)(4)H-4 and release 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroquinoxaline, respectively. Furthermore, Mo(PMe3)(4)H-4 serves as a catalyst precursor for the hydrogenation of quinoline, isoquinoline, and quinoxaline to 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroquinoxaline, respectively. Mo(PMe3)(4)H-4 is the first simple molybdenum complex to effect catalytic hydrogenation of these heterocyclic nitrogen compounds, a necessary step in hydrodenitrogenation.