Lanthanum-mediated C-H bond activation of propyne and identification of La(C₃H₂) isomers

n²-Propadienylidenelanthanum [La(n²-CCCH₂)] and deprotiolanthanacyclobutadiene [La(HCCCH)] of La(C₃H₂) 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(n²-CCCH₂) 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(n²-CCCH₂) 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(n²-CCCH₂), but it reduces considerably the metal-ligand stretching frequencies of La(HCCCH).