Axial Ligation in Ytterbium(III) DOTAM Complexes Rationalized with Multireference and Ligand-Field ab Initio...

The nature of the axial ligand coordinated to the Yb³⁺ ion in [Yb(DOTAM)]³⁺ has profound consequences in the magnetic anisotropy and optical properties of the complex, as evidenced by ¹H NMR and UV-vis spectroscopies. The pseudocontact shifts of ¹H nuclei and the ²F_5/2 ← ²F_7/2 absorption band were found to be very sensitive to the nature of the axial ligand (MeOH, H₂O, MeOH, or F^-). The energy levels of the ²F_5/2 and ²F_7/2 manifolds in [Yb(DOTAM)(X)]³⁺ (X = MeOH, H₂O, or dimethyl sulfoxide (DMSO)) and [Yb(DOTAM)F]²⁺ complexes were assigned from the analysis of the optical spectra and ab initio calculations based on CASSCF wave functions that considered dynamic correlation through perturbation theory (NEVPT2) and spin-orbit coupling effects. The magnetic anisotropies obtained with ab initio calculations are in good agreement with the experimental values derived from ¹H NMR spectral data, though for the [Yb(DOTAM)(H₂O)]³⁺ and [Yb(DOTAM)F]²⁺ complexes, the explicit inclusion of a few second-sphere water molecules is required to improve the calculated data. Crystal-field calculations show that the observed pseudocontact shifts do not correlate well with the crystal-field parameter B₂⁰, as predicted by Bleaney's theory. The change in the sign of the magnetic anisotropy from prolate (X = MeOH, H₂O, or DMSO) to oblate in [Yb(DOTAM)F]²⁺ is related to the relative energies of the 4f³ orbital and the 4f³/4f³ pair, which are affected by the coordination ability of the axial ligand.