Vanadocene de Novo: Spectroscopic and Computational Analysis of Bis(畏5-cyclopentadienyl)vanadium(II)

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• 作者：Timothy A. Jackson ; J. Krzystek ; Andrew Ozarowski ; Gayan B. Wijeratne ; Benjamin F. Wicker ; Daniel J. Mindiola ; Joshua Telser
• 刊名：Organometallics
• 出版年：2012
• 出版时间：December 10, 2012
• 年：2012
• 卷：31
• 期：23
• 页码：8265-8274
• 全文大小：427K
• 年卷期：v.31,no.23(December 10, 2012)
• ISSN：1520-6041

文摘

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^鈥?, 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鈥?00, 19鈥?60, and 24鈥?80 cm^鈥? are due to ligand-field transitions from the ⁴A_2g ground state to the ⁴E_1g, ⁴E_2g, and ⁴E_1g excited states, using symmetry labels from the D_5d 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鈥?.96 cm^鈥?), 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鈥?.90 cm^鈥?), where the dominant (80%) contribution to D arises from spin鈥搊rbit coupling between ligand-field states, with the largest contribution from a low-lying ²A_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^鈥? 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.