Commercially, nitrile hydrogenations are often conducted using noble metals or expensive nickel or cobalt. In recent years, Fe PNP complexes have emerged as excellent catalysts with high selectivity for primary amines but important aspects of mechanism remain unclear. We report a detailed mechanism study for an Fe PNP benzonitrile hydrogenation that demonstrates the surprising intermediacy of oligomeric structures in equilibrium with monomeric imines that lead to the desired primary amine product. The reaction has 1<sup>st</sup> order dependence on hydrogen and catalyst concentrations in 2-propanol but zero order dependence on benzonitrile concentration. An Arrhenius plot suggests an activation energy of 98.23 [kJ/mol] and pre-exponential constant of 2.46 x 10<sup>20</sup> [mL/mol/min]. An Eyring plot was used to characterize transition state energy and values were reasonable in comparison to literature density functional theory calculations for similar systems. These results provide a basis for catalyst optimization and future continuous reactor design. This mechanism addresses known gaps in the literature. Overall, this reaction provides excellent conversion (&gt;99%) and yield to primary amine (&gt;97%).

Mechanistic intrigue for an Fe-PNP complex for benzonitrile hydrogenation

ACS Spring 2022

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