Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by C—C bond cleavage and coupling of propene

Abstract

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.

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 ).…”

“…We consider the concerted H 2 elimination because previous studies have shown that step-wise dehydrogenation paths are less favorable for the metal atom-mediated dehydrogenation of small alkenes and alkynes. 63,[65][66][67][68][69][70][91][92][93][94][95][96][97] A plausible reaction mechanism for the formation of Iso A from the La + 1-pentene reaction consists of La addition to the C= =C double bond, La insertion into two C(sp 3 − −H) bonds, and concerted H 2 elimination (Fig. 4).…”

“…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.…”