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

Abstract

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 n H n ) ( n = 2, 4, and 6) and La(C m H 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(C 6 H 6 )] is formed by 1,3-butadiene addition to lanthanacyclopropene [La(C 2 H 2 )] 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[η 4 -(1-buten-3-yne)] [La(C 4 H 4 )] and 1-lanthanacyclopent-3-ene [La(C 4 H 6 )]. 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.