High-Resolution Electron Spectroscopy of Gas-Phase Metal−Aromatic Complexes

High-resolution electron spectroscopy combines pulsed field ionization zero-electron kinetic energy (ZEKE) detection with in situ laser-assisted synthesis and supersonic expansion. The technique offers sub-meV spectral resolution for the electron spectra of metal complexes and is a powerful tool to study their bonding and structures. This Perspective presents recent progress in single-photon ZEKE spectroscopy of metal−aromatic complexes and focuses on the determination of the electron spin multiplicities, metal binding sites and modes, rotational conformers, and conformational changes of these critical species in organometallic chemistry

High-Resolution Electron Spectroscopy and Rotational Conformers of Group 6 Metal (Cr, Mo, and W) Bis(mesitylene) Sandwich Complexes

Group 6 metal–bis­(mesitylene) sandwich complexes are produced by interactions between the laser-vaporized metal atoms and mesitylene vapor in a pulsed molecular beam source, identified by photoionization time-of-flight mass spectrometry, and studied by pulsed-field ionization zero-electron kinetic energy spectroscopy and density functional theory calculations. Although transition metal–bis­(arene) sandwich complexes may adopt eclipsed and staggered conformations, the group 6 metal–bis­(mesitylene) complexes are determined to be in the eclipsed form. In this form, rotational conformers with methyl group dihedral angles of 0 and 60° are identified for the Cr complex, whereas the 0° rotamer is observed for the Mo and W species. The 0° rotamer is in a <i>C</i>_2<i>v</i> symmetry with the neutral ground state of ¹A₁ and the singly positive charged ion state of ²A₁. The 60° rotamer is in a <i>C</i>_<i>i</i> symmetry with the neutral ground state of ¹A_g and the ion state of ²A_g. Partial conversion of the 60 to 0° rotamer is observed from He to He/Ar supersonic expansion for Cr–bis­(mesitylene). The unsuccessful observation of the 60° rotamer for the Mo and W complexes is the result of its complete conversion to the 0° rotamer in both He and He/Ar expansions. The adiabatic ionization energies of the 0° rotamers of the three complexes are in the order of Cr–bis­(mesitylene) < W–bis­(mesitylene) < Mo–bis­(mesitylene), which is different from that of the metal atoms. These metal–bis­(mesitylene) complexes have lower ionization energies than the corresponding metal–bis­(benzene) and −bis­(toluene) species

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

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

Lanthanum-Mediated C–H Bond Activation of Propyne and Identification of La(C₃H₂) Isomers

η²-Propadienylidenelanthanum [La­(η²-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(η²-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(η²-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(η²-CCCH₂), but it reduces considerably the metal–ligand stretching frequencies of La­(HCCCH)

Threshold Ionization and Spin–Orbit Coupling of Ceracyclopropene Formed by Ethylene Dehydrogenation

A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce­(C₂H₂) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativistic correction, dynamic electron correlation, and spin–orbit coupling. The MATI spectrum exhibits two nearly identical band systems separated by 128 cm^–1. The separation is not affected by deuteration. The two-band systems are attributed to spin–orbit splitting and the vibrational bands to the symmetric metal–ligand stretching and in-plane carbon–hydrogen bending excitations. The spin–orbit splitting arises from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The organolanthanide complex is a metallacyclopropene in <i>C</i>_2<i>v</i> symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f¹6s¹ and Ce 4f¹, respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a π* antibonding orbital of acetylene

Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum–Benzene in the Gas Phase

The reaction between La atoms and 1,3-butadiene is carried out in a laser-vaporization molecular beam source. Metal–hydrocarbon species with formulas La­(C_<i>n</i>H_<i>n</i>) (<i>n</i> = 2, 4, and 6) and La­(C_<i>m</i>H_<i>m</i>+2) (<i>m</i> = 4 and 6) are observed with time-of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum–benzene complex [La­(C₆H₆)] is formed by 1,3-butadiene addition to lanthana­cyclopropene [La­(C₂H₂)] followed by molecular hydrogen elimination. Lanthana­cyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La­[η⁴-(1-buten-3-yne)] [La­(C₄H₄)] and 1-lanthana­cyclopent-3-ene [La­(C₄H₆)]. The La­(benzene) complex exhibits distinctive metal–ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations