来源：ACS Publications
We report ground states, equilibrium structures, and both electronic and electron paramagnetic resonance (EPR) spectra of divalent rare-earth metallocenes Ln(CpiPr5)2 as obtained from scalar and exact two-component (X2C) relativistic density functional theory (DFT) calculations. While the results for metallocenes with Ln = Nd, Sm, Eu, Tm, and Yb are consistent with a conventional (4f)n+1 configuration of the metal atom, metallocenes with Ln = La, Ce, Gd, and Lu are best described as having (4f)n(5d)1 ground states. Strong configuration mixing is observed for Ln = Pr; for Ln = Ho and Er, two distinct states of (4f)n+1 and (4f)n(5d)1 character are energetically close, and comparison with available experimental data slightly favors a (4f)n(5d)1 ground-state assignment. Our results show that the ground states of the Nd and Pr compounds have strong or partial (4f)n+1 character, suggesting that previous assignments based solely on experimental data and atomic f-d splittings [McClain, K. R. J. Am. Chem. Soc. 2022, 144, 22193–22201. 10.1021/jacs.2c09880] should be revised. X2C calculations of the EPR parameters show that the magnitude of the isotropic hyperfine coupling (HFC) relative to an atomic reference is a poor descriptor of s/d mixing, and ground-state assignments based on this approach are unreliable for rare-earth compounds. A massive HFC constant of 4401 MHz is predicted for the Lu compound.
While ferrocene was discovered in the 1950s, the synthesis and characterization of its f-element analogs with substituted cyclopentadienyl ligands have only recently gained significant momentum. In 2000, Sitzmann et al. reported the structures of classical divalent bis(pentaisopropylcyclopentadienyl) metallocenes, Ln(CpiPr5)2, for Ln = Sm, Eu, and Yb. Although divalent bis(cyclopentadienyl) lanthanide species of Sm and Eu have been known since the early 1980s, the Ln(CpiPr5)2 (Ln = Tb and Dy) metallocenes reported by Gould et al. in 2019 were the first linear, divalent lanthanocenes with (4f)n(5d)1 ground states, and exhibit unique magnetic and electronic properties. Recently, McClain et al. extended the synthesis of these divalent metallocenes with bulky CpiPr5 ligands to yttrium and the rest of the lanthanide series(except radioactive Pm). In contrast to their trivalent counterparts, the electronic structure of divalent open-shell lanthanide complexes is highly sensitive to coordination geometry and ligand field due to the near-degeneracy of the 4f, 5d, and 6s shells, giving rise to a variety of states with often radically different properties and reactivity even within the same coordination geometry. Some of these compounds exhibit single-molecule magnetism, giant hyperfine coupling constants (HFC), or photocatalyic properties.
Density functional theory (DFT) calculations have played a critical role in the characterization of the first linear lanthanocenes and actinocenes of the chemical formula M(CpiPr5)2 by connecting the observed structural and spectroscopic properties to the electronic structure of these compounds. However, the electronic structure of the more recently synthesized lanthanocenes was inferred by empirical comparison of the structure and spectra with known precedents, as well as by comparing isotropic HFC constants to atomic data. Here, we critically assess these assignments using scalar and exact two-component (X2C) relativistic DFT calculations. Efficient implementations enabling accurate theoretical simulations of electron paramagnetic resonance (EPR) spectra have recently become available, allowing us to investigate in detail how the electronic structure affects HFC constants in these systems.