Original scientific paper

DFT Studies on Molecular and Electronic Structures of Cationic Carbene Complexes [L₂(η5-C₅H₅)Fe=CR₅]+ (L = CO, PH₃, dhpe, PPh₃; R = H, F, CH₃)

Isabella Hyla-Kryspin ; Organisch-Chemisches Institut der Universität Münster, Corrensstr. 40, D-48149 Münster, Germany
Christian Mück-Lichtenfeld ; Organisch-Chemisches Institut der Universität Münster, Corrensstr. 40, D-48149 Münster, Germany
Stefan Grimme ; Organisch-Chemisches Institut der Universität Münster, Corrensstr. 40, D-48149 Münster, Germany

Abstract

For the title complexes we discuss the results of DFT calculations for (i) the molecular and electronic structures, (ii) the rotational barriers of the carbenes around the Fe=C_carb bond (ΔE_rot), and (iii) the binding energies of the carbenes (De). Where available, the calculated properties of the Fe=C_carb bonds are compared with previous theoretical and experimental data of some prototypical carbene complexes classified as Fischer- or Schrock-type compounds. It is shown that the rotational barriers of the carbenes, the Fe−C_carb bond distances and bond strengths are sensitive to the carbene substituents and to the ligands L attached to iron. For complexes with given L the values of ΔE_rot diminish in the order: CH₂ > CF₂ > CMe₂ and an inverse ordering is obtained for the decrease of the Fe=C_carb bond distance. The ΔE_rot of dimethylcarbene are close to those of Fischer-type compounds, while ΔE_rot of methylidene approach values typical for Schrock-type carbenes. The replacement of the CO ligand by poorer π-acceptor ligand increases the values of ΔE_rot in the order: CO < PH₃ < dhpe < PPh₃ and diminishes the Fe−C_carb bond distance in the same order. The binding energies (De) of the carbenes are larger than the first dissociation energy of CO from the pentacarbonyliron complex. For complexes with given L, except one, the De values decrease in the order: CH₂ > CMe₂ > CF₂. The properties of the investigated compounds are traced back to the character of the Fe → C_carb π-backbonding interactions and their competitions with the Fe → L and R → C_carb π- interactions. It is also shown that the PH₃ ligand can only be considered with caution as a good model for the PPh₃ ligand in computational studies.

Keywords

iron carbenes; rotational barriers; binding energies; DFT calculations

Hrčak ID:

38628

URI

https://hrcak.srce.hr/38628

Publication date:

30.6.2009.

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