Nanosecond multiphoton ionization spectroscopy of transition-metal sandwich compounds: a comparative study of nickelocene, ferrocene and bis(η6-benzene)chromium

Ionization of Decamethylmanganocene: Insights from the DFT-Assisted Laser Spectroscopy

Metallocenes represent one of the most important classes of organometallics with wide prospects for practical use in various fields of chemistry, materials science, molecular electronics, and biomedicine. Many applications of these metal complexes are based on their ability to form molecular ions. We report the first results concerning the changes in the molecular and electronic structure of decamethylmanganocene, Cp*₂Mn, upon ionization provided by the high-resolution mass-analyzed threshold ionization (MATI) spectroscopy supported by DFT calculations. The precise ionization energy of Cp*₂Mn is determined as 5.349 ± 0.001 eV. The DFT modeling of the MATI spectrum shows that the main structural deformations accompanying the detachment of an electron consist in the elongation of the Mn-C bonds and a change in the Me out-of-plane bending angles. Surprisingly, the DFT calculations predict that most of the reduction in electron density (ED) upon ionization is associated with the hydrogen atoms of the substituents, despite the metal character of the ionized orbital. However, the ED difference isosurfaces reveal a complex mechanism of the charge redistribution involving also the carbon atoms of the molecule.

Laser spectroscopic and computational insights into unexpected structural behaviours of sandwich complexes upon ionization

Transition-metal sandwich complexes play key roles in various fields such as fundamental and applied chemistry; many of their unique properties arise from their ability to form stable or reactive ions. The first mass-analyzed threshold ionization (MATI) spectra of mixed sandwich compounds, (Ch)(Cp)Cr and (Cot)(Cp)Ti (Ch = η⁷-C₇H₇, Cp = η⁵-C₅H₅, Cot = η⁸-C₈H₈), presented in this work provide an extremely accurate description of the electron detachment. The ionization energies of the neutrals and stabilization energies of the metal-ligand interactions upon ionization are derived from the MATI data with an accuracy of 0.0006 eV. In combination with DFT calculations, laser threshold ionization spectroscopy reveals surprisingly different structural variations accompanying the detachment of the non-bonding d_z² electron from the sandwich molecules. The geometry of (Ch)(Cp)Cr remains practically unchanged while the ionization of (Cot)(Cp)Ti causes a noticeable shortening of the inter-ring distance, similar to that resulting from the ionization of a typical antibonding orbital. Electron density analysis throws light on the nature of these amazing effects.

Substituent effects on the electronic structures of sandwich compounds: new understandings provided by DFT-assisted laser ionization spectroscopy of bisarene complexes

Recent advances on substituent effects in transition metal bisarene complexes studied with high-resolution threshold ionization spectroscopy are reviewed to demonstrate new aspects of the ligand influence on electronic structures of sandwich molecules. Unprecedented accuracy in the determination of ionization energies provided by the laser techniques makes it possible to reveal and describe quantitatively such fine phenomena as isotope effects, the mutual substituent influence or variations of substituent effects on replacing the central metal atom with its Group analogues. In combination with DFT calculations, laser ionization spectroscopy unveils mechanisms of the ligand influence on unique redox properties of sandwich complexes which are of key importance for their practical use.

Rydberg state mediated multiphoton ionization of (η7-C7H7)(η5-C5H5)Cr: DFT-supported experimental insights into the molecular and electronic structures of excited sandwich complexes

The first REMPI/DFT study of a mixed sandwich complex reveals fine ligand effects on structural transformations accompanying electronic excitation.

TD DFT insights into unusual properties of excited sandwich complexes: structural transformations and vibronic interactions in Rydberg-state bis(η6-benzene)chromium

TD DFT calculations reveal specific features of the Jahn–Teller effect in the lowest Rydberg p state of a prototypical sandwich molecule.

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).