Lanthanum-mediated dehydrogenation of butenes: Spectroscopy and formation of La(C4H6) isomers

Abstract

La atom reactions with 1-butene, 2-butene, and isobutene are carried out in a laser-vaporization molecular beam source. The three reactions yield the same La-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage and coupling of the butenes. The dehydrogenated species La(CH) is the major product, which is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectrum of La(CH) produced from the La+1-butene reaction exhibits two band systems, whereas the MATI spectra produced from the La+2-butene and isobutene reactions display only a single band system. Each of these spectra shows a strong origin band and several vibrational progressions. The two band systems from the spectrum of the 1-butene reaction are assigned to the ionization of two isomers: La[C(CH)] (Iso A) and La(CHCHCHCH) (Iso B), and the single band system from the spectra of the 2-butene and isobutene reactions is attributed to Iso B and Iso A, respectively. The ground electronic states are A (C) for Iso A and A' (C) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and leads to the A ion of Iso A and the A' ion of Iso B. The formation of both isomers consists of La addition to the C=C double bond, La insertion into two C(sp)-H bonds, and H elimination. In addition to these steps, the formation of Iso A from the La+1-butene reaction may involve the isomerization of 1-butene to isobutene prior to the C-H bond activation, whereas the formation of Iso B from the La+trans-2-butene reaction may include the trans- to cis-butene isomerization after the C-H bond activation.

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

“…This may explain why no intermediates were observed in our experiments even though the adduct (IM1) and inserted species (IM2, IM3) are more stable than the product. This observation is similar to the previous studies of La reactions with other small unsaturated hydrocarbons, 63,65,69,70 except for propene where a La-inserted species was identified. 67 2-pentene dehydrogenation follows a similar path as 1-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.…”

“…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.…”