Pyridine(diimine) Molybdenum-Catalyzed Hydrogenation of Arenes and Hindered Olefins: Insights into Precatalyst Activation and Deactivation Pathways

Pyridine(diimine) molybdenum bis(olefin) and bis(alkyl) complexes were synthesized, characterized, and examined for their catalytic activity in the hydrogenation of benzene and a selection of substituted arenes. The molybdenum bis(alkyl) complex (4-tBu-iPrPDI)Mo(CH2SiMe3)2 (iPrPDI = 2,6-(2,6-(C(CH3)2H)2C6H3N═CMe)2C5H3N) exhibited the highest activity for the hydrogenation of benzene, producing cyclohexane in >98% yield at 23 °C under 4 atm of hydrogen after 48 h. Toluene and o-xylene were similarly hydrogenated to their respective cycloalkanes, with the latter yielding predominantly (79:21 dr) cis-1,2-dimethylcyclohexane. The molybdenum-catalyzed hydrogenation of naphthalene yielded tetralin exclusively, and this selectivity was maintained at higher H2 pressure. At 32 atm of H2, more hindered arenes such as monosubstituted benzenes, biphenyl, and m- and p-xylenes underwent hydrogenation with yields ranging between 20 and >98%. (4-tBu-iPrPDI)Mo(CH2SiMe3)2 was also a competent alkene hydrogenation catalyst, supporting stepwise reduction of benzene to cyclohexadiene and cyclohexene during molybdenum-catalyzed arene hydrogenation. Deuterium labeling studies for the molybdenum-catalyzed hydrogenation of benzene produced numerous isotopologues and stereoisomers of cyclohexane, indicating reversible hydride (deuteride) insertion/β-H(D) elimination, diene/olefin binding, and allylic C–H(D) activation during the reaction. The resting state of the catalyst under neat conditions was established as the η6-benzene complex (iPrPDI)Mo(η6-benzene). Under catalytic conditions, pyridine underwent C–H activation of the 2-position and furan underwent formal C–O oxidative addition to yield a "metallapyran". Both reactions were identified as important catalyst deactivation pathways for the attempted molybdenum-catalyzed hydrogenation of heteroarenes.