Mass-analyzed threshold ionization spectroscopy of lanthanum-hydrocarbon radicals formed by C—H bond activation of propene

Abstract

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.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by C—H and C—C bond activation of 1-pentene and 2-pentene

“…We have extensively used the DFT/B3LYP method and found that this method generally produced adequate results for spectral and structural assignments of organometallic radicals. [63][64][65][66][67][68][69][70]86 No symmetry restrictions were imposed in initial geometry optimizations, but appropriate point groups were used in subsequent optimizations to help identify electronic symmetries. The geometry of each stationary point was obtained through a relaxed potential energy surface scan along the proposed reaction coordinates.…”

“…Because of low ablation laser power, no La clusters were produced except for very small amounts of La 2 , La 2 C, and La 2 O 1,2 . La oxides could be formed by La reactions with oxygen that is present in the carrier gas as an impurity or by laser vaporization of La oxide impurity in the La rod, [63][64][65][66][67][68][69] whereas the La carbide could be associated with La reactions with the vapor of diffusion pump oil. The mass spectra from the two pentene reactions are essentially the same and show a predominant product La(C 5 H 8 ), a significant amount of La (C 10 H 18 ), and two minor species La(C 2 H 2 ) and La(C 3 H 4 ).…”

“…The failure of the MATI measurement on La(C 3 H 4 ) is largely due to the extremely low number density of this species, as sharp spectra were recorded for the species of the same stoichiometry formed in the La reaction with propene. 67 The reason for the lack of the MATI spectrum of La(C 10 H 18 ) is less clear. But, it could be due to a large geometry change upon ionization that leads to FC transitions with a weak origin band and weak vibronic bands at low vibrational quanta, an unfavorable case for the MATI experiment.…”

“…62 We have recently reported the mass-analyzed threshold ionization (MATI) spectroscopy and formation of the metal-hydrocarbon radicals produced by the lanthanide-mediated C− −C and C− −H bond activation of several small alkenes and alkynes. [63][64][65][66][67][68][69][70][71] Our studies have demonstrated that the combination of the MATI spectroscopic measurements with theoretical computations is a powerful approach to investigate transient metal-hydrocarbon species.…”

“…In our previous MATI spectroscopic measurements of La reactions with ethylene, 65 propene, 67,68 and butenes, 70 we investigated the structures and formation of La-hydrocarbon radicals formed by dehydrogenation, metal insertion, and C− −C bond cleavage and coupling reactions. For the La + ethylene reaction, we observed lanthanacyclopropene [La(CHCH)] from dehydrogenation and lanthanacyclopentene [La(CH 2 CHCHCH 2 )] from C− −C bond coupling.…”