We report the first example of metal-mediated acetylene bicyclopentamerization to form naphthalene in the gas phase. The bicyclic aromatic compound was observed in a complex with La. The La(naphthalene) complex was formed by the reaction of laser-ablated La atoms with acetylene molecules in a molecular beam source and was characterized by mass-analyzed threshold ionization spectroscopy. The bicyclo-oligomerization reaction occurs through sequential acetylene additions coupled with dehydrogenation. Three intermediates in the reaction have been identified: lanthanacyclopropene [La(C2H2)], La(cyclobut-1-en-3-yne) [La(C4H2)], and La(benzyne) [(La(C6H4)]. The metal-ligand bonding in the three intermediates is considerably different from that in the La(naphthalene) complex, as suggested by accurately measured adiabatic ionization energies.

η(2)-Propadienylidenelanthanum [La(η(2)-CCCH2)] and deprotiolanthanacyclobutadiene [La(HCCCH)] of La(C3H2) are identified from the reaction mixture of neutral La atom activation of propyne in the gas phase. The two isomers are characterized with mass-analyzed threshold ionization spectroscopy combined with electronic structure calculations and spectral simulations. La(η(2)-CCCH2) and La(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation, respectively. Both isomers prefer a doublet ground state with a La 6s-based unpaired electron, and La(η(2)-CCCH2) is slightly more stable than La(HCCCH). Ionization of the neutral doublet state of either isomer produces a singlet ion state by removing the La-based electron. The geometry change upon ionization results in the excitation of a symmetric metal-hydrocarbon stretching mode in the ionic state, whereas thermal excitation leads to the observation of the same stretching mode in the neutral state. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal-hydrocarbon radicals are lower than that of the neutral La atom. Deuteration has a very small effect on the ionization energies of the two isomers and the metal-hydrocarbon stretching mode of La(η(2)-CCCH2), but it reduces considerably the metal-ligand stretching frequencies of La(HCCCH).

La(CH) and La(CH) are observed from the reaction of laser-vaporized La atoms with propene by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. Two isomers of La(CH) are identified as methyl-lanthanacyclopropene [La(CHCCH)] (C) and lanthanacyclobutene [La(CHCHCH)] (C); La(CH) is determined to be H-La(η-allyl) (C), a C-H bond inserted species. All three metal-hydrocarbon radicals prefer a doublet ground state with a La 6s-based electron configuration. Ionization of the neutral doublet state of each of these radicals produces a singlet ion state by removing the La-based 6s electron. The threshold ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal-ligand and ligand-based vibrational frequencies of the neutral and ionic states. The formation of the three radicals is investigated by density functional theory computations. The inserted species is formed by La inserting into an allylic C-H bond and lanthanacyclopropene by concerted vinylic H elimination, whereas lanthanacyclobutene involves both allylic and vinylic dehydrogenations. The inserted species is identified as an intermediate for the formation of lanthanacyclobutene.

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

La reaction with propene is carried out in a laser-vaporization molecular beam source. Three Lahydrocarbon radicals are characterized by mass-analyzed threshold ionization (MATI) spectroscopy. One of these radicals is methylenelanthanum [La(CH 2)] (C s), a Schrock-type metal carbene. The other two are a five-membered 1-lanthanacyclopent-3-en [La(CH 2 CHCHCH 2)] (C s) and a tetrahedron-like trimethylenemethanelanthanum [La(C(CH 2) 3)] (C 3v). 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 6s 1-based valence electron configurations, and singly charged cations favor singlet states generated by the removal of the La 6s 1 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.