Vanadocene de novo: Spectroscopic and computational analysis of bis(η5-cyclopentadienyl)vanadium(II)

Academic Article

Abstract

  • The magnetic and electronic properties of the long-known organometallic complex vanadocene (VCp2), which has an S = 3/2 ground state, were investigated using conventional (X-band) electron paramagnetic resonance (EPR) and high-frequency and -field EPR (HFEPR), electronic absorption, and variable-temperature magnetic circular dichroism (VT-MCD) spectroscopies. Frozen toluene solution X-band EPR spectra were well resolved, yielding the 51V hyperfine coupling constants, while HFEPR were also of outstanding quality and allowed ready determination of the rigorously axial zero-field splitting of the spin quartet ground state of VCp2: D = +2.836(2) cm-1, g⊥ = 1.991(2), g∥ = 2.001(2). Electronic absorption and VT-MCD studies on VCp2 support earlier assignments that the absorption signals at 17 000, 19 860, and 24 580 cm-1 are due to ligand-field transitions from the 4A 2g ground state to the 4E1g, 4E 2g, and 4E1g excited states, using symmetry labels from the D5d point group (i.e., staggered VCp2). Contributions to the D parameter in VCp2 and further insights into electronic structure were obtained from both density functional theory (DFT) and multireference SORCI computations using X-ray diffraction structures and DFT-energy-minimized structures of VCp2. Accurate D values for all models considered were obtained from DFT calculations (D = 2.85-2.96 cm -1), which was initially surprising, because the orbitally degenerate excited states of VCp2 cannot be properly treated by DFT methods, as they require a multideterminant description. Therefore, D values were also computed using the SORCI (spectroscopically oriented configuration interaction) method, which provides multireference descriptions of ground and excited states. SORCI calculations gave accurate D values (2.86-2.90 cm-1), where the dominant (∼80%) contribution to D arises from spin-orbit coupling between ligand-field states, with the largest contribution from a low-lying 2A1g state. In contrast, the D value obtained by the DFT method is achieved only fortuitously, through cancellation of errors. Furthermore, the SORCI calculations predict ligand-field excited-state energies within 1300 cm-1 of the experimental values, whereas the corresponding time-dependent DFT calculations fail to reproduce the proper ordering of excited states. Moreover, classical ligand-field theory was validated and expanded in the present study. Thus older theory still has a place in the analysis of paramagnetic organometallic complexes, along with the latest ab initio methods. © 2012 American Chemical Society.
  • Authors

    Published In

  • Organometallics  Journal
  • Digital Object Identifier (doi)

    Author List

  • Jackson TA; Krzystek J; Ozarowski A; Wijeratne GB; Wicker BF; Mindiola DJ; Telser J
  • Start Page

  • 8265
  • End Page

  • 8274
  • Volume

  • 31
  • Issue

  • 23