Kinetics and Pathway Analysis Reveals the Mechanism of a Homogeneous PNP-Iron-Catalyzed Nitrile Hydrogenation.

Nitrile hydrogenation via the in situ-generated PNP-FeII(H)2CO (1) catalyst leads to a previously inexplicable loss of mass balance. Reaction kinetics, reaction progress analysis, in situ pressure nuclear magnetic resonance, and X-ray diffraction analyses reveal a mechanism comprising reversible imine self-condensation and amine-imine condensation cascades that yield >95% primary amine. Imine self-condensation has never been reported in a nitrile hydrogenation mechanism. The reaction is first order in catalyst and hydrogen and zero order in benzonitrile when using 2-propanol as the solvent. Variable-temperature analysis revealed values for ΔG298 K⧧ (79.6 ± 26.8 kJ mol-1), ΔH⧧ (90.7 ± 9.7 kJ mol-1), and ΔS⧧ (37 ± 28 J mol-1 K-1), consistent with a solvent-mediated proton-shuttled dissociative transition state. This work provides a basis for future catalyst optimization and essential data for the design of continuous reactors with earth-abundant catalysts.