Reactions of La and Ce with Propylamine: Formation and Threshold Ionization of Lanthanide Imido Radicals

Metal-mediated activation of aliphatic amines offers a versatile route to structurally diverse molecules in organic synthesis. In this study, reactions of lanthanide atoms (Ln = La and Ce) with propylamine were conducted in a pulsed laser vaporization supersonic molecular beam source. A series of dehydrogenated Ln-containing species were identified via time-of-flight mass spectrometry. Dehydrogenated complexes with the formula Ln(NC₃H₇) were characterized through single-photon mass-analyzed threshold ionization (MATI) spectroscopy, supported by quantum chemical calculations. Theoretical investigations employed density functional theory for both La and Ce species and relativistic multiconfiguration self-consistent field and second-order quasi-degenerate perturbation theory for the Ce species. The MATI spectrum of La(NC₃H₇) reveals a single vibronic progression, attributed to ionization from the doublet ground state of the chain-like radical La(NCH₂CH₂CH₃). In contrast, the MATI spectrum of Ce(NC₃H₇) exhibits two distinct vibronic systems, corresponding to ionization from the two lowest-energy spin-orbit coupled states of the Ce(NCH₂CH₂CH₃) radical. Both Ln-containing radicals are formed via a thermodynamically and kinetically favorable concerted dehydrogenation of the amino group.