PROBING SELECTIVE BOND ACTIVATION IN ALKYLAMINES: LANTHANUM-MEDIATED C-H AND N-H BOND ACTIVATION STUDIED BY MATI SPECTROSCOPY.

Abstract

In this work, La atom reactions with small alkylamines \chem{R-NH_2} and \chem{H_2N-(CH_2)_n-NH_2} (n=2, 3, 4) are carried out in a laser-ablation supersonic molecular beam source. Reaction products are observed with photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization (MATI) spectroscopy and DFT calculations. These reactions proceed via an exothermic \chem{H_2} loss from the ligand and form a metal complex. The \chem{R-NH_2} ligands favor the formation of acyclic metal complexes where La atom is doubly bonded to the N atom, whereas the \chem{H_2N-(CH_2)_n-NH_2} ligands prefer the formation of cyclic metal complexes where La atom is bonded to both N atoms.

The reaction between La atom and \chem{CH_3NH_2} produces \chem{LaNCH_3} in two low-energy isomeric forms: an acyclic C$_{3v}$ structure and a three -membered C$_{s}$ ring. The C$_{3v}$ isomer is formed by concerted elimination of two H atoms from the \chem{NH_2} group, whereas the C$_{s}$ ring is formed by H atom elimination from both the alpha C and \chem{NH_2} group. Both isomers prefer a doublet ground state with a La 6s$^{1}$-based electron configuration and a singlet ionic state by removing the 6s electron. The C$_{3v}$ structure is calculated to be more stable than the C$_{s}$ isomer but has a slightly lower adiabatic ionization energy (40022 (5) \wn) than the ring isomer (40399 (5) \wn) as measured from the MATI spectra. The MATI spectrum of the C$_{3v}$ isomer is dominated by the origin band and La-Ligand and C-N stretching vibronic bands. The MATI signal of the C$_{s}$ isomer is very weak and is only 10\% of the C$_{3v}$ MATI signal. This observation confirms that the C$_{3v}$ isomer is more stable and its formation is more favorable than the C$_{s}$ ring.