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(C_nH_n) (n = 2, 4, and 6) and La(C_mH_m+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[η⁴-(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.