Wavelength Dependence of Photooxidation vs Photofragmentation of Chromocene

Photofragmentation of Gas-Phase Lanthanide Cyclopentadienyl Complexes: Experimental and Time-Dependent Excited-State Molecular Dynamics

Unimolecular gas-phase laser-photodissociation reaction mechanisms of open-shell lanthanide cyclopentadienyl complexes, Ln(Cp)₃ and Ln(TMCp)₃, are analyzed from experimental and computational perspectives. The most probable pathways for the photoreactions are inferred from photoionization time-of-flight mass spectrometry (PI-TOF-MS), which provides the sequence of reaction intermediates and the distribution of final products. Time-dependent excited-state molecular dynamics (TDESMD) calculations provide insight into the electronic mechanisms for the individual steps of the laser-driven photoreactions for Ln(Cp)₃. Computational analysis correctly predicts several key reaction products as well as the observed branching between two reaction pathways: (1) ligand ejection and (2) ligand cracking. Simulations support our previous assertion that both reaction pathways are initiated via a ligand-to-metal charge-transfer (LMCT) process. For the more complex chemistry of the tetramethylcyclopentadienyl complexes Ln(TMCp)₃, TMESMD is less tractable, but computational geometry optimization reveals the structures of intermediates deduced from PI-TOF-MS, including several classic “tuck-in” structures and products of Cp ring expansion. The results have important implications for metal–organic catalysis and laser-assisted metal–organic chemical vapor deposition (LCVD) of insulators with high dielectric constants.

Wavelength and Metal Dependence in the Photofragmentation of a Gas-Phase Lanthanide β-Diketonate Complex

The time-of-flight mass spectra of tris(2,2,6,6-tetramethyl-3,5-heptanedionato) lanthanide(III) [or Ln(thd)3 with Ln = Eu, Tb, Gd] produced by laser-induced multiphoton ionization in a supersonic expansion were studied as a function of laser excitation wavelength. Resonance-enhanced multiphoton ionization (REMPI), monitoring the Eu(I) ion signal from gas-phase Eu(thd)3, was observed in three distinct visible-excitation regions, corresponding to electronic absorption transitions on neutral Eu(0) atoms. The confirmation of the presence of Eu(0) atoms in the beam supports the proposed mechanism for the production of Ln atoms through sequential dissociation of neutral thd ligands from the metal following photoexcitation into ligand-to-metal charge-transfer (LMCT) states. Evidence is also presented that the LnO+ and LnOH+ fragments observed in the mass spectrum are produced via a separate, competing fragmentation pathway. The branching ratios between the two fragmentation pathways are compared for Ln(thd)3 (Ln = Eu, Tb, Gd). The ligand-dissociation pathway that produces Ln atoms appears to be more favorable in Ln(thd)3 complexes with low-lying LMCT states. Finally, the observation of the Tb2(thd)6+ dimer and its associated fragmentation pattern, as well as the presence of metal carbides, which are relevant to carbon contamination in chemical vapor deposition, is discussed.

Multiple Photochemical Reaction Pathways in a Ni(II) Coordination Compound

The gas-phase photofragmentation of the mixed-ligand coordination compound trans-bis(trifluoroacetato)bis(N,N'-dimethylethylenediamine)nickel(II) (Ni(tfa)2(dmen)2) detected via time-of-flight mass spectrometry is reported. In contrast to most gas-phase studies of metal-containing compounds where fragmentation of weak metal-ligand bonds dominates, the data here show that the dmen ligands fragment while still coordinated to nickel. The manner in which these ligands fragment is highly specific, leading to mono- and diimine species that remain coordinated to nickel. Uncoordinated mono- and diimine species and various small dmen fragments are also observed with high intensities in the low mass region of the spectra. NiF+, a fragment that is formed by fluorine abstraction, is always observed, even though no Ni-F bonds exist in the starting material.

Wavelength Dependent Photofragmentation Patterns of Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Ln (III) (Ln = Eu, Tb, Gd) in a Molecular Beam

Laser photoionization and ligand photodissociation in Ln(thd)(3) (Ln = Eu, Tb, Gd; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) are studied in a molecular beam via time-of-flight mass spectrometry. The fragmentation patterns are strongly wavelength dependent. With 355 nm excitation, the mass spectrum is dominated by Ln(2+), Ln(+), and LnO(+) fragments. The bare Ln ions are believed to arise from photoionization of neutral Ln atoms. The Ln atoms, in turn, are produced from the Ln(thd)(3) complex in a sequence of Ln reductions (through ligand-to-metal charge-transfer transitions), with each reduction being accompanied by the dissociation of a neutral ligand radical. In contrast, under visible-light (410-450 nm) excitation, a significant Ln(thd)(n)(+) signal is observed (where n = 2,3 for Ln = Tb,Gd and n = 1-3 for Ln = Eu). Thus, with visible excitation, photoionization of Ln(thd)(n) competes effectively with the Ln-reduction/ligand-dissociation sequence that leads to the dominant bare Ln-ion signal seen with 355 nm excitation. The fact that monoligated Ln(thd)(+) is observed only for Ln = Eu is interpreted in terms of the relative accessibility of an excited ligand-to-metal charge-transfer state from the ground electronic state of neutral Ln(thd).

Wavelength-dependent photofragmentation and photoionization of gaseous (eta4-cycloocta-1,5-diene)(eta5-cyclopentadienyl)cobalt

Gas-phase photoreactions and photoproducts of the mixed-ligand compound (eta(4)-cycloocta-1,5-diene)(eta(5)-cyclopentadienyl)cobalt are reported. Significant amounts of the monoligated complexes CoCOD and CoCp are produced, and the relative amounts are wavelength dependent. The COD ligand (with the weakest metal-ligand bonds) is always preferentially labilized as expected, but the relative amounts of the CoCOD and CoCp fragments change by 1 order of magnitude as the excitation wavelength is changed. The gas-phase photoreactions exhibit other surprising features that are uncommon in the photoreactions of organometallic compounds in the gas phase. Significant amounts of the intact cation are formed, in contrast to most reported reactions where fragmentation of the weak metal-ligand bonds precedes ionization. Most surprisingly, fragmentation of the ligands occurs while the ligands are still coordinated. The resulting metal complexes contain ligand fragments that remain coordinated to the metal. Breaking several carbon-carbon bonds with retention of weaker metal-ligand bonds is unexpected. For example, C(5)H(5) undergoes fragmentation while still coordinated to the cobalt, yielding smaller compounds such as Co(CH)(+), Co(C(2)H(2))(+), Co(C(3)H(3))(+), and Co(C(4)H(6))(+). Correspondingly, coordinated COD yields Co(C(6)H(6))(+), Co(C(5)H(5))(+), Co(C(4)H(6))(+), Co(C(3)H(3))(+), Co(C(2)H(2))(+), and Co(CH)(+). The wavelength dependence of the ligand labilization is examined by utilizing mass-selected resonance enhanced multiphoton ionization spectroscopy. Both broad bands and sharp lines are observed in the mass-selected excitation spectra. The action spectra obtained in the gas phase while monitoring the cobalt ion follow the absorption onset found in solution. The changes in fragmentation pathways are interpreted in terms of the initially accessed excited state.