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 Yb ion in [Yb(DOTAM)] has profound consequences in the magnetic anisotropy and optical properties of the complex, as evidenced by H NMR and UV-vis spectroscopies. The pseudocontact shifts of H nuclei and the F ← F absorption band were found to be very sensitive to the nature of the axial ligand (MeOH, HO, MeOH, or F). The energy levels of the F and F manifolds in [Yb(DOTAM)(X)] (X = MeOH, HO, or dimethyl sulfoxide (DMSO)) and [Yb(DOTAM)F] 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 H NMR spectral data, though for the [Yb(DOTAM)(HO)] and [Yb(DOTAM)F] 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 , as predicted by Bleaney's theory. The change in the sign of the magnetic anisotropy from prolate (X = MeOH, HO, or DMSO) to oblate in [Yb(DOTAM)F] is related to the relative energies of the 4f orbital and the 4f/4f pair, which are affected by the coordination ability of the axial ligand.