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 B 2 0, 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 4f z 3 orbital and the 4f x 3/4f y 3 pair, which are affected by the coordination ability of the axial ligand.