La-mediated dehydrogenation and C-C bond cleavage of 1,4-pentadiene and 1-pentyne: Spectroscopy and formation of La(C5H6) and La(C3H4) radicals

La atom reactions with 1,4-pentadiene and 1-pentyne are carried out in a laser-vaporization molecular beam source. Both reactions yield a predominant La(C5H6) radical from ligand dehydrogenation and a minor La(C3H4) species from C-C bond cleavage. The metal-hydrocarbon radicals are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of each species formed by the two reactions are essentially identical and consist of a single vibronic band system for La(C5H6) and two band systems for La(C3H4). Each band system exhibits a strong origin band and several weak vibronic ones. Adiabatic ionization energies, metal-ligand vibrational frequencies, and low-frequency ligand-based modes are measured for the two species. La(C5H6) is identified as a six-membered lanthanacycle [La(CHCHCHCHCH2)] (Iso A, C-1), whereas two isomers of La(C3H4) are a four-membered metallacycle La(CHCHCH2) (Iso 1, C-1) and a three-membered ring La(CHCCH3) (Iso 2, C-s). The ground electronic state of each radical is a doublet and that of the singly charged cation is a singlet. The remaining two electrons that are associated with the isolated La atom or ion are spin paired in a molecular orbital that is bonding combination between a La 5d orbital and a pi* antibonding orbital of the C5H6 or C3H4 fragment. For the 1,4-pentadiene reaction, the formation of the six-membered metallacycle La(C5H6) involves La addition to a C=C double bond, La insertion into two C(sp(3))-H bonds, and concerted H-2 elimination, whereas the formation of the three- and four-membered La(C3H4) rings involves two hydrogen migrations followed by C-C bond breakage to eliminate an ethylene molecule. For the 1-pentyne reaction, the formation of each species requires isomerization of a La(1-pentyne) adduct to a La insertion species and then proceeds with the same elemental steps as those for the diene reaction. (C) 2018 Elsevier B.V. All rights reserved.