Gas-Phase Reactions of Doubly Charged Lanthanide Cations with Alkanes and Alkenes. Trends in Metal(2+) Reactivity

Yang

2018

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.

Section: Introductionmentioning

confidence: 99%

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

Yang

2018

“…Its reactions with metal ions were mainly studied with Fourier-transform ion-cyclotron resonance, guided-ionbeam, or other mass-spectrometry based methods. 8,[29][30][31][32][33][34][35][36] Neutral metal atom reactions are more difficult to study because reaction products cannot be followed directly by mass spectrometry. Alternatively, reactions with the neutral atoms were monitored by photoionization mass spectrometry of resultant species or laser-induced fluorescence measurements of the depletion of the metal atom concentration.…”

Section: Introductionmentioning

confidence: 99%

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

Kumari

et al. 2017

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.

“…35 Moreover, the reactivity trends of singly and doubly charged positive lanthanide ions with hydrocarbons, including propene, were investigated by several groups using mass-spectrometry based methods. [36][37][38] The general conclusions from those studies were that singly charged lanthanide ions with at least two non-f valence electrons (La + , Ce + , Gd + , Pr + , and Tb + ) in the ground or low-energy excited states and doubly charged ions with ground state or low-lying d 1 electron configurations (La 2+ , Ce 2+ , Gd 2+ , and Tb 2+ ) were more reactive toward hydrocarbons than other lanthanides. This work aims at the structural determinations of La(CH 2 ) formed by the C− −C bond cleavage and La(C 4 H 6 ) formed by the C− −C coupling of propene using mass-analyzed threshold ionization (MATI) spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

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

Kumari

et al. 2017

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.