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

J Chem Phys; 149(3): 034303, 2018 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-30037237
La atom reactions with 1-pentene and 2-pentene are carried out in a laser-vaporization molecular beam source. The two reactions yield the same metal-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage of the pentene molecules. The dehydrogenated species La(C H ) is the major product, whereas the carbon-carbon bond cleaved species La(C H ) and La(C H ) are the minor ones. La(C H ) is also observed and is presumably formed by La(C H ) addition to a second pentene molecule. La(C H ) and La(C H ) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of each species from the two reactions exhibit the same transitions. Adiabatic ionization energies and metal-ligand stretching frequencies are determined for the two species, and additional methyl bending and torsional frequencies are measured for the larger one. Five possible isomers are considered for La(C H ), and a C metallacyclopentene (Iso A) is identified as the most possible isomer. La(C H ) is confirmed to be a C metallacyclopropene. The ground electronic state of each species is a doublet with a La 6s -based electron configuration, and ionization yields a singlet state. The formation of the lanthanacyclopentene includes La addition to the C=C double bond, La insertion into two C(sp )-H bonds, and concerted dehydrogenation. For the 2-pentene reaction, the formation of the five-membered ring may also involve 2-pentene to 1-pentene isomerization. In addition to the metal addition and insertion, the formation of the three-membered metallacycle from 1-pentene includes C(sp )-C(sp ) bond breakage and hydrogen migration from La to C(sp ), whereas its formation from 2-pentene may involve the ligand isomerization.