Lanthanum-Mediated C–H Bond Activation of Propyne
and Identification of La(C<sub>3</sub>H<sub>2</sub>) Isomers
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Abstract
η<sup>2</sup>-Propadienylidenelanthanum
[La(η<sup>2</sup>-CCCH<sub>2</sub>)] and deprotiolanthanacyclobutadiene
[La(HCCCH)] of La(C<sub>3</sub>H<sub>2</sub>) are identified from
the reaction mixture of neutral La atom activation of propyne in the
gas phase. The two isomers are characterized with mass-analyzed threshold
ionization spectroscopy combined with electronic structure calculations
and spectral simulations. La(η<sup>2</sup>-CCCH<sub>2</sub>)
and La(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation,
respectively. Both isomers prefer a doublet ground state with a La
6s-based unpaired electron, and La(η<sup>2</sup>-CCCH<sub>2</sub>) is slightly more stable than La(HCCCH). Ionization of the neutral
doublet state of either isomer produces a singlet ion state by removing
the La-based electron. The geometry change upon ionization results
in the excitation of a symmetric metal–hydrocarbon stretching
mode in the ionic state, whereas thermal excitation leads to the observation
of the same stretching mode in the neutral state. Although the La
atom is in a formal oxidation state of +2, the ionization energies
of these metal–hydrocarbon radicals are lower than that of
the neutral La atom. Deuteration has a very small effect on the ionization
energies of the two isomers and the metal–hydrocarbon stretching
mode of La(η<sup>2</sup>-CCCH<sub>2</sub>), but it reduces considerably
the metal–ligand stretching frequencies of La(HCCCH)
