La reaction with propene is carried out in a laser-vaporization molecular beam source. Three La-hydrocarbon radicals are characterized by mass-analyzed threshold ionization (MATI) spectroscopy. One of these radicals is methylenelanthanum [La(CH2)] (Cs), a Schrock-type metal carbene. The other two are a five-membered 1-lanthanacyclopent-3-en [La(CH2CHCHCH2)] (Cs) and a tetrahedron-like trimethylenemethanelanthanum [La(C(CH2)3)] (C3v). Adiabatic ionization energies and metal-ligand stretching and hydrocarbon-based bending frequencies of these species are measured from the MATI spectra, preferred structures and electronic states are identified by comparing the experimental measurements and spectral simulations, and reaction pathways for the formation of the metal-hydrocarbon radicals are investigated with density functional theory calculations. All three radicals prefer doublet ground electronic states with La 6s1-based valence electron configurations, and singly charged cations favor singlet states generated by the removal of the La 6s1 electron. The metal-carbene radical is formed via multi-step carbon-carbon cleavage involving metallacyclization, β-hydrogen migration, and metal insertion. The metal-carbene radical formed in the primary reaction reacts with a second propene molecule to form the five-membered-ring and tetrahedron-like isomers through distinct carbon-carbon coupling paths.
Bibliographical noteFunding Information:
We are grateful for the financial support from the National Science Foundation Division of Chemistry (Chemical Structure, Dynamics, and Mechanisms, Grant No. CHE-1362102). We also acknowledge additional support from the Kentucky Science and Engineering Foundation.