Spectroscopic Characterization of Nonconcerted [4+2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum-Benzene in the Gas Phase

The reaction between La atoms and 1,3-butadiene is carried out in a laser vaporization molecular beam source. Metal-hydrocarbon species with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) are observed with, time of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum-benzene complex [La(C6H6)] is formed by 1,3-butadiene addition to lanthanacyclopropene.[La(C2H2)] followed by molecular hydrogen elimination. Lanthanacyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La[eta(4)-(1-buten-3-yne)] [La(C4H4)] and 1-lanthanacyclopent-3-ene [La(C4H6)]. The La(benzene) complex exhibits distinctive metal-ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations.