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Spectroscopic Characterization of Lanthanum-Mediated Dehydrogenation and C–C Bond Coupling of Ethylene

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journal contribution
posted on 20.06.2016, 00:00 by Sudesh Kumari, Wenjin Cao, Yuchen Zhang, Mourad Roudjane, Dong-Sheng Yang
La­(C2H2) and La­(C4H6) are observed from the reaction of laser-vaporized La atoms with ethylene molecules by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. La­(C2H2) is identified as a metallacyclopropene and La­(C4H6) as a metallacyclopentene. The three-membered ring is formed by concerted H2 elimination and the five-membered cycle by dehydrogenation and C–C bond coupling. Both metallacycles prefer a doublet ground state with a La 6s-based unpaired electron. Ionization of the neutral doublet state of either complex produces a singlet ion state by removing the La-based electron. The 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. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal–hydrocarbon cycles are lower than that of the neutral La atom. Deuteration has a small effect on the ionization energies of the two cyclic radicals but distinctive effects on their vibrational frequencies.