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

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 2F5/2 ← 2F7/2 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 2F5/2 and 2F7/2 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 4fz3 orbital and the 4fx3/4fy3 pair, which are affected by the coordination ability of the axial ligand.