Vanadocene de Novo: Spectroscopic and Computational Analysis of Bis(η⁵-cyclopentadienyl)vanadium(II)

Abstract

The magnetic and electronic properties of the long-known organometallic complex vanadocene (VCp 2 ), 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 51 V 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 VCp 2 :Electronic absorption and VT-MCD studies on VCp 2 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 4 A 2g ground state to the 4 E 1g , 4 E 2g , and 4 E 1g excited states, using symmetry labels from the D 5d point group (i.e., staggered VCp 2 ). Contributions to the D parameter in VCp 2 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 VCp 2 . 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 VCp 2 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 2 A 1g 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.

Similar ligand–metal bonding for transition metals and actinides? 5f1 U(C7H7)2−versus 3dn metallocenes

“…As expected, SOC has no qualitative impact on the electronic structures of the TM metallocenes, as it has nearly no effect in the free ions themselves (see Table S9 † ). Upon a closer look, of course, SOC induces slight zero field splittings (ZFS) in the GSs of vanadocene and nickelocene of ∼2 cm –1 and ∼39 cm –1 (PT2-SO), respectively, in good agreement with experimental ZFS parameters D of ∼2.8 cm –1 for vanadocene 39 , 91 and ∼34 cm –1 for nickelocene. 40 , 112 , 113 …”

“…For nickelocene, Ni 2+ (3d 8 ), the CF configuration is (3d σ ) 2 (3d δ ) 4 (3d π ) 2 and the GS is a spin-triplet . Note that the 3d-shell occupations are confirmed by experiments for both vanadocene 89 – 91 and nickelocene. 89 , 90 , 92 …”

Nonmagnetic single-molecule spin-filter based on quantum interference

“…The vanadocene molecule has a sandwich structure, with a core vanadium ion located between two cyclopentadienyl conjugated rings. In the gas phase, vanadocene has a total spin of S = 3/2 20 , which is essentially preserved in the junction, as confirmed by our ab initio calculations.

Fig. 1Measured conductance of bare silver atomic-scale junctions and Ag/vanadocene molecular junctions.

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Structural and electronic elucidation of a N-heterocyclic silylene vanadocene adduct

“…At 10 K, the EPR spectrum ( Figure S6 in ESI) significantly differs from the anisotropic splitting of a high-spin (S = 3/2 from 3d 3 ) complex in frozen solution. 19 In our case, neither the g-anisotropy nor the hyperfine anisotropy was completely resolved at X-band. Our initial hypothesis was that only the permissible transition (∆M I = 0) between m I = -1/2 and m I = +1/2 nuclear sublevels was resolved in the parallel (g) region at 334 mT as there were no resonant transitions in the perpendicular (g) region at 167 mT.…”