Threshold Ionization of Cobaltocene: The Metallocene Molecule Revealing Zero Kinetic Energy States

Abstract

Dedicated to Professor Gleb Abakumov on the occasion of his 75th birthdaySince the discovery of ferrocene 60 years ago, [1] metallocenes have represented one of the most intriguing classes of organometallics. Numerous theoretical and experimental investigations of electronic structures of these prototypical systems form a basis for understanding the nature and fundamental properties of the metal-ligand delocalized chemical bonds. In contrast, intense chemical interest has been focused on these compounds during the last decade because of their relevance to organic synthesis, catalysis, bioorganometallic chemistry, and materials science. [2] Redox parameters appear to play a key role in tuning the chemical reactivity of cyclopentadienyl metal complexes and properties of the metallocene-based materials. [3] Precise information on ionization energies (IEs) of these sandwich compounds is, therefore, of fundamental importance in predicting their chemical behavior.High-resolution IEs can be obtained with zero kinetic energy (ZEKE) and mass-analyzed threshold ionization (MATI) techniques [4] based on the laser excitation of jetcooled molecules to high Rydberg levels (ZEKE states) and subsequent ionization by an electric pulse. Recently, fascinating insight into the electronic structures of transition-metal bis(arene) derivatives has been provided by ZEKE and MATI studies. [5] However, no successful attempts to measure the ZEKE or MATI spectra of metallocenes have been reported so far. Multiphoton excitation of [Cp 2 Fe] and [Cp 2 Ni] (Cp = h 5 -cyclopentadienyl) gave no MATI signals, probably because of the instability of the corresponding ZEKE states. [5b] Herein we report the first example of a metallocene MATI spectrum. In this work, we studied cobaltocene which, like ferrocene, represents one of the key sandwich compounds. This complex attracts the attention of theoretical chemists [6] because of its degenerate ground electronic state, 2 E 1 '' in the D 5h point group (the [Cp 2 Co] 19electron configuration is …[3d(e 2 ')] 2 [3d(a 1 ')] 2 [3d(e 1 '')] 1 ). The neutral molecule should, therefore, appropriately lower its symmetry as a result of the Jahn-Teller (JT) effect. This makes it especially interesting to obtain a high-resolution photoionization spectrum of cobaltocene. Moreover, ionization of the [Cp 2 Co] molecule yields the 18-electron cation which is an isoelectronic analogue of the highly symmetric (D 5h ) ferrocene. The surprisingly well-resolved MATI spectrum of [Cp 2 Co] measured in this work demonstrates that the powerful MATI and ZEKE techniques can be efficiently used to investigate fine electronic effects in metallocene molecules.Indeed, when conditions for the one-photon excitation of the cobaltocene ZEKE states in a supersonic jet were found in our experiments, the MATI spectrum having a rich vibronic structure was recorded (Figure 1). In contrast to the classical UV photoelectron spectrum [7] (PES) showing a single broad 3d(e 1 '') ionization band with the full width at half-maximum (FWHM) of...

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“…We found that in vacuum, in the JT distorted ground state, the vertical −IP of cobaltocene was −5.33 eV within the PBE approximation and −5.45 eV when using the PBE0 functional. Both values are within 10% of the reported experimental values of −5.55 eV (from UPS studies) and −5.44 eV (from a zero kinetic-energy technique) . The cobaltocene family contains an even stronger reducing agent, decamethylcobaltocene, having a higher experimental vertical −IP of −4.71 eV .…”

“…Both values are within 10% of the reported experimental values of À5.55 eV (from UPS studies) 67 and À5.44 eV (from a zero kineticenergy technique). 68 The cobaltocene family contains an even stronger reducing agent, decamethylcobaltocene, having a higher experimental vertical ÀIP of À4.71 eV. 67 Our calculated ÀIP is À4.25 eV, again within 10% error.…”

Photoionization and photofragmentation in mass spectrometry with visible and UV lasers

“…Two years later, Ketkov and Selzle () published a study which described the analysis of metallocenes with the help of MATI spectroscopy, in this case cobaltocene, Cp 2 Co. It was the first time that MATI spectra of such structures could be obtained.…”

Metal Cluster Size-Dependent Activation Energies of Growth of Single-Chirality Single-Walled Carbon Nanotubes inside Metallocene-Filled Single-Walled Carbon Nanotubes

“…The metal–carbon distances obtained from the experimental data (electron and X-ray diffraction) and density functional theory (DFT) calculations were reported to increase in the line with Fe-Co-Ni. The Fe-C distances were found to be in the range from 2.007 to 2.064 Å [ 44 , 45 , 46 , 47 , 48 , 49 ], the Co-C distances from 2.070 to 2.120 Å [ 37 , 49 , 50 , 51 , 52 , 53 ] and the Ni-C distances from 2.148 to 2.196 Å [ 36 , 49 , 54 , 55 ]. This is explained by changing the electronic structure of the molecules in the line with FeCp 2 —CoCp 2 —NiCp 2 .…”